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    Rebuild your Dynaco ST-35 power amp - photos ..

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    Bob Latino
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    Rebuild your Dynaco ST-35 power amp - photos ..

    Post by Bob Latino on Fri Feb 20, 2009 12:58 am

    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

    wharf-creek

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    Age: 62
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    Re: Rebuild your Dynaco ST-35 power amp - photos ..

    Post by wharf-creek on Thu Apr 16, 2009 10:34 am

    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

    Bob Latino
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    Re: Rebuild your Dynaco ST-35 power amp - photos ..

    Post by Bob Latino on Thu Apr 16, 2009 12:52 pm

    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

    vleena

    Posts: 9
    Join date: 2010-01-23

    Re: Rebuild your Dynaco ST-35 power amp - photos ..

    Post by vleena on Fri Jan 22, 2010 7:55 pm

    Hi Bob,

    I just have my ST35 modified as per you suggestions, all points, I also added the Bias kit (from Dynapart) and replaced the filter capacitor by the Dynakit one, as the old one was leak. Mod was done by experience audio electrician and followed the given instruction.

    After connecting it, I found the sound is a lot londer than before, I used to turn the volum to 12 o'clock, but now around 8 or 9 o'clock only. Not sure what is wrong. I noticed the quad shape resister 95 ohms was also removed it was connceted to the filetr cap.

    I sound is not as clean as before and bass is a bit boomy. Image and Sound stage are also blur not as good as before. I hope after burnning in for 100 hours sound will improve.

    Looking forward to your advice.

    Bob Latino
    Admin

    Posts: 1933
    Join date: 2008-11-27
    Location: Massachusetts

    Re: Rebuild your Dynaco ST-35 power amp - photos ..

    Post by Bob Latino on Fri Jan 22, 2010 8:34 pm

    Hi,

    Are you saying that the 95 ohm resistor between section 4 of the quad cap and the grounding lug is NOT THERE ? That 95 ohm resistor sets the reference to chassis ground for ALL FOUR output tubes. That could be the reason why the amp does not sound right. On the other hand, I am not familiar with the Dynakitparts autobias circuit that you are using and maybe the 95 ohm resistor is not needed with this circuit.

    Bob


    Last edited by Bob Latino on Sat Jan 23, 2010 6:40 am; edited 1 time in total

    vleena

    Posts: 9
    Join date: 2010-01-23

    Re: Rebuild your Dynaco ST-35 power amp - photos ..

    Post by vleena on Fri Jan 22, 2010 11:35 pm

    Hi Bob,
    Thanks for your quick response.

    Yes the Resister was removed. But I just rechecked the diagram for Bias control upgrade schema, looks like the resister is removed. The output tubes and grouding lug are connect to the Bias control section instead.

    The sound is big, too lound, cloundy. I expected a hersh sound right after changing the C3, C4, C5 caps to Auricaps.

    I will try to add the photo to show you.

    regards
    Vichai

    vleena

    Posts: 9
    Join date: 2010-01-23

    Re: Rebuild your Dynaco ST-35 power amp - photos ..

    Post by vleena on Fri Jan 22, 2010 11:48 pm

    Hi Bob
    Sorry, I can not copy photos. Can I have you email address ?
    regards
    Vichai

    Bob Latino
    Admin

    Posts: 1933
    Join date: 2008-11-27
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    Re: Rebuild your Dynaco ST-35 power amp - photos ..

    Post by Bob Latino on Sat Jan 23, 2010 6:42 am

    Hi,

    Send the photos to me at my Email address > Bob01605@aol.com

    Bob

    Bob Latino
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    Re: Rebuild your Dynaco ST-35 power amp - photos ..

    Post by Bob Latino on Sat Jan 23, 2010 2:23 pm

    vleena wrote:Hi Bob
    I have forwared you some photos, and really appreciate your feedback.
    regards
    Vichai

    Vichai,

    Since I am not that familiar with the Dynakitparts autobias system for the ST-35, I have forwarded your photos to Kevin at Dynakitparts. He should be contacting you by Email soon ..

    Bob

    vleena

    Posts: 9
    Join date: 2010-01-23

    Re: Rebuild your Dynaco ST-35 power amp - photos ..

    Post by vleena on Sat Jan 23, 2010 7:30 pm

    Hi Bob
    Thank you very much. Kevin contacted me. I will semd him some more information, and will keep you in the loop.
    Regards
    Vichai

    briank

    Posts: 1
    Join date: 2010-04-30

    Re: Rebuild your Dynaco ST-35 power amp - photos ..

