Crate VC5310 power tube failure ,and fuse blown.

[gtrplayr1976]

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.

Maybe where the jumper J03 is ? That's where I've been reading the current. Wait , it IS marked on the board. So just remove the jumper ,and add the resistor ?

[Enzo]

Use a calculator. What is the voltage upstream of R77? Schematic says 15v at TP 27. With R77 at 100 ohms, calculate what the bias would be with the lower resistance at stock 15k. Now calculate it for say 50k. Difference? less than a tenth of a volt. So the stock circuit is as high a voltage as it is going to get. You could eliminate R78 entirely, and still nt get more than a tenth of a volt increase.

[gtrplayr1976]

Use a calculator. What is the voltage upstream of R77? Schematic says 15v at TP 27. With R77 at 100 ohms, calculate what the bias would be with the lower resistance at stock 15k. Now calculate it for say 50k. Difference? less than a tenth of a volt. So the stock circuit is as high a voltage as it is going to get. You could eliminate R78 entirely, and still nt get more than a tenth of a volt increase.

So ,leave it as it came ,or try the R43 fix ?

[Enzo]

JW03? I'll believe you, I don;t see it on my drawing offhand.

The jumper wire will either be across the same gap as R43 or it will be in a different location. Is there a place for a large resistor betwwn the two center power tubes? And the bypass cap that went with it C29 right next to one of the sockets?

[Enzo]

R43 would work as far as I can figure. But so would ordering cooler tubes. Just short of adding a voltage doubler, you won't get cooler operation modifying the bias adjustment.

[gtrplayr1976]

JW03? I'll believe you, I don;t see it on my drawing offhand.

The jumper wire will either be across the same gap as R43 or it will be in a different location. Is there a place for a large resistor betwwn the two center power tubes? And the bypass cap that went with it C29 right next to one of the sockets?

It's J03. Here is a pic. Cap is there too ,or the space is.

[Enzo]

Yes then, my idea would be to remove JW03 and install R43. That space is for 10 watt size. I bet 10 ohms would make sufficient difference, but I'd be interested in other views. One could also make the bias supply adjustable too. not the resistor gets us part way there, and the adjustable bias gets us the rest of the way.

[gtrplayr1976]

Yes then, my idea would be to remove JW03 and install R43. That space is for 10 watt size. I bet 10 ohms would make sufficient difference, but I'd be interested in other views. One could also make the bias supply adjustable too. not the resistor gets us part way there, and the adjustable bias gets us the rest of the way.

So ,leave the existing R77,R78 ,and just add the 10watt to R43 ,or add the pot to R77 ,R78 along with R43 ?

[gtrplayr1976]

Yes then, my idea would be to remove JW03 and install R43. That space is for 10 watt size. I bet 10 ohms would make sufficient difference, but I'd be interested in other views. One could also make the bias supply adjustable too. not the resistor gets us part way there, and the adjustable bias gets us the rest of the way.

OK, I put a 10ohm 10watt in R43. I also put the bias pot in as in figure #1. The resistor dropped the current to 108,and makes the wattage around 10.5. The pot still changes nothing where it is located. Should I put it in series with R43 ,and put R77 ,and R78 as they were ? I also noticed while checking pin 2 that the current changed to 123ma.

[g1]

Enzo has demonstrated that we are at the upper limit of the bias supply range, there is no room to increase it.
So by installing the bias pot you will only be able to increase the idle current, not decrease it beyond where you are now.
To enable the pot to do this, you will have to decrease the value of R78 to 5K as shown in the bias #1 drawing.
If you still have R78 at 15K in series with your bias pot, that is why you don't see any change in idle current.
The change in current when you have your probe on the grid is normal, you would probably hear hum with a speaker connected.

[gtrplayr1976]

Enzo has demonstrated that we are at the upper limit of the bias supply range, there is no room to increase it.
So by installing the bias pot you will only be able to increase the idle current, not decrease it beyond where you are now.
To enable the pot to do this, you will have to decrease the value of R78 to 5K as shown in the bias #1 drawing.
If you still have R78 at 15K in series with your bias pot, that is why you don't see any change in idle current.
The change in current when you have your probe on the grid is normal, you would probably hear hum with a speaker connected.

Thanks g-one. Something else I noticed when I had the bias pot in ,and the 10 ohm 10 watt resistor in R43 there was no hum in the amp. There was hum before. I decided to see if adding a second 10 watt in parallel withe the one in R43 would get me a little hotter. I removed the bias pot ,and added the second resistor. The current came up to 120 which brought the watts to 11.7 ,BUT the hum was back. Was the cooler setting eliminating the hum ? Will the amp sound ok at 10.5 watts? I can try changing the resistor value at R78 also. I meant to do that anyway.

[g1]

Yes, I would think the hum is due to the higher current. Either mismatched tubes at that current level, or the increased demand on the power supply filtering.

[gtrplayr1976]

Yes, I would think the hum is due to the higher current. Either mismatched tubes at that current level, or the increased demand on the power supply filtering.

I think I'll go back to the pot ,and 5k along with the 10 ohm 10 watt at R43, and see what happens. I'll report back. Thanks g-one.

[Enzo]

All these things work together.

Th reason we are looking into R43 is that we cannot make the Bias voltage any higher with the stock bias circuit. Also, unless we increase R77 substantially from its present 100 ohms, the adjustment mod will not do much. But if we alter R77 for that purpose, teh bias gets even hatter, the wrong direction.

The point of adding back in the formerly "missing" R43 was to sorta pre-bias the tubes enough your adjustment would have room to move without burning up the tubes. Or at least we might hit upon a resistor that worked by itself luckily.

If 10 ohms in R43 worked well, maybe 20 works better? Without a larger R77, there is little point in making the bias adjustable. Raise R77 and the adjustable part will work, then whatever we do with R43 is to bring it into line with our needs.

The bottom line is to make this amp run cooler, we need more bias volts than we have. The choices I see are to make a new bias SUPPLY from either a voltage doubler or HEY a new transformer just for that - OR - gain a few volts from a cathode resistor added.

[gtrplayr1976]

All these things work together.

Th reason we are looking into R43 is that we cannot make the Bias voltage any higher with the stock bias circuit. Also, unless we increase R77 substantially from its present 100 ohms, the adjustment mod will not do much. But if we alter R77 for that purpose, teh bias gets even hatter, the wrong direction.

The point of adding back in the formerly "missing" R43 was to sorta pre-bias the tubes enough your adjustment would have room to move without burning up the tubes. Or at least we might hit upon a resistor that worked by itself luckily.

If 10 ohms in R43 worked well, maybe 20 works better? Without a larger R77, there is little point in making the bias adjustable. Raise R77 and the adjustable part will work, then whatever we do with R43 is to bring it into line with our needs.

The bottom line is to make this amp run cooler, we need more bias volts than we have. The choices I see are to make a new bias SUPPLY from either a voltage doubler or HEY a new transformer just for that - OR - gain a few volts from a cathode resistor added.

If I increase R77 what would be a good starting be ? By adding the resistor to R43 it did bring the current down to 107 ma total which made the wattage around 10.5. Is that too cool ? Thanks enzo