Crate VC5310 power tube failure ,and fuse blown.

[Enzo]

Oh, I don;t recall what your target might be. If the new resistor alone makes it run about where you want it, then leave it like that. I was only describing what goes into making it adjust within range.

The stock VC112/5212 runs the four tubes at about 12 watts, which seems really warm to me, but apparently works. That is 60 ohms in R43 for a 9.7v drop. The fairly similar PV Classic 30, when I worked up the cathode bias mod you can still find over at Blue Guitar, I went with 100 ohms, which I felt a bit more secure with.

So myself I;d still consider 10 watts pretty warm, but certainly within precedent, and not at all "too cool." If it sounds good, and the tubes are not red plating, then I;d have to say your bias was within the reasonable range.

[gtrplayr1976]

Oh, I don;t recall what your target might be. If the new resistor alone makes it run about where you want it, then leave it like that. I was only describing what goes into making it adjust within range.

The stock VC112/5212 runs the four tubes at about 12 watts, which seems really warm to me, but apparently works. That is 60 ohms in R43 for a 9.7v drop. The fairly similar PV Classic 30, when I worked up the cathode bias mod you can still find over at Blue Guitar, I went with 100 ohms, which I felt a bit more secure with.

So myself I;d still consider 10 watts pretty warm, but certainly within precedent, and not at all "too cool." If it sounds good, and the tubes are not red plating, then I;d have to say your bias was within the reasonable range.

I never really tested it to see what it sounded like at 10.5 watts. I may do that before I do anything else ,and at that level I had no hum which I was getting at 11.8 watts ,and above.
I wonder why they eliminated R43 ,and C29 ,and went with what I now have R77 ,R78 C49 ?

[Enzo]

The difference is it is the same board used in more than one model. Your amp used the board with the fixed bias supply. The other model put in the cathode resistor and bypass cap, and left out the jumpers to the bias supply, and for that matter left out the bias supply itself too. And it was thus a cathode bias model. The board was designed to do both, it wasn;t a matter of they made it one way then changed their minds. Your tubes might be running hotter than we'd like with the amp stock, but both bias systems work, and both models worked.

[gtrplayr1976]

I figured that's what they did. Other than the speakers they should be the same amp. I was just curious as of why they changed it. Isn't important though. I just want to get this one back to good working order. Once I get this bias thing straight I can get back to the issue it was having originally.
Thanks Enzo

[Enzo]

That is the thing, they didn;t "change" it.

[gtrplayr1976]

Oh I know they didn't change the board just those few components.

[g1]

The point (I think) is that it is not a change within a model, rather 2 separate models that use the same basic board, but stuffed with some different components. This is not all that uncommon, and is a lot more economical for the manufacturer.
There is probably another model amp that is similar but cathode biased.
We tend to think in technical terms, but often marketing is also a big factor in the design. Cathode biased can be a trendy buzzword that sells more amps. Covering more angles by having a model for the cathode biased crowd, and another for the fixed biased bunch may result in more amp sales.

[gtrplayr1976]

My cousin has a VC5212. Same amp with 2x12. I would be curious to see the board in that one just to see which it used.

[Enzo]

Yes, exactly. The other model is the VC3112/5212, and that is cathode biased.

This isn;t any different from the Fender amps using the same boards for the Hot Rod DeVilles and Hot Rod Deluxes.

[gtrplayr1976]

Once I get the bias good I want to see why there is channel bleed in this amp. At least that is what he has told me it is doing. I will have to check it for myself to get an accurate description.

[Pedro Vecino]

I doubt that you can raise the bias voltage if TP27 and TP30 indicate same voltage. When I had to adjust the bias in these amps I have always built a new circuit.

[gtrplayr1976]

I doubt that you can raise the bias voltage if TP27 and TP30 indicate same voltage. When I had to adjust the bias in these amps I have always built a new circuit.

Thanks Pedro. By just adding the resistor to R43 it cooled the tubes down from 13 watts to 10.5 ,so I may just leave it at that if it sounds ok.

[Pedro Vecino]

I do not know if we're referring to the same scheme. What is R43? Thanks!

[Enzo]

There are two models using this board. The thread is about a model that grounds the power tube cathodes and has a fixed bias supply. The problem is the bias supply will not go negative far enough, and there is zero room in the existing supply to grow. So the normal options are to remake the bias supply with a voltage doubler, or to add a small aux transformer to make a new bias supply.

But the other model sharing this board is cathode biased. So the bias supply section of the board is empty, and instead of a ground jumper to the tube cathodes, there is a space for a 10 watt R43, the cathode resistor. There is also a space for a bypass cap on the resistor.

Even though the R43 is not used in the thread model, the board is still marked for it.

So just as a simple alternative, I suggested he remove the jumper from across R43 position and install a resistor. 10 ohms was suggested and seems prety close to what we need. So now he has the stock fixed bias PLUS 10 ohms of cathode resistor. That added enough voltage to the overall bias to get him on track.

I have attached the layout for the board. All the parts are shown, regardless of which model it is stuffed for.

[52 Bill]

I have attached the layout for the board. All the parts are shown, regardless of which model it is stuffed for.

They actually have a GOOP guide! And they have standard goop procedures.

There's usually so much hot melt in these things, I figured the workers were paid by the volume of goop they ran through in a day.