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Solid State Amplifiers => Schematics and Layouts => Topic started by: Gustaf on May 06, 2016, 03:31:52 PM

Title: Ibanez TSA30 Schematic and PCB Layout
Post by: Gustaf on May 06, 2016, 03:31:52 PM
Here is the circuit diagram and PCB layout for Ibanez Tube Screamer Amplifier TSA30.

Thanks to vitrolin for sharing!

BR,

Gustaf
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: bajaman on September 09, 2016, 03:13:38 AM
Thanks - extremely helpful - these amplifiers have been a curse to fix from day one - I have seen many with the same intermittent buzzing fault - almost pulled what is left of my hair trying to fix the suckers. The fault is capacitor C20, a 470uf 25v filter capacitor - I replaced this with a 2200uf 25v type - problem fixed ! Hope this helps anyone else out there :)
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on September 09, 2016, 08:00:04 AM
Thanks Bajaman, handy to know little details like that. :tu:
I wonder why they run the preamp heater filaments at 10 volts ???
Phil.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: J M Fahey on September 10, 2016, 11:46:45 AM
I also wondered that the first time I saw it.
Not sure whether calling this weird or plain stupid.

They use a single regulator, here shown as a 10V one, to power both filaments, which require 12V (in fact, 12.6) and the Tube Screamer, which requires 9V .
So somebody finds himself very intelligent because he "split the difference" and saved one regulator  :loco, using a single 7810 instead of one 7812 (which to be perfect requires a diode to ground at the grund leg so it outputs 12.6V) PLUS a 7809 for the TS.
Maybe they gave him a bonus for the incredible savings  :duh

Personally I´d use a 7812, and increase the value of the resistor feeding the TS (R46) from 220r to about 1k2, which should give it poorly regulated 9V , more like a 9V battery would do.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on September 10, 2016, 11:50:57 PM
Thanks for the detail,,Yes I guess it's no surprise as everyone is trying to scratch out every last cent.
I figured it would run fine just by changing the reg to 12Volt and the TS circuit would sound much the same.
Phil.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on October 12, 2018, 04:14:47 PM
Reviving because I need to know - did anyone swap the regulator without changing r46? Haven’t verified everything in the path is rated over 12.6v so thought i would check if anyone already did it. If so, did it change the sound?
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on October 13, 2018, 08:49:58 AM
I don't have that Amp but a read of the schematic tells me it will matter little as 12 volts is still within the 16 Volt cap ratings.
I doubt you would hear any difference in the sonic result.
Though V1 will last longer if heater is running 12Volts :tu:
Phil.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on October 16, 2018, 01:58:18 AM
I don't have that Amp but a read of the schematic tells me it will matter little as 12 volts is still within the 16 Volt cap ratings.
I doubt you would hear any difference in the sonic result.
Though V1 will last longer if heater is running 12Volts :tu:
Phil.

Thanks! Along with making v1 last longer, might it reduce the hiss? It adds white noise as I turn up the volume currently.  It seems to be coming from v1 as it disappears when I pull v1. I replaced the coupling caps and grid and plate resistors with metal film 1 watt resistors. Decreased the grid to 10k while I was at it. Reduced the hiss some but still not as quite as my tsa15... that amp has proper power to the preamp so wondering if it would help with noise too...?
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on October 16, 2018, 08:31:37 AM
No way to really know without an A/B test circuit. :-\
But I'm guessing that with the Valve preamp section running wide open and then gain at 1C3A you are bound to have some hiss.

You can always reduce R4 and 5 too a more sane 100k that will reduce some excess hiss but be aware that U3A is active even when the TS part is switched out.
As far as the Valve section goes it's a Marshall/Bassman type circuit but volume is after tone stack which means the preamp is full on all the time so hiss will be evident even at lower setting of volume.
R17 could be made as a pot for preamp gain, that might have worked better.

Sorry I can't think of anything else,
Phil.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on October 16, 2018, 02:13:24 PM
No way to really know without an A/B test circuit. :-\
But I'm guessing that with the Valve preamp section running wide open and then gain at 1C3A you are bound to have some hiss.

