Comments for page: PS Mods for ST-35

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Posted by Art Grannell February 11, 2015 - 12:58 pm
Bill,
If the “guru” is Shannon Parks, then I’ll just say he’s a “good guy” and has done a lot for tube electronics. However, as I mentioned in the very first comment after posting this article, no two people are going to see things exactly the same way. Audio and electronics are certainly great examples; otherwise everyone would own or build identical amplifiers and loudspeakers. How boring that would be!
To make a decision, it often helps to rely on “old fashion” common sense. Does it make sense to you that the B+ voltage will actually increase from the first filter capacitor to the second, with a choke or resistor placed between them?
Regarding Dave’s EFB bias: I honestly don’t understand what “more sag and less swing” means. Go back and read his original post “A New Look at an Old Friend”. Soon after this was published, I made my own independent measurements and they closely approximated the power increases that he discovered. I didn’t have a distortion analyzer at the time, so my results were not as accurate as Dave’s, but they were close. If you read “An Installation of the EFB™ Bias Circuit in a Dynaco ST-35”, you can see my impressions of the sonic improvements from EFB Bias in an ST-35.
If you check Dave’s post in the DIYtube “Stereo 35” forum, you’ll see that it currently has 44345 views, far more than any other posting. There’s a reason for that.

Keep in mind that if after building the amplifiers, you decide that the voltage is too high, or you want to change the EL84 bias circuit, changing either or both of these is quite simple, and only involves a few dollars in parts, since you already have the chokes.

I encourage to you proceed. It should be a valuable learning experience, and very satisfying to say “I built it myself”. Think things out as you work, double check your efforts, join the thousands of other “home builders”, and enjoy the fun! I don’t think that in 55 years of building electronics circuits, speakers, and computers that I’ve ever got anything precisely “right” on the first try---but that’s how I learned…………. and you can too!
Electronics, as a hobby should be “fun”. If you want to spend the money, there are 1000’s of manufactured electronic items to purchase “off the shelf”. But the appreciation and enjoyment of these isn’t the same, evidenced by the fact that many are disposable.

---Art Grannell

Posted by Bill February 09, 2015 - 12:52 pm
I posed similar questions to the guru at diytube and was told that B+ would be around 420 with the same choke I'm using. Also, EFB was not encouraged since it would not be necessary and there would be more sag and less swing if I did. This is getting a little too confusing for me, so I may retire any thoughts on doing mono versions.

Posted by Bill February 07, 2015 - 08:03 am
Thanks a lot Art, this has been very helpful. I'll be using the Triode PA-774 in the mono build and of course the EFB - which I use in my current amp. That is, if I do build the mono version!

Posted by Art Grannell February 06, 2015 - 08:38 pm
Bill,
Chokes oppose AC current and fluctuations in DC current. This filtering characteristic is the reason for use in this application, and their lower series resistance improves regulation, which is significant. I would not place a resistor is series with a choke as doing so would slightly decrease the choke’s inductance, and otherwise defeat much of the benefit of using a choke. Should you find it absolutely necessary to add the 50 ohm resistor, I would add it after the choke, and would add another electrolytic filter cap from the junction of the choke and resistor to ground creating a beneficial additional stage of filtering.
To some extent, a choke’s inductance is dependent upon the current flowing though it, so for a monoblock application, a slightly smaller one may be better suited. But if you already have them, “use ‘em”. As stated, there’s always “trade-offs”, and there is no perfect choke in any circuit.

I’m guessing that with the choke alone, you will see a B+ voltage to the OT of less than 390VDC, depending upon your power transformer and AC line voltage at the time. I wouldn’t be concerned at all unless it exceeds 425VDC. Due to high AC line voltage, I’ve run EL84’s at that plate voltage and higher for extended periods, with no issues.
However, what is important at elevated plate voltages is the tube current, which if not adjusted, can rise along with the plate voltage. Of course, I recommend Dave’s EFB circuit for adjusting the current. Don’t forget the control grid and screen grid resistors.

Also keep in mind that unshielded chokes radiate electrical noise, like other inductors. Place them away from sensitive low level circuits.

Good Luck!

---Art Grannell

Posted by Bill February 06, 2015 - 10:17 am
I use a Hammond choke (1.5 H, 200mA, 56 ohm, 400VDC) in my ST-35 and I get readings of 389 (385) at the 1st stage, 383 (380) at B+, and 322 (320) at the driver stage. ( Dynamo specs from the manual in brackets). Wall voltage 118.5.

I'd like to use the same choke in a mono version, but, as you say, I will probably need more voltage drop. This is my only concern: I could use a high wattage 100 ohm resistor and be done with it, or source a different choke, or ..... possibly combine a resistor with a Hammond choke? I have a couple of chokes lying around (same Hammonds). So it would be a C-R/L-C-R-C with the mono version - if I build it. Make sense?

Posted by Art Grannell February 05, 2015 - 10:33 pm
Bill,
I would need a little more information, concerning your objectives to be able to make a recommendation. It sounds as if you have replaced the 50 ohm series resistor with a choke in a stereo ST-35. Is this causing a problem? If so, is the issue excessive 120Hz noise or do you now feel that the supply voltage is too high? If it is the latter, what voltage readings do you have (to ground) both before and after the 6.8K resistor? How does this relate to a “monoblock” ST-35?

---Art Grannell


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