ST70 bias pots question

Hi all, I'm new to the forum but thanks to the many helpful users on the net (and the long history of the ST70) I have been able to proceed with rebuilding my dad's old ST70 without much help. Having never heard the difference between an original build and a modified build, I'm going to keep the amp audio path as close to original as possible but I am modernizing as I see fit. For the bias section I have two 10 turn pots on hand. My question is whether the original bias pots are "touchy" to adjust. In other words, is it worth while to replace the original pots with 10 turn pots for more accuracy or are the originals good enough?

While I'm on the subject, instead of one pot adjusting two tubes, would it be worth installing 4 pots and 4 bias resistors to be able to adjust each tube individually? Unless there is more to it, it doesn't seem to be a very hard modification. But is it worth it? Thank you all in advance. Rick

PS, I'm doing all of the normal upgrades like a new 80/40/30/20 cap, a new diode, new bias components, new driver board, new RCA jacks. I am going to omit the wiring to the pre amp sockets except for the bias measuring points of course. Oh and omit the stereo/mono switch too.

n3ikq wrote:Hi all, I'm new to the forum but thanks to the many helpful users on the net (and the long history of the ST70) I have been able to proceed with rebuilding my dad's old ST70 without much help. Having never heard the difference between an original build and a modified build, I'm going to keep the amp audio path as close to original as possible but I am modernizing as I see fit. For the bias section I have two 10 turn pots on hand. My question is whether the original bias pots are "touchy" to adjust. In other words, is it worth while to replace the original pots with 10 turn pots for more accuracy or are the originals good enough?

While I'm on the subject, instead of one pot adjusting two tubes, would it be worth installing 4 pots and 4 bias resistors to be able to adjust each tube individually? Unless there is more to it, it doesn't seem to be a very hard modification. But is it worth it? Thank you all in advance. Rick

PS, I'm doing all of the normal upgrades like a new 80/40/30/20 cap, a new diode, new bias components, new driver board, new RCA jacks. I am going to omit the wiring to the pre amp sockets except for the bias measuring points of course. Oh and omit the stereo/mono switch too.

welcome to the world of tubes!

Have you considered upgrading to the VTA input/driver board?
It will make a HUGE difference in the overall sound, and it comes with 4x BIAS trimmers on the pcb, one for each power output tube. You will need to change the BIAS adjusting wiring in each power tube and add a 10OHM 2W resistor on each power tube socket, but it is pretty straight forward and wiring diagrams are easy to get from this forum.
The investment in this VTA pcb is worth EVERY $!

Thanks for your quick reply. I learned of the vta board only after I purchased and installed a nice new quad cap! I think I'm now committed to keep close to the original design which is fine given that I have nothing to compare it with. If I really drink the tube kool aid I'll build a second modified model to compare it with. I was thinking that being able to adjust each tube bias separately might allow me to mix and max tubes to a certain extent and allow me to NOT have to buy matched pairs of tubes. anybody know of a vta-like board that does not have the power supply components but has four bias adjustments?

if you want to keep the original board, but modify the bias for individual adjustment,
check out this mod available here:
http://www.dynakitparts.com/dynakit-products/bias-kit-parts/Bias-Balance-Control-Upgrade-Kit
but consider that it's $48 and you can buy a complete VTA70 board with all parts for $99 and your amp will sound at least 4X better

n3ikq wrote:Hi all, I'm new to the forum but thanks to the many helpful users on the net (and the long history of the ST70) I have been able to proceed with rebuilding my dad's old ST70 without much help. Having never heard the difference between an original build and a modified build, I'm going to keep the amp audio path as close to original as possible but I am modernizing as I see fit. For the bias section I have two 10 turn pots on hand. My question is whether the original bias pots are "touchy" to adjust. In other words, is it worth while to replace the original pots with 10 turn pots for more accuracy or are the originals good enough?

