Dynaco ST-70 distortion

[moodydan]

I've got an ST-70 that I've re-capped (including the multi-section can), replaced the driver board with a new board from Dynakitparts, and replaced the EL34's with a matched quad. The 5AR4 isn't an issue as I've swapped it with a new tube and nothing changed. The 7199's (as well as the rectifier) all tested good.

The issue is I've got distortion in both channels. It sounded good for a few hours of play, then gradually got terrible. Bias is set and steady, and the distortion issue was there before I installed the new driver board. I suspect the choke, just because it's all I can think that would effect both channels, but this is the first ST-70 I've played with. Any suggestions?

Thanks,
Dan

EDIT: and apparently I can't spell distortion. or change the title. sorry.

 

[gadget73]

if there was a problem with the choke, I'd expect to see voltages dropping somewhere. If that were happening, the bias voltage would drop with it.

Check the voltages at the driver board. If there was a problem with the voltage feed to the front, it could cause distortion without affecting the output stage.

 

[moodydan]

I will post pictures this afternoon, and will check voltages at the driver board.

 

[NOSValves]

I doubt it is the choke. Never rule out a brand new part failing prematurely, a wiring error or cold solder joint. Since the problem is identical in both channel it usually means its power supply related.

 

[primosounds]

If everything seems to be ok, and you have some distortion i would check the grid leak resistors.

 

[peterh]

I've got an ST-70 that I've re-capped (including the multi-section can), replaced the driver board with a new board from Dynakitparts, and replaced the EL34's with a matched quad. The 5AR4 isn't an issue as I've swapped it with a new tube and nothing changed. The 7199's (as well as the rectifier) all tested good.

The issue is I've got distortion in both channels. It sounded good for a few hours of play, then gradually got terrible. Bias is set and steady, and the distortion issue was there before I installed the new driver board. I suspect the choke, just because it's all I can think that would effect both channels, but this is the first ST-70 I've played with. Any suggestions?

Thanks,
Dan

EDIT: and apparently I can't spell distortion. or change the title. sorry.

Does your pre deliver DC on it's output ? That could explain distorsion.
try to add a cap at the st70 input , 0.33uF is enough.

 

[BinaryMike]

Does your pre deliver DC on it's output ? That could explain distorsion.
try to add a cap at the st70 input , 0.33uF is enough.

That's an excellent suggestion!

 

[moodydan]

Does your pre deliver DC on it's output ? That could explain distorsion.
try to add a cap at the st70 input , 0.33uF is enough.

I tried 2 different preamps - a PAS 2 and a Marantz SC-6 - as well as used a Marantz 1060 as a pre, and distortion was consistent. I couldn't get to it today to measure voltages, but I'll bring it up on a variac tomorrow and report what I find. Adding caps at the input is probably smart either way, so I'll probably do that while I'm in there measuring voltages.

 

[moodydan]

In the meantime, here are pictures:










online free hosting

 

[moodydan]

It does look very close, I'll check this afternoon after work and see.

 

[NOSValves]

That's an excellent suggestion!

I'll second that.

 

[moodydan]

Well, while checking voltages the sound cut in and back out - then I got a spark at one of the 20uf posts on the can cap when measuring. I checked and double checked, but I wasn't grounding to the chassis. I suppose there's a short inside the can, so I'll pop the old can back in (it measured within specs, anyway) and see how that works. Hopefully I can get replacement for the bad one, don't know AuthentiCaps failure rate...

 

[gadget73]

see if it wasn't just a bad solder connection suddenly making contact again.

 

[moodydan]

see if it wasn't just a bad solder connection suddenly making contact again.

I did before last post, cleaned off original solder and re-soldered. I was hopeful that was it, but smoke started coming off the 22k resistor, too.

 

[gadget73]

fair 'nuff. Sounds like something is shorted then.

 

['goodtimes']

Hello,
My St-70 has a left channel problem. The left channel is not as loud as the right and also is distorted. I have changed tubes (one at a time-rectifier, output, and driver) with no change whatsoever. I have tried known good used tubes and NOS NIB known good tubes with no change. I have checked resistors and caps and all seem to be in spec. Another odd thing is it will bias and hold the setting on both channels.

Upon checking voltages I get the following;

GZ34-
Pin 2 to ground - 454 VDC

Pin 8 to ground - 454 VDC

Pin 4 to ground - 374 VAC

Pin 6 to ground - 375 VAC

All EL 34's are the same give or take 1V
Pin 1 to ground - 1.5

Across pins 2 and 7 - 6.0 VAC

Pin 3 to ground - 437 VDC

Pin 4 to ground - 438 VDC

Pin 5 to ground - -24 VDC
Pin 6 to ground - -24 VDC
Pin 8 to ground - 1.5 VDC

Quad Cap
Section #1 - 454 VDC Half circle symbol back side of amp - one choke lead and wire to pin 8 of the rectifier tube

Section #2 - 442 VDC square symbol left side of amp - another choke lead and the two wires from the two output transformers

Section #3 - 373 VDC full circle symbol on right side of amp

Section #4 - 287 VDC triangle symbol toward front side of amp

I have installed the new "upgraded" quad cap where
half circle = 80uf
square = 40uf
triangle = 30uf
full circle = 20uf

Any help or suggestion would be greatly appreciated.

 

['goodtimes']

You might want to create a new thread for this.

No problem, I was not sure if that was the better way to go about this.

 

[eiraved]

Hello, My St-70 has a left channel problem

Welcome to AK!

As Peter recommended, create a new thread to get the answers you need, This thread is for moodydan's issue.

 

['goodtimes']

Welcome to AK!

As Peter recommended, create a new thread to get the answers you need, This thread is for moodydan's issue.

Thanks!
I am on it.