Some ST-70 newbie questions.

[mrted]

First of all thanks to everyone for their DIY experimentation on the ST 70 amp. There is a lot of good advice on this site. I’ve had tubes in almost every piece of equipment I have owned. The first was a Regency tube amp I bought at a garage sale when I was 11. Now I am building my first tube amp, an ST70 clone. The parts lineup is mostly from this web site. I’ll post the lineup later. Now on to the questions:

1. Since the audio inputs and outputs are isolated from the chassis can you use a 3-prong power cord for the 120AC in and ground the frame? I know this questions sounds trite but I would like to ground the frame for noise and safety purposes but I’m also concerned about introducing hum.
2. Are all “winged C” Svetlana EL 34’s the tubes to use? Or is this topic snake oil.
3. What wattage should the new 10-ohm cathode resistors be? And should I use four or two? I’m using Shannon’s driver board.

Thanks
Ted
Waiting on the last shipment to arrive.

[WA4SWJ]

Hi Ted,

I have an ST-70 clone and I used a 3-wire power cable and grounded the chassis. The amp has a very low hum level - barely audible. I don't think you'll have a problem at all.

I've used SED and Mullard (the new ones, not NOS) EL34's in my ST-70. I think the Mullards sound better by a small amount. It's subjective and I can't offer any direct evidence via measurements, etc.

I used 2 watt resistors in the cathodes. I^2*R = (.050 A)^2 X 10 ohms = .025 W -- 2 watts is plenty at 50 ma bias.

BTW - my ST-70 is my best sounding amp with my Poseidon Mark III's a very close second.

Regards,

[Shannon Parks]

I have an ST-70 clone and I used a 3-wire power cable and grounded the chassis. The amp has a very low hum level - barely audible. I don't think you'll have a problem at all.

Hi Guys,

1) Ed, where did you connect the safety ground?

On my original ST70, I connected a safety ground at one of the rear power transformer mounting lugs with a star washer. All the other grounds star ground at the can cap, except for the EL34 current sense resistors which ground at the sockets. With the safety ground attached, there was unacceptable hum so I disconnected it. Hum retreated to very low levels (way under 500uV at 8 ohms) and is totally inaudible on my horns - probably my quietest amp.

I never tried messing with it after that. I think the Dynakit folks in particular would want to research this and make this lone mod to their kits.

2) Strangely, I've never used the Svetlanas. Seems people either swear by them or at them. Just like any other tube. I use JJ's because they are inexpensive enough to have a few quads laying around, and they test well for low distortion. I hate their crappy sockets, though.

3) Regarding the EL34 current sense resistors, I've read that a few people like to size these as 'fuses' for when things go wrong. So it comes down to wanting to replace a tube or a replacing a resistor. I try to size the resistors inbetween. :) I would use four: separate sense resistors on each tube to be able to use the balance pot on the board.

BTW, I seem to remember reading where the original 15.6 ohm resistor is a magical value to match the tubes perfectly for best performance. That is pure hokum as far as I can tell. It was a handy feature for cheap VOMs, but that was it. Matching tubes with the balancing pot is a far superior method and assures lowest DC leakage current in the A-470s, thus assuring best bass performance.

Shannon

[Wmacky]

Hi,
I'm also new here, and new to tube gear for that matter. I'm waiting for an "upgaded" St-70 chassis that I have purchased from Soundtastic, and will do a ground up build for my first project. I believe you also purchased the same chassis from that auction a few day's ago? Perhaps we could help each other out with issues, as we will be doing the same project at the same time! I plan to use Shannons board along with the PS board from SDS or the completed unit on ebay from Tubes4pro. I've been doing tons of reading, but the learning curve is steep.

[WA4SWJ]

Hi Guys,

I connected the safety ground right at the star ground on the filter capacitor ground lugs. Everything is grounded there. My cathode resistors all are connected there too via wires. I don't have an underside picture of my ST-70 although I could take some I guess. However it would be a few days before I could upload them. I will not say that it is professional looking underneath but it looks good on the outside and it works like a champ!

The 15.6 ohm resistor, as I understand it, was intended to work like this:

15.6 X .1 A = 1.56 volts - the voltage of a fully charged carbon zinc dry cell. You could compare the reading of the dry cell with the voltage across the resistor with your inexpensive VOM and know that your bias on both tubes combined (100 ma) was set correctly if both voltages were equal. But I do like Shannon's approach. I like being able to balance the current through the tubes and measure each one individually. I don't think there is a huge sonic benefit for them to be exactly balanced, but keeping them fairly close is good. It would be interesting to try some balancing distortion measurements once I get my analyzer up and running. I check the bias and balance on my amp fairly regularly. It drifts most when the EL-34's are new and then settles down and is fairly stable from then on. Mostly it just gives me an excuse to mess with it. I'm listening to it right now through the new speakers I just finished building and it sounds wonderful. You guys that are building ST-70 clones with Shannon's driver are in for a treat.

Another suggestion - if you use solid state rectification consider placing 100 - 470 ohm resistors in series with the screen leads right at the EL-34 tube sockets to help prevent arcing. I had some trouble with arcing at first when I put in my solid state rectifier, but since I've added those resistors I have had no problems. Has to do with screen stability and preventing oscillation. There is an article about this on the audioXpress web site - an article about protecting your power tubes but I've forgotten the title. Go there and check it out. I've done this on all my amps now including my Mark III's even though those use 5AR4 rectifiers. Overkill probably, but those are expensive output tubes.

Just for info - I used Triode Electronics Dynaclone transformers and have solid state rectification with considerably more filter capacitance. I use a 5751 voltage amp and 12BH7's as phase inverters. I have the beefier Dynaclone power transformer. I love this amp!

Hope this helps. I'll be glad to help you guys that are building if I can.

Regards,

[Shannon Parks]

Thanks for the info, Ed. I'm playing around with the Ike & Boonton right now - rainy days are good for toobin'.

FWIW, with the Rev B parts list for my ST70 driver, I used 600V values for the coupling caps due to the higher B+ if folks used solid state rectification. So SDS users can experiment with SS without fear of blowing something up. Yellow_Light_Colorz_PDT_07

Shannon

[erichayes]

Hi All,

Just a couple of comments . . .

If you're going to be completely anal about your grounding scheme--be it star or bus--the chassis ground point should be as close to the input jack as possible. A combination of star and bus grounds is preferable, with starring being done according to current draw (i.e.: output and P.S. grounds separate from small signal grounds). I know it's kinda silly, but it's how Bud and I got 0.0027% THD (@ 1W) out of the 100W. monoblocks.

As far as the output cathode resistors go, I finally relocated a little gem by the late Bruce DePalma--don't know how or why I discovered it in the first place--regarding his take on analog amplifier design.

http://depalma.pair.com/Analog/analog.html

You'll find the answer to the magic cathode resistor, along with a bunch of interesting philosophy that I don't necessarily agree with (especially his choice of output tubes).

[EWBrown]

I've used both JJs and Svetlana EL34s in my ST70s. I have in reserve, a NOS quad of GE 6CA7s (that I bought many years ago at a hamfest for dirt cheap) , and a quad of not-yet tried JJ KT77s, of which I have read lots of very good reports, here and elsewhere.

/ed B in NH