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Offline tpatton

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Parts upgrades, small circuit changes?
« on: December 28, 2011, 05:55:26 AM »
I just got the Stereomour circuit diagram from a friend who's just received the kit and is shy about asking questions on the Forum (which I hope to get him past), so I'd like to ask a couple for him.

(1) Are there places where "boutique" capacitors would help the sound?

(2) Are there resistors that could be replaced by chokes, as I did very successfully (thanks to advice from Caucasian Blackplate) in my two Seductions, and then did again in the S.E.X. amp?

(3) Is there anything else along these lines that's worth considering?

Many thanks to any who can advise.

Offline Grainger49

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Re: Parts upgrades, small circuit changes?
« Reply #1 on: December 28, 2011, 06:28:33 AM »
Tom,

I don't have a Stereomour manual but do have the Paramours.  I'll assume the circuit is much the same and answer:

1) Yes, the Parafeed Cap could be upgraded.  As I upgraded my iron I used an Obbligato (black) film in oil 10uF in my Paramours.  I upgraded the output transformer and plate choke thus needing that value.  A stock value will be fine for a straight upgrade.  If the Stereomour uses an interstage 0.1uF that is upgradable too.  I also upgraded the power supply caps, both.  The cathode bypass cap is in the signal path and is an electrolytic.  Film is great or a better electrolytic is a good upgrade.

2) The power supply is probably CRC, that resistor could be "choked" with the same DCR value, well, close to the same is fine.

3) you can convert to a power amp by replacing the input volume control with a fixed input resistor, or upgrade the pot.

4) The R1 on the C4S board is a final tweak you can make.  (I don't know this resistor value)  Tantalum was suggested to me. 
« Last Edit: October 08, 2012, 10:25:52 AM by Grainger49 »

Offline Paul Joppa

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Re: Parts upgrades, small circuit changes?
« Reply #2 on: December 28, 2011, 07:21:27 AM »
Grainger is correct; I'll just add some details.

The parafeed cap is 3.3uF, 630v or higher. Anything from 1.6 to 4uF is OK with the 2A3, and 3.3 to 8uF for the 45. The interstage cap is 0.1uF, 630v or higher. These caps are located on the frontmost terminal strips and there is a good bit of room for larger parts.

On the power supply board there are two 1200 ohm resistors and two 360 ohm resistors (the latter only for the 2A3) in parallel. For the 2A3, these can be removed and replaced with a choke that has about 270 ohms resistance. I would suggest at least 5 henries, and at least a 60mA current rating; a widely available spec is 10 henries 90mA such as the Triad C7-X. For the 45, you need 1200 ohms DC resistance and 40mA rating. I can't find any with enough resistance so you would need to add a resistor in series. The Hammond 154G (9 henries, 40mA, 700 ohms) in series with a 500 ohm 5 watt resistor should work.

The last power supply caps (100uF, 450v) and the cathode bypass caps (220uF 250v) are isolated from the signal current loop by the plate choke impedance, and hence less important than the signal-path caps above. Nevertheless, they are not insignificant and can be replaced with better parts. The 100uF can be smaller if you have installed the filter chokes, I'd guess 27uF minimum - otherwise use at least 100uF. The bypass caps can be smaller, say 47uF minimum, and the minimum voltage rating would be 100v - we use the larger values because we use them everywhere and buy tons of them. Both of these caps can also be bypassed with smaller high-quality caps, leaving the electrolytics in place.

All of the above power supply parts are located on the PC board and there is no room for larger parts on the board - so you'd have to get kind of creative. I would keep the chokes well away from the audio transformers so you don't accidentally pick up some added hum. Remember the power supply is pseudo-dual-mono so you need two of everything!

The cathode bypass caps are in parallel electrically with the cathode resistors, under the C4S board, and there is a little room there for replacements, or for a smaller, high quality bypass while leaving the original electrolytics in place. You might find room there for a high-voltage bypass cap as well - notice there are extra B+ holes on the PC board for just this purpose. Be careful about the heat from the cathode resistors - heat will damage caps quickly, so maintain as much space between caps and resistors as you can.

Hope that helps. Perhaps it should become a sticky.
Paul Joppa

Offline tpatton

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Re: Parts upgrades, small circuit changes?
« Reply #3 on: December 28, 2011, 04:21:59 PM »
Grainger & Paul: many thanks, from me (for my general education) and for my friend Eric who's going to build the Stereomour.  If he follows even most of these suggestions, he'll end up with a wonderful-sounding amp, I'm sure.  We may have follow-up questions, if we may.  Thanks again!

Offline ericatflc

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Re: Parts upgrades, small circuit changes?
« Reply #4 on: December 29, 2011, 09:08:18 AM »
Good afternoon,

I'm the guy Tom is speaking of and I want to thank Paul and Grainger for all the excellent advice.  I'm kinda leaning to getting a couple Triad C7-X chokes to replace the 1200 ohm and 360 ohm resistors.  I'll just have find a good spot to place them, following Paul's advice about keeping them as far away from the audio transformers.  I had really good luck using Hammond chokes to replace power supply filter resistors in the Seduction I built last year.  I made quite a few part substitutions in the Seduction build, most from recommendations made by Tom.

If the 0.1 microfarad Russian teflon caps that Tom likes so well have a high enough voltage rating, I may want to them as interstage coupling caps. 

I do have a question however.  I order the Stereomour kit without an enclosure because I plan to make one using Lexan.  (I built a Lexan enclosure for my Seduction and I think it is killer!).  I know size the Lexan pieces have to be to properly to accomodate the 10" x 12" top plate, but what height should I make the enclosure so that there is plenty of space inside for the completed amp?  I suspect I can figure it out once I get the amp built, but I want to get started on the enclosure.  I have a part time job (I'm semi-retired) at a local glass company and the shop foreman will cut scrap 3/8" Lexan for me to the sizes I tell him.  He'll also cut the rabbets for me.  So how tall shall I have him cut them?