You can always reduce R4 and 5 too a more sane 100k that will reduce some excess hiss but be aware that U3A is active even when the TS part is switched out.
As far as the Valve section goes it's a Marshall/Bassman type circuit but volume is after tone stack which means the preamp is full on all the time so hiss will be evident even at lower setting of volume.
R17 could be made as a pot for preamp gain, that might have worked better.

Sorry I can't think of anything else,
Phil.

Awesome. Thanks for the advice!
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: g1 on October 17, 2018, 11:38:16 AM
Once I worked on a tube mic that had a higher noise floor because the heater voltage was too high.  Bringing the heater back down to 12.6V had a noticeable impact.
Try it and find out, it's probably something that varies from case to case as far as how much difference it makes.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on January 27, 2019, 10:04:57 PM
No way to really know without an A/B test circuit. :-\
But I'm guessing that with the Valve preamp section running wide open and then gain at 1C3A you are bound to have some hiss.

You can always reduce R4 and 5 too a more sane 100k that will reduce some excess hiss but be aware that U3A is active even when the TS part is switched out.
As far as the Valve section goes it's a Marshall/Bassman type circuit but volume is after tone stack which means the preamp is full on all the time so hiss will be evident even at lower setting of volume.
R17 could be made as a pot for preamp gain, that might have worked better.

Sorry I can't think of anything else,
Phil.

Hey.  The gain knob pointer was great.  Got it on 50k right now and seems to have good range.  I actually bypassed the entirety of the solid state front end now on a switch and it sounds way better.  Most of the noise is gone and it sounds much clearer now. 

I have another question.  Is this how I would add a presence control?  5k pot in series with a 100nf cap coming off between r25 and c16?
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on January 28, 2019, 01:10:32 AM
Sorry but
Unless you tell us Where Exactly in the circuit have you inserted the new input,,it's all meaningless?
could be  VR9?  or at R33?

Forget the presence idea,, bad for the PI stage. xP
Phil.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on January 29, 2019, 12:40:17 AM
Sorry but
Unless you tell us Where Exactly in the circuit have you inserted the new input,,it's all meaningless?
could be  VR9?  or at R33?

Forget the presence idea,, bad for the PI stage. xP
Phil.

I just added an input jack with a 1meg resistor and went straight in to cn3b which eliminated the TS, boost, and solid state always on gain.  Actually took that whole input PCB out for now, which means the "effects loop" does nothing now. 

Replaced r17 with the 50k pot for gain.  It sounds good now.  Incredibly loud.  It was oddly quiet stock for 30 watts - quieter than my blues junior by a good bit.  But its not pedal friendly really now.  Thought I could tame some highs by cutting back the presence maybe?  Or maybe adding a tone control earlier between v1 and v2?

Just learning and experimenting with this one.  Will take any advice.  Thanks!
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on January 29, 2019, 07:35:18 AM
You could replace R72 with 560k and change your new gain pot with 250k.
Also replace R4 and R5 with 100k,, that will back off the gain.
Is R13 (56k) still in circuit?

You can alter the tone stack might help back off excess treble.
Make C23 100nF and C22 22nF might work,, not sure how well but suck it and see.
Phil.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on January 29, 2019, 12:17:09 PM
You could replace R72 with 560k and change your new gain pot with 250k.
Also replace R4 and R5 with 100k,, that will back off the gain.
Is R13 (56k) still in circuit?

You can alter the tone stack might help back off excess treble.
Make C23 100nF and C22 22nF might work,, not sure how well but suck it and see.
Phil.


Cool thanks.  I will try it!  r13 is still there.  I have a 510k metal film on hand to replace r72 with and a 250ka pot.  So I will probably try that as well as the 2 100k changes for r4 and r5.  i already replaced those with metal film in my quest to get the noise floor down but will sub in the 100k and see how that impacts it too.  That 50k difference in your recommendation at r72 won't impact much will it?  Slightly more gain?

I already changed the tone stack to more standard fender cap values.  C23 to 100n and C24 to 250p.  Left the mid cap alone at 47n for now.  The tone is good.  A tiny bit bright but treble knob is just running lower than normal and sounds good clean.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on January 29, 2019, 08:28:52 PM
I went with 510k.  That with the 100k for r4 and r5 and the 250k gain pot changed the eq of the amp quite a bit.  I think i am going to up the gain pot to 500k now and thats it!  Works great with pedals.  If anything now it is too bassy and the the treble is way more tame.  I may play with values a little more but I think its a pretty happy medium now.

