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EWBrown wrote:
For the output coupling caps (the board accomodates two sets, with independent outputs, I used some of the Green Monster Russian 2 uF PIOs, and for the other set, the included 0.68 uF 400V WIMA caps. Those were intended for PSU bypassing, but I have more similar ones to choose from...

Sounds good :) my two outputs will probably be 220uF 450V 105C electrolytics bypassed with 2.2uF 630V Solens for low-impedance headphones, and some smaller caps for linestage (high-impedance) duty. Maybe some .1uF Black Cats. Just kidding. Yellow_Light_Colorz_PDT_11

I'd consider those "black cat" caps as being in the same league as "black cat" firecrackers... They do have a certain "coolness factor" look, but they were the cheap parts of the day, and were never expected to last 40-50 years.

On the Aikido board, I used some double-sided foam tape to help secure the big green monsters to the board, they are too heavy to safely depend on lead support only. I top mounted everything on the board.

For the two DC bypass caps, I used a 0.1 uF Russian PIO, and a 1 uF "green mosster" Russian PIO. THe board has plenty of room for these huge "Kommie Kaps" as one forum member over at Bottlehead calls them. They definitely have the "coolness factor" look...

/edB in NH

Hey Ed B ... (and anyone who wants to jump in of course)

If you have a second, could you look at Soren's schematic, which is
basically the White Cathode Follower, and help me figure out why
my built version isn't working right ?

The driver works fine, the oscilloscope shows a nice voltage
gain at the 100K resistor on the grid of the second stage.

The second stage isn't drawing any current across the plate
resistor, it's 320 volts either side. The cathode is about 64
volts. If I jump that stage out --- I get output ...

There is also (obviously) about 64 volts on the plate of the
last stage ... seems kind of low ?

Everything is connected right ... the resistor values have been
changed slightly to what I had, but not much off the schematic.

thanks for any thoughts !

... tom

Hi All,

I guess, because I wasn't all that interested in headphone amps, I didn't follow this thread as closely as I do others.

Short Wave Junkie, run your sim without that 37.4K "plate" resistor in the circuit; Doctor D, do the same. That resistor has to be raising all kinds of Hell with the operation of the lower half of the totem pole. Lessee what happens . . .

No, Tom; git 'em outta there! All shorting them does is short out the previous stage as well. The White, or "totem pole" CF lower half derives its plate potential from the current flowing through the upper half. If you look at the circuit, Soren has the lower half plate connected to B+ through the 34.7K resistor as well as the cathode of the upper half. In effect, that clamps the lower half's plate to B+, preventing it from following the signal on the upper half's cathode--kind of an electronic Denver Boot. Get rid of the resistor and things should improve . . . at least to the point of getting signal to the output jacks.

Ahhhh ! I knew one of the resident gurus would have the answer.
Removing those resistors did the trick ... I'm listening to the amp
as I type this. It sounds pretty good, some clipping if I drive it
hard, but these AKG K-240 headphones always sucked anyway ...
I am going to run out to the car and get the Sony phones I use at
my weekend DJ job and see how they sound.

Thanks Eric !

Hey, thanks for all the help so far. I just read an interesting article about White CF's in general and I think that this may help.
http://www.tubecad.com/october99/page4.html


Specifically, this page:
http://www.tubecad.com/october99/page7.html

"Thus, the only way the Vg of the top triode can equal Vg of the bottom triode is if the plate resistor equals the inverse of the transconductance of the triodes being used. (The test to put any tube circuit equation through is to try the equation with a 6AS7 and then with a 12AX7 to check the equation for absurdities.) "

It makes a bit of sense to me but since I'm using a slightly different topology, I'm not positive that it applies. If I wanted to try it, the transconductance of the 12AU7 is 2.2mA/V, so 1 divided by that gets around 450 ohms? 470 is probably close enough.. However, TubeCAD says that the output impedance shoots up to over 200 ohms

TomMcNally wrote:Ahhhh ! I knew one of the resident gurus would have the answer.
Removing those resistors did the trick ... I'm listening to the amp
as I type this. It sounds pretty good, some clipping if I drive it
hard, but these AKG K-240 headphones always sucked anyway ...
I am going to run out to the car and get the Sony phones I use at
my weekend DJ job and see how they sound.

