Design using EL42 output pentodes

[Eli Duttman]

I would prefer to keep things simple

Phil, we'll keep things as simple as satisfactory performance permits.

Keep the OEM power trafo and B+ rectifier topology. You will use a 6DJ8/ECC88 family tube in the small signal "hole". The current production JJ E88CC/6922 is both decent and affordable.

Remember my previous remark about sufficient gain. CCS, instead of resistive (R7), loading will be used on the voltage amplifier. The least complicated CCS is the IXYS 10M45S. Let's look at the E88CC data sheet. If I_B is forced to be 4 mA. by the CCS and -3 V. are on the grid, 110 V. are on the plate. That's a very suitable operating point. Ohm's Law tells us that 750 Ohms drop 3 V., when the current present is 4 mA. 100 of those Ohms are already accounted for by R6. Set R5 as close to 650 Ohms as you can, in an available part. C2 should be 100 μF. and not less than 6 WVDC. Now we wait for SY or other knowledgeable person to compute the R9/R11 value.

[SY]

R9/11 are a piece of cake to compute. Select a current for the second stage- I'd probably use 6 or 7 mA, it's not that critical. The cathode will be just a couple volts north of the grid. With 110V on the plate of the voltage amp, the cathode voltage of the split load will be roughly 112V. That also means that the drop across the plate resistor is 112V, so you've only got 250 - (2x112) = 26V across the tube. Tilt!

Let's try it again. Redo the current in the first stage so that its plate voltage is closer to 80V. That means 82V on the split load cathode, 82V drop across the plate resistor. Now we have 250 - (2x82) = 86V across the split load tube. MUCH better.

The split load resistors would then be R = V/I = 82V drop/6mA current = 13.7k. Anything from 12-15k will work fine, but the two resistors should be closely matched.

If you want to be a little tricky in the first stage, substitute a red LED (1.7V drop) for R5/C2.

[Eli Duttman]

R9/11 are a piece of cake to compute. Select a current for the second stage- I'd probably use 6 or 7 mA, it's not that critical.

My gut feeling is that "less is more", in this case. How much current can safely be pulled from the B+ PSU? Remember, the OEM small signal tube is a 12AX7/ECC83. Would 18 KOhm "concertina" resistors be acceptable? Reworking the filter the rectifier drives into a CLC with 10 μF. in the 1st position followed by a low DCR 5 H. inductor should keep the rail voltage up, while allowing a little more current to be squeezed out. Maximize small signal B+ voltage by using a "check valve" style decoupling network of 22 Ohms and UF4007 in series followed by a 15 μF. capacitor.

Let's try it again. Redo the current in the first stage so that its plate voltage is closer to 80V.

Does I_B = 3 mA. in combination with 2.5 V. of bias meet with your approval? 2.5 V. is already too low for handling the full 2 VRMS O/P of a "standard" CDP. An 18 KOhm resistor between the I/P and the "top" of the volume control deals with that. However, if the padding resistor becomes larger, we might run out I/P drive. What's the best compromise?

[SY]

Would 18 KOhm "concertina" resistors be acceptable?

Sure, it'll work fine.

Regarding the bias, remember that there's feedback to that point, so that the grid-cathode swing is not limited to just the bias voltage- the cathode will swing in the same direction as the input signal.

[Eli Duttman]

Phil,

SY has finished things off very nicely for you. Per his remark about I/P voltage swing limit, forget a padding resistor at the "top" of the volume control.

I_B = 3 mA. and bias at -2.5 V. make R5 = 733 Ohms. C2 remains 100 μF.

Use 1% 18.2 KOhm 1/2 W. metal film parts for R9 and R11.

A Hammond 156L or something similar is appropriate for the B+ PSU filter choke.

It seems you get to recycle all 8 tube sockets and both power trafos.

Please draw up a revised schematic and post it.

[phil_2]

Thanks a lot Eli and SY!

I will post up a revised schematic ASAP

[phil_2]

Ok here is the revised schematic as I understand it, I think there is a good chance I've misinterpreted you here, with the power supply and CCS... sorry if I have. I've also assumed those changes you mentioned to the feedback loop if using the hammond OPT aren't necessary now, correct me if I'm wrong.

[SY]

On my way out to cook dinner for a couple of lovely ladies, but I promise I'll look at it tomorrow, if Eli doesn't nail all the points first.

[Eli Duttman]

Phil,

R2 = 470 KOhms and C8 = 1800 pF. Those values will give a little added protection against O/P trafo core saturation and small signal circuitry slew limiting. 470 KOhms combines with 12 nF. to give a 3 dB. down point of 28.2 Hz. IIRC, that's below the lowest note a double bass can produce. 1100 Ohms combines with 1800 pF. to "corner" at 80.3 KHz. Stray wiring capacitance will lower that value slightly. Still, for "Red Book" CD as the signal source, things are fine. CCS loading the voltage gain triode prevents the use of "peaking". Let's avoid a problem in trying to do too much. "Perfect" phase compensation requires the use of an oscilloscope. This circuit gets pretty good via brute force. Thankfully, the 6922/E88CC has a high gm.

The schematic symbol for a CCS is a pair of interlocked circles. You state the current present, which in this case is 3 mA. You need a few passive parts in combination with a 10M45S to set the current. Please refer to the data sheet I previously linked.

The B+ PSU filter is CLC. A 47 μF. cap. follows the 5 H. choke.

The rectifier's heater wiring is yet to be corrected. None of the heaters get connected directly to the chassis.

BTW, use a good quality, low impedance, 'lytic for C2. Panasonic and Nichicon offer suitable parts. A low quality part will not properly transmit the high freq. error correction signal.

[phil_2]

Thanks Eli

I mistakenly took R5 and R6 as providing the resistance referred to as Rk on the data sheet. I have added Rk, which from the graph = 1140 Ohms. The data sheet does not show any other parts, however you said "You need a few passive parts in combination with a 10M45S to set the current" Please let me know if anything else is needed. I have kept the incorrect symbol for the 10M45S for now just for clarity for myself when I'm connecting it up.



What would you suggest for a good cap. type for C1?

Also you mentioned the voltage rating for C2 to be not less than 6V, am I right in thinking this was a typo? I've not seen such a low voltage rating on an electrolytic.