Dr Z Mini hum?

[chuckb]

ok steve, could you elaborate on why you think that the 180ohm resistor and 25uf/50v bypass cap is within the limit for this amp with the voltage @ 295v and the voltage drop across the resistor @ 9.35v?

[eschertron]

hey steve, i disconnected the bypass cap and checked it and it's the proper value. i changed it to another new cap of same value and the results are as follows; CR= 182.3R, voltage drop across the resistor is 9.35, plate voltage is 294.5vdc. V/R= .0512 which is 51.2 ma. i mulplied 0.0512 by the plate voltage (294.50)=15.1 w which is to high. anything else i can try?

Don't forget to subtract the cathode voltage from the plate voltage (resulting the actual voltage dropped across the tube). See posts #22 and #23.
Also you are seeing the screen current in the reading you get. Take 15% to 20% off the reading for that.
294.5 - 9.35 = 285.15v across tube
285.15 * .0512 = 14.6W
14.6 * 80% = 11.68W
14.6 * 85% = 12.41W
so you are dissipating at the plate 12W +/- a little bit. I'd say play the amp and if it sounds good (and doesn't eat tubes too fast) it's OK.

[Justin Thomas]

One nice thing about EL84s: they are still cheap! Smoke 'em if you got 'em...

Justin

[chuckb]

hey eschertron, thanks for the info it's much appreciated! i'm constantly learning from you guys. to all that chimed in to help me here ,thanks so much. As for el84's i'll smoke em when i got em,lol.

[eschertron]

hey eschertron, thanks for the info it's much appreciated! i'm constantly learning from you guys. to all that chimed in to help me here ,thanks so much. As for el84's i'll smoke em when i got em,lol.

Glad to help. And glad you got the hum problem to go away.

[Steve A.]

ok steve, could you elaborate on why you think that the 180ohm resistor and 25uf/50v bypass cap is within the limit for this amp with the voltage @ 295v and the voltage drop across the resistor @ 9.35v?

Are those the readings you got after replacing the EL84 as per mhuss's suggestion in post #13? At least for me the GT EL84's always drew more current than other tubes which is why I still have a quartet of them that I haven't been able to use in any of my amps.

The AC30 uses a 50 ohm cathode resistor for four EL84's which would work out to 200 ohms for one. 180 ohms is within 10% tolerance of that. I like to run my power tubes a little bit cooler than that so I'd probably go with 220 ohms but that is me... Who am I to second guess Dr. Z?

Steve Ahola