12ax7 - 334V schem says 185V ???

[hasserl]

Thanks, R. I did a jumper wire across those resistors and the heater voltage looks good. I have only two 5881s in the amp right now, so this voltage drop is quite puzzling to me.

Do you know if it's normal/common in these amps for the 12ax7 plate voltages to be so far off from the schematic? For example, on V2 pin 1 I have 325v but the schematic says it should be 185v! That's a huge difference! Other plates are pretty far off, too (see my table and link to schem above). I checked all the resistances on each tube (top and bottom) and they all look right.

No, I don't know if the low voltages are normal. I was assuming that once you got the filament voltage correct the plate voltage might correct itself as the tubes started to conduct current as they should.

Keeping Ohm's Law in mind, E = I * R. So if the voltage is high one of the other two is off. If the resistance is correct, that leaves current. For whatever reason it appears the tubes are not conducting as much as they should be. I'm thinking it's still an issue with the filaments. I'm curious, is the clean channel louder than the lead channel?

[Krwkka]

I don't have a place where I can really blast this thing, but at pretty decent levels (95db average) it doesn't appear that there is any difference between the channels as far as the potential sound level.

I wonder if it might make a difference if I supply the heaters on V1 & V2 with AC and from a separate transformer?

[Krwkka]

Is 110dB good?

Had an opportunity to make some noise today, so I put in my ear plugs and cranked her up to where sustained notes were averaging 110 dB (measured unweighted at about 3' from Marshall 1960a cabinet - two speakers only [G12T75])

Not the slightest problem with the relative volumes (clean / dirty channels).

I still have some crazy voltages, but I do believe that the distortion channel is sounding better. Maybe I'm just growing accustomed to the sound???

[Krwkka]

If anybody has any ideas on why these voltages are so crazy, please post!

... and if I figure it out I'll let you know.

Much thanks to all who have helped!

[Krwkka]

Schematic is Wrong?

My, my, my. I've been playing this amp for a year since this subject was dropped! It sounds great, by the way.

I would appreciate it if you guys would please check me on this...

I concluded that the high voltage on V2a is normal for this amp, despite what the schematic says. What I did to come to this conclusion is I changed the value of the cathode resistor to match the value of V3a (which is on the same supply node). That made the configuration of V2a almost identical to V3a (100k plate, ~1.5k cathode). The result was that my plate voltage on V2a was within 10 volts of V3a.

Link to Schematic

My guess is that Carvin was looking for a tone for the distortion circuit, so they paired a 100K plate resistor with a 3.3K cathode resistor for a very cold running tube.

Does this sound like good logic?

As to the low DC heater voltages, that also appears to be normal for this amp (not saying good - just "normal for this amp"). I left the jumpers across the two .56 ohm resistors and I am happily toasting electrons at 6.1vdc. I do believe that the tone of the distortion smoothed out after I did the jumpers.