TSL100 Woes: PT2

[fdesalvo]

After fixing the heater supply to V6 I installed the new KT77s a few minutes ago and prepared to bias the damn amp up. I hooked up my meter to the bias points and took her off of standby.

As I took standby off, the left pair red-plated faster than hell. I cranked the bias pots down to minimum and the sons of bitches only cooled down a little bit, the plates still have a little red in them.

My meter will not give me a steady reading for anything. I think my meter is screwed up. It's only flopping back and forth between zero and some random digits regardless fo the setting.

What the hell is going on? Is my bias supply cooked on this absolute piece of crap Brittish turd?

[Enzo]

Take a deep breath.




Now exhale.

Take a 9v battery and measure it with your meter. DO you get a steady reading? If so, the meter is probably fine.

Pull all the power tubes. Turn the amp on. STandby too. It is now just like it was working except for no power tubes.

Turn the meter to AC volts and stick the probes down pins 2 and 7 of one of the power tube sockets. DO you get more or less 6 volts of AC? Now we know the AC volts scale works. (If there was NOT 6VAC there, the tubes woukld not have cocnducted current to red plate inthe first place)

Set the meter to DC volts. Ground the black probe to chassis and red probe into pins 3, 4, and 5 of each power tube socket. You can do all this from the top side, just remember the pins count the other direction from underneath. Check EVERY power tube socket, make no assumptions. There should be B+ on pins 3 and 4 of each. Maybe 475-500v, whatever. Now check pin 5 of each to see if there is about -50VDC on each, more or less.

MAKE SURE THE MUTE BUTTON IS NOT PRESSED IN.

Verify that the bias controls do adjust pin 5 voltage. And recall that the right control works the left tubes and vice versa. Looking at it from the rear.

If any of those voltages are missing we can find out why.

If and only if those are all OK, then we proceed to tubes.

Since the two left tubes are wired together and the two right tubes are wired together, all it takes is one of a pair to fail and they BOTH can red plate. And no amount of bias adjusting will compensate for a shorted tube. So if you know which was which, insert ONE of the tubes that was NOT red plating earlier into one socket and power up. Does it red plate? Can you set its bias? Remember to set it for half the normal amount because ther is only one tube.

If it works in one socket, then we know that both the tube AND that socket are OK. Now move THAT tube down one socket and retest. Use this one known good tube to test each socket. Always have a speaker connected, but no need for a signal through the amp. We have just demonstrated that all four sockets work with a known good tube. Or we discover a socket or two that don't work.

Now pull the good tube, and try another tube in a socket. At this point it doesn't matter which socket, they all work, right? If it works, pull it, and try another. We are testing each tube individually. If any tubes red plate in sockets we know aer OK, then that is a bad tube.

I don't care how new the tubes are, any tube can be bad at any time, even right out of the box.

If all four tubes test OK, and all four sockets, then try everything all together and see what happens.

[fdesalvo]

Thanks, Enzo. My days are so busy and after work is no exception. This is the last thing that I needed, haha. I will check these things and let you know what I have. I know right off that my meter is screwed, so tomorrow I'll pick up another.

Regards,

[fdesalvo]

Alrighty, here are some measurements:

V5-8 Pins:
2/7: ~6vdc
3/4: ~510vdc
5: -39vdc

I put in some old Svetlana =C= EL34s for the hell of it and checked the voltage on the bias prongs and I got around .15 on either pair with the bias as low as possible. With the bias maxed, I got .07?! WTH??

[Enzo]

Are those pin 4 voltages present on all four sockets?

[Steve Conner]

As far as I know, the TSL100 can't take 6L6 type tubes without mods to the bias circuit. Maybe your KT77s are more like 6L6s than EL34s. What brand are they? Do you have a datasheet for them with curves?

I looked at the curves for the JJ KT77, and it seems to me they would indeed redplate like hell with only -40V bias and 500V on the screen.

[Enzo]

Aha! Good call.

[fdesalvo]

Well, the KT77 is supposed to drop right in place of the EL34, so I don't understand how it's comparable to a 6L6.

All voltages were consistent across each pin from socket to socket.

Thanks, guys.

[Enzo]

The science here is straightforward - drop a set of EL34 into the amp and see if it likes them. Even just a pair would probably be good enough.

[fdesalvo]

Yeah, Enzo, I think it's come down to that, as well. I'll let you know what's up after I get soem fresh ones in. Thanks for sticking with me through this.

Can anyone confirm what the stock bias suppy should produce in the TSL100?

I have to admit for the first tim, I totally dig these high gain JJ 12ax7s in the TSL circuit. I hated them in fender type amps and even my former boogie, but the TSL loves these.

This amp is terribly thin, hopefully some E34Ls, these preamp tubes and my 400w cab with g12k100s will help fatten this amp up.

[fdesalvo]

Closure?

Well, last night I popped in the E34Ls and made sure the bias was as low as it could go. I turned the amp off of standby and hoped for the best.

No red plating. With my MM set to 200vdc, I measured the bias. It measured: .20. I slowly turned it up stopping intermittently to let them heat up. No red plating. I remeasured with the bias pots cranked to max and they actually read lower: .10!

I'm not sure what's going on here. I turned the pots down to minimum and rehearsal began. I kept my eyes on the tubes and no red plating. Just beautiful sound from the JJs.

For all I know the bias could be way the hell off. I hate flying blind, but they seem to be stable and cool enough to work with.

Another thing I hate is admitting how good these JJs sound. I have them all around- High Gain "Kit" and E34Ls. It made this TSL100 sound beautiful. The overdrives are meaty, liquidy, and just shimmering. The clean is not much beter, but that's typical Marshall. Plus, my Fulldrive sounds great through it.

Before, this amp was way too bright and thin. Now it's totally useable and I've been getting inspiring tones from it. I'll have to see what happens with these tubes....

[Shea]

As far as I know, the TSL100 can't take 6L6 type tubes without mods to the bias circuit.

Not true. I put a quad of JJ 6L6s in mine last weekend. With the trimmers set to max bias voltage they were way cold, so I had plenty of adjustment to work with.

I gigged with the amp Sunday. Played it louder than hell, and the tubes and amp held up fine.

Shea

[Amp Kat]

You must be measuring something wrong on the connectors as you should get around 70 mv's frome the center leg to each opposite side. Do you maybe have your meter on AC instead of DC ?

[fdesalvo]

^No sir- DCV. I'm from NOLA, btw. I used to enjoy BR back in the day!

[Shea]

You'd probably get a more accurate reading if you set it to the millivolt range. My meter has a setting for 0 - 300 mv.

But I doubt that would explain why a reading would go down when it ought to go up.

I had a weird bias-checking experience with my TSL. When one of the EL34s went bad, I left the other three EL34s in there so I could figure out which direction to turn the bias trimmers to set them for the coldest setting before I replaced the tubes. When I'd measure the current on the side that had only one tube plugged in, I'd get a really high reading, like in the 70's. But when I'd measure the side that had two tubes, I'd get reading in the 40s or lower. I'd switch the tubes around, but no matter how I had them arranged, I'd get readings indicating that two tubes would draw in the 40s of milliamps whereas one of them would draw in the 70s. It wasn't a fluke with one of the sockets, and it wasn't a fluke with one of the tubes. Very strange. I expect the B+ to go up a little when I remove one tube, but I don't think that accounts for it.

But when I put four 6L6s in the amp, I got normal bias readings, I was able to set them to 90 per side, and they work and sound just fine.

Shea