Ampeg Gemini I - Intermittent volume loss

[rockinrob]

I've been working on this amp off/on for a few weeks, working out the bugs, and have it narrowed down to one final, annoying issue. I was really hoping to do this overhaul solo without asking for advice, but I think I'm getting into the "replace parts just because" stage, which I am completely against doing.

I also just bought my first scope and am learning how to use it. So far I have seen a squiggly line that squiggles differently sometimes, meaning I can't understand if I'm doing it right for this problem, or if it is in fact showing me something. :rimshot

Anyways, here is the issue: Amp comes on, everything is fine for a few seconds if you don't play or if you play lightly, but if you hit any note or chord hard, the amp "clamps", and loses probably 50% of the volume. Sometimes it stays "clamped" for a few minutes, other times it will come right back if you quit playing, or even if you continue playing. Sometimes it will not clampdown at all for a few minutes, even with hard playing, but it inevitably goes back to clamping mode.

It does this in both channels, so I am assuming it can't be in either preamp, the trem circuit, or the reverb circuit.

I have done the chopstick test multiple times, with no result. It has all electrolytics replaced. I have found no voltages that change when it does the clampdown, including the bias voltage for the power tubes and 7199. I have used 3 different 7199 phase inverter tubes, and two sets of 7591 output tubes. I have removed the not used preamp, reverb and trem tubes. I have turned reverb and trem on and off. I have measured all voltages and they are all reasonable. The resistance on the output transformer looks good (100 ohms on both side, if I recall correctly). I replaced all the signal caps in the phase inverter stage, and it changed nothing. All the resistors are measuring good, and I swapped a few that were off a little, but that also did nothing. I put all original parts back in.

I have the original rectifier diodes in the amp, but the voltage coming from them is steady when the amp clamps, so I think they are okay.


I put a 1k sine wave from my iphone and traced it out on the scope. It showed up okay, but I didn't get much out of the readings on the scope. I am reading some articles about how to use them properly so I guess I will figure that out eventually.

I made a dummy load out of two 15 ohm 20w resistors in parallel, and I could clearly hear the 1k tone coming from somewhere (assuming the OT vibrating?). I didn't realize the OT would do that with a load on it. It this okay?


Ideas? I think I need to verify the point that the voltage of the sine wave drops, but I am unsure how to do that when it is being amplified by the amp, changing the voltage at each stage.



BTW, my scope is an analog phillips pm226

Before:
[IMG768]http://i235.photobucket.com/albums/ee22 ... 97b24e.jpg[/img]

[xtian]

With your sine generator and scope, you have great tools to find out WHERE the signal is failing when in fault mode. When the amp has "clamped," we can assume the signal is not present at the output transformer's secondary. But where does it stop? At the power tubes grid? At the PI input? At the tone stack?

Finding this info out will help you locate the fault.

[rockinrob]

Thanks for the tips xtian!

However, I have a few problems...

The big one is that the "clamp" last for only a few seconds, and I cannot make it do it reliably, so while running a test tone into it, I have no way of telling if it has occurred, and when it does occur, there is little to no te to diagnose anything.

I did check it out with music playing and a probe I built with a pad in it, but that came up inconclusive due to the quick timing of ithe fault...

[gui_tarzan]

Does it have a 6CG7 tube? Have you verified heater voltages, tube socket connections, etc.?

[rockinrob]

It has a pair of 7591 outputs, 7199 phase inverter, 6cg7 reverb and 12ax7 trem and preamp tubes.

I pulled the reverb and trem tubes to narrow down the possibilities. Heaters measured right on and sockets match the schematic.

With an intermittent problem, I am looking for a faulty part, solder joint or broken wire, right?

The chopstick test rules out the wires. The ot probably wouldn't switch back and forth (right?). I've reflowed all the solder joints.

Should I measure for ac? Should I be thinking of an oscillation type issue?

I think a clue is that it is related to how hot the input signal is...

[Stevem]

You do not even need the scope to do a lot of signal tracing!

Grab your volt meter and set it for ac volts , put your test tone into the amp and hook your meter across pin 6 of each output tube and crank the amp, you should read at least 12 volts of drive signal to the grids ( pin 6) of the output tubes.

When the amp craps out does this 12 volt signal level drop off also, if so then then issue lays in the PI or the preamp section assuming all the B+ voltages stay about the same.

Note that as the output stage produces output and draws more current from the power supply that you will see the B+ voltages in the amp drop down by some 25 volts or more!

Also make note of how many volts of audio output the amp is feeding into your load resistors and report back on all of this!

[rockinrob]

I tested this by playing guitar instead of a test tone, so I could hear through the speaker when the event happened.

At pin 6, I averaged 50vac while playing hard chords. The dropout had no affect on this on either tube.

I checked the vac on the ot secondary at the speaker out jack, with a speaker connected. For this measurement, I averaged 12 vac, until the event occurred, where it dropped to 6vac.

Shorted winding in ot? I'm assuming this is indicating it is either in the tube sockets, both pairs of output tubes (They came with the amp) or ot, correct?

[Stevem]

Ok, if the dc voltage across the output cathode resistor stays the same when the output to the speaker craps out then your OT is bad, if that cathode voltage changes a lot , but the drive voltage to pin 6 holds steady than you have bad output tube sockets.
You should use a steady test tone to prove this out if you can!

Fliptops sells good replacement OTs for Ampeg amps.

[pdf64]

Ok, if the dc voltage across the output cathode resistor stays the same when the output to the speaker craps out then your OT is bad

The tubes should still respond to high levels of grid signal voltage by increasing cathode current, which should cause the cathode voltage to rise, irrespective of whether the OT primary is shorted or not.

[rockinrob]

I tightened the tube sockets when I first started working on this, but they weren't really loose at all, and are in really nice shape.

I will spray some deoxit in there just to be sure though...

Also, I measured the resistance of all sides of the OT at the beginning, and found both sides of the primary to be at 100 ohms or so. I guess this can change with the OT at operating voltages?

Stevem, I'm a little confused by what you typed..

the dc voltage stays steady at all points I've measured so far, that is the first thing I was looking for, because I assumed this was a biasing or similar issue related to a resistor in the beginning.

DC is steady at pin 6. AC was also steady.

The only change I have been able to measure is at the speaker terminal, where I saw it go from around 12V to around 5v when it dropped, and then back to 12V when it came back.

[Stevem]

When you are reading 12 volts ( 18 watts) on the speaker, note what the voltage is across the cathode resistor, if that cathode voltage stays the same when your voltage at speaker drops to 5 volts than the OT is shorting intermitant for some reason and is a gonner!

[pdf64]

Ah, sorry Stevem, I see what you mean now.

[rockinrob]

Well, last night I put in the new transformer. I got 10 minutes of loud playing with no drop out, so i think (knock on wood) this ampeg is back in bidness!

thanks for the help!

Lesson learned: measuring AC voltage is a helpful diagnostic tool

[Stevem]

Good deal she's back up!

If you want to protect that new OT get a inline fuse holder and wire it into the center tap primary and install a 250 ma fast blow fuse in it.