Crate Turbo Valve 60 help

[Denny_A]

Does anyone happen to have a schematic for one of these you could send my way? I'm trying to help a friend with his Crate TV60 that is blowing fuses. I found 2 connected leaky diodes off of 1 of the OT's primaries. I know these amps were supposed to be self biasing. Are these diodes part of that set-up?

Thanks for any help you can provide.

Denny

[Enzo]

REmove those diodes - and the power tubes - now power it up. Does it still blow fuses?

Those aer called flyback diodes, and are there to protect the transformer from voltage spikes that usually occur when the amp is run without a speaker load. If they short or leak, it is B+ to ground through them, so fuses blow.

"Self biasing" merely means the amp is not adjustable, nothing cosmic or high tech.

The amp will run fine without them, they are just protection. But I would replace them. If one is damaged, they all were stresssed. Replace ALL of them. DOn't try to be cheap for a couple 5 cent diodes. You can run without them while workin gon the amp though.

Once the amp no longer blows cuses, THEN install power tube. How old are the original tubes? Isn't it time for a new set now?

[Denny_A]

"Nothing Cosmic" HA Ha Ha!:p

Truely thanks for your reply Enzo. I know what fixed bias and cathode bias look like in a point to point amp, but on this pc board and without a schematic I was very unsure. The amp had been making crackly, crunchy noises until I put new power tubes in and everything was fine for about an hour of playing, in fact it was sounding the best that crappy amp ever sounded and the poof , out went the fuse. I will replace the diodes tomorrow and procede with start-up without the power tubes and if all is good, with the power tubes.

I am also questioning the output tranny. Everything measures good except on the secondary side, there is very little resistance (2 or 3 ohm) between the 3 leads. That is not normal, right? It is a 4 or 8 ohm selectable impedance.

Thanks Again,
Denny

[Enzo]

I have no idea what impedance the thing has, but impedance is not resistance. Your secondary is a winding of relatively few turns and is of heavy wire. it will normally read less than an ohm in resistance.

Whatever the circuit is built on, if you wonder what hte bias type is, simple. measure resistance from the cathode of the power tube to ground. If it is zero or a few ohms, then we must have fixed bias. If you get something like 250 ohms, then it is cathode biased. Alternatively, power up and see what voltage is on the power tube grid and cathode. If negative voltage is on th grid, it is fixed bias. if the grid is at zero, then see if the cathode has some positive voltage. if the cathode is positive, then we have cathode bias. I purposely did not enter voltages because a little EL84 amp might have 12v bias - or either type, while a 100 watt with 6L6 might have 55 volts of bias.

I can make that determination without even opening the chassis. Pull a power tube and measure at its socket.

Now then, everyone say it together:

just because tubes are new doesn't mean they are good. New tubes can be bad right from the box, and they can fail in a few minutes. No guarantees. Don't assume. SO whuile your diodes might be bad, make sure to try different tubes.

[Suburbanite]

Now then, everyone say it together:

just because tubes are new doesn't mean they are good. New tubes can be bad right from the box, and they can fail in a few minutes. No guarantees. Don't assume. SO whuile your diodes might be bad, make sure to try different tubes.

theres a song in that somewhere

[Denny_A]

Enzo,

Diodes are replaced and it powers up without power tubes and also with a known good set of power tubes without blowing fuses. There is still an intermittent popping, cracking, screeching sound like before the diodes became leakey. I have also replaced all the preamp tubes with known good ones.

Could it be possible that an intermittent short in the OT caused the flyback diodes to go bad? I have a spare OT I will try subbing in. Any more ideas?

Thanks!

[Enzo]

I keep a spare OT on my bench, ready to clip into any amp that is suspicious, do it.

The transformer wouldn't kill the diodes. But if the diodes are shot, then they no longer protect the transformer.

I'd be looking for a noisy resistor. In any case, isolate the problem. Does this noise persist even with all controls at zero? If so it suggests power stage or PI. If not, then earlier. ANy control that affects the noise in any way is either at or after the source of the noise. COntrols having no effect on it are before it.

ANother trick is to pull the girst tube. ANy help? Pull the second tube. etc etc. Start from the front and pull tubes until the noise stops. That way you know how far along the amp the trouble lies.

Get out your scope. Go through the amp stage by stage. Where does the noise come in?

[KapnK]

I've got them if you still need them.