OTL Asylum

OTL, Output Transformerless Amplifier User Group.

Return to OTL Asylum

Message Sort: Post Order or Asylum Reverse Threaded

12sx7's in the new MA1 Silver Edition

Posted on October 27, 2004 at 09:50:54
Hello all

I presently have M60's. They have the 12SX7's installed instead of the 6SN7's. (This is an easy change in the new M60's. These tubes require 12.6V instead of 6.3V). The 12SX7's replaced some very good NOS 6SN7's with the improvements in the M60's being quite dramatic.

Does anyone know if the auto biasing of the Silver Edition will prevent this change?. The M60's almost give me all the power I need (Coincident Total Victory speakers) and I would hate to lose the gains in the 12SX7's, just to get more power.



Hide full thread outline!
Re: 12sx7's in the new MA1 Silver Edition, posted on October 27, 2004 at 10:30:09

Posts: 10488
Location: Bethesda, Maryland
Joined: December 11, 2000
The questions to ask, and which I cannot answer, are as follows: (1) Does the Silver edition have a DC filament supply? If so, it is certainly regulated at 6.3V, so you would have to find a way of deriving a separate 12.6V filament supply in order to use 12SX7s. (2) If the Silver edition has an AC filament supply, is 12VAC available by tapping the proper transformer leads (as it IS available in your M60 and my MA240s, for example). As for the autobias circuit in the Silver edition, I don't know much about it, but it no doubt regulates bias and/or plate voltage, which have nothing to do with filament voltages, so I am fairly certain there will be no problemo with 12SX7s on that score.

But if you have Coincident Total Victorys and if you love your M60s with 12SX7s, why make a change at all? Indeed, the M60s ought to be ALL the power you need, not just "almost" all.


Re: 12sx7's in the new MA1 Silver Edition, posted on October 27, 2004 at 15:59:35

Posts: 503
Location: Saint Paul
Joined: October 22, 2004
Cousin B, Lew,

I feel that I can answer these questions..

The Silver Edition MA-1 has a DC filament supply @ 6.3VDC. The supply is derived with a full-wave rectifier (2 x 8A Schottky diodes) from the main dual-winding filament toroid. Complimentary leads from each winding are joined to form a virtual 'center tap', which serves as the negative (ground, if you will) side of the DC filament supply. The supply uses a simple pi-network filter, with three 2,200uf@16V caps, and a small (~0.16ohm) resistance between the poles of the filter. Schottky diodes are needed, as the voltage drop across standard diodes would be too large to create a useful pi filter; we need the small extra headroom that the schottkeys afford us.

I don't see any problem replacing the basic full-wave setup with a full-wave bridge rectifier, and deriving your 12.6V from that. As I mentioned, the caps are 16V devices, so will be fine to re-use for a 12.6V supply. One could also try a full-wave voltage doubler; though that type of circuit tends to be less reliable, and I don't suggest it.

A little experimentation will be required, to see if the bridge rectifier can be a standard type, or built from schottky devices; same reasons as stated earlier. Also, the resistance value between the filter poles will need to be recalculated.

Finally, and most daunting, two resistor values in the fixed-bias circuit will most likely need to be recalculated. The 12SX7 doesn't behave *exactly* like a 6SN7, so the grid voltages in the output section will probably be off. Measure them accurately with the stock 6SN7 configuration, then plug in new resistance values when the 12SX7 are in place, such that the output section grid voltages are the same as before.. roughly -66V in class A, and -92V in class AB.

Hope these comments are helpful. As always, this is my own personal opinion and does not reflect the views of anyone but myself. Your mileage may vary. Void where prohibited. You burn it, you bought it.. etc.



Re: 12sx7's in the new MA1 Silver Edition, posted on November 1, 2004 at 13:01:49

Lew, thank you for your reply.

Yes the M60's are/where great, but there is no comparison. The Siver's control the base a lot better, the music simply fills the room. The drums on Patricia Barber's Modern Cool are in my living room, as is almost Patricia herself (though not quite) The same cannot be said for the M60's.

I can't stop listening to music, my poor neighbours.


Yahoo, there in. 12SX7 heaven., posted on November 17, 2004 at 17:52:15

I just received my modified MA1 Silvers. The 12SX7's are installed. And there is no comparison. Everything is more full bodied and goose bumpy. Thank you all who swear by these tubes.

These may be the last amps I need to own. Though I would like to try the Tenors with the Coincident Total Victory???. Are we never satisfied.


Page processed in 0.022 seconds.