Bottlehead Kits > Stereomour

Parts upgrades, small circuit changes?

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tpatton:
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.

Grainger49:
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. 

Paul Joppa:
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.

tpatton:
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!

ericatflc:
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

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