6G15 build problem

[mozz]

Ok, here is the problem. Will post schematic of you are not familiar. Homemade 6G15, some parts scrounged, Hoffman turret board, Mojotone chassis. Built the unit and it works. Sometimes. If you turn the unit on, no sound comes out. Muted. This is going into a battery operated headphone amp for the time being. If you unplug the output jack, then plug it back in, it now works. Works exactly as it's supposed to work. If you shut it off and then turn it back on immediately, it comes back on for 1/2 a second, then mutes out again. If you unplug the output jack, then plug it back it, it now works.

Here is the funny part, when it mutes out like this, either from a cold start or like above when you just turn the power off and back on for a second, i can also get it to turn back on if i unplug the out going into the headphone amp. Not touching the amp output jack at all, the 2 ft wire going into the amp input.

All voltages are within 5-10%. It mutes no matter where the controls are set. Just changed the 2 0.1ufs, the .047uf, the .01uf on the tone, and the 250pf.

Hoffman schematic shows shorting output jack, had that, fender schematic shows regular output jack, now switched to that. All tubes tested and swapped.

[nickb]

The problem could be with either the reverb or the headphone amp. Have you tried feeding the output to an amplifier rather than the headphone amp to eliminate one or the other?

I'm also thinking that transient or leakage voltage on the output of the reverb might be upsetting the headphone amp.

[mozz]

Pulled the 12at7 input tube, and the 6k6 driver. Still mutes and i can purposely get it to mute by touching a meter to pin 7 of the 7025. I have to then reset it to get output again. Something with those 2.2meg resistors. That tube is cutting out. I do have a 12ax7 in there but the same pinout.

Also, i changed the output cord, from 3 ft to 15 ft, seems worse as it wont' reset unless i plug it in and out 5 or 6 times.

[martin manning]

Pulled the 12at7 input tube, and the 6k6 driver. Still mutes and i can purposely get it to mute by touching a meter to pin 7 of the 7025. I have to then reset it to get output again. Something with those 2.2meg resistors. That tube is cutting out. I do have a 12ax7 in there but the same pinout.

I'm betting your 12AX7 in the 7025 position has suffered heater insulation failure. Swap that one out for a known good tube, preferably one that has a high Vh-k rating.

[mozz]

Swapped out the 12ax7 earlier and no difference. Tried a different amp and it now works fine. Some with the headphone amp that it has rather low input impedance or high capacitance and it's loading down the output and somehow it mutes. Have 3 or 4 other amps will try it tomorrow. The problem amp is a Rainbo effects unit, from the mid 80's. I've never even seen one before but i use it as a headphone amp for practice. Weird. Thanks.

[nickb]

Swapped out the 12ax7 earlier and no difference. Tried a different amp and it now works fine. Some with the headphone amp that it has rather low input impedance or high capacitance and it's loading down the output and somehow it mutes. Have 3 or 4 other amps will try it tomorrow. The problem amp is a Rainbo effects unit, from the mid 80's. I've never even seen one before but i use it as a headphone amp for practice. Weird. Thanks.[ATTACH=CONFIG]36952[/ATTACH]

The meter to pin 7 reinforces my output voltage DC leakage and/or transient concerns. Try adding a 100k across the output jack and see if it helps.

[Enzo]

Just a suggestion: in the future, go ahead and post or link the schematic, so we don't have to look for it elsewhere.

[mozz]

Yes, i said that in the first post. Wanted replies for people who has worked on these or built one.
http://www.prowessamplifiers.com/sch...Schematic.html
Could you tell me by looking if a circuit on the output would load this down and cause it to mute? Maybe 10k or less? There may be a problem with the headphone amp i am using. The lower half of the 7025 has odd voltages, not a usual gain stage setup.

[Enzo]

I have worked on many of them, I have been a Fender gold tech for 30 years, but I don't have the circuit memorized.

