Dynaco St-70 Mod ID please

I am listening to all your advice and will probably revert to stock and do the Curico Triode when the 100 ohm resistors arrive.

There is another clue, I just noticed the wires to the the pots were moved from the pots to ground, they are still connected to the circuit board. I have ran it the way it is now for at least half an hour, The tubes look normal--there is no hum-the outputs stay cool-the power transformer is only warm, there is just the problem of the static over the music.
 
I finally discovered that the mod in question is for the Sylvania JAN 6B4G, not the older model. I have completed the work on an unmodified amp and it sounds great with those tubes.
 
I wish I could give a better answer but I have not been able to find voltage info about this tube, I do hope to do a reading soon based on the standard schematic. For now all I can say is that it has been on for several hours with no red plates or bad signs. There is no preamp,(for now) just an IphoneX to an AudioengineB1 streamer hooked directly to the St-70, about 80% of the volume on the Iphone is a comfortable listening level. The amp was recapped about twenty years ago then put on the shelf, the bias caps are 100/100 Sprague atoms. I keep thinking there is more I should do, but I am basically a recapper not as knowledgeable as I would like. I keep wondering about the feedback wire being disconnected, and if something should be done in place of that. I do know that I wish I had read Dave's baseline testing for the St-70 earlier, although I don't remember discussion of this mod, it rekindled my interest in the amp. I am lucky I found a couple on the cheap back in the 90's. There is more info and pics at the thread below.

http://audiokarma.org/forums/index....ruments-dyna-st-70-6b4g.853253/#post-12346393
 
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"To fill a military order Philips internally triode connected some 6AV5GA tubes, which when triode connected are pretty close to a 6B4G. These have been available for a number of years. It would seem Philips either couldn't or chose not to make a run of "real" 6B4G. IIRC these date from the 80s, and I doubt a real 6B4G could have been made at that late date."

I believe you are referring to the original 6B4G which these are not, I posted a pic on the thread I mentioned above. I think the Sylvanias are like the ones from Phillips.
 
If 3 and 8 are the plate and screen, when I put the leads of my Fluke meter on those pins there is no signal. I do not know what to do otherwise to determine that.
 
Here is a pic of the tube. There is no signal from the Fluke from 3 to 2 or 7 or any other pin. I really don't know what I have, I do know it sounds really nice in the St-70, with the few changes made.
 

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from what I read so far, and my guess: this unit went from fixed bias to cathode bias (the bias pot
connections disabled or rendered out of circuit, second, the output tube was changed to a 6BG4
with some changes to accommodate this new tube.

this needs a schematic drawn up then we can 1) fix it if needed, 2) indicate the changes to revert
it back to an original ST70.

I had a modified unit where the entire driver circuitry was changed ( no more use of the 7199 or
equivalent use of 6GH8/5u8/etc) so this is something that the does the output stage.

or just keeping running it as long as it doesn't damage itself.
 
Joeriz: Thank you so very much for solving the mystery 6B4G mod, I now remember this article, that picture did it, 20 years ago I did this mod and recently restored the one I did it on to regular Triode mode. I had a problem back then and put it on the shelf, I recently discovered that the problem was a bad 7199 that leaked and did not even move the needle on one of the plates. The one I have since converted to 6B4G still does not have the 1.5 meg resistor changed, but I did it on the first one, I will swap those around. Also on the first one I used 500 ohm resistors but the one I just did I used 800 ohm I may change that back, In this latest version I did not remove the tap from pin 4, I did the first time I am now wondering if I should do it again. Even as is it still sounds good and is more powerful than I anticipated. Thank you, Thank you, mystery solved.

Joeriz responded to this thread: http://audiokarma.org/forums/index.php?conversations/dynaco-st-70-6b4g.943204/#message-1599726
 

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