Dynaco ST-35: Getting Ready To Rebuild

I rebuilt my own ST-35 five years ago, and have thus far retained the stock circuitry (albeit with new capacitors on the driver boards). I may install EFB mods at some point, or at least put in a C-354 choke, but have yet to feel the need to do so. I agree with what others have said about using Sovtek EL84Ms, though I did find a quad of close-matching Mullard-made EL84s recently. Here's a picture of my ST-35, complete with old Dymo labels from when it was apparently part of KLH's research & development department, as it came from the estate of Daniel von Recklinghausen (who also worked for H.H. Scott, among other companies):
st35_2.jpg

I probably am going the EFB route, but seriously, the choke is a wonderful option. That was my plan when I first got here. I don't know, maybe put it off board and do both?
 
I built an ST-35 with new parts and transformers from an SCA-35. Those are the same transformers used in an ST-35. New D. Gillespie boards along with the EFB board went into it. I can't speak for the sound of an original ST-35, but the sound of this amp compares very favorably with my reference amplifier. Namely, very clean and detailed, within its power limitations. Add some reasonably efficient speakers, and it'll do the job great.
Just adding a yes amen and Halleluja!
 
I built an ST-35 with new parts and transformers from an SCA-35. Those are the same transformers used in an ST-35. New D. Gillespie boards along with the EFB board went into it. I can't speak for the sound of an original ST-35, but the sound of this amp compares very favorably with my reference amplifier. Namely, very clean and detailed, within its power limitations. Add some reasonably efficient speakers, and it'll do the job great.
Just adding a yes amen and Halleluja!

Using Altec Model 19 and 604-8G . So around 99 and 100 db effecient I think. My other main amps are a 45 and 300B. This amp is over twice the power of the 300B and of course way more powerful than the 45. So no worries there! I have been wanting to get a pair of Bozaks which this Dynaco should drive well also I think. AR is another wish list, might use the ST-70 for that...
 
As your amp was coming to you stock (I guess), it makes sense to plan what replacements to make to the power supply (and elsewhere) at the start. I would check/replace the can cap and coupling caps even before a first power up. If you are replacing the can caps, it makes sense to consider the EFB version at that time.

Some of the discussion about 'Who are the "we" here?' might also reflect that not everybody on AK has English as a first or even second language. And with different languages come different communication styles which might come across unintentionally as 'attitude'.

Looking forward to some pics of your ST35 journey. Even though I have an SCA35 with Dave's boards, I'm still curious about the legendary sound of a well-serviced ST35.
 
As your amp was coming to you stock (I guess), it makes sense to plan what replacements to make to the power supply (and elsewhere) at the start. I would check/replace the can cap and coupling caps even before a first power up. If you are replacing the can caps, it makes sense to consider the EFB version at that time.

Some of the discussion about 'Who are the "we" here?' might also reflect that not everybody on AK has English as a first or even second language. And with different languages come different communication styles which might come across unintentionally as 'attitude'.

Looking forward to some pics of your ST35 journey. Even though I have an SCA35 with Dave's boards, I'm still curious about the legendary sound of a well-serviced ST35.

Not a bad idea, I will see what I can do to check those out and make sure they are operating well. It is supposed to be in operational condition. I went ahead and ordered unpopulated EFB driver and power supply boards this morning and put in an order for some European 12DW7 (those are quite expensive).
 
Sure it would help. But, doing it in this sequence you won't know the sonic potential of the original circuit. To get closer to what the original circuit is capable of sonic-wise, you'd need to refresh a bunch of parts 1:1 well before dropping-in the EFB, IMO. The playing field has to be level before a more meaningful comparison can be made. Of course, if you aren't really interested in doing this, then by all means go for the EFB right off the bat.

Why bother? It's been very well documented how hamstrung the original circuit was with regard to power supply. Seems like spending a bunch on the original PSU, then upgrading would be money better spent doing it right the first time and buying more records. :)

-D
 
Might it be that the original power transformers are a bit under-specified? Even with Dave's fine new boards, the 240V power transformer in my SCA-35 vibrates too much, but this may be because of the 50Hz frequency in the UK. And my ST70 sounds much better with a bigger 240V PT designed for 50Hz, than the original did with drop down voltage. But I can't comment on how the original PTs operate with 117V 60Hz line voltage.
 
EFB demonstrates a well-considered and desirable technical improvement to the Stereo 35 from someone with obviously real engineering expertise who respects the original Dynacos (all very refreshing, by the way - thanks, Dave). But, none of this renders the original Stereo 35 impotent.

If I may, "hamstrung" seems like an exaggeration. Yes, the original Stereo 35 will be tougher on the power tubes and have less power available, but operated within its capabilities, it is still a viable, fine-sounding amplifier. Just don’t run it into Magnepans and expect concert hall levels. Choose speakers of sensible efficiency to use with it, and things should be just fine.

What we have now in various “mods” is a sonic alternative to the sound quality of the original Dynacos. If someone wants to use EFB or any other “mod” for technical reasons alone, that’s fine. But, technical issues aside, a refreshed, properly functioning original Stereo 35 could still be more to someone’s personal sonic taste.

I try to avoid a polarized view of tech vs. subjective interpretation of the sonics. I try to take an eclectic approach and blend the two, because I realize that personal sonic taste inevitably varies. However, what I do think is totally off-base is the ongoing undertow of belief throughout different forums that all of the original Dynaco circuits suffer from some kind of hopeless erectile dysfunction.

