Going by the underside, typical build quality and the cloth leads on the transformers say this one is on the earlier side. (Nothing wrong with that, just saying.) Rolling up a comment or two above, I would do the minimum to get this working (seems like just replace one bad power tube?), listen to it, check some voltages, and have a plan. There is no law that says ST-70s have to be rebuilt.
Dynakit ST 70. A Restoration Journey
- Thread starter Stereodiver
- Start date
Stereodiver
Well-Known Member
Thank you SS904. Now I understand, what those caps do. I’m sure it’s apparent that I’m not very good at reading a schematic. Your simple explanation helps me a lot!
I am familiar with that web site. Unfortunately it doesn’t give the uf value of the capacitors. As I’ve said I’ve seen both 47uf and 100uf installed there. In my experience it is rarely recommended to increase the uf value. But, there are exceptions. The phono board of Marantz receivers for example. I’m just wondering if this is also one of those exceptions.
Stereodiver
Well-Known Member
That is exactly what I’m going to do. Listen to it first. I’ll prolly order up tubes tomorrow.
Formulating my “Plan” is the reason I’m asking so many questions. The tubes are going to be the largest expense. The rest of the stuff I may need is relatively inexpensive, until you add shipping costs. I’d much rather order everything I ”May” need from the minimum number of suppliers to minimize the shipping costs. I don’t like a .38 cent resistor costing me $6.00! LOL.
Thanks guys, I really appreciate all your help. Mark
I always have received great service from Dynakitparts and Antique Electronic Supply (Tubes and More). They are very helpful along with knowledgeable.
Get the tubes you really want over settling on just saving money. There is a difference in tubes. I never as a rule use any expensive vintage tubes when decent cost effective modern ones are readily available. Saving the high dollar vintage ones.
Get the tubes you really want over settling on just saving money. There is a difference in tubes. I never as a rule use any expensive vintage tubes when decent cost effective modern ones are readily available. Saving the high dollar vintage ones.
The VTA 70 driver board many use have 100uF caps for the bias power supply with the whole thing mounted on the board along with individual bias pots for each tube. Again though, if you're doing a resto, I'd probably stick with the 50uF caps. Not sure what is included with the kit from that website, but 47uF would also likely work given the tolerances. You can get 50uF 100V electrolytics cheaply and easily if you are just replacing the caps and not interested in a whole kit.
John Ess
Well-Known Member
Before doing any mods, other than a new set of EL34's, give it a good listen...as long as there's nothing else wrong with it. Check the bias and play it for several hours and get a sense of how it sounds. My first real tube gear was Dyna MkIII's and MkIV's....and I stuck with the 7199 circuit...I just rebuilt the boards using plexiglass and new parts. And I'd do it again if I had a ST-70. Try to experience the amp as Hafler intended.
John Ess
Well-Known Member
Loved the pair that I had. Worth pairing with the right speakers for sure.
Lavane
Lunatic Member
I covered the driver board in my ST-70 with a plexiglass cover. No more exposed circuit board. I've seen a plexiglass cover for the stock driver board before on Ebay.
Attachments
John Ess
Well-Known Member
That looks good. I used 3/16" plexi to actually replace the boards in my MkIII's. Bought new parts and drilled holes for the leads. New tube sockets mounted on top. Actually pretty easy job. Plexi is nice to work with.
Stereodiver
Well-Known Member
Moving on to the driver board.
I will do my initial listening with this board as is. I think I remember reading that those “Black Cat” caps are supposed to be pretty good.
I know there is much controversy as to weather or not one can “Hear” the difference between different types of caps. I do not want to add to that controversy. If you say you do,,,than you do. If you say you don’t, than you don’t.
To the folks who have replaced these, what did you use and why?
Thanks, Mark
I will do my initial listening with this board as is. I think I remember reading that those “Black Cat” caps are supposed to be pretty good.
I know there is much controversy as to weather or not one can “Hear” the difference between different types of caps. I do not want to add to that controversy. If you say you do,,,than you do. If you say you don’t, than you don’t.
