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tube problem

tweeks5

Member
Messages
20
hey guys, i have a quick question that i think some of you may be able to help me with...
i recently upgraded my peavey classic 50 212 with new power tubes and speakers. i went from the stock sovtek el84 power tubes, which admittedly didnt sound too bad to jj el84s, which ive heard great things about. i was dissapointed when i chucked the new tubes in and switched over to the distortion channel: it was really thin and unpleasant sounding. i would have thought that new tubes would make the amp sound better, because before these tubes the amp sounded reasonably good. do you guys think it might be a biasing issue, as the peavey classic 50 is fixed bias and i thought it would run well on any type of tube?
the price of tone = many headaches and many dollars!
 

tweeks5

Member
Messages
20
its quoted asbeing non adjustable fixed bias, ive heard its biased very cold and thats why i get the ****** distortion, as cold biasing creates crossover distortion, at least thats what ive been told
the amps uses el84 tubes.
 

TweeDLX

Member
Messages
3,756
its quoted asbeing non adjustable fixed bias, ive heard its biased very cold and thats why i get the ****** distortion, as cold biasing creates crossover distortion, at least thats what ive been told
the amps uses el84 tubes.
Non-adjustable fixed bias just means you have to change components (bias resistor) to change the bias. There are no means of manually adjusting the bias settings like with an exterior or interior pot.

Mike
 

PRNDL

Member
Messages
497
How does it sound on the clean channel?

I believe the distortion adds a boost without tone controls, which means its tone depends on the clean channel.

I had the same (sort of) issue with a Peavey Delta Blues - a new set of tubes (JJ power and EH preamp) sounded a bit thin, but it did have a lot of punch and clarity, which I thought would make it cut through the mix with a Marshall vibe. Turning down the treble helped, but hitting the preamp boost button really opened the middle.

I thought the JJ's really let the guitar tone shine through, which is a bit on the harsh side for those used to a smooth blues tone.
 

59Vampire

Platinum Supporting Member
Messages
4,439
Fixed bias means that you DO have to rebias.

Is that EL84s or EL34s?

This brings up a situation I am trying to handle. My 18 watt clone uses el84s. I can pop any tube in it i want without doing anything. I was told it was cathode biased. I was thinking about getting one of those current measurers that plugs into your amp socket hooks up to a multimeter and then you plug your tube in. The guy who sells them told me that I cannot do it in this amp. It needs to be a fixed bias amp.

Soooooo, is a single ended amp like the EPi Valve Jr fixed bias? If it has only one tube what is there to bias? Would a single ended amp give accurate measurements with that type of device?
 

TweeDLX

Member
Messages
3,756
Soooooo, is a single ended amp like the EPi Valve Jr fixed bias? If it has only one tube what is there to bias? Would a single ended amp give accurate measurements with that type of device?
Even a single tube would need to be biased. Simply put, biasing is adjusting the plate voltage/cathode current ratio (dissipation) to conform to the tubes specified operating range. I'm sure there are others around here who can (and probably will) give you a better explanation. Bottom line is, yes, you have to bias even one tube. If the device in question measures both voltage and current, it should work on any single-ended amp with a fixed bias.

Mike
 




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