Bottlehead Forum
General Category => General Discussion => Topic started by: absolutk on July 25, 2020, 10:02:41 AM
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Hi all,
Long time DIYer here, but mostly solid state and digital gear. I inherited a pair of these Bottlehead amps. Built by a detail-oriented friend of a friend, they exchanged hands with no money involved. Upon cursory search, these appear to be Paramounts, but I couldn't be sure. Were 2A3s a direct swap for 300Bs?
I wish to polish them up (proper enclosure) and put them in permanent service with my newly acquired Omega Super Alnico Monitors, but had two questions before I embark on the journey:
1) They sound superb, but one of the channels sounds stronger than the other. How should I approach balancing them? Replace signal or power tubes? Adjust bias?
2) Have there been any updates to the design that I can incorporate as I rework them?
Thank you!
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Those appear to be Paramour II, which shared a chassis plate with Paramount. The PT-2 power transformer makes it pretty certain - look at the sticker on the transformer, there is only one 2.5v winding, so 2A3 only.
For balancing the channels, you'll need to figure out why they are unbalanced. That's a separate adventure, but you can start by swapping the tubes between channels to see whether the unbalance follows the tube or not.
Yes, there are upgrades available. The plate choke and output transformer can be replaced with the current-production Stereomour II iron which will give a substantial improvement in the bass extension and power, and the driver can be upgraded to a shunt-regulated stage using either the stock 12AT7 or the more linear 5670.
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Thank you Paul, for such a thorough response.
I hooked them up today, and enjoyed them for the first time since the pandemic started. What a joy.
The adventure to chase the unbalance was short; the power tubes are OK, but it followed the 12AT7. Time for new ones I suppose. Can my current configuration also support a 5670? Or is that only with the newer, shunt regulated design? Tube recommendations are always appreciated.
I’m curious about the upgraded plate choke, output transformer and driver stage. What would these parts cost and would there be a market for any of the parts I swap out? These Paramour IIs will only be for occasional listening and, eventually someone else’s to experience and enjoy. I would hate to create waste in the process.
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I'll have to research it to see whether some resistors would need to change, but otherwise the 5670 should be quite practical. You'd have to rewire the socket anyway...
In some later designs, the 12AY7 (or it's high-end version 6072) would plug in successfully, but I'd have to check on the Paramour II.
Or if you want to go extreme, how about a 6J5? Not much gain, but way more linear than any higher-gain tube I've found. :^D
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I’m curious about the upgraded plate choke, output transformer and driver stage. What would these parts cost and would there be a market for any of the parts I swap out?
For new iron, see here: http://bottlehead.com/product/iron-upgrade-kit-for-s-e-x-kit-2-0/ (http://bottlehead.com/product/iron-upgrade-kit-for-s-e-x-kit-2-0/)
As for resale value of the old iron - I can't tell what output transformer that is, but the choke is a Triad C-7X which retails new for around $10.
I'll have to research it to see whether some resistors would need to change, but otherwise the 5670 should be quite practical. [snip]
how about a 6J5? Not much gain, but way more linear than any higher-gain tube I've found.
@Pj: will follow what you have to say about both options with great interest!
cheers, Derek
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Part of the iron upgrade for the Paramour II is to repurpose the Triad C-7X as a power supply choke. I believe the mounting holes in the top plate are already there to accommodate the move. Then its just a matter of removing one of the resistors in the power supply (270 ohm as I recall) and wiring the Triad C-7X in its place.
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Part of the iron upgrade for the Paramour II is to repurpose the Triad C-7X as a power supply choke. I believe the mounting holes in the top plate are already there to accommodate the move. Then its just a matter of removing one of the resistors in the power supply (270 ohm as I recall) and wiring the Triad C-7X in its place.
Thanks for the reminder - I had forgotten that!
Yes, the C-7X can mount behind the power transformer. There are no detailed instruction for re-wiring (unless someone has posted their mod?) but we can address that later. The upgrade kit that Deke609 posted is for a different amp [edit: it has the right choke and transformer, but the instructions are for an older version of the S.E.X. amp.]
