Stereomour II mods

[hmbscott]

I have a Stereomour II on order. I have been eyeing the Bottlehead power amp lineup for some time now, and have been learning towards the Stereomour. The Black Friday sale pushed me over the edge. However, since I have a Moreplay preamp, I don't really need or want the integrated features of the Stereomour.

Therefore, I am planning to build it as a stripped down power amp with no volume, balance or selector features. Although, I do want to build in top surface output impedance switching. I will be designing and building a custom chassis plate to support the modifications.

My calculations are that the input balance and volume pots should be replaced with a 20K/33K voltage divider. Another question, would changing the cathode resistor value from 365ohm to 360ohm be significant? I would like to change it to a Vishay naked z-foil resistor and that's the closest value I can find (see the input circuit diagram).

For output impedance switching of OT-2, I plan to use 4 DPDT switches, two of which are center-off (see the impedance switch circuit diagram).

Your review and critique will be much appreciated.

Thanks, Scott

[Paul Birkeland]

I will be designing and building a custom chassis plate to support the modifications.

That's a great idea!

My calculations are that the input balance and volume pots should be replaced with a 20K/33K voltage divider. Another question, would changing the cathode resistor value from 365ohm to 360ohm be significant? I would like to change it to a Vishay naked z-foil resistor and that's the closest value I can find (see the input circuit diagram).

You can just use a pair of 100K resistors at the input without any issues. Usually we provide Vishay cathode resistors stock, so you might consider sticking with those.  The naked foil resistors aren't always easy to use in point-to-point wiring.

For output impedance switching of OT-2, I plan to use 4 DPDT switches, two of which are center-off (see the impedance switch circuit diagram).

What's the goal with switchable output impedance?

[Paul Joppa]

The usual recommendation, as always, is to build the amp as designed until the de-bugging is done, since it's much more difficult to diagnose a non-standard configuration. I would suggest wiring in at least the volume control to start with, so you can set your normal loudness to be in the middle of the preamp's control range - then measure the pot and substitute an L-pad. The balance control can be left out since Moreplay also has one.

As PB says, 360 ohms is close enough to 365 ohms.

I did not check your switch arrangement - it's a little unclear to me. I see it allows 6 combinations for only 4 impedances. Incidentally, you show two center-tapped secondaries, but the tap is actually at the 2/3 point. But I'll attach the diagram of the switch array we used to offer, which uses two DPDT switches. (It also allows a third switch to select balanced or single-ended outputs.)

You could use a 4-pole 3-position rotary control (per channel) using the balance and volume mounting holes, if three impedances are enough.

[hmbscott]

That's a great idea!

Thanks Paul! I will share what I came up with when I have finally hashed out the details.

You can just use a pair of 100K resistors at the input without any issues.

Got it. Just for my understanding, that seems like it would increase the input impedance from 53K to 200K, and would drop the voltage presented to the Rstop from 63% to 50% of the value at the RCA jack. I think I understand why that probably wouldn't make any practical difference. Are there limits on where the input impedance should fall?

Usually we provide Vishay cathode resistors stock, so you might consider sticking with those.  The naked foil resistors aren't always easy to use in point-to-point wiring.

I used naked foil resistors on my Moreplay without any difficulty, but I won't make a decision until I am assembling.

What's the goal with switchable output impedance?

Well, fun of course! It's fun to design the circuits and the hardware to implement it. And, it can look cool if I do it right. And, I am a little curious to hear the differences between output impedances in my system. I'm sure that after a few weeks (or a few minutes) I will settle on a specific choice and likely never change it again. So it's not super practical. But, this is a hobby right? practicality isn't the point, rather maximum enjoyment value is my goal! That has a lot to do with the entire modification exercise. I don't honestly expect I can improve on your design. If I am careful, I can probably avoid making it sound any worse. :-) But, but in the process I can have lots of fun with some engineering creativity, and I can make it uniquely my own in the process.

[Paul Birkeland]

Just for my understanding, that seems like it would increase the input impedance from 53K to 200K

No, just solder a 100K resistor across each input jack and call it good.

