Original Dynaco ST70 with VTA driver board - Bias Voltage Drops After a Few Minutes

knotscott wrote:Hi all!  The title pretty much says it all.  I've added a VTA PCB to my uncle's old Dyna 70.  Test 1 passed with flying colors, but test 2 is not fairing as well.  Once it warms up, I can set the bias to 0.40 volts, but within a minute or so it quickly starts to drop.  I'm guessing the GZ-34 rectifier tube is weak.  My tester is too old to have settings to test it, but I tried the settings for a 5V4, and it tested bad.  A new rectifier tube is ordered, but I wondered if there's anything else I should be looking into?

TIA for any help!

knotscott wrote:The current quad cap is original.  Is it best to just replace that regardless of how it tests, or can I simply add some outboard caps to help it along?

knotscott wrote:The current quad cap is original.  Is it best to just replace that regardless of how it tests, or can I simply add some outboard caps to help it along?

Bob Latino wrote:Since the Dynaco ST-70 came out in 1959, your amp could be 61 years old? If that quad cap is the ORIGINAL cap, I would absolutely just replace the cap. The original quad cap was a weak spot on all the original Dynaco ST-70's. I would use the 550 volt 80, 40, 30, 20 Authenticap as a replacement cap. This is sold by Dynakitparts and other vendors. When you install this cap, have the 80 section face the rear of the amp. If you do that all the other sections will be in the correct position. This will also raise the capacitance of your amp from 90 uF to 170 uF. The 550 volt Authenticap is the same quad cap used in all VTA amps.

Bob

knotscott wrote:

It was built by my uncle in 1961, so yeah, it's pretty old, plus has a fair amount of sentimental value to me.  I understand that the Authenticap is a great replacement, but can I ask what that cap offers for an additional $20 over the CE Manufacturing 525v cap (other than the obvious 25v extra capacity)?  With 2 kids in college and one getting married this fall, my wife isn't exactly encouraging me spend more money on this old amp right now!     That said, I don't wanna be penny wise/pound foolish with this thing either.

tubes4hifi wrote:you didn't mention what tubes you are using, but some tubes (like KT88s) aren't happy with the original values of R29-32 on the driver board of 150K (which works perfectly fine with EL34 tubes).
So if you are using KT88s, change those four resistors to 100K.   If it's the rectifier tube or quad cap, you would notice the B+ (your can check it where it comes onto the driver board where
the four large resistors in the center are) that voltage will sag for more than a minute (it will sag the first minute anyway, as the output tubes warm up and conduct current).
Bias should be set after the amp has warmed up for 3-4 minutes.