SVT-CL - another weird one.

[TimmyP1955]

Speaker out distorts at a very low level.

Preamp out is also distorted. This would seem to show where the trouble is - but does it?

Inserting an open plug into the Poweramp In jack - to separate the Pre Out and Power In - cleans up the preamp!

Or:

Removing the 12AX7 from the power amp also cleans up the preamp! Tried two other 12AX7 with the same result.


What would:

Cause the 12AX7 to have such a low Z-in that it loads down the preamp?

Or:

Cause the 12AX7's grid to have enough voltage on it to turn on one of the clipping diodes that are in parallel with the input?


Before you ask, no, we have not yet tried another tube in the preamp, measured the voltages on the amp 12AX7's socket, nor removed the clipping diodes to see if one is faulty - that's for Monday.

[nosaj]

Speaker out distorts at a very low level.

Preamp out is also distorted. This would seem to show where the trouble is - but does it?

Inserting an open plug into the Poweramp In jack - to separate the Pre Out and Power In - cleans up the preamp!

Or:

Removing the 12AX7 from the power amp also cleans up the preamp! Tried two other 12AX7 with the same result.


What would:

Cause the 12AX7 to have such a low Z-in that it loads down the preamp?

Or:

Cause the 12AX7's grid to have enough voltage on it to turn on one of the clipping diodes that are in parallel with the input?


Before you ask, no, we have not yet tried another tube in the preamp, measured the voltages on the amp 12AX7's socket, nor removed the clipping diodes to see if one is faulty - that's for Monday.

What about the resistors on the Phase Invertor?

nosaj

[nevetslab]

What about the resistors on the Phase Invertor?

nosaj

I wouldn't suspect the resistors on the Phase Inverter causing the loading of the preamp output, since the input preamp stage on the power amp is buffering that. I too would suspect the anti-clipping diodes D55/Df6 or somehow the grid bias resistor R8, though with it being 470k, that would have to change substantially to load down the preamp output.

I haven't run into this one before.

If you have an oscillator and a variable resistor in the range of 0-1M that could be placed in series with the Power Amp input, you could dial the variable resistor to see where you get a 6dB drop in level, and then measure that resistance, which would be roughly the current input resistance of the power amp. Sweep the Osc frequency to see if it's frequency dependent. Too bad it's in the power amp, as getting at the parts is a lot of effort.

[Enzo]

What if it isn't loading? Idly thinking, what if a ground is missing and the tube currents are causing a DC in tehre. Exit the tube, no currents?

[TimmyP1955]

Thanks, we'll try the variable resistor idea, and pull the diodes. I'd think it has to be a loading issue, as when the preamp is running OK without the poweramp connected, it's feeding the line input of a Mackie mixer, which I think is 10k. (I forgot to mention that out signal generator also goes distorto when connected to the amp input. I'm leaning toward the diodes, which test OK with a DVM, but.....)

[Enzo]

The two diodes, D55,56 are there just to clip excess signal level. If you think they may be faulty in some fashion, simply unsolder one end of each and lift it from the circuit. The amp should function normally without them. If nothing changes, then they were not at fault. If the amp wakes up and sings, then they are the issue.

Make sure you are not feeding the power amp too strong a signal. It is normal for the two diodes to clip at a volt or less. SO if you stuff 2v of signal into the amp, it will come out all clipped.

[g1]

You might want to try disconnecting the slave jack as well, it looks to be paralleled with the amp-in jack.
I guess it's possible someone could put the wrong one in or something too.

[TimmyP1955]

My young cohort found a ribbon cable that we have never previously exercised in these amps. Doing so exorcised the demon. He did not note which connector this was.

[nevetslab]

My young cohort found a ribbon cable that we have never previously exercised in these amps. Doing so exorcised the demon. He did not note which connector this was.

Since your problem didn't appear to be in the power amp, that eliminates the ribbon that passes the cathode signals & driver signals to/from the power tube PCB from the main power amp PCB. That leaves the ribbon on the output side of the power amp PCB that lands in the Preamp...the I/O cable. Ampeg has changed that cable from dual 10x2 conductors to a single-in-line that has much smaller male/female pin/sockets, with the cable side at both ends crimped. I often find solder problems on that in both generations. In going thru the brand new SVT-CL's that have the in-line single row crimped cable, in the process of de-soldering to re-solder discrete pins that looked like they would go 'bad' in time, my Pace Desoldering Iron actually sucked two pins right out of the plastic header! Never had that problem before. I've only had intermittent issues with that ribbon I/O cable, not low impedance loading of the preamp. Glad you've uncovered the culprit