Bottlehead Forum

General Category => All things MagneQuest => Topic started by: Fred_P on January 20, 2015, 10:15:50 AM

Title: Steromour OPT upgrade
Post by: Fred_P on January 20, 2015, 10:15:50 AM
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 ( 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

Title: Re: Steromour OPT upgrade
Post by: Paul Birkeland on January 20, 2015, 10:47:09 AM
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)

Title: Re: Steromour OPT upgrade
Post by: Paul Joppa on January 20, 2015, 11:41:10 AM
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!  :^)
Title: Re: Steromour OPT upgrade
Post by: braubeat on January 21, 2015, 04:59:53 AM
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.

Title: Re: Steromour OPT upgrade
Post by: Fred_P on January 21, 2015, 06:58:36 AM
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?

Title: Re: Steromour OPT upgrade
Post by: Paul Joppa on January 21, 2015, 09:13:35 AM
The current SEX and Stereomour use the same plate choke and output transformer, and is (IMHO) a big improvement over the old versions. In fact we sell an iron upgrade kit for the older SEX amps, consisting of these parts.
Title: Re: Steromour OPT upgrade
Post by: chard on March 22, 2015, 09:31:54 AM
 I have a stereomour configured with a 2a3 output. I think I would like to upgrade my current plate choke which provides 20 henries to a magnequest plate choke that provides preferably 50 henries and gets more bass. I am planning on dealing with the exposed taps with a 3 sided wooden shield, open at the top and at the side facing the power transformer. What are the henries and ma. ratings for the BH6?  If I get a nickel version of the BH6 or the BCP15 will the fact that I don't have a nickel output transformer negate the treble enhancement effect of the nickel choke? Will the difference between a silicon steel and a nickel choke effect the bass much? Can either of these cokes fit on the top of the stereomore chassis? My 2a3's draws about 50ma. The BCP15 is rated at 50ma 40 henries for the nickel plate choke and 40ma 50 henries for the silicon steel choke. Will either the silicon steel or the nickle BCP15 choke have any trouble handling 50ma.

Title: Re: Steromour OPT upgrade
Post by: Paul Joppa on March 22, 2015, 12:25:41 PM
Here are a few clarfications:

BH-6 has the same specs as a BCP-15 gapped for 50mA (i.e. 40 henries), but with superior winding techniques. The winding technique - not the core material - provides some treble enhancement.

Neither is offered with a nickel core, since that would reduce the current capability with little other effect. Core material is much less important in plate chokes because they have many more turns of wire.

The solder terminals are exposed, making it dangerous since the choke terminals are at the full power supply high voltage. If mounted topside, the terminals will be on the top - a box "open at the top" will be poor protection. I cannot recommend it, but a much less unsafe arrangement would place the output transformer on top. Its primary is at the 2A3 cathode voltage (60 volts), still above the nominal 24vDC safety threshold but way better than 380vDC (!!!) You would still want to insulate and protect the two primary terminals, since accidentally shorting them to the chassis will remove the bias from the 2A3, rapidly damaging the tube. The secondary, including the impedance switch board if you are using it, is safely grounded when wired correctly.

Both chokes have the same mounting dimension as the stock choke and output transformer.
Title: Re: Steromour OPT upgrade
Post by: chard on March 22, 2015, 01:01:30 PM
  Thanks Paul,
    What would be the parafeed cap value since the plate choke value is changing from 20 henries to 40 henries? The current parafeed cap value is 3.3uf. Also how big a change in bass would this be, hardly noticeable or pronounced?

Title: Re: Steromour OPT upgrade
Post by: Paul Joppa on March 22, 2015, 05:26:03 PM
My target value is 5uF - as always, 2.5 to 10 is a good range, it's not too critical.

It should get you nearly an octave more depth, but it's all below 30Hz. You're more likely to hear cleaner bass than deeper bass, unless you have really unusual speakers....
Title: Re: Steromour OPT upgrade
Post by: chard on March 22, 2015, 05:30:01 PM
Thanks Paul for this information.