Here is another thing I plan on playing with - a switch to bypass the u3 solid state front end instead of using 2 inputs with a switch to select between them.  But... that will leave the buffers in place from the TS and boost so that may change the tone of having a input straight in to r13 to V1...

I'll take ideas if you have them.   :tu:
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on January 31, 2019, 05:59:51 AM
If R13 is (56k) then it's likely done to tame the amp down as the two SS dirt stages would be too much for the Amp section. :-X
Normally R13 would be at least 220k,,more common value is 1Meg.

With 1meg at R13 will open the amp up huge but you will then have to re eq the whole amp circuit. :'(

You can play with a few sections;
Remove C51 for a bit more top end, if you still want more top remove C50 as well but watch out for feedback at high volume.

If R72 is 500k you can add a treble bleed around it  ~470pF is common.
(just go find all the classic valve amps for clues)

You can fiddle with the values of R18 & R25,, dig up some bassman/Marshall schematics for ideas. That changes how the PI distorts amoung other things.

If too much bass make C10 smaller ,try 1n5.

Other stuff;
You don't need metal film for plate resistors normal carbon is fine.
you use metal film for signal path not DC stuff.
As to your ideas of the SS crap in front,, it's anyones guess how it will help,, frankly the whole thing is a bit of a miss mash.
I'd focus on getting the Valve Amp section to work well then down the track you might be able to tweak the TS part.
I'd bypass the the gain section around u3A as it's likely too much,, hence they had to turn down V1 (R13 56k)

With a pair of 6L6 running from 560VDC would be way over 30Watts so it should be loud if not then they don't know how to build Valve circuits. :duh
Phil.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on January 31, 2019, 05:26:18 PM
Good advice.  Question:  I managed to bypass the u3a but I bypassed all of u3 so lost the boost too.  I pulled the leg of r33 and went to cn3a from that spot.  Can I keep the boost in place by starting somewhere else?  Not seeing where that would be.


Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on February 01, 2019, 07:37:06 AM
My advice remains the same.
You need to get the amp to work FIRST. 8|

To do that;
Lift the R62 side of c30 use a shielded cable grounded at one end and join up with pin 2 of CN3a.

Lift one end of C27 to remove all the SS stuff.
Make R13 1Meg.
Now you will have an all Valve circuit with a normal input.
Work with ideas already mentioned until you get the Valve circuit to work and sound good,,,,
Then try ideas with the SS part,, IF it needs it. ;)
Phil.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on February 01, 2019, 11:16:16 PM
Interesting... so i bypassed all the SS by literally removing it from the chassis (unplugged the connectors for CN1a, CN2A, CN3b, and CN7a) and bolting a new input with a 1meg from input to ground just acting as a pull down resistor to the existing input hole.  Then ran that input and ground to CN3a. 

Sounds like we accomplished the same thing because i then had the input straight in to CN3b which is r13 (which I had changed it to 68k because that was what i found in other fender schematics).

But I am missing your 1meg in line before v1 right?  I don't see that in other fender schematics.  Seems very high.  Is it serving a purpose there?  Doesn't seem like it would do anything to the tone right?  68k in series with 1meg before v1...

Thanks for your help!  Learning a lot.

It sounds good without the 1meg in series, so i think I have the tube amp part solid.  Just wondering how the 1meg would effect it.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on February 02, 2019, 04:42:31 AM
I think you missed something,, The 1 Meg Replaces R13,, leave R62 in place.

Here is what it should look like as a schematic,, there are 1,000 of schematics on the net and almost as much info. I've numbered the resistors so they match your schematic.
Google, Valve wizard,, Tone Lizard,, Rob Robinette,, and many other who have good info.
Phil.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on February 02, 2019, 06:32:57 PM
I knew i did something wrong!  Thanks.  Totally misread and thought R13 was in series in to v1 and didn't notice it was going to ground!  Stupid... i think I have it now.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on February 03, 2019, 02:52:03 AM
Thanks for all the help.

