st70: sound level in one channel greatly diminished

[kheper]

Sound is still diminished on the right channel, though things are still warming up.

[Ty_Bower]

How does it sound with the Dynaco 12AX7?

Have you tried swapping the left and right RCA input cables? Maybe the problem is in your source or preamp?

[TomMcNally]

I wasted a lot of time thinking an amp had a weak channel,
only to discover the balance pot on the preamp was dirty and
needed to be rocked back and forth a couple of times.

[kheper]

How does it sound with the Dynaco 12AX7?

Have you tried swapping the left and right RCA input cables? Maybe the problem is in your source or preamp?

The sound is still diminished in the right channel.

The Dynaco 12ax7 makes the sound a little brittle.

I swapped rca and speaker cables a long time ago.

[kheper]

Maybe the problem is in your source or preamp?

I tried alternative sources.

I'll go direct into the amp later.

[Ty_Bower]

Question for the gurus: Can he disconnect the NFB at J12 and J13?

The amp will be louder overall, and damping and distortion will both get worse. But, it might tell us if something is wrong with the feedback circuit. Maybe something is coming back in that is pulling down the right side.

I'm still raising an eyebrow at those 12AX7 cathode voltages. It should be around 1.2 volts. Not 7.5. Not 3.24.

[kheper]

Question for the gurus: Can he disconnect the NFB at J12 and J13?

The amp will be louder overall, and damping and distortion will both get worse. But, it might tell us if something is wrong with the feedback circuit. Maybe something is coming back in that is pulling down the right side.

I'm still raising an eyebrow at those 12AX7 cathode voltages. It should be around 1.2 volts. Not 7.5. Not 3.24.

I put a 12ax7 RCA grey plate in the amp.

pin 3 = 1.25V
pin 8 = 1.25V

The right channel is still diminished. I always suspected the NFB, but it seems to be ok.

Maybe replace R7, R9, R110, and C3?

[EWBrown]

1.2 to 1.3V on the 12AX7 cathodes is normal, higher voltages indicate the tubes are drawing excess curent. Also chack the 12AX7 plate voltages, there can be some variation from "normal" but not too much, either..

Diminished output could be that the NFB is set too high, check the resistance of the NFB loop (may have to disconnect from the OPT secondary to get a correct reading).

Also make sure that all four EL34s are biased correctly, usually 50 mA each.
if one is far out of spec, kits G1 coupling cap, it could have some high resistance leakage, and be skewing the grid bias voltage.
HTH

/ed B

[kheper]

1.2 to 1.3V on the 12AX7 cathodes is normal, higher voltages indicate the tubes are drawing excess curent. Also chack the 12AX7 plate voltages, there can be some variation from "normal" but not too much, either..

Diminished output could be that the NFB is set too high, check the resistance of the NFB loop (may have to disconnect from the OPT secondary to get a correct reading).

Also make sure that all four EL34s are biased correctly, usually 50 mA each.
if one is far out of spec, kits G1 coupling cap, it could have some high resistance leakage, and be skewing the grid bias voltage.
HTH

/ed B

The plate voltages on the 12ax7 were 184 or 186V.

My bias supply cannot supply 50ma per tube. The bias is set at 45ma per tube.

[Ty_Bower]

My bias supply cannot supply 50ma per tube. The bias is set at 45ma per tube.

That's an odd statement. The bias supply doesn't provide the current that idles through the tube. The idle current comes from the high voltage B+ supply. The bias supply is usually -30 to -50 VDC, and needs to source no more than a couple milliamps.

Applying a negative bias voltage to the grid of the tube reduces the amount of current the tube will conduct. The greater the negative bias voltage, the less the tube conducts. If you apply a great enough negative voltage, you can shut off the tube completely.

If you cannot get your amp to bias any higher than 45 mA, that tells me the voltage divider in your bias circuit is providing too great a negative voltage. The suggested bias circuit in the DIY ST70 manual has a 10k fixed, followed by a 10k pot, then finally 10k fixed to ground. You ought to be able to get between -25 and -50 volts out of it. I'd think that at -25 volts bias, your output tubes would conduct well in excess of 50 mA.

You could probably replace the 10k resistor between the pot and ground with a 5k part instead. That ought to get you a bias voltage between -15 and -45 volts. Maybe that will get your tubes up to 50 mA.

By the way, all of this has absolutely nothing to do with your original problem.

edit... A few posts back you said your bias voltage was roughly -40 volts:

J21= 40.3V close to left channel

Are you saying you can't dial that bias voltage any closer to -35 volts or so?

[kheper]

My bias supply cannot supply 50ma per tube. The bias is set at 45ma per tube.

That's an odd statement. The bias supply doesn't provide the current that idles through the tube. The idle current comes from the high voltage B+ supply. The bias supply is usually -30 to -50 VDC, and needs to source no more than a couple milliamps.

Better phrasing:
My bias supply is defective. It does not play well with the amp to bring up the required 50ma per tube.

I have been running 45ma per tube for a while.

Applying a negative bias voltage to the grid of the tube reduces the amount of current the tube will conduct. The greater the negative bias voltage, the less the tube conducts. If you apply a great enough negative voltage, you can shut off the tube completely.

If you cannot get your amp to bias any higher than 45 mA, that tells me the voltage divider in your bias circuit is providing too great a negative voltage. The suggested bias circuit in the DIY ST70 manual has a 10k fixed, followed by a 10k pot, then finally 10k fixed to ground. You ought to be able to get between -25 and -50 volts out of it. I'd think that at -25 volts bias, your output tubes would conduct well in excess of 50 mA.

You could probably replace the 10k resistor between the pot and ground with a 5k part instead. That ought to get you a bias voltage between -15 and -45 volts. Maybe that will get your tubes up to 50 mA.

By the way, all of this has absolutely nothing to do with your original problem.

edit... A few posts back you said your bias voltage was roughly -40 volts:

J21= 40.3V close to left channel

Are you saying you can't dial that bias voltage any closer to -35 volts or so?

I forget what the highest negative voltage I reached with the pots was.

The bias expansion mod from Dynaco Doctor would work as well.

http://www.dynaco-doctor.com/ST7%20Bias ... %20Mod.pdf

The tubes measure 450mV at 5 and 8 of the pre-amp takeoff sockets, which translates to 45ma.

50ma is not some ideal number. Many advocate running the EL-34s at 40-45ma.

[Ty_Bower]

50ma is not some ideal number. Many advocate running the EL-34s at 40-45ma.

Agreed. I'm convinced that plate temperature is the critical factor, not any specific idle current. Although it's nearly impossible to accurately measure the plate's temperature, if they're starting to glow red they are too hot.