The Red Light District - another PP EL84 amp

[jackinnj]

SY -- want to dimension for me the size of the DPST switch that you are using to short the 100K input resistors? The ones I have from DK with the long bat-handles measure about 500 x 460 mils.

I think everything should fit without having to notch the board:

[SY]

I'll measure mine, but if yours are easily available, go with them. I used off-the-shelf from Radio Shack.

[sonata149]

I’m building this amplifier with different power transformers from those shown in the PS schematic (post #2) or used in the original design (Dyna SCA35). My problem really, is with regards to the filament wiring. I have a straight 6.3Vac winding (i.e. without centre-tap) to which all the valve filaments will be connected in parallel. Now, the ECC81 driver/phase-splitter valves must be floated above GND.

I am thinking of series connecting 2 x 100R || 10nF across the 6.3V points and from the junction make a connection direct to the GND lift network on the B+2 PS output. I believe this should be fine for the ECC81s, but how do I connect the 4 x EL84s to the same 6.3V transformer (ie in parallel with the ecc81s) without lifting them also from GND?

I hope I’ve made myself clear and would appreciate help from other builders and/or competent diy-ers.

Thanks,
Joe A

[SY]

That means of creating an artificial CT should work fine.

As long as the EL84 heaters are within their heater-to-cathode limits (which they are in this design), there's no problem lifting them from ground.

[sonata149]

Thanks, SY. Will keep you posted.

Joe A

[sonata149]

I'm in the first-testing phase of my RLD 84 build (as per SY's article) but I'm getting ridiculously low voltages (4 volts) on the input valve plates and phase-splitter plates and cathode. This voltage is taken without the output tubes. The voltage on the respective plate resistances is OK - around 380v. The voltage from the screen regulators is also OK - around 285v - but these are obviously out of the ecc81 circuit.

I also measured the heater-float voltage and again the voltage is similar (in the 10v range). Again the voltage prior to the heater float RC network is 380V. I made resistance checks for the OV line of the amplifier circuit (but not, I am now realising, for the power supplie(s) ground).

Does anyone have an explanation for such low voltages at the valve plate pins?

[SY]

If I understand correctly, this is just for the input tubes? Do you get the same result with the input tubes removed? If you used the protection diode on the phase splitter grid-to-cathode, make sure it's not backwards or shorted.

[dsavitsk]

Originally Posted by SY

If you used the protection diode on the phase splitter grid-to-cathode, make sure it's not backwards or shorted.

If something like this is the case, it sounds like you need a properly rated fuse, too ...

[SY]

Not so much as you'd think. If the diode is shorted or backward, the max current that can flow is the B+ divided by the sum of plate and cathode resistors. That's still only a few milliamps so a fuse won't save the day.

This is why I love, love, love my variac.

[sonata149]

If you used the protection diode on the phase splitter grid-to-cathode, make sure it's not backwards or shorted.

Hello SY, I have no diode on the phase splitter grid-to-cathode. I just built the amplifier as shown in your article on Bas' website.

Yes, this is just for the input tubes (i.e. without the el84s). With (all) the valves removed I still get the same low voltages.

By the way, dsavitsk, my 1 amp fuse (on 230Vac mains supply) survived several turn-ons/offs, if that's what you mean.

Thanks for your help.