Bad Cat Black Cat 30R Low B+

[vintagekiki]

On DC30 and AC30, signals from two preamp are came to both PI inputs.

[Helmholtz]

On DC30 and AC30, signals from two preamp are came to both PI inputs.

Thanks. If that's also the case here we have a missing ground path for the 100k? tail resistor. In any case, I don't think that the 1.2k cathode resistor should be directly grounded.

[bobloblaws]

I think the 100k is the tail resistor and is grounded together with the upper PI input (assuming there is no NFB).

(Using a continuity tester can be a big help in finding hidden connections.)

The Matchless DC30 schematic I uploaded in post #1 obviously isn't as great of a reference as I initially thought.

In any case, on this amp there is no NFB and no 100K resistor. There is a grounded 47K tail resistor. Here's my revised in-progress drawing.

[Helmholtz]

Now it looks right. PI is similar to the 2xEL34 version :https://music-electronics-forum.com/...4&d=1597882902
Let's get back to troubleshooting.

[vintagekiki]

Thanks. If that's also the case here we have a missing ground path for the 100k? tail resistor. In any case, I don't think that the 1.2k cathode resistor should be grounded.

No NFB no 100k.
The R3 (1.2k) cathode resistor is grounded via R1 (47k).

http://dealers.korgusa.com/svcfiles/ac30p1.pdf

[bobloblaws]

Ok, now we're getting closer.
So we have unwanted positive grid voltages. There's no doubt that this causes the high cathode currents and low B+.

Please proceed as follows:

1) Measure socket pin 2 voltages with all EL84s pulled.
2) Make sure that the junction of the two 220k grid leaks is well grounded (0VDC).
3) Lift both 47nF coupling caps and remeasure socket pin 2 voltages.

OK, I followed your methodology to the letter. In a nutshell there was 6-8V on the grids with the tubes pulled. After lifting the caps grids were 0V. I took the obvious further step of swapping in new coupling caps. So with the new caps and tubes back in the grids are at 0V and the cathodes at 10.4V. So that's all good. However, B+ is still low, around 330VDC, should be closer to 360V. I still haven't done any kind of leakage test on the filter caps. Do you recommend that as logical next step?

[Helmholtz]

OK, I followed your methodology to the letter. In a nutshell there was 6-8V on the grids with the tubes pulled. After lifting the caps grids were 0V. I took the obvious further step of swapping in new coupling caps. So with the new caps and tubes back in the grids are at 0V and the cathodes at 10.4V. So that's all good. However, B+ is still low, around 330VDC, should be closer to 360V. I still haven't done any kind of leakage test on the filter caps. Do you recommend that as logical next step?

From your measurements I calculate a leakage resistance of around 5M of your coupling caps (it is rather uncommon that both coupling caps are leaky). Could you verify this with your Ohmmeter?
The EL84s' cathode current is 43mA per tube, still hot but acceptable.

What makes you think that the B+ of 330V is too low? Did you ever measure a higher value? A B+ variation of 10% is not uncommon due to components' and line voltage tolerances.

[bobloblaws]

From your measurements I calculate a leakage resistance of around 5M of your coupling caps (it is rather uncommon that both coupling caps are leaky). Could you verify this with your Ohmmeter?

I'm unclear how I can verify a specific leakage resistance with the ohmeter alone, but what I am observing is that one of those caps shows "OL" on the meter while the other one initally reads around 2M and the meter quickly counts up to 6-7M then briefly reads OL and then starts the cycle over and does that continuously. I have a leaky cap I removed from another amp recently that exhibits the same behavior as the latter and the new caps both show OL. So I would be inclined to think that when the meter cycles between 2M up to 6M, OL, and all over again that that indicates a problematic cap and that steady OL indicates a good cap, but please correct me if I'm wrong. That would support your idea that it is unlikely that both caps are leaky but on the other hand I saw positive voltage on all 4 grids. Maybe the voltage "leaked" through the "Cut" circuit, but that would mean the .0022 cap was leaky as well.

The EL84s' cathode current is 43mA per tube, still hot but acceptable

My understanding is that the Matchless DC30's and Black Cat's have a reputation for running hot bias and going through tubes quickly.

What makes you think that the B+ of 330V is too low? Did you ever measure a higher value? A B+ variation of 10% is not uncommon due to components' and line voltage tolerances.

I have this amp in a couple of times before. The first time I recorded B+ as being 361V in full power mode. Six months later (a little more than a year ago) it was brought in again because it fell off the back of his SUV and some tubes were broken etc. At that time I recorded B+ of 350V. Now it's down to 330V so it seems to be getting progressively lower.

