Leak TL12+ Oscillates

[Audiowize]

Hello there,

I am working on a pair of Leak TL12+ amps to get them up and running to enjoy. So far I repacked the power supply cap with fresh lytics, replaced the coupling caps, and replaced a bunch of resistors that measured out of whack. I did notice more leakage than I was comfortable with across the 1M grid leak resistors, so I brought those down to 470K, which I thought would be a cause for the problems I'm having, but doubling the coupling cap value to correct the time constant did not resolve this problem.

One amp works perfectly and sounds great, but the other amp oscillates like crazy as soon as it is partially warmed up, though it did not initially do that after the rebuild. It will even oscillate with the feedback network disconnected which I find really unusual. It is dead quiet with the input shorted. The EF86s I use have pins 2 and 7 internally connected as well.

Both amps have power transformers marked 3973 and output transformers marked 3921. There is a single 25pF cap across the EF86 plate load, the feedback resistor is 12K with a 220pF cap across it.

I can take the tube set out of the other amp and trade it for the tube set in this amp and the problems remain in this amp.

Both amps are earthed through the mains cord and I get good continuity between the earth pin on the power cord, the chassis, and the audio ground.

Any help is greatly appreciated.

[Radio Wrangler]

You've not said anything that gives any obvious clues.

One thing to be careful about with hooting audio amps is to be sure what frequency it really is oscillating on. This means putting a good scope on it.

I've seen people chasing what they thought was audio frequency instability, and it really was a squegging RF oscillation.

You can also have issues with ultra-linear stages benefiting from RF taming resistors in the screen connections and sometimes even in the anode.

David

[snowman_al]

What does 'oscillating' mean?
High pitched squeal, howling, burping noises, motor boating or low frequency cone extension?

The Leaks are normally stable so check what you have done first.
Have you measured voltages and compared to the 'good' one?
Pictures would help us see too.
Are the output transformer primaries okay?
Have you checked the 'impedance' selector plug for high resistance? etc.
What value of coupling caps did you use? C4 and C9 should both be 0.1uF and C8 and C10 should be 0.22uF. The grid leaks R16 / R17 need to be 470kΩ not 1MΩ.
Alan

[GrimJosef]

Since you've disconnected the NFB you could also try disconnecting the EF86 output from the ECC81 grid, and/or the ECC81 outputs from the EL84 grids. When/if either of those stops the noise you could check to see whether the earlier, now disconnected stage(s) are still oscillating.

As Alan says, the TL/12+ isn't usually marginal for stability. So this has to be quite a gross fault.

Cheers,

GJ

[Audiowize]

Originally Posted by snowman_al

Have you checked the 'impedance' selector plug for high resistance? etc.

I owe Snowman a pint when the apocalypse is over. Someone had plugged in the impedance selector backwards so the small dingle end was rattling around in the big hole and the big end had expanded the small hole. I pried them back together and made sure the speaker selector didn't have any play, then the amp fired right back up.

Thanks so much for the help! Naturally I got scope captures and a bunch of photos before I got to this response and addressed it

[snowman_al]

Glad you found the problem.

Just a 'heads up' regarding the little selector plug.
Increasingly, as they are getting older, they develop a high resistance contact between the pins and metal plate. It can be low ohms to a few 100s of ohms. It leads to very odd distortion and drop outs.
Advice is to solder a wire bridge into the 2 pins. And of course clean and tension the spring sockets.
You can remove the shell of the transformer quite easily with the transformer still on the chassis to inspect and clean / tension the selector sockets.

Alan

[bikerhifinut]

Good tip Al, I may just do that with my venerable Stereo20 as a preventive maintenance.
I have to say that Al's knowledge of the Leak Valve amps is Comprehensive, along with a few others here.
The old impedance selector is a known issue with this sort of Amplifier, not just Leaks and also the voltage selectors on mains transformers can cause a bit of grief.

Andy

[Audiowize]

I'll get that plug soldered up, that's a good idea.