While I'm on the subject, instead of one pot adjusting two tubes, would it be worth installing 4 pots and 4 bias resistors to be able to adjust each tube individually? Unless there is more to it, it doesn't seem to be a very hard modification. But is it worth it? Thank you all in advance. Rick

PS, I'm doing all of the normal upgrades like a new 80/40/30/20 cap, a new diode, new bias components, new driver board, new RCA jacks. I am going to omit the wiring to the pre amp sockets except for the bias measuring points of course. Oh and omit the stereo/mono switch too.

If you want to keep the original board, then installing 4 pots and separate cathode resistors
is the way to go. As regards to cathode resistors you might install 10 for each tube and get
used to set bias to 0.5V ( correspons to 50mA ).

No, you don't need 10-turn pots, although if you have them by all means use them. But
keep in mind that every adjustment of one pot wil affect the other, adjusting bias
usually means a lot of back-and-forth .

Ok so it looks like I still use an 80/40/30/20 cap even with the vta driver board. It does look like the most cost effective way to get all of the time proven upgrades while replacing my ratty original board. Thanks everyone.

n3ikq wrote: I was thinking that being able to adjust each tube bias separately might allow me to mix and max tubes to a certain extent and allow me to NOT have to buy matched pairs of tubes.

The extra expense, if any, for properly matched tubes is ultimately worth it.  Sure, tubes are forgiving of some amount of mismatch and the casual listener may never notice or care.  But severely mismatched tubes will simply not perform well and no amount of biasing will fix that.  Ultimately their life will be shortened and whatever cost savings you were trying to achieve will be negated by the cost of a replacement tube.  There's no audiophile magic to it.  A properly matched set of tubes will perform better and if biased conservatively will last many years.  The best solid state gear uses tightly matched transistors and other parts.  Why should tubes be any different?

I also recommend the VTA driver board. It's one of the best things you can do for these old amps.

Thanks for the tip. I'm hoping the original tubes will be good enough to get me up and running at least! One last thing, when I buy the vta board, does it come with the four 10 ohm and one 2200 ohm resistors? Just asking to see if I need to start scrounging for them in advance.

No, you do not need ten-turn pots.  The originals, if they are in good shape without intermittents, are fine (you do NOT want an open circuit in the bias pots, the tubes will red-plate immediately!).

As for splitting the cathodes and providing independent adjustment, I'm of mixed feelings about that.  You can set the bias for each tube individually and thus use tubes that are mismatched for DC current; but the patented Dyna mechanism for reducing distortion depends on both cathodes being connected together AND being lifted above ground by a small resistance...the common cathode resistor.  It is possible to AC-couple the cathodes to restore the distortion-reduction mechanism to the circuit while maintaining independent DC paths for each tube; a high-quality capacitor in the 10μF range is required to do it.

thanks for posting this..............GP49
As for splitting the cathodes and providing independent adjustment, I'm of mixed feelings about that.  You can set the bias for each tube individually and thus use tubes that are mismatched for DC current; but the patented Dyna mechanism for reducing distortion depends on both cathodes being connected together AND being lifted above ground by a small resistance...the common cathode resistor.  

One bias pot pr ch for output tubes also gives full output......to me it sounds better....
when the  output tubes cathoids are not talking to each other .....it like....dancing with one  foot nailed to the floor....hehe...................................
I have had an worket on a lot of tube amps an have set minny up with one pot that were sold with a pot for ea output tube an found most like the sound better.
4 new tubes today come match.....the only way to get new unmatch tubes would buy one at a time?
An even  this has never been any bias prob for me.....i have MK3s with stock 6AN8 drivers....work an sound great....two tube front end do sound diff....better yes maybe sometimes....more noise for sure...less you can match frontend tubes with a tube tester....then youll need a nother tube tester to ck the frist tube tester....less you get a new tube tester.....an that more money than most amps an preamps put togather...

An one more thing...... it looks like Jim McShane is one of the only people realy Matching tubes..... an from what i see he sales tubes at the same price as minny otheres that dont do the right matching......looks like any tubes he sale like KT120s that i may need well come from him.........I do not know him but seems  vary fair......an well like others.....Pass,Carver,give info on audio an how thay see it works best......
.tube in 2014 gofig.........all just one mans finding..


Thanks for the site Bob..................................