Transistor phase splitter (Yaqin MC-84L)

[Cobra2]

I noticed this Yaqin amp has a transistor for phase-splitter, is this a good solution? Unusual "modern" for a chinese amp?
http://yaqin.slickpepper.org.uk/wp-c...4l_circuit.jpg
Datasheet 2SD669:
http://www.datasheetcatalog.com/data...6/2SD669.shtml

Arne K

[Eli Duttman]

I'm not enthusiastic about that design, in general.

The BJT is a "concertina" of sorts and should not do too much sonic damage. Perhaps the load is truly split. Still, non-symmetrical resistances in a "concertina" phase splitter raise doubts in my mind.

I don't like the non-symmetrical 12AX7 SRPP voltage gain block at all.

IMO, the design has a "Cheap Charlie" aroma.

[SY]

No kidding. And to make things worse, the SRPP uses up a potentially good section of the 12AX7 to no real advantage.

If it were my amp, I'd rewire it for a grounded cathode input stage direct coupled to a split load tube. Because of the high impedance input of the split-load, you can use a CCS on the first stage, which gives you plenty of gain to make the feedback effective. 12AX7 is a better choice than its reputation would suggest as long as the plate load is VERY high, but a 12AT7 would be my first choice there.

Given the slipshod design, you might want to examine the output transformers carefully to see if it's worth trying to polish a turd.

[audiowize]

It might almost be easier to just implement a simple/classic 12AT7 directly coupled cathodyne phase splitter. Of course, an amp like that might be built on a board, or have iffy output transformers as some have suggested.

[Cobra2]

So, if I have an amp with little space, and limited filament-supply, this is still a bad option...maybe I can squeeze in one 6111 sub-mini tube (as gain-stage?).

Arne K

[audiowize]

You can rework this amp with one 12AT7 per channel. I think you will be ok in terms of filament supply. If you are still hurting, use a single triode and a transformer to minimize filament requirements. How about you post some shots of the inside and transformers.

[Eli Duttman]

Originally posted by Cobra2
So, if I have an amp with little space, and limited filament-supply, this is still a bad option...maybe I can squeeze in one 6111 sub-mini tube (as gain-stage?).

Arne K

Arne,

The Chinese design you showed SUCKS. However, there is nothing wrong (IMO) with a SS "concertina" phase splitter, to save space and filament power.

DC couple a high voltage enhancement MOSFET to the the common cathode voltage gain tube. Employ identical source and drain resistors of a value appropriate to the I_D desired. If a wimpy 12AX7 or similar type is the voltage gain device, use a ZVN0545A as the splitter. Other tube types in the voltage gain "hole" can drive the reverse transfer capacitance of the IRFBC20, which is more capable than the little ZVN0545A.

[Cobra2]

Thanks for the advice, - it is really for an EL34 amp.

Chinese EL34 amp to modify

I'm just looking for options, and wondered if it could be an good idea.
I left this amp on the shelf, when I ran out of parts....and now hope to get it running again. I managed to squeeze a couple of ST-70 output irons on top instead of the cut-up-Hyundai and stolen phone-wire OPT's ;-)

Arne K

B.T W.: Any more "common" mosfets I can use?

[Rockmore]

Received a MC-84L as a xmas present. It's stock. To get this thread rolling again, would like those of you interested, owning, or modding them to show exactly what and how you've done and/or what needs to be done.

[kenpeter]

base fed from cathode end of srpp resistor is not symetrical use of triodes, but errors to the side of providing needed base current.
I dont see a problem with it.