Original Dynaco ST-70 has sound gone in one channel

Hello! I have an original Dynaco ST-70 that has had a few mods done to it (replaced driver board/components with an upgraded original layout board, replaced can cap with Triode under-chassis cap board) that has been operating with no problems for a few years. A couple days ago, I was listening to it and got a very loud pop with a short high-pitched blast of static in the left channel, followed by that channel producing no output and the sound from the right channel sounding kind of "thin" (not much bass). Today, I replaced the tubes - I had had some old tubes in it and figured one of them had just failed - but the sound was the same (left channel dead, right channel sounded off). I checked the biaset on the tubes via the front sockets and the right read more-or-less normal (a little high), but the left read 38.4v! I immediately powered the amp down, since I figured that would damage the tubes if it went on very long. I'm now worried something else is damaged - maybe an output transformer? Any suggestions where I should start troubleshooting?

Thanks!

Dan

If this amp was a ST70 i would start with examining the 15.6 ohm resistor between the
combined cathodes on the failing side and ground.
If the resistor is blown, junk the EL34 that was installed there, get a new resistor and a pair of
matched EL34. In addition check the voltages and compare with the dynaco manual.
That said, this amp is reportedly munged, these suggestions might not work for this amp.

.

Thanks! The only other mod I've made to this amp is replacing the original bias bots with Bourns 10-turn units, making it much easier to adjust as the amp heats up when I have it on for a few hours. Attached is a picture of the inside of the amp.

Update! I found the multimeter and checked the 15.6 resistors right after posting the above. The one on the channel that's out is reading "open circuit" so it's clearly bad; sounds like that's the culprit. Thanks for the help! Is there anything else I should check that might have been damaged by this failure? Would having tubes (not the ones that were in during the failure) installed on that channel for about 30-60 seconds of operation (as described above) have damaged them?

I just noticed that the tube that was installed in that socket when the channel went out seems sort of loose in its base, like it has come unglued or something; I will definitely be junking that one...

dtwright wrote:Update! I found the multimeter and checked the 15.6 resistors right after posting the above. The one on the channel that's out is reading "open circuit" so it's clearly bad; sounds like that's the culprit. Thanks for the help! Is there anything else I should check that might have been damaged by this failure? Would having tubes (not the ones that were in during the failure) installed on that channel for about 30-60 seconds of operation (as described above) have damaged them?

I just noticed that the tube that was installed in that socket when the channel went out seems sort of loose in its base, like it has come unglued or something; I will definitely be junking that one...

Tubes used at the broken resistor is not damaged by that.
But both tubes at the affected side when resistor blow should be junked,
they might work but will have much easier to flash-over again.

.

Glad to hear I didn't bake my second set of tubes while testing. Double checked the fuse and it's 3A.

Thanks both of you for your help!

A couple of things you might want to do that would extend the life of the tubes and the system:

1.) Measure the voltage across pins 2 and 7 of the output tubes with a meter set to the 20 volt or so AC scale on a voltmeter. I'm going to assume that it is higher than 6.3 volts. Since you have a new power transformer on that unit (can see that by the lack of bell covering on the underside and the plastic covered leads from the power transformer) you might want to put a 3 amp CL-80 NTC on the power cord hot (black) lead (maybe between the power switch and the fuse). Measure again, and if still above 6.3 volt, put a second NTC across the white return lead. Having 6.3 volts or slightly less leads to long life of the tubes, additionally the NTCs inhibit the turn on power surge, putting less strain on the power transformer, tube heaters and the circuit in general.

