Bottlehead Forum
Bottlehead Kits => Stereomour II => Topic started by: hmbscott on December 04, 2023, 05:41:30 PM
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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
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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?
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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.
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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.
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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.
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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.
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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!
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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.
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Doc,
I totally understand. That is entirely fair.
Thanks, Scott
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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.
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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.
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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
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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
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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.
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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!
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I'm definitely surprised that they took that on and I'd be interested in seeing how it looks when you get it!
If the socket are feeling a little squishy, you could draw up a filler plate to mount underneath that, then just paint that a contrasting color to add some stiffness to the top plate.
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I received my custom chassis plate today. SendCutSend did their usual amazing work! I did a couple of strength tests, and far as I can tell my custom plate has approximately the same stiffness as the standard plate. There was no issue with inserting a 2A3 (I realize I installed the socket backwards). Note the degree of deflection in each plate while supporting a 10-lb barbell weight and after it was removed.
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That came out well!
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I like it!
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Thanks! I’m happy with how it turned out.
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Well done, it will truly be a functional piece of art. Thanks for the photos and inspiration.
Karl
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Looks cool! Would you mind sharing the cost of the custom chassis plate?
Thanks, and good luck with the build,
Pete
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Thank you!
Yeah, so it's not cheap. There's a lot of cutting on this one, so a lot of machine time. For comparison, SendCutSend quoted me for the standard chassis plate, $68.85 in qty of one in 5052 aluminum 0.100 thick. They have a cool instant quote feature just with uploading a design file. So that gives you an idea of what you could expect for something simple like holes resized or moved around. But, suppose you wanted the standard layout, but in C110 copper 0.125 thick. That quotes at $352.31.
My custom chassis plate is 5052 aluminum 0.100 thick. The cost was $356.74 (less tax and shipping). If I had ordered my design in 1/8" copper it would have set me back $679.57. I thought about it, but that's both more than I wanted to spend, and because its thicker it would have required an increase in the minimum feature size, which would've changed the look of the art.
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A unique plate in every sense of the word. Nice expression of creativity!
I'm still trying to figure out how Doc got his head inside that bottle ......
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Sweet.
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I'm still trying to figure out how Doc got his head inside that bottle ......
He had to hold his breath.
Have you considered, once it's done, experimenting with a little under hood light. Maybe a gentle "filament orange" glow or something...John
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That could look pretty slick. Shortly after I built my Moreplay, I was messing around with under-lighting, before I just settled for an illuminated switch. I temporarily placed a red bicycle tail light under the chassis to see what it would look like. Gave it an interesting look for sure. Strategically placed, subtle lighting could really make it pop IMO.
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Cool, looks nice! how did you wire that?
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Cool, looks nice! how did you wire that?
I didn't end up going that direction, it was just a simple test with a red light I had laying around. Of course the unit was off when I placed it in there and well away from the IEC socket.
I never got to the point of wiring it up but it could be done in a way when you turned the unit on the lighting would come on with it. I would definitely get some advice on the best (and safest) way to integrate the power needed for the illumination in a way to obviously not affect audio performance or worse ... electrocution.
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2wo, ... light under the hood ... Yes, Have been kicking that idea around. On a related note I have been thinking about the paint job that might go well with lighting, perhaps an orange to purple to black gradient "evening sky" like this (w/o stars), which I think I would have to get professionally done because I'm not that talented. If done well it could be awesome.
I'm torn because my other Bottledhead gear is Rust-Oleum Forged Hammered Antique Pewter, and I like the fact that they all match, but I doubt Antique Pewter would be complemented by under lighting, so ... I'm procrastinating on painting for now.
Mucker, that under lighting looks very cool!
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Thanks! I think a tube glow amber under the bottlehead logo would look superb. Other colors in the other areas. Whatever you do with it, I'm sure it's going to follow what you've been doing with it, which is excellent!
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Awesome or Just Too Much?
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Um ..... AWESOME!
The top of your buildings need red flashing lights for airliner safety ... LOL.
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You deserve some kind of bonus if you do a vinyl wrap on that panel and hand trim all those cutouts.
