Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Schematics and Layouts => Topic started by: galaxiex on September 13, 2015, 12:33:41 PM

Title: National GA920P Schematic
Post by: galaxiex on September 13, 2015, 12:33:41 PM
Another eBay acquisition.  :)

Here's pics of how it came to me.
This ones gonna need some work.  ::)

Right out of the shipping box it was/is filthy!

In case it wasn't noticed...
I seem to have a weird affinity for these cheap/trashy/small/throw-away/old/SS practice amps.   <3)

I have about 8 of them now and the supply of reasonably priced units on fleabay seems to have dried up.
I wonder who could be responsible for that???  :lmao:
All I'm seeing now on thebay is guys that want a fortune just because it's "vintage"  ::)

(http://i58.tinypic.com/n7t77.jpg)

(http://i58.tinypic.com/erli4m.jpg)

(http://i57.tinypic.com/27yo47s.jpg)



Gonna need a new rear panel for the speaker cab.
This one looks homemade.   :o

(http://i57.tinypic.com/24e86cx.jpg)


Speaker jack on the left. Tremolo and Reverb foot-switch jacks on the right.

(http://i62.tinypic.com/2s92c6v.jpg)

Got a Radio Shack full range 8" with whizzer cone in the top location.

What is possibly??? one of the original speakers in the lower spot.

Gonna need some new speakers and a new baffle.

Perhaps some nice 10 inch units since I have to replace the baffle anyway.

Opinions on the ports?
Should I cut them on the new baffle or not bother?

(http://i62.tinypic.com/2cekuu9.jpg)


To my surprise and delight... what appears to be the original schematic and parts list was tucked inside.    8)  :dbtu:

(http://i62.tinypic.com/13zbk38.jpg)

Here is a scan of the original schematic. 
Probably the only one on the whole Intergoogle thing...  :lmao:

In the process of redrawing this to make it more readable. Will post that when I have it done.

(http://i62.tinypic.com/34zdis6.jpg)

More...

Looks like someone messed with the input jack wiring cuz on a quick glance it doesn't seem to match the schematic,
but other than that the amp looks relatively untouched. Yay!

(http://i58.tinypic.com/317cpl0.jpg)

(http://i60.tinypic.com/qyfiiw.jpg)

(http://i61.tinypic.com/wl3m83.jpg)


I know I should make some changes to the mains wiring for safety reasons,
but the eBay seller said he powered it up to see if it worked so I just had to try it out.

Plugged it in and yeah, it works great!
Very clean even at full volume, albeit not very loud.
Practice amp bedroom level.
Tremolo and Reverb work ok.
Actually a very nice rich sounding SS amp.
Seems like lots of harmonics and "shimmer".   8|  :dbtu:

More...

(http://i59.tinypic.com/14kb2v8.jpg)

(http://i62.tinypic.com/6f2ve0.jpg)

(http://i58.tinypic.com/20zppxj.jpg)


Cheers!


Title: Re: National GA920P Schematic
Post by: galaxiex on September 14, 2015, 08:22:23 PM
Hmmm, something is not right in schematicville...

So I've been slowly redrawing the "original" schematic that I found inside this amp...
...all the while glancing at the circuit board... and now noticing that things don't seem to quite line up...

Bloody 'ell!!!

That schema is NOT the same as the actual circuit board!!!

 :grr :grr :grr

Now I have to trace yet another board to create a schematic.  :grr :grr :grr

(heavy sigh) Oh well... Might as well bloody get on with it then....

I wonder who could have left the "wrong" schematic inside this amp???  :trouble
Title: Re: National GA920P Schematic
Post by: galaxiex on November 01, 2015, 08:33:11 PM
Finally got back to this project... so-far have the power amp section schematic drawn from tracing the circuit board.

I must confess to *not* understanding the 3 diode network...
If anyone can shed some light on the function of this part of the circuit I'd much appreciate it.  :)
Title: Re: National GA920P Schematic
Post by: Enzo on November 01, 2015, 11:40:05 PM
The three diodes are your bias string, but I have to have doubts it is drawn correctly.

Make SURE where the cathode of D3 is wired.  I'd be much happier if D3 were wired to the base of Q4, rather than the emitter, and likewise, the 22 ohm seems more like it ought to go to the emitter.

I could be wrong.
Title: Re: National GA920P Schematic
Post by: galaxiex on November 02, 2015, 07:33:35 AM
Thanks Enzo.  :)
It's possible I got the base and emitter mixed up on Q4.
I did manage to dig up data sheets for all these ancient transistors
But I did not pull them from the board to verify pin outs.
I'll double check.
Title: Re: National GA920P Schematic
Post by: galaxiex on November 02, 2015, 04:41:52 PM
Thanks to Enzo for the help.  :)

I did indeed have the emitter and base mixed up on Q4.