To the folks who have replaced these, what did you use and why?
Thanks, Mark
Pio1980
AK Member
630 VDC metallized polypropylene film types by Illinois or Panasonic.
Best values for reliable quality modern technology.
Lavane
Lunatic Member
A agree. Both of those are cheap and sound fine. I tend to use both.
Also polypropylene Orange Drops are very affordable.
I was going to use something "better" in my Dynaco ST-35 clone I built, but the yellow Illinois caps that came with the kit sounded great so I left them.
But, you can get fancy and use Russian PIO caps. They're pretty cheap and I used them in my VTA ST-70 and my VTA M-125 mono blocks.
Stereodiver
Well-Known Member
I have tried some Russian PIO caps while voicing a crossover in a DIY speaker build. I didn‘t like them for that application. What they did in the tweeter circuit was certainly audible to me though.
I have read where a few people recommend upping the .1uf caps to .22uf and the .05uf caps to .1uf. Claiming the lower value was only used because of budget constraints.
Does anyone have an opinion about changing to the higher values?
Thanks again! Mark
I have read where a few people recommend upping the .1uf caps to .22uf and the .05uf caps to .1uf. Claiming the lower value was only used because of budget constraints.
Does anyone have an opinion about changing to the higher values?
Thanks again! Mark
The stock values are just fine. Within a feedback loop like these are, the cap values are not as critical to frequency response as basic math might imply, but the 0.1uf coupling caps have a -3db point of 6hz. 0.22 would move it to about 3hz. This isn't going to be audlble anyway. The 0.05 is a screen decoupling cap, and its good down to 2hz. About all you might achieve with bigger ones is upsetting the LF stability.
kvflyer
Super Member
I'm not an engineer. That said, I don't change values that were chosen by the person(s) that designed an amplifier. Often there are reasons for choosing a particular capacitor such as motorboating, subsonic oscillations etc. But each is entitled to do what they want. It's just my experience that when I restore an amplifier (I've done many HH Scott) and use the design, it just works.
Your Mileage May Vary...
Your Mileage May Vary...
Stereodiver
Well-Known Member
Thanks for dispelling a seemingly long held myth. And, thanks to all for hanging in with me.
Moving on, Resistors.
I‘ve checked them all using their color codes. A few test slightly low but most test a little high. Than there are a few that must be tied to a capacitor so I can’t get an accurate read without lifting a leg. I didn’t feel it was necessary.
In this position,
A seemingly high quality tight tolerance resistor was used. Why only here?
Some recommend changing all the resistors to 1% metal film. I have done this a few times. The Marantz 2252 that I listen to now as an example. I certainly like the way it sounds but commenting as to weather or not it’s truly better is above my pay grade. One thing I will say is that it is indeed quiet. Nothing comes through the speakers until fully rotating the volume control, even than, very little.
Than there’s this,
According to the color code, these resistors should be 1k ohms. Yet you can see they measure 48.6 and 49.2 respectively.
Is there something going on here that I don’t know about?
Should I be worried about this?
Is changing to metal film resistors something I should think about?
Lastly, in dcgillespie’s write up it was mentioned to add, “Screen Stability Resistors“. Disconnecting the wires from pin #4 on the output tube sockets, adding a 100 ohm 1/4 resistor than of course reconnecting the wires to the resistor.
Should I do this?
Thanks again, Mark
The one resistor shown is for the bias adjustment. For this reason Dynaco wants it to be precise do to the 1.56V IIRC.
The testing may be off do to in circuit.
Personally I go for PM when going thru a unit. There are so few parts in this amplifier replacing them with new is not that expensive. Last thing wanted is to get it all hooked up and a few hours in troubles show up.
The testing may be off do to in circuit.
Personally I go for PM when going thru a unit. There are so few parts in this amplifier replacing them with new is not that expensive. Last thing wanted is to get it all hooked up and a few hours in troubles show up.