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Paul, that's just crazy talk! I'm going to see if I can locate a shop that can air brush it. I have a local shop that has done some pretty fancy cerakote work. https://rcrefinishing.com/cerakote/
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I dropped off the chassis, transformer cover and chokes at RC Finishing today. Robert said it will be 2-4 weeks to complete. So hopefully I will be able to start the build in February. Getting excited!
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Finally! I just picked up my finished chassis plate and transformers. I think Robert at RC Refinishing did an amazing job I asked him to sign the back side. The last slide is my backlight assy.
Now I can start the build.
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That had to take some time!
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Awesome...
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Insane! So nice ....
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Um...Wow!
Thanks for continuing to share the journey, looking forward to seeing it once it's singing.
Karl
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Wow! That's great work.
Alonzo
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I am happy to report that I completed the build late last night. It passed all tests first time through, and I had it singing minutes later. It sounds amazing, naturally ;), thank you Doc, Paul, Paul. The output impedance switching is interesting, very audible differences. More on that later when I have had a chance to digest it.
All that white wiring is 16ga braid-shield twisted-pair multi-stranded silver-plated teflon-insulated wire (sounds fancy on paper), but is an absolute bear to work with. Seemed like a good idea at the time, but I cannot recommend it. It does neaten the appearance of the 2A3 heater wire and the output spaghetti bowl. And, contributes to the amp being very quiet, the tiniest bit of heater hum from the left channel, but I need my ear 2" from the driver to hear it with very efficient speakers.
I am not completely finished. At a future point I will be adding the Shut Regulator upgrade, and I have some RCA 2A3 black plate NOS waiting in the wings. Oh, and I have some figured Claro Walnut I plan to use to make book-matched bases for the Stereomour and the Moreplay set.
Thanks for all your encouragement!
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Is there a good source for pre-milled claro walnut? I rebuilt some amps a while ago that came with bases made out of that, and people keep asking if I can make more.
On that filament wiring, adding the DC filament upgrade will remove both the heater hum and most of the potential noise induced by having those heater lines running around, though it sounds like your speakers aren't so extremely sensitive that it's an issue.
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Hi Paul,
I bought my Claro Walnut here: https://nwtimber.com. They specialize in highly figured, musical instrument grade, etc. So pricy, but they sell very nice stuff. Another good source for more "mundane" Claro is https://www.macbeath.com. Both sell lumber, so I am doing my own milling of the base.
I'm hoping the hum isn't from heater wires radiating, because my right channel is silent, and the 2A3 heater leads are shielded with similar construction to the 12AT7 heater wiring. That said, I haven't spent much time fiddling with it so I don't really understand what's causing it. And, it's very minor anywise.
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I'll have to pop by since they aren't all that far away. I bet they could just whip out bases and finish them in house very easily.
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Wow! Gorgeous.
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Wow! Gorgeous.
Yeah, everyone was like "wow, good job building those bases", and I had to break the bad news to them that I have no idea who made them or where they came from. I sure would like a few more though!
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Nice stuffs fellas! Shunt regulator .... yeah it's cool you are waiting on that because it will give you a good idea just how great the Stereomour II can be. It transformed mine.
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... Shunt regulator .... yeah it's cool you are waiting ...
Yeah, I'm looking forward to experiencing that!. It's going to be a while. I'm currently breaking in my tubes, and boy did they need it! The first hour of listening was a bit ... it sounded good, but something was definitely a bit off, because I just couldn't settle down to listen through an entire song. It seems like 75% of the break-in happened in that first hour though, and now that I've got maybe 15 hours on it, it is sounding like butter! I'm going to give the stock Psvains a full month or so, and then swap in my RCAs. And then ...
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We need a night shot...John
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It's taken forever, but I have finally finished the Claro Walnut bases for the Stereomour and Moreplay. It was a painstaking process because I am terrible at mitering and this was an amazing piece of wood that I didn't want to F-up (required building a custom jig to do it properly). I wanted to book-match and grain wrap the bases and to try a french polish. It took somewhere between 30 and 40 coats of shellac, but I really wanted to do justice to the spectacular grain.
