'68 Bassman amp volume

[Doc Stitches]

Just finished the amp rebuild.

Very low volume on bass channel and no volume on normal channel

I will do a pin voltage test

Doc

[Chuck H]

That's the best place to start. Provided nothing starts smoking while you're at it! Which can happen if it's a miswire causing the problem. I hope diagnostics go well and you can play it at midnight

[Doc Stitches]

Pin readings

That's the best place to start. Provided nothing starts smoking while you're at it! Which can happen if it's a miswire causing the problem. I hope diagnostics go well and you can play it at midnight

So here are the pin numbers ( right to left, from the back )

tube 1
1-205v
2-0
3-2v
4/5-16v
6-198v
7-0
8-2v
9-17v


tube 2
1-0
2-0
3-0
4/5-16v
6-271v
7-0
8-2v
9-17v


tube 3
1-207v
2-0
3-1v
4/5-16v
6-273v
7-0
8-2v
9-17v


tube 4
1-283v
2-74v
3-113v
4/5-17v
6-276v
7-74v
8-113v
9-17v


tube 5


1- -60v
2-14v
3-450v
4-450v
5- -60v
6-449v
7-17v
8-0 grd


tube 6
1-3v
2-14v
3-450v
4-450v
5-3v
6-449v
7-17v
8-0 grd

74v on pins 2 & 7 tube 4 ?

Is the heater voltage OK at 17v ?

[Justin Thomas]

Which Bassman circuit did you rebuild it into? So, are you measuring AC or DC on the heaters? I could see having DC if someone elevated the heaters in an attempt to reduce hum. Is the amp modified in any way? Measure the AC <ACROSS> the heaters of the tubes - that would mean one probe on pin 2, the other on pin 7 of a 6L6. Should be 6.3V or so. What matters for heaters is the voltage across, not to ground.

The ~70V on the grid of V4 is no biggie - the way that phase inverter works.

Your power tubes DEFINITELY look wonky.

Justin

[Doc Stitches]

sorry AB165 circuit

woops .... I was on DC for the heaters. I'll check em again.

[Chuck H]

Those are some of the most wonky voltages I've seen. Too much wrong here to even start with a diagnosis.

Are you certain these are accurate" Have you checked your meters accuracy. Is it possible the battery is dying?

The first thing that stands out above all else is the filament voltage. Did you measure those for AC or DC? Either way there's something really wrong there. You should have 6.3ishVAC (possibly 3.15ishVAC depending on how you measured it) and 0VDC. The second thing that stands out is the bias voltage. Unless something is wired wrong you should have the same negative voltage on pin 5 of both power tubes and it's a pretty straight forward circuit that puts it there. Since you only have -3V on pin 5 V6 I think there must be a mistake.

You should pull those tubes and correct wiring errors. Take voltage readings with tubes out, repost your findings and don't plug tubes back in before they are reviewed here please.

[nosaj]

Those are some of the most wonky voltages I've seen. Too much wrong here to even start with a diagnosis.

Are you certain these are accurate" Have you checked your meters accuracy. Is it possible the battery is dying?

The first thing that stands out above all else is the filament voltage. Did you measure those for AC or DC? Either way there's something really wrong there. You should have 6.3ishVAC (possibly 3.15ishVAC depending on how you measured it) and 0VDC. The second thing that stands out is the bias voltage. Unless something is wired wrong you should have the same negative voltage on pin 5 of both power tubes and it's a pretty straight forward circuit that puts it there. Since you only have -3V on pin 5 V6 I think there must be a mistake.

You should pull those tubes and correct wiring errors. Take voltage readings with tubes out, repost your findings and don't plug tubes back in before they are reviewed here please.

What kind of meter are you using? Also put a fresh battery in and see if the voltages change. I know I get some weird readings when my battery gets below a certain level.

Nosaj

[Doc Stitches]

Again ... I built this to AB 165

6.6v on heaters ... was set to DC

Meter is Pros Kit/ Eclipse ...new battery, used on 20 + amp repairs

Amp head was partially stripped when it got it and was also modded some.

