Musicians Roadhouse

I can get trim pot2 to read 76mv but i get an OL reading on my DMM from trim pot1. I can adjust trimpot1 to get trim pot 2 to read differently. What is happening?

Set your meter to DC volts and check it . Sounds like a possible bias resister blown .

WaRLaX wrote:I can get trim pot2 to read 76mv but i get an OL reading on my DMM from trim pot1. I can adjust trimpot1 to get trim pot 2 to read differently. What is happening?

What JMD to you have 50 watt or 100 watt? If it's a 50, swap the power tubes and see if the problem moves to the other pot. If it's a 100, swap the left pair of tubes with the right pair of tubes and do the same. If the problem does move to the other pot, you may have a bad tube. If not, something else is wrong (fuse, resistor...). Oh, and yes, the pots do affect each other.

It is the 100w head. I am going to ask a dumb question here but which two tubes are paired? The two right next to each other as in V2 and V3 or would it be every other one as in V2 and V4?

Usually outer tubes are being paired (1&4) and so inner tubes (2&3)
I don't see why would JMD be different. But I could be wrong

sikter wrote:Usually outer tubes are being paired (1&4) and so inner tubes (2&3)
I don't see why would JMD be different. But I could be wrong

That is correct the inner 2 are paired and the outer 2 are paired. This does not mean that they share the same bias pot, they don't. The two left tubes are on one bias pot, and the two right tubes on the other. That's why I suggested swapping tubes left to right, to see if the problem moves to the other bias pot (follows the tube).

Ok I swapped the tubes and still the same problem with an OL reading on pot1. So I put them back where i originally found them and set the one pot i could read at 76mv. I forgot to mention that the amp sounds great and pushes loudly. I just wanted to bias to make sure I wasn't going to eat my tubes up. The fuses are intact so it seems i need to save a few bucks and take it to an experienced tech.

One point to remember: when biasing in pairs any increase/decrease on one pair will affect the other. As current increases/decreases as you raise/lower the bias voltage for one pair this affects the B+ voltage for ALL 4 tubes, so it changes slightly on the other pair.

Ohm's law: When current increases, voltage decreases. In this sense when you raise/lower the bias current on one pair you are raising/lowering the plate voltage on all 4 tubes, which affects the bias on the other pair. That's why you have to go back and forth to balance it out on those setups.