Want more bass for Gibson GA15RV (Trace Elliot Velocette 12R)

[clintonb]

Compared to all my other amps, my Gibson GA15RV (Trace Elliot Velocette 12R) lacks bass. One problem is that the cabinet is just too small. Plugging into a larger external cabinet helps a lot, but even then, I have to use darker speakers with a lot of low end. But that makes my cabinet sound too boomy with all my other amps. So I'm hoping to tweak the circuit to get the Gibson's bottom end more in line with the rest of my amps.

Here is a schematic and operating instructions:
http://www.gibson.com/Files/schematics/GA15RV.pdf

I want more bass but I don't want to sacrifice the clarity and articulation the amp has. Based on the circuit description, it seems a lot of the circuit is designed to keep things from getting muddy.

So I'm wondering if changing the capacitor leading into the phase inverter with a higher value would be the best place to start. I remember when I blackfaced my silverface Twin Reverb that changing that cap had the most effect on the tone.

I'm assuming C15 is the one I want to change. Right now it is 22nF (.02uF). I wonder how much bigger I should go?

The parts list says that cap is 22nF 100V axial. I'm assuming it is ceramic because it looks similar to the axial capacitor on this page:
Amazon.com: AVX 1.0uF 50V Thru-Hole Ceramic Capacitor Axial +80-20% Z5U (Bag of 10): Industrial & Scientific

I'm assuming Radio Shack doesn't have 100V capacitors like these, they never seem to have anything that handles high voltages. Do you guys know where I can get something similar without having to order large quantities?

[Jazz P Bass]

I would change the V1 cathode bypass capacitor.
Presently it is 1 uf/ 5 volt.
For full "passing" a 25 uf / 25 volt is called for.
Go to Radio Shack & purchase a 25, 15 10 & 4.7uf.

[pdf64]

C15 isn't rolling off any low end. Whereas C3 at the input probably is - most amps don't have a cap there, so you could try just linking it out.
And try increasing C1 and C8 the cathode bypass caps, as suggested - the combination of the 2 shelving filters in series will reduce the bottom end 10dB below the mids. Pete.

[clintonb]

Thanks for the suggestions!

I noticed that C15 was already pretty big compared to say my Silverface Twin Reverb. I really need to study up more and experiment with filter circuits to see where frequency cutoff points are.

I wonder if this amp rolls off a lot of bass in the preamp because it was designed for the type of player that cranks the amp and rolls off the volume for cleans. I noticed that the owners manual for the GA30RVS (which is two GA15RVs combined into one) recommends turning the amp volume all the way up and using the guitar volume to go from clean to dirty and kicking in the boost for solos. But cranking the amp is too loud. I use the amp as a clean platform for distortion pedals.

[clintonb]

C15 isn't rolling off any low end. Whereas C3 at the input probably is - most amps don't have a cap there.

I replaced the C3 cap (.0022uF) at the input with a jumper wire. There was a slight change, barely noticeable. I suppose any extra bass I'm getting by replacing that with a jumper wire is still mostly getting rolled off by the cathode bypass caps. So I'll try the cathode bypass caps next.

[J M Fahey]

Don't mess with this amp.
There's nothing "cutting bass" there, as you are already discovering; so if you lack bass, look elsewhere.
All you'll get is, at best , to muddy it with no real bass increase.
In fact, it has *two* treble killing capacitors: the 100pF plate to cathode on V1A and the much more drastic 1000pF at V1B plate.
The "tone" control, in fact is a Classic Fender TMB type, with Bass already on full (fixed 470k) and Treble linked to Mid; you go from full treble and no mid to the opposite , with a single knob.
Fact is, you have a Fenderish 15W, small box, open back combo; not much chest thumping power there.
Sorry.

[clintonb]

Don't mess with this amp.
There's nothing "cutting bass" there, as you are already discovering; so if you lack bass, look elsewhere.
All you'll get is, at best , to muddy it with no real bass increase.
In fact, it has *two* treble killing capacitors: the 100pF plate to cathode on V1A and the much more drastic 1000pF at V1B plate.
The "tone" control, in fact is a Classic Fender TMB type, with Bass already on full (fixed 470k) and Treble linked to Mid; you go from full treble and no mid to the opposite , with a single knob.
Fact is, you have a Fenderish 15W, small box, open back combo; not much chest thumping power there.
Sorry.

Thanks for your input.

So are you of the opinion that nothing can really be done?

Yes, a small open back combo isn't going to give me much bass. Some speakers help a little bit, but I gave up on that. But even when I plug it into a bigger cabinet, it seems to have less bass than my other amps.

I was wondering why some of my other small amps seem to have more bass.

My 70s silverface Princeton (which is a 12 watt 1x10 combo) had a fuller sound than my Gibson GA-15RV 15-watt 1x12 combo does.

I also have a Reason Bambino Grande 1x12 combo. That is a 20-watt/10-watt amp. It has a fuller sound than the Gibson. But it also has a wider and deeper cab. But when I plug it into the same external cabinet as the Gibson, the Bambino has much more bass than the Gibson. Actually the Bambino has two channels, normal and bright. The normal channel, which they describe as Vox-like, has less bass and highs and more mids than the Bright channel, which they describe as Fender-like, which has more bass and highs and less mids.

