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Steromour OPT upgrade

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Fred_P:
While my stereomour kit is on the way I thought it would be a great time to explore the MQ opt upgrade options.
I had already started this thread http://www.audioasylum.com/forums/magnequest/messages/1/11450.html on audio asylum
where Paul shared the necessary information for the design of a drop-in replacement, but Mike said in the end he is currently too busy with other projects to attend to this request.

Since this has become clear, I was wondering what other options would still be viable...
Would for example converting my stereomour to 45 and freeing the OPT of the bass duties put something like the EXO45 + BH2 within reach?
I plan to use open baffle speakers which will not go deeper than 200 Hz anyway.
Below that a pair of active subs would take over, which were fed from an external LDR volume control unit in front of the amp.
This would probably necessitate the recalculation of the parafeed caps as well...

Thanks for your help

Paul Birkeland:
IMO, there is something to be gained in the low frequency response by going to a Magnequest plate choke when using the 2A3, but you'd want one with flying leads (not sure Mikey will make that for you).

If you are using the 45, the upgraded plate choke becomes less important, as the stock one has a great amount of inductance for the 4K primary. 

I would bet that Sowter would make you a Nickel/M6 4K output transformer.  You would need to send them some dimensions though to see if they could use the correct laminations and channel frame to make it drop in.  (These will be about $500-750/pair)

-PB

Paul Joppa:
You can substitute an EXO-45 or 46 (5000 ohm primary) when using the 45. The stock plate choke is 40 henries, while the BH-2 is 50 henries, a small but probably audible difference in the bass, less so for the projected above-200Hz operation. Capacitor of 1.6 or 2.0uF is the target value respectively, with a range from half to twice the target being a reasonable range for experimenting.

I would expect 20% reduction in power, i.e. 1.6 watts.

The EXO-45 cannot handle more power than 2 watts in the bass, but if the bass is restricted before the power amp then the power capacity goes up and it should work fine with the 2A3. Here the 20 henry plate choke (2A3 tap) could be replaced with a BH-6 or a BCP-15, but only if you can get Mike to put on flying leads like the BH-2 - you do NOT want exposed terminals at 400 volts! Expect -20% power as above. Or you could keep a 45-rated plate choke (BH-2 or stock) and run the 2A3 as a 45, keeping the higher plate voltage but using the 45's 1600 ohm cathode resistor.

If you have the $$$ I recommend the all-nickel laminations for their delicacy in the treble; bass is their weak point. But lamination preferences are all over the place - YMMV.

Not that we have not tried any of these combinations - no promises or guarantees!  :^)

braubeat:
If you really want to go wild the Dowdy plate chokes have flying leads. You can get them in 40ma. or 60ma. Of course they are huge. Over 100H.

Michael

Fred_P:
What a wealth of useful information  :)
This is why I love this place....

The easiest way seems to be the 45 conversion, and a pair of full nickel EXO-45.
I suppose the 45 conversion itself has been discussed in some other threads already...

As for the power reduction, I am currently totally happy with the 2 Watts from my SEX driving 88dB sensitive fostex OB speakers.
The new speakers I plan to buy are rated 100dB, so absolutely no issues there. 

Could the stock OPT of the Stereomour then be used to upgrade my SEX 2.0?
That would be a great side benefit. Would I want/need to upgrade the choke in this case as well?

Thanks

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