Still in need of cooler 6L6's....advice please

[Chuck H]

"When I dialed counter clockwise the plate voltages would rise close to 700 so I backed off. I was able to get -33volts with the trim pot half way"

So it's not so much that you can't get enough bias voltage. You just can't bias it that cool without the plate voltage rising too high?

Chuck

[Garydean]

True. Here is what I did last night.

I removed the 5K 2W dropping resistor on B2, and replaced it with a 2.2K in series with a 4.7K 2W resistor, which helped drop the voltages on the preamp tubes. I subbed in for the 33K resistor, a 15K resistor on the bias adjust trim pot in series to ground.

Below are my findings

116 line voltage

6L6

pin 3.....633V
pin 4.....320V
pin 6.....-20V
.....................................................
adjusted bias via trim pot. Line voltage still at 116
20% orange glow on plates
pin 3.....635
pin 4.....322
pin 6.....-22
.....................................................
line voltage raised to 120V
20% orange glow on plates
pin 3.....652
pin 4.....331
pin 6.....-23
.....................................................
adjusted bias via trim pot. Line voltage still at 120
60% orange glow on plates
pin 3.....628
pin 4.....321
pin 6.....-17.7
.....................................................
adjusted bias via trim pot. Line voltage still at 120
no orange glow
pin 3.....686
pin 4.....346
pin 6.....-28.9

B+ (B1) has now elevated to 696

I'm sure I left out pertinent information. Please advise.

[Chuck H]

Well, I've already been poo poo'ed by Enzo and Bruce, and their perception should probably always be taken over mine, but I'd say you have a very specific problem that needs a different kind of solution from what the original manufacturer did.

Let's say, for example, that with 115v from the wall AND a different 6L6 type tube that is not available today you could get that amp stable. But you don't have the same circumstances to work with as the original manufacturer so you can't get the amp stable.

Change the circumstances. Modify the bias supply to provide up to -60 volts and use 6550s.

This isn't the only amp to have a problem like this. The Fender Deluxe Reverb is fine at 110v, OK at 115v, your pushing it at 120v. Now some people have 126v or more at the wall and it's just too much. You can find discussions one after another, people trying to find the best brand of 6V6 that can take it. The Traynor Guitarmate has the same issue with it's EL84 tubes.

I will take my beating now.

Chuck

[Garydean]

Thanks for sticking your neck out Chuck....LOL. Suppose, for discussion purposes, I do switch to 6550's. I then would need to beef up the filter caps from 100/350's to 100/450's, or would the 350V be OK?. Any other changes required?

Wouldn't my B+ be excessive with the bias beyond -35V?

thanks,

Gary

[Enzo]

Oh dear, I am poo pooing. I hate that shit... All friends here.

I myself worry about dissipation. Note that while the B+ climbed, at the higher bias setting your red plates went away. Now if the resulting elevated B+ is too much for the tubes, then they will arc or otherwise self destruct. But voltage alone does not cause red plating, it is the power dissipated.

There are various techniques for dropping the B+.

YOu can pull the output tubes and measure the B+. That will be as high as it can go.

[Chuck H]

"Wouldn't my B+ be excessive with the bias beyond -35V?"

No, 6550s require more bias and drive voltage for the same current draw as a 6l6. Also, 6550s have a max vp of 600 (IIRC). So your not going to be near as close to max as you would be with a 6l6 (max 375v) or even a 6l6gc (mav 500v).

Enzo is right. It takes both voltage and current to create watts. If you keep raising the bias voltage your plate voltage will go up (to a point) but your current draw will go down by a greater degree. I suppose in that case that you may end up needing beefier filters even if the 6l6s can be made to work. But, since your already rigged for it, try Enzo's idea. Just keep raising the bias voltage until the plates stop glowing red. If nothing arcs or otherwise fails, your done. If that makes you too nervous then order a big diode and drop the B+ by 80volts or so. You may need to change some other dropping resistors in the rail to bring the preamp voltages up. But then you would surely be O.K. as long as you use 6l6GCs and not just plain ol' 6L6s.