Again, thanks for all the well thought-out ideas from you experts; I appreciate it a lot.

Eric

Offline InfernoSTi

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Re: Parts upgrades, small circuit changes?
« Reply #5 on: February 10, 2012, 02:36:01 PM »
Just to let you know, the two things that I really love most after tweaking my Stereomour are:

Number 1 tweak: Mundorf Silver in Oil capacitors in parafeed and coupling cap locations (I recall it was about a $200 investment with results that are simply stunning for such a simple part change).

Number 2 tweak: removing the volume pot and selector switch from the input path.  I am using a Warpspeed Optocoupler for volume control and input selection duties instead.

I would be remiss if I failed to mention the lovely JJ 2A3-40 tubes and the Pvsane 12AT7 tube that really are the real deal to my ears.

Enjoy your new Stereomour!

John
John Kessel
Hawthorne Audio AMT K2 Reference Speakers
Paramount 300B w/MQ All Nickel Iron,  Mundorf S/G 5.5 uF,  and  Vcap Teflon .1 uF
Auralic Vega DAC
CAPS V3 w/SOtM Linear Power Supply & PPA USB

Offline Grainger49

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Re: Parts upgrades, small circuit changes?
« Reply #6 on: February 10, 2012, 03:29:46 PM »
Eric,

The Triad C7-X is a resistance of 270 ohms.  That is a good replacement for the 360 ohm resistor, it could be a higher resistance.  The C7-X at 270 ohms is not a good replacement for a 1200 ohm resistor, but the Hammond 155J (1026 ohms, good for 65mA) I think it would probably do well there.  My hesitation is I don't know what current it will see.

The Stereomour will have some heat under the lexan enclosure.  Be certain that there is an air space to allow convection cooling of the cathode resistors under the top plate.  

The base is 3 or 4 inches tall but I don't have the Stereomour so I'm not quite sure.  Someone will measure it for you.
« Last Edit: February 10, 2012, 03:39:38 PM by Grainger49 »

Offline shelby1420

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Re: Parts upgrades, small circuit changes?
« Reply #7 on: February 21, 2012, 08:02:22 AM »
Hey Guys, I have ordered the Trida C7- X chokes and will swap them for the 360 ohm 5 watt resistors...... any advice as to where they should be placed and also has anyone had any luck finding  a choke for the 1200ohm resistors?? If not is it worth only doing the c7-x's???
Enjoying the music

Rick

Offline Paul Joppa

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Re: Parts upgrades, small circuit changes?
« Reply #8 on: February 21, 2012, 08:31:09 AM »
You must remove BOTH the 360 ohm resistors AND the 1200 ohm resistors, in order to install the 270 ohm choke for 2A3 use. They are in parallel (360 ohms in parallel with 1200 ohms is 277 ohms). If you only remove the 360, then it will be shunted by the 1200 ohms and will lose most of its effectiveness.

The purpose is so that by removing the 360 ohm resistor you are left with 1200 ohms, which is the value needed for a 45.

As I said before, I have no advice on where to put a choke - the layout was not designed with that in mind. And yes, if you do the mod for a 45 you will need at least one more terminal somewhere to connect the series resistor. You could use a 900 or 920 ohm resistor (10 watts preferred) with the C7-X. Remember all these parts are at full (lethal!) high voltage and must be inaccessible in operation!
Paul Joppa

Offline shelby1420

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Re: Parts upgrades, small circuit changes?
« Reply #9 on: February 21, 2012, 08:53:33 AM »
Ahhhhhhhh, ok!!!!  So am I right to assume that I attach either ( not directional ) wire from the choke to removed 360ohm resistor end ( empty hole on board)  and the other wire from choke to the other end of same removed 360ohm resistor (empty hole on board)-----and then repeat for other side 360 ohm resistor with second choke ?   I am running 2a3, do I need to do anything with the holes left from removing the 1200ohm 10 watt resistors???  Oh, and thanks for answering my questions, clueless but eager to learn!!!!
Enjoying the music

Rick

Offline Paul Joppa

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You got it! (nt)
« Reply #10 on: February 21, 2012, 09:23:26 AM »
nt
Paul Joppa

Offline shelby1420

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Re: Parts upgrades, small circuit changes?
« Reply #11 on: February 21, 2012, 10:10:47 AM »
Hey Paul, thanks!!!! Last question the holes on the PC board left by removing the 2 1200ohm resistors......what do I do with them do I need to run a jumper from them to the 360ohm holes or simply remove them and leave them???
Enjoying the music

Rick

Offline Grainger49

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Re: Parts upgrades, small circuit changes?
« Reply #12 on: February 21, 2012, 10:14:50 AM »
Leave those holes open.  Putting in a jumper creates a short where you really don't want one.  I'm saying don't replace a resistor with a jumper.

The ends of the 1200 ohm and 360 ohm resistors are already jumped together on the circuit board.

Offline shelby1420

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Re: Parts upgrades, small circuit changes?
« Reply #13 on: February 21, 2012, 10:26:31 AM »
Awesome, thanks!!!
Enjoying the music

Rick

Offline shelby1420

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Re: Parts upgrades, small circuit changes?
« Reply #14 on: February 22, 2012, 05:50:01 AM »
Ordered the chokes and Elna Cerafine caps to replace existing caps on board, I will update when it is done..........
Enjoying the music

Rick