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I'm thinking about building a Rev. D as a charity auction item. I have a salvaged PA774 transformer, but don't have any extra Z-565s. I've been looking at the Edcor open-frame transformers as an option, but the GXPP15-8-8K and XPP15-8-8K are only rated for 15W. I could go for the 25W 7.6K XPP25-8-7.6K, but that's not the optimal resistance for the circuit. Also, it's $10 more per transformer (not a big concern, but a factor).

So the big question - do you guys think the 15W Edcors can handle the additional 2.5W of power that the ST35 will put out, or should I just get the 25W Edcors and be 400 ohms lower on the OPTs?

Thanks!

-D

I used a set of Hammond 1620 on a ST35. They worked just fine for the most part, and sounded quite good. The 1620 has a rated primary impedance of 6600 ohms and secondary windings/taps for 4, 8 or 16 ohms.

The 1620 suffered a bit on the low end. They would distort noticeably on the scope if you tried to push them more than a few watts at 20 Hz. I don't know if this was due to the relatively low primary impedance, or because of something else. To the ear they sounded fine, especially when played at reasonable levels.

I'd say go for whichever Edcor fits your budget, and don't worry about whether one is 7600 ohms or the other is 8000 ohms. It's all about the turns ratio anyway, and the rated primary impedance only counts if the speaker keeps a constant impedance - which it never does.

The only thing I'd worry about is the maximum DC current rating for the primary. In the case of the Hammond 1620, it's rated for 158 mA. I'll never hit that with a pair of 6BQ5 tubes.

I'd go with the Edcors. Dynaco's original rating of 17.5 W/ch was just a "wee bit" optimistic, anyway. 14 to 15 watts is a little closer to reality, in many cases!

Thanks, guys!

There's no rating on the XPP15-8-8K, but the GXPP15-8-8K is rated at 75mA DC. Which current (plate or grid) should I be concerned about? The plate when running at 300VDC can see up to 92mA at maximum, but at the grid it is a lot less.

-D

15W GXPP are on 25W cores. No probem.

Cheers!

I've got a pair of XPP15-8-8K that I'm just about ready to use with the st-35 board, give or take a week or two. Stay tuned.

whitelabrat wrote:I've got a pair of XPP15-8-8K that I'm just about ready to use with the st-35 board, give or take a week or two. Stay tuned.

That's the perfect moniker then... Yellow_Light_Colorz_PDT_02

I'll be interested to see the results!

-D

Tonite I did a first smoke test. The results weren't good. I set the initial feedback for 20.3k and the bias to 400ohm as suggested. I didn't bother fine tuning the bias since I don't have an insulated screw driver suitable for the job yet.

Sound only came from one speaker. It was accompanied by a horrific squeeling sound! Argh! :o

I'll have to check the 12AU7 tube, cause it's real old. I don't think that's the problem. Time to search the forum.

whitelabrat wrote:Sound only came from one speaker. It was accompanied by a horrific squeeling sound! Argh! :o

Horrible squealing noise usually means you've got positive feedback instead of negative. Squealing from only one channel suggests you flipped the plate and screen taps on that channel, but not the other.

Double check the leads between the output transformers and the board. Compare your wiring to the diagram, and compare the left channel to the right channel.

Labrat,

I'm not sure there <is> a diagram with the Edcors out there. Easy test: disconnect the negative feedback then re-power up. If that solves it, swap the UL taps and secondaries and on the Edcors. Your phase is inverted.

This is always a 50/50 venture on unknown trannies. Just happened to me and Thermion the other day with Hammonds after he <swore> he had the phases figured out from datasheet.

I just finished reading a post about positive feedback. It definitely sounds like positive feedback. I figured I'd try removing the feedback connections as a last resort, but I also believe my phase is backwards. I wasn't using my head.

I wasn't getting any sound from the right channel, but there was a bit of humming. I didn't bring things up to full voltage. I'll have to triple check my wiring.

I'm hoping to take another stab at it tonight. I'm happy that my 5U4 rectifier works at least! I've also wired the choke where the CL90 varistor normally goes. That way I can attach the choke via the terminal strip pins 9 and 10. Appears to work well so far.

I've also wired the choke where the CL90 varistor normally goes. That way I can attach the choke via the terminal strip pins 9 and 10. Appears to work well so far.

NOOOOOOO !

The CL90 is in the primary of the AC to the power transformer !

The choke is in the high voltage DC after the rectifiers !

Get that choke out of there ... you need to remove the resistors
at R-34 and R-35, and you can either put a 2 pole barrier strip there,
or stick the choke wires in the holes and solder them.

... tom

Really it's OK. My AC primaries don't go to the board. I was a bit baffled at first about why the CL90 is on the board and not on the power switch. I won't really need it with a 5U4GB rectifier. This also gets my heaters going faster before the B+ kicks in. I try to keep as much AC power away from the board as possible. If you look carefully at the ST35 board the traces for the CL90 go to only pins 9 and 10.

Let me explain. I've run wires (heavy guage teflon insulated) from the R-34, R-35 position over to where the CL-90 would normally reside. That way I don't need the extra terminal strip.

Ahhh .. ok ... sorry !

Anyway (for others reading) there is a spot for a
terminal strip for the choke - make sure to order
an extra 2 position strip when you buy your parts.
(attention Triode - you should include one with the kit too)

TomMcNally wrote:NOOOOOOO !

I thought the same thing myself, Tom, when I first read his post. After re-reading it several times, slowly, I realized what he was doing.