Why is my MRA3-L6 amp whistling?

[mauro]

Greetings Ron, Rob and All,
I've recovered "enough" to resume work on this amp and with Ron and Robs pointers I have it sounding pretty damn good! It played perfectly last night but I played it as soon as I got home from work and it immediately started whistling or maybe scratching is a better term?
I've cleaned and tightened the speaker socket and all the tube sockets and thought that fixed it last night but low and behold it's back with a vengeance today!
Could this be caused by a poor connection in a tube socket?
Mauro

[mauro]

I meant to say screeching not scratching.
Also wanted to mention that when I turn the speaker output switch from 16 watts to 8 watts the whistling gets considerably louder.

[Ron Rich]

Hi Mauro,
Simple answer--It whistles on accountacause,--- it can't sing !
Yep--poor connection, bad tube, somewhere -- wiggle things around while it's playing--you'll find it, or replace tubes one at a time--
Didja clean ALL rotary switches with a safety solvent, or pencil eraser-- THEN, "Deoxit-5", as per can directions ! ??
Ron Rich

[mauro]

Hey Ron,
Thanks for the speedy response!
Yes I did all the above but perhaps I didn't get them as well as I thought.
I'm going to do it all again, just make sure.

[mauro]

I think I have to teach this amp the words!
I generally do not use profanity but this one is trying my patience!
I've recleaned and checked every connection point and switch with my analogue meter and everything was perfect!
I even resoldered a bunch of caps just to make sure. I systematically changed one tube at a time.
Still whistling!!
Not sure where to go from here?
Could a capacitor cause this?

[mauro]

Only other thing I can think of is at pin 1 of V6 6SL7 the schematic calls for a 250mmf cap to ground. There was a 0.001 or 0.01mf cap there in the amp.
I put in a 250mmf cap.
Long shot but could this be the culprit?

[Ron Rich]

Mauro,
Is the "cap" on the 6J6 tight ? Have you changed it ?? If you have spare tubes, change them, one at a time--could well be a bad tube--- Ron Rich

[mauro]

Ron,
Cap on 6J7 is clean and tight.
I changed tubes one at a time until I went through them all.
I guess at this point I'll have to use a scope to trace the signal?
It would be helpful if someone could tell me which pins on which tubes I should be probing as i believe the signal goes from grid to plate in each tube in the path, just would be nice to know where to look.
Thanks,

[mauro]

Here are some more clues; when I pull any tube from V2 to V5 the squeal is still there.
When I pull V6 6SL7 phase inverter the squeal disappears but so does the sound.
When I pull one of 6L6's there is no sound but if I pull the other 6L6 there is sound and the squeal.
Does this give any clues?

[Ron Rich]

Mauro,
Did you jumper that B+ connection on the speaker socket ? Have you checked voltages ? --sounds to me that the problem is somewhere in the out-put circuit ? (poor ground, on a new cap ??) Ron Rich

[mauro]

Thanks Ron,
The speaker plug is jumpered and making contact
All voltages are where they should be.
I'm thinking a contact point as well.
I removed the amp cleaned and really tightened all contacts and moved all switches back and forth, I was going to start probing with a scope but then, just like that, it started with the squeal and then squeal is gone and I'm back to poor volume!

[benp]

Hi Mauro,
Check the connections on the output transformer to the 6V6 tubes are correctly connected if you removed any of those wires and reversed them it will squeal and also motorboat. It happened to me on a JL and took me a while to figure out I had one tube wired in reverse from the transformer.

Ben

[mauro]

Thanks Ben but this amp has never been touched before me.
I'm thinking the output section as well.
What I'm wondering is if the amp should play with either 6L6 removed?
I know from past experience that it will but dont know if it makes a difference which 6L6 is pulled?
If the answer is yes, should play with either/or removed then I found the area to concentrate on.
Thanks for your reply.

[mauro]

By the way, I'm now back to good sound reproduction accompanied by the loud squeal!

[robnyc]

May 24, 2018 21:04:16 GMT mauro said:

Here are some more clues; when I pull any tube from V2 to V5 the squeal is still there.
When I pull V6 6SL7 phase inverter the squeal disappears but so does the sound.
When I pull one of 6L6's there is no sound but if I pull the other 6L6 there is sound and the squeal.
Does this give any clues?

Mauro. it sounds like a bad output tran. This kind of thing can be cause vs. effect. By that I mean you clearly have oscillation occurring but this can kill an output tran if the amp was allowed to operate this way for a long period. If the oscillation goes ultrasonic, which is common, it can drive the tubes to saturation. The only manifestations will be lower and distorted volume and a sagging B+ due to the heavy conduction.

If you have signal at the G1's of both 6L6 tubes, then the tran is bad.

BTW: One point to check are all the common-point grounds that are made to riveted tabs. These are known to develop resistance between the tab and the chassis over time. When I was a kid hanging out in a service shop there were numerous service bulletins from TV and audio manufacturers advising servicers to drill out the rivets and replace them with screws. We used to just scrape the area around the tab, clean with alcohol and sweat a nice island of solder across the tab and a bit of the surrounding area. I still have a 300 watt weller that takes no prisoners for that purpose.

Good luck.

RobNYC