Crate Stealth GT50 bias help

[rockon1]

Ive got a Stealth GT50 combo. With the EH 6V6's I can only get the bias up to about 12ma. The Plate voltage is 390. To get more toward an acceptable 60% I need to get them around 18ma. Im thinking all I need to deo is bump up the bias resistor value? If somebody could possibly point it out with a suggested new value on the attached schematic that would be great. Thanks,Bob

http://music-electronics-forum.com/a...5&d=1218189291

[Mars Amp Repair]

Hello Bob,
If your bias current isn't high enough, you would need to reduce the bias voltage on the grids of the power tubes.

You would first need to verifiy that the filters in the bias ckt are not loading it down (unlikely anyway) and that the resistor values there are good.

Then decreasing the value of R95 would there by load the bias supply more through the bias adj pot.

I'd just trial & error adding a resistor in parallel with R95. Start with say a 220K or 150K. Center the pot & try to find a value that sets the bias correctly at that point on the pot.

If you start getting much lower that 100K & your bias is still not high enough, you might try increasing the value of R94 instead as you don't want to load the bias supply & get too much current through your bias pot.

I have to think if you're as close as 28ma with pot fully CW then you'll be able to increase the range by bridging R94.

Glen

[rockon1]

Thanks I only want to get it to around 18 ma so I will try a minimal change of adding the 220K first I guess. Thanks again Bob

[rockon1]

The board wasnt marked so I had a bit of a struggle figuring out which resistor was R95. It dawned on me that the one connected directly to the bias pot was it. I soldered in a 220K resistor (the butcher way above the board)and with the pot fully hot got to 18 ma. Good enough! Thanks Bob

[David Ray]

Crate Stealth GT50H Bias Settings?

You're looking at 18ma for your bias?. I have around 400volts at pin three and the lowest I can get is around 28ma. The schematics state to add a sine wave to the effects return and an 8 ohm load on the speaker output. It also states that the proper negative voltage should be between -34 and -37. Am I off base here? Can someone help me?

[rockon1]

You're looking at 18ma for your bias?. I have around 400volts at pin three and the lowest I can get is around 28ma. The schematics state to add a sine wave to the effects return and an 8 ohm load on the speaker output. It also states that the proper negative voltage should be between -34 and -37. Am I off base here? Can someone help me?

The EH's I was using ran pretty cold-I guess. I simply use a bias probe and adjust the bias to a suitable ma range. 12/400= .03 x 60% =.016 or 18 ma. so Im idling at 60%. Ive since switched to running 2 6L6's instead since I ran out of 6V6's. I donr know about the sine wave thing-maybe Im missing something but I cant imagine what. Bob

[David Ray]

Just curious

I was just curious about that. I had the Lee Jackson video for the gt50 and he adjusted the bias by checking the resistors near the ground nut of each power tube. He also had his at around 26ma I believe. I may be not checking things right. I read somewhere that 6v6's with over 375volts at pin three would be more in line with your 18ma, but my bias adjustment pot won't go below 27 or 28. Bias is confusing to me as there are several different methods to use. Sorry for the confusion, it's probably my bad...I'm like a mad scientist with these things at times.

[rockon1]

I was just curious about that. I had the Lee Jackson video for the gt50 and he adjusted the bias by checking the resistors near the ground nut of each power tube. He also had his at around 26ma I believe. I may be not checking things right. I read somewhere that 6v6's with over 375volts at pin three would be more in line with your 18ma, but my bias adjustment pot won't go below 27 or 28. Bias is confusing to me as there are several different methods to use. Sorry for the confusion, it's probably my bad...I'm like a mad scientist with these things at times.

Simpelest wayu to do the math-
1)Know how many watts your tube(6V6,6L6,EL34 etc) is capable of producing.A 6V6 is a 12 watt tube.

2) Take that # and divide it by the plate voltage. That will give you the ma at 100% dissapation. Typically for a fixed bias amp you want to be in the 50-70 % range of that number so...

12watt tube/ 400plate voltage = .030 or 30 ma. max dissapation

3) Take your max dsissapation number and X by your target diasapation

30 X .6 =18 ma.

I dont know why he has the number at 26 ma. Bob

[David Ray]

I see what your saying... that makes more sense. The board has 20ma written in ink on it near the bias pot. I think my bias set-up is wrong. How can I check the bias and see if I need to change a resistor or something to bring the bias in line. I'm sure that having mine so high has attributed to the harshness of the amp somewhat.

[rockon1]

I see what your saying... that makes more sense. The board has 20ma written in ink on it near the bias pot. I think my bias set-up is wrong. How can I check the bias and see if I need to change a resistor or something to bring the bias in line. I'm sure that having mine so high has attributed to the harshness of the amp somewhat.

Not sure but I think your reading the negative bias voltage not the actual current draw. On the schematic I have it says set the bias voltage between -37 and -34 volts. Thats not the same as your idle current in ma. The easiest way I know is to have a simple bias probe. Bob

[David Ray]

You're right. I am reading the negative bias. I don't have a bias probe, but I don't know how to read the current draw or ma. Any suggestions?

[rockon1]

You're right. I am reading the negative bias. I don't have a bias probe, but I don't know how to read the current draw or ma. Any suggestions?

Well set it to the suggested spec and later on get a bias probe. They are cheap for a simple single unit type. Bought mine for 25$. Putting 1 ohm resistors on pin 8 on an amp with pcb mounted sockets(which is what most probes do without the fuss) seems like too much of a hassle. There are other methods like the transformer shunt method Im not keen on. Bob