Chuck

[Enzo]

Looking back at your schematic, what sort of voltage is on that pink wire from the PT? COuld we swap the pink wire for the red one and reduce the voltage that way?

Aside from the points I was making, I do agree that 660-700v is way too high, just not the cause of the red plates.

[J Martin]

I have used 6550 tubes in those amps without a problem. Just got another one into the shop with a blown output transformer. The original was long gone and the recent replacement was way undersized. Here is my chance to optimise.

[Dave Curtis, dB AudioTech]

At 660vp your going to cook those tubes pretty fast, even if you bias them cool. In fact it sometimes happens that the cooler you bias them, the higher the vp will go, making things even worse.

Are you sure the amp is designed to run 6L6s? Where did that schematic come from?

Doesn't matter. I'd change to 6550s or el34s at least.

Chuck

I doubt the tranny can supply the extra filament current.

-DC

[Dave Curtis, dB AudioTech]

I've fixed up more than a few Univoxes very similar to this one; get the dissipation down to less than 70%, R&R the baked OP tubes, crank it up and stand back. ;o)

Be careful of the board traces when swapping components...

-DC

[Chuck H]

Well, I've never seen one of these so I cannot say. But the 6550s will only draw an extra 1.4 amps. I hould hope the filament winding is overdesigned by at least that much.

Also, Didn't another poster already indicate that he swaps those amps to 6550s regularly.

I can say this. If the amp is designed to run that close to specs then the designer had no business putting 660Vp on a pair of 6l6s.

Chuck

[Dave Curtis, dB AudioTech]

Well, I've never seen one of these so I cannot say. But the 6550s will only draw an extra 1.4 amps. I hould hope the filament winding is overdesigned by at least that much.

Also, Didn't another poster already indicate that he swaps those amps to 6550s regularly.

I can say this. If the amp is designed to run that close to specs then the designer had no business putting 660Vp on a pair of 6l6s.

Chuck

Although the trannies are fairly good sized, I'd want to see how hot the PT gets during a 4 hour gig with 6550s in it (could always spring for a filament XFMR). They don't have to be biased very hot anyway (nor do the 6LGCs). I'm just saying I have happy customers running good 6L6GCs in these amps; as they were designed. It won't sound the same with 6550s or EL34s (I do like the sound of 6550s in my bass amps, though). The Univox U-1226 and U-1061 I worked on early last year had 640V and 625V B+ respectively, and they're still going strong. Just make sure the circuit is good (bad caps & resistors galore; watch those PCB pads), and don't bias it hot. Oh, and definitely no upward impedance mismatches.

[Garydean]

Thank you everyone for all the information. I've tweaked the bias at -28.9v with no orange plating. The voltage #'s are a bit higher that normal (6L6's ..684V) but the line voltage is at 121v rather than 117v as suggested by the MFG.

Enzo, to answer your question about the pink wire.... despite what the schematic shows, this amp has no pink wire. The schemo shows 11 wires coming from the PT, this one has 9.

if I'm crazy.... replies are most welcome.

Gary

[Amp Kat]

That schematic also supports 6L6GC's which are 30 watt tubes and should take the voltage. What I'm confused about is why is changing the bias voltage effects large plate voltage swings. Somethings not right there. -33 is way to hot for those tubes. With that much voltage you should be up in the -50 to-60 range and you may have to create a voltage doubler to get it there but you really need to get up in that range. 6l6's will take it if you get the bias right but I'd make sure you use a GC type in the 25 to 30 watt range or the 5881 Sovtek that will take up to 800V or so even if it's just for a test.

[Garydean]

I have Sovtek GB's in there now. I have a set of EH GC's I'm gonna throw in...... so I'm not done?

thanks Amp Kat