6F6G Amplifier

[sorenj07]

How does it look? I have the Z565's, PA774, and the tubes. I'm also considering using a single 6EM7 per channel, the high gain half direct-coupled to a concertina formed by the low gain/low Ra half, Williamson style.




I've got 8 Russian 6F6G types on hand, but am prepared to try new production 6V6's as well..

[Miles Prower]

Originally posted by sorenj07
How does it look?

Looks like a good start.

First off, the 6SL7 LTP phase splitter needs a longer tail in order to work right, preferably a CCS. Here, you don't have much of a tail at all, just the AC impedance of that diode. I also have doubts as to how well this type can drive the grids of a final, with its higher Ci + Cmiller + Cstray. I'd install a 6CG7/6FQ7/6SN7 (I'd suggest a 12AU7 for this, but 12AU7s aren't any cheaper, and sometimes even more than the more linear types) cathode follower grid drivers. If you don't want to make more holes for sockets, then a MOSFET source follower. That way, you can direct couple the cathode followers and also avoid that blocking problem that overdrive tends to cause when capacitor coupling to a fixed bias stage. You already have a negative rail, so that's NBD to add.

[jechentau]

First off, the 6SL7 LTP phase splitter needs a longer tail in order to work right, preferably a CCS. Here, you don't have much of a tail at all, just the AC impedance of that diode.

the diode is reverse biased while in operation.
it seems it is there to prevent cathodes initially 150 V negative relative to grids at startup, which would be bad.
so actually it has a 150K tail.

agree with rest though; cf drivers would work nice here

[sorenj07]

I do have two extra EH 6SN7GT's that I could throw into this. I also have some badass 5687's but a) they're noval and b) probably overkill. My one concern is that it's a bit bulkier at 8 octals in one chassis for MAYBE 12-13W/ch...

None of them should be getting too hot though (6F6's are only running at 12W) so close spacing combined with ventilation holes should be good. Depending on financial constraints I'm going to try and CAD-design a custom stainless chassis.

One last question though: How do you design CF's into an amp? Should I just peruse a few schematics that use them? Got any good examples? How much extra current would the 6SN7 CF's take? I'm using 162mA quiescent as this amp stands out of the PA774's 180mA. It's a lot closer to choke input, which should free up some extra juice, but if the '774 already runs hot, I don't want to push things too much. Might have to go monoblock with a 150mA+ Hammond per chassis

[FranStar]

I would always look at what has been done

You will find the 12ax7 the tube used in acres of push-pull amps with the old split phase circuit used mostly.

A 6sl7 will provide plenty of drive to 6f6 tubes. These tubes do-not need a lot of drive as they are higher impedance pentodes, lower grid current demands.


If you forget the diode, use a 2.2K cathode resistor, and the neg feedback has to go to the input triode, a basic circuit will do the job fine.

Part of the Magic sound of older tube circuits is the variables in the circuit so no real need to negate everything.

I look at circuits of vintage gear to see how tubes were used before in gear that was sold and had good sonic character.

You can go to the library and get copies of SAMS photofacts, or look on the NET at schematics.

Also 6f6, 6k6, 6y6, 41, 42 and others are kind of lost tubes that have very good sonics and should be explored again.


Look at this link to see samples of some of my recent restortations and special audio projects.

http://photobucket.com/albums/b121/SalesBoy/

George

[Miles Prower]

Originally posted by sorenj07
One last question though: How do you design CF's into an amp?

What is the Q-point bias for the 6K6s? Once you have the Vgk for these, then you know what the static cathode voltage is. Add enough Vpp at the plate for headroom, and enough standing current to provide the charging current for the Ci + Cmiller + Cstray, and draw a loadline.

Should I just peruse a few schematics that use them? Got any good examples? How much extra current would the 6SN7 CF's take?

