• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Dynaco ST-70 bias resistors

jsg1469

jsg1469

Member
My 1962 stereo 70 is all original with the exception of recently replacing the circuit board (stayed with original 7199 circuit) the selenium rectifier to a 1N4007 and the two bias filter caps to 100uF 100v. My right bias pot is loose and not stable. My meter is all over the place if I touch it. I tried cleaning but didn't work. Replacing both 10k pots today. I read Bob Latino recommended changing the 10k resistors on the bias bar to 5.1 but cannot find the thread. Question: all I have available is 4.7, 6.8k and 10k resistors at 1 watt. What will replacing to a 4.7 or 6.8k do to bias settings? Should I just test and keep the 10k or replace with 10k if necessary? I only use EL34 and occasionally KT77 in the amp. Thank you
 
Register to hide this ad
I did the Great Bob Latino's fix and its probably in my rebuild thread
I added a second 10k to effectively get the proper 5k

It was relevant for sure

DynaKit ST-70 two of them

So these look a little shabby even by Arizona standards Amp #1 Amp #2 I got these off of CL They came with cages I guess I'll clean them up a bit I'm gonna need another can of De-Oxit Looks like 4 Sylvania EL34's on one amp 2 x RCA GT Britian EL34s 2x GE GT Britian EL34s mix of...
audiokarma.org
Post #101 has all the information

DynaKit ST-70 two of them

Here is something I read When you replace the original selenium rectifier with a diode, the bias voltage adjustments sometimes go out of range. The reason for this is that a diode has more "forward voltage" than the selenium rectifier. There is an easy solution. On the 7 lug terminal strip to...
audiokarma.org

Good luck
 
Omegaman said:
I did the Great Bob Latino's fix and its probably in my rebuild thread
I added a second 10k to effectively get the proper 5k

It was relevant for sure

DynaKit ST-70 two of them

So these look a little shabby even by Arizona standards Amp #1 Amp #2 I got these off of CL They came with cages I guess I'll clean them up a bit I'm gonna need another can of De-Oxit Looks like 4 Sylvania EL34's on one amp 2 x RCA GT Britian EL34s 2x GE GT Britian EL34s mix of...
audiokarma.org
Post #101 has all the information

DynaKit ST-70 two of them

Here is something I read When you replace the original selenium rectifier with a diode, the bias voltage adjustments sometimes go out of range. The reason for this is that a diode has more "forward voltage" than the selenium rectifier. There is an easy solution. On the 7 lug terminal strip to...
audiokarma.org

Good luck
Click to expand...
Thank you. I didn't think of adding a second resistor in parallel. Thank you for the info.
 
The original thread from some NAK audio site where Bob says this is so obscure, I doubt I could find it again.
I just did exactly what Bob did

My rebuild kit did have a 4.7k resistor but kinda obscure instructions on what to do with it

but its all good now
 
Only time you need to do anything is if the pot ends up at the far end of travel and the bias voltage at the test points is still low.

the trick is knowing which 10k resistor to sub. There are two, and if you sub the wrong one it makes the problem worse. The highlighted one will make the bias voltage less negative which increases current flow and gives more voltage at the test points. Changing the other one does the opposite.


1707843418383.png
 
gadget73 said:
Only time you need to do anything is if the pot ends up at the far end of travel and the bias voltage at the test points is still low.

the trick is knowing which 10k resistor to sub. There are two, and if you sub the wrong one it makes the problem worse. The highlighted one will make the bias voltage less negative which increases current flow and gives more voltage at the test points. Changing the other one does the opposite.


View attachment 3117627
Click to expand...
Mine goes from extreme highs to extreme lows. I have several matched quads of EL34's I've tried. I know that faulty bias pot is a lot of the problem.
 
Omegaman said:
The original thread from some NAK audio site where Bob says this is so obscure, I doubt I could find it again.
I just did exactly what Bob did

My rebuild kit did have a 4.7k resistor but kinda obscure instructions on what to do with it

but its all good now
Click to expand...
Did lower both resistors?
 
jsg1469 said:
Mine goes from extreme highs to extreme lows. I have several matched quads of EL34's I've tried. I know that faulty bias pot is a lot of the problem.
Click to expand...
bad pots do what they do, but if the range is wrong, thats where the resistor change comes in.


Another thing thats worth doing, tie a 100k from the wiper of the bias pot to the side going to the capacitor. That will prevent the tubes from losing bias in case the pot loses contact.
 
gadget73 said:
bad pots do what they do, but if the range is wrong, thats where the resistor change comes in.


Another thing thats worth doing, tie a 100k from the wiper of the bias pot to the side going to the capacitor. That will prevent the tubes from losing bias in case the pot loses contact.
Click to expand...
Good information
 
BIAS SUpply - JCM 800, jpeg.jpg
I've done this mod to some Fender amps. This is how the old-school Marshall JCM 800 bias supply is built.

If the 22k-ohm pot fails open, the tubes still sees C- voltage.
 
thats a better way to do it. If the pot fails it just puts the grids full negative. The resistor to the wiper does it too, though the total resistance from grid to the bias supply ends up higher than tube spec. Not the end of the world, and still better than losing bias supply entirely.
 
Per the thread

I have two 10k resistors. I will parallel 10k resistors to the two existing 10k resistors to make 5k resistance in each. This should bring the bias back in line and raise the B+ to the correct level
 
Last edited:
gadget73 said:
thats a better way to do it. If the pot fails it just puts the grids full negative. The resistor to the wiper does it too, though the total resistance from grid to the bias supply ends up higher than tube spec. Not the end of the world, and still better than losing bias supply entirely.
Click to expand...
And having the tubes go ballistic.

One "should" hear a difference with full negative bias, but you'd be surprised. ;)
 
I appreciate all of your feedback today. I have successfully made the repairs and biased up two quads of NOS Teslas and 1 new production Mullard. Everything is holding steady at 1.45, with 116 volts at the wall. Thanks again.
 
You must log in or register to reply here.
Back
Top Bottom