Guessing which will be a good load for a 6AF11 tube in SE (~175V B+)

[kagliostro]

The 6AF11 tube wasn't planned to be used in AF, but someone did

The first amp I've seen that uses such a tube is the Squirrel Monkey

http://solorb.com/elect/musiccirc/squirrelmonkey1/

I know that there is also a Kit by Zeppelin called Percolator that uses the same tube

https://zeppelindesignlabs.com/product/percolator-2w-amp-kit/

also I've find an old radio project that uses the 6AF11 tube

http://www.rfcafe.com/references/popular-electronics/one-receiver-all-bands-jan-1963-popular-electronics.htm

On the Squirrel Monkey amp

B+ is 180V with a load of 3.5K (the autor say this give the better power performance (??) )

On the Zeppelin Percolator

B+ is indicated on the istruction to be 170V but there is no info about the OT primary impedance

On the Radio Project B+ is around 175V and the load is 10K

On the 6AF11 datasheet there are not indication about use in AF, so no OT load are indicated

http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_1/6AF11.PDF

https://frank.pocnet.net/sheets/049/6/6AF11.pdf

B+ is similar on the 3 different circuits, but what to say about the load ?

one use a 3.5K load, the second is unknown and the third use 10K of load

Plate dissipation of the pentode is 5W, so, if I'm not wrong, Z = Va^2 / Pa

175V x 175V = 30,625

30,625 / 5W = 6,125 Ohm Load

Did I argued correctly ?

(the 3.5K load used on the Squirrel Monkey confuses me more than I'm by myself)

Thanks

Franco

[jjasilli]

Here's  a load calculator for a pentode:  https://www.vtadiy.com/loadline-calculators/power-stage-calculator/


Hopefully it's helpful

[DummyLoad]

6K is optimum with those parameters. perhaps simply what they had available at the time or had in their possession? the 3K part if you double the speaker load then is proper reflected. the 10K loads is not optimum but will work with 1/2 speaker load - 5K is close enough?


--pete

[PRR]

It looks like a whole guitar amp in one bottle. But I think if you cram a gain of 5,000 (20mV to 100V) in less than an inch it will oscillate. Unless you have a Tube Tax (Germany did once), get a second tube.

And note that 12AX7+6V6 heaters are 0.3A+0.45A= 0.75A which is -less- than the heater demand of one 6AF11.

Assume the "Characteristics" suggestion is a pretty good (show-off) operating point.
200V 24mA plate
4.8 W Pdiss (96%)
200V/24mA= 8.33k equivalent resistance
If the pentode has low-low losses, the optimum swing will be very similar to the equivalent DC resistance.
But if we were really starving for power, we would pick a bigger tube!
So anything 5k to 10k, more or less.

We would like more data about V and I. We can put the plate higher but then the G2 probably should be lower, and it may be awkward to get a solid 100V source. And 5W at 300V means 17mA and ~~18k load, an awkward value. We can't really make the voltage lower and use all the power, because at zero-grid the pentode passes 55mA, so the idle should be 27mA, which is not really different from 24mA. Say 180V 17mA and 10k.

[kagliostro]

Many Thanks to All

I think I've a ~7K SE OT rated for 6W, May be I can use it


---

I was able to find a reverse engineered schematic of the Percolator

Where I find the schematic there is the indication that the OT primary impedance was guessed, and they guessed for 6K as I did

On the schematic you can see that the values of components on the triodes are lower than in the Squirrel Monkey and I think that it was done to have less gain and so a circuit less prone to autooscillation

I would like to know which will be your opinion on adding the Tone Control (on the NFB) present on the Squirrel Monkey to the Percolator circuit

also, did you think it will be feasible and give an effective Tone Control ?

Here the Squirrel Monkey & the Percolator circuits





BTW on pin #7 of the Squirrel Monkey 6AF11 there is connected a 15K resistor .... do you think it is correct or it will be 1.5K ??

Thanks

Franco

[jjasilli]

FWIW there's also a 15AF11, identical except for heater voltage.  Presumably this would cut current demand.

[DummyLoad]

It looks like a whole guitar amp in one bottle. But I think if you cram a gain of 5,000 (20mV to 100V) in less than an inch it will oscillate. Unless you have a Tube Tax (Germany did once), get a second tube.

looks like C4 and C7 are there to address that issue in the percolator plan.


--pete

[kagliostro]

Thanks Pete

--

jjasilli

unfortunately I think I've never seen one 15AF11, however I've 2 or 3 6AF11 and a pair of sockets 

Franco

[jjasilli]

Stereo 4W amp!

[kagliostro]

Stereo 4W amp!

For that I've a pair of EL95 with theyr SE OT



Franco