Even to my untrained eye this just looks better.

Looks like R13 is a feedback resistor and could bear some experimenting with values, or even a pot.
Could this be used as a "Presence" control?
Title: Re: National GA920P Schematic
Post by: Enzo on November 02, 2015, 06:09:38 PM
More likely there for stability, I'd bet.  get your tone from the preamp.
Title: Re: National GA920P Schematic
Post by: Loudthud on November 02, 2015, 06:14:12 PM
R13 is not a feedback resistor, it's part of a bootstrap current source load for Q2. It uses the output capacitor and the speaker as the parts you normally see connected to the opposite rail of Q2's emitter which, because Q2 is a PNP, would be ground. A cute circuit that eliminates one resistor and one capacitor at the expense of a little DC current in the speaker. A similar circuit to the one in a recent MEF thread. Look at the last post here, link: http://music-electronics-forum.com/t40551/

The feedback network is R7, C2, and R5.
Title: Re: National GA920P Schematic
Post by: galaxiex on November 02, 2015, 07:56:42 PM
R13 is not a feedback resistor, it's part of a bootstrap current source load for Q2. It uses the output capacitor and the speaker as the parts you normally see connected to the opposite rail of Q2's emitter which, because Q2 is a PNP, would be ground. A cute circuit that eliminates one resistor and one capacitor at the expense of a little DC current in the speaker. A similar circuit to the one in a recent MEF thread. Look at the last post here, link: http://music-electronics-forum.com/t40551/

The feedback network is R7, C2, and R5.

Very interesting, and thanks for that.  :)

I see it now (the feedback network).

Slightly OT...
I actually have one of those Heathkit combo amps (Heath TA-16) that is mentioned in the 2nd post of that thread.
It uses that mix of NPN si and PNP ge transistors in the output stage. TA2577 NPN si, 2N2148 PNP ge.
Title: Re: National GA920P Schematic
Post by: galaxiex on November 02, 2015, 08:25:51 PM
More likely there for stability, I'd bet.  get your tone from the preamp.

Yup... actually this amp sounds very good just the way it is.
I'm probably not gonna muck with it too much, other than rebuilding the speaker cab
and installing some decent efficient drivers.


At most, I might put a buffer in front, I don't need a full schematic to do that but...
I'm drawing the schematic anyway cuz I can't find one anywhere, and I like to do this.  :duh
Title: Re: National GA920P Schematic
Post by: galaxiex on November 08, 2015, 08:35:23 PM
Ok, I finally got a schematic drawn up for this amp.
Hopefully no mistakes but I'm a little cross-eyed from squinting at circuit boards...  :P

No mods to it..... yet.... Not even sure if I will mod it, except to fix up the input jacks.
Someone was in here before, and I'm fairly sure the input jack wiring is not original... see pics above...

The amp works quite good as-is... but I need to play it for awhile....

On the schematic I only placed one input jack, but all three are wired to R1.
Likewise I only drew a rudimentary power supply, not showing the switch, fuse etc...

Anyway... if someone needs a schematic for one of these, here it is!
The only one on the whole Intergoogle. :)
I could not find a schem for these amps online despite extensive searches,
and no offense, but the file teemuk posted is corrupted and won't display properly.
I'm not even sure it's the same circuit.

The reverb circuit seems odd to me.
I've never seen one where the input of the tank is connected directly to V+.
The resistances for the tank are what I measured with my Fluke DMM.


Title: Re: National GA920P Schematic
Post by: Loudthud on November 08, 2015, 11:18:55 PM
Slightly OT...
I actually have one of those Heathkit combo amps (Heath TA-16) that is mentioned in the 2nd post of that thread.
It uses that mix of NPN si and PNP ge transistors in the output stage. TA2577 NPN si, 2N2148 PNP ge.

Most of the designs from the mid to late 60's didn't pay much attention to thermal feedback. Many times the bias diodes weren't mounted on the heatsink. A number of designs just used fixed bias like the Thomas/Vox amps. The Si/Ge designs were an attempt at a complementary output stage using parts that were actually available at reasonable prices.
Title: Re: National GA920P Schematic
Post by: Enzo on November 09, 2015, 06:16:52 PM
I bet you have seen that reverb circuit before, just not in a reverb.  The transducer in the reverb is a coil, just like the winding of a transformer.  So draw an inductor there instead of a box.  Now it looks just like a single ended output stage driving a transformer, like any Champ with a tube, or in SS amps with a transformer driving the outputs' bases, there is often a single ended transistor driving the primary side.
Title: Re: National GA920P Schematic
Post by: galaxiex on November 09, 2015, 11:48:05 PM
I bet you have seen that reverb circuit before, just not in a reverb.  The transducer in the reverb is a coil, just like the winding of a transformer.  So draw an inductor there instead of a box.  Now it looks just like a single ended output stage driving a transformer, like any Champ with a tube, or in SS amps with a transformer driving the outputs' bases, there is often a single ended transistor driving the primary side.

