Help! What could possibly be wrong?

[kevinkr]

Originally posted by sonata149
Hello again! I wish to thank you all for your suggestions. I have today fixed the problem and as some of you asked me to report on the fix, here I am.

After almost losing all hope of getting the 2 amplifiers to work I did one last thing which I didn't write about because I considered it immaterial. At the input of the first stage (ef86) on the circuit (go to 1st post link) is a 1M resistor. I had earlier connected a 120K across it to bring close to 100K impedance which I had often seen on modern valve power amplifier circuits.

Bringing back the 1M input impedance solved the problem and now I'm listening to it again. However I still cannot understand why this should have created the problem - no sound. Can some of the more knowledgeable members of the forum explain, please.

Thanks for being with me throughout.
Joe A

You attempted to modify the design without understanding the relationship between plate and screen current in the EF86 - too bad you didn't mention this sooner, a lot of people here could have warned you of the consequences. Adding the resistor you mention increased the screen current by an order of magnitude and as a result the EF86 probably saturated - alternately all of the plate current was diverted to the screen, either scenario would lead to little or no signal at the plate. (Without actually probing the specific circuit I am not sure exactly which of these two scenarios is the case.)

[sonata149]

Thanks kevinkr for your feedback. I didn't realise that the 1M resistor between the grid and 0V of the first stage was that important. I've seen many circuits with a 100K resistor. (BTW if I wanted to insert a volume control does this mean it has to be a 1M pot?)

The EF86 is connected in triode mode (screen connected to plate). I still don't understand why the change in the input resistor should produce a no-sound situation. Athough I understand that there is some interaction between the electrodes in a valve, I cannot see why this happened.

Maybe you can help me understand this. Thanks & Regards.

[kevinkr]

Color me confused, I did not realize until I just read your response that you had triode connected the EF86, so I am not sure what you did with R6.. Changing R1 as you proposed should have been ok. You might want to recheck your work and make sure what you think you have done is actually what you have done.. I've done this more than once..

Can you measure the voltage present at the grid with the 1M grid resistors, and what about the pre-amp driving this - cap coupled tube with dc resistance to ground present at the output of your pre or source.

I am thinking that there is some grid leak bias going on currently or something else odd like dc on the output of the source. The operating point of the EF86 is shifted by triode connection, and other component value changes for optimum performance might be necessary. I'd put it back the way it was, measure cathode, plate and screen currents (determine by voltage drop across resistors) and then reconnect in triode mode and adjust the cathode resistor until the plate current is about the same. (That's the best I can come up with for now.)