Over the past few months I have fully burned in both the original Psvane tubes (good) and a pair of matched NOS RCA Black plates (amazing), and damn! The Moreplay & Stereomour can really sing!
Up next, and I'm a bit afraid, because the Stereomour already sounds perfect, the Shunt Regulator upgrade and DC filament supply.
At some point I will talk about the output impedance switching I built in to my Stereomour, it's really interesting. I use it a lot more than I ever expected.
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Your continued attention to detail has really paid off. Lovely.
Karl
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Great job, love the corner spline's.
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Night Shot.
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"Hot time, summer in the city"
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I installed the shunt regulator board a couple days ago, and the DC filament board today. The shunt regulator impacted high frequency performance the most obviously, introduced a touch of sparkle and high end clarity. To me, sonically similar to adding a Speedball to a Crack. Sounds even more awesome!
The DC board completely silenced a pretty loud heater hum, which prior to, could be heard from 10 feet away, so very nice!
Like a dunce, I was bending the lead of the very last 0.13 ohm 3W resistor needed to complete the DC filament install, and suddenly the lead snapped off, WTF ?!?! Because, as I discovered, I was "bending" it with my side cutters, doh. F it! Decided to voltage test the amp anyway even though it was missing that one resistor. After staring at the circuit for a couple minutes I realized the added resistors increase the voltage feeding the heater, being paralleled with the existing 0.15 ohm resistors, dropping the resistance to about 0.07 ohms. So, testing without one should be safe.
Turns out in my case my heater voltages are already running a bit hot, the socket with both of the new 0.13's installed was running too high (2.71 VDC), vs almost too high on the socket where I'd only installed one of them (2.62 VDC). I ended up removing all of the new 0.13 ohm resistors to get the correct target 2.5 VDC on both sockets. So accidentally cutting off that lead was the accidentally right thing to do. Hah.
And with that, my stripped-down pimped-out Stereomour II is officially finished. The Stereomour/Moreplay is an absolutely delicious sonic combination, thanks to the awesome crew at Bottlehead for creating them! And thanks to all who have offered encouragement and advice. Much appreciated!
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I made a couple of upgrades recently: 1) rolled in matched Telefunken ECC81's (Used tested "near" NOS) for the original Sylvania JAN 12AT7WC. I'm becoming a fan of the Telefunken sound! 2) Upgraded my output caps, which were Audyn Cap Plus 3.3uF 800V MKP to V-Cap ODAM 3.3uF 630V. The Telefunken's were put in first and were very nice right off the bat. The caps are more subtle, but don't have a lot of runtime yet. Together the tube and cap upgrades are a nice addition to the amp, most notable is increased dynamics. The amp is also equipped with matched NOS RCA black plate 2A3s, which sound amazing. At this point I don't contemplate making further upgrades.
And, way back, I had promised to discuss my experience with output speaker impedance switching. Initially the amp was connected to 8 ohm speakers. I played around with it a lot at first and noted both louder and more high-frequency emphasis the higher I switched the impedance. Most notable, it sounded more exciting at 16 ohm, but also a bit more fatiguing, suggesting maybe more distortion. It may have reduced the bass at higher impedance settings, but that was hard to detect due to the higher output. 4ohm was a little muted, and at 2 ohm it just sounded lifeless and dull. I think a significant part of that impression might be due to reduced volume at lower impedance settings. I spent a lot of time listening at 16, 8 and 4 ohm, and eventually settled on 8 ohm as the best balanced.
More recently I have switched to 4 ohm speakers and after a relatively brief audition at various settings landed on 4 ohm setting as the best balanced although 8 ohm is not bad sounding either. A side note regarding my speakers, they present a flat impedance load at the nominal value to the amp from 0Hz up to about 5K gently rising to about double nominal at 20kHz.
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Jon,
Here is my speaker impedance switching circuit, and how I would implement an 8-16 ohm switch. Note these circuits are specific to Stereomour output transformer wiring and might need modification for other output transformers.