MrDave; I started to build this preamp, and switched to the MM EO design. I have a few extra bits for this that I'll sell cheap. The most important is probably the Decor power transformer. The EO design uses a much lower plate voltage, so I had to go to a different power transformer. If you are interested, I'll go figure out what I paid for it, and sell it to you for some fraction of the original price. It is new, but it has the leads cut to length, since I built it and then changed my plan.

Scott

Yeah, shipping the transformer to the UK is probably not cost effective. That sucker is heavy!

I have the Edcor XPWR100-120, so that clearly is not going to help you (I was hoping it might have a dual primary, but no such luck.

Good luck on your build! I also have four NIB/NOS Siemens gold pin D3A (7721) tubes (the EO only uses EC88's). I got them from a guy in Germany on eBay. IIRC they were about $US110 including shipping for the set. Let me know if you are interested in those.

thanks guys, ill bear the tubes in mind.

Ive found studio electronics on ebay but the cinemag 1254 is even more expensive than the sowter! £140 ($180) per can plus postage and import etc.
Sowter also do a slightly cheaper model, the 9570 which i think is suitable but this wouldnt allow me to drop to 1:5 if im finding it all a bit hot with my cartridge.
looking at the stereophile reviews, bobs devices come up a lot but he gets the 1131, a hand 'made by dave' version of the 1254 and other models use cinemag sky, but theyre not on the cinemag website so im wondering if he got them custom made.

Havent found out much about the shure.

It does look like the sowter 1480 is quite a bit better than the 9570 though.

ha, yes. Just used the way back machine. in 2010 about £71 +vat (15% i think). and £81 +vat (20%) in 2015.
I suspect this is quite a critical part of the whole project so probably not worth skimping on. Ill build it once and thatll be that!

Any news on the boards?

Thanks

D.

I have a few questions regarding the power supply for the EO preamp.

SY says that the balanced heater supply can be used for the EO, but that one needs to change the voltage for the tubes used. He gives an example in the EO article about using +/- 3.15 volt regulators for a 6.3 volt heater (which is what I need for my ECC88 tubes.

HOWEVER, his schematic shows the heaters connected between each regulator output and the heater common. If that was the case, then the voltage on each heater would only be 3.15 volts.

Like this:



I built up the heater regulators and the preamp boards. I have hooked up the tube heaters ONLY (not ready to try connecting the plate and cathode supplies). As expected, I measure 6.3 volts (OK, it's actually 6.4) between the positive regulator output and the negative regulator output.

If I connect one heater connection to the negative regulator, and the other to the positive, the tube heater glows (as expected). If I hook it up between one regulator and ground, the heater doesn't heat up (as expected). So this seems to argue for hooking them up n this manner (and it also makes more sense relative to running them balanced to eliminate common mode noise (otherwise how is it that running one tube form one reg and the other from the other reg would provide any noise benefit? ).

So my initial questions are:
1) Is the HMN heater schematic incorrect, and should I be running one heater terminal from the negative reg and the other from the positive reg?
2)If not, then should I set the regulators to 6.3 volts each, and run one tube from each one?
3) if the answer is #2 above, then how is it that this balanced heater scheme provides any noise rejection?

My other issue is that if I let the regulator run without being connected to anything, it is stable and runs cool. HOWEVER, when I put the tubes in, the the regulators get super hot, and after only about 1 minute they go into thermal shutdown. I suspect this is because I used the original HMN raw supply that puts out about +/-13 volts, and at 365 mA of heater current per tube, I get about 3.6 watts of dissipation. I'll try using a lower voltage raw heater supply, and see how that works.

Scott