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ST-70 strangeness

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I picked up a cheap St-70 on Ebay and it was delivered yesterday. I checked everything out and it all seemed to be intact so I attached some speakers and hooked it to the Variac and a meter on each bias plug. The bias on the left had side kept on going up to 3 volts and sure would have went higher if I had kept adding more voltage. I have had this problem before and a cleaning of the tube socket and tube plugs cured it. After cleaning the sockets and tubes I was able to power it up to 115 volts and showing 0.51 on the bias turned all the way down. I checked and the amp has the stock 15.6r resistors so I attempted to bias it up to 1.56. The right hand side biased easy but the left hand side gets to exactly 1.21 where it holds steady but if I go further it runs away quickly climbing over 2+ volts. What would cause this? I am guessing that the cathode resistor is shot and it gets to a certain operating point and temperature and just starts to fry up letting full current go through to the bias plug. I was planning on replacing the old single resistors with individual 10 ohms on each tube and biasing at 1 volt but would like to know if I am correct in my assessment of what is wrong.
 
The cathode resistors could be a problem, though probably not the root cause, so replace them. If this is a stock unit, without question the caps will have gone dry (electrolytics) or leaky (coupling and bypass caps). Replace them with modern units. The bias supply has a selenium rectifier in it. Replace it with silicon.

And make sure you have good tubes. Any ST70 is going to need some refurbishment to be reliable, but the cost to do so is pretty reasonable, certainly under $100 if you avoid "fashion" parts.
 
I recapped 3 Dynaco ST-70's, they are very easy amp to work on. You can easily recap the whole amp including the quad capacitor for less than $70.00.

About your problem, make sure there are cracks on the Printed circuit board traces and above all, make sure you have a good set of tubes.

One of the ST-70's I recapped biased perfectly with my tubes. I gave the amp back to its owner and one channel would not bias. He swapped the tubes around and found out he had a bad tube although it tested fine in a tube tester.

He put in a new quad and all was fine.

Regards,
Sal Brisindi
 
So, the bias is at 1.21 for a while & the jumps to 2+? That is probably a bad bias pot.
You can verify this (carefully) w/o having to buy new ones by just switching the center tap wires from the pots to the bias resistors (I don't remember, but I think maybe these are back on the circuit board ).
A thorough cleaning of the pot might patch things up, but a bad bias pot can fail open = zero bias = bright red plates. You can replace them with similar types with the same or a bit less total resistance.
The cathode resistors are just 15.6 Ohm, 1%, to give 1.56V for 100 MA (both tubes). Dyna chose 1.56 V because this was the output of a standard Zinc-Carbon dry cell, & you could use this to calibrate your meter. Meters had pretty wide tolerances back in the '50's.
Better caps & a refurb of your bias supply are also a good idea, but I would attend to the bias pots first.
 
Sal Brisindi said:
I recapped 3 Dynaco ST-70's, they are very easy amp to work on. You can easily recap the whole amp including the quad capacitor for less than $70.00.

About your problem, make sure there are cracks on the Printed circuit board traces and above all, make sure you have a good set of tubes.

One of the ST-70's I recapped biased perfectly with my tubes. I gave the amp back to its owner and one channel would not bias. He swapped the tubes around and found out he had a bad tube although it tested fine in a tube tester.

He put in a new quad and all was fine.

Regards,
Sal Brisindi


you mean no cracks on the PCB traces, I assume?
 
Thanks,

I never even thought of looking for a break on the circuit board. That is a good idea. The main cap appears to be a recent addition to the amp as well the rectifier has been replaced with diodes. I do have some IXYS 11A 1200V Ultrafast, Soft Recovery Freds’s I can replace them with if it would make a difference. I am pretty sure that the tubes ready for the big tube display in the sky. I don’t have a tube tester but they really give of lots of violet glow and they are not Russian tubes. All four have the same construction 6CA7/El34 Made in Great Britain but two are labeled GE and Two RCA. They have exactly the same single halo construction as a quad of Mullards I have but I can not find a date code on them and they don’t have a hole in the lock post. All four tubes glow very violet inside, so much that where the slots are in the plate, half way down, the light shines out and makes a perfect yellow and violet clover leaf pattern on each side of each tube. I have never seen anything like it. I will take a picture of it when I get home from work
 
Phee said:
So, the bias is at 1.21 for a while & the jumps to 2+? That is probably a bad bias pot.

I considered that as well as it is always exactly the same spot in turning the pot that the voltage runs away. Off hand do you know the value of the pot. I can pick one up on the way home. I am very lucky to live within a few min drive of Parts Connexion and a Sayal outlet.
 
Here are the pictures of the tubes with the violet shapes.
 

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Long ago I built a Stereo 70, then some MkIIIs. Can't remember which one or both had the violet glow, but IMO it's perfectly normal. If the pots are wire wounds, squirt them with tuner cleaner and cycle 'em. Wire wound pots don't really wear out. If the grids have coupling caps (hey, it's been a while), *any* DC leakage is a disaster. Leakage you can't see with a DVM is still serious. To properly test coupling caps for DC leakage, you have to put a reasonably large voltage across them and measure the current with a microamp meter.
 
Well it looks like it was the bias pot at fault. I tested everything and it was all within the normal tolerances but the pot on one side only went up to 5k and the other got to about 2k then just stopped conducting all together. 4 bucks and a little soddering and its all fixed. Im going to replace the driver board and do some mods but I really wanted the amp to be working before I started making any changes.

Thanks for the advice it really helped me in fixing the problem. I thought it was the bias resistor.

Cheers
 
Conrad Hoffman said:
Long ago I built a Stereo 70, then some MkIIIs. Can't remember which one or both had the violet glow, but IMO it's perfectly normal.

It doesn't just happen on the big boys. Lots of my EL84s get a blue glow near the point where the mica meets the plates. It used to bother me, now I think it's pretty. As mentioned above, I believe it's caused by electrons leaking past the plates, hitting the glass causing it to fluoresce.

An externally hosted image should be here but it was not working when we last tested it.


If you zoom in, you can see it a bit in this picture too. It's on the left hand tube near the top. Ignore the red hot plate that's trying to melt down - this amp has other problems.

An externally hosted image should be here but it was not working when we last tested it.
 
Interesting, I wonder if it is just certain brands or makes that used a certain metal or someting. In my amps I have only seen it on newer made Russian tubes. I have not seen it on any US or GB tubes up till now. The picture is 4 Russian 6L6gc's on my Audio Research ST70C3 they really light up violet.
 

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Pyre said:
Interesting, I wonder if it is just certain brands or makes that used a certain metal or something. In my amps I have only seen it on newer made Russian tubes. I have not seen it on any US or GB tubes up till now.

It's not brand or local specific. It depends on the tube and how hard you are running them. Usually biased up next max dissipation is when you see the electron glow the most... and some tubes do it more than others regardless of manufacture. It's not a bad sign per se... but could be if it's a tube that hadn't done it before and then all of a sudden if was glowing blue a great deal.
 
Blue glow on the glass is normal, and as posted earlier somewhat indifferent to brand and type. I have however noticed it more often on new production tubes, especially Russian tubes.

Blue or violet glow in side the plate structure used to be reason enough to discard a tube back in the 60's and early 70's. It is however common on most current production tubes. I believe that this has to do with contamination of the vacuum.

A deep violet to pink glow that fills up most of the area between the electrodes is generally the sign of a gassy tube (vacuum contaminated with air or molecules outgassed from the internals).

The tube in the photo is bad, really bad, blow up the power supply kind of bad!
 

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