Dr Z Mini hum?

[chuckb]

hey guys, thanks for all your help. I switched out the filter caps and problem solved. Now i have the tube running @ about 12w and i know it's the maximum dissipation for an el84. It was the highest value of cathode resistor i had on hand.I'll adjust the bias when i get the new resistor. The tube is a GT el84s and i'm wondering if they can handle more than 12w? i know some brands people run up to 14w, won't last long though i guess. once again thanks for the input, it was greatly appreciated.

[g1]

Glad to hear you got it working. For future reference:

not to sure how to measure the ripple.

In your post #4, you measured the DC voltage at the filter caps. Set your meter to AC volts and measure the same places. The AC voltage will be the ripple.

By the way, has the DC gone up with the new filter caps?

[chuckb]

Hey G1,yes the DC has gone up. Thanks for the heads up on how to measure the AC ripple.

[chuckb]

hey g1 i have a question, if the caps fail in my cathode biased amp for some reason, why then when with replaced identical caps do i have to re-bias? shouldn't the bias be the same. The reason i'm asking is because i just replaced all three filter caps in my amp and the bias was way high forcing me to change out the cathode resistor to a larger one to bring the bias down. is this a common thing to do after a cap change? and why does that happen/

[Dave H]

why then when with replaced identical caps do i have to re-bias? shouldn't the bias be the same./

The bias with the new caps should be the same as the bias was with the old caps before they failed. From the voltages on the schematic the EL84 is running at 13.9W total dissipation (plate +screen). Knock off 15% for screen dissipation and the plate dissipation is 11.8W. It's normal to run single ended class A amps at close to 100% PD as the PD goes down when a signal is being amplified. If your bias probe measures cathode current (plate + screen current) then you should be OK.

[chuckb]

ok here's what i got.Voltage across the bias R is 180ohms, voltage drop across that resistor is 9.28v V/R=9.28/182.7 =0509 which is 50.9ma, 50.9 ma x295.7 plate voltage =15 ? is this to high?i also change the grid resistor to a 2.2K ,it was the closest i to the 2.7 on the schematic. the other schematic posted is a 5.6K. will this make any difference to my bias readings/

[g1]

The grid resistor should not affect the bias.
So your question about the increase in bias is because you measured 45mA before, and now it is 51mA?
Have you subtracted the cathode voltage from the plate voltage in both cases for your calculations?
And also, the 45mA was measured with the bad filter cap, so we don't really know what the bias was before the cap went bad?

[chuckb]

hi g1, didn't know i had to subtract the voltage drop across the cathode? i did it and it gave me 14.5 watts, which is still to high for a el84. i need to get the bias down any suggestions? this bias is way to high, i need help in getting this down without increasing the cathode resistor.

[chuckb]

somethings causing the bias to be so high(15w). with new filter caps installed, it must be something else causing this high bias?

[The Dude]

It's not uncommon in many of the newer EL84 amps for companies to run the hell out of tubes. I've changed more cathode resistors in recent years than all the others put together. I'd be willing to bet the amp was "over-biased" from the beginning.

[chuckb]

ok, sounds like it to me as well. is there anything else that could possibly cause this other than being biased to high from the factory? I contacted Dr Z and they told me it was way to high but also said they bias their amps correctly.

[Steve A.]

With an 180 ohm cathode resistor and 22uF/50v bypass cap the bias should be well within limits of EL84. Have you confirmed the value of both components? I would try disconnecting one lead of the cap...

Steve Ahola

[chuckb]

ok steve i'll disconnect the bypass cap and check to see if it's the right value.

[chuckb]

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?

[chuckb]

i have no other choice but to go with a higher cathode resistor and see if that does it. Now i have to determine the value of the resistor.