Modding Gibson GA-5 Tone Stack

[critterkllr]

I have a Gibson GA-5 guitar amp that I've been slowly working on over the past year. It was in pretty pour condition and missing a lot, so it needed some work. I'd like to try replacing the bridged t tone stack with the tone switch that is used on the second channel of a Matchless DC30. The treble knob would be replaced by a the cut circuit found in Vox amps (as well as the Matchless).

My plan is to remove the tone stack starting at C3 and to reattached it at pin 7 of V1B. I'd be implementing the 6 pole switch from the Matchless schematic, as well as the 1M pot and the 470K to ground following it. Whether or not I'll need the bright cap will depend on how it sounds. Is there something else that I would need to take into account? For example, does this circuit require a specific output impedance from an EF86 to work correctly? Am I fine using a 12AY7? My knowledge on these things doesn't touch what you guys know.

Some mods that I've already done to the amp:

Removed R3 and jumped C1. Changed the cathode resistors to 1.5K and bypass caps to 25uF. I'm using a 12AY7 instead of a 12AU7. I've given the amp a cap job and lowered some of the filtering. I also added a switchable negative feedback loop. I've tried modding the tone stack by adding 100K between C4 and ground, as well as changing some of the cap values, but I'm just not getting along with it.

 

[critterkllr]

Matchless DC30 schematic

 

[walter]

It looks like that would work, you can take out C3 as well. Hook it up, hear how it sounds, and tweek if needed.

 

[abbey road d enfer]

I have a Gibson GA-5 guitar amp that I've been slowly working on over the past year. It was in pretty pour condition and missing a lot, so it needed some work. I'd like to try replacing the bridged t tone stack with the tone switch that is used on the second channel of a Matchless DC30. The treble knob would be replaced by a the cut circuit found in Vox amps (as well as the Matchless).

My plan is to remove the tone stack starting at C3 and to reattached it at pin 7 of V1B. I'd be implementing the 6 pole switch from the Matchless schematic, as well as the 1M pot and the 470K to ground following it. Whether or not I'll need the bright cap will depend on how it sounds.

Beware that the Matchless/Vox tone stack has more loss than the Gibson. You may find that gain is lacking

does this circuit require a specific output impedance from an EF86 to work correctly? Am I fine using a 12AY7? My knowledge on these things doesn't touch what you guys know.

In the DC30, the tone stack is fed from a low-impedance voltage-follower; since ther is no voltage-follower in the GA5, the response will be somewhat different.
This tone stack is similar to the Fender one, that is used with or without volttage-follower depending on which model.

 

[CJ]

here is some stuff from Gerald>

 

[critterkllr]

I'm hoping that if it loses too much gain, a 12AX7 in V1 will do the trick. I guess I'll have to see.

Any advice on how to anticipate the differences in the response?  Am I looking at something small or drastic? I'd be using the same tone stack on a little 12 watt 1x10 amp instead of a 2x12 30 watt amp with massive transformers. I'd expect that it'll be a pretty different outcome regardless.

Thank you!

 

[critterkllr]

here is some stuff from Gerald>

There are quite a few different models. If it were any early one, I'd leave it stock. I think mine is a late 60's. This is when they were trying to go after a scooped blackface sound, but were using a push pull EL84 output section like a Vox AC15 or Marshall 18W. Unfortunately, I think Gibson was trying to make their amps sound like Fender guitars were being used Fender amps. The high pass filters, small cathode bypass caps and tone stack really made this amp sound horrible haha. With some mods this thing is sounding pretty awesome. I just can't completely get with the tone stack, specifically the "bass" control.

 

[CJ]

got a GA 18 Explorer in today, has the original N alnico jensen, 12ax + black plate 6V6, small opt, should be a hoot, Lab teport coming up...semi Vibrolux,

 

[critterkllr]

got a GA 18 Explorer in today, has the original N alnico jensen, 12ax + black plate 6V6, small opt, should be a hoot, Lab teport coming up...semi Vibrolux,

Nice! I take it it's the model with the 10" speaker then?

Both the 12" and 10" GA 18s are great amps. Especially if you find 5E3s or 5E11s to be a bit too mushy on the low end. And if not, it's only a few components away.

 

[abbey road d enfer]

I'm hoping that if it loses too much gain, a 12AX7 in V1 will do the trick. I guess I'll have to see.

Any advice on how to anticipate the differences in the response?  Am I looking at something small or drastic?

Small in terms of tone control, but the signal loss may not be easily compensated by just swapping tubes. 

 

[critterkllr]

Small in terms of tone control, but the signal loss may not be easily compensated by just swapping tubes.

Hmm, I'm not sure I want to drill and add another gain stage. The Gibson is using half a 12AX7 with a choke as the phase inverter. Is this giving me less gain than a traditional phase inverter circuit that uses both sides of the 12AX7? Any recommendations?

 

[fazer]

Another idea is to add a mosfet follower like an IR 820.  Very easy because it does not require a bipolar suppy, and is suppose to still have a tube sound to- it, not an SS quality.  Do a google search.  I believe Keene wrote the article (mosfet follies).    That should drive the tone stack without loss.  Also it could be kludged in to check for tone without a major design effort to make sure you like it.

 

[abbey road d enfer]

Another idea is to add a mosfet follower like an IR 820.  Very easy because it does not require a bipolar suppy, and is suppose to still have a tube sound to- it, not an SS quality.  Do a google search.  I believe Keene wrote the article (mosfet follies).    That should drive the tone stack without loss.  Also it could be kludged in to check for tone without a major design effort to make sure you like it.

Although it may help gain a dB or two, a source-follower won't compensate the loss introduced by a Fender/Marshall/Trainwreck tone stack.

 

[fazer]

Agreed it won't reduce the loss of the tone stacks action but its generally agreed that there is a different quality to a follower driven tone stack ala Marshall compared to a blackface fender.  A follower will not provide gain just lower impedance.   

Also some of the sound of a marshall is due to the 190 to 200 volts on the plates of the 1st preamp tube which distorts when driven compared to the 260 volts on a blackface.    Its also easy to use a clean booster pedal to drive that 1st tube harder.  Lots of ways to do this if your ok with adding a few solid state devices.  That might be sacrilegious to some tube amp purist.  Works well on a Dumble however.
]

 

[DaveP]

See if you can model the frequency response of the existing controls first using the tone-stack calculator.
http://www.duncanamps.com/tsc/download.html
You may be able to alter some components to get you in the right ballpark.  You can see the results against a fender profile to compare.

Good luck
DaveP

 

[critterkllr]

I did spend quite a bit of time with the tone stack calculator. I modded it to make it more usable for me, but it still isn't quite right. It's pretty close if I leave the bass up high. I might just use something else to shape the signal before it hits the amp.

 

[CJ]

you might want to sweep that innerstage xfmr,  it could be stealing a lot of tone,

 

[abbey road d enfer]

you might want to sweep that innerstage xfmr,  it could be stealing a lot of tone,

It can't really be bypassed, since it's the PI.