• TGP is giving away a Strat, Tele, and Jazzmaster. Click Here for full details.
    Click Here to upgrade your account and enter today!

Hiwatt fixed bias

Echoes

Senior Member
Messages
6,220
Any tips on replacing tubes for my 78 DR504 50 watt head? The amp has no adjustable bias pot. I was told NOS EL34 Mullards or matched set of tubes that can take higher plate voltages....is this correct?
 

fusionid

Member
Messages
922
shoot Tubedepot or Dough an email. Those being my prefer providers. I have a DR504 clone with bias adjustment so can't really help you there but I can tell you I prefer the sound of the winged Cs in this amp.

Best place to check for details being vintage hiwatt restoration forum.

I don't believe the plate voltage is too high for current production ie about 500 volts DC per my readings. I believe EL34s are ok with place voltages exceeding 550v DC (do not quote me on this though). However the problem to my understanding has to do more with the fixed bias (resistor as opposed to a pot) setting which is simply set too high.
 

zenas

Member
Messages
8,263
Long gone now but on my 80ish DR504 I put a bias pot in one of the extra power tube holes. Just sandwitch some fender washers to hold the pot.
I don't rember now how the hell I hooked it up but when I sold it I removed the "mod". If I rember right it was just two wires tack soldered.

Any halfassed tech should be able to install one for you. I'd been working on amps for maybe a year when I did mine. (and that was before the internet)
 

Echoes

Senior Member
Messages
6,220
Tried a bunch of different EL34 tubes and all read between 65 ma and 80 ma....roasting hot! Does anyone know a good tech I can send the amp to have a bias pot added to the circuit?
 

phsyconoodler

Member
Messages
4,315
No it's not an easy addition at all.The stock bias supply can only provide about -37v of bias voltage.The only way is to build a new bias supply with a doubler so you can get the needed negative voltage so you can use modern tubes.
Not hard to do,but not easy either.You need to add a full wave bridge to the existing bias tap so you can get -60 or -70v to get new production tubes into the ballpark current-wise.
 

mark norwine

Member
Messages
17,150
you don't have to do that....

You could just build off the HT winding & use an aggressive divider to put the B- where you need it to be.

Look at a Princeton Reverb.....that's how it's done.
 

gldtp99

Member
Messages
3,737
No it's not an easy addition at all.The stock bias supply can only provide about -37v of bias voltage.The only way is to build a new bias supply with a doubler so you can get the needed negative voltage so you can use modern tubes.
Not hard to do,but not easy either.You need to add a full wave bridge to the existing bias tap so you can get -60 or -70v to get new production tubes into the ballpark current-wise.
What you're saying is true about some DR103's--- But the OP's DR504 is a different story----- the DR504 uses a full wave (not bridge) rectifier and the bias supply is taken from one high voltage leg of the center tapped HT secondary---- so there's no lack of potential for the bias circuit.
DR103's use a fullwave bridge rectifier, and have no HT center tap------ they use a separate PT bias winding that is often too wimpy to do the job with today's EL34's-----this model is where the bias voltage doublers come into play in Hiwatts......................gldtp99
 
Last edited:




Trending Topics

Top