Weird issue with voltage Redding plating one half of the power tubes

MWChapel wrote:I searched everywhere, but I have yet to see this issue.

So this is what I'm seeing. When I first turn on the amp, after about a minute of perfect bias left and right, I feel a vibration, and left side of the amp voltage bias drops, and the right sight bias blows up and starts to red-plate the power tubes for several seconds then the vibration stops, bias goes to normal. Then after a minute it does it again.

However, after the amp "warms up" I guess after about 5 minutes it stops, and from that point on for as long as the amp is on, it biases perfectly, and runs completely normal.  It's only at start up.

I have the original driver board, and replaced the four coupling caps, and the two .05 caps. And I have tried a couple 7199, even have NOS RCA 7199's. But the behavior is exactly the same.

Any ideas?

Bob Latino wrote:

MWChapel wrote:I searched everywhere, but I have yet to see this issue.

So this is what I'm seeing. When I first turn on the amp, after about a minute of perfect bias left and right, I feel a vibration, and left side of the amp voltage bias drops, and the right sight bias blows up and starts to red-plate the power tubes for several seconds then the vibration stops, bias goes to normal. Then after a minute it does it again.

However, after the amp "warms up" I guess after about 5 minutes it stops, and from that point on for as long as the amp is on, it biases perfectly, and runs completely normal.  It's only at start up.

I have the original driver board, and replaced the four coupling caps, and the two .05 caps. And I have tried a couple 7199, even have NOS RCA 7199's. But the behavior is exactly the same.

Any ideas?

Yes - Since you did mention 7199 driver tubes, I will assume that you are working on an original Dynaco ST-70. If that is the case start by rebuilding the bias system.  (1) replace the selenium rectifier with a diode (2) replace the two 50 uF @ 100 volt capacitors on the 7 lug terminal strip. Replace the two 10K resistors between pins 1 and 2 and pins 3 and 4 with two resistors of a lower value (if you are using a diode instead of the original selenium rectifier). Try two 7.5K resistors or two 5.0K resistors (with a 1 or 2 watt rating)

More about replacing the original Dynaco selenium rectifier with a diode Here > Forum post on replacing the selenium rectifier

Now > Test the bias system. Center the two bias pots, remove all the tubes, turn the amp on. Now measure the DC voltage on pins 5 and 6 on all output tube sockets to chassis ground. You should get maybe -32 VDC to -40 VDC at all measuring points. If you do shut the amp OFF. Now you can place all the tubes in the amp and try the startup again.

As a side note > For anyone with an original Dynaco ST-70, despite what Dynaco said, I would bias each output tube pair at 1.25 volts instead of the 1.56 it says in the original manual. 1.56 VDC gives 50 milliamps bias current per each output tube. That may have been fine in the 1960's. Dynaco used the rugged Mullard EL34 output tubes and they took that relatively high bias setting in stride. Modern output tubes are better served with a lower bias setting. (1.25 VDC or about 40 milliamps per each output tube on an original Dynaco ST-70)

Bob

MWChapel wrote:I searched everywhere, but I have yet to see this issue.

So this is what I'm seeing. When I first turn on the amp, after about a minute of perfect bias left and right, I feel a vibration, and left side of the amp voltage bias drops, and the right sight bias blows up and starts to red-plate the power tubes for several seconds then the vibration stops, bias goes to normal. Then after a minute it does it again.

However, after the amp "warms up" I guess after about 5 minutes it stops, and from that point on for as long as the amp is on, it biases perfectly, and runs completely normal.  It's only at start up.

I have the original driver board, and replaced the four coupling caps, and the two .05 caps. And I have tried a couple 7199, even have NOS RCA 7199's. But the behavior is exactly the same.

Any ideas?

deepee99 wrote:

MWChapel wrote:I searched everywhere, but I have yet to see this issue.

So this is what I'm seeing. When I first turn on the amp, after about a minute of perfect bias left and right, I feel a vibration, and left side of the amp voltage bias drops, and the right sight bias blows up and starts to red-plate the power tubes for several seconds then the vibration stops, bias goes to normal. Then after a minute it does it again.

However, after the amp "warms up" I guess after about 5 minutes it stops, and from that point on for as long as the amp is on, it biases perfectly, and runs completely normal.  It's only at start up.

I have the original driver board, and replaced the four coupling caps, and the two .05 caps. And I have tried a couple 7199, even have NOS RCA 7199's. But the behavior is exactly the same.

Any ideas?

Hi and welcome, MWChapel
.
For the effort in new parts and labour and measurably improved sound, give one of Bob's and Roy's VTA driver boards, with all new parts, and keep the original for display purposes or something. The VTA driver board, properly tubed, is the essence of the "VTA house sound" we've all come to know and love.
Cheerio,