Two examples:

Vixen Main Schemo

Le Renard Main Schemo

6K6s shouldn't present a particularly difficult load, so the required current for the cathode followers shouldn't be over 5.0mA. So far as the Vixen is concerned, the 6SL7 LTP didn't have anywhere near the drive to charge up the 807 grid capacitance for a good slew rate at the higher frequencies, and it sure didn't have enough to deal with transients that drive the grids positive. This would make for some nasty sonic degradation, even if an overt clip weren't actually heard. The cathode follower presents a much more agreeable load since there is no Miller capacitance, the Cgk is "reverse bootstrapped" to a smaller effective value, leaving only the Crt as the only capacitance that draws the lion's share of the current at 30KHz.

The situation with the Le Renard design is somewhat different. Here, the 6BQ7s would seem to have the current sourcing capability, as the gm is much higher than for 6SL7s. However, these are cascoded, and that makes for a higher than normal Zo. The grids of the 6BQ6s would drag it down too much, negating the frequency performance of the cascode. Again, the cathode follower driver represents a Hi-Z load that doesn't ruin the performance.

In both cases, having current sourcing capability greatly improves overdrive behaviour since the finals can transparantly slip a bit into A2 instead of clipping at the grid.

[sorenj07]

Thanks for the tips! the 6F6's (not 6K6's) are biased at around -20.6V 40mA. Unfortunately I couldn't find any ultra-linear curves for 6F6 tubes, so I based my calculations off some UL curves for 6V6 types. Here is my load-line.



I'll spend some time figuring out what you said about CF's. Maybe Morgan Jones can help as well...

As a side note: Do you know who I could ask to plot some UL curves if I send them a few of my extra Russian 6F6S tubes? It'd be very useful for everyone, I think.

[Miles Prower]

Originally posted by sorenj07
I'll spend some time figuring out what you said about CF's. Maybe Morgan Jones can help as well...

This isn't all that difficult to figure out. Given a Po= 17W, and a Rl= 8K (P-2-P) you can figure:

V= sqrt(17 * 8E3)= 368.8Vrms (P-2-P)
V= (368.8 / 2) * sqrt(2)= 260.8Vp (per phase)

Av= 260.8 / 22= 11.6

For the 6K6, Crt= 0.5pF, so Cmiller= 0.5 * (1 + 11.6)= 6.3pF

Cgk= 5.5pF, so

Ci= 5.5 + 6.3= 11.8pF, guesstimate Cstray to be: 30pF, and

Ct= 11.8 + 30= 41.8pF

At 30KHz:

Xc= (2pi * 30E3 * 41.8E-12)^-1= 126K9

I= V / Xc= 22 / 126.9E3= 0.17mA

By the "Rule of Five", the driver plate current should be 0.85mA or better. Given that you have a -150Vdc rail already, you're good to go there. The plate of the cathode follower can be set to +150Vdc for headroom.

As a side note: Do you know who I could ask to plot some UL curves if I send them a few of my extra Russian 6F6S tubes? It'd be very useful for everyone, I think.

Yves just might have some already.

[sorenj07]

Finally resurrecting this project. I've redesigned the 6SL7 LTP, feeding 2.5mA into the pair. By the rule of five, 1.25mA per 6F6 ought to be OK, right? In any case I still have enough heater current to spot an extra 6SN7 as a CF and I'm beginning to think it's worth it.

One question I have is the 150V that seems to feed the CF's in both schematics. Where should it come from, and how much plate current will each channel's 6SN7 need? I've got about 310V to work with but an extra 20mA total, say, will drag it down to 300V or so.

The two reference schematics also use different means of biasing the CF's. They both have capacitors to ground in common. What is their purpose/what is going on?

What is the resistor/capacitor from each output tube's plate to the CF input grids? Some form of feedback?

Also, what is the inductor/resistor network connected to each output tube's plate in both schematics?

Thanks a bunch. I really want to turn this amp into a reality.

P.S. Yves says he's super busy at the moment, and his curve tracer isn't in operation...

[sorenj07]

The new 6SL7 LTP has an output impedance of 28K or so. Is this objectionable?

The details (rounded a bit):

B+ 310V
Ia total: 2.5mA
C- 155V
Ra = 80K 1W on top, 82K 1W below
Rk = 62K 1W
Va-k 210V
Vg-k = 2.11V
Gain = 22.25
Vo max = +-47V

(Thanks TubeCAD)