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Noisy Squeal from SE amp

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I built the attached SE amp circuit using the EF36 (a 6J7 variant) as driver and a 6L6G as output valve. The circuit is my adaptation of JC Morrison's Fi-Primer 6L6 SE-UL. I used the EF36 instead of the EF86 / 6267 driver and as my OPT does not have a UL point I'm feeding the screen from a second CLC section on the PS.

Today I fired up the amp and as soon as the valves warmed up I got a loud noisy squeal (oscillation?) from the loudspeakers. I changed / swapped the speaker connections on OPT and removed the NF connection. This did not solve the problem.

I then replaced the 0.1uF coupling capacitors in case the original ones were leaky. Same result. I haven't taken any voltage readings because I'm using the speakers as the OPT load so I have to eliminate this horrible noise from the speakers first.

Can some members provide any clues or guidance as to what is happening and how to solve the problem.

Thanks in advance for any input / suggestions.

Joe A
 

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Isn't it more normal to return 4.7uF the screen bias cap to ground? (Don't know if that is causing the problem though)

I have an SE amp with these old pentodes in it. I love the odd arrangement of the input grid being on the top cap! My output tubes are EL34s though.
 
Thanks Psychobiker and Hotbottle for your input.

Since you are pointing fingers at capacitors I might as well add that I used a 1000uF across the EF36 cathode bias potential divider (upper resistor) instead of the 100uF specified in the original circuit. I don't know how such a change would affect the performance.

I will attend to this area at the first opportunity.

Thanks and regards.
 
Thanks korey and SHiFTY but I have already strapped the screen to the anode (of the 6L6) and the sound improved dramatically. The only noise I'm getting now seems to be coming from the topcap connection, especially the one which is close to the AC mains switch.

Do you think that a shielded cable (for the topcap) will help? If yes, how and where should I connect the braid (to ground or loose)?

Thanks and regards.
Joe A
 
sonata,

You mention that you have triode strapped the 6L6 (by connecting pin 4 to pin 3 via 100R resistor). korey and SHiFTY are talking about something different. They are talking about the phase of the negative feedback. If that phase is wrong, the feedback will be positive, causing squealing/motor boating etc. If this is the case you can "try switching B and P around on the output tranny". This will reverse the phase of the feedback and (hopefully) cure your problem.

Cheers,

Chris
 
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