One thing Reason mentioned to me was that a Bambino sounds fuller than other small amps because they use fixed bias instead of cathode bias which is what my Gibson uses. He says fixed bias gives you more bass (more highs too?).

I did find one online discussion where a person did get more bass out of the amp. But I'm not sure if it was because of the mods (remove cap at input, use larger cathode bias caps) or the speaker change. See posts #35 and later from this thread:
Gibson tube amp mod for JAZZ - diyAudio

Could one increase bass, by changing the 470K resistor which sets the bass in the tone stack? That's kind of unusual, a lot of amps have a fixed mid, but I've never seen a fixed bass before. I think I'll experiment with the Duncan Amps Tonestack Calculator.

Perhaps the lack of bass is all relative. Maybe the bass isn't too low, perhaps the mids and highs are too strong, so at any given volume, the bass sounds shy in comparison. Perhaps I could attenuate the mids and/or highs so the bass sounds louder in comparison?

[J M Fahey]

But even when I plug it into a bigger cabinet, it seems to have less bass than my other amps.

It *does* have those 1uF cathode caps, which cut relatively deep bass; but I think they are there for a good reason, meaning they are not cutting bass the speaker can easily reproduce.
Obviously a designer optimizes response thinking on what *he* es actually using, rather than on what an eventual external cabinet might do.

I was wondering why some of my other small amps seem to have more bass.
My 70s silverface Princeton (which is a 12 watt 1x10 combo) had a fuller sound than my Gibson GA-15RV 15-watt 1x12 combo does.

Probably different designer choices. Do they sound so tight at high level as this one?

One thing Reason mentioned to me was that a Bambino sounds fuller than other small amps because they use fixed bias instead of cathode bias which is what my Gibson uses. He says fixed bias gives you more bass (more highs too?).

Never experienced that. Don't say it's impossible, just can't find arguments either for or against that theory.
Personally I feel the influence, if any, would be too small.

I did find one online discussion where a person did get more bass out of the amp. But I'm not sure if it was because of the mods (remove cap at input, use larger cathode bias caps) or the speaker change. See posts #35 and later from this thread:
Gibson tube amp mod for JAZZ - diyAudio

Ok, I'll read it later, although the user might have pulled some brightening caps for a smoother, sweeter Jazz sound.
Fine with me, but it's not the same as "increasing bass".

Could one increase bass, by changing the 470K resistor which sets the bass in the tone stack? That's kind of unusual, a lot of amps have a fixed mid, but I've never seen a fixed bass before.

It's already twice the value of the usual 250K Fender bass pot.
What you may try, besides it's very easy, is to "Marshallize that tone stack, which will give you *much* increased low mids and not_so_deep Bass.
Maybe that's all you need.
You can even add a mini-toggle switch to change from one to the other.
R11 must go from 100K to 33K.
C14 must go from 47n to 22n.
The difference will be astounding.
To fully Marshallize it you should replace C12 (220pF) with 470 pF but on a hunch I would leave it as-is.
You are not after more high-mids, I think.
I would not mod it beyond this point.
Try it and comment.

I think I'll experiment with the Duncan Amps Tonestack Calculator.

*Excellent tool*
I think we all must be grateful to Duncan for so kindly releasing such useful (and user-friendly) software.
Plug the original values and the modded ones into it, and see for yourself.
Use the Fender model, bass pot 470K, always on full, both extremes of the "tone" curve are full treble, 0 mids, and the opposite.

Perhaps the lack of bass is all relative. Maybe the bass isn't too low, perhaps the mids and highs are too strong, so at any given volume, the bass sounds shy in comparison. Perhaps I could attenuate the mids and/or highs so the bass sounds louder in comparison?

Maybe, perhaps that's what the Jazz player did, but it will give you less overall useful volume.
Another option would be using your other amps for what you want, and using this one, unmodified, on its own; after all it's an excellent amp.
Good luck.

[clintonb]

Through a little experimentation and comparison with my other amps, I discovered that the Gibson doesn't lack bass compared to my other bass, it is just significantly brighter.

So I changed the treble cap from 220pF to 120pF and that mellowed the tone a bit. It was definitely an improvement.
Perhaps I may change the bright switch value from 220pF to something smaller since I just want it to add a little shimmer. Although, in triode mode, I do need more brightness.

I remember trying the mod where you remove C6. The designer said it was there for a "presence lift". I tried it, but I didn't like it. I think it removed clarity if I remember correctly.

Here is a brightness mod I found in a Harmony Central review of a Trace Elliot Velocette 12R:

at first I found the amp very bright;I switched tubes and the speaker but did not like it;I came back to the original tubes(Sovtek 12AX7WB) and the Celestion Vintage 30,but changed the capacitors of the tone stack:C13-47n,C14-33n,C7-100p,C12-150p,R12-300k;that did the trick. The tone is clear and has good mids and a tight bass.

I might try that someday if I get the parts.