Right you are Enzo!
I guess what also confused me is the cap, C7, where one would expect to see a load resistor.
I realize now that the tank transducer coil is the actual load, and the cap is simply across the coil for....
Stability?
Frequency compensation?
Other?

It just looks odd to see they drive the tank coil directly without a transformer.
I'm so *used to* seeing a transformer at the input of the tank.
Also the tank transducer coils being almost equal resistance, I had not come across before.

When I first drew that circuit my first thought was...
WTH! this can't be right!....
but the reverb functioned just fine before I took it all apart to trace the board...
and hopefully will still function when I put it all back together.  :cheesy:

The other thread with the Audition amp has an identical *looking* transformer driven tank,
but much lower input transducer coil resistance as well as relatively high output transducer coil resistance.
That all appears more *normal* and what I am used to observing.
Title: Re: National GA920P Schematic
Post by: J M Fahey on November 11, 2015, 06:56:38 AM
Quote
I guess what also confused me is the cap, C7, where one would expect to see a load resistor.
I realize now that the tank transducer coil is the actual load, and the cap is simply across the coil for....
Stability?
Frequency compensation?
Other?
Correct.
The collector load is the drive coil.
Collector current passes straight through that coil, and is modulated by signal driving the transistor.
Since that current is "pushing" the drive magnet towards one side by a fixed amount, depending on coil turns, actual current, magnet strength, distance, etc, varying (modulating) said current will make the magnet mofe further or closer, vibrating/twisting following audio input.
Said twisting runs along the spring, takes some time to reach the other end (delay time, usually between 30ms to 80 ms, depending on tank) and produces the reverb effect.
Current  (at idle) is between 8 and 10 mA , a lot considering the coil has hundreds of turns.
Quote
It just looks odd to see they drive the tank coil directly without a transformer.
What for?
Maybe you mean in *tube*  circuits, where they must adapt tank low input impedance (in old Fenders around 8 ohms) to high tube impedance (12AT7) , but not here.
In fact, most modern Tube amps also drive the tank coil without transformers , either straight with an Op Amp or with a more powerful discrete amp .
Our friend Phatt hits them with a sledgehammer (TO220 transistors fed +/- 35V rails  :trouble  )
Quote
I'm so *used to* seeing a transformer at the input of the tank.
Browse a couple dozen modern schematics.
Modern meaning anything from a late 60's Kustom on  :lmao:
Quote
Also the tank transducer coils being almost equal resistance, I had not come across before.
Same.
In a Fender Black/Silverface reverb reverb drive is a small 1W power amp, parallel single ended 12AT7 , and the tiny output transformer can drive a small speaker, so it was plate to speaker wound, so input impedance was around (nominal) 8 ohms, typical speaker values, while output transduces uses many turns of very fine wire to have higher output voltage, so high impedance.

Now an Op Amp or a tiny transistor are not happy driving an 8 ohms load, but feel fine in the range of, say, 200 ohms to 2k to 10k .
Now, once the reverb factory set up the automatic winder for high impedance (a couple kohms) coils and it's churning, say, 1000 coils an hour, it's a very attractive idea (for the production manager)  to be able to use the exact same coil at the driving end.

In a nutshell:
* output coils will be as high impedance as possible to increase signal.
Not actually very high because wire becomes impossibly thin and breaks all the time (it sure ruins the manager's day) but a couple kiloohms is an acceptable compromise.
* input coils will be around 8 ohms for Fender type tube+transformer drives
* around 2000 ohms for single Op Amp driven tanks .
Regular Op Amps (741/RC4558/TL072)can't drive (well)  loads below 2k .
* around 150/200 ohms for small transistor boosted Op Amps (check the Lab Series reverbs).
(http://www.rru.com/~meo/Guitar/Amps/Lab/Images/Lab_Series_L5_L7_L9_L11_scheme.jpeg)

Here they use a full sized power amp, fed +/-25V, with TIP29/30 output transistors (1 Ampere instead of typical 5mA available from an Op Amp) , so loud that they use an 8 ohms input tank but they must attenuate it with a series 56 ohms resistor.

Phatt does a similar thing but with +/- 35V rails  :loco  and mid/high impedance tanks.

