Catastrophe, HELP!

[ioginy]

This morning I had a brain wave!

The tubes I have in the amp are cruddy ones I had laying around that I put in for the initial test but never took out. So I swapped my power tubes for a set of Groove tubes from my other amp, rectifier with a NOS raytheon 12at7 and my pre-amp with a Telefunken 12ax7 (as well a on the octal pre-amp socket with a NOS sylvania 6SL7WGT) and the squeal went down significantly until I got to higher volume.

When I flick the switch to pull the ground on the tone stack however I get a really bad oscillating squeal as well. The volume jumps a HUGE amount as well so i figure that is part of the problem.

Also I was thinking about the rectifier tube. I have a old 5U4 in there right now, but I have a bunch of nice old NOS 5Y3's and a 5V4 at my disposal. I didn't want to just swap them in though because I couldn't find any definitive cross reference on these tubes. If anyone has ideas on that I would be grateful. As a side note, the voltage across my filter caps seems really low. I am using a recycled power transformer, could this be my problem.

I'm going to change the bias setup right now. I'll be back:)

[ioginy]

Alright, did the bias change, noticed I had 22K instead of 220k (swapped to 220k) off the bias to the 1.5k grid resistors. I put the 10 ohm cathode resistors back in to replace the 820 ohm ones I stuck in to bring down the horrid noise....

Now, there is no radio, no squeal, but there is a HUGE amount of hum, and the tone stack, volume, bright switch and tone stack ground lift switch make no difference to the hum.

I should try it with the guitar plugged it to see if there is any signal actually going through.

brb
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edit
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Alright, tried plugging a guitar in and it has no effect until the loume is maxed, at which point it sort of burps but makes no discernable guitarish sound. I am thinking self bias may be a good temprary (or permanent fix) at this point.

[dhuebert]

Plug your signal generator into it and use your scope to trace the signal thru the amp. Only way I know of to do it. What is the cathode current thru the 6L6s at idle? What is the idle voltage on the plates of the triodes? Look at the way I grounded the preamp here:

http://diytube.com/phpBB2/viewtopic.php?t=2395

Very quiet.

Don

[ioginy]

The burp is gone, in my altering I didn't reattach the ground to the bias circuit. Done that, and it works again but still sounds like bunk.

I sadly don't have a signal generator or a Oscilloscope, but I do have a Dmm.

With the Bias adjust all the way down:

Cathode across 10ohm resistors:
V1 - 1.40v
V2 - 1.84v

Plate from ground buss to pin3
V1 - 275v
V2 - 271v

With Bias turned all the way up:
Cathode
V1 - 1.69v
V2 - 1.89v

Plate
V1 - 265v
V2 - 264v

Those number seem small. I always thought the plate voltage should have been around 350v

I haven't put in the self bias circuit yet, but I am thinking it might be a good plan. Also, I put in a shielded wire from the input to the 68k grid resistor and the radio has come back a bit.

[ioginy]

Installed self bias and it is much better. The voltage across the 470ohm resistors from the cathodes is aprox 7.5v

The volume is rather quiet comparatively but I am chocking that up to the low voltage reading I am getting. I have a feeling it's the transformer... thoughts?

Also, I am going to try and switch the opt leads to see if that takes care of the squeal... kept meaning to do that but am getting to it now.

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edit - 4:47
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Switched the opt leads and the squeal went away for the most part. When the bright switch is engaged the amp is now quite clear compared to how it was before. However when the bright switch is off it gets some hum back and a little radio.

[ioginy]

I haven't tested it out yet, but I had to get my mind into other things so I built a faraday box for my filter caps. Looks cool. Hope it makes a difference.

[ioginy]

Interesting discovery.

Faraday box made no difference, sadness.

However I did discover that all of these problems must be in my volume and tone controls. I turned the volume up slowly and it squealed at points and picked up radio at points, and then when I got it maxed it sounded great. Clear, great tone and decent volume. That was with all the tone knobs down, when I turned them up at the full volume points I went back into noisy territory. Then when I flicked the ground pull switch for the tone stack it all went really bad. I think i am going to have to redesign this from the ground up.