Speaking from experience (I've had the same amplifier both with the original Dynaco implementation as well as with EFB), I'll have to take the side of EFB. And I think hamstrung is an apt description, given that you were good for about 12WPC and still eating output tubes. However, everyone has their own preferences.

-D
 
This is the chassis I would like to use. I know its not the classic all cramped up on a small Dynaco chassis, but I would think the amp would have more room to run cooler. st-35-improved-chassis-base-6.gif
 
Got the amplifier a few minutes ago and, contrary to advice and my better judgement, I plugged it in and turned it on. Couldn't stop myself. It is now playing and I must say, playing very sweetly. Even with JJ and regular Sovtek tubes I am extremely impressed. The sound for such a little amplifier is astounding! And given that my other amplifier I am using is a 45 SET amp (my 300B has a hum problem) I had to turn the volume WAY down and it is just blasting out the sound still. Anyway, if I can get even better sound from this amp I will be amazed, it sounds that good off the bat. I suppose I should get my order placed with Jim McShane for some tubes. Trying to decide between JJ and Sovtek EL84M. The EL84M should be sturdy but I am familiar with JJ and remember liking them well enough. If the modified amplifier isn't going to eat them I think I will go that way if I can't bring myself to spend the cash for some EI.

Boards arrived in the mail today today too. Doug's turnaround was lightening.

 
Congrats!
Of course you should connect and use it! What else did you intend to use it for ?
While listening to your favorite recordings and building up a audioble memory of the
equip, you can start figuring what needs to be done .
Matched el84 is one such thing. 6dw7 ? check the voltages and compare with schematics, if
close there is no need. B+ cap ? possibly ( or go for EFB where the cap in included on the board).
Maybe EFB will cost less then a new B+cap ?
Some folks mounts a circuit breaker on front, i rather install a SSR and control via DC from
the preamp.

Again, enjoy your amp, given it's power it's an amazing gem!
 
Got the amplifier a few minutes ago and, contrary to advice and my better judgement, I plugged it in and turned it on. Couldn't stop myself.

A dim bulb tester is a half-way safety for situations like these in future. For your next amp. :),or for after you do the rebuild. Great to hear that it's working well
 
I have ordered all the parts. Boards came in immediately but Mouser is saying some of the parts are back ordered until December(!!!). Oh well, gives me plenty of time to get used to it.

I had noticed that it is a tad fatiguing on initial turn-on. But as it warms up it seems to lose some stridency that I was worried about. Plus my speakers might be a bit bright with certain amps. Put in a pair of Mullard 7247 to replace the pair of JJ tubes it came with. Didn't notice any change in the sound oddly enough. None. But somehow I feel like I am enjoying the amp a tad more? I don't know, hard to describe. The change, if it is there (and one assumes it has to be) appears to be minimal. But they are NOS so I will see how it develops.

And I was extremely pleased to find when I went through my tube stash that I already have a matched quad of JJ EL84 tubes as well as quads of Tungsram and GE oval plates. Woo hoo!
 
Lost the left channel. Red plated an output tube. Looks like I am going to be waiting for parts before I can do any more listening. Here is to hoping I didn't damage a transformer.
 
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Lost the left channel. Red plated an output tube. Looks like I am going to be waiting for parts before I can do any more listening. Here is to hoping I didn't damage a transformer.
Yikes! :eek: If you're lucky, it was a tube that just happened to go bad all on its own. After that, my next guess would be a coupling cap on the driver board which has gone bad. I replaced the green caps on both of the driver boards in my ST-35 before I put a volt to it, even though I thought they might be OK (vs. the molded-paper caps used in the ST-70 and Mark III, which I don't trust at all). If you shut the amp down shortly after the tube started red-plating, the output transformers should be fine. The Z-565s seem pretty robust to me.
-Adam
 
Yikes! :eek: If you're lucky, it was a tube that just happened to go bad all on its own. After that, my next guess would be a coupling cap on the driver board which has gone bad. I replaced the green caps on both of the driver boards in my ST-35 before I put a volt to it, even though I thought they might be OK (vs. the molded-paper caps used in the ST-70 and Mark III, which I don't trust at all). If you shut the amp down shortly after the tube started red-plating, the output transformers should be fine. The Z-565s seem pretty robust to me.
-Adam

Thanks for the input. I shut it down very quickly so hopefully the transformers are ok. I won't learn as much but I am leaning towards replacing the driver boards and putting in the the EFB at the same time just to be on the safe side and getting the amp up and running again.
 
So I finished the first stage of the build! And everything is working. Sound is marvelous. I decided due to the red plating of one of the EL84 to just go ahead and update the power supply and the driver boards. So I can't know what did what specifically, but I didn't feel like taking a chance on destroying any transformers. And amazingly I was able to do both the rebuild of the driver boards and alterations for the EFB power supply board at the same time without any issues. Also put in old stock RCA jacks and speaker posts from Dynakits to replace the ones in the amp which were pretty corroded. I am going to listen to it for a while then replace the 50 ohm resistor with a choke, so second stage is pretty easy.

Also ordered a matched quad of EL84M from Jim McShane. I don't know what the output tubes are driven at now, but I assume it is still fairly hard. Listening to some Tesla tubes for now but will switch them out for the Sovteks eventually.

 
Good to hear its working! I want to build one of these so bad. You should like the Sovteks. :thumbsup:
 
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