Otherwise the tiny drive coil would burst in flames ... not kidding

Quote
When I first drew that circuit my first thought was...
WTH! this can't be right!....
Does it look better now?  ;)

As of the parallel cap, the coil is an inductor, and in theory it will have infinite impedance at some high frequencies ... the transistor will not be very happy ... the parallel cap avoids that.
Title: Re: National GA920P Schematic
Post by: galaxiex on November 11, 2015, 02:47:11 PM
Quote
I guess what also confused me is the cap, C7, where one would expect to see a load resistor.
I realize now that the tank transducer coil is the actual load, and the cap is simply across the coil for....
Stability?
Frequency compensation?
Other?
Correct.
The collector load is the drive coil.
Collector current passes straight through that coil, and is modulated by signal driving the transistor.
Since that current is "pushing" the drive magnet towards one side by a fixed amount, depending on coil turns, actual current, magnet strength, distance, etc, varying (modulating) said current will make the magnet mofe further or closer, vibrating/twisting following audio input.
Said twisting runs along the spring, takes some time to reach the other end (delay time, usually between 30ms to 80 ms, depending on tank) and produces the reverb effect.
Current  (at idle) is between 8 and 10 mA , a lot considering the coil has hundreds of turns.
Quote
It just looks odd to see they drive the tank coil directly without a transformer.
What for?
Maybe you mean in *tube*  circuits, where they must adapt tank low input impedance (in old Fenders around 8 ohms) to high tube impedance (12AT7) , but not here.
In fact, most modern Tube amps also drive the tank coil without transformers , either straight with an Op Amp or with a more powerful discrete amp .
Our friend Phatt hits them with a sledgehammer (TO220 transistors fed +/- 35V rails  :trouble  )
Quote
I'm so *used to* seeing a transformer at the input of the tank.
Browse a couple dozen modern schematics.
Modern meaning anything from a late 60's Kustom on  :lmao:
Quote
Also the tank transducer coils being almost equal resistance, I had not come across before.
Same.
In a Fender Black/Silverface reverb reverb drive is a small 1W power amp, parallel single ended 12AT7 , and the tiny output transformer can drive a small speaker, so it was plate to speaker wound, so input impedance was around (nominal) 8 ohms, typical speaker values, while output transduces uses many turns of very fine wire to have higher output voltage, so high impedance.

Now an Op Amp or a tiny transistor are not happy driving an 8 ohms load, but feel fine in the range of, say, 200 ohms to 2k to 10k .
Now, once the reverb factory set up the automatic winder for high impedance (a couple kohms) coils and it's churning, say, 1000 coils an hour, it's a very attractive idea (for the production manager)  to be able to use the exact same coil at the driving end.

In a nutshell:
* output coils will be as high impedance as possible to increase signal.
Not actually very high because wire becomes impossibly thin and breaks all the time (it sure ruins the manager's day) but a couple kiloohms is an acceptable compromise.
* input coils will be around 8 ohms for Fender type tube+transformer drives
* around 2000 ohms for single Op Amp driven tanks .
Regular Op Amps (741/RC4558/TL072)can't drive (well)  loads below 2k .
* around 150/200 ohms for small transistor boosted Op Amps (check the Lab Series reverbs).
(http://www.rru.com/~meo/Guitar/Amps/Lab/Images/Lab_Series_L5_L7_L9_L11_scheme.jpeg)

Here they use a full sized power amp, fed +/-25V, with TIP29/30 output transistors (1 Ampere instead of typical 5mA available from an Op Amp) , so loud that they use an 8 ohms input tank but they must attenuate it with a series 56 ohms resistor.

Phatt does a similar thing but with +/- 35V rails  :loco  and mid/high impedance tanks.

Otherwise the tiny drive coil would burst in flames ... not kidding

Quote
When I first drew that circuit my first thought was...
WTH! this can't be right!....
Does it look better now?  ;)

As of the parallel cap, the coil is an inductor, and in theory it will have infinite impedance at some high frequencies ... the transistor will not be very happy ... the parallel cap avoids that.

Wow JM! Thanks much for all that!  :)

I freely admit to having little understanding of the design of circuits.
Some (simple) stuff I can figure out or intuit from surrounding circuitry.
(I guess it's all simple when you have years of experience/training...)
I'm mostly a tinkerer, tho I do study and try to understand this stuff.  :)

I very much appreciate you explaining and putting in language anyone can understand how this works and the reasoning behind it all.

This is what the "textbooks" lack, and they often assume a high(er) degree of knowledge/training than what I possess,
so it can be difficult for me to get "understanding" from textbooks.

Not to mention, the texts are often rather thick with math.
I can use Ohm’s Law and often do, and simple stuff like RC time constant.
But when I want to know how to bias a NPN bipolar transistor and the textbook goes into pages and pages of esoteric (to me) calculations and formula I get cross-eyed and dizzy.

