PT question - JE Labs Stereo 2A3

[gable]

I've built a few tube amps, but haven't modified any psu designs. I have a solid understanding of the basics, but I know when and where I need a bit of help

I'm putting together the needed parts to build a JE Labs Stereo 2A3. I have some nice vintage PT's and I would like to use one of them for the build. I know it has more than enough current capability, but I believe the secondaries output higher voltage than the PT spec'ed in the schematic(1).

The transformer is FREED 720VCT @300mA, 6.3VCT@8A, 5V @4A.

A few images:




So I know I'm going to need to modify the power supply some. I have some 166Q2's on the way for the 2A3 filaments and the main PT should handle the rest. I have a giant 6.3v @~10amp transformer I could use if I needed/wanted to for the 76 and 6sn7gt's.

If anyone has any insight or direction on the modifications needed, I would love to hear it. I am going to work on simulating in psud as well.

Cheers,
~Gable

1 - http://4.bp.blogspot.com/-jFYCl3UUOLM/T97-LTfVRII/AAAAAAAAB7c/gEMdchMUCjg/s1600/JELsc-ST2A3DX.jpg

 

[marantzfan]

Huh. I have a pair of Angela PT’s laying around that I may never use. How cool that this schematic calls for them by name.

 

[je2a3]

If memory serves me right I used a 720VCT power transformer on this amp. You just have to forgive me since I no longer remember the value for the B+ dropping resistor I used between the output of the rectifier and the 1st filter cap. But that shouldn't be a problem since you know your way around PSUD sims (I don't ).

You have ample filament current for a set of 76s and 6SN7s. If you use 2x6SN7s, I'd suggest "mirror image" wiring of the octal sockets so that you can interchange the 2 tubes periodically so each "triode half" is used.

Freed + 2 x Hammond 2.5V @3A and you're all set.

Happy soldering and listening!

JE

 

[joe83]

Im also about to address the mains transformer for the same amplifier, i have a mains transformer from an audio note kit 1 (actually two of them, so thought i could do a pair of mono amplifiers) and i have no idea if its suitable (ive only put kits togeather in the past). The voltages on the kit one and the JE Labs 2a3 are quite different so perhaps not suitable... I know i can ofload the spare AN mains transformer quite easily but then i need to get a suitable one.

I know Hammond is generally recommended but ive been told that the Hammond mains transformers arent great at dealing with UK mains as it can vary quite a bit so im unsure if it would be wise to go with hammond however the other option is to go with Sowter who are UK based so i would assume would be OK but they are about 3x the price. Ive ordered from sowter several times in the past and will prob buy my output transformer from them.

Can anyone offer any advice?

Cheers

PS the schematic calls for 0.22uf 630V coupling capacitors, i have some 600v ones will that be sufficient?

 

[gable]

You just have to forgive me since I no longer remember the value for the B+ dropping resistor I used between the output of the rectifier and the 1st filter cap. But that shouldn't be a problem since you know your way around PSUD sims (I don't ).

No worries! I think between some PSUD sims and a bit of on the bench testing, I'll be able to find the right spot.

You have ample filament current for a set of 76s and 6SN7s. If you use 2x6SN7s, I'd suggest "mirror image" wiring of the octal sockets so that you can interchange the 2 tubes periodically so each "triode half" is used.

Freed + 2 x Hammond 2.5V @3A and you're all set.

Happy soldering and listening!

JE

Oh, awesome idea, thank you !

Cheers,
Gable

 

[gable]

Im also about to address the mains transformer for the same amplifier, i have a mains transformer from an audio note kit 1 (actually two of them, so thought i could do a pair of mono amplifiers) and i have no idea if its suitable (ive only put kits togeather in the past). The voltages on the kit one and the JE Labs 2a3 are quite different so perhaps not suitable... I know i can ofload the spare AN mains transformer quite easily but then i need to get a suitable one.

I know Hammond is generally recommended but ive been told that the Hammond mains transformers arent great at dealing with UK mains as it can vary quite a bit so im unsure if it would be wise to go with hammond however the other option is to go with Sowter who are UK based so i would assume would be OK but they are about 3x the price. Ive ordered from sowter several times in the past and will prob buy my output transformer from them.

