Crate Turbo Valve 120 Schematic? TV-120

[RWood]

Does anyone have one?

Just took the amp in and [what might be] R115 has blown apart so badly I can't even read the value!

Thanks,

RWood

[RWood]

I found one for the TV60 amp; I assume this is similar but with two power tubes instead of the four that are in the TV120.

The resistor that blew apart is a 150 ohm, 5 watt in the B+ line, just after the first set of filters.

RWood

[Enzo]

Which probably happened when a power tube shorted its screen to something.

[RWood]

Thanks Enzo.

The tubes (4 x EL34) all tested reasonably strong (mids 70s out of 100) on my cathode-conductance PACO tester. Would a tube with a shorted screen still do this?

On further investigation of the circuit board, I see that a 250mA fuse on the power tubes' cathode circuit is also blown. And the wire from the standby switch to the circuit board had gotten so hot that it burned the insulation right off it.

Before I found the fuse my first suspect was that the initial filter caps were shorted. Now I am wondering if the lack of a cathode connection caused runaway B+ voltage/current, frying the components that it did. Does that make sense?

RWood

[Enzo]

No. Opening the cathode lead is one way to shut off a tube. Some amps switch tubes in and out for hi/lo power switching that way. Some amps use a cathode lead switch for standby.

A blown cathode fuse and a burnt resistor between plate and screen B+ nodes pretty much screams shorted tube to me. A tube short can be intermittent and maybe not show up on a tester. Shake each tube in front of your face and rap on the side of each witha knuckle. Is there any looseness? ANything rattling around inside the tube? Remember, the tube tester holds the tubes upright, while many amps holt them upside down. Loose crap can fall out of place inside while moving from one to the other.

And are these tubes the same ones that were in it when it failed?

Any of the tubes have a broken off center post so it could have been put in the socket the wrong way?

[RWood]

Is there any looseness? ANything rattling around inside the tube?

Yes indeed, in the very first one I tried. I small blob of something.

I should have been suspicious when I tested it, as I now recall the needle went strong into the mid 80s but began deflecting in the 82-85 area and would not settle. I guess that is the definition of intermittent!

Thanks much for your diagnosis, Enzo. Time to order some replacement parts.

RWood

[RWood]

A quick question before I replace the 150R power resistor. Should I stick with the 5 watt value of the original, or increase to a 10 watt?

Even though the 5 watt blew apart like a stick of dynamite, I am glad that it did versus remaining intact and causing further damage (to the transformers I suspect). It worked like a fuse. So I wonder if a 10 watt would be less likely to blow up, but more likely to cause damage to the other parts.

Thoughts?

[found]

I am trying to find what pre-amp tubes go where on tv60. I have all 12ax7, 5 of them and it has a loud squeal. I read somewhere that it requires 4-12ax7 and 1-12au7? I cannot find any information on this and would really appreciate some.
Gerry

[RWood]

V4 should be a 12AU7. It was a handwritten addition to the schematic.

V1 is nearest the input, count to V4 from there.

[ManRay]

No. Opening the cathode lead is one way to shut off a tube. Some amps switch tubes in and out for hi/lo power switching that way.

Sounds like a great mod! Would anyone care to elaborate on how I could go about doing just that for my Crate TV 120?

Thanks!

[Double00Buck]

You can call 800-258-6883 Loud Technologies Support Techs. They can help you.