JCM2000 - How hot is Too Hot???

[mikeboone]

First the history...

I am looking at an amp (TSL122) for a friend that at first was making lots of strange noises and blowing the HT and Mains fuse.

Upon examination, I found some bulging on the 330uf caps and some leaking fluid from the 100uf caps. I re-capped the amp - (Thanks Enzo for directing me to Mouser for the parts)

So then I looked at the tubes and noticed that on two of them the orange GT silkscreening was brown. These came from V5 and V6. With all the tubes back in their respective places the bias adjust point was 165mV on V5&6 and 130mV on V7&8 and making plenty of noise. I took the scorched tubes out and left 7&8 in place. I set the bias point to 90mV as suggested in a thread that I saw here regarding JCM2000's. I moved these two tubes to 5&6 and did the same. No noise or other silliness now.

I then split the tubes up to V6 and V7 and each side is reading 45mV and the amp sounds great. I split them to V5 & V8 and them same is true.

I replaced all 4 tubes with a new set and the bias point did need to be changed and the amp sounds fine.

The owner says the amp is getting too hot now. All I have is his word but no other practical way to know.

It is getting pretty hot. The mesh back cover gets pretty hot and the chassis is almost too hot to touch. But this is a 100W amp and amps get hot. And when all this is going on the bias point is steady as she goes, amp sounds fine.

So - How hot is too hot???

[melvin]

Are you reading voltage off a cathode bias resistor? What's the current running through each side? I usually set the initial bias at 35mA for each 6L6, el34's or 6550's somewhat higher (sometimes as high as 75mA per tube, which gets hot!), but then I adjust by tone. As long as you don't exceed the max dissipation wattage of the tube, your fingers aren't getting burnt when you touch the front, and the amp doesn't smell from the plastic melting off the wires, I think you are probably okay. But if a customer says the amp is too hot, I would bring the current down until he's happy. If his tone suffers, you could always tell him his bias is set too cold...and start charging him to re-bias.

[bluesfreak]

the TSLs do get hot indeed. However, Marshall suggests 80mA per side for the Power Tubes (which is a bit low in my ears) but if the customer complains just get it down and you will hear complaints soon about a loss in dynamics *g* but anyhow, that's now his problem...

[mikeboone]

Are you reading voltage off a cathode bias resistor? What's the current running through each side? I usually set the initial bias at 35mA for each 6L6, el34's or 6550's somewhat higher (sometimes as high as 75mA per tube, which gets hot!), but then I adjust by tone. As long as you don't exceed the max dissipation wattage of the tube, your fingers aren't getting burnt when you touch the front, and the amp doesn't smell from the plastic melting off the wires, I think you are probably okay. But if a customer says the amp is too hot, I would bring the current down until he's happy. If his tone suffers, you could always tell him his bias is set too cold...and start charging him to re-bias.

There are test points on the back of the chassis that are fed from 1 ohm resistors between the cathode and ground.

I will bring the draw down to as low as the tone can stand and see if it makes a difference.