Bad Cat Black Cat 30R Low B+

[bobloblaws]

I have a Black Cat 30R (Matchless DC30 clone) from Bad Cat here with low B+, it should be about 350V but is in fact around 310V. There is about 14V AC ripple. The power transformer is providing about 560VAC which lines up with measurements I took last year when there was no issue with the B+. The amp has a 5AR4 tube rectifier as well as 2 different diodes for rectification. The 5AR4 tests OK on my tube tester and I also popped in a brand new 5AR4 just to be sure. One diode is labeled D420C and measures 0.490 forward voltage. The other is labeled GI828 and measures 0.360 forward voltage. I have not located a datasheet for the D420C, but the datasheet for the GI828 indicates forward voltage of 1.05 so I'm wondering if that could be the problem. The filter caps all test good. I have 6A6 diodes in stock rated at 600V and I am wondering if that would be a suitable replacement for one or both of the existing rectifier diodes. Thoughts anyone?

Edit: I should add it has choke as well. Here's a schematic for the Matchless amp.

[vintagekiki]

Smaller +UA (310V instead of 345V) with 14V AC ripple can be cause of poor filtration (defect elko 33uF / 450V); defective or worn 5AR4 tube (not diodes replacement); high consumption some of output tube 4 x EL84 due to defect or improper bias; and damaged contact on stby sw due to sparking (higher pass resistance).

To specify the cause, first remove 4 x EL84 output tubes and measure voltage and ripple at the +UA point (first elko 33uF / 450V)

[bobloblaws]

Smaller +UA (310V instead of 345V) with 14V AC ripple can be cause of poor filtration (defect elko 33uF / 450V); defective or worn 5AR4 tube (not diodes replacement); high consumption some of output tube 4 x EL84 due to defect or improper bias; and damaged contact on stby sw due to sparking (higher pass resistance).

To specify the cause, first remove 4 x EL84 output tubes and measure voltage and ripple at the +UA point (first elko 33uF / 450V)

Like I said, I ruled out the rectifier tube itself and the filter caps. Could be the choke though, right? Are you saying it for sure is not a problem with one or more of the rectifier diodes? To be clear, the Black Cat rectifier is different than what you see on the Matchless schematic. The Black Cat has a diode in series with both transformer secondary legs and pins 4 and 6 of the 5AR4 tube.

I can try your troubleshooting ideas when I get a chance to do more work on it, hopefully in a few days.

Edit: I was perusing 5E3 mods on Robinette's page and happened to find this section where he talks about wiring in backup diodes with a tube rectifier https://robrobinette.com/5e3_Modific...m#BackupDiodes That appears to be what I am seeing in the Black Cat. It makes me more confidant that the 6A6 diodes I mentioned above will work as replacements if it comes to that, but please someone correct me if I'm wrong.

[Helmholtz]

If wired as you describe, the diodes are in series with the GZ34 and only serve a protection purpose. Means that they are not essential for rectification, amp should work the same if you temporarily short the diodes.
Choke would need to have a short between winding and core/bracket to lower B+. Not very likely but easy to test with an Ohmmeter (out of circuit).

How did you test filter caps?

What is the EL84s cathode voltage (pin 3)? And what do the cathode resistors measure? Check the cathode cap for leakage/short.

Otherwise follow vintagekiki's advice.

[bobloblaws]

If wired as you describe, the diodes are in series with the GZ34 and only serve a protection purpose. Means that they are not essential for rectification, amp should work the same if you temporarily short the diodes.

Right, I edited my last post in this regard right around the time you posted.

Choke would need to have a short between winding and core/bracket to lower B+. Not very likely but easy to test with an Ohmmeter (out of circuit).

I will when I get the amp back. Owner has a gig tonight and amp seemed to still work adequately, so fingers crossed.

How did you test filter caps?

With meter, out of circuit.

What is the EL84s cathode voltage (pin 3)? And what do the cathode resistors measure? Check the cathode cap for leakage/short.

Cathode voltage was 18V, resistors were about 122 ohm. I did not check the cathode bypass caps, I will ASAP.

[Helmholtz]

With meter, out of circuit.

So you probably only verified capacitance? We are looking for leakage (not related to capacitance).

Cathode voltage was 18V, resistors were about 122 ohm.

I guess this was in low power mode (only 2 EL84s activated)? Cathode voltage should be around 10V in both modes. 18V is way too high (means excessive plate current). Cathode resistance should be 68R in high power and and 2x68R = 136R in low power mode. High plate current could be caused by a leaky coupling cap. Or could mean a defective hi/lo power switch.

[bobloblaws]

So you probably only verified capacitance? We are looking for leakage (not related to capacitance).