[ioginy]

Update:

I took out the bright switch and the tone stack ground drop. They seemed to be causing more trouble than they were worth for the moment.

No more radio and the hum has diminished quite a lot, however the sqeal... the squeal!!!! As the volume gets louder, so does the squeal. I am totally stumped here. Anyone?

[ioginy]

update:

I swapped the opt leads to the 6L6's and the squeal went away unless the volume is maxed. It squeals when I turn the standby on as well. Not constantly, just a quick one then fades out. Kinda wierd.

I read on the diyaudio forum that squeal can also be caused by the power transformer not delivering enough juice, which would ring true with my readings. If anyone has run across that I would love to hear about it.

Anyway. There is still a decent amount of hum which I am working through.

[mesherm]

I seem to remember someone else had a similar problem that was caused by connecting the negative feedback from one speaker terminal but forgeting to ground the other speaker terminal.
In any case, try disconnecting the negative feedback and see what happens.

[dhuebert]

Put a 100 pF cap across the plate resistor on the first gain stage.

It is your input that needs the farraday cage, that's cute BTW.

7.5v/470 ohm = 16 mA half what it should be.

1.4v/10 ohm = 140 mA YIKES!

You are looking for 35 to 45 mA at idle and 70 to 90 mA at full power.

What size of bypass cap have you got on the cathode resistors on the 6L6s?

Don

[ioginy]

turns out my bias circuit was way off base. I have fixed that up and most of my voltages are good. I had some squeal though and so I dig into it. it started going well and then it went south FAST. I found the squeal would go away when I used a 12au7 phase inverter, but if I threw the 12at7 in I would get the squeal coming at about half volume. I did some research and found that most of the things people suggested I had already tried, except for the specific locations of the wires going to the preamp tube. I tore it down and rewired it the way that seemed to be the most common suggestions and tried it. The squeal is WAY worse and I am getting a bit of radio again. I took it apart again and just wired everything loosely so I could move stuff around and see what happened. The input wire is shielded for the sake of reference.

So, it appears the wired from the volume to the preamp tube is the one that effects he squeal most. I can't get it to go away, but when I move it it is definitely effected. Also when ever I touch the grid wires, I can hear it through my speaker, like you would a microphonic tube.

Help please :)

[nyazzip]

maybe you have one or more crap preamp tubes...?

[ioginy]

I checked into that, bought a bunch of tubes, mixed and matched with old ones and new ones and there is only a difference when I use ones with lower MU ratings. lower MU = no squeal.

[ioginy]

I did a full voltage check against what the schematic says. I changed the resistors between the filter caps to get closer to what they should be. Before, they were really high.

Anyway, the amp works great with a 12au7 in the preamp socket, and a 12at7 in the phase inverter.. OR with a 12au7 in the phase inverter and a 12ax7 in the preamp socket. It is supposed to have a 12at7 in the phase inverter and a 12ax7 in the preamp socket, but when that is the case, I get squeal in the last third of the volume. Even with the 12au7 in either place when the volume is all the way up i get really flubby distortion when the volume is full.

Also when I set my bias to a good level of 30mA the voltage to the grids is around -50. This seems very odd to me. Anyway, here are all my voltages beside what the schematic says it's supposed to be.

First cap - 425 / 420
second cap - 421 / 410
Third cap - 370 / 340
Fourth cap - 228 / 230

(6l6 = v1 and v2)
V1 Grid - 416 / 405
V2 Grid - 416 / 405
V1 Plate - 304 / 410
V2 Plate - 290 / 410
Bias voltage - -50 / -35

(V4 is 12ax7 and V3 is 12at7) measurements taken with these tubes in.
V3a plate - 199 / 180
v3b plate - 203 / 180
v3a grid - 55 / ?
v3b grid - 55 / ?
Cathode - 82 / 90

V4a plate - 156 / 170
v4b plate - 155 / 170
V4a grid - 0 / ?
V4b grid - 0 / ?
Cathode - 1.04 / 1.6

Thanks for any help anyone can give.