4145 JCM800 Club and Country, 100W 4x10" combo

[gbono]

Just finished getting this amp back together and with a new set of JJEL34's I'm getting half the static plate current then the original Mullards (there were only two original tubes with the amp). How to modify the bias since I'm only getting about 12-15mA per tube (470VDC plate voltage)? Crossover distortion is noticeable even with bias pot turned to max.

FWIW This is basically a JCM8000 in combo format - schematic here: http://drtube.com/schematics/marshall/4140.gif

[Pedro Vecino]

You must reduce the value of the resistor in series with adjustable resistor. If it is 56K, for example, you can install on it a 220K paralleled to get 45K approximately.

[1ampman]

Bias is negative volts....Too much neg voltage cuts the current draw in the power tube. It seems your getting too much neg volts and the power tube is not conducting (drawing enough current and your getting crossover distortion) . You need to cut back on the bias voltage by adding or enlarging the resistor after the bias supply diode. Or install a higher value bias pot. Pedro has it backwards. He's increasing the neg voltage. Look at other amp schematics to get some ideas...

[gbono]

Understand the device physics already just looking for some advice from someone who has experience with resistor change for the current EL34 (JJ in this case) and this series of amp. I'm going to use a decade box to find the closest value to get a decent plate current and be done with it........

[g1]

You need to cut back on the bias voltage by adding or enlarging the resistor after the bias supply diode. Or install a higher value bias pot. Pedro has it backwards. He's increasing the neg voltage. Look at other amp schematics to get some ideas...

No, Pedro does not have it backwards. He is decreasing the resistor in series with the bias pot, which will make the bias voltage less negative and increase idle current.
Installing a higher value bias pot is backwards. You need to lower that resistance, not increase it.
Increasing the resistor in series with the bias diode is a valid suggestion.

Those are the 2 best ways to make the idle current range hotter, either increasing the resistor in series with the bias diode, or decreasing the resistor in series with the bias pot.

[gbono]

If you look at the schematic - very poor reproduction on the site - you can barely make out a range of values (R43?) down to 15K - the magic number for the batch of JJ's, in the amp now, is 33K ohms.

[Pedro Vecino]

In these cases manipulating the input resistor of the bias circuit to reduce the voltage range at the end may be more dangerous than manipulating the resistor in series with the adjustable resistor. The reason is that the first one has to be lifted and that means removing or cutting it necessarily to raise its value while in the second it has to be reduced and can be done by installing one in parallel.
The difference is that if the connection is interrupted by a defect in the work the first formula can be catastrophic while the second simply misalign the circuit to the safe side.