granger.frederic said:
1:clean all the HiZ parts first with isopropyl alcool
2:measure the capsule polarization voltage , should be around 60V
3: measure bias , should be around -2,2v (before 220MOhms)
4:measure anode voltage after 47K : should be around 75V with EF12
ifyou have bad measure, then :
2:HT is the culprit, or you have to avoid 80V (1/2,2mohms) if you choose it before ... return to 1/1Mohms, or 0,01uF is leaking...
3:heater psu is the culprit (-6v should be stable) , or divider bridge contains wrong resistor values , or 100nF is leaking...
4:tube is bad if psu is good
check also twice the transformer wiring
but 7:1 ratio is perhaps a bit low for an EF12 ...You could try to wire primary sections in paralel
regards
Fred
Just address the points, Fred...
1) cleaned all Hi-Z multiple times very well.
2) polarization V is around +77V or so. Schematic calls for ~80V
3) Bias. Hard to tell on this. Measuring immediately prior to the 220M resistor (I am using a 200M instead) I have about -1.1V. After a 10M resistor I have -2.1V. Not sure which is the correct point to measure, but I can't make the point just before the 200M resistor to be -2.2V without drastically changing the values of the voltage divider resistors. Makes me think something must be wrong, or I am measuring it correctly after all.
4) Anode voltage is around +72V
5) What is HT? I am using a voltage divider per schematic of 1M/2.2M. The .01uFuF is brand new Mundorf
6) PSU puts out a very stable -6V; it's built by a 3rd party vendor. Divider bridge has right resistors. 100nF is a brand new V-cap teflon.
7) Have tried 3 different tubes; all the same problem: distortion even at low amplification levels. It actually sounds like a kazoo! Like the capsule membrane is overloaded, yet even at low (below clipping) preamp levels. Normal to soft speaking/singing sound OK, but not stellar.
Regarding Om's suggestions:
[quote author=O3misha]Of course, you may try another Ef12( all tubes differ).
If all values and voltages are normal, I would try to "play" with
1. reducing polarisation voltage
2. changing the anode current ( higher current =less distortion)
3. Increasing ( to 125-130VDC) anode feeding[/quote]
All values are normal except I'm not sure if I got the bias voltage right; all resistor values are right and all the rest of the voltages are right.
1) I tried reducing polarization voltage. Schematic calls for 80v; I reduced to 72V no difference - still distortion.
2) I tried the opposite - reducing anode voltage, which didn't help. I'll see what I can do about trying a higher value
3) I was afraid I might fry something to go this high, so haven't yet tried 125-130V.
Thanks for any more thoughts. I have reached out to A.Grosser to see if he has any other thoughts as well. I may have to send this to someone, but I'll keep trying any suggestions that come in. Thanks.