Dyna ST-70 6B4G mod rescue

[ronton3]

Last month I posted trying to ID a mod on an ST-70. I now know that it was for 6B4G tubes, I found 4 that I had tested 20 years ago. I was confused about what I had done because I forgot the 6B4G is a triode and I kept thinking that I had just planned to put the 70 in regular triode mode for the EL-34s. I also removed the 500 ohm cement resistors replacing the .02 cap in the bias supply thinking they were not needed, and replaced the .02 cap.
I was thinking of returning the ST-70 to stock operation but I would like to try the 6B4G's first after having done all the work long ago. I did put them in, and moved the Voltage up slowly, at about 60 volts the static started and around 85 volts a bit of music began coming through, this lasted about a minute and then it all stopped and there was a bit of hum. I shut it down. There had been no burn smell, no flashes or red plates, nothing obvious. I believe I need a schematic and instructions for the 6B4g mod so I can make sure things are right before I proceed. These pics show the connections to the front sockets. I now believe the wires to the bias pots had been moved to ground because of the bias arrangement for the 6B4Gs. The Cerifine cap is 47uf, and is the first B+ cap from the 5AR4. Where can I find info about the 6B4G mod. Thanks for all the help. Happy New Year.

 

[Palustris]

The only 6B4G ST70 modification that I know of was part of the Triode Electronics replacement driver board. In the manual Ned Carlson explained how the conversion worked. There may not be enough voltage drive with the 7199 to drive the 6B4Gs to full output, so the Triode board had provision to use a tube like the 5687 or ECC99 to drive the output tubes with an EF86 as a voltage amp. You need to add a 6.3V supply for the output tube's filaments. The bias supply isn't used. Instead a 800r 20W resistor is installed between the center tap of the output tube's filament transformer and ground, bypassed by a 100uF cap. Current draw for the two tubes is 80mA which is about 64V across the cathode resistor so the driver tube needs to be capable of generating at least this voltage for full output from the amp.

 

[Pio1980]

I had one in the works with Battradio, using Fisher ppp 6V6 opts and Mullard type driver boards, but it's on hold while he takes care of more important matters.

 

[ronton3]

This fully explains why I had the 20 watt cement resistors(500 ohm)in the bias supply and should have left them. I still hope to find a Schematic or complete instructions if possible. I looked over the Triode site, and could not find a specific mention of the 6B4G, many driver board options. If possible I would like to keep the original.

 

[Palustris]

Well, 20 years ago Ned Carlson owned Triode Electronics and he sold the ST70 board with extensive documentation. I don't have any idea what comes with the Triode board today, but one is still available. Note that Ned's market was "audio amateurs" who were readers of Audio Amateur Magazine and Glass Audio Magazine. These readers were very willing to learn basic AC and DC electronic theory and experiment with different ideas and techniques to improve the sound of widely available equipment like the Dynacos.

So, a complete schematic was not available for the ST70 using the 6B4G output tubes since Ned assumed that each user would use his documentation as guidance to do it their own way.

It was a different time. There was a series of articles about the Marantz 7T which was a dog of a preamp. The entire guts were eliminated to use the attractive case, front panel and chassis.

 

[ronton3]

Well, 20 years ago Ned Carlson owned Triode Electronics and he sold the ST70 board with extensive documentation. I don't have any idea what comes with the Triode board today, but one is still available. Note that Ned's market was "audio amateurs" who were readers of Audio Amateur Magazine and Glass Audio Magazine. These readers were very willing to learn basic AC and DC electronic theory and experiment with different ideas and techniques to improve the sound of widely available equipment like the Dynacos.

So, a complete schematic was not available for the ST70 using the 6B4G output tubes since Ned assumed that each user would use his documentation as guidance to do it their own way.

It was a different time. There was a series of articles about the Marantz 7T which was a dog of a preamp. The entire guts were eliminated to use the attractive case, front panel and chassis.[/QUOTE

What a coincidence: Last night I finally found 24 copies of Glass Audio the first one I picked up was Vol4 #1 1992, and there was an article by Norman Koren on a mod for the ST-70 for Triode mod, only problem he was talking about the board and drilling holes in the chassis to add 12AU7s, among other changes. It then occurred to me, I've been confusing Converting to Triode mode for the EL34's and using a true Triode like the 6B4G, which requires a different set up for bias, and probably a more powerful driver than the 7199. I am also a bit upset with myself for not checking the 7199s, which I thought were okay, one has a leak and no movement on the pentode with a Hickok 752 tester, out of 6 7199s I checked three had very low readings on the pentode or triode, and will now be out of service.
I did see the Triode mod board you were referring to and now understand it will set things up for the 6B4G, they say there is a manual, and I have emailed them to make certain it will accommodate the 6B4G, I will probably order it. But first thing this morning I carefully noted all the changes I made 20 years ago and then returned it to triode mode for the EL34s. Things were going well up to about 90 volts, then at 117V the 47ohm/and620 ohm tied together on the board burned up. A rather shocking and unexpected event, since it was sounding okay. I now believe it was probably caused by the bad tube.

I still have 23 more copies of Glass Audio to go through, who knows maybe I will even find the right schematic for a single ended 300b amp I constructed back then. I would also like to revive a small 45 amp I did that sounded great. Thanks for the help.

 

[ronton3]

I just discovered a difference between the Curico Triode set up and the Triode USA electronics set up. One says to remove the wires from pads 4 and 11 that ran to the
El34 sockets I replaced the wires from 11 and 14, maybe that is why the resistors burned up. Which of these is correct, for triode operation of the EL34s

Curico: At pin 4 of V6 disconnect the GREEN Output Transformer wire. Cleanly cover the exposed end of the wire with high quality electrical insulating tape or heat shrink tubing. This wire will remain un-terminated therefore dress appropriately near the chassis corner. Obtain a 100 ohm, 1/2W, Carbon Film resistor (Mouser # 293-100). 12.Refer to figure 1 – Connect one lead of the 100 ohm resistor to pin 3 of V6. Connect the remaining lead of the 100 ohm resistor to pin 4 of V6.


Triode USA: Triode operation of 6CA7,EL34, 6L6-GC , and tubes with same pinouts (ST70 or Mk3 or Mk2 with our boards or original boards ONLY)

Find the green (not green/white) wire (one from each output transformer) hooked to pin 4 on an EL34 on each side (or on each amplifier in the case of Mk2/Mk3) of the amplifier, there is also a wire leading from that pin to pad 11 (on one side) or pad 14 (other side) in an ST70, pad 4 on a Mk2 or Mk3.
Disconnect the green wires from their respective sockets\par and run directly to pad 11 (on one side) and pad 14 (on the other).

Remove the wires that previously ran from pad 11 & 14 (pad 4 on Mk2 & Mk3) to the EL34/6550 sockets.

Disconnect the screen tap leads from pin 4 of each of the other two output tubes, then insulate the ends of the wires so that the bare leads are not exposed.
Install a 100 ohm (1/2 watt minimum) resistor between pin 3 and pin 4 of each of the output tube sockets.