So the tube amp portion now sounds excellent and plays nicely with pedals and is much quieter now.  1meg is in place for r13.  Switched r18 to 470r.  left r25 at 15k since that is between a bassman and deluxe reverb.  May play with it someday but sounds good now.

Still pretty bass heavy.  Could play with c10 (or c54?) coupling cap but don't have any on hand rated at high voltage.  Just running bass control around 30% and it is good clean but as the new gain is turned up it distorts pretty bassy.

Now thinking about how to work the SS portion back in to the circuit on a switch.  Have an spdt on-on I am going to use maybe at the point you mentioned with r62 going to middle lug and one side going straight to cn3a and the other to C30.

Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: mikemagyar on February 16, 2019, 08:59:25 PM
Hey Mprall00,

How did you go about wiring up the 500k pot to replace r17? Did you just wire it as a variable resistor?

I've followed this thread and modded my tsa30 to have the all-tube signal path I'd hoped the amp would have when I bought it! However, without some means of attenuating the signal before the factory volume knob(i.e. the r17 pot mod), the signal is slightly distorted.

What's the advantage of replacing r17 with a pot vs replacing r13 with a pot? Could both r13 and r17 be replaced with pots to control signal gain at both stages?

Thanks in advance for any and all knowledge shared!
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on February 16, 2019, 11:38:26 PM
I just wired it as a variable resistor.  Actually just got a 1meg pot because the 500k pot at R17 was not breaking up much when cranked and even though i almost always play it clean as a pedal platform I thought having the option of more preamp gain would be nice.

I guess you could replace r13 with a pot as well to control v1 too.  Let me know if you try it!

After tweaking a lot of resistor values and replacing the tonestack, the SS front end actually doesn't sound bad either.  I got the solid state section to be switchable with the stock input jack but it doesn't sound good.  When the SS is bypassed it sounds like the impedance is way off - like something at the input needs that buffer from the SS sections.  I even tried adding a 1 meg pull down on the stock input jack...  It just sounds really flubby with no presence when the SS is bypassed but sounds perfect when i removed the whole daughter board and added my own input jack straight to CN3b.

SO... i am about to drill a hole next to the existing input for a second input direct to CN3b and a hole for the gain pot above and between the gain and bass knobs.  That way I can just plug in straight to the tube section with the new input or plug in to the TS>Boost>SS front end if i want to use it.

Side note: replacing the signal path resistors with metal film and replacing the coupling caps and tone stack made this amp a lot less noisy.  Not sure if one of those thing did more for the noise floor than any another - i just know the whole change helped.  i got mine very old and very used, so it could have been noisy due to age, but this really helped as did a proper bias.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on February 17, 2019, 10:14:03 PM
The problem with the SS section is that the gain section is on all the time.
Which means those Leds (D11-D15) will add some clipping (Dirt) to the signal even when boost is OFF.

You could try removing the gain section.
Break connection at Q7 pin1, also the top of C27.
That removes the drive section.

Now just join pin2 at CN2a and then join that to pin2 at CN3a.
That gives you the always on input buffer as well as the switchable TS circuit and FX loop is still working.

Remember I don't know this amp so it's anyone's guess as to how well this would work,, personally I'd remove (and bin) the SS section and use pedals.
Phil.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on February 17, 2019, 11:35:18 PM
I agree... But if i toss it then i have 5 open holes in the front.  And if I keep it and break the connection as you outlined then i still have to drill a hole for the new gain pot for v2.  And if I am drilling anyways I am going to just solve it with a second input.  So my new input will bypass all of it - buffers, ts, SS, everything - and go straight to the CN3a.  And the stock input will still be there and use the stock daughter board as is.

Truth is, once we changed all that stuff (i changed a lot with your help but the biggest impact was the 12v regulator, R4, R5, R13, R72, R17 as a pot, and the tone stack caps), the SS front end actually doesnt have a bad gain tone.  So therefore I think I will drill one extra hole and have both options, even if I don't really ever use the stock option... Just turn the SS gain all the way down, plug in to the new input, and it will be like it isn't there.

Once done I will try to remember to take pictures and write up an easier to follow list of changes in a new post.  These can be fun to learn with since they are relatively cheap for 30 watts.  I got my TSA30 combo for just under $200 used...
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on February 19, 2019, 06:47:20 PM
Yep a fair point. :tu:
As I don't have that Amp I'm only guessing.

