Bottlehead Forum
Bottlehead Kits => Stereomour II => Topic started by: lordvader on January 31, 2016, 05:21:59 PM
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Hey all.
So I really want to build a 45 amp, and was wondering if the Stereomour II (or possibly the Kaiju) can be converted to use 45 tubes ? I know the previous version did, but have yet to read anything about such a mod for these amplifiers.
Thanks !
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It is interesting that there is no mention of running the Stereomour II with 45 tubes. I'm sure it can. The main differences between the I and II are the universal power transformer and a second 12AT7 to add two triode sections that will be used in future upgrades. But lets wait for someone at Bottlehead HQ to give the official answer.
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http://bottlehead.com/smf/index.php?topic=7804.msg75442#msg75442 (http://bottlehead.com/smf/index.php?topic=7804.msg75442#msg75442)
PB's answer to this question from the launch/intro thread. SPOILER: the answer is yes. :)
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Thanks Brent!
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We have not yet generated a description for how to make the change, but we did allow for the possibility in the design phase. We decided to leave it out of the manual because relatively few make the change, and it just adds confusion for the many who don't.
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We have not yet generated a description for how to make the change, but we did allow for the possibility in the design phase. We decided to leave it out of the manual because relatively few make the change, and it just adds confusion for the many who don't.
Makes sense, Paul. Will the conversion instructions be made available on the forum via a "sticky" or something? I haven't pulled the trigger on a Stereomour II yet. I'm waiting a little to see what you guys have in mind for the upgrade goodies. Chances are good that I'll be trying the amp in both configurations since I have a nice stash of 2A3 and 45 tubes.
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Thanks for the update !
What about the Kaiju (didn't want to ask the same question twice, btw) ?
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The Kaiju will not have a 2A3 or 45 option.
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Just to clarify the Kaiju 300B amp is, in single ended tube terms, a high power amp. The Stereomour II is a lower powered SE tube amp, and a 45 wants to live in a Stereomour II. There is not really any point in putting an Aprilia RSV4 engine in an Alfa Romeo 4C. It will work a lot better in an Aprilia.
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to give some incentive to the designers I wont buy a stereomour 2 until you can convert it to #45 tubes, lol from the time PJ found the error in the 45 conversion in another topic , I have just been thrilled with my stereomour 45
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The ability to convert to 45s is built into the design, we just haven't written instructions yet. It is as simple as the original - swapping out a few resistors and exchanging a couple plate choke wires. We left out the instructions in the main manual to reduce confusion.
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no , I'm in no hurry to get the stereomur 2 , I'm just in disbelief that 1.75wpc is making such wonderful sound after 40 of spending thousands of dollars foolishly to get nothing even close to this. which has been around in radios 70, 80 years. cant get over it, and again thank you, thank you, thank you!!!!!!!!!!!!!!!!!
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Just made the change myself. Bliss
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi285.photobucket.com%2Falbums%2Fll58%2FMKane63t%2FDSC_0487%25203_zpsszea8wc4.jpg&hash=cad7dd193819f3b7bb5ae4258e8f277d2dd14431) (http://s285.photobucket.com/user/MKane63t/media/DSC_0487%203_zpsszea8wc4.jpg.html)
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So what do I need to do to get this working ?
My Stereomour II should be arriving any day now :)
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So what do I need to do to get this working ?
My Stereomour II should be arriving any day now :)
I'm really curious about this too. I really prefer 45s, and am looking forward to building one of these. Would this mod use the same output transformers as for the 2a3? What is the impedance on the OTs in the kit?
rockdoc
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The output transformers in Stereomour II are 4K ohms, and will run with 45s very well.
