Need BIAS for HXDA Retube

[Burtrola]

My HXDA needs new tubes. Started sounding bad with my Les Paul bite sounding something like a trumpet. Just lifeless otherwise. Happened fairly quickly, though I did notice a slight hiss that would appear and disappear over the last month or so. Anyway, I assumed it was the power tubs and got some Mullard el34's from tube-store online. They just came in finally. (Hope it was the right choice).

Anyway, does anyone know how to bias one of these guys? I checked the manual, but it does not tell you the recommended bias settings. Guessing around 30? This will be my first re-tube/bias and don't want to screw up my glorious amp.

Thanks in advance for any help.

 

[Bluesboy998]

30 to 35 MA is safe

 

[Burtrola]

30 to 35 MA is safe

Great thanks! Just to taste I guess? How much difference will 5ma make?

 

[Bluesboy998]

The tubes will run a little hotter at 35 some people prefer that I run mine at 35/37

 

[László]

I was told about 35 is a good setting.

 

[justmund]

Was looking for pics for another reason and came across this

So looks like 30mA is the go (and interestingly, PRS say use different tubes in there if you wish)

 

[Burtrola]

Thanks everybody. I will try different amperages and see how it sounds. Why is the bias adjust a 1/4" plug? Do I need something special to make the adjustment? Assumed there would be a dial or something like that. Was going to buy a MM today at lunch. Guess I need to research on YouTube first.

 

[Bluesboy998]

That's not a 1/4 inch plug that's the slot for the screw driver

 

[Burtrola]

That's not a 1/4 inch plug that's the slot for the screw driver

Thanks. Have to say that had me a little confused. D'oh!

 

[Bluesboy998]

Ok you take off the rear panel plug in your speaker don't put anything in the front 1/4 jack NO GUITAR remove old tubes install new tubes plug in your voltmeter black plug in the middle leave it there.turn amp on stand by plug in red probe left for left tube right for right tube doesent matter which tube you bias first.Turn amp on full power set voltmeter install screwdriver and proceed to turn left or right until you get the proper reading 30 ms for instance when you are satisfied remove probe and install in the other port repeat process good luck you should be fine.

 

[Burtrola]

Ok you take off the rear panel plug in your speaker don't put anything in the front 1/4 jack NO GUITAR remove old tubes install new tubes plug in your voltmeter black plug in the middle leave it there.turn amp on stand by plug in red probe left for left tube right for right tube doesent matter which tube you bias first.Turn amp on full power set voltmeter install screwdriver and proceed to turn left or right until you get the proper reading 30 ms for instance when you are satisfied remove probe and install in the other port repeat process good luck you should be fine.

Awesome thanks. Pretty much what I was thinking. Just didn't know where the trim-pot was. BTW, how does the trim pot know which tube to bias? By which jack I plugged in? My MM only came with standard leads. Should also get some banana clips?

 

[Bluesboy998]

Yes by which jack Is plugged in if your meter has the pointed probes they will work fine

 

[Burtrola]

Awesome! Great to have the extra info so very much appreciated. Have not seen what the factory put in there yet, but hope these Mullards do the trick.

 

[Bluesboy998]

I use mullards in my Marshall very reliable I think they use jj's Iam not sure some amps came through with different tubes

 

[justmund]

I believe factory is JJ pres and NOS Svetlana Winged C's for power tubes