Problem with Premier Twin 8

[bluzmn]

When my Premier Twin 8 gets hot, it starts to lose volume and gets a distortion that sounds like a blown speaker. A few weeks ago, I put in a new(ish) Sovtek 6L6GC; I played the amp for 1/2 hour, and it sounded OK. However, I played the amp for a longer period today, and it started doing the same thing. Where should I look for the problem?

[The Dude]

Considering the age of the amp and your statement "... gets a distortion that sounds like a blown speaker" it's likely that your electrolytic caps are failing. A couple of questions:

1) Do you notice an abnormal amount of hum with nothing plugged in?
2) Is the distortion more or less noticeable at higher volumes?

[Bruce / Mission Amps]

Are you even sure yours is a 6L6 amp? Many of them were not.... you might actually have a 6V6 or 7591 version ..?
Also, the funky old 10K 1 watt screen resistor gets hot as heck in these when non tech players have been plugging the wrong tube into them...and they can go way up in resistance, effectively reducing the output quite a bit and sounding distorted.
And, the cathode biasing resistor is only a 2 watter... not many 6L6 cathode biased amps can run a little two watt cathode biasing resistor.

[Steve Conner]

Yup. You might also want to check for leaky coupling capacitors.

[Bruce / Mission Amps]

Yup. You might also want to check for leaky coupling capacitors.

Yeah, I forgot about that.... most of the old late 50's Premier amps... including the Twin 8's, had paper in oil "waxed" coupling caps... which after all this time (50-60 years, yikes)... well, I'd replace them all with either a modern PIO or a cheap, generic, axial polyester plastic cap.
95%-98% of the players I've worked with over my entire life time would NEVER hear the difference between an amp using good wax PIO caps and generic polyester caps but, after replacing them all, their vintage amps would work great.

[km6xz]

If it sounds ok for a while when turned on from cold but get bad, but gradually deteriorates, sure sounds like bias is shifting, just what would happen if a coupling cap is leaking. But there are other possibilities but more information is needed to create a list of logical causes. The more basic data supplied the narrower the list and with enough information on symptoms, only one solution will explain ALL the symptoms. Asking for blue sky guesses without more information will just confuse you with a dozen different trails to follow.
Do you have a voltmeter, or oscilloscope? A few voltage measurements will help a lot.

The amp has cathode bias so should be able to use either a 6V6 or 6L6 since the heater winding can support a 6L6. The plate impedance is different however so the speaker impedance for optimum output needs to be changed.

[bluzmn]

@ The Dude - 1.I've already replaced the filter caps and the cathode bypass cap on the power tube, and no, it doesn't have the characteristic hum of dried out electrolytics. 2. The distortion seems to increase proportionately with the volume.
@ Bruce - The amp came with an old Sylvania 6L6GB, so I assumed that was the correct tube to replace it with. Anyway, I have 6 different schematics for this amp; the ones with a 6L6 or 6V6 and a 6X4 rectifier are nearly identical; the ones with a 7591 have a 5Y3 rectifier, and my amp has a 6X4. The "funky old 10K 1 watt screen resistor"; is that the one between the 2 10mfd filter caps? Should I replace it with a 2 watt (or more)? I do have some 270ohm 5 watt resistors on hand, so I'll replace that cathode biasing resistor.
@Bruce & Steve Conner - I also have 2 0.022mfd polyester caps, so I can replace the coupling caps; should I replace the grid leak cap also?
@ km6xz - Where should I check the voltages?
@ Everyone - Thanks for your help.

[Bruce / Mission Amps]

First resistor across the FP filter cap lugs is usually 270 ohm one and the second one is the 10K. I always use a Mtl Ox 2 watter there.
Unfortunately, at this stage of the amp's life, coming with a 6L6 tube doesn't really mean much.
The schematics you are looking at are very likely consumer drawn and not original artwork from "Premier".
I worked on a lot of these old amps over the years and I still can't say for certain...but I think the 6X4 rectified Twin 8 is almost always a 6V6gt amp.
The 6X4 has about half the current handling capability as a 5Y3GT.
Does it have negative feedback from the OT? Does it have a metal strap in the middle of the chassis top to bottom? Is it all point to point with all paper in oil wax caps... etc.

OK I just checked... better then half the current handling but still not what a 5Y3 could handle.