I just want a (hopefully) simple, "plain language" explanation of what the parts in the circuit do,
so I can *by-guess and by-gosh* make something that works.
The texts are for a engineering point of view... I'm just tinkering... I end up copying others work and tinker from there.

Yeah, older Fender tube reverb circuits are what I’ve mostly looked at.
I will look/study/pay attention to, some “modern” circuits.  ;)

What you say about the tank coils certainly makes sense.
I get the impedance thing, and also how the “suits” like to make everything easy and cheap to manufacture.

Ya, it all looks better now…. Amazing how just a little understanding can clear up so much!

Thanks again!  :)
Title: Re: National GA920P Schematic
Post by: g1 on November 11, 2015, 08:42:44 PM
Here's some more reverb tank info you may find interesting, includes some sample circuits:
https://www.amplifiedparts.com/sites/all/modules/custom/tech_corner/files/spring_reverb_tanks_explained_and_compared.pdf
Title: Re: National GA920P Schematic
Post by: galaxiex on November 11, 2015, 09:02:03 PM
Thanks g1! That is indeed interesting.

I notice that document shows a Blues Junior partial reverb schem which does not use a matching tx.
The tank is directly driven by a TL072.
Interesting... because I have a Blues Jr. and never noticed that before, or,
more like, I wasn't paying much attention to the reverb circuit whenever I looked at the schematic.  ::)
Title: Re: National GA920P Schematic
Post by: galaxiex on November 21, 2016, 09:18:04 PM
Necro bump for a small update.

I created a crude board layout for this amp.
Title: Re: National GA920P Schematic
Post by: galaxiex on November 21, 2016, 09:22:25 PM
Also slightly updated the schematic.
Title: Re: National GA920P Schematic
Post by: clintrubber on November 01, 2017, 09:05:23 AM
Thanks for sharing this info on the GA920P, much appreciated!

Have one here as well, no idea if it still works, I've never plugged it in after taking it home from a fleamarket (never dared to..., have to check it for safety first)

Mine didn't come with a schematic.

Was wondering, any info on the reverb tank they used ?  Input- & output-impedances ?
Then possibly another circuit could be used to drive the tank.

I'll admit I was intending to use the enclosure forsomething else, but who knows it's a fun amp by itself.

Bye/thanks   
Title: Re: National GA920P Schematic
Post by: clintrubber on November 01, 2017, 09:10:23 AM
Was wondering, any info on the reverb tank they used ? Input- & output-impedances ?

Oops, apologies, just saw the 225R & 222R indications in the last version of the schematic. Are these as measured with a DMM ? So the R_DC values ?

Will check as well on my unit.

Bye/thanks
Title: Re: National GA920P Schematic
Post by: galaxiex on November 01, 2017, 12:58:53 PM
Was wondering, any info on the reverb tank they used ? Input- & output-impedances ?

Oops, apologies, just saw the 225R & 222R indications in the last version of the schematic. Are these as measured with a DMM ? So the R_DC values ?

Will check as well on my unit.

Bye/thanks

Thanks, glad this is of some use.  :)

That's correct, DC resistance values measured with a DMM.

Yup, they are kinda cool little amps.  8) Actually sound pretty good for what they are.

Be aware there are at least 2 different versions of this amp,
both have the same name (National) and model number (GA920P) but different circuit boards/schematics.

I have one of each but not posted anything about the "other one" yet.
Title: Re: National GA920P Schematic
Post by: clintrubber on November 01, 2017, 02:14:59 PM

Yup, they are kinda cool little amps.  8) Actually sound pretty good for what they are.

Just curious, how 'wet' can the reverb get at maximum?

Quote
Be aware there are at least 2 different versions of this amp,
both have the same name (National) and model number (GA920P) but different circuit boards/schematics.

I have one of each but not posted anything about the "other one" yet.

Hi,

Thanks, I'll check the R_DC of the spring unit here as well.

Mine looks alike (outside & inside) to your pics on page 1 at first I thought, but the placement is actually drastically different.
The sping tank is directly behind the front panel and the power devices are mounted on the base of the unit. The rear jack panel location is also different. Perhaps this is exactly describing your other unit ;-)

Mine has the blue section on the front as well, I've seen some pics of amps without.  I don't have the speaker enclosure.

FWIW/IIRC, I recall these amps being shown in mailorder catalogs. Here in The Netherlands the combo version went for 199 (Dutch guilders), and there was a 'Hondo II'  Les Paul copy for another 199 to be had, same catalog page, got us drooling :-)

OK, I'll see if it works, and if so, how it sounds. Curious to the spring reverb configuration. Too bad the enclosure doesn't permit a full sized tank.