One thing I should add is that the Schematic shows no Grid Stopper resistors on the power tubes and although you won't notice much at low volumes when the Amp is working at high volumes it will most likely spit,, sometimes called power stage choking.
I'd suggest at least 5k and right at the pin if possible. you may have to cut tracks to do it but worth it if you play with a lot of heavy loud distortion.
This very problem drove me nuts on some of my early valve amp builds years back as I only had 2k2 resistors. sounds gawd darn awful. :grr
I replaced them with 10k and bingo no more harsh blocking distortion.  <3)

Explained well here if interested;
http://ozvalveamps.org/choking.htm
Phil.   
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on February 19, 2019, 11:15:40 PM
Good advice!  Didn't notice that.  I bet it was never a problem with the original design because it was never getting loud enough to get the power amp cooking anyways.  it was so quiet stock...

I doubt I will get it cranked that loud anyways but in case I do...

Would that mean essentially cutting the trace between R14 and pin 5 of v4 and R10 and pin 5 of v5 then adding something between 1.5k and 10k where i cut the trace?  Is there any downside?  Nothing mentioned in what I am reading... did ibanez just design it without to save $.05?
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on February 20, 2019, 06:49:27 AM
Yes highly likely you will have to cut tracks.
As I don't have a track layout I have no idea of how easy that would be.
I've found with pcb's you often have to get inventive with solving issues. I find a week may pass sometimes before an idea comes to mind, other times it comes to me straight up.
If the tracks are wide enough you may be able to cut and drill mounting holes.
but a lot of those pcb's have very narrow tracks which makes it hard to find a spot.
If tracks are too narrow I drill beside the track and solder the pig tail along the track.

No down side to grid stop resistors that I've read about.
I think you will find it's Mandatory on nearly all good designs. 8)

As to why they omitted those resistors,, anyone's guess. xP

Be aware a lot of these amps are just whipped up in a mad frenzy with little or none R&D.
The whole amp is likely out sourced to a far off country,, read cheap labor and no QC

There was a HiWatt Amp question on here long ago that was so badly designed it was unusable because of noise due to bad PCB layout. :duh
turns out HiWatt had little to do with that Amp and hence had no records. :loco

AFAIK, Marshall still make amps that use EL34 tubes and run them at elevated voltages,, which is no problem for the plate of EL34 but the screens are then way past their max voltage limit. For most brands of EL34 the screen limit is 420volts.
They don't last very long with the screen at 470volts.
Do they care? I doubt it. :trouble
The RCA manual states quite clearly that AX7 will last for 10,000Hr's and power valves about half that.
The Valves in a well designed amp would/should last for at least 10 Years. (with heavy use)
I've witnessed Amps that are 30 plus years old and still using original Valves.
Today they just slap it together and know that the valves will last at least a year.

A chap who came to me years back had asked the local music shop,, How often do you replace the valves?
The shop owner replied,, oh every year or so. :lmao:

He was so glad he came to me for some advice. 8|

Phil.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on February 20, 2019, 10:08:26 AM
I'll have to look tonight but i feel like C13 and C5 being so close to pin 5 means i could almost cut the trace and then just solder the resistor to the existing hole that the leg of C5 is already in then directly to pin 5 without modifying the board at all.  Any problem not using the board at all here?  More noise?
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on February 20, 2019, 01:20:24 PM
Another question while I am at it - V1 and V2 don't have a cathode bypass cap.  Worth adding one to pin 3 of both?  Or at least V1?  would raise gain... and depending on value, on specific frequencies?
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on February 23, 2019, 12:35:37 AM
Coming back with another question - can anyone tell me how the 2 triodes of v1 and v2 are working?  Staring at it realizing they aren't parallel.  And they aren't in series... are they?  is the signal coming from the cathode of both v1 and v2?
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on February 23, 2019, 06:50:50 AM
You have 2 Cathode followers, one at V1 and another at V2.
Implementing a Cathode follower at V1 is likely just a hair brain idea by someone in design who reads too much internet gossip. probably no great advantage over a single triode. :-\

Obviously you did not google Valve wizard. :trouble
It's all there for you to learn. Hint!!! 8|

http://www.valvewizard.co.uk/dccf.html

Main Index;
http://www.valvewizard.co.uk/index.html
Phil.

