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Old 29th August 2007, 03:33 PM   #1
Pyre is offline Pyre  Canada
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Default ST-70 strangeness

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.
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Old 29th August 2007, 03:41 PM   #2
SY is offline SY  United States
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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.
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Old 29th August 2007, 04:16 PM   #3
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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.

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Old 29th August 2007, 05:00 PM   #4
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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.
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Old 29th August 2007, 05:15 PM   #5
chrisb is offline chrisb  Canada
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Quote:
Originally posted by Sal Brisindi
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?
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Old 29th August 2007, 05:16 PM   #6
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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 Fredsís 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 donít 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 donít 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
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Old 29th August 2007, 05:21 PM   #7
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Quote:
Originally posted by Phee
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.
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Old 29th August 2007, 09:05 PM   #8
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Here are the pictures of the tubes with the violet shapes.
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File Type: jpg pics 008-800.jpg (10.9 KB, 318 views)
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Old 29th August 2007, 09:56 PM   #9
Phee is offline Phee  United States
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According to the schematic at Dr tube,
http://www.drtube.com/schematics/dynaco/st70.gif
They're 10K wirewound pots (linear, right?).
You might want to get two.
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Old 29th August 2007, 09:58 PM   #10
SY is offline SY  United States
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They're carbon, not wirewound. Linear taper.
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