Hiwatt bias really too hot : best or worth amp ?

[billou_35]

Hello & Happy New Year 2013 full of Hiwatt Stacks to Everyone

I checked the bias on my DR504 '72 with Svetlana Wing =C= and I got 70mA & 80mA @ 414Vdc !!!

I read that an amp too hot is sounding like hell but of course won't last long. Like this, the amp is crunching when dimed but that's not incredible crunch...

My question is : the amp will become a lot less crunchy with proper bias ? I don't have the sag I got with my old '71 DR103 (that I sold) when master pushed in this DR504...

In fact, I found it a bit cold sounding...

I guess I will need Mark Huss voltage doubler circuit to get the proper bias.

[mhuss]

You're right, the tubes won't last long at all at those levels! The good news is, 504s use the Marshall style bias circuit, so all you'll have to do is change a resistor, or add a trim pot if you want it adjustable.
http://hiwatt.org/Layouts/DR504bias.png

--mark

[billou_35]

Wooowww, Mark, the Hiwatt Guru, I'm not worthy !!!
Thank you Mark, I will do very very soon...

Could you please tell me what can I expect having it colder ? Less crunch for sure but maybe more sag ??? Nothing in benefit except longer tube life which is a lot by itself ?!

[mikhailwatt]

Is 414vdc a bit low? Seems to me my '80 '504 2-holer runs at around 450vdc, on 115vac mains.

(Old stock valves bias perfectly in this one with no resistor swaps.)

If my math is correct: 70mA @ 414vdc puts you at about 29 watts dissipation or 83% of max. A bit on the hot side, yes. I like 65-70%.

[mhuss]

I don't think you'll have less crunch or less sag, the Hiwatt power supply is really stiff. Mikhailwatt makes a good point, is that the total current for both tubes? If so, it's a bit high but quite not meltdown levels, particularly if the B+ is low. What is your mains voltage?

--mark