6SN7 Driver in Paramount 1.0 (Original with 12AT7)

[galyons]

I am buying a pair of original Paramounts w/300B's.  I will need to rework them because the original build was poorly done and the cosmetics are more than tired.

PJ mentioned several times in the old and new forums that his plan was to take the drivers to 6SN7's on his.  That would be my preference as well.  But I could not find any needed circuit changes for the original 12AT7 driver boards in my forum searches.

I also plan to implement the B+ switch on the PS board, (switch on the exterior), as described on the forum by PJ:

“For the stock Paramount in 2A3 configuration (direct coupled), we have seen no problems with old stock 2A3s, or with the supplied Sovtek tubes. A few modern-production tubes have shown a visible and/or audible arc and "pop" through the speakers; this has been known since the ParaGlow days (predecessor to the 2A3 Paramount) and the usual solution was to implement a standby switch on the high voltage supply - or use a tube that does not have the problem. This is, incidentally, why the Paramount power supply circuit board has a place for that high-voltage switch - it's not mentioned in the manual where a jumper closes the circuit, but the jumper is in the circuit at a place where a normal 240vAC switch can be safely used to switch on the high voltage.”

My reading of  the information on the softstart V1.1 upgrade is that it is primarily to accommodate 2A3's being directly coupled and the 5760 driver.  (I like the 5760 and have them in a non-BH headphone amp, but have too many 6SN7 & 7N7 tubes to not use them!)

So the change to octal/loktal and the B+ switch should get me everything that I want/need. (I have Paramour 45's w/ MQ iron and Paramour 2A3's with Altec OPT's, (yeah I know, maybe a couple too many P's!))  If I decide to give up the 300B’s down the road for 2A3’s, then I would definitely do the soft start, but maybe leave the driver as a 6SN7 or 7N7.

PJ said to post so the information would be available to the forum.  His initial response was:

“ The pinout of the 6SN7 is obviously different  :^) but other than that the SS board should work. You'll be adjusting for something around 6 volts bias on the driver section instead of 4v on the 5670, but the range is 2.5 to 7.5 volts so there should be no problem.

I do definitely recommend the SS board for any direct coupled version, 2A3 or 45. The ability to adjust the driver plate voltage is very important to get the best out of the design, and was left out of the original Paramount only because it repeated the legacy (Afterglow/Paraglow) design as closely as possible.

I haven't done it myself yet, so no direct experience. But I'll help out on the forum.”


The old driver boards for the 12AT7 don't have a trim pot,  but it looks like the current requirements are close for the 6SN7, as well. Will there need to be resistor or other changes to the old boards to make them work with 6SN7's?

I will start the rehab on the amps early next week.  Will need a few days to get them running to original spec's with the 12AT7's. Then switch out the 9 pin socket for an 8 and let the games begin!

Cheers,
Geary

[johnsonad]

Hi Geary,

I can't speak to the old boards but on the new it was as easy as plugging it in and adjusting the bias pot. You are going to loose a lot of gain going from the 12AT7 to a 6SN7, something like 12dB.  Give the 12AT7 a shot, you may like it.

Regards,

Aaron

[xcortes]

What operating point do you use for the 6sl7?

[galyons]

Hi Geary,

I can't speak to the old boards but on the new it was as easy as plugging it in and adjusting the bias pot. You are going to loose a lot of gain going from the 12AT7 to a 6SN7, something like 12dB.  Give the 12AT7 a shot, you may like it.

Regards,

Aaron

Thanks Aaron,
I have AT7 family tubes in my Paramours.  It is not that I don't like AT7's, I do. But the 6SN7's are just a bit nicer to my ears...sweeter..better timbre...better  imaging. I don't need all of the AT7's gain as I use a preamp. 

I haven't changed out the AT7's in the Paramours for octal/loktal because I don't want to permanently enlarge the socket hole, (and I am not a big fan of the adaptors).  I have considered changing the pin outs in the Paramour 45 for 6CG7's. (has the PT with higher heater current).  Just haven't pulled them out of the system to do so. 

The Paramounts are designed for 8 pin sockets so that makes the decision that much easier! (And PJ's seemingly constant tease about doing so on his!  )

Cheers,
Geary

[galyons]

What operating point do you use for the 6sl7?

Xavier, part of my desire is to reduce the gain on the Paramounts, so 6SN7's rather than 6SL7's.  The plate current is about the same as 12AT's. It think it is the bias that I may need to tweak and I am seeking guidance to do so.

Cheers,
Geary

[xcortes]

You're right. 6SL7s. Aaron, what voltage you adjusted the trimpot for? You used the same bias as the 12at7?

[johnsonad]

I had a thread on this a while back. Keep the bias per the specs.  For the 300B amp this is 175v with the SS version. PJ recommended changing the trim pot from a 10k to 20k but I have plenty of range with the stock pot.

[xcortes]

Thks

[galyons]

I had a thread on this a while back. Keep the bias per the specs.  For the 300B amp this is 175v with the SS version. PJ recommended changing the trim pot from a 10k to 20k but I have plenty of range with the stock pot.

Please bear in mind, this amp is not V1.1 with soft start. This is the original Paramount with the 12AT7 driver boards, no trim pot. 

