Topic review

Tom,

Dynakitparts will be coming out with a bias system for the ST-35 amplifier by the end of this month. Email Kevin at Dynakitparts for more details on his new "ST35-BCU" ST-35 bias system.

I have used the Russian 6P14P EL84 substitutes and they sound OK to me. I haven't had any trouble with the 8 that I have. I have them in my personal ST-35 and another custom EL84 amp based on the ST-35. They are inexpensive and seem to be holding up well after about 2 years and a couple of thousand hours service.

Bob

Hi Bob,

I resolved my Login ID problem!! Sorry for any inconvenience there!

I found your article here on the ST-35 to be very interesting and informative. I just happen to have an 'extra' new choke around here, and I also have a 'new' ST-35 that I'd completed in Kit Form from Dynakitparts.com. I think I'll install that choke, and perhaps just change-out the RCA plugs as well. However, what I'm MOST curious about is your comment about the ST-35 NOT having any adjustable bias controls. This, in fact, seems to be consistent on virtually every EL84 based amp that I've ever seen! The opposite seems to be true of the 'big-tube' amps, where they almost always have a bias control of some kind. So, my question here is simply this: Do you know of a modification to the ST-35 that would facilitate the inclusion of a bias control on that amp? I might mention, since my completion of that kit, I've noted the importance of having Matched Quads on my ST-35, and my SCA-35s....as well as a few other EL84 based amps I have here. So, I'm actually 'in-process' of building some kind of EL84 Tube Measuring device. I had purchased a single-ended 6BQ5 amp off ebay and was all set to start this project, but when the amp arrived, the PT was dead....and so went my project for now. But, I'm still looking for something to build that I can rotate a bunch of EL84s into...either one at a time or more if possible...and to measure their current draw, then match 'em up accordingly. I've been far more involved with EL34s, KT88s, and 6L6 type platforms and I never really paid attention to the bias aspect of any 'little EL84' amp. But, I am now!!

So......if you've got any plans to re-engineer that part of the ST-35 circuit...I'd LOVE to know about 'em. I'll even offer to guinea-pig my amp for initial testing! I think it's a great idea to 'add' a bias control circuit to the ST-35...and with the number of old 'un-matched' EL84s out there that are GREAT tubes...just not adequately sourced for installing as matched-quads in this amp... I think creating a way to individually bias each tube would be a GREAT addition to this amp! Also, this modification could be easily applied to all the old SCA-35s out there....which use essentially the same output circuit.

With that......good luck, best wishes....and hope to hear back from you.

Oh...one other thing I wanted to mention....the 6P14P tube; the Russian equivalent to the EL84: Do you have any thoughts about that tube? I've tried a few of them, and I find them 'nice'! Not 'Stellar'...but not bad either. I'd put them into the same category as the original Sovteks or current JL tubes. Just wondered if you had tried them and if so, what your thoughts were?

Regards,

TSD

Dynaco ST-35 with new boards and C-354 choke added



Power switch added to front of another reworked ST-35



The Dynaco ST-35 was one of the last tube power amps designed by Dynaco. Many feel, and I concur, that this little amp, within its power range, is probably the best sounding ALL STOCK power amp that Dynaco ever put out. Only 17.5 watts per channel but, even unmodified, a very clean sounding 17.5 watts. EL84 output tubes, a 12DW7/7247 all triode driver system and a solid state rectifier gave this little amp excellent frequency extension and very low distortion figures. Dynaco did cut a few corners to save a buck. There was no on/off switch, no choke and no adjustable bias system like on the ST-70. With a little time and a few modifications you can still improve the sound of this little gem.

1. Put in an ST-70 C-354 choke in place of the 50 ohm resistor on the quad cap. There is a nice place for it on top of the chassis right in front of the left output transformer. Just drill two 9/32" holes that are 2 3/8" apart. The choke will cut down on AC ripple current and lower the noise floor of the amp somewhat.
2. Remove C1 on both channels and replace it with a jumper wire. C1 is a DC blocking cap but isn't really needed since all modern preamps have a DC blocking output cap.
3. Pull C6 (18 pF) and C7 (27 pF), two disc ceramics, from the feedback circuit and replace them with better silver mica units. These two capacitors have usually drifted pretty far off their original values in 40+ years. If you still have the original ones in there now both channels probably have different amounts of feedback now.
4. Replace the main coupling caps C4 and C5. Dynaco used relatively cheap caps here. Auricaps will fit in here nicely. Be careful which replacement caps you buy because there is limited space on this smallish sized amp.
5. Replace C3. You don't really need a premium cap here but you might as well replace it since the cap in there is 40+ years old.
6. Replace the input jacks with gold plated input jacks. The original ones were aluminum and over the years they develop a hazy coating and sometimes cause "non contact" issues. Do you sometimes have to jiggle your interconnects to get sound out of the amp ?
7. Replace the output terminals with modern 5 way binding posts. Dynakitparts sells a replacement set.
8. Check the resistance of all those carbon composition resistors on the old board. (The resistor values may be found in the ST-35 manual - Link below) They usually drift high and sometimes start to make noise. (Do you hear a little "hisssss" that maybe shouldn't be there ?) Replace them with metal film resistors. You could also replace both boards and all parts with the Dynakitparts ST-35 replacement driver boards and parts set.
9. Add a power switch. Any smaller 1/4 inch AC switch will work just fine. Just drill a 1/4 inch hole in the side of the chassis somewhere on the front of the amp.

Dynaco ST-35 assembly manual

Bob