Dynaco st70 troubles

Endspec

Endspec

Super Member
Dyna st 70 with all new caps and power tranny as I was testing it one channel had a click sound almost like a relaey and the left channel stopped playing, I had my meters on the bias test point and that read ok. It did this 2 times so far with in 5 min of power up.
I have checked all my solder joints be fore the 2nd test and no problems there.
Any insight would be nice.
Thanks Steven A.
 
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I would try swapping the 7199 tubes between the two channels and see if the problems follow the tubes.
Another thing to check is the mono-stereo switch on the front. This has caused me problems in the past. My opinion is it is better removed.
 
Good Idea on the stitch, It is missing the button and I was thinking of replaceing or removeing it but I did not think of it being the problem.
I will check it and bypass it tonight and see how it does.
 
Ok I bypassed the stereo switch and tryed it, it still drops the left channel so I awapped the 7199's and the same on the same channel, The next thing is I will swapp the output tubes from side to side. It is dropping the bias voltage on the left side when it does it too.
The bias holds till that point.
 
Endspec said:
Ok I bypassed the stereo switch and tryed it, it still drops the left channel so I awapped the 7199's and the same on the same channel, The next thing is I will swapp the output tubes from side to side. It is dropping the bias voltage on the left side when it does it too.
The bias holds till that point.
Click to expand...


I assume that by "...dropping the bias voltage..." you mean that the bias voltage disappears when measuring at the bias check point. If so then you have already proven that the 7199 is not involved. It is somewhere in the biasing, B+, or output tubes of that channel. Swapping the output tubes is that right next step. If the problem moves then clearly it is one of the output tubes and replace them. If it stays on the same side you need to track down where you do and do not have the right DC voltages on that side:

When doing the following checks DO IT SAFELY:yes:. These voltages could be LETHAL:yikes:



Check for voltage on the plates and screens of the Output tubes,
Pin 4 - Screen - 415 Volts​
Pin 3 - Plate - 410 Volts​
Check for voltage on the Grids of the Output tubes,
Pins 5 and 6 of each tube - -32V​
(That's Negative 32). (This is per the manual) This should be done with a High Impedance Meter. They specify a VTVM but a good DMM should work OK also.

If any of those voltages are missing post back. We will need to walk you back through the circuit that is broken to see where it is broken.
 
Found the problem

One of the 6ca7 tubes goes bonker after warm up, replaced the pair and it has been singing for about 30 min.:banana:
 
Raqn it about an hour and checked the bias and it was still spot on, so I dug out my little dj mixer I use for a pre amp and also for the phono pre in it(I know not real good unit but it was free) and wired it up and ran the laptop into it and let me say this, with the eq off on the mixer it is fantastic sounding, but on some dance music we use in the gigs pop the eq on and this st70 will come alive and shake the crap out of the house with the sansui sp 3500 speakers and the bass is not tubby at all. I am getting to like stereo again lol.
 
Endspec said:
Raqn it about an hour and checked the bias and it was still spot on, so I dug out my little dj mixer I use for a pre amp and also for the phono pre in it(I know not real good unit but it was free) and wired it up and ran the laptop into it and let me say this, with the eq off on the mixer it is fantastic sounding, but on some dance music we use in the gigs pop the eq on and this st70 will come alive and shake the crap out of the house with the sansui sp 3500 speakers and the bass is not tubby at all. I am getting to like stereo again lol.
Click to expand...

The Stereo 70 is a great amp:thmbsp: given the right operating conditions. What you need is a hookup that keeps DC and VERY low frequencies out of the input. (Haven't done any comprehensive testing but I screen out anything below about 15 Hz and it seems to clear up ALL the issues I've had with the amp.) I'm guessing from the description you provide of your setup that you are keeping those very low frequencies out of the amp.

The basic circuit is (IMHO:D) a really good implementation:yes: of the Williamson design.
 
shelly_d said:
The Stereo 70 is a great amp:thmbsp: given the right operating conditions. What you need is a hookup that keeps DC and VERY low frequencies out of the input. (Haven't done any comprehensive testing but I screen out anything below about 15 Hz and it seems to clear up ALL the issues I've had with the amp.) I'm guessing from the description you provide of your setup that you are keeping those very low frequencies out of the amp.

The basic circuit is (IMHO:D) a really good implementation:yes: of the Williamson design.
Click to expand...
Well it has the stock pc board with all new caps and the multi cap so I don't know abut keeping the 15 hrtz out but as I play it more it seems to get better yet as the new caps get broke in, I used could get local for cheap lol and it seems to be ok, later I would like to upgrade the board to a better one with out the 7199 tubes though.
 
