Bias setting for original ST70

I posted this topic in the Dynaco photos area of the forum, but wanted to repost here to get a few more eyes and comments.

I got a factory built ST70, bought with no tubes.

Tubes arrived today: matched JJ EL34 quads, Sovtek 5AR4, 1 NOS GE 6GH8A, 1 NOS Zenith 6GH8A, both in adaptors. Speaker are Vandersteen 2CE Signatures on the 8 ohm tap.

Following the instructions for initial setting in the Dynaco ST70 instruction manual, I was able to bias both channels at 1.67 V, higher than what the manual says, 1.56V. Both bias pots were turned all the way to the left, which I think is the minimum, as turning them to the right increases the voltage measured at the power takeoff sockets. My line voltage is 121V, higher than the 117V called for in the manual, but not too high, so I wonder if this is the reason I can't get the bias voltage to go lower.

After about an hour or so, the bias voltage dropped down to 1.56 V or so on both channels, with the bias pots on the right at its lowest setting, and the one on the left turned up a bit beyond it's lowest setting to match the voltage on the right. I just measured bias voltage again, after the amp has been on 2 plus hours, I got 1.52V on the left and 1.53V on the right without touching the bias pots.

I wonder if this is normal for a stock unit using the 6GH8A's instead of the 7199's? Or is this due to new tubes settling in? Should I be concerned at all, or just being a newbie worry wart???

Being new to tube gear, I'm also surprised at how hot the amp runs. However, it does sound great, and it's quiet through both channels no hums or other extraneous noises.

Any comments would be appreciated, thanks!

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erniegiro wrote:I posted this topic in the Dynaco photos area of the forum, but wanted to repost here to get a few more eyes and comments.

I got a factory built ST70, bought with no tubes.

Tubes arrived today: matched JJ EL34 quads, Sovtek 5AR4, 1 NOS GE 6GH8A, 1 NOS Zenith 6GH8A, both in adaptors. Speaker are  Vandersteen 2CE Signatures on the 8 ohm tap.  

Following the instructions for initial setting in the Dynaco ST70 instruction manual, I was able to bias both channels at 1.67 V, higher than what the manual says, 1.56V. Both bias pots were turned all the way to the left, which I think is the minimum, as turning them to the right increases the voltage measured at the power takeoff sockets. My line voltage is 121V, higher than the 117V called for in the manual, but not too high, so I wonder if this is the reason I can't get the bias voltage to go lower.

After about an hour or so, the bias voltage dropped down to 1.56 V or so on both channels, with the bias pots on the right at its lowest setting, and the one on the left turned up a bit beyond it's lowest setting to match the voltage on the right. I just measured bias voltage again, after the amp has been on 2 plus hours, I got 1.52V on the left and 1.53V on the right without touching the bias pots.

I wonder if this is normal for a stock unit using the 6GH8A's instead of the 7199's? Or is this due to new tubes settling in? Should I be concerned at all, or just being a newbie worry wart???  

Being new to tube gear, I'm also surprised at how hot the amp runs. However, it does sound great, and it's quiet through both channels no hums or other extraneous noises.

Any comments would be appreciated, thanks!

The 7199 / 6gh8 has nothing to do with bias adjustments.

That the pot's adjusting range don't work indicates that the Se rectifier has detoriated.
Replace it ! Use a bias kit from dynakitparts.com .

Too high bias might damage your power tubes, don't use the amp until this issue
is fixed.

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Thanks for the advice! The SE rectifier is the original one. It looks like the amp has the original resistors on the 7 lug terminal strip, but the caps on it look like replacements. I'm at work now so later I'll give them a closer look later and see if the value is different from the original value.

Prior to receiving the amp and tubing it up, I had read the threads regarding the SE rectifier.

The posts I read indicate removing the SE Rect, replacing it with an IN4007 diode mounting it on a 3 lug terminal strip with the banded end appropriately oriented, per the pic I saw on the post. The posts also mentioned that it might be necessary to add a 10K resistor in parallel with the 10K resistor between lugs 1 & 2 of the 7 lug strip and a 20K in parallel with the other 10K resistor between lugs 3 & 4. I have the diode and ordered the 3 lug strip and a 1W 10K and a 1W 22K metal film resistor, just received. I guess the question now is am I on the right track? Should I replace the SE rect with the IN4007, and try it without the added resistors, or go ahead and add the resistors at the same time?

One note, I wrote the above before I read the Curcio Bias expansion mod. He calls for 3W 10K and 15K resistors, instead of the 1W resistors I have. Will the difference in watt and resistance ratings matter? Or should I order the 3W rated resistors?

Thanks for your attentions!

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Peter is correct. Mount the 1n4007 and see what happens.
Chances are that the bias voltage ( measured in the "biaset" tap) is too low , this comes
from the lesser losses in the diode, which makes the (negative) voltage more negative.
If so start adjusting the resistors until the adjustment range of the bias pots is comfortable.