Conversion to the 45 is moderately simple, but there are a few non-obvious things involved. Some of the resistor values must change, and to confirm the right value we'll have to make the changes ourselves and measure the results. I THINK this is the list of changes:
Reduce power supply voltage (increase the 130-ohm 2-watt resistors)
Adjust the filament voltage (increase the 0.15 ohm resistors)
Change the cathode bias resistors
Use the higher-inductance tap on the plate choke
[Replace the parafeed output cap - optional]
Incidentally, I originally designed it to have some other output stage options, not just the 45. It's not clear which of them we might support, at what level of support. So I won't say more just yet.
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sounds good, thanks Paul!
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What operating points are the 2a3's running at (so I can math the rest) ?
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The target operating point for the 2A3 in Stereomour is 300v plate to cathode, 50mA plate current, and 60v bias.
The 45 should run 275v, 35mA, 56v bias. Be aware that at the reduced current, the PSU voltage will increase slightly.
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Conversion to the 45 is moderately simple, but there are a few non-obvious things involved. Some of the resistor values must change, and to confirm the right value we'll have to make the changes ourselves and measure the results. I THINK this is the list of changes:
Reduce power supply voltage (increase the 130-ohm 2-watt resistors)
Adjust the filament voltage (increase the 0.15 ohm resistors)
Change the cathode bias resistors
Use the higher-inductance tap on the plate choke
[Replace the parafeed output cap - optional]
Incidentally, I originally designed it to have some other output stage options, not just the 45. It's not clear which of them we might support, at what level of support. So I won't say more just yet.
Paul, any updates on the 45 conversion? This thread from a year ago is the most recent I've seen on it. Thanks!
Steve T
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The power supply voltage available needs to go from 380 to 350. The Stereomour I used a total of 1200 Ohms of series resistance in the power supply to accomplish this, you could start by replacing the four 130 Ohm/2W resistors with 600 Ohm 5W resistors. There's plenty of space in the layout for this. If you find that you don't quite have room for four 5W resistors, you can leave one 130 Ohm/2W resistor in each channel and use a 1.1K dropping resistor in place of the other 130 Ohm resistor to get your voltage drop, but it should be a 10W part.
The filament voltage is dropped from 3.15V to 2.5V through two 0.15 Ohm resistors. I would try switching these to 0.25 Ohm resistors. Be prepared to check your AC voltage after you do this and adjust as necessary. If I was assigned this task, I would buy 0.22, 0.25, and 0.27 Ohm 3W resistors, but I still wouldn't be surprised if I needed a different value.
The cathode bias resistors need to be changed to 1.6K/10W resistors.
The higher inductance tap on the plate choke means disconnecting the black wire and connecting the red wire in its place for each plate choke.
The parallel feed cap should go somewhere around 8-10uF.
If you end up doing this, please post the voltages that you end up with after the changes (including your value for the filament voltage resistors and resulting AC voltage), and PJ can look it over and see what might need to be tweaked.
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Thanks PB!
Steve T
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Parafeed cap should be around 5uF.
The stock power tube cathode resistance is 2000 ohms in parallel with 3000 ohms, giving a net 1200 ohms. For 1600 ohms, you can replace the 3000 ohm resistance with 8000 ohms, keeping the 2000 ohm resistor. In the practical real world, 8200 ohms is close enough, and a 2 watt or greater rating is adequate. I chose this design specifically for the possible 45 conversion. Later we decided that there were likely to be very few conversions actually done, so in the interest of keeping the manual simple we did not describe the change.
A third possibility at the initial design stage was to get 1000 ohms (by replacing the 3000 ohm resistor with a 2000 ohm resistor) for use with a 3000-ohm output transformer such as the one used in Kaiju, or a Magnequest TFA-2004. This has some other complications, serious enough that we are not planning to pursue this option at this time.
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Parafeed cap should be around 5uF.
[...]
A third possibility at the initial design stage was to get 1000 ohms (by replacing the 3000 ohm resistor with a 2000 ohm resistor) for use with a 3000-ohm output transformer such as the one used in Kaiju, or a Magnequest TFA-2004.
Oops, I used 3K when calculating that parafeed cap value.