Bugera 333 212 combo, very low output

[The Dude]

......When I hooked the line out from the other amp into the effects return I get full power, very loud volume, but it is very distorted even on the clean channel.....

Can you explain what you mean by that? If you are feeding the effects return directly, the preamp shouldn't have anything to do with the test, yet you mention "the clean channel".

At any rate, if I correctly understand your post, we have determined the power amp works and the preamp has problems? If that's the case, I would next do a quick check of preamp plate voltages. You may have an open or out of tolerance plate resistor.

[bwheat]

Can you explain what you mean by that? If you are feeding the effects return directly, the preamp shouldn't have anything to do with the test, yet you mention "the clean channel".

At any rate, if I correctly understand your post, we have determined the power amp works and the preamp has problems? If that's the case, I would next do a quick check of preamp plate voltages. You may have an open or out of tolerance plate resistor.

I was referring to the amp that I had hooked up to this amp being on its clean channel, not the actual amp that I am working on. I have done all the voltage measurements with tubes in and out as well as plate resistors. The first page of this has all the results from those tests.

Actually just looked again, those measurements were from the power tubes and phase inverter. I will get some numbers from the preamp tubes tomorrow. The plate resistors I tested were for the preamp and all were in spec according to the schematic I found, even though it is a little different as it is for a 333xl and I have just the 333.

[bwheat]

I got a little free time today and measured all the pins for the 3 preamp tubes.

Valve6
P1 207v, P2 0v, P3 1.5v, P4 16v, P5 5.2v, P6 211v, P7 0v, P8 1.5v, P9 10.8v

Valve7
P1 213v, P2 0v, P3 1.5v, P4 -5.6v, P5 -16.8v, P6 156v, P7 0v, P8 1v, P9 -11v

Valve8
P1 174v, P2 0v, P3 1.3v, P4 -5.4v, P5 5.2v, P6 150v, P7 0v, P8 1v, P9 0v

[bwheat]

Could anyone tell me if these numbers seem fine.

[nosaj]

Could anyone tell me if these numbers seem fine.

Someone correct me if I'm wrong but pins 4,5,9 The numbers don't seem right or even correct polarity.
To me your heater (filament) voltages are not correct for a 12ax7.
nosaj
Bugera_333XL_Schematics.pdf

[bwheat]

Someone correct me if I'm wrong but pins 4,5,9 The numbers don't seem right or even correct polarity.
To me your heater (filament) voltages are not correct for a 12ax7.
nosaj
[ATTACH]45533[/ATTACH]

I will measure those again, just to verify, I thought that the heater voltages seemed all funky.

[Enzo]

The heaters are fine, I bet if you look at them they are glowing. Your voltages on the triode pins all look reasonable, which means the tubes are conducting - which they do with proper heater.

Look at the schematic, right in the middle, there are your 12AX7 heaters. They are wired in series across +18 and -18vDC.

[nosaj]

The heaters are fine, I bet if you look at them they are glowing. Your voltages on the triode pins all look reasonable, which means the tubes are conducting - which they do with proper heater.

Look at the schematic, right in the middle, there are your 12AX7 heaters. They are wired in series across +18 and -18vDC.

And the reason V8 pin9 is zero is because it is directly at Earth ground?

Thanks,
nosaj

[bwheat]

The heaters are fine, I bet if you look at them they are glowing. Your voltages on the triode pins all look reasonable, which means the tubes are conducting - which they do with proper heater.

Look at the schematic, right in the middle, there are your 12AX7 heaters. They are wired in series across +18 and -18vDC.

Should valve 8 have more than 5 volts present on either pin 4 or 5 then like valves 7 and 6. I believe that there

[Enzo]

DO it this way: measure voltage from pin 4 to pin 5 of each tube. I bet you find close to 12v on each tube. VOltage to ground doesn't matter, what matters is the voltage across the heater - its working voltage./

They are all three in series from +18 to-18v. That means one tube will have +18 and +6 on pins 4 and 5 (or 5 and 4) The next tube will have +6 and -6v on pins 4 and 5, and the remaining tube will have -6 and -18v on pins 4 and 5. The tube with 6v on each end will have pin 9 grounded.

If the three tubes have glowing heaters, and the amplifier amplifies, then move on from the heaters, they are fine.

[bwheat]

DO it this way: measure voltage from pin 4 to pin 5 of each tube. I bet you find close to 12v on each tube. VOltage to ground doesn't matter, what matters is the voltage across the heater - its working voltage./

They are all three in series from +18 to-18v. That means one tube will have +18 and +6 on pins 4 and 5 (or 5 and 4) The next tube will have +6 and -6v on pins 4 and 5, and the remaining tube will have -6 and -18v on pins 4 and 5. The tube with 6v on each end will have pin 9 grounded.

If the three tubes have glowing heaters, and the amplifier amplifies, then move on from the heaters, they are fine.

I will give that check. I appreciate your advise. This tube audio stuff is a bit different than a lot of the electronic stuff I have expierence with. I have a real good understanding if I am looking at as far as the circuit looking at say a handwired jtm, or a twin reverb, or even some of the modern stuff with pcb boards that are arranged similar. The way these are built with the switching circuitry, the effects loop, and partially correct schematic, has me a bit confused because I can't see the actual traces on the preamp board as they are on the bottom.

Where I am at is I guess going thru and testing the components in the signal path, grid stopper, coupling caps, and the grid leak resistors. I assume having a scope and signal generator to trace the ac signal would be very helpful for this.

[bwheat]

Alright, I decided to make a signal probe to track the signal thru the circuit, and am getting something which doesn't seem right. I am getting signal to the first tube pin 7, and an amplified signal out of 6, so the tube is working to amplify the signal. What doesn't seem right is that the signal into the second stage of the first preamp tube is dropped to what it is coming into the amp. Shouldn't the signal going into the second stage (pin 2) be amplified from the first stage? Than from there further amplified by the second stage and onto pin 7 of the second preamp tube? The signal going into the second preamp tube is also equal to the input of the first stage, no amplification. There should be more than one stage of amplification in the preamp correct?

[Enzo]

The tone stack has huge losses. So I don't think this sounds wrong at all.

[bwheat]

The tone stack has huge losses. So I don't think this sounds wrong at all.

So should the final amplified signal coming from the preamp to the power amp be about the same as coming out of the 2nd stage of the preamp tube. I am going to check at the point where the signal enters the power section to see if it is weak there vs the signal coming out of the preamp tube.

[bwheat]

On what is x17 on the schematic, which comes from the front board to the power board I have just a very faint signal that is lower than what the signal coming into the front of the amp from my laptop signal generator. Very strong signal on the few 2 pin harness's around that one on the front board, but little to nothing coming out to the power board.