ODS Issues

[dorrisant]

Let's just say that I have a 2x 6V6 OTS... I got it after a customer tried to build. Wires were everywhere... couldn't see for the forest of wires let alone trace anything. So I rebuilt.

I have built a few ODS amps before... No real problems. I always seem to forget to hook something up or cross up a couple of the coax... always got them up rather quickly for such a complicated amp. This one has me stumped!

I've checked through everything about 3 or 4 times already.

I rolled tubes to get the preamp voltages the same as the #124 preamp voltages but they are all over the place depending on the tube.

Very garbled guitar signal can be heard but only if the volume and master are nearly all the way up. Much lower volume than should be.
I can hear oscillation in the form of a variable- frequency whine radiating from the OT. With a dummy load the OT is relatively quiet until about 8:00. Then the oscillation intensifies till about 2:00. After that it seems to jump in intensity and the frequency rises from there on as well.

Bias is set at 25mA per tube. This goes erratic as the MV gets turned up.

Plate voltages with master all the way down:
6V6 - 406
6V6 - 407
PI - 293, 245
V2 - 171,179
V1 - 224,214

Plate voltages with master all the way up:
6V6 - 385
6V6 - 385
PI - 248, 249
V2 - 164,137
V1 - 203,194

Last time I heard something similar, It turned out to be a bad OT. Does this sound like just an oscillation maybe? With the voltage dropping as the MV gets turned up it makes me think that an oscillation is using up the B+.

[dorrisant]

More pics...

[dorrisant]

One more...

Edited to remove duplicate pics.

[xtian]

Mmmmm, lunch. I love that compact thing. But then, I don't have to work on it!

Yes, I'd agree that a dropping plate voltage as MV opens is likely oscillation eating your current. Got scope?

I would start by tracing signal from the input forward. Signal gen and scope. Figure out which parts of the amp are working or not.

[larsvictor]

It seems that the oscillation is already present in the preamp gain stage since it disapears when you´re grounding the signal path between preamp and power amp.
Remarkable mismatch in PI plate voltage with master volume down. Is this reproducable with a different tube ?

[dorrisant]

Xtian... Blew up my scope probe I guess. Was limping along with that one but it drives you mad when the probe goes intermittent. Scope shows no signal, I test at the input jack... no signal. Slap the generator... no signal. Connect generator to signal tracer and there is audio.

USPS delivers new set today or tomorrow... Oh crap! I just found the 100x probe I could have used.

Thanks for confirming the idea of an oscillation. You are on the spot!!

larsvictor, I have a jar of 12xx7 tubes that I can quickly pop in a circuit. Tried a few and the voltages are all over the place. I will find a 12AX7 with balanced triodes and play with the trimmer too. I have just set this trimmer right in the middle for startups in the past. Don't know what's causing it though, so the brain is open. Gonna learn today!!

Thanks

[67plexi]

V-2 is way off that is the most vile looking so called ODS I have ever seen
V-1 should be around 185-195 VDC V-2 190-200 VDC
Any way to look under the main board and look for burnt tracings.

[dorrisant]

No burnt traces... I had the board out and replaced most of the wire to and fro.

Vile... Damn, I thought I made it look pretty.

[67plexi]

dorrisant, Not your work the amp it self, Check your dropping string
and find out why V-2 is so low.

[dorrisant]

Was just kidding... will do.

[larsvictor]

With an error in the dropping string V1 plate voltages should be low too. Somehow it seems that V2 has a much higher idle current than V1 (more that one would expect with the slightly higher B+ - Ra´s and Rc´s are equal). Bad snubber caps ?

[dorrisant]

Ok... got the probes mentioned above (gonna order another set and put them away).
The oscillation would come and go depending on the setting of the master. It was made worse by turning up the volume. The OD trimmer on the back of the amp was making it way worse as it was turned up. Even a lower setting sent the amp into oscillation. There was also a something else going on with the volume knob too... it had a huge jump in volume at the last 5% of the taper and too low of a volume level outside of that 5%.

Turns out this was two problems:

1) I noticed that I had forgot to ground the Effects loop jacks which is also tied to the output jacks and the grounds for the coax feeding to and from the OD trimmer as well as the ground for the trimmer pot itself. I tacked that in and the amp was suddenly 90% better. The oscillation was gone and all but the volume knob worked correctly. All of the preamp voltages stabilized.

2) I tested the volume pot and it just ain't right. New one on the way, didn't have one with the push-pull on hand.

Not out of the woods yet but suspect it will be when the pot comes in.

Thanks again to all!!

PS: 1000th post!! Now I have my sights set on Tom and Mark...

[xtian]

PS: 1000th post!! Now I have my sights set on Tom and Mark...

Happy birthday, old man!

PS, high post counts do not mean you know what you're doing. Ask me how I know.

[fenderbender]

This may or may not have anything to do with it... I noticed, in one of your pix, that there's a red wire that appears to not be soldered properly, to the pcb. check the solder pad, near the edge of the board, close to V2 and the large orange drop cap.
Glad you're getting a handle on it, should be a sweet little amp!
Best regards,

Tom

[dorrisant]

This may or may not have anything to do with it... I noticed, in one of your pix, that there's a red wire that appears to not be soldered properly, to the pcb. check the solder pad, near the edge of the board, close to V2 and the large orange drop cap.
Glad you're getting a handle on it, should be a sweet little amp!
Best regards,

Tom

Amp is fine now... the solder pad on the top side of the via is missing. Previous bad soldering by the owner. The wire is properly soldered/connected, it is just underneath where it connects to the trace that matters here. It is a nice sounding little thing. Owner picked it up today and I thought he was pretty stoked.