ST70 Trying to understand stereo / mono switch mod

I bought this Dynaco ST70 a few weeks back.  Everything seems to be working OK, but a few minor issues - can't seem to get bias above ~1.4V on both sides with the bias turned all the way up. Also the left speaker won't output at equal levels (it almost sound like midrange isnt there) until I turn the volume up quite high or let it warm up for ~30 mins.  Once it's warm / I turn it up it sounds good to my novice ears.

I replaced the rectifier tube with a Mullard 5AR4, replaced the quad cap, replaced the output tubes with KT77s and it looks like the selenium rectifier / bias was replaced with kit.

I'm also trying to understand what was done to the stereo mono/switch.  Pictures below. Any help is appreciated.  I had to take the "link" off the images below as I couldn't post as new member. 

Bob Latino wrote:I altered your post above to make the photos show up in the post ..

Your amp is an original Dynaco ST-70 that was factory wired. Note the rivets on the tube sockets and the serial number badge on the rear face of the amp. This is characteristic of an original Dynaco ST-70 factory wired amp. Those capacitors and resistors on the input jacks and stereo/mono switch is an old mod mostly associated with Frank Van Alstine. It is done to restrict the bandwidth of the audio signal to make it easier for the output transformers to handle the audio signal. Personally, I don't like this mod as it introduces an extra resistor and capacitor directly into the signal path.

The quad cap has been replaced. Good - Probably 90% of those original Dynaco ST-70 quad caps are leaking some voltage. The selenium rectifier has been replaced with a diode. Good ..

Advice > try replacing the driver board with either a Dynakitparts original type driver board OR one of our VTA ST-70 driver board from Tubes4hifi. The carbon composition resistors on that original Dynaco board have probably strayed a bit from their original value. You have to remember that the original Dynaco ST-70 amp came out in 1959. Some of these amps still in circulation are now over 60 years old.

Bob

Bob is correct.  The old carbons have no doubt strayed and I would venture to guess that the board has dozens of cold joints in it as well.   Also, those 7199's are probably half dead or dead.  Replace that board with a VTA type, which will clean up the bias supply. And touch up all of the solder joints around the tube sockets.   Your issues should disappear.  You'll like the sound much better too!
Chris

Hey all - took your advice and replaced with a Dynakit parts board.  The left channel issue has gone away.  Looks a lot cleaner and once I took that old board out you can definitely tell its seen better days.