I just bought a Fender twin reverb ri. Its quite old so I replaced all the tubes (pre and power) with a matched set of JJs from Eurotubes.
I set the bias to around 39 - 40 ma. All the tubes were close to each other. Using the amp shortly after I noticed it was very hot to the touch. I rechecked the bias and found that it was around 44 ma but when I left it for a while (15+ mins) it would start to creep up. After a while it reached the mid 50's and was still climbing. I kept backing the bias down but it would keep climbing. I turned off the amp and allowed it to cool before rechecking the bias. I got a reading of 6ma and it soon started to climb back up towards 30 and up.
I switched off and reinstalled the old fender groove tubes and set them to a nominal 30ma. After a while they too started to climb (although maybe a little slower).
To check bias I use a biasrite tool and a fluke DVM set to the 300ma dc range. All tone and volume controls to zero, reverb off, nothing plugged into the input. Speaker plugged in as normal. I leave the amp to warm up for a few minutes before turning on the ht.
First off, is this normal? (I cant see how it can be). If not then what do I need to check? From my rudimentary knowledge it seems as though either the bias supply is unstable or there is a signal coming from somewhere (which presumably is DC or the dvm wouldnt read it - leaky cap?).
Can anyone offer some advice?
Many thanks
Neil
I set the bias to around 39 - 40 ma. All the tubes were close to each other. Using the amp shortly after I noticed it was very hot to the touch. I rechecked the bias and found that it was around 44 ma but when I left it for a while (15+ mins) it would start to creep up. After a while it reached the mid 50's and was still climbing. I kept backing the bias down but it would keep climbing. I turned off the amp and allowed it to cool before rechecking the bias. I got a reading of 6ma and it soon started to climb back up towards 30 and up.
I switched off and reinstalled the old fender groove tubes and set them to a nominal 30ma. After a while they too started to climb (although maybe a little slower).
To check bias I use a biasrite tool and a fluke DVM set to the 300ma dc range. All tone and volume controls to zero, reverb off, nothing plugged into the input. Speaker plugged in as normal. I leave the amp to warm up for a few minutes before turning on the ht.
First off, is this normal? (I cant see how it can be). If not then what do I need to check? From my rudimentary knowledge it seems as though either the bias supply is unstable or there is a signal coming from somewhere (which presumably is DC or the dvm wouldnt read it - leaky cap?).
Can anyone offer some advice?
Many thanks
Neil