Can anyone offer any advice?

Cheers

PS the schematic calls for 0.22uf 630V coupling capacitors, i have some 600v ones will that be sufficient?

I've not used Hammond on 230 mains, but my experience has always been positive. I do generally buy their transformers and chokes over specified for the application. It may not be technically necessary, but it puts my mind at ease, and seems to make re-using transformers for other potential future projects easier. In my head at least!

The 600v capacitors should be fine, a quick glance of the schematic tells me the voltages there are <400v. I'm certainly no expert, but I believe you'll be quite fine. In fact, the caps I have set aside for that spot are 600v Russian FT3's.

If your audio note transformers aren't suitable for the project, I would think you can sell them to pay for an appropriate one (or two for mono blocks).

Cheers,
Gable

 

[joe83]

I've not used Hammond on 230 mains, but my experience has always been positive. I do generally buy their transformers and chokes over specified for the application. It may not be technically necessary, but it puts my mind at ease, and seems to make re-using transformers for other potential future projects easier. In my head at least!

The 600v capacitors should be fine, a quick glance of the schematic tells me the voltages there are <400v. I'm certainly no expert, but I believe you'll be quite fine. In fact, the caps I have set aside for that spot are 600v Russian FT3's.

If your audio note transformers aren't suitable for the project, I would think you can sell them to pay for an appropriate one (or two for mono blocks).

Cheers,
Gable

Thanks for your reply! I suspect i could use the AN transformers but on the kit 1 scematic there is something like 480v where on the JELABS 2A3DX i see 370v at a similar spot (after the 5u4g and before the choke, but i think id rather get something better suited first time as i like to keep it as simple as possible.

Ill contact sowter and the distrubutor of Hammond in the UK and see what they suggest...

Thanks for confirming the the capacitors will be OK, I saw nothing higher than 345v around the caps area so thought it would be ok but i once used a pio cap rated for 100v where 350v was needed. I dont want a repeat of that!

Good luck with your build, id be interested in seeing how it goes if you get the chance to take a few pics along the way.

Cheers

 

[gable]

Thanks for your reply! I suspect i could use the AN transformers but on the kit 1 scematic there is something like 480v where on the JELABS 2A3DX i see 370v at a similar spot (after the 5u4g and before the choke, but i think id rather get something better suited first time as i like to keep it as simple as possible.

Ill contact sowter and the distrubutor of Hammond in the UK and see what they suggest...

Another UK option may be JMS transformers. Ale Moglia speaks highly of them.
http://www.jmstransformers.co.uk/contact-us/
http://www.bartola.co.uk/valves/

Thanks for confirming the the capacitors will be OK, I saw nothing higher than 345v around the caps area so thought it would be ok but i once used a pio cap rated for 100v where 350v was needed. I dont want a repeat of that!

Good luck with your build, id be interested in seeing how it goes if you get the chance to take a few pics along the way.

Cheers

Ouch! I bet that was a mess.

Thank you, I will definitely post pics along the way. Will likely be a few months, for now I'm saving parts as I come across the right ones at the right price!

Cheers,
Gable

 

[joe83]

Another UK option may be JMS transformers. Ale Moglia speaks highly of them.
http://www.jmstransformers.co.uk/contact-us/
http://www.bartola.co.uk/valves/



Ouch! I bet that was a mess.

Thank you, I will definitely post pics along the way. Will likely be a few months, for now I'm saving parts as I come across the right ones at the right price!

Cheers,
Gable

Thanks for the info on JMS, will contact them as well.

Im also in no real rush, but have most of the parts now and so no excuse (although still not sure about what other capacitors to use; electrolytic or something in oil) the knowing how to construct it will be the biggest challange but im quite looking forward to it

 

[gable]

Thanks for the info on JMS, will contact them as well.