2.) Put a 3-5 watt resistor between the output transformers screen lead and the screen of the power tubes (Pin 4). While this may very slightly reduce power output, the tubes will be much less likely to arc and the overall effect is a better sound. Minimum value is 100 ohms but I personally like about 500-750 ohms. See the following websites for more on this issue:

http://www.tronola.com/html/maximize_tube_life.html

And much more on this issue with some other enhancements:

http://www.oestex.com/tubes/screens.htm

http://www.oestex.com/tubes/ul.html

3.) Finally, I would highly recommend getting the individual tube bias adjustment kit from DynaKit Parts. This makes it possible to adjust the current flow to each tube rather than the pair which may not be balanced:

https://www.dynakitparts.com/shop/bias-balance-control-kit/

Good luck, I really like my ST-70 which is highly modded and very listenable.

Solder Slinger wrote:A couple of things you might want to do that would extend the life of the tubes and the system:

1.) Measure the voltage across pins 2 and 7 of the output tubes with a meter set to the 20 volt or so AC scale on a voltmeter. I'm going to assume that it is higher than 6.3 volts. Since you have a new power transformer on that unit (can see that by the lack of bell covering on the underside and the plastic covered leads from the power transformer) you might want to put a 3 amp CL-80 NTC on the power cord hot (black) lead (maybe between the power switch and the fuse). Measure again, and if still above 6.3 volt, put a second NTC across the white return lead. Having 6.3 volts or slightly less leads to long life of the tubes, additionally the NTCs inhibit the turn on power surge, putting less strain on the power transformer, tube heaters and the circuit in general.

2.) Put a 3-5 watt resistor between the output transformers screen lead and the screen of the power tubes (Pin 4). While this may very slightly reduce power output, the tubes will be much less likely to arc and the overall effect is a better sound. Minimum value is 100 ohms but I personally like about 500-750 ohms. See the following websites for more on this issue:  

http://www.tronola.com/html/maximize_tube_life.html

And much more on this issue with some other enhancements:

http://www.oestex.com/tubes/screens.htm

http://www.oestex.com/tubes/ul.html

3.) Finally, I would highly recommend getting the individual tube bias adjustment kit from DynaKit Parts. This makes it possible to adjust the current flow to each tube rather than the pair which may not be balanced:

https://www.dynakitparts.com/shop/bias-balance-control-kit/

Good luck, I really like my ST-70 which is highly modded and very listenable.

This is great additional information, thanks!

One other suggestion that involves a lot more work, if necessary... The original ST-70 had a 30/20/20/20 mfd cap. At the time of manufacture, that was a reasonable value. Today things are different. The original Triode power supply board had four sets of 100 mdf caps resulting in a 50/50/50/50 mfd setup. An improvement, but not great... The latest Triode board is 40/195/135/135 mfd which is much better. The following recommendation ONLY applies to the older board (if installed, which I can't tell from the picture).

The design can be made better by changing out some of the caps. I would recommend changing the first pair of caps after the 5AR4 downward from 100 mfd each to 60-80 mfd each, at at least 300 volt rating. This will bring the first cap value down to 30-40 mfd, 40 mfd is considered the max the current versions of the 5AR4 can handle, the individual 300 volt rating will put you at a 600 volt max which won't ever be reached in an ST-70. The current value is 50 mfd which is slightly too large, stressing the 5AR4. If you have used solid state diodes, then the original first set of caps are okay but I think a 5AR4 is still installed based on the picture.

Likewise the second pair of caps can be increased. This will take some research, but if you look at "mouser.com" you can find some larger value caps with a 250 volt rating that will increase the main energy store significantly. I don't remember the exact value I purchased but believe it was about 200 - 300 mfd, resulting in a 100 - 150 mfd @ 500 volt overall value (get the largest value that will fit). Make sure the height, diameter and lug spacing are appropriate to fit on the board and not exceed the max height available, increasing this value will help with bass reproduction as well as adding extra filtering for the front end tubes.

Finally, add a 22 mdf Solen 630 v metalized polypropylene cap connected to the 2nd stage you just increased, a 22 mfd @ 630 volt will just fit into a Dynaco chassis. Adding this cap will clean up the higher frequency notes. You can glue it in with either hot glue or silicon sealer.

Good luck and enjoy your amp.