Too High Bias ST-70

Hi, just joined this forum. This is my first post. I just bought a ST-70 off the bay. It is a later model with Z326 output  transformers and smaller or shorter power transformer. It looks all original right down to the original diode (which I need to replace).

I bought it with no tubes. I have two RCA black plate 7199's NOS, Mullard 5AR4/GZ34 and 4 new Tung Sol EL34B's.

Hooked up a set of speakers to it. Slowly brought it up on a Variac and checked the power transformer voltages with no tubes in it and it checked fine at 117 volts. I then brought it up on a Variac with the rectifier tube in and starting at 50 volts and then 70, then 90 and stopped at 117 volts. Waiting about 15 minutes each step. All seemed fine. Turned it off and pulled the rectifier tube and put the rest of the tubes in and checked the filament heater voltages again. Checked fine. Put the rectifier tube in and turned it back on. Measured all the voltages as per the build manual and they all measured good.

I set the bias to 1.56 on both sides. A little bit of jumping around but seemed fine. Hooked it up to a PAS-3 and FM-3 I have had for like 15 years. It didn't sound the best at first but after a couple hours started to sound better. The next day it sounded like the day before when I first fired it up, but not too long after started sounding good. The tubes are burning in and the amp is starting to sound really good.

I checked the bias again after a few hours running and it was 1.91 on one side and 1.93 on the other side. OK, just adjust the bias again. But it won't go below 1.89 on either side. But the bias is rock stable. I suspect the new tubes are drawing too much current. That sound right?

How should I go about fixing the problem? I want to leave this as stock as possible. I thought when I fix the problem I would add the diode then. Can I change the 15.6 ohm resistors for 10 ohm to fix the problem. Then would I have to set bias to 1.00?

Or should I get a set of output tubes with lower amp draw?

Thank you, Kenny

Kennybugs wrote:Hi, just joined this forum. This is my first post. I just bought a ST-70 off the bay. It is a later model with Z326 output  transformers and smaller or shorter power transformer. It looks all original right down to the original diode (which I need to replace).

I bought it with no tubes. I have two RCA black plate 7199's NOS, Mullard 5AR4/GZ34 and 4 new Tung Sol EL34B's.

Hooked up a set of speakers to it. Slowly brought it up on a Variac and checked the power transformer voltages with no tubes in it and it checked fine at 117 volts. I then brought it up on a Variac with the rectifier tube in and starting at 50 volts and then 70, then 90 and stopped at 117 volts. Waiting about 15 minutes each step. All seemed fine. Turned it off and pulled the rectifier tube and put the rest of the tubes in and checked the filament heater voltages again. Checked fine. Put the rectifier tube in and turned it back on. Measured all the voltages as per the build manual and they all measured good.

I set the bias to 1.56 on both sides. A little bit of jumping around but seemed fine. Hooked it up to a PAS-3 and FM-3 I have had for like 15 years. It didn't sound the best at first but after a couple hours started to sound better. The next day it sounded like the day before when I first fired it up, but not too long after started sounding good. The tubes are burning in and the amp is starting to sound really good.

I checked the bias again after a few hours running and it was 1.91 on one side and 1.93 on the other side. OK, just adjust the bias again. But it won't go below 1.89 on either side. But the bias is rock stable. I suspect the new tubes are drawing too much current. That sound right?

How should I go about fixing the problem? I want to leave this as stock as possible. I thought when I fix the problem I would add the diode then. Can I change the 15.6 ohm resistors for 10 ohm to fix the problem. Then would I have to set bias to 1.00?

Or should I get a set of output tubes with lower amp draw?

Thank you, Kenny

I suggest you replace the Se (bias rectifier).
The simplest way is to mount an 1n4007 diode on the Se tabs ( leave the
Se as solder stand it won't do any harm for now).

You might even get the too little bias with the 1n4007 diode, in that case you
need to adjust the 10K resistor ( the one between the 2 50 uF caps in the bias
circuit) to 15k

The above gives you a quick fix, the permanent solution is to
get a bias kit from dynakitparts.com it will include all needed parts.

I had the same problems with my two ST70's before I replaced the driver boards with VTA boards a couple of years ago. On one amp I replaced the output tubes which solved the problem and on the other I replaced the selenium rectifier in the bias circuit with a 1N4007 silicon diode and I also replaced the caps in the bias which fixed that amp. If the amp hasn't been recapped you might want to consider doing that as that can eliminate problems later on.

Thank you Peterh and Big Harry. It looks like I may just rebuild the whole amp and also upgrade it. I just got done rebuilding two Heathkit W5M's. They turned out good. I put a bucking transformer in each one inside the chassis and also put in a high voltage delay board made for a Dynaco ST-70. Put in all new resistors and capacitors. Had to buy new output transformers because one measured high resistance. I used Classic Tone 40-18080 Hi Fi style transformers. Put in RCA 12au7's black plates, RCA 5R4 rectifier's and Tung Sol 7581A output tubes. They sound good. But the ST-70 sounds darn good also even old and stock.  I remember a guy trying to sell me one back in the mid 70's. I really liked it, but didn't buy it.

Thank you both, Kenny

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PeterCapo, I will clean the sockets and tighten them like you suggest. If that doesn't work I will change out the resistor like Peterh suggests. And if it still doesn't work change out the caps like Big Harry said. Then decide if I want to rebuild or upgrade it. But it will be a little while before I can do that. It just sounds so nice now. And is quiet. And has good solid bass and pretty nice highs. But man that power transformer sure gets hot. But I think it is OK and isn't getting too hot.

