Fixed-Bias Williamson EL34

[sorenj07]

I'm looking to make a fairly powerful yet compact tube amp. I've decided (once again) on the classic Williamson topology , mostly because it's been so popular. I like the looks of the fixed-bias version here from UTC:



the current draw of 110mA/ch idle for the EL34's is reasonable, given that I'd be using a single Hammond 282X with UF4007's into a 193M choke. Current capability for this PT is 280mA, so a little headroom is nice.

Ironic that I'm building a stereo amp with one of these suckers when my 6L6 monoblocks (all they need is the chassis now) use one 282X per and probably won't have 35W .

Anyway I'll probably be doing some work to adapt this schematic to use 12BH7's instead of 6SN7's. The Ra is lower, they're a tiny bit less powerful, and not quite as linear but they are much more compact and should be OK. The usual coupling-capacitor time constants fix will also take place. Check for updates in the next few weeks.

[frank754]

Yes, I'd be interested too on this. These old circuits are a nostaglic interest for me, and though they may suffer some criticism, I'd like to see you final impression on the performance across the audio spectrum. Squeezing the max out of these, I've read that it's more of a PA than a hi-fi, but in those early days there was a lot to recommand them, before the audiophile era got really sophisticated.

[Salas]

What is the dynamic current headroom that must a PT provide in such a class A/B amp? 220mA needed for bias only, say 250mA for all stages stereo. One PT 282X has 280mA capability. Is there any formula telling us the true dynamic headroom for containing really good, modern sounding, signal peaks?

[sorenj07]

Originally posted by salas
What is the dynamic current headroom that must a PT provide in such a class A/B amp? 220mA needed for bias only, say 250mA for all stages stereo. One PT 282X has 280mA capability. Is there any formula telling us the true dynamic headroom for containing really good, modern sounding, signal peaks?

Not sure. I figure that the PT doesn't need to be THAT much more robust than the max. current draw because it's mostly the (hopefully large) filter capacitors' jobs to power through transients and such. At least that's my impression.

In any case, parts are on their way in. I picked up a pair of Edcor 6K6 60W output transformers for use with four regular JJ EL34's. Apparently 6K6 is the ideal impedance for these tubes. Power supply is a 282X (1000VCT 280mA choke-input) solid-state rectified into a 193M filter choke followed by a few hundred uF's of capacitance. Input/splitter/driver tubes are two E-H 6CG7/6FQ7's (noval 6SN7 equiv's). This will make out to be an absolutely beastly stereo chassis. Should be fun!

one other thing. any recommendations for a bias transformer?

[jrdmedford]

Ultralinear EL34s will require way more feedback than triode
beam tubes... I'd replicate the Williamson ckt first before
attaching screen grid taps for EL34s.

The orig Williamson used a nice cathode bias network with
two pots - one to set the DC bias and another to set the bias
balance. You tweak the balance so that you get zero sum DC
current through the OPT primaries. This lets you milk the most
performance out of your output transformers and you'll notice
it on bass transients. 100 ohm 2W pots are cheap and easy
to buy. Radio Shack sells 50 ohm 2W pots... Try this before
going for the fixed bias approach.

http://www.bonavolta.ch/hobby/en/audio/wilamson.htm

12BH7 is good! 6GU7 is a 6.3V only version of the same tube.

If you truly want to go for power, start accumulating 807s.
There's plenty of WW-2 surplus still left about... You can get
807s to do things that you'd not dare with EL34s. 807s love
to get the crap kicked out of them. You can safely run them up
to 600V on the plate.

-- Jim

[sorenj07]

i want to keep fixed bias. to keep DC balance, could I instead stick a 25 ohm 2 watt pot between the two EL34's cathodes and put the center tap to ground?

[Tweeker]

In any case, parts are on their way in. I picked up a pair of Edcor 6K6 60W output transformers for use with four regular JJ EL34's. Apparently 6K6 is the ideal impedance for these tubes.

If your using four tubes, should that not be 3K3? "Ideal" is a matter of taste anyhow. Power versus distortion and operating point etc etc.

[sorenj07]

Originally posted by Tweeker


If your using four tubes, should that not be 3K3? "Ideal" is a matter of taste anyhow. Power versus distortion and operating point etc etc.

nah - 2 OPT's, 4 tubes, regular push-pull.

[Johan Potgieter]

Originally posted by jrdmedford
Ultralinear EL34s will require way more feedback than triode
beam tubes... There's plenty of WW-2 surplus still left about...

Er, Jim,

Not quite sure why you say that? Not to go into matters too finely now (it was done elsewhere), but at 15W output (the max. for triodes in this circuit) UL connected EL34s give lower distortion with some power to spare. They have about 20% more at their usual maximum of some 35W, according to Mullard graphs.

I prefer 6L6GCs to EL34s myself (efficiency etc.), but the above is a fact.

On power requirements it must be realised that for music the average demand is much lower than peaks (about 4 - 5 times, depending). The transformer may thus be quite smaller than required to supply continuous power for say, a sine wave. It is true that supply capacitors, or at least the filter output capacitor, need to be quite large in order to supply peak power.

Regards

Edited: Typo

[jon_010101]

Originally posted by sorenj07
i want to keep fixed bias. to keep DC balance, could I instead stick a 25 ohm 2 watt pot between the two EL34's cathodes and put the center tap to ground?

No need, really ... that 10k pot in the grid circuit does just that for you.

You may wish to add a small carbon resistor on each cathode, however. Then, just measure the voltage across the resistors to calculate the bias current. These could help offer some protection in case an output tube shorts itself, since the resistor will pop like a fuse (which I see is specified in the schematic). 10 ohm would allow for easy mental calculation of current, and if you used a 1/8th watt 10 ohm carbon comp resistors it is fairly likely that they will blow when trouble arises. Or just put a 10 ohm 1/4 watt in series with the fuse.

... also, 55mA at 445V is basically max dissipation for an EL34 in fixed bias ... I'd consider backing that down a bit. It is okay in cathode bias since 50 of those volts would be on the cathode resistors. 45-50mA may be "safer".