[Helmholtz]

I'm unclear how I can verify a specific leakage resistance with the ohmeter alone, but what I am observing is that one of those caps shows "OL" on the meter while the other one initally reads around 2M and the meter quickly counts up to 6-7M then briefly reads OL and then starts the cycle over and does that continuously. I have a leaky cap I removed from another amp recently that exhibits the same behavior as the latter and the new caps both show OL. So I would be inclined to think that when the meter cycles between 2M up to 6M, OL, and all over again that that indicates a problematic cap and that steady OL indicates a good cap, but please correct me if I'm wrong. That would support your idea that it is unlikely that both caps are leaky but on the other hand I saw positive voltage on all 4 grids. Maybe the voltage "leaked" through the "Cut" circuit, but that would mean the .0022 cap was leaky as well.

The Fluke 179 should be able to measure DCR up to 50M. So OL will indicate a DCR above 50M, which should mean a good cap. But there is a possibility that the caps shows increased leakage only at elevated voltage and you meter only measures with its low internal voltage. You could use the method I proposed for ecaps using the PI plate voltage as supply. Or skip it as it seems to be fine with new coupling caps.
BTW, always make sure a cap is completely discharged before you connect it to an Ohmmeter. Might damage the meter otherwise.

My understanding is that the Matchless DC30's and Black Cat's have a reputation for running hot bias and going through tubes quickly.

If you care about tube life, you may replace the 120R cathode resistors with (tube manufacturer recommended) 130R or two 68R in series, or even 150R.

I have this amp in a couple of times before. The first time I recorded B+ as being 361V in full power mode. Six months later (a little more than a year ago) it was brought in again because it fell off the back of his SUV and some tubes were broken etc. At that time I recorded B+ of 350V. Now it's down to 330V so it seems to be getting progressively lower.

This indicates still another problem and it makes sense to test the supply ecaps for leakage.

[vintagekiki]

Amplifiers with automatic bias, for bias have a cathode resistor that defines their cathode current. The cathode current is quite high and these amplifiers belong to the hot amplifiers and are said to work in class A.

It should be noted for the cathode bias that the value of the bias voltage is not constant as with amplifiers operating in class AB1 (bias define by -Ug1) but depends on the tube to tube, and the degree of wear of the cathode emission.

For example Vox AC30 for 4xEL84 and Rk = 50 Ohm, has bias Uk = 10V which corresponds to a current of 50ma per tube, or 200mA for quartet.

[bobloblaws]

...

By the way, I just noticed you mentioned the possibility of a leaky coupling cap way back in post #6. I got so hung up on trying to figure out how to test electrolytics for leakage and the math behind it that I completely overlooked that.

[Helmholtz]

Amplifiers with automatic bias, for bias have a cathode resistor that defines their cathode current. The cathode current is quite high and these amplifiers belong to the hot amplifiers and are said to work in class A.

It should be noted for the cathode bias that the value of the bias voltage is not constant as with amplifiers operating in class AB1 (bias define by -Ug1) but depends on the tube to tube, and the degree of wear of the cathode emission.

For example Vox AC30 for 4xEL84 and Rk = 50 Ohm, has bias Uk = 10V which corresponds to a current of 50ma per tube, or 200mA for quartet.

I have never seen a PP guitar amp that operates in true class A. As soon as the cathode voltage in a cathode biased amp increases at large signal, the amp operates in class B. So it's a class AB amp. All class AB amps operate in class A at low signal and class B at large signal. In a true class A amp cathode voltage does not change with signal level.
The Vox AC 30 is definitely a class AB amp, even though it's biased at 120% or so. To turn an AC30 into a true class A amp would require to limit its plate supply to around 250V.

[vintagekiki]

The cathode current can be reduced by increasing the cathode resistor (Rk) power tube (s), but the cost of such regulation is less amplifier dynamics.

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/8554-about-biasing-tube-guitar-amplifiers
About biasing tube guitar amplifiers

https://www.aikenamps.com/index.php/is-the-vox-ac-30-really-class-a ¹⁾
Is the Vox AC-30 Really Class A?

http://web.archive.org/web/20101229213911/http://diyguitarist.com/Images/BiasChart-EL84.jpg ²⁾
EL84/6BQ5 Bias Chart

[bobloblaws]

By the way, I just noticed you mentioned the possibility of a leaky coupling cap way back in post #6. I got so hung up on trying to figure out how to test electrolytics for leakage and the math behind it that I completely overlooked that.

Speaking of which, I'm still confused about the leakage resistance subject. In post #9 you wrote "If an ecap shows a DCR (out of circuit - watch polarity) in the megohm range it should be ok for most applications." I'm having trouble squaring that with "So OL will indicate a DCR above 50M, which should mean a good cap".

Are we talking about different methodology or standard for electrolytic caps as opposed to non-electrolytics? Also, theory wise, is there a linear relationship between DCR as measured by ohmmeter and leakage resistance such that a cap that measures e.g. 1M DCR has half the leakage resistance than does another cap that measures 2M DCR?

[Helmholtz]

[QUOTE=vintagekiki;n911689]The cathode current can be reduced by increasing the cathode resistor (Rk) power tube (s), but the cost of such regulation is less amplifier dynamics.[QUOTE]

I don't believe in such generalized statements.