Kenny

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That's the page and test points I used. It measured real close to those voltages.

Kennybugs wrote:Thank you Peterh and Big Harry. It looks like I may just rebuild the whole amp and also upgrade it. I just got done rebuilding two Heathkit W5M's. They turned out good. I put a bucking transformer in each one inside the chassis and also put in a high voltage delay board made for a Dynaco ST-70. Put in all new resistors and capacitors. Had to buy new output transformers because one measured high resistance. I used Classic Tone 40-18080 Hi Fi style transformers. Put in RCA 12au7's black plates, RCA 5R4 rectifier's and Tung Sol 7581A output tubes. They sound good. But the ST-70 sounds darn good also even old and stock.  I remember a guy trying to sell me one back in the mid 70's. I really liked it, but didn't buy it.

Thank you both, Kenny

No! It's not time for any large scale "rebuild". It's time to fix the
problem you have, which seems to be a insufficient bias.

When this is repaired, use it for a while so you know it's working ok. Then
it's up to you if you want to use the solder iron.

Kennybugs wrote:PeterCapo, I will clean the sockets and tighten them like you suggest. If that doesn't work I will change out the resistor like Peterh suggests. And if it still doesn't work change out the caps like Big Harry said. Then decide if I want to rebuild or upgrade it. But it will be a little while before I can do that. It just sounds so nice now. And is quiet. And has good solid bass and pretty nice highs. But man that power transformer sure gets hot. But I think it is OK and isn't getting too hot.

Kenny

No again, you should replace the Se rectifier , replace with a Si diode, an
1n4007 will do just fine. Then you _might_ need to change a resistor to
be able have the bias adjustment range suit all tubes.

Don't replace items at random , you risk to get into a morass of problems.

Kennybugs wrote:
That's the page and test points I used. It measured real close to those voltages.

Unfortently there is no figures of the bias (-) voltage. The weak point is the
rectifier itself, they tend to go "blank" when they fail. Parallelling with a
Si diode will show if this is your problem.

Why is the bias voltage omitted ? Noone knows, but this might have been
considered a "fail-proof area" where no problem is expected ever. I do not
have a working st-70 thus i cannot tip of what voltage to expect.

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OK, I'll replace the diode first. I was going to replace one part at a time. It's just I have the resistor so I was going to do it first, but I'll get the diode and do it first.

Once I get it running right then I'll play it for awhile. Probably just leave it like it is then.

Kenny

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Both of my ST70's sounded good before I replaced the driver boards. My issue with the original boards was the 7199 tubes and their scarcity. I have a good supply of 12AU7's so the change to the VTA boards made sense to me and their cost is reasonable. I still recommend replacing the caps and any out of tolerance resistors on the original boards as the amps are now somewhere close to 50 years old and if they haven't reached the end of their service life, they will in the near future.

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Big Harry wrote:Both of my ST70's sounded good before I replaced the driver boards. My issue with the original boards was the 7199 tubes and their scarcity. I have a good supply of 12AU7's so the change to the VTA boards made sense to me and their cost is reasonable. I still recommend replacing the caps and any out of tolerance resistors on the original boards as the amps are now somewhere close to 50 years old and if they haven't reached the end of their service life, they will in the near future.  

The original caps and resistors have shown their reliability and will not need
replacement unless one of them is found broken.
Most Resistors can be measured in place ,like most caps. None needs to be
removed for sanity check.

Most of the trouble st70 owners get comes from wild replacements and improvements !

Things that need change is EL34 and 5AR4, they are "burned out" after a few
1000h and should be replaced.

Thanks for all the tips and help. Will let everybody know what was bad or needed changed and how it goes when I get the parts and fix it.

the bias value has absolutely nothing to do with the driver board (why has nobody posted this yet?)
First, replace the selenium diode with a new 1N4007 that costs 10c (well, might be $8 with shipping).
Then, with NO tubes in the amp, turn on and see if you can adjust the bias pots to read from around -25 to -50vdc on each output tube pin 5.
If that works, the bias voltage and the bias pots are good, if not, well replace the bias pots, or better yet, buy the VTA70 board
and fix ALL of the problems instantly and have amp that sounds 10X as good as anything using a 7199 tube.
Likely the final problem is the output tubes are worn out.

Thanks for your reply. The tubes are new. I have diodes coming. I also bought some CL-80 surgistors. I did replace the 10k resistor with a 15k resistor but the bias went up. So I paralleled the 10k and 15k to make 6k and that worked. But i want to replace the diode and see if that fixes the problem with the 10k in place.

I will do what you suggest when I get the diode and measure the voltage at pin 5.

Kenny

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Peter,
yes, and I know you haven't ever bought any VTA products so you don't have a clue how good they are,
unlike 90% of the other people who are on this forum

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published specs are nearly worthless, as most any piece of "hi-fi" gear has specs so good that they way below listening thresholds.
I'll just say that Bob and I consistently (and this means like every week of every year for the past 15-20 years)
get emails from customers saying that our equipment is so far superior to others they have owned and heard.
This includes McIntosh, Audio Research, Cary, Pass Labs, and others, most of which cost 3-10x as much for equivalent gear.
I'm not talking about joe-blow customers with a $2000 system, I'm talking about customers with $50K+ systems.
Good enough for me and our customers !!

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