Magnequest Iron for 45 / Top Plate Biasing / Chokes in the Power supply / Etc…

[Paully]

Just wanted to take a minute to update a couple of different threads for anyone who cares.  Thought I would just do one post instead of posting all over the place.  First, we (Grainger and myself) installed chokes in the power supply of the Paramounts.  Nice upgrade as I have stated elsewhere.  Having converted my amps to 45 I also ordered Magnequest output transformers in full nickel with a 5K impedance and also ordered some plate chokes.  As Paul Joppa has indicated, a very nice upgrade.  I realized just how fantastic the transformers are that are stock in the Paramounts but this was an improvement.  Don’t know what impact the new nickel transformers had versus the chokes versus the change in impedance, but I am happy with the package.

Second, we took out the bias pots from the driver boards and installed some 10-turn pots that sit on the chassis as well as put in a small tube socket to read the bias on.  We followed Paul’s advice from the previous thread and used 1.1K and 3.3K resistors and also paralleled some diodes for safety.  Also noticed that you have to pay additional attention to make sure things stay grounded on the driver board.  The really neat thing is that if I stick one meter lead in one socket and the other in the other socket I can just turn the pot until the meter reads 0 and the tubes are biased.  That is awesome!  Thank you to Grainger as always for walking me through it and keeping me from electrocuting myself!  I am really, really happy with that tweak.

Also, from a previous thread, I upped the resistance for the cathode resistors from 6K by putting in an additional 680 ohms.  As Paul pointed out, the gargantuan 25 watter I used isn’t really necessary.  But anyway, my line voltage is 120v through my P500.  That seems to have gotten my readings for the output tubes where I need them.  As those readings were requested by another Bottlehead I will list them below.

A1  210    210
A2  464    463
A3  162    160
A4  208    208

We also put in some ancient C4S boards I had sitting around into the Mother Foreplay and also converted the Eros to 6BR7 permanently, but I digress.

[Grainger49]

It was pretty gratifying to get so many projects knocked off in 2 1/2 days.

Edit: In the second picture down from the top you can see the red and green wire we used since we didn't have any STP left over from Paul's builds.  I chucked the two ends into a battery powered drill and slowly started to twist them together.  Then sped up the drill and made 3 feet of tightly twisted wire in about 10 seconds.  My first try and it worked beautifully.

[shelby1420]

It was pretty gratifying to get so many projects knocked off in 2 1/2 days.

I'll bet guys, thats a boat load of work, nice work!!!!!

[jrebman]

Hi Paul,

Can you post the changes you made for the 45 conversion with the exo-46 -- or 45 as the case may be?  Just in case I et the itch to try this at some point down the road with a 5k opt?

[Paully]

Do you want a step-by-step?  No problem if you do.  As far as changes, as Paul Joppa pointed out in another post, changing the output transformers requires a change in the coupling caps, if I have it right, due to the change in impedance.  Unless you change the chokes too.  But if you do one or the other you need to change the coupling caps.  Since I tend to buy decent coupling caps it was the right move to go ahead and buy the plate chokes and not have any other expense.  That way you are just putting in the chokes and transformers, no other changes necessary.  I ended up using the EXO-45 with 8 ohm impedance.  If you want a write-up of the hook-ups let me know and I can get it for you.

[jrebman]

Paul,

Don't need the pictures :-).  And I also don't need any step-by-step info, just what component value changes you made to cathode resistors, power supply, soft-start board, etc. to accomodate the 45 with a 5k opt.  I'm asking because PJ mentioned in another thread that some things would have to be reworked to use the 5k opt, and I'm just wondering what those component changes are.  I've got a paramount manual now, but still have to wait to order my soft-start boards.

Thanks,

Jim

[Frank Mena]

Just out of curiosity.... what speakers are you planning to use after the conversion to the 45?

Cheers
FM

[jrebman]

Frank,

If you mean me, I'll be using them with an upgraded pair of Cornwalls, which will actually be Cornscalas in cornwall enclosures and will use Klappenberger crossovers to give a 6-8 ohm impedance that is almost purely resistive.

Paul has some kind of Altecs.

-- Jim

[Grainger49]

Paul has the Valencias with new crossovers he built with the best components he could afford.  It was a noticeable step up when he did it.

[Paully]

I had a nice post written up, but I hit the wrong button and lost it.  Just read this http://www.bottlehead.com/smf/index.php/topic,895.0.html.  Only thing I would add is at 120V from the wall I needed to add another 680 ohm resistor in series with the two 3K to get my voltages down.

I have the Altec 846A.  Very efficient and no problems with the 45.  I also played some Klipsch Chorus II with them and found volume to be more than sufficient so your Cornwalls should be fine.

[Paul Joppa]

I can't find the post right now, but as I recall I made some calculations specific to Paully's power line voltage as well as the 5K transformer impedance. (Or was it someone else? Seems to have been more than one thread on the 45 conversion lately!)

Anyhow, in most cases you can reduce the driver plate voltage, which will increase the 45 plate-to-cathode voltage and reduce the current so that it matches a 5K impedance. In some cases you might get too much current or voltage or power in the driver, in which case the 6000 ohms cathode resistance can be increased. Since the increase is small, you can get away with a third resistor that has a lower power rating, and this third resistor can replace the wire that connects the two 3000 ohm resistors in series. That makes is a simple modification. I have a clunky and unreliable spreadsheet, not good enough to post but I'll do the calculations whenever they are needed.

I hope everyone understands that the Paramount was designed for the 300B and 2A3; the 45 modification is not a standard thing. It's theoretically possible to run a Type 50 too, though you'd have to use a grid choke which has some hum risk. The Paramour II and Stereomour were designed for the 45 and 2A3, and as I've said I have plans for a shunt regulated 45 amps as well.

[johnsonad]

and as I've said I have plans for a shunt regulated 45 amps as well.

This is what I'm waiting for   Yesterday I was reading posts back to 2004 on the Magnequest board regarding a SR 45 amp at least in theory. 

Hey Paully, do you have less bass with your Altec's compared to driving them with a 2A3?  In my Fi amp the 45 sounded wounderful except for the lack of low end. 

[Paully]

I have noticed no loss in bass with my Altecs, but the speakers never dug that low to begin with.  Maybe, maybe the really low bass but my audio memory is such that I haven't really noticed (and thus I can choose not to care).  I had been planning on a powered sub-woofer for a while now anyway, even before the change.

Just out of curiosity, Paul, what would you do differently with an amp to optimize it for 45/2a3 versus one that is 2a3/300B besides the output transformers, plate chokes, and cathode resistors?  Already did that.

Build a 45 amp and I might just have to do something that will get me in more trouble.  I can hear it now, "Why do you need another amplifier?  Is something wrong with the one you have?"  I usually just answer yes, yes there is.

[johnsonad]

LOL, it's often easier that way right Paully!  My wife does help me from collecting too much stuff in closets though!