Inherited monoblocks - What model is this and how to improve?

[absolutk]

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!

[Paul Joppa]

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.

[absolutk]

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.

[Paul Joppa]

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

[Deke609]

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/ 

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

[Natural Sound]

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.

[Paul Joppa]

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.

[Natural Sound]

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.

[Deke609]

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.

[Paul Joppa]

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.

[absolutk]

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?

[Paul Joppa]

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.

[Paul Birkeland]

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.

[Natural Sound]

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.

[Natural Sound]

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