Evolution of an old Ciuffoli Head amp but need more help/adv

the thermionic watercooler

Evolution of an old Ciuffoli Head amp but need more help/adv

Postby glen » Sun Nov 27, 2011 6:27 am

Put together a very simple yet overkill SE spud amp for headphones.

Many here may not be familiar with headamps but basically this one is for a 32 ohm load, needing at the most 0.5Vrms so ~less than 15mA current swing < 10 mW ball park. I come here cause you guys seem to have always given the best advice over the years when it comes to tube fundamentals ;)

Here is a basic outline (sorry its not a schematic but I thought this would be quicker to read), there are two identical channels (yes separate from including the power transformer as I had two of these on hand.)

Image

I know the WE417 is prone to oscillation, but I have this fairly well stoppered I believe as all the resistor stoppers are directly on the pins.
The heater is DC from an LM1085 at 12.6V with the two tubes in series, both tubes measure close to 6.3V across their heater pins. There is no measures to stop RFI on the heater pins (not sure how one would do this.)

From RMAA souncard measurements the 2H is a respectable -60dB down and 3H -80dB down, the noise floor is well below -100db except a small bumb at 60 hz which is there with the loopback test as well. This is just output straight to the soundcard which is around 5k load. Absolute black background with these sensitive phones :))

So THD at 1.4V out measures a nice 0.12% but the IMD looks bad at 0.469 , and it isn't noise, the graph has very distict peaks all over the place with the IMD test :'( .


The WE417 operating point is 17mA @ 1.9V cathode bias. I tested the shunt reg and all is good as far as how it is setup according to the designer. He questioned the operaing point so I tried 20ma bias and 15 ma bias, maybe 15mA was slightly better but not much.

But I guess I suspect oscillation robbing power, I mean I am only asking for 10mW of lean power? I also supect the OPT, it is gapped for 20ma (don't know if that is a max or how that works with regard to AC swing across the primary.)

The thing is absolutely beautiful at what I would call normal listening levels (.2Vrms out) but just a little bit higher and then it just drops off a cliff with what I would call shouty solid state type distortion.

Looking for adice on how to get just a little more clean power out of my frankenstein.

thanks.
glen
 
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