Bye!
Title: Re: National GA920P Schematic
Post by: galaxiex on November 01, 2017, 06:48:49 PM

Quote
Just curious, how 'wet' can the reverb get at maximum?


It can get pretty wet, but it doesn't sound good.  :(
I'm sure this is due to the limitation of the small tank.
It also doesn't help that there is only 1 spring.

I played with part values in mine and got a "fair" sounding reverb,
but it will never be a Fender Twin.

If you "over do it" with driving the tank too hard,
or increasing the gain on the recovery side, bad noises happen.
Screeching/feedback/metallic "clank" sound, etc.

Changing some cap values and possibly adding some caps in strategic places can smooth out the sound.
Limiting the frequency range that is fed to the tank helps. 

There are other small multi spring tanks around that would fit the chassis.
I never tried any, and there may be impedance matching problems with some of them.
No guarantees they will sound any better than the existing tank.

 
Title: Re: National GA920P Schematic
Post by: galaxiex on November 01, 2017, 06:51:23 PM


Quote
Mine looks alike (outside & inside) to your pics on page 1 at first I thought, but the placement is actually drastically different.
The sping tank is directly behind the front panel and the power devices are mounted on the base of the unit. The rear jack panel location is also different. Perhaps this is exactly describing your other unit ;-)


I don't recall the layout of the other amp.

Easy enough for me to open it up and get some pics.
I'll do that later.  ;)

Cheers!
Title: Re: National GA920P Schematic
Post by: clintrubber on November 01, 2017, 07:17:12 PM
I don't recall the layout of the other amp.

Easy enough for me to open it up and get some pics.
I'll do that later.  ;)

Cheers!

Nice, who knows it's like mine :-)  And who knows that other one has the right schematic of your first one, and vice versa :-)     

 I'll post some pics as well (this weekend)

Bye!
Title: Re: National GA920P Schematic
Post by: clintrubber on November 01, 2017, 07:25:42 PM

Quote
Just curious, how 'wet' can the reverb get at maximum?


It can get pretty wet, but it doesn't sound good.  :(
I'm sure this is due to the limitation of the small tank.
It also doesn't help that there is only 1 spring.

OK, too bad. Only 1 spring, I see. So that would actually mean that a better small tank could still give some room for improvement...  but like you say, it'd be unrealistic to expect Twin Reverb sounds, let alone the superwet stuff as from the Fender Reverb unit.


Quote
I played with part values in mine and got a "fair" sounding reverb,
but it will never be a Fender Twin.

If you "over do it" with driving the tank too hard,
or increasing the gain on the recovery side, bad noises happen.
Screeching/feedback/metallic "clank" sound, etc.

Changing some cap values and possibly adding some caps in strategic places can smooth out the sound.
Limiting the frequency range that is fed to the tank helps. 

Thanks for sharing the results of your experiments, much appreciated!

Quote
There are other small multi spring tanks around that would fit the chassis.
I never tried any, and there may be impedance matching problems with some of them.
No guarantees they will sound any better than the existing tank.

I have a broken Danelectro Spring King, but the tank itself is assumed to be OK.
That's a two spring medium length one, still not Fender-close, but probably a step up.

I realized that this 'Dano tank' matches the Fender TR & Reverb unit specs, w.r.t. the in- and output- impedances. Not the length obviously
Adding say the Surfy FET Reverb PCB (you'll have heard of it) would easily fit this enclosure.

Bye!
Title: Re: National GA920P Schematic
Post by: galaxiex on November 01, 2017, 11:23:02 PM
Here's some pics of my "Blackface" version along with the schematic that matches this version.

I did change a few things... notably the tone control.
Sorry, I didn't document the changes but I used ideas from the AMZ notch tone control.

http://www.muzique.com/lab/notch.htm (http://www.muzique.com/lab/notch.htm)

Modding the tone control makes a significant difference to the tone of the amp.
For the better IMHO.

Having a little "scooped mid" helps for less muddy tone. Esp with distortion pedals.

I have seen the Surfy Bear reverb and if it could be incorporated into this amp
I think it may go a long way to improving the reverb tone.

Of the differences between this and my other "Silverface" version,
the input jack circuitry, reverb is transformer driven, and the different output devices.

As with all these amps, I added a 3 wire grounded cord and clipped the death cap.
Title: Re: National GA920P Schematic
Post by: clintrubber on November 02, 2017, 12:41:16 PM
Hi,

Thanks for the pictures! Interesting to see that 'they' (assuming it was all coming from the same factory) went through all these versions, colour schemes, component re-locations etc 

Mine is with the reverb tank located between the front and the PCB, so all pots are wired, so not directly on the PCB.
No reverb-TX.