    Post by briank on Fri Apr 30, 2010 12:12 pm

    It's my wild guess, but the only way that the amp sounds that much louder and worse is if the negative feedback is disconnected somehow.

    DynakitParts
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    ST-35 Problem

    Post by DynakitParts on Sat May 01, 2010 10:58 am

    Briank,
    That was exactly the problem...NFB was not connected properly.

    I had asked our friend Vichai to post a follow up as a courtesy to all
    that posted advise,including yours truly. This is free forum and help
    is always generously offered by all members.

    Regards,

    Kevin @ Dynakit

    vleena

    Posts: 9
    Join date: 2010-01-23

    Re: Rebuild your Dynaco ST-35 power amp - photos ..

    Post by vleena on Tue May 04, 2010 9:57 am

    Kevin, Bob and all

    Yes correct, the faulty caused by no NFB connection. Actually, we used the incompleted schema as a guideline. Infact my upgrade included everthing suggested by Bob, except resistors replacement. In additions, new electro cap, bias controller, AC/IEC, Cardas RCA and SPK connections were also installed at the same time.

    Good experince indeed. Actually, I should have taken few wiring photos before changing anythings.

    I am not a DIY guy. and my technician diud not have time to go thru the original schematic. That's why we missed out the NFB connection part.

    I am very happy with the more details, better dynamic, more mid range clarity than ever before.
    My ST35 is driving Quad ESL53 with no problem. I do not have PAS2 or 3 preamp, but connect st35 to Tube Hovland HP100 pre, which I think should be a lot better than PAS2/3 pre. Any recommendations.

    I just prchased Fluke 115 from Ebay for bias measurement, and now I can play with EL84 Tele NOS, EL84M, and Golden Lion EL84 (recommended by Kevin) are on the way. To be honest I prefer EL84M to Tele NOS interms of speed, detail, and openess of high frequecy.

    I had been in up(wow !)and down (shit what's wrong!) mood during past 2-3 months. But a lot of funs indeed. It is worthwhile to upgrade and push the ST35 to it limit.

    I will have the ST35's resisters checked soon.

    Any advices are very well come.
    Thanks Vichai

    kost

    Posts: 32
    Join date: 2009-05-12
    Age: 50
    Location: athens

    EL84M

    Post by kost on Thu May 06, 2010 4:39 am

    vleena wrote:Kevin, Bob and all

    Yes correct, the faulty caused by no NFB connection. Actually, we used the incompleted schema as a guideline. Infact my upgrade included everthing suggested by Bob, except resistors replacement. In additions, new electro cap, bias controller, AC/IEC, Cardas RCA and SPK connections were also installed at the same time.

    Good experince indeed. Actually, I should have taken few wiring photos before changing anythings.

    I am not a DIY guy. and my technician diud not have time to go thru the original schematic. That's why we missed out the NFB connection part.

    I am very happy with the more details, better dynamic, more mid range clarity than ever before.
    My ST35 is driving Quad ESL53 with no problem. I do not have PAS2 or 3 preamp, but connect st35 to Tube Hovland HP100 pre, which I think should be a lot better than PAS2/3 pre. Any recommendations.
    Hi

    You mean sovtek el84m?

    kost


    I just prchased Fluke 115 from Ebay for bias measurement, and now I can play with EL84 Tele NOS, EL84M, and Golden Lion EL84 (recommended by Kevin) are on the way. To be honest I prefer EL84M to Tele NOS interms of speed, detail, and openess of high frequecy.

    I had been in up(wow !)and down (shit what's wrong!) mood during past 2-3 months. But a lot of funs indeed. It is worthwhile to upgrade and push the ST35 to it limit.

    I will have the ST35's resisters checked soon.

    Any advices are very well come.
    Thanks Vichai

    Hi

    You mean sovtek el84m?

    kost

    vleena

    Posts: 9
    Join date: 2010-01-23

    Re: Rebuild your Dynaco ST-35 power amp - photos ..

    Post by vleena on Tue May 18, 2010 8:14 am

    Hi
    They are EL84M and printed 6P14P on tubes.

    However, I just have my Quad ESLs sit on the new 14 inches stands instead of original legs and the sound changes dramatically (Telefunken),leaner, sweeter mid range, better image and sound stage, and more dynamic.

    So I will need to experiment all tubes again, 6P14P, Tele and new issued Golden Lion. Definitely the result would be different this time. I suspect 6P14P may be harsh and fatiq for long hours listening.

      Current date/time is Tue Sep 16, 2014 8:29 pm