Trying to find replacement valves for an oldish Musicman amp

Mr songwriter said:
Yes, that makes sense, I think I will go for a matched pair.



What does getting my bias set involve? would I be better off getting a qualified amp tech to do it?
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Some amps have adjustable bias, some don't. If yours doesn't, then it's a non-issue, but if it does, it really should be set every time you change power tubes.

Setting the bias determines the operating range of the power tubes, and it makes a pair of tubes in push-pull smoothly hand off the switchover from one to the other for the positive and negative sides of the output waveform. An incorrect bias can give you a flat, distorted sound and/or burn out the power tubes very quickly.

Every individual tube has its own "comfort zone" that is determined by bias voltage, and a set of power tubes shares one bias circuit, so that is why using matched pairs (or sets, if more than one pair is in the amp) is important.

A tech will use an oscilloscope and a calibated dummy load to set the bias, so unless you have that stuff and know what to look for, I'd send it to a tech.
 
Rockdoc & GGunn: thanks for the info, I've just bought a matched pair of Marshall EL34's, so I'll try them out and see if I'm getting the flat, distorted sound you describe, if so, I'll get a tech to do the bias.
 
After all this MM talk I went home last night and proceeded to try and record with my MM hd12. Good God almighty is that thing loud! I had my monitors pushed and the amp was still way overbearing. I look at the master volume and it was on 2! This thing is silly loud, but sounds wonderful once she gets hot. What a score for $400 Canadian.
 
Mr songwriter said:
Rockdoc & GGunn: thanks for the info, I've just bought a matched pair of Marshall EL34's, so I'll try them out and see if I'm getting the flat, distorted sound you describe, if so, I'll get a tech to do the bias.
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OK, but an incorrect bias setting can also run the tubes too hot, which might sound good to you but can burn up your new power tubes pretty quickly. A bias set by a tech is not that expensive. When last I changed tubes on my old Super Reverb, I saved a few bucks by pulling the chassis and just bringing that in.
 
ggunn said:
OK, but an incorrect bias setting can also run the tubes too hot, which might sound good to you but can burn up your new power tubes pretty quickly. A bias set by a tech is not that expensive. When last I changed tubes on my old Super Reverb, I saved a few bucks by pulling the chassis and just bringing that in.
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Cost of new set of tubes = £20 - £40, which is roughly the same as what I would expect the tech to charge me, so I'll have to think about that one, though I do like the idea of taking the electric bits out of the chassis.
 
Thanks, Mr ggunn.

I had the MM site bookmarked before my hard drive decided to bail. Looks like it has been expanded quite a bit.
 
Thanks for the link, they seem to recommend 6CA7's over EL34's for MM amps, I'll try and remember that next time I buy a set. I put the EL34's in and tried it out and it did sound quite a bit better, definitely sharper, clearer and more consistent. The volume fluctuations I was getting with the bright switch on seem to have disappeared too. I haven't really cranked it up yet, but it doesn't sound as though it needs biasing (I *hope* - I'll see how long these tubes last)

As people have said, these amps aren't really known for a heavy metal sound, but I can get quite a reasonable one out of it if I use my EH Big Muff with it.
 
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