Help. Circuit diagram/schematic for Paramount 12at7 board needed

[pcutter1]

Hi, first post from a newbie to your forum. I'm not tech minded so please forgive me if I'm referring to things wrongly. I have a pair of Paramount amps, the original 2a3 version. Due to the builder assembling(one of) the amps with extremely tight (negative) clearance between the power transistor heat sink and the case the transistor has sustained damage (legs bent and snapped) which has likewise damaged the PCB, and I therefore need to either obtain a replacement board, or assembly, or a circuit diagram for the board. I may, when and if the UK comes out of lockdown, be able to get a PCB made as I'm a member of a Hackspace with PCB making facilities, however, I would much prefer to obtain a board rather than this option.
I'm not new to Tube amps, having been using some form of tube amp almost continuously since the late 1980's. but I've never really been of a mind to build them. So a question, I've never seen a circuit diagram (or the inside of an amp) with a power transistor in the circuit, most of the amps I've had have featured 2 driver tubes, the ones I remember being ecc82 and ecc83 types, so I'm presuming that the power transistor takes the place of the first of the 2 tubes, am I correct? If this is the case then is this for relliability, space saving or some other reason? Thanks in advance for any information you may have.

[Paul Birkeland]

I may, when and if the UK comes out of lockdown, be able to get a PCB made as I'm a member of a Hackspace with PCB making facilities, however, I would much prefer to obtain a board rather than this option.

Can you post a picture of the board and the damage you see?  The Paramount used two different driver boards.  One of them is still in production, and the other board is out of production, but I have a few spares still.  If you want to spend the time and money to try to copy it, that's OK too, but making a single board is expensive!

So a question, I've never seen a circuit diagram (or the inside of an amp) with a power transistor in the circuit, most of the amps I've had have featured 2 driver tubes, the ones I remember being ecc82 and ecc83 types, so I'm presuming that the power transistor takes the place of the first of the 2 tubes, am I correct? If this is the case then is this for relliability, space saving or some other reason? Thanks in advance for any information you may have.

No, the transistors are not taking the place of tubes.  On that board, there are two constant current sources, and these take the place of what would otherwise be plate loading resistors.  Depending on which revision of the Paramount you have, there will also be some components on the board that work in tandem with half of the driver tube to provide a shunt regulated DC supply for the other half of the tube, and the newer version of the circuit has some components to provide adjustable bias (handy for the 2A3 version).

Constant current sources approximate a very high AC impedance (like using a 700,000 ohm resistor) while at the same time allowing for a sensible DC voltage drop.  The high impedance maximizes tube performance (THD and gain) while also rejecting a significant amount of power supply noise. 

[Paul Birkeland]

PJ pointed out that although this got posted in the Paramount board, your thread title is for a Paramour. 

Can you still post a photo of the board?  You may just need half of one of our legacy boards, and I can send one to you.

[pcutter1]

Hi, sorry my mistake I should have said Paramount. I have a pair that looks like the one in your photo, I will take a couple of photo's as soon as I can, if the boards I have in my amps are different to the ones you have then I will need a pair so that I can have both amps the same, correct? Are the other components readilly available, I managed to get a number of NOS power transistors (the correct type isn't available in the UK) which involved a long wait for them to arrive.
Thanks for answering my question about the use of the power transistor in this application, I can't say I understand everything you explained but I get the general gist.

[Paul Birkeland]

All of the transistors are still in production, so they shouldn't be hard to get. Here are all the transistors that go on that board available in large quantities in the UK.
https://www.mouser.co.uk/ProductDetail/ON-Semiconductor/MJE5731AG?qs=sGAEpiMZZMutXGli8Ay4kK2TqWlAzC1zZfBibKFYt6U%3D
https://www.mouser.co.uk/ProductDetail/Central-Semiconductor/2N2907-PBFREE?qs=sGAEpiMZZMshyDBzk1%2FWi0MKeUdJDRlzE7fcKKCKftcdg3%2FR2QXFLA%3D%3D
https://www.mouser.co.uk/ProductDetail/ON-Semiconductor-Fairchild/LM431ACZ?qs=sGAEpiMZZMvAvBNgSS9Lqn4E3LoDYvrk

You will not need two boards.  If you are particular about the resistors being completely identical between boards, I can probably assist you in ordering the correct parts, or I may have some surplus here.

[pcutter1]

Hi, thanks very much for your replies. I've taken a couple of shots of the board hopefully they will help identify it.
I hope I understand your reply stating that I would not need to obtain 2 of the boards, that would seem to indicate that there would be no difference in (sonic) performance between the existing board and the new one.
Kind regards, pcutter1
Oops, notification is telling me I can't post external links (the photo files)

[Paul Birkeland]

You can upload them to our site or just paste a link as text.

What I'm telling you is that I have some of the old boards sitting in a box, so you could replace what you have with something that's identical.

[pcutter1]

Hi, again many thanks for your help, after reading your original reply about the function of the circuit board I was emboldened to have a go at a repair, as I was almost certain that the damage was entirely physical. I replaced the power transistor with a new one and left the legs in place,  connected the legs to the circuit board using some very fine pure silver wire so that the transistor is now wired point to point asit were. Result: total success, the amp now functions perfectly, I kept it running for about 32 hours. This doesn't mean I don't need to consider new board(s), as I know the reason for the damage happening was entirely down to a longstanding injury that means I have little grip with my thumbs I feel it would be safer to have a pair ready to use just in case. So could you supply a matching pair of boards, obviously as long as they match the version doesn't matter. The cost? How to pay (PayPal would be best). Shipping to the UK.
Best Regards,

[Paul Birkeland]

Can you post a photo of the board in question?