Zenith to Deluxe conversion Oscillation

[Blindog]

I've managed to get this thing sounding pretty good with Andy and others help. https://tubeamparchive.com/viewtopic.php?t=6282

I thought I would rewire the octal for a 6SL7, and thats where the fun begins! As soon as I turn the volume past 2 it starts a high frequency oscillation and turns into a skreeching fizz as you go up. The tone control seems to affect it as well. If I cup my hand around the 6SL7 without touching the tube and touch the chassis, it goes away!

I added a piece of shielded cable to the grid of the 6SL7, but no help. I've read through a bunch of posts on oscillation and did some chopsticking, but nothing seems to help. Keep in mind it was stable with the 6J5. The lead dress didn't change much at all with the rewire. I think the biggest movement in wires was I had to move a pair of heater wires from pin 2 to pin 8. I've include some pics. Any suggestions?

The 6SL7 is running at about 145v. The PI about 175V a side, 6l6 (or 6V6) voltages look OK. The layout you see is pretty much how it was wired way back when, obvious exceptions are the input and speaker jacks.

Thanks in advance!

Mark

[Andy Le Blanc]

sometimes the phenolic of old tube sockets does some pretty funky things
if it becomes a partial conductor...... it might need to go..... octal tubes can
be noisey, microphonic, you can hear the tube heating up...etc.. but not that
check the socket for fuzz.... stray stands of wire.... carbon burn marks
it might just be that the contacts in the socket need to be re-tension'd...

[Blindog]

Thanks for the reply Andy.

Things I have tried:

Put in a new socket for the 6SL7.
Cleaned and retensioned the output tube sockets.
Shielded cable to input jack.
Chopsticked countless wires.
Exchanged the 6SL7 for another thinking it was microphonic.
Switched the OT primary wires.

These things have had no effect. The only thing I can figure is the higher gain of the 6SL7 (70) vs the 6J5 (20) is destabilizing the circuit somewhere. Maybe I can't use this tube in this layout?

Mark
Anyone have any more suggestions? I'm about to say screw it!

[skyboltone]

Thanks for the reply Andy.

Things I have tried:

Put in a new socket for the 6SL7.
Cleaned and retensioned the output tube sockets.
Shielded cable to input jack.
Chopsticked countless wires.
Exchanged the 6SL7 for another thinking it was microphonic.
Switched the OT primary wires.

These things have had no effect. The only thing I can figure is the higher gain of the 6SL7 (70) vs the 6J5 (20) is destabilizing the circuit somewhere. Maybe I can't use this tube in this layout?

Mark
Anyone have any more suggestions? I'm about to say screw it!

Are you using the same plate/cathode resistor pair that the 6J5 used? Look in the back of the tube manual for the recommended resistance values for a 12AT7 at that voltage and try those. Can you put a 250K linear pot between the wire to the following stage and ground; after the blocking cap, thereby throttleing the gain?

[Blindog]

Are you using the same plate/cathode resistor pair that the 6J5 used?

The circuit right now is pretty much following the 5D3 Deluxe schematic in my original post. I'm using 820r bypassed with a 22uF 50v.

I thought about trying 1k5 grid stoppers on the 6V6's.

Only other thing that was changed that I can think of is that I used a 25uf to bypass the 6v6 cathodes from my old Fender that I gutted for my ODS clone. Could a bad bypass cap cause this?

Thanks for the reply.

MArk

[Alexo]

Funny, I just built a 6SL7 champ and had a similar problem, a high freq. oscillation when gain was applied. I was surprised because it wasn't an especially high gain circuit, and I used pretty good lead dress. I shielded several leads and kept on chopsticking , but nothing worked until I swapped out the power tube for another 6V6. Still a bit puzzled by that, might help in your case, but probably won't. Looks like your output transformer leads might be running kind of close to your preamp socket and your control pots, that could certainly do it if that's the case. Have you tried chopsticking those leads?

[Blindog]

Alexo, thanks for the input. I have tried different output tubes, even a pair of 6l6's. the oscillation just gets louder faster with the 6l6's

The OT leads in the original layout actually ran even closer to the preamp circuit, to the terminal strip right inbetween the 6SL7 and the 12AX7. They then ran out a hole in the chassis to a terminal strip on the left side for the speakers. I have chopsticked those leads as I was suspect of those right off the bat. All I can say is ARGGH!

Mark

[Alexo]

Haha, I know that Argh well!

Does it go away if you zero out the volume control on the guitar? That might help you determine if it's entering at the first gain stage or the second.

A 6SL7 does have more gain than a 6J5, so it's conceivable that a layout that didn't oscillate with the J5 might with the 6SL7.

[Blindog]

Yeah, it'll oscillate without anything plugged in the input jack. I think I'm gonna change the bypass cap and try some grid stoppers on the 6V6's and see what that does. I'm not giving up yet

Mark

[Blindog]

Ok, added 1k5 grid stoppers, still oscillates. I'm about to scrap the ground scheme of this beast and try a buss bar type scheme. Any other bright ideas out there?

Mark

[Alexo]

Have you tried adjusting these?





But seriously....

If it squeals with V1's grid grounded out, that points to the second gain stage's grid, which I assume is connected to your vol./tone controls, which run extremely close to those signal-radiating OT leads!

In my experience, squeals don't come from bad grounding, but I could very well be wrong.

[Blindog]

Have you tried adjusting these?

Nope, mine's not adjustable lol!

I was gonna change the ground scheme anyway, so it couldn't hurt.....but it definitly did not help

I dont know if this is a clue or not...in the pic there is a piece of aluminum that I installed that divides the volume and tone controls from the rest of the amp. Without that there I get a fairly loud 60hz hum with or without a guitar plugged in. It did this before I changed out the 6j5. With that shield in there it almost goes away....you can hear it but not annoying.

Just thought of something...I'm only using 1/2 of the 6SL7. Do I need to ground out any pins of the other half? Maybe getting some signal leakage?

[Andy Le Blanc]

double check the ground scheme for loops..... a bus gets the ground where you
want them and off of the chassis which helps but doing it after the fact is a chore
double check your filament supply too.... it could be comeing thru there
it could be a result from capacitances to ground from your dress......
look around and see if there are any leads or wire runs that could be cleaned
up or shortend....

[Blindog]

Alright, after dicking with this thing all day, I've decided that this layout will not work with a higher gain input tube. I switched it back to the 6J5 and it sounds great. The Zenith dudes knew what would work I guess

The good news is that after all the tweeks I've done it's as quiet as it's
ever been. Thanks to everyone that chimed in. I'll try and get a sound clip of the old beast up.

Mark