Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chip on April 21, 2010, 09:03:23 pm
-
I would like to put in 1 ohm resistor for biasing.
Do I have to re-route the plate diodes directly to ground (#1)?
Or, can I leave them where they are (#2) and just add the 1 ohm?
-
#1
Why not install 1Ω resistors for each tube?
-
Since you only want to measure cathode current I don't see why you have to tamper with the diodes at all. Just install the one ohm resistor in the cathode ground line and be done with it. I usually install a separate one ohm resistor in each cathode just so I can monitor each tube separately but since your bias isn't individually adjustable for each tube it's not a big deal.
I see a 1 ohm 10W resistor in your diagram. I would just use the standard 1 ohm 1W 1% resistor that Doug sells. It's only drawing milliamps through that resistor. 10W is overkill to me. These resistors should to be precision tolerance for accuracy's sake. If you have sourced 1 ohm 10W 1% resistors then disregard this part.
-
I bet the diodes are mounted directly on the tube sockets between pins 3 and 8. If so, when he puts the 1Ω between pin 8 and ground, he'll want to move the diode anode to another ground since pin 8 will no longer be connected to ground..
-
It makes very little difference to the diodes. They set some limit to the flyback spikes when the OT is overdriven. If B+ were 500V and the OT were perfectly balanced, the flyback could not exceed 1,000V, so the OT insulation could be "1000V" and never break down (except it aint that simple). One Ohm and a half Volt on the bottom doesn't really change anything, maybe 1,002V kicks. There's much more slop in a real OT than that.
As Slucky says, it is probably more about clean wiring than about electricity.
-
Yes, currently the diodes are mounted directly from 3 to 8 on the 6L6s. I forgot to mention that "A" gets 500V and "B" gets 490V. This amp was a 100W with 4X6L6s w/ solid state preamp but I am converting it to a 2X6L6s w/ tube preamp, need tone not loudness.
The Peavey SR-2873, shown in the above pic, is a 2KV PIV 250mA fast recovery diode. I was worried about over powering the 1 ohm resistor when the diodes are protecting against flyback spikes. That’s why I originally put in 10W just to be on the safe side, but not really knowing what I could expect from the flyback spikes I thought I should ask you guys.
But it sounds like I should be ok if I just leave them mounted directly from 3 to 8.
Thanks for the help guys.