1955 Gibson GA-30

[boroman]

Cap value doesn't matter as a cap doesn't pass DC (except if it's leaky).

What does the cathode resistor measure?
What is cathode voltage?

255 Ohm measure without cap
16.5 V across it

[Helmholtz]

Means a plate dissipation of 8.8W per tube (assuming a matched pair) or 73% with a 12W tube like an original 6L6GT.
If you think you need higher PD, lower the cathode resistor to 220R or 180R.

[boroman]

Means a plate dissipation of 8.8W per tube (assuming a matched pair) or 73% with a 12W tube like an original 6L6GT.
If you think you need higher PD, lower the cathode resistor to 220R or 180R.

I know changing cathode resistor will change dissipation (but will also affect PV, right?) - question is, was the amp designed to run close to 100% like most of other cathode biased amps - please educate me here...

[Helmholtz]

What we do know is that the amp was designed with a 250R cathode resistor.
Cathode resistor sets the cathode current.
Changing its value means redesigning the amp.

[glebert]

"Amp sounds great" but yet you are considering making changes? If PD was too high I could see it, but for what you described I would leave it aloooone....

[g1]

was the amp designed to run close to 100% like most of other cathode biased amps - please educate me here...

'Most' cathode biased amps are not running near 100%. Unless they are single-ended (one power tube).

[pdf64]

My thinking is that anode dissipation often ends up necessarily high, near the limiting value, due to anode voltage being up near its limit. ‘Necessarily’ because otherwise there can be bias shift (cathode voltage increases) at high power output, resulting in the amp’s sound degrading a bit.
But as your anode voltage is well below the limit, it may be things panned out (eg with the load impedance) such that the idle dissipation could also be well below the limit but still sound good.

[boroman]

'Most' cathode biased amps are not running near 100%. Unless they are single-ended (one power tube).

So, is GA30 a Class A or shifts more towards AB1?

[Helmholtz]

So, is GA30 a Class A or shifts more towards AB1?

Not possible to tell from just looking at the schematic.
But you can easily find out: If cathode voltage increases (by more than say 10%) at large output, it's class AB.

In fact I've not yet seen a PP guitar amp running in pure class A (this includes the Vox AC30).

[LarBal]

Reviving this thread to add a schematic , as I lucked into one of these rare 12AY7 GA-30 amps. Got it for $400 in non-working condition. 100% original (including tubes), and the 8" speaker was shot at the 12" Jensen P12S (10 watts!) was very tired and farty. After restoring the amp(Cap job, and had to rebuild very noise paraphase), I put in an Alnico cream and it sounds great, if not a bit dark. I still need to get a good 8" for it.

Turns out this amp is very similar to the GA-20 with the 12AY7 preamp. I'll attach it for reference, as well as my amp's schematic I drew out on a marked up GA-20 schematic.

Of note:
Mic input is the loudest with the most headroom, and looks to be grid leak biased, though the grid leak is only 1M. ??
The other 3 inputs share a single 100K plate resistor (drifted to 130k), and a shared 470r Rk
Check out the tone stack. I found this in one of the other octal variants of the GA-30, as well, it seems to function almost like a variable coupling cap. All the way up you get the 250p cap bypassing a 2M resistor essentially, and all the way down basically a .05 cap
Tone expander(i didnt draw the switch, it puts it in series with the 220k fb resistor) doesn't seem to do anything

Even with the tone control all the way up, the amp is a hair dark with a tele. It sounds wondedeful, syrupy, thick, compresses very nicely under the fingers, but I'm a bit baffled as to why it would be so dark.
I'm going to experiment with a basic tone control, but I can't see how that would add any sparkle to the amp, unless I'm misunderstanding how the current tone control works

I'm wondering if adding the 8" will bring some sparkle out...what about the entrance to the paraphase, would 470K darken up the inverted signal considerably as opposed to 220K?

I'm interested to hear your thought on this, and how to brighten up the amp

[Helmholtz]

Mic input is the loudest with the most headroom, and looks to be grid leak biased, though the grid leak is only 1K. ??

How 1k?
Grid leak R1 is 1M.
The voltage table doesn't fit the (modified?) schematic.

[LarBal]

My mistake , I meant 1M on the grid leak. Usually you see at least 5M there though. On the original GA20 schematic the leaks are 470K. I just thought that was low for grid leak biased stage

I haven't recorded voltages yet, though I imagine raising them on the rail would brighten it up eh?

[Helmholtz]

My mistake , I meant 1M on the grid leak. Usually you see at least 5M there though.

Grid leak biasing with only 1M works with very low plate voltage.

[Helmholtz]

The somewhat dark sound of the amp could be due low input impedance.
When tubes are operating with a plate voltage significantly lower than 100V, grid leak current increases and tube input impedance significantly drops.
Low input impedance of say 100k or lower will make the PUs sound dull.
To verify you could try a buffer or buffered effects pedal before the amp.

[LarBal]

Interesting.. I think the schematic shows 75V plate for the mic channel first stage. I'm at 85V or so.

The amp sound really really good till it gets to about 4. I'm getting what I thought to be speaker fart initially, but I have put 4 different 12" speakers on it today, and they all are exhibiting the same sound....nasty farty breakup only on the E and A strings, when hit hard when it starts to get loud. Note that it's starting to breakup in a pleasing way around 4 with no issues, but when it really starts to crunch, weird sounds happen..
I even changed the 1st coupler to .02 with no change.

I'm starting to think think there's something in the chassis causing it. Are those 470K grid leaks for the 6V6 too much?