Low Volume Output and Hypersensitive Grid Connection on Phase Inverter

[Cwazy_Wabbit]

I’m trying to resurrect an ’80s MTI-era Ampeg V5. Started by replacing all caps and any resistors that were more than 5% out of tolerance. Also cleaned all pots and replaced a mixed bag of mismatched power tubes with matching set of Tung-Sol 6550's. After confirming that the voltages on pins 3, 4, and 5 of the power tubes and the bias readings on my TAD Bias Master were all within spec and fairly well balanced, I fired her up.

Everything looked good until I plugged in a guitar, whereupon, with Pre-Volume set to 1 and Master Volume set to 8, I was met with either no, or very low, output, depending mostly on where I set the Pre-Volume. Cranking the Master up to 10 didn't make any difference. The first thing I tried was changing the V3 phase inverter tube, which definitely helped, but still didn't give me the output level I was expecting. Nowhere near what I would expect from 100 watt amp with the master volume cranked up to 10. And even this result could only be obtained if the Pre-Volume was turned up to at least 3 or 4, which I'd rather not do, because I'm not a big fan of distortion. So these were -- and still are -- the symptoms that caused me to start poking around with a wooden chopstick.

The only thing I discovered was a hypersensitive grid connection on pin 2 on the phase inverter tube (V3). When I tap or move the wire that connects pin 2 to the PCB, the tapping echos through the speakers (kind of like the sound of a microphonic tube, but lower pitched and without the tink-tink sound of tapping on glass). This sound isn’t extremely loud, but still very noticeable. And when I touch the solder joint at that pin with the probe of my multimeter… OH MY GOD! The volume of the “POP” is almost deafening! This does not happen when I touch the other end of that same wire (i.e., where the solder joint connects it to the PCB board)… only at the pin connection for V3 (phase inverter). Doesn't happen on V1 or V2, either.

Here’s what I’ve tried so far:
1. Confirmed that the tube (originally 12AX7, but now 12AT7) is not microphonic.
2. Re-soldered the solder joints at both the pin and the PCB connection.
3. Checked the continuity on the wire, which was fine, but replaced it anyway, just for good measure.

Any thoughts about what might be causing these problems would be much appreciated.

[xtian]

Likely the PI is working as it should but signal is not arriving at input grid. Confirm working PI voltages. Confirm continuity from grid to 1M/0.1u. Have a scope? Trace the signal.

[danman]

I was going to ask if the PI cathode resistor connected to pins 3 and 8 is the correct 470 ohm value. Many builders have accidentally installed a 470k resistor in this spot and the result is very low volume. From your pic, it appears that you used the correct resistor though.

Did you bundle the grid/plate/cathode wires together with zip ties on each socket or was this done previously? I have never seen these sensitive wires bundled together like that because of the risk of oscillation.

[Stevem]

Did the amp have a problem before you started working on it?

Try this if you have a way to generate a audio tone / sine wave.

Input .150 mv @ 1khz.

Open channel gain to 5.

Dime the master.

Set your test meter for AC volts.

Hook your meter across the PI output after the coupling caps ( cap to cap not cap to ground) to the output tubes.

You should read at least 40 volts of audio there.

If you do read at least 40 volts then your output transformer is shorted.

If you do have good voltage then move back to the PI input and hook the meter from there to ground and you should see at least 3 volts there.

In general as you move back up stream from the output tube grind to the input and touch each tube gain stage grid with a screwdriver you should hear a pop and that pop should get louder as you move back up stream.

Where you lose the volume of that pop is the section/ stage where your problem is.

[lonote]

Likely the PI is working as it should but signal is not arriving at input grid. Confirm working PI voltages. Confirm continuity from grid to 1M/0.1u. Have a scope? Trace the signal.

This was my guess too, before reading this reply. I would start working backwards from that point.

[pdf64]

A bad connection between valve pin and its corresponding socket female contact seems a likely suspect here. Most likely V3 and most likely the one for pin 2.

[B Ingram]

I’m trying to resurrect an ’80s MTI-era Ampeg V5. Started by replacing all caps and any resistors that were more than 5% out of tolerance. ... with Pre-Volume set to 1 and Master Volume set to 8, I was met with either no, or very low, output ...

Did the amp have a problem before you started working on it?

It is a bitter pill we all swallow some time: if we change a bunch of stuff & the amp doesn't work right, we probably made a poor-connection/solder joint in one or more places.

As Stevem suggests, if the amp wasn't "Dead" before the parts-changes, I would assume my changes created the problem. When I've assumed this about my own work in the past, I was able to find & correct my mistake after close visual inspection of everything I swapped.