#1
I am new to the re-tubing thing, having recently converted to the more expensive, yet even more awesome world of tube amplifiers. However, It's been a little over a year of moderate use, and I know it's about time to change the power tubes. I have an Ampeg R50H, and have no idea IF or even HOW to bias the amp after I change the Tube.

I have read the owner's manual, and have found nothing about re-tubing or biasing the amp. There are no ports to connect a voltmeter to, and no pots to adjust the bias (that I can find anyway).

Also, are there any recommendations for a good matched pair of EL34's? The current ones are sovtek, and I like the tone, but I also know they are the "low end" of tubes, and if I invested so much in buying a great amp, why would I put the "low end" or "stock" tubes in it? I mean REALLY?

That aside, when I re-tube the amp, do I have to bias, and if so, how do I adjust the bias with no pot? Also, I'm told that if you only need to replace power tubes every year or two, and pre-amp tubes will go for substantially longer, is this true?

Thanks in advance for your help.
#2
If you're really not sure what to do then take it to a shop. It's better to fork up the money than screw something up. I've never had to change tubes, but from what I've read you should rebias when you do.
#3
If the manual doesn't say anything about biasing then there is a good chance these amps are either cathode biased or non-adjustable fixed bias. In either case, if that is true, then you don't need to bias it. Just plug n play. I recommend SED =C= or Ruby STRs
Last edited by 311ZOSOVHJH at Dec 20, 2011,
#4
Ampeg schematics are normally pretty easy to find but I can't find one for the R50H.
Bummer. Find a schematic and we can tell you all you need to know.
Gilchrist custom
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#5
Last edited by 311ZOSOVHJH at Dec 20, 2011,
#6
Ok, it looks like it is an adjustable fixed bias. You could do a rough and ready rebiasing by measuring the voltage at the grid and adjusting the bias pot until it reads -34Vdc. Personally I'd still us a bias probe and do it properly.
PS: you have to do this with the chassis out and the amp turned on (with a speaker attached). So you'll be poking around inside on a live circuit containing lethal voltages. If you aren't used to working on high voltages I'd leave it to somebody that is.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#8
Just to be sure, the EL34's should read the same -34 Vdc as the 6L6 tubes? I only ask because the old thread involved a R-212 which had two 6L6's in it as opposed to the EL34's in mine.

I also found a schematic. Here is the link: http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/ampegamps/ampeg_r-12r_212r_50h_reverbrocket_ri.pdf
Last edited by SEALSniper1152 at Dec 20, 2011,
#9
The bias point in these amps is the junction between R39 and R40, and you should adjust the bias to -34 VDC, according to the schematic and according to Ampeg when I talked to them.

The schematic doesn't make a distinction between the EL84s and 6L6s, so I would guess that -34 V is correct in both cases.
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