Transformers for ST35

[Blooze]

Hi ya'll.

A friend of mine gave me his stuffed revB pcb and a pair of old OPT's pulled from a Fisher amp (stamped T-799-116) he thinks may have been from an old X-100. I measured the primary and got 6.6K. They are about the size of the Hammond 1620's.

I have the option to get another pair of OPT's that are believed to be from a pair of old Heath A-7's for cost of shipping. I have no idea of the impedance on those or whether they would work for this circuit.

Would either of these be worth pursuing?

Other options include buying a pair of Edcor CXPP25-MS-8K with 25% screen taps or dynaclones (I know someone with a pair of older ones that have never been used for just a little more than the edcors).

On the PT front, I'd like to go ahead and use a tube rectifier and was wondering what would be recomended. On a side note the coupling caps are 400V rated PIO, FWIW. I see most everyone has used a 5AR4 as their rectifier. Has anyone tried a 5V4G? Reason I ask is that I've got a couple of those and only one 5AR4 (Amperex Great Britian Bugle Boy).

Thanks for all the help in getting me started!!!!

[Ty_Bower]

A friend of mine gave me his stuffed revB pcb and a pair of old OPT's pulled from a Fisher amp (stamped T-799-116) he thinks may have been from an old X-100. I measured the primary and got 6.6K. They are about the size of the Hammond 1620's.

I used a pair of Hammond 1620 on my ST-35. Coincidentally, it also happened to be a Rev B. The 1620 are specified as 6.6K primary load and offer 40% screen taps. Neither of these are "ideal" for the Stereo 35. The original Dynaco Z565 should have a primary impedance around 8K and screens at 23%. Regardless, I was extremely pleased with the way the Hammond 1620 worked in my amp. I got the Hammonds for a very good price and they performed well. The only room I might have for complaint was that they could not deliver the same kind of low frequency performance as the Dynaco transformers. I also noted they were "louder" than a Z565. Some adjustment of the NFB loop might be in order.

I did a quick search for the Fisher X100, and noticed there are no UL screen taps on the schematic. If that's the case with your transformers, you might have to rethink a few things.

Good luck with the project!

[Shannon Parks]

Do the Fisher output trannies have UL taps? If so, I'd use them. What are you planning to use for your power transformer (thinking of your rectifier voltage drop)?

--Shannon

[Blooze]

Do the Fisher output trannies have UL taps? If so, I'd use them. What are you planning to use for your power transformer (thinking of your rectifier voltage drop)?

--Shannon

I don't believe I saw any UL taps on the Fisher's.

Was thinking possibly something along the lines of a Hammond 273BX might work for a PT. I used a 273X on my Darling to get a B+ of 265V, but had to use a bucking transformer because the mains was so high (their new trafos have a 125V primary option). But it's a different PS altogether. You can see it here:
http://shanemoorephotography.smugmug.com/Electronics/My-DIY-Equipment/13836992_F9nQR#1014042710_sATHT

[Blooze]

FWIW, without the bucking trafo I was getting around 300V IIRC with the 5V4G. Unloaded the 273X was giving me right at 400V with 122V mains.

[Shannon Parks]

I think a Hammond 273BX with a 5V4G sounds pretty reasonable. I've been stockpiling ST shaped 5V4s for future use myself.

--Shannon

[Blooze]

I think a Hammond 273BX with a 5V4G sounds pretty reasonable. I've been stockpiling ST shaped 5V4s for future use myself.

--Shannon

I think that trafo would probably work with either the 5V4G or 5AR4. I've got that Amperex 5AR4 and it's supposed to be good, but I hate to burn it up for some stupid reason as it's costly to replace OTOH I've got 2-3 ST shaped 5V4's.

Edcor has a similar transformer in the XPWR155-120 at 200mA.


The A-7 OPT's supposedly have UL taps according to this schematic (of course there are about 6 different versions of the amp). I guess my question is whether they will take the VA should the primaries prove useful.

[Shannon Parks]

Yeah, the earlier ones *don't* have the UL taps.
http://www.audiophool.cjb.net/Audio.html

But if they did, I'd use them.

--Shannon

[Blooze]

Well, we'll see when they get here. I'm getting them for the cost of shipping so if they won't work here I can use them sometime down the road. Hopefully they are from a D or E model.

[msmpe]

I thought the the Heathkit A7s used a 7.5K OPT, which if correct would be doable with the EL84.

[Blooze]

Asked Edcor what their CXPP25-MS-7.6K OPT's max mA rating was out of curiosity. Phyllis said 80mA.

[Blooze]

Will the 5V4G rectifier be adequate with it's 175mA output? I know the max voltage at 500V for choke loaded will be fine. Seems like the 175mA will be close.

[Geek]

Asked Edcor what their CXPP25-MS-7.6K OPT's max mA rating was out of curiosity. Phyllis said 80mA.

For a CXPP?

I know the XPP and GXPP are rated 80mA, but when I talked to Brian about a PP EL86 project where peak currents can hit 1/4 amp, he said it'll be fine

Cheers!

[Blooze]

Looking at ordering the PT this week and just want to double check the specs with someone.

Seems like a 5V4G might be getting pushed pretty hard with a 175mA rating? So if tube rectified maybe stick with the 5AR4?

IF going tube rectified with the choke, would it be better to go with a 600VCT, 700VCT, or something in between? At least 180mA? 200mA better?

If going with SS rect. go with the 600VCT?

And last but not least, is there a trafo rating that would work with both types? I could always wire an extra octal socket on the chassis to use later if wanted, just remove the diodes and plug in the tube rectifier.

[EWBrown]

For the two different DIY35s in which I used 6600 ohm OPTs, (Hammond 1620s in one, and Triode TF110-48-ULs in the other) I used a lower B+ voltage, approx 325VDC instead of 360 to 380V, and I also cranked up the cathode curernt for each EL34 up to 40 mA, instead of the "normal" 35 mA..

This still stays safely within the EL84s' plate dissipation ratings, and also effectly lowers the tubes' operating RP slightly.

Works for me

/ed B