Regards bypass caps at V1&V2,, yes try some values and see what happens.
Not a lot of advantage at V1 but at V2 will pull more tone change.

As it's an oddball circuit (with the SS preamp)
It's anyone's guess how much advantage adding Cathode caps will help?
Phil.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on February 23, 2019, 10:30:41 AM
Haha. Late night dumb question. I’ve even added a cathode follower to a blues jr. Think I was thrown off because I’ve never seen one on v1 as I was looking up schematics... and it was late. Thanks for the response to the dumb question!
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: Mprall00 on March 02, 2019, 01:22:09 PM
Any advice on adding NFB so I can add a presence control?  Tried a couple things and actually deleted a post from yesterday that didn't work.  I tried deluxe reverb style later and got a loud screeching noise.  No damage done but definitely didnt work.

For that I tried lifting the grounded side of R15 and C16 and connected them. Connected 820r resistor from the speaker output to the lifted sides of R15 and C16. Put one end of a 47r resistor to ground and the other end to the connection of the other 3.

The end goal was to add more like a bassman style with a presence control.  Just tried without to start and didnt work at all.

Edit:  Flipped primary because that sounded like positive feedback - same problem.  Hmm....  Definitely seems like I am getting positive feedback either way...  It did firm up bass and treble seems to sustain better.  It is also in phase with my blues junior when run in stereo now.  So all wins, but still NFB does work.

Edit 2: Tried the NFB of an AB165 bassman (47k in series with .1uf fed back into the phase inverter after C10).  It looks like weird compared to most NFB set ups that i tried originally.  It worked this time since it is actually negative feedback, but it reduced signal way to much.  Not sure why the much more common place NFB from other fender models squealed like positive feedback.  Research continues...
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on March 06, 2019, 07:11:50 AM
well you had me wondering as I noted a posting and then it was not there. :duh
Not to worry, in one sentence;
If you want lots of distortion then don't bother with NFB. :-X

The NFB of AB165 bassman is not going to work without major mods. you need those 220k resistors from the plates of power tubes. It was not a good design. :loco
(look up Rob Robinette site for lots of clues on what works and what is a waste of time)

I'm not a big fan of presence systems as you need NFB and that will run the amp cleaner so if you want lots of OD then you have to run the amp at much higher level.
If that is what you want then go for it.
If you just want more treble then lower the coupling cap in front of the PI stage.

Look at some fender schematics for clues,, a lot of the AB763 circuits have very small value caps at the PI input.

Those big fender twins are so loud due to the 1nF cap at PI input, Change that to 22nF and they would just fart out at high volume.
Remove the NFB and they would not be so clean at high volume. xP

Regards to Bassman,,, Which one???? :loco

Old valve amp schematics might look simple but you need to build a few to realise that a couple of changed values can make a huge difference. 8|

At the heart of most of them is the Power supply resistor values, they are often very different and that has a massive impact on the sonic result.

As example;
Your Amp circuit has R3 (22k/2Watt) now go look at most Bassmans and you will see they are 1k to 5k.
That alone makes a big difference as to how early the PI distorts.
Your circuit is closer to Plexi Values which means the PI OD's early.

Most Bassman's use AT7 while nearly all marshall's use AX7. Again that changes how the amp responds.

I could go on here as the options are endless but maybe you can just print out a dozen Schematics and look closely at the Power rail values.
Meantime read up Robs pages.
https://robrobinette.com/Amp_Stuff.htm

At 750 pages you might be there for a long time ;)

HTH, Phil.
Title: Ibanez TSA-30 REAL FX LOOP
Post by: lighthunt on June 10, 2020, 02:19:12 AM
First of all, thanks for the schematics! ;-)

Gentlemen, did anyone tried to mod/add real FX loop to tsa-30? IMO it's the biggest weakness of this amp from player point of view. (I'd like to gently overdrive the signal with TS  and push the tube with higher volume/gain to get sound I like, which works well on it's own but adding reverb in the front or into the "fake fx loop" ruins it).

