ST70 Question

Just got this. The question I have is about the upgrades. There’s 2 12BH7A's the board in front and 2 12AU7's under the tube shields. Has anyone seen this upgrade?

Hi --- In the early 2000's there was Magazine called Glass Audio which no longer exists.  They published several issues exploring the Commercial Upgrades to the ST-70 that were available at that time( 1990's thru early 2000's ).   I just checked my copy and what you have is not one of the Upgrades that Glass Audio reviewed.

Also, your circuit board does not look like the Std St70 .... parts placement is different and it looks there were parts removed? 

Any chance you could take a picture of the underside of the Chassis showing how the tubes were connected to the PCB and post it?  

Then maybe someone on the Forum could tell what you have.

If, I had to guess, I would say that your upgrade is not a commercial one. I can see the solder holes in the Upper Center of your PCB where parts were removed. 
The commercial upgrades usual provided a brand new PCB and Circuit. Just my opinion.

Here's what the previous owner said about the upgrades.

. I took the ST 70 to my local McIntosh Certified Repair Shop and had them upgrade it with the following changes/upgrades.
The Selenium rectifier was replaced with “FRED”
Original capacitors were replaced with larger, modern capacitors
Driver circuit was replaced, adding two 12AU7 tubes
The stereo/mono switch was converted to a sensitivity switch
The upgrades result in better sound and a cooler running amp that extends tube life.

Very interesting. As mentioned above, if you could send some pics of the underside wiring it would be useful to determine what you have there. My guess is that the two added 12au7s are being used as drivers for the + and - signals to the power tubes, and the original two tubes are configured as voltage amp followed by phase inverter as in Williamson circuit. This would indeed be a significant upgrade from the original. I suggest studying the circuit board and chassis wiring and drawing a schematic to see just what you've got there. Looks promising!

Hi Again ---- I think you just verified my comment about the Non-commercial Upgrade.

The upgrade was done by a Tech in a McIntosh Service Shop.

(1) He replaced some 50+ year old parts (Selenium Rectifier and Caps) with modern parts (FRED and caps).

(2) He replaced the 7199 tubes with 12BH7s and 12AU7s and then physically modified the Dynaco PCB/Circuit to accommodate the 12BH7/12AU7 tubes.

I am guessing, but maybe there was a McIntosh circuit which used the 12BH7/12AU7 combo and that was the basis of the changes??

I would have to say if the mods sounds good to you, that is all that matters. Either way, 12BH7s and 12AU7s are fairly inexpensive tubes and are still manufactured but 7199s are no longer manufactured.

If you could post a few pictures of the wiring side of the chassis that would be interesting.

Have fun listening.
Stew