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Thoughts on restoring a Dynaco ST70

The 7199's are becoming more scarce. Buying what's left on ebay, etc, can be a crap shoot getting good quiet examples. The 6GH8A's Are more abundant. I wound up going with Latino's VTA ST-70 kit that uses three 12au7's and there variants. With that driver board it is a bit of a different animal compared to the original ST-70 though. But there are many choices in tubes that can be used that are still available and still being manufactured. But as I said
Blackdog61 said:
I think 716 but don’t remember. A lot of regulars here like the little yellow caps in tube amps. I found them to be a bit smoother and transparent than the ODs.
Regarding driver tubes, there is a lot of info on the net, but my take is the 7199 and 6GH8A, 6U8 and other variants are essentially the same other than the pinout. Some recommend minor changes to circuit values but there’s good enough, perfect and blowing it all up trying for the elusive. After trying a bunch I’m very happy with the 6G8H8As. If there is a difference it’s very slight.
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I've started using those yellow caps. I never got into spending a.lot.of cash on fancy boutique caps to begin with, but after trying the yellow caps in a couple builds and used the ones that came with my ST-35 kit I'll be using them a lot more. Nothing wrong with cheap and good. I was going to put some Solen or Mundorfs in my ST-70, but it sounded so good with the yellow ones Ieft them alone. I use the cheap Panasonic film caps as well.
 
This might be an odd request but does anyone have a layout already made of the original driver board? Then I can just mill it on the CNC router and etch it.
 
I'd also recommend a pair of CL-90 soft start MOV devices in series with the primary of the power transformer, especially if it is to be powered straight from the AC line, and especially if a solid state rectifier substitute is used in place of the 5AR4. Another change would be to replace the two 1.5 meg resistors with 780k-820k 1/2 W resistors for better splitter bias balance.
 
Wouldn't the rectifier tube encourage a soft start by design? My understanding is that the tube comes in conduction slowly as it's warming up, over a period of several seconds.

Are the 1.5M resistors you're referring to the ones that are pulling up the second grid in the driver? I don't quite understand what would changing the 1.5M resistors improve with. Can you please explain a little bit, I'm interested in improvements over the original design.
 
radu_c said:
Wouldn't the rectifier tube encourage a soft start by design? My understanding is that the tube comes in conduction slowly as it's warming up, over a period of several seconds.

Are the 1.5M resistors you're referring to the ones that are pulling up the second grid in the driver? I don't quite understand what would changing the 1.5M resistors improve with. Can you please explain a little bit, I'm interested in improvements over the original design.
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The 5AR4 is underrated for full power output from both channels, the silicon rectifier substitute allows full power availability. Two CL-90 in series provides a ca 5 VAC voltage drop to the power transformer primary when hot to compensate for generally higher modern line voltages than the ca 117 VAC the transformer was designed, bringing output voltages closer to design tolerances. The soft start is a bonus and makes the solid state rectifier option combined with the larger capacity quad cap option practical without excessive initial current draw surge stressing the transformer and fuse.

The plate voltage of the voltage amp stage pentode section determines the bias point of the directly coupled triode section phase splitter grid, the distribution ratio of the cathode resistance, triode section, and anode resistance is determined by the triode section grid bias point. Optimum distribution of ratios is 25%,50%, 25% of available B+ to the splitter anode resistor relative to bottom of cathode resistor to develop max linear drive swing and avoid asymmetrical driver clipping. The value of the G2 screen resistor determines the splitter triode grid bias point by setting the pentode plate anode voltage.
It's all in the excellent thread on base line testing the Dyna Stereo 70 by Dave Gillespie.
https://audiokarma.org/forums/index.php?threads/dynaco-st-70-base-line-testing.578485/
 
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petercapo said:
The original Dynaco PC boards had a phenolic substrate. The top (component) side had some kind of glossy finish.
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Bakelite is a phenolic resin, just with a brand name. The parts are cast in a mold and heated until it sets. The stuff used in the circuit boards is probably paper or cloth impregnated with the resin but same basic process as making a radio cabinet.
 
