Re Bias?

Grand Pappy

Silver Supporting Member
Messages
280
If you replace the matched pair of stock GT 6V6 power tubes in a PRRI w/ a matched pair of JJ 6V6 power tubes do you have to re bias the amp?
Thanks
 

makerdp

Member
Messages
786
Yes absolutely. Always rebias, even if the "matched pair" is rated exactly the same. Note that the JJ 6V6S has different specs than a typical 6V6 so be sure to check a bias calculator that has a specific choice for them.
 

jthomas666

Member
Messages
1,065
I suspect that you will get 2 different answers to your question: "yes" and "no."

In all likelihood you can just replace with the same type of tube without re-biasing. If you look around the interwebs, people note that amps techs over the years have just popped in new tubes and done nothing more. JJ tubes have a reputation of being pretty rugged and unless the amp burned up the last set of tubes, so you are probably ok.

That said, components will drift in value and it is unlikely to do harm if the bias is checked to see if it is within acceptable limits. If you have enough experience with high voltage you can do this yourself. If you are not so experienced, it would be wise to have someone else do it.
 

swiveltung

Member
Messages
14,483
If you buy Groove Tubes with the same rating as the current ones you can probably be fine. That's what their rating system is for... not everyone wants to be a tech. Otherwise... rebias... or let your ears/eyes tell you if it sounds ok and the tubes aren't redplating.
 

Grand Pappy

Silver Supporting Member
Messages
280
Thanks for your answers.
I have no clue how to rebias an amp.
What can I expect to pay an amp tech to do the job?
 

makerdp

Member
Messages
786
You'll just have to call around and ask or ask at your local music store what their guy charges.
 

Muttlyboy

Member
Messages
3,163
If you don't feel ready to learn how to bias your own amp just yet...

Post a new question here and/or in "Amps and cabs" asking for a recommendation for a good amp tech in your area.
 

Blue Strat

Member
Messages
30,722
Does anyone know if the PRRI has a bias pot? If so, it's not too difficult to bias yourself IF you have a bias probe (sold separately).
 

Grand Pappy

Silver Supporting Member
Messages
280
Does anyone know if the PRRI has a bias pot? If so, it's not too difficult to bias yourself IF you have a bias probe (sold separately).

i don't know if mine has a bias adjust pot. It's a new FSR tweed 65 PRRI 15W model / a 12" speaker.
I'll pl the back panel off when I get a chance and check it out
 

Blue Strat

Member
Messages
30,722
i don't know if mine has a bias adjust pot. It's a new FSR tweed 65 PRRI 15W model / a 12" speaker.
I'll pl the back panel off when I get a chance and check it out
I did a quick search and it seems that there isn't one installed. One guy talked about installing one in his amp.
 

zenas

Member
Messages
8,871
Thought the PRRI would've had a bias pot. But I suppose the originals didn't.

Somewhere on the Hoffman amps website there's a page about bias circuits that might be helpful for adding one. I have no idea how easy or hard it is to do on the reissues though.
 

OaklandA

Gold Supporting Member
Messages
392
That's funny. The PRRI I once had definitely had a bias pot. It was from the first year of issue if that makes any difference..
 

Blue Strat

Member
Messages
30,722
Cool! The link I saw mentioned something about mounting a pot on the back panel. Maybe his DID have the pot...or maybe it was a different year's model... I wonder if there are test points.

Most electrical engineers under 70 didn't learn about tubes, or work with them, so your neighbor MAY have to do a little research about biasing.
 

makerdp

Member
Messages
786
I'd be shocked if there wasn't a YouTube video on how to bias your exact amp. Your neighbor will have access to the tools and the ability to follow the video and get it done.

You can sit-in while he does it and learn a thing or two about it too. Then maybe next time you can handle it yourself.
 




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