The original output transformer is a 10-watt 70-v line output on the 1.25-watt tap, originally intended for small distributed PA systems. It originally cost $3.95 if I recall correctly - it just happened to sound quite good, though limited in deep-bass power handling. You can toss it with a (relatively) clear conscience.
For historical accuracy, I should mention that back in the day there was an upgrade for this amp, using a Magnequest plate choke and a custom output transformer, neither of which are on the market today.
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Thanks for the reminder - I had forgotten that!
Yes, the C-7X can mount behind the power transformer. There are no detailed instruction for re-wiring (unless someone has posted their mod?) but we can address that later. The upgrade kit that Deke609 posted is for a different amp.
The original output transformer is a 10-watt 70-v line output on the 1.25-watt tap, originally intended for small distributed PA systems. It originally cost $3.95 if I recall correctly - it just happened to sound quite good, though limited in deep-bass power handling. You can toss it with a (relatively) clear conscience.
For historical accuracy, I should mention that back in the day there was an upgrade for this amp, using a Magnequest plate choke and a custom output transformer, neither of which are on the market today.
Paul, wasn't there also an increase in the parafeed cap value? I know that on my iron upgrade with 45's there was. May not apply to the 2A3 though.
I may have some more detail on the PS choke upgrade. I'll have to dig around a bit.
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The upgrade kit that Deke609 posted is for a different amp.
Oops. Apologies. I figured that since it is the same iron used in the Stereomour 2A3, it would work in the Paramour.
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Oops. Apologies. I figured that since it is the same iron used in the Stereomour 2A3, it would work in the Paramour.
It is the same iron, but the instructions are not for the Paramour II. I'll edit my post - it wasn't clear.
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Thanks Paul, Derek and Natural Sound for all the guidance!
So if I understand correctly, the upgrade path, should I choose to explore, would involve:
1) Repurposing the current C-7X to become a power supply choke (replacing a resistor. There’s a healthy 5w, 270 ohm joining P5 and P11. Is this the one?). The C-X7 moves to the top side, adjacent to the power transformer, giving two of the empty holes a purpose.
2) A $200 kit provides a plate choke to replace the C-7X and an upgraded output transformer to replace the current, inexpensive, yellow-tape output transformer. These will still offer 4 ohm and 8 ohm taps.
3) The topic to change from 12AT7 to a 5670 or 6J5 is completely orthogonal. It, at the very least, requires a re-wiring of the socket (but the socket itself can remain the same - they both appear to have the same 9-pin mount), but may also need additional modifications/components.
4) The precise instructions for rework will need some investigation, so I better start earning some positive karma.
Did I grasp this right?
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You got it.
To clarify - the C-7X goes on the underside of the chassis - all the iron components do, so the appearance does not change. It would use the existing two holes, behind the power transformer if the amp is oriented with the tubes in front.
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I am going to recommend that you leave the 12AT7 in place. The cost and the amount of work to go to the 5670 will be somewhat significant, and the amount of support that we can provide to debug your amps if there is an issue will be rather limited.
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The precise instructions for rework will need some investigation, so I better start earning some positive karma.
I have a pretty comprehensive upgrade procedure that I copied from a Bottlehead WEB page that no longer exists (that I know of). Out of respect for Bottlehead's intellectual property I won't be posting it here. I wanted to send it to P.J. But didn't have his e-mail address so I sent it to Doc B. to forward. Be advised that the holes in the top plate work great for the Triad choke but require a bit of rework for the PC-3 / OT-2 Bottlehead Iron. The hole pattern on the top plate is for discontinued Magnequest iron that Mr. Joppa eluded to earlier. If you decide to go this route I can photograph my system to show you how I dealt with this. It's pretty trivial and well worth the effort.
