Fixed Bias 2-EL84 amp suggestions needed for tube matching

59Vampire

Platinum Supporting Member
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4,576
I have a boatload of anos and nos el84s that I need to match. My previous matcher was an old bedrock but I blew it up a while back and stopped matching.

Can anyone suggest an inexpensive amp that fits the bill that I can cannibalize for testing purposes?

Thanks.
 

pdf64

Member
Messages
8,929
Change any cathode biased amp to fixed by swapping the cathode resistor/s for zener/s, and then add a couple of 1 ohm cathode current sensing resistors.
The zener/s would need a similar dissipation rating to the resistor/s they replace.
 

59Vampire

Platinum Supporting Member
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4,576
Thank you but I am not capable of altering anything! Using a bias probe is one thing.......
 

59Vampire

Platinum Supporting Member
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4,576
I have read that these amps run tubes very hot. Would that be an issue when matching tubes? Keep in mind I am not a tech but can read a meter with the bias rites that I have. Thanks.

Can anyone build a testing rack?
 

Blue Strat

Member
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30,686
I have read that these amps run tubes very hot. Would that be an issue when matching tubes? Keep in mind I am not a tech but can read a meter with the bias rites that I have. Thanks.

Probably not an issue for a one minute, or less, test since these amps seem to run for 100s of hours as is.

Can anyone build a testing rack?

Of course, but I can almost guarantee you won't like the R&D, build, and test cost.
 

59Vampire

Platinum Supporting Member
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4,576
Inexpensive? Lol. Certainly not 500. I just need something to match tubes. I saw pro juniors going for 250ish.

I just need a functional amp that will not crap out and cause me to spend money to have it fixed. I just need to match a large amount of nos and anos el84s I have been hoarding
 

Jeff Gehring

Gold Supporting Member
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7,430
So would a fixed bias single-ended little power section work for you? Will you basically just be loading a tube in it, then checking where it biased out? I'm thinking something could be put together from junk drawer/spares parts that would serve for that purpose pretty cheaply. So all you'd really need would be a power supply and the output stage?
 

JJman

Member
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994
Epiphone valve jr. It is SE and less than $100 used. Just use a good voltmeter and measure the cathode dc once warm for a while. Tubes with the same cathode dc in there should be dc matched in a PP amp. If you use a PP amp for matching, you have to leave one tube in side A, changing out the tubes in only side B each time. Otherwise you're changing an extra variable. Either approach should work for matching.
 

59Vampire

Platinum Supporting Member
Messages
4,576
Jeff and JJman,


I dont think a single ended amp would work for matching. I am not a tech, but my research in the past, and in all fairness, I cannot remember the specifics, told me that in order to accurately match by plate current I would need a fixed bias 2/4 tube amp. I previously used a bedrock four el84 amp but blew it up a few years back and sold it off as a project.

I am under the imnpression that running a single tube does not accurately reflect the measurements needed for matching.

PLEASE correct me if i am wrong beefore i buy an amp. I have a vht with the el84 converter i can use but again, I do not think that is the way to go unless i am wrong. Which of course wouldnt be the first time!
 




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