My low watts projects

[EWBrown]

You have it right, just leave the 12B4A pin 3 and the 12AX7 pin 9 unconnected. The filaments will be running from 12.6VDC, so this will work out well.

Even though 12B4As and 1626s are different, they have a "coincidental" compatibility in that they will be "happy" with the same B+ voltage (appx 240-245 VDC, and plate / cathode currents (25-27 mA) and cathode resistor (1K, 3 to 5W), and SE OPT primary impedance (5K to 7K).

The 12B4As will deliver about twice the output power (1.6W) as the 1626 (750 mW). Interestingly, in the 1626 data sheet, it states that these tubes are not suitable for purposes other than RF oscillators or Class C RF amplifiers. I guess they forgot to tell the tubes about that, they sound quite nice in a low powered SET like the "Darling".

I suppose that one could build up an amp with both octal and 9 pin sockets properly interconnected, and have the choice of operating it with either type of tube.

I've seen a similar technique used before, in one of teh Angela Instruments DIY projects, it was wired up wit both types of sockets, so that either a 12AX7 (or 5751), or a 6SL7 could be used as the VA stage.

If you prefer using octal tubes for the VA stage, just use a 12SL7 in place of the 12AX7, they are close enough that no changes (other than the tube socket) is needed.

/ed B

[katabatic]

Thank you, Ed, I appreciate the help.

George

[EWBrown]

I worked out the 12B4A SET on SE CAD about "seven ways to Sunday" and after numerous iterations, I found that the "sweet spot" re output power and THD is as described in my previous posting.

12B4As can also run with a higher plate voltage (around 310VDC, but with lower plate curent (18 mA, requires a cathode resistor of around 2.2 to 2.4K), but then it needs a much higher impedance OPT, at least 11 or 12K primary, and better with 22-24K. This gives a very nice sounding result, but the output power will be significantly lower, around 800-900 mW maximum - this is the methodology used in Jef Larson's "Abraxas" SETs.

I've copied this design, and used the vintage German made SE OPT which are salvaged out of 1950s vintage Grundig and Telefunken table radios (these are nominally 11-12K to 4 ohms, but with an 8 ohm load, they will reflect back as being 22-24K. These OPTs can be found on E-bay, "testedtubes" is one such seller, in Germany.. These trannies were designed for use with EL95/6DL5.

/ed B

[soundbrigade]

May I throw in a towel or glove or a dice ... cannot remember what.
I haven't really (well, in a way I have) started a project using VT25. Got the OP trannies from Edcore and am about to order a VERY NICE R-core tranny from HongTjong. However the driver tube, 417A, seems to be slightly difficult to find, so I am a bit interested to hear what tube I could possible use as driver.
Some suggestions are 6S45Pand 6E5P. I was thinking of 6N6P or even 6N1P.
It would be very fun (I don't know why, really ) to use an octal tube as a driver, but which one? 6SN7??

Instead of the 417A I wanted from AES, I ordered a pair of 1626. I already have a (almost) finished a matrixed Darling, so maybe I should try to solder together a QUIT Darling-SE.

[EWBrown]

I had to refer to "Tube Lore" for this one, it's not a very common tube these days. Philips produced a close equivalent:

The 5842 comes very close to the WE417A, high-gm (24 mA/V) RF triode , which was originally intended for grounded-grid (cathode drive) RF or IF service.

http://frank.pocnet.net/sheets/009/5/5842.pdf

The Russian 6C45 (6S45P-E) is domewhat similar, but it has a significantly higher gm (about 45 mA/V), and a completely different pinout, and it can be a real "screamer" in audio usage... . Its original purpose was as a pulse generator / modulator in Russian Radar equipment, and the high transconductance was required in order to generate very fast rise and fall transition times.

http://frank.pocnet.net/sheets/112/6/6S45PE.pdf

These "hyperactive" high transconductance RF triodes just LOVE to oscillate in a grounded-cathode VA circuit, so very careful circuit layout, and the use of grid- and plate- stopper resistors is a necessity.

The grounded-grid topology nicely side-steps the instability issues, but itisn't really applicable to audio usage, as it has a fairly low input impedance, typically 50 to 600 ohms in RF usage).. Having said that, the "second" triode in the standard Schmidt type LTPI is in fact, a grounded-grid amplifier, which generates the "non-inverted" output phase.


HTH

Спасибо

/ed B in NH

[soundbrigade]

I will propably make a trial with my 6E5P, universal and very linear tetrodes. They will also play an active role (one day) in my EL156SE.

Prosce bardze!

[EWBrown]

All of these low powered SE designs make for a very tasteful, and a heavily laden table of a virtual smörgåsbord of tube-ology

A smörgåsbord is like an American all-you-can-eat buffet, but without all of the wimpy "food for food" salad-bar stuff.
Maybe some he-man veggies like 'taters, beets, onions and cabbage, just keep the limp and leafy rabbit food off the menu, please

More dedicatd to a variety of meats, and meat by-products
(sausages, pickled mööse testicles, etc), fish, cheese, breads, etc,
and, mercifully, no aragula, bean sprouts or tofu, or other
"bottom of the food-chain" stuff

Definitely NOT recommended by cardiologists, worldwiide

/ed B in NC

Dig in!

[kheper]

All of these low powered SE designs make for a very tasteful, and a heavily laden table of a virtual smörgåsbord of tube-ology

/ed B in NC

On the other hand, it is a very short distance from the smörgåsbord table to the garbage can

[soundbrigade]

But I always keep a sick bag ready ...

[dhuebert]

http://en.wikipedia.org/wiki/Sm%C3%B6rg%C3%A5sbord

Those crazy Swedes, what next?

Don

[soundbrigade]

Lutefisk ....

I have a serious question. What's the limit of "low watt"?? Is a 25W SE-amp a low-watt amp? Or a modest 8W KT88-based amp?

[EWBrown]

In ham radio, ten watts is considered the upper limit of QRP "low power", so I suppose that could also apply to audio projects

/ed B

[EWBrown]

Started the next permutation of the 6F3P / 6Ф3П design, this one with PSE-UL.
Got the metal mangling completed and mounted the trannies, tube sockets, I/Os, etc. today.

/ed B

[kheper]

http://en.wikipedia.org/wiki/Sm%C3%B6rg%C3%A5sbord

Those crazy Swedes, what next?

Don

The Swedes get deserved credit for their meatballs. However, Lutefisk (codfish marinated in lye) is a culinary atrocity

[soundbrigade]

I will take a deep look at my 5998A-PSE next week. Hopefully the GroundedGrid-amp is at least that complete that I can measure voltages, have the tubes glowing and turning the (remote controlled) potentiometr both ways.


More fine young Swedish cannibal stuff: The Nobel Comittee gave today the Peruvian author Mario Vargas Llosa the price in literature. He's a great writer - highly recommended.