Help With A JCM2000 TSL 122!

[Mr. Droopy-Drawers]

Help! I have a blowed-up TSL 122, and I have the board mostly cleaned up and parts replaced, but one resistor is holding me up . On the board it is designated 'R9', but no schematic I find seems 'right'. If anyone has this amplifier, and can peek at the resistor, I would sure appreciate it. It is located right between two output tubes (labeled V7 and V8). Here is my best description;

- Between V7 and V8 you'll see a purple wire, R8, R9, R10, and a fuse F4.
- R8 is a 100-ohm/2-watt resistor that sits above the board about 5/8".
- R10 is a 220K, 1/4-watt resistor flat on the board.
- R9 is right between R8 and R10, and is also about 5/8" above the board. It looks to be 1-watt.

Any help identifying this resistor is greatly appreciated. This arrangement is not mirrored for V5 and V6, so I can't just look over there. There is R6, which looks to also be about 1-watt, and is 5/8" above the board. I might just try this value for the heck of it, but thought I'd give you guys a chance to prevent me from doing something that'll just blow it up again. Thanks !

[tim]

It looks like a 1 ohm 1 watt resistor on my almost unreadable schematic...it's the same as R6 assuming you're looking at the power amp board which makes sense seeing where it is in the circuit.

[Mr. Droopy-Drawers]

R6 on my board is 1-ohm, and it looks like 1-watt, and physically matches the shape and how high it is off the board compared to R9. That's why I thought it likely was 1-ohm. But what the heck is it in the circuit? At best I think it is a current sense for the Cathode of that particular tube, but there is only one resistor for both tubes. Wait a minute; you set the bias at back for each side (90mA?), so having one resistor for two tubes makes sense, although for two tubes peaking at only maybe 150mA the 1-watt is overkill. I think I am going to give it a shot. I won't hold anyone responsible if it blows again. It'll just be a sign from Jehovah that JCM2000's are to be avoided.

[Blinker]

There's a picture http://www.lynx.bc.ca/~jc/TSL122sockets4.gif here.

[Sock Puppet]

There's a picture http://www.lynx.bc.ca/~jc/TSL122sockets4.gif here.

That's exactly the picture I was looking for... the whole article's worth a good read since you have the amp apart:

http://www.lynx.bc.ca/~jc/TSL122.html

Schematic: http://www.drtube.com/schematics/mar...tl10-60-02.pdf

S.

[Mr. Droopy-Drawers]

Bingo! And that blowed-up picture way down towards the bottom is exactly what mine looked like. Weird. But let this be a lesson to all you engineers out there; when you take a perfectly good tube amplifier design and @#$%! it up, you only think you made it better.

[Enzo]

But let this be a lesson to all you engineers out there; when you take a perfectly good tube amplifier design and @#$%! it up, you only think you made it better.

What are you talking about. These are 1 ohm resistors in series with the power tube cathodes. This is nothing new. We here on this board add them to amps all the time. If you follow the schematics, you will see the top end of those resistors runs up to the bias adjust board - they are the test points you use to set the bias.

These resistors burn up when a tube shorts out. Not the engineers' fault.

[Mr. Droopy-Drawers]

What I was refering to was the whole mess Marshall calls an amplifier. Geez, it's enough to drive a nun to drink. But if you have to use 1-ohm resistors, put a diode across it, and avoid meltdowns later. Using 1-ohm resistors is fine, if you have a handle on it, but let me play Devil's advocate. In a 100-watt head (which I have), one resistor is used to monitor two tubes. So far so good? Marshall wants you to set it to 90mVDC, which is also fine (45mA per tube). Are you still with me? I have had it happen more than once (but usually with Sovtek tubes, so I rigged this quiz) monitored the Plate current my own way and ignored the 1-ohm resistors, and saw one tube measuring 55mA and the other tube measuring 35mA. The grand total? That same 90mVDC was measured at the back pins. This happens a lot with EH and regular Sovtek EL34's, so I buy dozens to match up a quad. And I always end up with two tubes left over; one might measure 25mA or 30mA (leaving the bias setting alone) while the other measures 45mA or 50mA. I save them of course, and match them to what I buy on my next tube shopping trip. So use 1-ohm resistors, it won't bother me a bit . Y'know, as I type this, I know you're asking 'Why buy Sovtek, then?' Because in my itsy bitsy, teeny weeny corner of the world, that's all the music stores carry. Order JJ online, you say? I should, but I am a slow learner. And I still have four EH quads (nicely matched) left from two years ago. I won't have to buy EL34's for some time.

[Mr. Droopy-Drawers]

Hey! To see if this Karma thingy works, let me try and repay the help from Tim. Maybe this will help someone else as well; as a habit learned from waaaaaay back in my TV service days, after putting the whole mess back together, I checked for any wiring mistakes (no schematic or layout, but my own drawings and notes). I did the light-bulb-in-series trick, and then used a Variac to slowly bring it up to line voltage (I know, it's like wearing suspenders and a belt). No warm resistors, no smoke. But then I did a leakage check on the chassis. I had a whack of current on the chassis . Just for the heck of it, I went to the AC input board. W7 is the brown wire, and W13 is the blue wire; these go from the AC input to the switch/fuse/power transformer. I swap them around, and the chassis leakage is gone. The wires were not removed to take the main board out, and they have always been in that original position. Why I haven't been zapped while grabbing a mic stand is anyone's guess.
Well, that's my tip-of-the-day. Everything else seems OK, but I just had preamp tubes in. Signal to all EL34 pin #5's looks fine on an oscilloscope. Tomorrow I'll pick a set of EL34's and see what happens.