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I should have mentioned this earlier. If you want an instantaneous pop in performance without changing anything else consider replacing the .1 interstage cap with something a bit better. For about $21 plus shipping I recommend a Theta .1uf 600v from Parts Express. I've used these in several Bottlehead designs and they sound great for the price. Others may chime in with other recommendations.
https://www.parts-express.com/audiocap-ppt-theta-010uf-600v-film-foil-capacitor--027-710 (https://www.parts-express.com/audiocap-ppt-theta-010uf-600v-film-foil-capacitor--027-710)
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Fortunately, Natural Sound forwarded to me the old instructions for the Magnequest iron upgrade package for the Paramour II. That package is no longer available, but the instructions are mostly the same. I'll see if I can edit them or at least provide a side commentary to adapt them to the currently-available upgrade. I think it can all work out without drilling holes or changing the appearance. It may take me a while, I have a lot of things on my plate at the moment - but if the OP decides to go ahead with that upgrade, I'll raise the priority.
I agree with PB, keep the 12AT7. Sometimes I get carried away with possibilities and lose track of what's practical :^)
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Sincere gratitude for all the knowledge.
Given the sheer number of open projects, I think it is prudent to shelve the iron upgrade for now, and the tube change permanently. Though, I will get replacement 12AT7s, redo the casework and upgrade the 0.1uF per Natural Sound's advice. Is that value critical? I found a pair of JantZen 0.15uF 2% caps (Red) in my parts bin. Would those work?
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0.15uF would work just fine in those.
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Excellent - thank you!
0.15uF would work just fine in those.
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The improvement from the iron upgrade would be monstrous. It goes well beyond the subjective terms. You will get more power out of the amps and significantly deeper/cleaner bass.
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The improvement from the iron upgrade would be monstrous. It goes well beyond the subjective terms. You will get more power out of the amps and significantly deeper/cleaner bass.
+1 for the iron upgrade. My Stereomour compares well with my amps with Magnequest iron.
With the iron upgrade, would he have the option of converting to 45 output tube op points?
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I'll clarify - I would *love* to pursue the iron upgrade. It's just that I'm stuck in a rut amongst two audio projects (this and a small solid state integrated); I need a simple win to get unjammed. On both, it's the chassis.
Is there a universal method used to secure the Paramour plate to a chassis? Internet pictures don't seem to indicate so. Right now they sit atop two rough rectangular frames, unsecured. I suppose I'd have to measure/drill holes and screw them into the chassis I design?
Also, would it be a major faux pas to move the I/Os to the rear instead of the top? Is having them on the top a Bottlehead signature?
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The iron upgrade does indeed open up the #45 more so than the stock iron.
No, it's not a faux pas to mess with the enclosure design. We use the plate and base method because it's incredibly easy to go back in and fix issues or apply modifications down the road compared to having components mounted to multiple panels at different angles. Here is an example of a pair of Paramour monoblocks that I converted into a stereo amp.
You can get binding posts intended for speakers and just install them through the wood base. I sell a metal plate and IEC power entry module on eBay that can be installed into wood with a hole saw and some wood screws. To move the RCA jacks to the wood base, I install the RCA jack on the biggest fender washer I can find, prewire the RCA jack, drill a hole in the base, then use polyurethane adhesive to glue the fender washer to the wood. If you have an oil finish on the base, you'd need to sand it off first.
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Is there a universal method used to secure the Paramour plate to a chassis? Internet pictures don't seem to indicate so. Right now they sit atop two rough rectangular frames, unsecured. I suppose I'd have to measure/drill holes and screw them into the chassis I design?
Most people are fine with using gravity to hold the top plate in place. But I understand that there are some instances where this is insufficient. What I've seen most commonly here is to glue some wood blocks in the corners of the wood base and then drill some holes in the top plate. Then it's just a matter of screwing the top plate down into the newly installed blocks.
Alternatively you can purchase some bamboo bases from Blumenstein Audio. Their design captures the top plate in a groove around the top of the base.
https://www.blumensteinaudio.com/all-products/wooden-amplifier-bases-for-bottlehead-amplifiers (https://www.blumensteinaudio.com/all-products/wooden-amplifier-bases-for-bottlehead-amplifiers)