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Author Topic: National GA920P Schematic  (Read 14732 times)

galaxiex

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National GA920P Schematic
« 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"  ::)









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




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



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?




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



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.



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!








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...








Cheers!


« Last Edit: September 13, 2015, 05:00:48 PM by galaxiex »
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galaxiex

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Re: National GA920P Schematic
« Reply #1 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
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galaxiex

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Re: National GA920P Schematic
« Reply #2 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.  :)
« Last Edit: November 01, 2015, 08:51:03 PM by galaxiex »
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Enzo

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Re: National GA920P Schematic
« Reply #3 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.

galaxiex

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Re: National GA920P Schematic
« Reply #4 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.
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galaxiex

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Re: National GA920P Schematic
« Reply #5 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?
« Last Edit: November 02, 2015, 04:49:45 PM by galaxiex »
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Enzo

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Re: National GA920P Schematic
« Reply #6 on: November 02, 2015, 06:09:38 PM »
More likely there for stability, I'd bet.  get your tone from the preamp.

Loudthud

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Re: National GA920P Schematic
« Reply #7 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.
« Last Edit: November 02, 2015, 06:22:24 PM by Loudthud »

galaxiex

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Re: National GA920P Schematic
« Reply #8 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.
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galaxiex

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Re: National GA920P Schematic
« Reply #9 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
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galaxiex

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Re: National GA920P Schematic
« Reply #10 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.


« Last Edit: December 20, 2015, 07:40:43 PM by galaxiex »
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Loudthud

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Re: National GA920P Schematic
« Reply #11 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.

Enzo

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Re: National GA920P Schematic
« Reply #12 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.

galaxiex

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Re: National GA920P Schematic
« Reply #13 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.
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J M Fahey

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Re: National GA920P Schematic
« Reply #14 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).


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.
« Last Edit: November 11, 2015, 07:00:24 AM by J M Fahey »

 

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