12W Push pull w/6C4C

[yagoolar]

After having succesfully built SY's RDL I am hungry for more.
I am planning to design and build a push-pull amp with 6C4C DHT triodes.
Some reqs / limitations
1. Tubes easily available
2. Dual mono approach
3. Use of 2 PSU transformers. each 2 x 320V 0.1A, 6.3V 2A, 12,6V 1A
4. 12 W output push-pull
5. 6C4C to be easily replaced with 6L6 (6p3s-e)
6. Silicon devices allowed: LEDs, FETs or BJTs.
7. CCS for heaters
8. Regulated voltage for a voltage amp and a phase splitter
9. LED biased output tubes
10. Size of a chassis

[yagoolar]

I think of some tubes I own to be used:
- voltage amp Phillips Miniwatt ECC81 and E80CC, Tesla E88CC, Russian 6H9C, Siemens C3g
- phase splitter Edicron E182CC, Russian: 6C45, 6C4, 6H8C
However, PT has 12,6V winding which limits the use to smoe of the above.
I feel E80CC as a voltage and E182CC as an LTP is a starting point.

[yagoolar]

As an OPT vintage Polish TGp-12-26-666 will be used:
Rp-p = 7k @ Ro = 15k.
n1 = 3200, n2 = 140
L = min. 90H
P = 12VA
It was designed for an EL84 push pull amp. If used with 8 ohm load it is approx. Rp-p = 3,7 kohm.

[SVStube]

6C4C have Ri = 800 ohms, and Rp-p = 3,7 kOhm is enough for it. For your PSU transformers Rp-p=5 - 6 kOhm would be better with a self-bias. With fixed bias I would do Rp-p=6 - 7 kOhm. Power output will decrease slightly, but the distortion will be much less.

[yagoolar]

Thanks SVStube for a piece of advice. I have those transformers at hand. Meanwhile I will have custom ones made.

As for the operation point:
Ua = 320V
Ug-c = -65V (LED) self bias
Ia=40 mA

then
Ub = Ua + |Ug-c| + Uw1 = 320V + 65V + 370 ohm * 40 mA = 399,8 V

[Bigun]

I have a bunch of these tubes so I'll be watching your development !

I've listened to a SE fixed bias 6C4C driven via an interstage trafo and it sounds good. Main complaint is hum, but with PP you can wire the two heaters in anti-phase.

[SVStube]

You need to rectify out the heat winding and hum away. Or you can make the heat winding of the 2 halves and include balancing potentiometer of small nominal among them.

[Bandersnatch]

hey-Hey!!!,
The last few DH PP amps I build with 1 heater trans for each tube, and one hum pot across it for each tube. 2.5V to 12V to heat( 1619, HY69, 4E27, 813, HY1269 ) and no hum at all. Don't go to trouble to rectify DC for DH PP. Watch dissipation in hum pot.
cheers,
Douglas

[yagoolar]

Originally Posted by Bigun

I have a bunch of these tubes so I'll be watching your development !

I've listened to a SE fixed bias 6C4C driven via an interstage trafo and it sounds good. Main complaint is hum, but with PP you can wire the two heaters in anti-phase.

A bit OT, still you may be interested in S. Bench's approach to hum cancellation
Effects of AC Heating Power Applied to Directly Heated Triodes

[yagoolar]

Now, I need to choose proper HT PSU for output tubes. I am not certain what is current requirements while voltage at grid changes from 0 to max.

Assume ug = 0 (or Ug = -65V), then Ib = 2x 40 mA and Ub = 400V.

When Ug1=0V and Ug2 = -130V, then Ib = 75 mA + 0 mA, because one triode does not conduct? Shortly, can I use a current source of 80mA as a model for a load to PSU?

Why is it important? One reason is choke impedance in series with the load, the more current, the more voltage drop.