The higher you go with input impedance, the more sensitive the input of the amp can become to external interference, and eventually you can exceed the limits of the 12AT7 and get movement in the operating point that isn't desirable.  If you go too low, you will end up loading down your sources.

[hmbscott]

No, just solder a 100K resistor across each input jack and call it good.

Thanks Paul,
Oh, I had misunderstood, I thought you were describing a voltage divider. So this is just 100K input impedance and full strength signal into the amp. I think I understand it.

[hmbscott]

The usual recommendation, as always, is to build the amp as designed until the de-bugging is done...

As PB says, 360 ohms is close enough to 365 ohms.

I did not check your switch arrangement - it's a little unclear to me ...

Because I will be making a custom chassis plate with no provisions for mounting components I won't be using, the recommended build-as-designed best practice would be nearly impossible. I'm not an EE, but I do have quite a lot of practical electronics experience, so I have confidence I can get away with this (hopefully not misplaced :-).

Good news on the 360 ohms working. I thought it might. I have done some dabbling in designing tube circuits and those calculations seemed to indicate that Rc values should have some wiggle room.

Yeah, that impedance switching is a mind bender. it took me several days of staring at it to come up with my crapy circuit.  I like your switching circuit much better than mine. It looks like I can use two 4PDT toggles to switch both speakers that way. Thanks!

[Doc B.]

I am obligated to step in here and advise that we might not be of help if this custom layout doesn't work. We're very willing to support you, however we may not be very helpful in pinpointing construction errors due to our lack of familiarity with your layout. Doing the debugging at a distance that we do on the forum depends a lot upon our knowledge of the kit layout and its potential sticking points. As long as you are willing to accept responsibility for your changes we are happy to support as we can.

[hmbscott]

Doc,

I totally understand. That is entirely fair.

Thanks, Scott

[hmbscott]

So this is what I am planning for my modified Stereomour. This is what I call a virtual build. I'm not finished with it yet, but it's one of the ways I try to avoid making mistakes with a highly modified kit, and especially how I avoid mistakes on the custom chassis plate. I don't finalize it until the kit has arrived and I can verify all the dimensions and components.

[hmbscott]

Here's my finalized virtual build and custom chassis plate. I'm pretty satisfied with the vent-perf art that expresses my impressions of Bottlehead and the Stereomour.

I maintained the relative location and orientation of the transformers, tubes, lug strips, etc. and so the basic wiring layout is unchanged, except for the simplified input and impedance switched output. Inputs are up front to minimize the input signal path, idea stolen from my Eros II. One notable wire change, 16GA shielded twisted pair for 2A3 heater and speaker-level wiring.

[Karl5150]

Scott, I have done a couple of custom builds on BH amps, but not near as ambitious as yours. I agree with your thoughts on taking a proven design and thoughtfully making it your own. Are you doing the chassis plate yourself? Good luck and keep us posted, I'm sure I'm not the only who would be interested in tracking your progress.
Karl

[hmbscott]

Karl, I've been using sendcutsend.com for laser cutting my chassis plates. All my Bottleheads are custom. I appreciate the interest and will post occasionally. Currently I have the kit, and all of the upgraded components except for the chassis plate itself, which I have been too chicken to order yet. It needs at least a couple (or 10) more passes for error checking. SCS is projecting shipping it Jan. 2 if I order today.
Scott

[Paul Birkeland]

SCS will reject that design as is if all the black regions are cutouts.  I would also not be surprised if you pushed down a 2A3 into a socket and the panel tore or the socket reoriented itself in unusual and unpredictable directions. Also keep in mind that the middle lug of the hum pot touching the chassis will cause amplifier meltdown, so you want the metal there to be sturdy too.   

[hmbscott]

Paul, Yes, all the black areas are cutouts. I did carefully follow all SCS's design rules. Ordered the chassis last Thursday. SCS accepted it without comment and completed laser cutting yesterday.

The plate is 0.100 thick aluminum, so despite the cutouts, I think it will be plenty sturdy. Regardless, I will test it carefully in light of your concern before committing to the build and report back mechanical integrity. It sounds like heat-shrink over the hum pot leads could preclude inadvertently building an arc welder!