radu_c said:
This might be an odd request but does anyone have a layout already made of the original driver board? Then I can just mill it on the CNC router and etch it.
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It's easier of course to just order a populated polyamide board for 6CG8 based family tri-pent small signal drivers. I used the one offered by Vintage Electron for its quality and parts choice, changing only the 1.5 meg resistors for 1 meg Cermet pots, wired as rheostatic variable resistors to set splitter balance bias.
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@Pio1980 what brand/series are the axial capacitors on your PCB?
I'm not as savvy as you are when it comes to tubes so I'll have to read a little more into the spliter balance bias. I like understanding what I'm doing so I appreciate you giving me references to where I can find information about it.
 
radu_c said:
@Pio1980 what brand/series are the axial capacitors on your PCB?
I'm not as savvy as you are when it comes to tubes so I'll have to read a little more into the spliter balance bias. I like understanding what I'm doing so I appreciate you giving me references to where I can find information about it.
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I think the board source used Illinois axial metallized polypropylene for the signal caps, and Illinois polyester for the screen cap. These Illinois polyprops may be the yellow caps mentioned in other postings.
 
For the higher power resistors, do you guys recommend wirewound resistors or just the same metal film just higher power? Can't tell exactly from the pictures how big those resistors are but I was thinking to use 1W for the fatter ones and 1/2W for the skinny ones.
 
Thanks everyone for your input. I'm getting ready to start the restore. I might ask for your advice as I'm progressing. One thing I forgot to ask, the resistors inside the chassis, what power are those? I'll go ahead and replace those too with newer metal film. Or should I use wirewound?
Thanks again
 
Alright, first hiccup. My st70 came with this nifty screen that fits over the PC board. But it will not fit the replacement board from dynakitparts. It seems the position of the tube sockets is about 1/4" wider that the original on each side. It makes me sad because the screen is really nicely made, perforated stainless and nicely bent. Definitely not shoddy craftsmanship making that. It looks like I'll have to replace it if I want to use the replacement board but dang it the shop isn't well set for sheet metal fab' and milling it on the Haas milling center will probably make even less sense.
 
Maybe make one out of acrylic, that should mill OK if you're set up for it.


most resistors will be 1/2w unless the schematic notes otherwise. The stuff in the power supply and the cathode resistors are larger, but offhand I don't know the ratings. Its in the manual and/or schematic someplace though.
 
I think it'll look pretty to make it out of acrylic but then it won't have the screening properties the metal one has. It's very nicely made and I think I'd hate myself if I ditch it. It looks like the fastest way out of this is to make my own PC board (which was my original intent but figured my time is more valuable than the 30 smackers dynakitparts charges for the PC3-B). Last evening I made the schematic and drafted the board. Just need to position the tubes accurately \wrt the tube holes in the screen then everything else should just fall in place. It's pretty cheap to send out the gerber files nowadays even if you only order two or 3 boards.

@petercapo I haven't ignored your suggestion. I completely agree it's very important to watch the voltage ratings on tube amps. I actually measured the voltage on all resistors both at idle and at full power and the highest voltage on the old board was across the 330K resistor at 292VDC. Luckily I have a pretty good collection of Dale CMF60's which are rated at 600V and I was planning to use those.

The bias resistors seem to have drifted a bit. They are a bit out of tolerance (about -1.3% from 15.6ohm) but they drifted equally and measure the new value within 0.65% of each other, so they track the new value at 0.33% which is well within the 1.0% specification. So for now since I can't find a replacement unless I cough up $5.25+shipping for just one piece at dynakitparts, I'll just remember the new value and adjust for that voltage.
 
There was resistance in the leads but when I'm actually doing metrology I turn to my Agilent bench meter and tare the reading with a 10ohm calibration resistor
 
wire resistors tend to be very stable over time. The carbon comp resistors, not so much. Those also typically drift high.
 
Depending on if you have a VTA or stock Dynaco driver board clear acrylic covers are available. The film caps would have to be mounted on the underside of the board though. I put one on my VTA board. I've seen them for sale on ebay for the stock Dynaco board.
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