Thanks Eric !

Hey, I'm reassured to see that at least SOME sound is possible with this topology :) Anyway, try tossing some 470 ohm resistors in place of the 37K ones, maybe something good will happen!

I just tried the Sony MDR-7506's and they are better than the
AKG's ... not quite right yet, but certainly tweakable. I only used
47 mfd output caps, so the impedance is high. Anyway Soren,
we're off to a good start now. I have a bunch of earbud phones
from my iPod and Inno ... but no 1/4" adapter handy with a flat
top, only the threaded type and they won't fit.

Cool - I'll mess with it tomorrow !

OK, sounds good! I'll expect some more updates later.. today then :) I wired the EL34 amp from just after dinner to now, the PSU is done and one half of one channel's signal is done. Pics to follow.

Soren,

If you want to post another schematic with the mods I'll run another SPICE analysis on it this weekend and let you know how it looks. I was suspicious when the analysis failed the first time but wasn't sure if my tube models were OK or not.

I was gonna run this White CF circuit on Tube CAD, to see what flay rod may be askew on the treadle (Old Monty Python reference).

The main bugaboo here will be that the output Z will be too high for most headphones, and lower frequency response will be diminished.

This may be a good place to experiment with a couple of those SPECO 7010 line matching trannies, Doc B uses these in a couple of the BH amp kits, and they work quite well. The 70Volt, 10W primary has taps that work out to 8K, 4K, 2K, 1K and 500 ohms (or 0.625, 1.25, 2.5, 5 and 10 Watts, respectively) , and the secondary is 4 and 8 ohms.

The main thing to remember with these particular trannies is that no DC voltage / current should be present on the winding, and another good tip is to wire them up as auto-transformers, connect the black "zero" wire of the primary to the 8 ohm tap of the secondary, and then the secondary's black "zero" lead connects to ground. This actually improves the overall freq response somewhat. Not too shabby, for $4 cheapo iron. It's definitely in the league witn Magnequest iron, but then the cost is about 2% of the MQ goods.

I know, the whole idea behind using the WCF circuit is to get away from using "iron", but I'd just mount the trannies in or on a separate small chassis, think of it as an "impedance adapter".

I've got a couple of spares of these SPECOs I can send you, if you want.

/ed b in NH

The output stage of Soren's schematic is the "Symmetrical White" straight out of TubeCad. I ran it, changing the values to what I had on hand, and TubeCad came back with the same results. I ran it this morning before I came to work and the normal White circuit is what we are basically doing by removing that resistor. The remaining plate resistor needs to be lowered in value to make this thing work right ... I'll mess with it later today and see what happens.

What do you think the target voltage at the plates of the first tube should be ?

It's got plenty of volume at this point, but lots of distortion I need to deal with.

Tom/Soren,

I've done some additional simulation and the circuit as posted without the 37.4K plate resistor on the last tube section has a gain of about 6. Fairly low gain. That's why you need to drive it hard to get any volume. However, it appears to be significantly resistant to clipping and essentially doesn't clip until about 20 volts pk-pk on the input at the grid. Distortion is pretty low when it's not being overdriven. It flat tops on the positive half cycle beyond about 20 volts pk-pk at the top of the waveform.

I changed the plate resistor on the first stage to 30K in the simulation and it significantly increased the gain (I didn't calculate that) without impacting distortion. So you might try that. Increasing the plate resistor value on the second tube section did essentially nothing. So, try 30K on the first plate. I'll play with it some more and if I find anything significant I'll let you know.

BTW - in case you didn't know - it's an inverting amp.

I'll post some screen shots in a few minutes.