Im also in no real rush, but have most of the parts now and so no excuse (although still not sure about what other capacitors to use; electrolytic or something in oil) the knowing how to construct it will be the biggest challange but im quite looking forward to it

My plan for the power supply capacitors is to use ASC X386 polypropylene in oil motor run caps. I have 4 or 5 of the 50uf 440vac caps. They are pretty big, so bear that in mind if you go that route as well. My build will have a separate power supply chassis, partly because of the iron and partly because of the giant caps.

As for the 100uf/100v off the 6SN7's, I have some Elna Silmic IIs that I plan to use.

Cheers,
Gable

 

[joe83]

My plan for the power supply capacitors is to use ASC X386 polypropylene in oil motor run caps. I have 4 or 5 of the 50uf 440vac caps. They are pretty big, so bear that in mind if you go that route as well. My build will have a separate power supply chassis, partly because of the iron and partly because of the giant caps.

As for the 100uf/100v off the 6SN7's, I have some Elna Silmic IIs that I plan to use.

Cheers,
Gable

Exactly what I had in mind after browsing some of the threads on building amplifiers here.

I have 4 x 220uf 160v audionote kelesi electrolytic capacitors left over that i thought i might be able to use but the values are a bit far out

 

[Redboy]

Here’s a question... if one builds a separate power supply for a 2A3 amp, where should the 2.5V filament transformers go? In the power supply chassis or the amplifier chassis?

 

[mhardy6647]

My friend Alan's working on one -- I think his are outboarded, but I'll ask him


Three chassis -- but it is a PP 2A3 stereo amp.
(the other two chassis are his preamp on the right and his all octal reboot of Hagerman's Cornet phono preamp in the front)

He's now workin' up a SE 2A3 amp, too, actually.

 

[mhardy6647]

So, I corresponded with Alan & he corrected my memory of the photo above and added his perspective on the FTs.

First, the photo. There are two separate power supples; one for the PP stereo 2A3 amp, and the other for the preamp.
I think the amp's P/S is the chassis directly behind it, so the other one (on the left) is the preamp's P/S. Also worth noting -- the preamp's active components are 76 tubes. Mea culpa.

Herewith, Alan's comments on his PP 2A3 amp P/S layout (edited a bit):

"My rationale is that if the FT are in the audio chassis, you need to wire 110 into it, which increases the possibility of hum. Also, you have two transformers generating magnetic [fields] near the audio components. Also bad.

"Putting the FT on the PS chassis gives you two extra pairs of wires (AC or DC, depending on the design) to run to the audio chassis. FWIW, I'm using AC filaments and don't feel the need to use DC. In order to minimize hum, I'm using one run of cat5 for each filament, using the "whites" [stripeys - ed. ] of each twisted pair as one pole and the [solid] colored [wires] of each pair as the other.

"... Since my PP amp is fixed bias, I grounded the center tap of the FTs (left and right) in the PS and was done with it.

"My PP amp has eight wires in the umbilical: Left filaments pair, right filament pair, left B+, right B+, bias, and ground.

"With the FT in the PS, you still have to address where to put the cathode grounding stuff (hum pot, cathode resistor, cathode cap). That can go on either chassis. There's probably a technical argument about which is superior (ground loops, safety in case the umbilical gets disconnected, etc), but basically you can ground the two wires on either end; by the FT or by the tube. I'd make the decision based on where there's more room, and where it looks better."

Alan is also building an SE 2A3 amp (currently breadboarded, so to speak) and offered some comments 'bout it, too.

"...my SE amp is breadboarded as one chassis, and I think it'll stay that way (about 15x17 so it will fit in my equipment box). But with what I've learned on my PP (which IS two chassis), were I to split it, I'd definitely put the filament transformers with the PS."

 

[Redboy]

Now that’s the kind of answer and information that I really like! My thanks to Alan.

 

[mhardy6647]

Alan is a fine fellow who can build or fix just about anything! Upstairs in his barn is hifi; downstairs are his jukeboxes (of the 78 rpm kind) and a pipe organ(theater-type), in pieces, that he is bent on restoring

Note that I have cleverly "helped" with this thread without knowing a damned thing about the subject matter myself!