Yes, I only checked capacitance. Is there a simple/practical way to check for leakage? I've been meaning to watch this: https://www.youtube.com/watch?v=OykjSOprsAE

I guess this was in low power mode (only 2 EL84s activated)? Cathode voltage should be around 9.5V in both modes. 18V is way too high (means excessive plate current). Cathode resistance should be 68R in high power and and 2x68R = 136R in low power mode. High plate current could be caused by a leaky coupling cap. Or could mean a defective hi/lo power switch.

Unfortunately I'm going off (faulty) memory as I didn't jot down everything. I should get the amp back in a day or two and I can give you better info.

[bobloblaws]

I guess this was in low power mode (only 2 EL84s activated)? Cathode voltage should be around 9.5V in both modes. 18V is way too high (means excessive plate current). Cathode resistance should be 68R in high power and and 2x68R = 136R in low power mode.

Sorry, another difference between this amp and the Matchless Dc-30 is the hi/lo scheme is different. With the Black Cat the two power tubes on each side of the OT share a 120 ohm cathode resistor. When switched to Low power mode the cathodes of two of the four power tubes are disconnected from the circuit (one on either side of the OT of course).

Edit: I just found a diagram I sketched last year. The two outside power tubes share a 120 ohm cathode resistor and the two inside tubes share another 120 ohm resistor. So two tubes, one from each side of the OT, share a resistor, and each resistor is bypassed with a 250uF cap.

[Helmholtz]

Sorry, another difference between this amp and the Matchless Dc-30 is the hi/lo scheme is different. With the Black Cat the two power tubes on each side of the OT share a 120 ohm cathode resistor. When switched to Low power mode the cathodes of two of the four power tubes are disconnected from the circuit (one on either side of the OT of course).

So the 18V is for 2 EL84s sharing one 120R resistor? If so cathode voltage and current are too high by almost 100%. Cathode current per tube should not be above 40mA.

Is there a simple/practical way to check for leakage?

If an ecap shows a DCR (out of circuit - watch polarity) in the megohm range it should be ok for most applications.
For a more reliable test you need a high voltage close to the intended operating voltage (e.g. the B+ of a tube amp). Connect your voltmeter in series with the cap (polarity!) to the high voltage. Voltage reading should be as low as possible (takes some time). If you know the input resistance of your meter you can calculate the leakage resistance of the cap. Cap is ok if the leakage resistance is not below 1M. So if your meter has an input resistance of 1M and the supply is 350VDC, the meter reading should be below 175V.

[bobloblaws]

So the 18V is for 2 EL84s sharing one 120R resistor? If so cathode voltage and current are too high by 100%. Cathode current per tube should not be above 40mA.

I see what you mean, if my numbers are right then it is drawing 75mA per tube. I'd better tell him to use another amp tonight. Thanks.

[vintagekiki]

Sorry, another difference between this amp and the Matchless Dc-30 is ...

Not to wander in the dark, the principles are there
the rest are just a nuances

https://music-electronics-forum.com/filedata/fetch?id=852064&d=1542104896

http://schems.com/bmampscom/badcat/badcat_hotcat_30.pdf

https://music-electronics-forum.com/forum/schematic-requests/48912-need-bad-cat-black-cat-30r-schematic/page2

https://tonesmiths.wordpress.com/2013/09/01/bad-cat-amplifier-models-simplified/

[Helmholtz]

And don't forget to also check the cathode voltage of the other EL84 pair.

[bobloblaws]

If an ecap shows a DCR (out of circuit - watch polarity) in the megohm range it should be ok for most applications.

Does this work with an auto-ranging meter?

[Helmholtz]

Does this work with an auto-ranging meter?

Sure, but observe correct polarity and wait for final DCR value. Make sure the cap is completely discharged and disconnected from circuit before connecting the meter. This method typically works for me. But the high voltage method described above should be more reliable.

[vintagekiki]

You have enough schematics in front of you, so make an effort to draw the right Black Cat 30R scheme and upload it to portal so that we can help you specifically without guessing.

When repairing amplifier must follow principle from output to input, and the rule step by step.
This means, first examine and measure vital operating voltages at rectifier (diodes, capacitors) and at power amp (tubes).
Preamp tested at the end, when you are sure that rectifier and power amp are correct.

A good rectifier and filtration is the heart of every amplifier.
The safest and fastest way to check the capacitor if you do not have adequate measuring equipment is to temporarily replace capacitor with a new one (not like new)
When replacing caps, make an effort try to main caps are low-esr and for 105 ° C

Unnecessary philosophy (what it would be, if it were) and unnecessary jumping from part to part should be avoided. Follow the procedures step by step.

If the Black Cat 30R on rectifier has switch solid state/tube , when switch is in solid state position the 5U4G tube should be removed.

Step by step
Rectifier switch put in solid state position.
How much is voltage and ripple voltage at the +UA point (first elko 33uF / 450V) with and without 4 x EL84 output tubes?