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Bob Latino

ViperZ

6 posters

Troubleshooting Dynaco ST-70

ViperZ: Posts : 100
Join date : 2010-03-26

Post n°1

Troubleshooting Dynaco ST-70

by ViperZ Thu Apr 01, 2010 4:02 pm

Hi,

Firstly, I did read Bob's short troubleshooting guide...

I have pretty much all-original Dynaco ST-70 amp. I replaced the two coupling capacitors at the bottom and the multi-cap with the new higher-voltage higher-capacitance equivalent from Dynakitparts.

Originally, previous owner "modded" the amp to be in triode mode by installing 100-Ohm resistors between pins 3 and 4 of all power tubes and disconnecting the power wire from pin 4. This way, one of the channels would go into oscillation (crazy high-frequency whistle), unless preamp was hooked up to it. So looks like some kind of grounding problem. Anyway, going back to standard/original pentode configuration fixed the issue.

Now, the amp works, but I can hear noticeable main transformer hum through the speakers (when the music is not playing, but I am sure it's obviously still there when the signal is on).
I installed the new 3-prong power cord, but it did not change anything (i.e. grounding the case with the third prong). I tried a different set of tubes (from my other ST-70, which is silent), but nothing changed. The power tubes are properly balanced too.

So is it a grounding issue somewhere in the amp? Where should I start? Thank you!

Bob Latino: Admin; Posts : 3277
Join date : 2008-11-26
Location : Massachusetts

Post n°2

Re: Troubleshooting Dynaco ST-70

by Bob Latino Thu Apr 01, 2010 7:16 pm

ViperZ wrote:Hi,

Firstly, I did read Bob's short troubleshooting guide...

I have pretty much all-original Dynaco ST-70 amp. I replaced the two coupling capacitors at the bottom and the multi-cap with the new higher-voltage higher-capacitance equivalent from Dynakitparts.

Originally, previous owner "modded" the amp to be in triode mode by installing 100-Ohm resistors between pins 3 and 4 of all power tubes and disconnecting the power wire from pin 4. This way, one of the channels would go into oscillation (crazy high-frequency whistle), unless preamp was hooked up to it. So looks like some kind of grounding problem. Anyway, going back to standard/original pentode configuration fixed the issue.

Now, the amp works, but I can hear noticeable main transformer hum through the speakers (when the music is not playing, but I am sure it's obviously still there when the signal is on).
I installed the new 3-prong power cord, but it did not change anything (i.e. grounding the case with the third prong). I tried a different set of tubes (from my other ST-70, which is silent), but nothing changed. The power tubes are properly balanced too.

So is it a grounding issue somewhere in the amp? Where should I start? Thank you!

Hi,

First make sure it is the amp. Disconnect the RCA interconnects on your preamp side and then short betwee the center pin and outer shield on the side that you disconnected. Use jumper wires or just the metal end of a screwdriver. If the hum goes away the amp is not the issue. If the hum is still there then the amp IS at fault.

If all your ground connectors are good and you tried swapping out the rectifier tube with a new one then maybe your power transformer has seen better days. Most of those original PA-060 power transformers are 40 - 50 years old now and sometimes they do start to vibrate, hum, buzz etc.

One other thing. The main coupling caps are on the TOP of the STOCK driver board in an original ST-70. the two capacitors on the 7 lug terminal strip are part of the ST-70's bias system. Try replacing the 6 black capacitors on the TOP of the driver board. There are four .10 uF and two .05 uF. You can replace the four .10's with .22 uF and the two .05's with a .10 uF if you want also.

IMHO you should probably just REPLACE the driver board. Go to Dynakitparts if you want a stock driver board replacement. If you want an upgraded driver board I recommend the VTA driver board.

Bob

wolverine: Posts : 59
Join date : 2010-02-20

Post n°3

Re: Troubleshooting Dynaco ST-70

by wolverine Thu Apr 01, 2010 8:43 pm

I have purchased a replacement board from Kevin myself and I can tell you first hand he sells excellent quality boards.

I have 4 amps running in triode mode with no problems at all. One of those amps has a replacement board that I bought from Kevin. I would not hesitate to purchase another board from him, he really knows his stuff, will be there to help you if you need it, and it is not that expensive to replace the old board. The one I purchased was populated and the work was top notch.

