Dynaco ST-70 MKII

Hi all, first time posting here. I was looking for a forum dedicated to Dynaco and this seems to be the one.

I'm not one for long intro's so I'll jump right into it. I purchased a used ST-70 MKII and I have some questions and possibly, concerns. Right off the bat, the amp works. It sounds good so I don't see any issue there. It came with JJ's as spares (used) and Mullards in the amp.

I'm not afraid to check out what hardware is under the skirt so it didn't take long to take off the top cover and the bottom as well and this is where I found a few things that were questionable.

1. The back of the amp is labeled for 220v, no mention of 120v? Also, the fuse is labeled as "1.5 amp slo-blo" however, it's got a 3A 250 installed. Not sure what to make of those.

2. After taking the top cover off, I noticed two things. Capacitors C25, C26, C15 and C16 were soldered on the bottom of the PCB. Looking at the original schematic, the best I can make out for those caps is .68uf 400v? What was installed instead are .22uF 630v?

3. C21 and C11 were replaced as well with Russian caps. The specified values should be .18uF 50v and what's installed is 0.047uF and no V value.

4. There are several jumpers that were installed on the underside. They look rather ugly and I would care to fix those as well but am curious to know if they were factory and why were they added?

5. Can the 6GHBA tubes be changed out for any other tube? Just curious.

That's all I have off the top of my mind. Who ever did the butchering on this amp with their custom trimmings hasn't has much time behind an iron, the work is sloppy. Butchered at best. The solder isn't even fully adhered on some caps. The factory work looks good.

I will be replacing the power cord as well. Looks like someone tried to fix it in the past, there is a replacement plug on it. There was an attempt to remove the cord from the case which completely damaged the train relief so that's gotta get fixed.

Over all, I am happy with the amp. It is certainly in good condition. No rust at all and the cover is excellent at well. I would love to hear some input from your guys. Should I keep the caps that were installed or return them to original values? Anything else I can change out to improve the circuitry? There are some components that looks like they were slapped in there carelessly that I can rework. I am an anal bastard when it comes to soldering. I am in the process of building the Engineers Amp using TV sweep tubes if anyone cares to take a look, ill drop the link to that thread -apparently, I can't drop external links. Please go to diyaudio.com and search Build Log: P Millett's DCPP "Engineer's Amp". I am "M3Roc" on that thread.

Please take a look at the pics I've included of the ST-70 for reference. I can't leave an external link so ill try to upload all the pics here.

The schematic is hard to read, its missing a lot of info. If i'm reading it correctly, C25, C26, C15 and C16 are coupling capacitors and should be .68uF 400V according to the schematic. Can anyone tell me why someone put .22uF 630V instead?

Hi --- Your question about the 0.68 versus 0.22 coupling caps on the Mark 2 St-70 came up before on the Forum. Probably about 4-5 years ago (or more).
If I remember correctly there was some discussion on 0.68 vs 0.47 vs 0.22 caps and the fact that people substitute what they have available in parts.

Hope this helps.

Thanks. I'm trying to find more info on the S2 amp but it seems like there are barely any out there.. Tons of the original ST-70 amps but very little info on the S2.

I'll have to do more scouring. I want to return the amp back to factory spec if that's best. I also really would love to get my hands on a new circuit board as well but I feel like that'll be impossible.

Please note the interpolations.  And, for the record, I have seen, over time, two (2) ST7- v. II and looked under the hood of one (1). I have never taken an iron to any one.

[quote="Tiny Anakim"]Hi all, first time posting here. I was looking for a forum dedicated to Dynaco and this seems to be the one.

I'm not one for long intro's so I'll jump right into it. I purchased a used ST-70 MKII and I have some questions and possibly, concerns. Right off the bat, the amp works. It sounds good so I don't see any issue there. It came with JJ's as spares (used) and Mullards in the amp.

I'm not afraid to check out what hardware is under the skirt so it didn't take long to take off the top cover and the bottom as well and this is where I found a few things that were questionable.