Endspec said:
Well it has the stock pc board with all new caps and the multi cap so I don't know abut keeping the 15 hrtz out but as I play it more it seems to get better yet as the new caps get broke in, I used could get local for cheap lol and it seems to be ok, later I would like to upgrade the board to a better one with out the 7199 tubes though.
Click to expand...

The stock board is what I use as well. What Dynaco did when they made this was they used a direct wire between the inputs and the input to the first tube (7199). This allows the DC from the preamp to enter the first tube. Then the feedback from the outputs is fed into the cathode. Since the DC is missing from the output because the output transformers cannot respond that low, the feedback mechanism tries to compensate by bosting the frequencies missing from the feedback. This drives the tube into satuation and muddies the mid to high frequencies through the amp. In the original they ment this amp to be mated with another Dynaco preamp (PAM1) which had coupling caps on the output. This would have served the same purpose as input caps on the Stereo 70.

If you have very clear and crisp sounding mid and high frequencies on your output then it is probable that you preamp/mixer is a very well beheaved device that is designed to very carefully screen out those frequencies from gtting to the power amp.

This article has a very well writen article on what happens in this amp and some sugestions on what to do to "fix" it. I've followed his advice (my preamp does let those very low frequencies through) and have been VERY happy:yes: with the results.

http://www.avahifi.com/root/equipment/amplifier/70_rebuild_plans.htm

You will hear a great many negitive things about this "fix", but the components to impliment are very cheap, and if they don't make an audible difference in your output then I would suggest new board won't either. The folks that sell the boards will tell you that this fix in not good enough but remember that they don't make their money when they don't sell you their boards.

Good Luck and happy :music:

Shelly_D

P.S. I'll be glad to answer any questions you might want to ask about implimenting this or what happens to the sound quality.
 
I can't speak for the 70 boards Steve, but many I have taken out of PAS preamps are very leaky (electrically). They put some sort of waxy coating on them that attracts dirt and moisture something terrible. I finally figured out to clean the boards in an acetone bath, followed by a denatured alcohol bath. They come out better than brand new, and look great to boot.:yes:
 
Thanks for the link Shellyd and to jay on how to clean the board better.
I am going to sit down and read it tonight Shelly and If I got any questions I will post up here.
Thanks Steven A.
 
Just curious, was the output tube that went bad the back one on the left side, looking at the amp from the front? Also, I'm assuming the original selenium bias rectifier has been replaced? If the bias circuit has even minor issues, for some reason an ST70 will chew up that back left tube first.
 
BrassTeacher said:
Just curious, was the output tube that went bad the back one on the left side, looking at the amp from the front? Also, I'm assuming the original selenium bias rectifier has been replaced? If the bias circuit has even minor issues, for some reason an ST70 will chew up that back left tube first.
Click to expand...

That is a good thing to know.:yes: Thanks
 
BrassTeacher said:
Just curious, was the output tube that went bad the back one on the left side, looking at the amp from the front? Also, I'm assuming the original selenium bias rectifier has been replaced? If the bias circuit has even minor issues, for some reason an ST70 will chew up that back left tube first.
Click to expand...
When i got the amp it had no tubes and a fried power tranny and choke, I orderd the dynakit bias upgrade and there better multi cap and a new choke for it.
I replaced two tube sockets on that side as they did not look good, I got a power tranny from Jay and picked up some good used tubes(well the tube tester said there good) and replaced all the caps on the pc board and brang it up on the veryac and it would play fine for about 3 to 5 min, than you could here a tink in a tube and that channel would loose sound so after some tube rolling I replaced those two 6ca7 tube and it plays fine now, got over 10 hours of play time yesterday on it. Just need to improve the high end as it sounds a bit flat.
 
Well about 15 hours on the amp and in the middle of one nice classical tune on the tuner as I sat here with my eyes closed I though my wife or kid messed with the treble control as all of a suden the mids and highs just came to life so I lookd and there still in the other room. I guess the caps broke in but never herd a change that fast, This thing is sounding so nice now.
A few tunes later they had a nice sapranor singer on and she sounded like she was in the room with me so clear and crisp. I can't wait to take it over to a friends and try it on some altec valencias he has waiting for this unit, just need to find a nice tube pre amp for him now to complete the system.
 
updated

Well 100 hours on it in the offoce so I moved it to the living room and wired it to a pair of rat shack T100's and useing a pioneer sx-838 as a pre amp as the tuner is dead in it, I am useing the phono and cdp with a seperate sansui tuner.
The sound from the T-100's shocked me as I never herd them on a strong tube unit only the cheap sony theater crap. I like the sound of the system now, nice clear highs and mids and realy tight bass and not tubby at all.
 
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