I agree with Bob, I would replace the board unless you are trying to maintain a museum piece to sit on the shelf. If this is an amp you want to use for yourself then replacing the board is the best option, and you certainly will not regret buying from Kevin at Dynakit.

wolverine

DynakitParts: Posts : 215
Join date : 2008-11-30

Post n°4

Re: Troubleshooting Dynaco ST-70

by DynakitParts Thu Apr 01, 2010 10:19 pm

Hi Viper,

If you suspect the power transformer, try mounting rubber
washers under the power transformer between the chassis and the power
transformer. Do not over-tighten the (4) 8-32 hex nuts..just snug em
up and add a second hex nut to keep everything from coming loose.

I have these on my website if you should need them.

Regards, Kevin

ViperZ: Posts : 100
Join date : 2010-03-26

Post n°5

Re: Troubleshooting Dynaco ST-70

by ViperZ Thu Apr 01, 2010 10:50 pm

Hi Kevin,

I bought both of my "rebuild kits" from you. I will be in the States for a couple of weeks starting Monday, and will order at least the replacement coupling capacitors (for both amps) at that point.

The power transformer (PA 060) on this particular amp was raised by the previous owner, so that it is at the same height as the output transformers. He used longer bolts with nuts, but not sure if rubber washers were used. Will obviously get a set from you.

I am looking at the possibility of a getting a new board from you too. What is the difference between the boards with metal film resistors and boards with carbon composition resistors? Wouldn't the carbon composition resistors deteriorate over time, whereas metal film are supposed to last much longer? I am asking because I'm pretty sure we don't use the carbon composite resistors for that very reason in our space designs any more.

Thanks again!

Eugen.

DynakitParts: Posts : 215
Join date : 2008-11-30

Post n°6

Caron Composition Resistors

by DynakitParts Fri Apr 02, 2010 10:53 pm

Eugen,
Yes, I recall your name and parts order.

Metal film resistors are IMHO the better choice for audio when
compared to carbon composition although some of my customers
still prefer the warm sound of these carbon comps. I have yet to
hear this so called "Warm Sound" but to each their own. I do not
wish to start a long debate on this subject.

I understand a customer who wishes to retain the authenticity of
their amplifier, however carbon comps were commonly used on all
electronic gear back in the 1950's and on through the early 60's.
If David Hafler (RIP) were designing audio equipment today, I can
assure you that not one carbon composition resistor would be in
his audio circuit.

Happy Easter

Kevin

ViperZ: Posts : 100
Join date : 2010-03-26

Post n°7

Re: Troubleshooting Dynaco ST-70

by ViperZ Sun May 23, 2010 8:25 pm

I am at a loss of ideas at this point.

The amp is still humming on both channels. After ~15 seconds left channel starts to hum noticeably, then after another 2-3 seconds, right channel starts to hum slightly differently.

- Started with humming slightly modified ST-70 (in triode mode). Changed everything back to original and started updating.
- All caps are new, including the upgraded big can multi-cap.
- Brand new board with all new parts.
- All new input and output conectors.
- New 15.6 Ohm resistors, even new stereo/mono switch (since I had one, I just put it in).
- New 3-prong power cord.
- Rolled the tubes from my other ST-70, yeah, hum changes, but not by much.
- Rewired a bunch of things, checked the ground connections.

What am I missing? Is it a big transformer humming? How do I check that it is? Thanks...
(The amp is humming with and without input connectors connected).

natoe: Posts : 35
Join date : 2010-03-06
Location : Pittsburgh, PA

Post n°8

Re: Troubleshooting Dynaco ST-70

by natoe Sun May 23, 2010 11:20 pm

recheck your ground connections the tube sockets were the 15.6 res mount. re touch the solder joints on all grounds.

Bob Latino: Admin; Posts : 3277
Join date : 2008-11-26
Location : Massachusetts

Post n°9

Re: Troubleshooting Dynaco ST-70

by Bob Latino Mon May 24, 2010 5:11 am

Try removing the third GROUND wire from your 3 wire cord by using a "cheater plug" or a "3 to 2" AC connector. Sometimes the addition of a 3 wire cord to an amp like the ST-70 can increase the hum level of the amp.

Bob

ViperZ: Posts : 100
Join date : 2010-03-26

Post n°10

Re: Troubleshooting Dynaco ST-70

by ViperZ Mon May 24, 2010 10:11 am

Bob, yes, I forgot to mention this - it's the first thing I've done (it's taped off right now). I also even tried switching wires between fuse and switch just to see if it will help.

Of course, I'll keep playing with the grounds, especially considering that there are very few grounding points, but I am at a loss of ideas at this point.

