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Dynaco ST-70: tubes

as long as it has the main in / pre-out connections. My tester preamp out in the workshop is actually a Mitsubishi receiver that the power amp section is dead in. Works fine as a preamp if you don't mind the bulk.
 
ordered a quad of shuguang el34b's. so cheap and seemed like a good place to start.

trying to figure out exactly which capacitors to order for the 6 black ones on the board but having a hard time. if anyone has specific recommendations for a basic replacement (don't want to get too crazy with mods yet), i would appreciate it. thanks!
 
One thing you can do to improve the power supply on those amps is to increase the capacitance a fair amount. The first cap after the rectifier tube has to be kept close to stock or you will stress that tube. An affordable way is to leave the original can cap disconnected for looks. Study what the aftermarket cap boards use on the Triode Electronics site etc and google the subject. It looks like you can't hurt by 5X to 10X capacitance after the first cap as mentioned. JJ caps are nice and affordable.

Also some of the replacement. can caps fail early as they may be too old etc.

Gegg has PCB's and schematics also if you want those.

http://www.classicvalve.ca/tubepcb.html

A great mod is the diode mod found on the tubes4hifi forum near the top using the near faster UF4007 in case you can hear the difference.

http://dynacotubeaudio.forumotion.com/f2-basket
 
pontoon said:
ordered a quad of shuguang el34b's. so cheap and seemed like a good place to start.

trying to figure out exactly which capacitors to order for the 6 black ones on the board but having a hard time. if anyone has specific recommendations for a basic replacement (don't want to get too crazy with mods yet), i would appreciate it. thanks!
Click to expand...
Leave them for the time beeing.
It might be that you will replace the driver board to something that does not use a 7199, then those caps will be automatically replaced.
In addition : They arn't broken so there is no need to replace them.
 
peterh said:
Leave them for the time beeing.
It might be that you will replace the driver board to something that does not use a 7199, then those caps will be automatically replaced.
In addition : They arn't broken so there is no need to replace them.
Click to expand...

I agree with peterh, but just to make sure you know what's what look at the picture of the original board here: http://dynacotubeaudio.forumotion.com/t191-dynaco-st-70-pc-3-driver-board-parts-layout-photo

The four tubular caps 0.1 uF at 400V are the grid coupling caps. If you are ordering a replacement can, or newer board with bigger electrolytic caps for the power supply, you might as well order four replacements for these and install them too. They will not add much to the cost of the other caps and are quite easy to solder in as they are bipolar (can be installed without worrying about polarity).
 
Replace the can cap, those don't hold up well. Check all the carbon resistor values as they drift high. The Black Cat caps are film, Mylar I think. I'd leave them alone for now, maybe change to polypropylene when every other thing has been fixed. Selenium rectifier definitely has to go. At that point live with it a while and see what you think. Brings back memories of mine from many years ago. At some point I changed all the resistors to RN70D metal films and didn't like the resulting sound, so use caution with mods.
 
ConradH said:
Replace the can cap, those don't hold up well. Check all the carbon resistor values as they drift high. The Black Cat caps are film, Mylar I think. I'd leave them alone for now, maybe change to polypropylene when every other thing has been fixed. Selenium rectifier definitely has to go. At that point live with it a while and see what you think. Brings back memories of mine from many years ago. At some point I changed all the resistors to RN70D metal films and didn't like the resulting sound, so use caution with mods.
Click to expand...

Well spoken!
 
thanks for all the info. that picture of the board helps a lot. i spent some time cleaning the amp today and i replaced the silenium rectifier with a 1N4004 diode. i ordered 4 shuguang el34b's and a sovtek gz34 so i think i will try the amp with those when they arrive and the 7199's that were in there and see if it works and how it sounds.
 
yes the fuse is a 3 amp. i do still have the tube that was in there which might work. but wanted to get another one to test too. i will check out the pins. thanks for the tip. my fingers are crossed.
 
If you fire up your amp with the old can cap still in place, why not do with a dim bulb tester in circuit at first? Use a 60W bulb.
 
It's up and running! I first attempted replacing the silenium recitifier with a diode myself, got the new Shuguang tubes in, and then it wouldn't even turn on! So I decided to bring it to an expert. They fixed my soldering job, cleaned it up, and replaced some resistors (which they said was unusual). Anyways, it was an inexpensive fix and seems to be working now. I have it hooked up to the pre-amp outs on a Sansui 2000x and JBL Centurys. Sounds great! I'm planning on getting a pre-amp for it. I'm leaning toward the Kenwood Basic C2.
 
Great! Another happy member of the big ST70 club. :) Did your tech show you how to check the bias on the tubes, if you decide to roll in others?
 
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