Dynaco ST-70 Triode Conversion Techniques

[brosenau]

I finished my ST-70 about 3 weeks ago and I'm extatic. It sounds absolutely heart-warming. The only mod I made was to replace the p/s cap and a couple of the electrolyitcs under the chassis.

Now, I'd like to install dual dpdt switches to switch from pentode to triode mode. Does anyone have any suggestions for some nice switches? I'm not sure how much voltage is coming from the feedback loops on the trannies, so I thought it would be best just to ask...

I'll post pictures when I get home tonight...

Brian

[Original Burnedfingers]

Now, I'd like to install dual dpdt switches to switch from pentode to triode mode.

This was how I did it on my first Dynaco Stereo 70. Its a quad pole double throw switch. Its not rated for the voltage it sees but I never had a problem with it. I also never switched it on the fly. I switched to triode mode before turning it on.

Since I have experienced the triode mode on the stereo 70 my personal comment it that I don't see anything particularly special about the amp in triode mode. Maybe that is something that I need to look at again in one of the amps I have now.

[quadtech]

On some other forums, I have seen the following recommendations -

Shin Chin p/n R13-25B1-05 (Mouser 112-R13-25B1), rated 6A @ 250VAC.

Radio Shack # 275-636

While you are doing this mod, it may be a good idea to put the 100 ohm, 2W
resistor right at the tube socket on pin 4 so that it is always connected in both UL
and triode mode.

This was recommended on the diytube list to help prevent tube arcing/flashovers.
While this may be an issue seen more with SS rectification and newly manufactured
tubes, it does not hurt to have it.

I made this change recently, and I do not observe any change in the sound,
but then I have not been blessed with magic ears.

More info here -

http://www.audioxpress.com/magsdirx/...lespie2544.pdf

[Original Burnedfingers]

The switch that I used was also rated 6A/250 volt. While it worked ok I still recommend that it never be flipped while the amplifier is on.

[brosenau]

Thanks for the quick response although it took me a few days to get back in the swing of things again. QT, just for clarification sakes, did you mean to place the 100 ohm resistors on lug 4 in series with the feedback leads from the trannies?

I read the article you recommended, and I hate to mis-read anything. I understand about the stabilization of the tubes, but the arc thing (as a whole) was a little new until I read the article.

Before attempting the conversion, I thought I'd try just to hardwire it over to Triode mode for a while. Boy, did THAT open a can of worms. At first, I noticed a significant difference in the midrange and presence. With a smile on my face I walked away to answer a beckonning call. When I returned, the right front tube was glowing brightly! I unpugged it as fast as I could (there's a lot to be said for a front-mounted power switch).

It took me a while of investigation, but I never actually did identify the culprit. I cleaned the 10K bias pots and resoldered a good deal of the PC-3 just be on the safe side, and I swapped the EL-34s to see if the problem followed the tube. It turned out that the right front position was the only one affected, but I'm a little leery about leaving it on without me being there (kind of like a small child).

For now, I think I'm happy with UL mode, but I am interested in the 100 ohm resistor placement. If you would be so kind...

Brian

[brosenau]

Originally posted by burnedfingers
The switch that I used was also rated 6A/250 volt. While it worked ok I still recommend that it never be flipped while the amplifier is on.

I often wondered what the result would be if you actually DID flip the switch with the power on though. [just kidding]

Did you have a good holiday?

[quadtech]

did you mean to place the 100 ohm resistors on lug 4 in series with the feedback leads from the trannies?

Exactly.
You need to solder one end of the 100 ohm resistor directly to lug 4,
and the other end to the leads (the Ultra-linear tap) from the output tranny,
so the resistor is always in series.

[quadtech]

It took me a while of investigation, but I never actually did identify the culprit. I cleaned the 10K bias pots and resoldered a good deal of the PC-3 just be on the safe side, and I swapped the EL-34s to see if the problem followed the tube. It turned out that the right front position was the only one affected, but I'm a little leery about leaving it on without me being there (kind of like a small child).

I also had a similar issue and it was due to a bad switch. One
of the triode/UL mode toggle switches was not making good contact.
Replaced the switch and the issue went away. But you say you
hardwired it to triode, so there is no switch, I guess. Perhaps a
bad solder joint?

[DigitalJunkie]

Originally posted by brosenau



I often wondered what the result would be if you actually DID flip the switch with the power on though. [just kidding]

Did you have a good holiday?

I've done it on my little 6V6 amp...It's quite a nasty "POP" from the speakers,I don't recommend it.

[brosenau]

As always, very good advice. Thanks for the clarification, and I'll get back to the forum later on this week. I have a music gig for one of our volunteer fire departments to spin up for and later this week, my better half and I will be going to see the Trans Syberian Orchestra for the first time.

Ya'all have a super new year, and we'll do the same.

Brian