Cheers,
Geary

[Paul Birkeland]

Hello Geary,

I would strongly suggest going with the Soft-Start PCB's instead of the high voltage switch if at all possible.  If you plan to stay with 300B's, the Soft-Start feature and the high voltage switch will be unnecessary. 

Using the soft-start PCB's, you'll have room for heatsinks on the MJE5731A's, and the adjustable driver bias trim is extremely useful if you plan to convert to 2A3's in the future.

If you're willing to add a hole, I'd advocate trying the 6GK5 as a driver tube, then use a 6SN7 or 6J5 in an octal socket as the shunt reg tube.  You could also run the 3S4 as a pentode driver with some additional tweaking.

[galyons]

Thanks PB!

I would strongly suggest going with the Soft-Start PCB's instead of the high voltage switch if at all possible.  If you plan to stay with 300B's, the Soft-Start feature and the high voltage switch will be unnecessary. 

 The plan is to stay with the 300B, at least for now.  I have tried 300B's in the past, and go back to 2A3/45's.  So if I do go 2A3's, yes, I would consider it imperative to get the new board.

Using the soft-start PCB's, you'll have room for heatsinks on the MJE5731A's, and the adjustable driver bias trim is extremely useful if you plan to convert to 2A3's in the future.

Although I don't have the amps in my possession, (tomorrow evening I will), there are heatsinks on the MJE350's.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FUmYf0LN.jpg%3F1&hash=b0b256dfaf3794d49b965874ee035a9ad9f8fc17)

If you're willing to add a hole, I'd advocate trying the 6GK5 as a driver tube, then use a 6SN7 or 6J5 in an octal socket as the shunt reg tube.  You could also run the 3S4 as a pentode driver with some additional tweaking.

You're killing me!    I have quite a few 6J5 metal and the usual "Quickie nut" bin full of 3S4's.  I thought about a 76 as the driver and a 6J5 or 7A4 as the shunt regulator.  I just hate to drill socket holes in the top plate to accommodate.

Cheers,
Geary

[Paul Birkeland]

Alright, if the driver boards are operational, I believe you will want to:

1.  Install 431 Regulator on the empty side
2.  Install 2.49K resistor in R3
3.  Install 3.5K resistor between the bigger "+" pad (fed from Rc) and the left-most R4 pad (resistor stands up vertically)
4.  Wire jumper from "K" to hole on Rc that feeds "+"
5.  Wire jumper from "-" to "-" on other corner of PCB
6.  Empty "+" pad goes to driver stage cathode

If you want to keep things somewhat easy, you could adjust the heater wiring and try a 6CG7 first, ensuring that the PCB's work, then do the gut and resocket. 

-PB

[Paul Joppa]

In PB's post, he suggested 2.49K and 3.5K resistors to create the 6v bias. In the soft start, those are a 4.99K resistor and a 10K trimpot - the old boards are not designed to take a trimpot.

I got the 6v value by reading the published curves by eye - no measurements were made, so no promise that I read the curves right! There is a likely +/-30% variation in plate voltage from tube to tube as well. That's why the trimpot is essential for direct coupled applications.

Some have remoted the trimpot for greater convenience. For anyone contemplating this, a few cautions - the wires must be twisted together and a non-inductive pot (not wirewound!) used, since this resistance is in the chip's feedback loop and it is prone to oscillate unless care is taken. I would use shielded twisted pair with the shield grounded to the chassis at the pot end to minimize noise or hum pickup at this sensitive node.

Just to clarify, the new board is larger primarily to accommodate a larger heat sink, the Aavid 5793. There were a few problems with the earliest version using MJE350s and no heat sink, and I wanted this board to handle a wider variety of applications.

[galyons]

Alright, if the driver boards are operational, I believe you will want to:

1.  Install 431 Regulator on the empty side
2.  Install 2.49K resistor in R3
3.  Install 3.5K resistor between the bigger "+" pad (fed from Rc) and the left-most R4 pad (resistor stands up vertically)
4.  Wire jumper from "K" to hole on Rc that feeds "+"
5.  Wire jumper from "-" to "-" on other corner of PCB
6.  Empty "+" pad goes to driver stage cathode

If you want to keep things somewhat easy, you could adjust the heater wiring and try a 6CG7 first, ensuring that the PCB's work, then do the gut and resocket. 

-PB

Thanks PB!  I like the idea of 6CG7 first, after installing the modified driver boards.

Is this an accurate "pic" of your scheme? (Corrected per PB) Leave everything on "A" the same? Populate the rest of "B" per manual?
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FOFeU0Ke.jpg&hash=97088df9e7fc35622e5b79c5ee3aed7579a74b6d)

If so, I'm on it. I'll have to get a couple of the LM413's, but have the resistors.

Cool! I'm excited.

Cheers,
Geary

[galyons]

In PB's post, he suggested 2.49K and 3.5K resistors to create the 6v bias. In the soft start, those are a 4.99K resistor and a 10K trimpot - the old boards are not designed to take a trimpot.

I got the 6v value by reading the published curves by eye - no measurements were made, so no promise that I read the curves right! There is a likely +/-30% variation in plate voltage from tube to tube as well. That's why the trimpot is essential for direct coupled applications...

PJ,
Given the +/- 30% swing possible in plate voltages, then in my capacitive coupled scenario I should be OK? (OK = close enough for jazz)  Or do you think I should try to get a trim pot wired in? 

Cheers,
Geary