Gibson GA-30RV tone tweak

[Randall]

Customer asks me to try to brighten up his GA30-RV. I told him we could try subbing a cap or two. It is a pretty simple circuit. What would you do to squeeze a little more brightness out of it? I initially was looking at the .005 cap on what I think is the tone pot at the grid of V3, but with that pot turned all the way up the cap is essentially shorted, so if it still isn't bright enough, changing that cap won't make a difference, correct?

http://www.prowessamplifiers.com/sch...n/Ga_30rv.html

[Enzo]

No help there, but look just above it at the 0.001uf. That goes from signal path to ground and so could be rolling off some highs. Disconncet it and see if it matters.


But you won't like my real answer: a brighter speaker.

[Randall]

I already thought about a speaker change, but I know this guy won't do that.

[Jazz P Bass]

I just don't get 'mods'.
With all of the different amps to choose from, why not buy what you like?

[DrGonz78]

Why not put treble boosting pedal into the amp? It worked great for Jimmy Page and Brian May.

[Randall]

I know. This guy is a good dude, but he busts my balls with stuff like this all the time. He has great vintage gear, but he is always trying to tweak it. It drives me nuts.

[nosaj]

I know. This guy is a good dude, but he busts my balls with stuff like this all the time. He has great vintage gear, but he is always trying to tweak it. It drives me nuts.

So be bluntly honest with him instead of offering bandaids. offer the real option 1 then option 2. I he's cheap and chooses option 2 knowing it might not likely be the best candidate then when he comes back he already knows what has to happen. I used to try an think for a customer I thought I knew well and in the end it didn't work out so well for me.( this was when I was working on computers)
nosaj

[drewl]

I know. This guy is a good dude, but he busts my balls with stuff like this all the time. He has great vintage gear, but he is always trying to tweak it. It drives me nuts.

Ha!
I have a customer like that too!
I restored over a dozen vintage amps for him, a couple he had me tweak over and over.

[Leo_Gnardo]

What would you do to squeeze a little more brightness out of it? I initially was looking at the .005 cap on what I think is the tone pot at the grid of V3, but with that pot turned all the way up the cap is essentially shorted, so if it still isn't bright enough, changing that cap won't make a difference, correct

Correct. BUT let's have a look at the other end of that V3 triode. Hevvins, there's an unbypassed 1K5 cathode resistor! Slap a cap across it, say 0.1 to 0.5 uF, that oughta sizzle his bacon.

Happy Easter, bite a chocolate bunny.

[Helmholtz]

Any change of frequency response within the global NFB loop (if activated) will not be very effective and might cause instability.

[Leo_Gnardo]

Any change of frequency response within the global NFB loop (if activated) will not be very effective and might cause instability.

Drat, true.

Wonder how this amp would fare with no FB loop at all? Quick cheap experiment. If that doesn't pan out, what's left? A cap across one of the 100K mix/stop resistors on the user's favorite input jack, say 0.0022 uF. More or less to taste.

[g1]

How about, instead of doing what Leo said at V3, change the V1 cathode bypass to similar value like he suggested.

[J M Fahey]

"Way back then", "good tone" meant "LOTS of Bass" . those horrible 12dB resonance peak Wurlitzer record machines were king, all tone controls were treble cut only and since NOBODY could provide real Bass, all agreed on murdering treble.
Notice switchable "tone expander" .0033 cap is an NFB Bass BOOSTER (same as modern Resonance controls).

So your friend should be HAPPY with muddy tone, thatīs why he bought that amp.

If you buy chocolate cake, you canīt complain it tastes like chocolate, can you?

That said, and in no particular order, try one or more:

* like Enzo said, remove that treble killing .001uF cap to ground

* replace USELESS (what were they thinking?) .005 cap at V2 cathode (it boosts above 22 kHz, go figure) by a more realistic .1 one, boosting some 6 dB above 1kHz

* turn Reverb channel into a sort of Tweed Bassman Bright channel by adding a fixed bright cap. Try 120pF and 220pf, keep best sounding one.

* if you want to turn amp into an even brighter one, do as Leognardo suggests, add an extra .1uF cap across unbypassed V3 cathode resistor.

* agree with Helmholtz that end to end NFB (what were they thinking?) will reduce EQ tweak effects, and might even turn amp into a phase shift oscillator .
That said, maybe loop gain is not enough to get there.

ACTUALLY, proper NFB should return to V3 unbypassed cathode. What were they thinking?
You might need to switch power tube plate connections to OT to avoid positive feedback howling.

Ok, try them and post results.

And thanks for posting the schematic.

EDIT:

How about, instead of doing what Leo said at V3, change the V1 cathode bypass to similar value like he suggested.

V1 halves share cathodes, I guess itīs better to have a largish cap there so they stay separated at Audio frequencies.

[Helmholtz]

Any change of frequency response within the global NFB loop (if activated) will not be very effective and might cause instability.

I should add that my statement above only refers to the forward path of the NFB loop.

In fact a LP filter in the feedback path can be very effective to boost treble (e.g. presence control).

You might try increasing V2 cathode bypass cap to maybe 47nF to get some "presence" - provided this causes no stability problem.

[Randall]

I convinced him to try a clean treble boost pedal or an active EQ before we go modding this old amp. That should do the trick, and I won't have to get tortured half to death with this.

And thanks for all the good answers. Gentlemen all.