Here's another, let's see what it does within a few minutes, now at $41 with 2 minutes to go.
https://www.ebay.nl/itm/263282000680?ul_noapp=true


Bye & I'll post pics from my GA920P this weekend
Title: Re: National GA920P Schematic
Post by: clintrubber on November 02, 2017, 12:44:22 PM
Aha, that auction just finished, 8 bids, winning bid US $57,66

Perhaps better not using mine as a donor-chassis/enclosure ;-)

Title: Re: National GA920P Schematic
Post by: galaxiex on November 02, 2017, 07:34:00 PM
Ahh yes... I was also watching that listing.
Just curious to see what it sold for.

I already have two so I don't need another one.  ;)
Title: Re: National GA920P Schematic
Post by: clintrubber on November 05, 2017, 07:05:30 PM
Just curious to see what it sold for.

Same here, it didn't do too bad I guess.

OK, FWIW, a few quick and low-qual pics for now, this is mine with the 'blue left front', and the reverb tank right behind the front panel.

Bye


Title: Re: National GA920P Schematic
Post by: galaxiex on November 05, 2017, 07:30:42 PM
Thanks for the pics.  :)

Interesting layout compared to the others.

Can you do a overall shot that shows the whole chassis.
Thanks!
Title: Re: National GA920P Schematic
Post by: clintrubber on November 10, 2017, 08:25:48 AM
Interesting layout compared to the others.

All those iterations are a bit puzzling/amusing! :-)  ... why ?!  8)

Quote
Can you do a overall shot that shows the whole chassis.

Here we go, 3 more pics, have a good weekend!
Title: Re: National GA920P Schematic
Post by: clintrubber on November 10, 2017, 08:29:27 AM
BTW, imho 5 Fender knobs look way better on this little amp than the bit smallish originals... just personal preference
Title: Re: National GA920P Schematic
Post by: galaxiex on November 10, 2017, 10:36:24 PM
Thanks for the pics!   :tu:

Looks like the reverb tank is "upside down" compared to both of the other ones.

I agree, all those iterations are a bit puzzling/amusing! 

Why? Who knows.   8) Different factory, different production run etc.

I too like the look of Fender knobs, but on these amps my personal preference is for the original knobs.
To each his own.  :)

Thanks again for sharing!  :tu:
Title: Re: National GA920P Schematic
Post by: lotus1 on December 16, 2017, 09:37:15 AM
Hello, I just discovered this forum and thread. Very cool. Have a question tho. I have a national GA 960P amplifier.I am in need of a schematic or someone who has one as I have 2 resistors that need replacing but, have burned up to the point I can't read values. If anyone has one of this model, would very much appreciate any help. It uses 2SD428 output transistors. Thanks all.Mario
Title: Re: National GA920P Schematic
Post by: Jazz P Bass on December 16, 2017, 02:31:02 PM
It may help if you describe where the resistors go.

What points do they attach to?
Resistors burn up when another part of the circuit demands eccess current.
Title: Re: National GA920P Schematic
Post by: DrGonz78 on December 16, 2017, 02:50:30 PM
I remember this thread... If you look back through it there are schematics given by Galaxlex.

https://www.ssguitar.com/index.php?action=dlattach;topic=3850.0;attach=7551

https://www.ssguitar.com/index.php?action=dlattach;topic=3850.0;attach=7553

https://www.ssguitar.com/index.php?action=dlattach;topic=3850.0;attach=6703
Title: Re: National GA920P Schematic
Post by: Jazz P Bass on December 16, 2017, 05:12:13 PM
Wow!

Those two schematics are totally different.

As the OP is searching for a schematic that uses 2SD428 (TO3 mount) the schematics may or may not be of help.

Maybe the Gibson G30 is closer.
Title: Re: National GA920P Schematic
Post by: lotus1 on December 16, 2017, 08:39:02 PM
It may help if you describe where the resistors go.

What points do they attach to?
Resistors burn up when another part of the circuit demands eccess current.
Yes, I understand, Was looking for the schematic but, Here's a couple of pics. It's the red ones connected to the transistor itself. One has the red wird connected directly, not sure why that is. Thanks for the replies, really like this amp, would like to get it running again. Mario
Title: Re: National GA920P Schematic
Post by: phatt on December 16, 2017, 09:01:56 PM
Those resistors will most likely be half an Ohm. (OR47)
Most likely reason for them burning up is someone plugging in an extra speaker which puts more current demand on the circuit.
These type of power circuits are basic in design and don't handle low Z speaker loads well.
re the red wire Q.   A; who ever fixed it last was lazy. :trouble

Replace those resistors and use the light bulb limiter (Instructions at top of this forum)

