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amplifier with ebl21?

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Originally posted by Gijs
i've got some ebl21 tubes with the transformers and I want to build a amplifier with them, but I can't find any schematics. does anybody have one for me? the output transformers are singel ended....


the special thing in your component set are the sinlge ended transformers.

I do not want to appear as the old radio expert here, i am only translating from my German Ratheiser tube manual:
the EBL21 tube is a duo-diode/power pentode, a weird thing fro the early radio days. It is an enhanved version of the EBL 1, but not much enhanced. The diodes were used for reference voltage generarion and demodulation somewhere in the preceding circuitry, but due to unwanted capacitive coupling between them and the output pentode, this type soon was abandoned and the two diodes were put into the envelope of preamplifier tubes.

Thius tube should be dirt-cheap, it has an L-Octal base and, well, methinks this is not a particular audio tube and sounding extraordinalrily good. But one never knows.... if you want to use it, please do not use the diodes, let their anodes float or connect them to ground with a very high resistor, 100k or higher.
here is the tube's base pinout:
1: heater
2: plate of pentode
3: g1/control grid of pentode
4: g2/screen grid of pentode
5: plate of diode2
6: plate of diode1
7: cathode/stop grid of pentode
8: heater

E_p_max: 300V=
P_p_max: 11W
P_out: ~ 5W
G_m: 9 mA/V
µ (pentode): 450
R_p: 50kOhm
µ (triode): 10 ??

typical pentode operation:
E_p: 250V=
E_g2: 250V=
E_g1/bias: -6V=
I_p: 36mA
I_g2: 4.5mA
For triode operation you wire g2 to plate; you'll need much more negaitve bias voltage on g1 for triode mode, don't ask me how much.
-30V maybe?

So , no schematic, but with the parameters you should be able to get it running as an output stage. I suggest you wire the tube as triode if you can live with the reduced output power (50% ?). For the ouput trransformer i would expect an impedance of 6 kOhm. Triode-wired pentodes very often are happy with the same load impedance as the pentode itself. So, try it out, you should be free to decide whether you sonically prefer pentode or triode operation although :) mischievous me, i'm afraid sonics will leave you no choice. Triode mode.

If you are not terribly short with money, i would consider to use a triode-wired EL84 (=6BQ5) with your output trannie. This IS a terrific sounding audio tube and it should be obtainable easily and moderately priced. 3 triode-wired EL84 make a gorgeous replacement for a 300B on a 2.3 kOhm primary. So one of them should still run fine on 6kOhm. I would not be very surprised if a triode-wired EL84 vastly outperforms the triode-wired EBL21 sonically. But not in pentode mode, the EL84 has R_p of 38, the EBL21 R_p of 50. Pentodes are terribly picky concerning their correct plate load distortion-wise; not so with triodes.
I have used EBL21's to replace EL84's in a DC SE amp using 2.5K transformer loading - sounds excellent and (the big plus!) they look so good! (If you like the look of these loctal beauties I suppose!) DC SE amp uses 8532 driver (200R cathode res. and 33K anode load) EBL21 has 3K cathode bias res. - thats it really apart from 1.5K and 47uf ht decoupling to driver stages. HT around 350v. Well worth 'hooking up' ( I know loading should be 5k but 2.5 sounds better with my speakers.


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