1. The back of the amp is labeled for 220v, no mention of 120v? Also, the fuse is labeled as "1.5 amp slo-blo" however, it's got a 3A 250 installed. Not sure what to make of those.

That is, or was sold initially as an Euro-amp, then. And as current is measured in watts, 1.5 x 240 is the same as 3 x 120 in wattage. So the heavier fuse for US voltage is good. NOTE: Make sure what is in there is a DUAL ELEMENT fuse, and not a standard slo-blo. Standard Slo-Blo fuses are dangerous.

2. After taking the top cover off, I noticed two things. Capacitors C25, C26, C15 and C16 were soldered on the bottom of the PCB. Looking at the original schematic, the best I can make out for those caps is .68uf 400v? What was installed instead are .22uF 630v?

With film caps and other small-value non-electrolytic caps, there is no penalty for increasing the voltage rating. You will find that most after-market caps of this nature are typically rated for 630V. No issue going up in voltage, DO NOT go down in voltage.

3. C21 and C11 were replaced as well with Russian caps. The specified values should be .18uF 50v and what's installed is 0.047uF and no V value.

As above. For the record, I do not support PIO caps, from Russia or elsewhere. I do screen caps before installation, but after basic suitability and quality standards are met, a cap is a cap is a cap is cap. And, For the most part, coupling caps are pretty tolerant of different values - just endeavor to make the channels match left-to-right. At the same time, I would not switch back to OEM values unless/until I understood the full extent of the modifications.  

4. There are several jumpers that were installed on the underside. They look rather ugly and I would care to fix those as well but am curious to know if they were factory and why were they added?

Some jumpers may be factory, and some may be after-market modifications. I cannot imagine a responsible manufacturer installing those flying-splices to the resistors as standard, but stranger things have happened. I do not remember any such thing in the one example I saw. You will need to chase down a schematic if you can.

5. Can the 6GHBA tubes be changed out for any other tube? Just curious.

There are a host of tubes that will "Fit": 6EA8, 6AN8, 6U8 are similar tubes with the same pinout. The 6KE8 will work in a parallel-filament application. But, as you will find out, it is likely you will pay more for the alternatives.

That's all I have off the top of my mind. Who ever did the butchering on this amp with their custom trimmings hasn't has much time behind an iron, the work is sloppy. Butchered at best. The solder isn't even fully adhered on some caps. The factory work looks good.

I will be replacing the power cord as well. Looks like someone tried to fix it in the past, there is a replacement plug on it. There was an attempt to remove the cord from the case which completely damaged the train relief so that's gotta get fixed.

Over all, I am happy with the amp. It is certainly in good condition. No rust at all and the cover is excellent at well. I would love to hear some input from your guys. Should I keep the caps that were installed or return them to original values? Anything else I can change out to improve the circuitry? There are some components that looks like they were slapped in there carelessly that I can rework. I am an anal bastard when it comes to soldering. I am in the process of building the Engineers Amp using TV sweep tubes if anyone cares to take a look, ill drop the link to that thread -apparently, I can't drop external links.

Enjoy!

p.s.:

Here is a link to the manual - not exactly informative.

https://www.thetubestore.com/lib/thetubestore/schematics/Dynaco/Dynaco-ST-70-II-Schematic-Owners-Manual.pdf

Thank you for the info. I will leave the caps on the bottom then. As far as the PIO input caps on top that are Russian, should I replace them with something else? It looks like they are 0.047uF and 200V?

Tiny Anakim wrote:Thank you for the info. I will leave the caps on the bottom then. As far as the PIO input caps on top that are Russian, should I replace them with something else? It looks like they are 0.047uF and 200V?

If the caps-in-place are good, then leave them alone. I do not support changing good caps for questionable caps - and my specific and direct experience with Russian PIOs is that the defect rate is significant, to which the complication of a conductive case makes then simply not worth it. But if they are in place, leave them alone.

Exception noted and respected.