Phil.
Title: Re: National GA920P Schematic
Post by: lotus1 on December 16, 2017, 10:09:19 PM
1/2? Thanks.  Was told the transistors went bad, they were replaced but, resistors, no.and yes, he was lazy... Haven't gone in myself as without a schematic, I don't touch...or help. I will rectify both the resistors and shoddy wiring, will report back. Oh, and the amp was only used with the cabinet it came with, a 2x12...haven't measured cabinet Z yet, so...It does look like the original cabinet. Will post more pics in the morning,Thanks.<
Title: Re: National GA920P Schematic
Post by: lotus1 on December 17, 2017, 07:44:43 AM
More Pics.
Title: Re: National GA920P Schematic
Post by: galaxiex on December 17, 2017, 09:49:45 AM
1/2? Thanks.  Was told the transistors went bad, they were replaced but, resistors, no.and yes, he was lazy... Haven't gone in myself as without a schematic, I don't touch...or help. I will rectify both the resistors and shoddy wiring, will report back. Oh, and the amp was only used with the cabinet it came with, a 2x12...haven't measured cabinet Z yet, so...It does look like the original cabinet. Will post more pics in the morning,Thanks.<

Likely won't ever find a schematic, and that means you will have to draw it yourself.

The main board doesn't look very complicated, should be fairly easy.

What I do...

Take good pics of where everything hooks up to the board AND hand draw a picture of where every wire attaches.
Label accordingly. The more detailed you make this, the less chance for mistakes when re-installing the board.

Remove the board from the amp and scan the copper side.
Import the scan into a graphics program (I use paint.net).
Flip the image so you have a component side orientation, as if an X-ray from the top component side.
Print that, and hand draw the parts right on the picture.
Now you have a layout diagram.

Draw schematic from that.
I use Express PCB which is a free program and comes with Express SCH schematic drawing program.
You could also draw it by hand.
Several of my first attempts at drawing a schematic I did by hand.

HTH
Title: Re: National GA920P Schematic
Post by: lotus1 on December 17, 2017, 11:00:15 AM
Very cool ideas...Thanks..will attempt when I can get some time..Just effected the "repair" so, fingers crossed it'll work. discovered why the funky red wire attachment..the solder pad had lifted when the original repair was done. fixed that a little better. get to try it out a bit later. Than ks all, cool place here. Next up is my peavey session 400 which ate the speaker voice coil. Putting out something ugly. had a wind shear go thru woods around studio took power out while amp was on..we see.
Title: Re: National GA920P Schematic
Post by: lotus1 on December 20, 2017, 09:36:49 AM
Effected repairs, Had to use a 1ohm resistor instead of .5...It's alive!! Workd again and that very cool sound is being heard again, Once again, Thanks for all. Will try to post pics soon.M
Title: Re: National GA920P Schematic
Post by: galaxiex on December 20, 2017, 09:57:12 PM
Effected repairs, Had to use a 1ohm resistor instead of .5...It's alive!! Workd again and that very cool sound is being heard again, Once again, Thanks for all. Will try to post pics soon.M

Good to hear you got it working!
Looking forward to the pics.  :)
Title: Re: National GA920P Schematic
Post by: lotus1 on December 22, 2017, 05:00:34 PM
Pics.
Title: Re: National GA920P Schematic
Post by: galaxiex on December 22, 2017, 07:41:55 PM
Cool looking amp!  8)

Are those 12" speakers?
Title: Re: National GA920P Schematic
Post by: lotus1 on December 22, 2017, 07:46:33 PM
Yes, 12" ceramic magnet, no names...sound pretty good tho. Kind of a Gilmour sound...That clear but "warm" thing.
Title: Re: National GA920P Schematic
Post by: lapsteelman on January 13, 2018, 11:42:55 PM
Haven't posted in awhile, just wanted to chime in and say a guitar player I was playing with got his hands on one of these.. I think he put a couple different speakers in the cabinet... but it sounded fantastic! REALLY great surf tone..
Title: Re: National GA920P Schematic
Post by: galaxiex on January 14, 2018, 06:35:18 PM
Haven't posted in awhile, just wanted to chime in and say a guitar player I was playing with got his hands on one of these.. I think he put a couple different speakers in the cabinet... but it sounded fantastic! REALLY great surf tone..

Nice!
Ya, they are cool amps.
I have no doubt it sounds better with different speakers.  :)

I started a project custom cabinet for mine and I need to get back at it.
Life stuff got in the way....  ;)
Title: Re: National GA920P Schematic
Post by: mwdavis on August 19, 2019, 06:55:01 PM
Hi all,

If I need to replace the transformer on this little amp (National GA920P), can anyone tell me what I would need?  This particular unit was purchased in Germany, so it's wired for 220V.  I need 120V 60Hz.

Thanks to everyone!