Bass channel bass pot was 1 M so I put in the 250-K

All new resistors and caps

I'll check 5 and 6 without tubes and check back before going any further

[nosaj]

Again ... I built this to AB 165

6.6v on heaters ... was set to DC

Meter is Pros Kit/ Eclipse ...new battery, used on 20 + amp repairs

Amp head was partially stripped when it got it and was also modded some.

Bass channel bass pot was 1 M so I put in the 250-K

All new resistors and caps

I'll check 5 and 6 without tubes and check back before going any further

Do you still have DC on your heaters? Chucks post states it should be 0Vdc, maybe a tube connection is bent towards a heater?(I've done it myself)

nosaj

[Chuck H]

74v on pins 2 & 7 tube 4 ?

The ~70V on the grid of V4 is no biggie - the way that phase inverter works.

Look at the voltages on pins 3/7 though. That's -39 for the grid of the PI. That would be the third thing that looks way wrong.

So to sum up:

All voltages are high. So don't expect any voltages to be on spec with the schematic. (PT providing more voltage and/or low current draw due to cold bias in the power tubes)

DC on filament string where it shouldn't be. (no idea/wiring error? stray strand?)

No bias voltage on one of the power tubes. (almost certainly a wiring error)

PI is biased WAY too cold. (could be a 10X resistor value error?)

[Doc Stitches]

new info and photos

tube 6 Pins 1 & 5 ... -47v

Pins 3, 4, 6 ... 457v



No DC on heaters ( without tubes )

I found several other issues I fixed :

Loose ground on Normal volume pot

Wrong resistor on Bass pot of Normal channel

No resistor on bass pot on Bass channel

Wrong resistor on bias pot

Added 2 10K resistors and one 220K resistor to bias circuit ( as in AB165 layout )

Think I can fire it up now ?

Look at the voltages on pins 3/7 though. That's -39 for the grid of the PI. That would be the third thing that looks way wrong.

So to sum up:

All voltages are high. So don't expect any voltages to be on spec with the schematic. (PT providing more voltage and/or low current draw due to cold bias in the power tubes)

DC on filament string where it shouldn't be. (no idea/wiring error? stray strand?)

No bias voltage on one of the power tubes. (almost certainly a wiring error)

PI is biased WAY too cold. (could be a 10X resistor value error?)

[The Dude]

Those voltages look good for V6. What about V5? Are they the same?

[Doc Stitches]

V 5 is ( -57v on pins 1 & 5 )

[Chuck H]

Well... Everything is relative. -47V bias on V6 is definitely an improvement. Do you have the same bias voltage on V5?. Go ahead and plug in the preamp tubes so you can see if the PI is still biased cold Did you check that PI bias resistor value to make sure it's not 47k? When you mention that you added resistors to the bias circuit I have to guess that someone had altered it to be an adjustable current bias control rather than a bias balance control. Does that circuit work as intended? You should be able to "balance" the voltages on the sockets with or without tubes in.

It's possible that plugging any tubes back in may cause a problem. There is no DC on the filaments now without tubes in, but you measured DC on the filaments before when the tubes were in. That makes me suspect a faulty tube. You should plug the preamp tubes in one at a time and measure for DC on the filaments after plugging in each one. If plugging in a tube puts DC on the filaments, that tube should not be used, ever, by anyone for any reason including just testing.

Don't plug the power tubes in before:

You confirm the preamp tubes are not causing DC on the filaments.

The bias supply is working properly. It's a bias balance control so you should be able to twiddle it for matching voltage.

The PI voltages are in proportion to each other (about 2V more on the cathodes than the grids).

Before these things check out there's no point in plugging in the power tubes.

P.S. If none of the preamp tubes cause the DC condition in the filaments then you still need to check for DC when plugging in each power tube. Do not run the amp with any tube that causes DC on the filaments.

[The Dude]

Is that 57v positive, or did you forget the negative sign? At any rate, both output tube sockets should have the same voltages and bias should be negative.

Edit: Also, there should be a "tubes matching" potentiometer in there. Until you get the thing working, turn the pot to roughly center of the adjustment range. Then recheck pin 5 of each of the output tubes.