Crate VC5310 power tube failure ,and fuse blown.

[gtrplayr1976]

Here's the 2 alternate circuits drawn out. The #2 option may be less desirable as it effectively increases R77 at the same time it is effectively decreasing R78. Doing both at the same time may cause the pot to be more "touchy" as they both decrease the bias voltage. Method #1 does not increase the effective series resistance of R77.
[ATTACH=CONFIG]30600[/ATTACH][ATTACH=CONFIG]30601[/ATTACH]

So option #1 the wiper ,and one end of the pot are connected ,and the other end to the end of R77 ? Also change the 15k resistor to 5K ? I thought I need to add more resistance to get the dissipation current down ?

[gtrplayr1976]

In chosing between those two, consider what happens if the wiper contact fails in the pot.


Hint: I never take bias off a wiper.

So in option #2 if the wiper fails you lose bias ?

[Enzo]

Exactly.

No bias at all, tubes melt.

In the other example, if the wiper fails, then the pot max vale is the result, and that reverts bias to the max voltage, in other words teh coldest setting. Safe for tubes.

[Enzo]

The problem with option 1 is not so much dissipation in R77, but rather that R77 is way too low a value to make good voltage division. I would make R77 much larger, like 3k or something. Then you will get reasonable adjustment range with a pot.

[gtrplayr1976]

The problem with option 1 is not so much dissipation in R77, but rather that R77 is way too low a value to make good voltage division. I would make R77 much larger, like 3k or something. Then you will get reasonable adjustment range with a pot.

I'll give that a try Enzo. Thanks !

[g1]

Enzo, in this particular case, the problem is trying to get the tubes to run cooler. So we need to keep every last available drop of the negative supply. I'm worried the -V level may be too little to accomplish the goal anyway. In such case a bigger source of negative voltage would need to be built (like a doubler or tap off HV or something).

[gtrplayr1976]

Enzo, in this particular case, the problem is trying to get the tubes to run cooler. So we need to keep every last available drop of the negative supply. I'm worried the -V level may be too little to accomplish the goal anyway. In such case a bigger source of negative voltage would need to be built (like a doubler or tap off HV or something).

I won't go to a lot of trouble to make changes. If #1 solution above doesn't work I'll just put it back as it was, and try to correct the other issues with the amp. At least try to get it back to how it is suppose to sound

[g1]

Understandable. As I mentioned before you can always buy tubes that are picked to bias up colder.

[gtrplayr1976]

Understandable. As I mentioned before you can always buy tubes that are picked to bias up colder.

I don't think this amp is used a lot g-one. It isn't mine. Just trying to help a friend. I already bought the tubes ,and didn't think I would need to ask for cooler ones until it was tested

[Enzo]

OK, two things

With R77 at 100 ohms, it is never going to work worth a darn a adjustable bias, so increasing R77 is necessary for that. If the voltage needs to be higher, that is a separate issue. We could increase the raw supply to whatever we needed, but the 100 ohm upper resistor would still stop us from a good adjustment circuit. It is like digging with a shovel while holding the handle down at the blade. No leverage.


If you want higher voltage, then there is NOTHING you can do to the little bias circuit to make it adjust higher. You must create a higher raw voltage to start with. The existing bias comes from the V- supply. An alternate source would be the AC for that supply and then a voltage doubler or tripler. Once you have that higher raw voltage, then you can adjust it down to the range you want. And when you do that, your R77 will want to be larger than 100 ohms.

[gtrplayr1976]

Maybe I'll just put the resistor back in

[g1]

I was really hoping to see if you could increase the -B at the power tube grids .

[gtrplayr1976]

I was really hoping to see if you could increase the -B at the power tube grids .

If I were experienced enough ,or had specific instructions i could do it.

[g1]

Just try increasing R78 will tell us enough. Maybe 25K.

[Enzo]

There is another way. This circuit board serves several models. In this model we have fixed bias and the output tubes are grounded at the cathodes. But there is a jumper wire between those cathodes and ground. IN another model, the tubes are cathode biased, and there is a spot on the board for that resistor. (And over there, the bias supply circuit would be left out). One could remove the cathode jumper, and mount a cathode resistor. Now the amp would still have the fixed bias from before, but the added cathode resistance would also bias the tubes cooler. If the board is marked, it is a 10w resistor at R43 between the center two power tubes. Stock is 60 ohms, but for this project I might start a lot lower. It would then be using both bias methods to total what we want.

Just a thought.


View the alternative in the VC3112/5212 schematic.