The Red Light District - another PP EL84 amp

[SY]

You might want to check the latest version, which is on my website. No significant differences for the original version, but there's a few minor tweaks. In any case, pull out the driver tubes and recheck voltages.

[sonata149]

This is really a continuation of my previous emails. At the moment I stuck trying to get the correct heater-floating voltage, and all I’m getting at the 200K – 51K junction is 5Vdc! I’ve already spent many hours replacing/breadboarding/testing all components from the end of the choke onwards and each time I get these low voltages. It’s been all very frustrating, so I’m turning again to the forum for help!

I’m using one transformer with 0-270-310 volts AC for all B+ points. The 0-270V powers the output valves (EL84s) giving me around 320V on the plates (with OPTs in place) which is OK; the 0-310V taps power the ECC81 drivers/phase splitters and the heater-float dividing network. B2+ is around 400V. The curious thing about it is that I get very low voltages at the end of every resistor that is fed by B2+. This is the case at the heater-float junction (as already said), at the plate of the ecc81 driver (5V), at the plate of the phase splitter (12V) and at the cathode of the phase splitter (5V). I have not used the protection diode on the phase splitter grid-to-cathode.


Can someone please help explain what could be wrong in such a simple circuit? Thanks in advance!

[SY]

I'll repeat my earlier question- if you pull out the driver tubes and measure the voltage at the plate pins (1 and 6), is it still 5V?

[sonata149]

Yes, SY, with the driver tubes removed the voltage at the plate pins are as reported earlier: 12V at the plate of the splitter and 5V at the input/driver plate pin. Thanks a lot!

Joe A

[SY]

Possibly nutty idea on my part- try a different voltmeter.

[jackinnj]

Remember, a transformer transforms current -- so the idea of an artificial center tap may not work -- I think that this is why the little Dyna trafo didn't work well with the Impasse, but the unit from Allied works like a champ.

SY -- what did you "tweak" in the newest version? Is it sufficiently tweakified to qualify as a POOGE?

[SY]

No, nothing like that. Since the original article, I dropped the diode strings to six in series and added small current balancing resistors. Of course, if someone wants to use co-poly(ethylene norbornene) capacitors or sputtered rhodium resistors, have at it.

As soon as I get a little more time, I have some ideas for a redesign using some local feedback around the output stage and CCS loading in the first stage, but that would be a bit beyond "tweaks."

[weinstro]

Sy,

Could the RLD work with 6V6? Any disadvantages to note?

[SY]

The main disadvantage is that the 6V6 takes a higher bias, so the LED strings will be proportionately longer. That means higher parts count and a somewhat higher dynamic resistance. Still probably less than the customary 10R cathode resistors, though.

[Brett]

Originally Posted by SY

The main disadvantage is that the 6V6 takes a higher bias, so the LED strings will be proportionately longer. That means higher parts count and a somewhat higher dynamic resistance. Still probably less than the customary 10R cathode resistors, though.

I'm also looking at doing the same with troided EL38 and possibly in the Fisher 800B (7591 pentode), both are around -13V Vg so not too big a difference. The EL86 will be first for a STAX amp.

My friend is very pleased with his RLD.