6L6G Weirdness

[ALBATROS1234]

I have a 1949 Bogen H30 PA that I converted to guitar. It sounded really good with a pair of Sylvania 6L6G's, 6L6GC's from the 70's modern 6L6GC's . The amp originally came with 6L6G tubes and I have one of those I got in a tube lot that sounds great in a SE amp I built. So I wanted to try a pair of coke bottles in this amp so I found a pair on Ebay which were reasonably priced and claimed to test good. I bought them and when I got them I tested them and they test fine, no shorts etc. I tried them in the amp and got this unbearable squealing in an amp which sounds fine with all the 6L6 types and also sounded good with 6V6gt's and 6F6G's. So I narrow it down to one of the 6L6G's I bought squeals the other hums badly in PP circuits. OK fine so I contact the seller he gets pissed and acts like I am the dumbass, blames my amp etc. So to be sure I try the tubes in a SE amp and they sound fine.....I am dumbfounded and try again in the PP amp to horrendous squeals and noise. Back into the SE amp and they both sound good. WHY, on God's green earth would a 6L6G or any other power tube sound great in a SE and squealy noisy shite in a PP. Yes, I tried them in different sockets and in combination with different tubes. But, the one tube especially squeals like a banshee in PP amps but sounds damn good in SE. How is this possible? I would think it would be the opposite for one thing because SE amps use the PT constantly and PP only on its phase pulling or pushing as it were. Am I in the Matrix or is this something there is a legit reason for? I am just really dumfounded and don't get why a certain tube or 2 sounds great i SE but really horrible in PP amps. Any logic to this?

[sluckey]

How many different PP amps have you actually plugged these tubes into?

[uki]

Are those tubes matched, slightly matched or not at all?

[ALBATROS1234]

2 different PP amps 1 SE amp.
None are a "matched pair." so to speak. They are all old tubes that test good on my dynamic conductance tester. The one that squeals matches fairly close to my original RCA one. It squeals with the one it is closest to as well as every other tube it is placed in a PP circuit with but does not squeal in the SE amp. This is why it makes no sense. IDK I thought I would post about it to see if some tube guru had some insight. Seems strange to me.

[brewdude]

Fixed biased or cathode biased?
Did you check the bias and/or dissipation?

[pdf64]

The actual circuits involved may help to explain it.

[octal]

Wild ass guesses:

Is it possible that this amp uses pins 1 and or 6 on the socket as tie points and the replacement tube has an internal connection? (intentional or non-intentional)


Is it possible that the amp is on the verge of instability anyway and the higher or lower Gm of the tube tips it over the edge? (The gain of the tubes will impact the stability margin of the feedback loop.)

[ALBATROS1234]

The amp that was squealing the most is cathode biased. Its a 5 tube amp with a 5Y3 and 2 dual triodes ,the power section is similar to a 5F1 deluxe but set up for 6L6s and instead of the first tube using a triode for 2 channels they are in series. So 3 gain stages , a split load PI into a pair of cathode biased power tubes.I will check the bias and see what I find with the offending tube installed. The weird thing is I have tried several other pairs ow power tubes and only when this one tube is in either socket does it squeal and sounds fine in a SE amp.

[tubeswell]

When the squealing starts, try holding the tubes one at a time with a (dry) towel.

[ALBATROS1234]

Thanks, It just seems illogical for this tube to be acting bad in PP but fine in SE. I hoped to figure out why?