( the biaset voltage is measured between cathode and ground, the more current flows in the
tubes the larger this voltage will be. 1.56 Volt is the target.
The bias diode and circuitry will actually create a negative voltage. This voltage, adjusted with the
bias pots, are applied to the powertub grids. The more negative voltage in the grid, the less
current will flow)
After work with the bias, turn the bias pots fully ccw to avoid current avalanche at power on, then
when tubes are heated slowly turn the pots CW while measuring the biaset voltage.
Alternate between left and right as they affect each other.

Thanks again guys! I'll install the IN4007 this evening and see what happens. I'll post the results.

erniegiro wrote:Thanks again guys! I'll install the IN4007 this evening and see what happens. I'll post the results.

One more possibly importent tip:

pull the rectifier before power up. Then connect a voltmeter between ground
and pin5 on one of the powertube pins. Power on and make sure you have
a negative voltage ( 30-45Volt) A positive voltage indicates that the 1n4007
is mounted the wrong way.

The best with buying a kit is that good instructions comes with the kit and
you will have all you need.

peterh wrote:

erniegiro wrote:Thanks again guys! I'll install the IN4007 this evening and see what happens. I'll post the results.

One more possibly importent tip:

pull the rectifier before power up. Then connect a voltmeter between ground
and pin5 on one of the powertube pins. Power on and make sure you have
a negative voltage ( 30-45Volt)  A positive voltage indicates that the 1n4007
is mounted the wrong way.

The best with buying a kit is that good instructions comes with the kit and
you will have all you need.

Thanks peterh! Do I remove all tubes to check pin 5, or power up with the tubes in after replacing the SE rectifier with the diode?

erniegiro wrote:

peterh wrote:

erniegiro wrote:Thanks again guys! I'll install the IN4007 this evening and see what happens. I'll post the results.

One more possibly importent tip:

pull the rectifier before power up. Then connect a voltmeter between ground
and pin5 on one of the powertube pins. Power on and make sure you have
a negative voltage ( 30-45Volt)  A positive voltage indicates that the 1n4007
is mounted the wrong way.

The best with buying a kit is that good instructions comes with the kit and
you will have all you need.

Thanks peterh! Do I remove all tubes to check pin 5, or power up with the tubes in after replacing the SE rectifier with the diode?

No just the rectifier, thus no B+ will flow and no damage will occur.
Leave the tubes in place as much as possible.

Perfect, thanks again!

PeterCapo wrote:You know, it's curious that after changing the selenium rectifier for a silicon diode, some folks seem to run into difficulty getting the bias adjustment into range while others don't.  I didn't have any problem getting bias in-range after installing a silicon diode.  I'd suggest starting with the original resistor values and power ratings (1-watt, I believe) and not complicate the installation unless you still have problems getting the bias in-range.

BTW, the original bias electrolytics were 50µF @75VDC apiece.  With the silicon diode, the voltage rating on those electrolytics should be increased.  The go-to value and rating for replacing them is 100µF @100VDC apiece.

A UF4007 UltraFast diode may be a sonic upgrade over the 1N4007.

PeterCapo, I just opened the amp up to do the rectifuer swap out. The bias caps on my unit were upgraded to 150uF 75 VDC. Is this enough, or should I swap them out for 100uF/100VDC caps?

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Well, we have success. First, line voltage I have a Monster Cable made power center with an analog meter, my line voltage stays in the 121-122V range.

After the SE rectifier swap out for the IN4007 swap, I measured voltages on all power tube socket pin 5's at -46V. Before the swap it was -35V.

I've managed to successfully set the bias at 1.56V. I attach pics.

I have not had to add additional resistors to the bias circuit, and I have not replaced the caps either, but since I have the suggested 1W 10 & 22K resistors, and have ordered the 3W 10 & 15K ones called for in the Curcio mod I may in the future. And I've ordered 100 uF 100VDC caps. So in the meantime I can play the amp, and keep a close eye on it.

Thanks for all the comments, instructions and suggestions, you all are an invaluable source of knowledge!!! I hope as I learn a bit I can share my knowledge with some hapless soul who needs help!

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PeterCapo wrote:I'd also be careful around the flying K40Y-9 coupling capacitors on the PC board.  If there's ever any downward pressure applied to them, the pads and traces on the underside of the board where they are soldered could delaminate without too much provocation.  

I know what you mean, one leg of one of the 4 separated during shipping, I soldered it back up. This amp has some upgrades or repair work done in the past. It looks like whoever worked on it discarded the cage that Dynaco puts over the PC3 board on factory wired units probably when the original caps were replaced by the K40Y-9. But it's clean and the seller was very good at describing its condition.

My intention is to keep the cage on, to prevent accidental damage to those coupling caps.