ViperZ: Posts : 100
Join date : 2010-03-26

Post n°11

Re: Troubleshooting Dynaco ST-70

by ViperZ Mon Jul 19, 2010 8:34 am

Just a quick update on this amp.... Ordered a power transformer from Kevin.

The amp now has the new board, new power tubes (swapped other tubes from working ST-70 as well), new capacitor including the upgraded twist-on cap, new power cord, new connectors. I also re-wired most of it to find the ground problem... The buzz became quieter, but it still can be heard from 1-2 feet, which is quite unacceptable, considering that my other ST-70 is dead quiet.

I guess, since it's still buzzing on both channels, the only other thing common to both channels is the power transformer... Wish me luck!

Bob Latino: Admin; Posts : 3277
Join date : 2008-11-26
Location : Massachusetts

Post n°12

Re: Troubleshooting Dynaco ST-70

by Bob Latino Mon Jul 19, 2010 6:20 pm

ViperZ,

Getting the new power transformer from Kevin is a good idea. The original PA-060's barely had enough power transfer capability to run the amp. With a new power transformer you can now try KT88 or 6550 output tubes. You won't get any more power out of the amp but both of these tubes sound a little different than EL34 output tubes. Depending on your musical taste and other associated gear, you may like the slightly different sound "flavor" of these tubes.

Bob

ViperZ: Posts : 100
Join date : 2010-03-26

Post n°13

Re: Troubleshooting Dynaco ST-70

by ViperZ Sat Jul 31, 2010 2:51 pm

Just wanted to thank everybody who helped and Kevin from Dynakitparts. The new transformer is much more beefier than the original one. I followed the original installation instructions and replacement of the transformer was a breeze. Now the transformer is working fine and is very quiet. I now have two fully working ST-70 Smile

Smile

. Time to think about how to get them into mono and hook them up to my Maggies and Dynaco pas-3 pre Smile

Smile

.

GP49: Posts : 792
Join date : 2009-04-30
Location : East of the sun and west of the moon

Post n°14

Re: Troubleshooting Dynaco ST-70

by GP49 Sat Jul 31, 2010 5:41 pm

ViperZ wrote:Time to think about how to get them into mono and hook them up to my Maggies and Dynaco pas-3 pre .

Whatever you do: if you want to use each Stereo 70 in mono, DO NOT use the Dynaco method of connecting the two channels in parallel and feeding one input to both channels using the Stereo/Mono switch.

It does not work properly. Power does not double, and distortion goes up. One major problem is that the feedback signals from the two channels get mixed when you parallel the outputs. The resultant feedback to the input stages does not work to correct the distortion generated in each individual channel, instead becoming a mechanism for CREATING unwanted distortion and nonlinearity.

The only proper way to run a stereo amplifier such as the Stereo 70 as a mono amp is to bridge the two channels. You invert phase on one input, then run the loudspeaker off the two HOT connections. Note that if you do this, you can not have ANY common ground connections in the loudspeaker wiring.

The big advantage to bridged amp channels is that you get four times the output power as a single channel. The disadvantage is the phase inverter circuit needed to drive one of the two channels.

ViperZ: Posts : 100
Join date : 2010-03-26

Post n°15

Re: Troubleshooting Dynaco ST-70

by ViperZ Sat Jul 31, 2010 5:55 pm

GP49 wrote:

ViperZ wrote:Time to think about how to get them into mono and hook them up to my Maggies and Dynaco pas-3 pre .

Whatever you do: if you want to use each Stereo 70 in mono, DO NOT use the Dynaco method of connecting the two channels in parallel and feeding one input to both channels using the Stereo/Mono switch.

The big advantage to bridged amp channels is that you get four times the output power as a single channel. The disadvantage is the phase inverter circuit needed to drive one of the two channels.

I have no problems making an inverter circuit. There are tons of examples on the net. If I remember, Hafler had one of those. But if that's the case, maybe I'll just sell ST-70's and look for a pair of MKIII's.

ViperZ: Posts : 100
Join date : 2010-03-26

Post n°16

Re: Troubleshooting Dynaco ST-70

by ViperZ Wed Aug 11, 2010 12:43 pm

Finally posted pictures of both amps here:
https://dynacotubeaudio.forumotion.com/your-dynaco-sytem-f1/my-rebuilt-st70s-t454.htm#2267

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Post n°17

Re: Troubleshooting Dynaco ST-70

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