So I am considering to add fx loop in between second preamp tube (V2) and the main volume pot. This should be easy mod since there is shielded cable already, but ... will such simplistic/passive approach work well or do I need to add any buffer there (transistor or opamp)?  I've studied other amp schematics (e.g Peavey classic 30) and they have transistor on the return. Is it necessary? Why? (impedance matching or so ... ,??)

I have some, but still limited knowledge, so I really appreciate your recommendation. Thanks!!!
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on June 12, 2020, 02:51:32 AM
The circuit already has a loop.
Inserting it in the middle will likely have little benefit.
As to your idea the problem is not an fx loop it is the design of the circuit.
i.e. a ss front end designed badly and little you can do without major mods. xP

Now note I've never been close enough to smell one of there amps but I would not be interested after seeing the schematic.
It is possible some clever chap has found a way around it but I doubt it would be simple.
There is too much happening at the front end so as you note inserting a reverb FX in the loop does not work well.
If you are hell bent that it will make a difference do it at R72.
As I have already mentioned,, I would bypass the whole front end and connect the input to V1A and change R13 to at least 470k. THEN the circuit will (Likely) be more pedal friendly.
Better mind might have more knowledge,, Phil
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: lighthunt on June 22, 2020, 06:57:56 AM
Hi Phill, thanks for response!

> "The circuit already has a loop."
yeah, but it's between the tubescreamer and tube preamp. so pedals work well if I keep tube preamp clean or almost, but not great dirty (for reverb etc).

the SS frontend is the tubescreamer, nothing wrong with it and can by bypassed by pluging guitar into this fake FX loop return if you don't like it (well almost, I think there is still a clean buffer before tube, no truebypass switch).

To my ears, sound is great, lot's of tonal possibilities, just missing that real FX loop between preamp and power stage. not good for live playing if you want to switch between clean and (amp) tube distortion, cause you can never get the volume right for both it's one channel combo with swichable TS in the front. With real FX (between preamp and power) I would be able to control volume by pedal for both clean and dirty amp or proper reverb for both.

If I find time, I'd do the poor mans FX loop (https://robrobinette.com/Generic_Tube_Amp_Mods.htm#FX_Loop), should be easy mod since there is shielded cable leading to volume pot ...

Will report here later.

Cheers





Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: phatt on June 23, 2020, 07:14:09 AM
OK but some stuff that might explain why not.
The Passive FX loop is for a 5E3 while the tone setup in your amp is the later TMB setup. The simple treble cut tone of a 5E3 is not as lossy as the BF type tone in your Amp.
AFAIK
Any loading of the volume control output will impact the effectiveness of your tone controls.
Yes it will pass signal but at the cost of signal strength  and tone compromised.
Some pedals will certainly load down the signal if inserted there.

You would likely need to use another HiZ stage so the tone does not suffer needless loss.
But As I mentioned the previous stage is ideal for driving an FX loop as it's a CF stage and will not suffer from any loading,, but as it's a pcb that maybe a hassle for you. I would Break signal after R72 and use the passive loop there.
The problem with the idea of FX loops inside a valve circuit is that you interrupt the magic of how the power supply drops all through the amp. Valve power supplies are not stiff like SS Amps and a lot of the mojo has to do with the interaction of supply rails as signal goes up. It's not just sag it's how it effects the whole path from input to output.

If you care to look at all those old 6V6 deluxe schematics you will notice that the first 2 drop resistors in the R/C supply are 10k & 10k in some but then the 5E3 it's 5k & 22k.
Just by changing those values makes a big difference to how the circuit responds, dynamics and feel change.

So if you want to get that stuff to happen then DON'T break into the circuit,, use pedals in front.
If you want reverb then you need to design the mixer part like later models of fender deluxe.
Hope that helps, Phil.
Title: Re: Ibanez TSA30 Schematic and PCB Layout
Post by: lighthunt on June 29, 2020, 12:30:16 AM
The more someone tells me don't do that, the more I want to try. Or at least experiment to find out they are right ;-) So disappointed  :o

No, but seriously, thanks!! for the insight, the more I look at that your answer, the more I see it replies to my original question. If I do some experiments sometimes later, I will share here.

Cheers