The lower half of the 7025 is a cathode follower, it has no gain, CF circuits have slightly less than unity gain. You really need to plug this thing into a different amp to see, any amp will do. There is no mute circuit, so anything on the output would have to shunt the signal to ground somehow. And just for grins, plug your guitar into the headphone amp directly, does that make sound? We really need to determine which unit has the problem.

With your mix pot centered, there are two parallel paths through the unit, so unless both are affected at the same time, the problem has to be at the output, and there is little there to cause trouble unless the jack is faulty.

[mozz]

It works with another amp as said. When i do have the problem, i can reset it buy unplugging the output and plugging back in. Jack has been changed. I can also create the problem by touching a meter probe to pin 7 on the 7025, somehow makes the tube mute, or go into cutoff. With a different amp, it turns on fine. Will test my other amps today.

[nickb]

It works with another amp as said. When i do have the problem, i can reset it buy unplugging the output and plugging back in. Jack has been changed. I can also create the problem by touching a meter probe to pin 7 on the 7025, somehow makes the tube mute, or go into cutoff. With a different amp, it turns on fine. Will test my other amps today.

Have you tried the resistor across the jack yet? A quick and easy 5 min test.

EDIT: A bit of explanation. The point is the there is no DC path to ground on the reverb output. Therefore it will float to some higher potential due to inevitable leakage or static. When you plug the solid state headphone amp in the input cap ( if it has one) passes that voltage into the headphone amp for moment - just enough to upset the headphone amp. I suppose power cycling the headphone amp only would be a valid test for this problem. Anyway, this is pure speculation, just a hunch. We'll see.

[Alex R]

I guess it is also possible that a high frequency oscillation is loading down the power supply in the fault condition. But it does sound more likely to be the headphone amp having a problem, prob not enough gain in the stage to cause oscillation.

Nick's suggestion of a 100K resistor across the output jack would give a ground reference to the reverb unit output, which the amp might expect, without it the bias of the first stage in the headphone amp might be upset. It would be odd if the amp input didn't have one but it might have a fault. I think Fender just assumed the unit would be used with a standard tube amp input complete with resistor to ground keeping the input stage bias steady.

Just speculating. try the 100K resistor.

For the OP's future reference I am not at all sure looking for someone who has built one of these before will produce the best person to ask - they will tend to assume the problems you're having are the ones they had, and so their answers might be skewed by that. Providing a schematic makes it easier for people who really know what they're talking about and can understand any of the problems that might be leading to this issue, to help you - there are plenty of them on here (not including me, I'm just adding my two ha'porth because the problem interests me)

[mozz]

Tried the resistor across the output, works fine in the headphone amp now, except lower volume. Tried a 1meg and also works good. Dragged another amp in the basement to try, Park solid state, and it works fine without the resistor. Then tried my deluxe and champ and it works with both.

Back to the headphone amp, took it apart to look inside. The input goes into a Tl072 through a 10k resistor. There are 2 pads not populated, goes from the input to ground. I soldered a 1 meg resistor in there and now it works fine. I don't know if this got left out at the factory or it was a cost saving measure.

Also measured 1 patch cord and it came up shorted when moved. Multiple problems and none were the unit itself. Guess i was rushing since i just got it done and wanted to hear it. Thanks to all.

[nickb]

Tried the resistor across the output, works fine in the headphone amp now, except lower volume. Tried a 1meg and also works good. Dragged another amp in the basement to try, Park solid state, and it works fine without the resistor. Then tried my deluxe and champ and it works with both.

Back to the headphone amp, took it apart to look inside. The input goes into a Tl072 through a 10k resistor. There are 2 pads not populated, goes from the input to ground. I soldered a 1 meg resistor in there and now it works fine. I don't know if this got left out at the factory or it was a cost saving measure.

Also measured 1 patch cord and it came up shorted when moved. Multiple problems and none were the unit itself. Guess i was rushing since i just got it done and wanted to hear it. Thanks to all.

So the headphone amp input was floating AND the reverb output was floating. Wow! Good job