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Question about Dynaco ST-70 Original Tubes

My ST-70 still has its original C354 choke installed, and it seems to be doing its job just fine. I replaced the choke in one of my Mark IIIs, not because it had failed or I thought it was going to fail, but because it had leaked some of its wax at some time, and started to smell whenever I switched the amp on. Said Mark III had seen a rough life, having had the bias pot and the 11.2 cathode resistor replaced at some point in its life. My other Mark III still has the originals of those parts, and still has its original choke. By the way, if you haven't seen my page of ST-70 troubleshooting tips, you might want to take a look. Good luck!
-Adam
 
vtth said:
Thanks. I will eventually replace the choke too. I'm looking through some resistors, does it matter to get the right Watt rating? Also, if the old resistor is 1k 0.5W, could I replace it with 1K 1W or 2W piece? Lastly, the resistance should be match? (ie, 10k vs 12K) Thanks.

I also acquired the PAS3, so I'm looking to buy resistors and capacitors for both units. I did order a capacitor upgrade kit for PAS3 since the kit replaced the selenium rectifier, capacitors below it and can capacitor.

I haven't touch the dynaco FM unit yet.
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Going to a higher wattage is fine, as long as the resistor fits in the space you have to mount it. Doing that will reduce the temperature the resistor funs at slightly and therefore the noise generated, very slightly. You need to match the resistance exactly. DO NOT reduce the wattage rating.

On the schematic you will see 2 pair of matched resistors, one pair at 47K and the other at 270K. These were selected to be no more then 1% different. Getting 1% resistors will probably be closer then what is currently in there in those spots because of age and drift but they could wind up being as much as 2% off, one being 1% high and the other 1% low. This probably will not be the case but instead of buying multiple resistors and matching them by hand on a meter that may or may not be consistent between readings, you can go to 0.1% tolerance for those 4 without adding much to the cost. That will put you bang on for those.
 
If you leave the diodes in place you might consider a mod. It will depend on what the B+ you see it. I ran my ST70 w/ a solid-state rectifier but I changed the supply from CLC to LCC. (From my wall AC the B+ was way too high from the 117VAC PT primary.) Changing to choke input scrubbed the B+ down into the correct range. This is a simple mod. I also ran 6L6GCs rather than EL34s, a great combo with this amp and reduces the filament current too.
 
Thanks for all the inputs and suggestions. I will try to find all 1% tolerant resistor with the exact resistance and correct power rating. I want the least interference/noise possible. I have read many other mods, including high power supply, mod to use different tubes. But I want to keep it as original as possible. I will replace exact same choke, order a replacement rectifier tube since the unit came missing, replace a can capacitor with the provided link and replace the EL34 tube with KT-77 tube quad matched from Gold Loin (they are expensive, but highly rated). I'll leave the silicon diode mod alone. Other than that, should I do anything else to help retain the life of the KT-77 tube longer with any modification? Also, since the original owner placed a silicon diode, should I remove the selenium rectifier or it's ok to leave it in there. Thanks!!
 
vtth said:
Thanks for all the inputs and suggestions. I will try to find all 1% tolerant resistor with the exact resistance and correct power rating. I want the least interference/noise possible. I have read many other mods, including high power supply, mod to use different tubes. But I want to keep it as original as possible. I will replace exact same choke, order a replacement rectifier tube since the unit came missing, replace a can capacitor with the provided link and replace the EL34 tube with KT-77 tube quad matched from Gold Loin (they are expensive, but highly rated). I'll leave the silicon diode mod alone. Other than that, should I do anything else to help retain the life of the KT-77 tube longer with any modification? Also, since the original owner placed a silicon diode, should I remove the selenium rectifier or it's ok to leave it in there. Thanks!!
Click to expand...

Sometimes users will use the original selenium rectifier in place but not hooked up just as a mounting point for the silicon diode. If that is case, it is just inert hardware, not doing anything. OK to just leave it there.

Perhaps you might want to wait with the KT-77's until after you have lived with the amp a few months. Decide what you like and what you want. There are a great many modifications that can be done to this amp that will take it from good to great without spending a fortune on expansive tubes. I have always considered that change to be in the realm of diminishing returns for this amp. Get a feel for it as is first. Save the money for a later upgrade. If you decide the amp is not for you with the EL34's, the KT-77's will be unlikely to change that in a significant way.

Shelly_D
 
The JJ E34L's (not a typo) have provided great results for me in these amps. Not a comment on their other tubes either pro or con...
 
vtth said:
Thanks for all the inputs and suggestions. I will try to find all 1% tolerant resistor with the exact resistance and correct power rating. I want the least interference/noise possible. I have read many other mods, including high power supply, mod to use different tubes. But I want to keep it as original as possible. I will replace exact same choke, order a replacement rectifier tube since the unit came missing, replace a can capacitor with the provided link and replace the EL34 tube with KT-77 tube quad matched from Gold Loin (they are expensive, but highly rated). I'll leave the silicon diode mod alone. Other than that, should I do anything else to help retain the life of the KT-77 tube longer with any modification? Also, since the original owner placed a silicon diode, should I remove the selenium rectifier or it's ok to leave it in there. Thanks!!
Click to expand...
I changed over to JJ KT77's for my ST-70 for a similar reason. I built-in a reduced bias current standby function which lowers the power tube dissipation from the stock 80% to 45%. The issue with that is the B+ voltage rises to about 450v. (Even when using the standard bias current, with a 122v AC line, that's approx. 430v on the screen grids. They are only rated for 425v MAX. I had two different brands of EL34's red plate in this condition.) The KT77 is rated for 800v on both the plate & screen. Problem solved. Plus, they sound just as good as my German EL34's.
The ST-70 power tx. is rated for 720vCT or 360v AC (at 117v in) x 1.41 = 507Vdc, Correct? I have never tried this mod. can anyone measure this voltage for us?
 
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