One channel dead?

[valco]

So I did two mods on my Dynaco ST70 and now the right channel is silent. Any help woud be appreciated!

I replaced the rectifier tube with UF4007 diodes. and used a 10w/50ohm resistor on the output of the diodes to bring the voltage down a bit. I also added a CL90 on the AC wall line, before it hits the fuse.

I also added two .22uf input caps to remove with any voltage that may be on the input when using various preamps.

The voltages all over the amp are great. Except for the right channel. The two right channel EL34s both read only about 100vDC on pins 3 and 4. They should be around 410vdc each.

For the diode mod, I just put a diode from pin 4 and from pin 6 to one end of a 10w/50ohm resistor. Then ran the other end of the resistor to pin 8. The voltage reads as it should.

For the input caps, I followed audioregenesis's instructions:

1. Remove the two 470K resistors currently at the input jacks. If you do not have replacements, remove them carefully, as you will reuse them in step 3.
2. Remove the two wires currently connected between eyelets 7 & 17, and their respective input jack. Install two .22 uF 100 volt caps of your choice in their place. The voltage rating of the caps should be kept low to keep the size small.
3. Reconnect one of the removed 470K resistors directly between eyelets 10 & 17 for the Right channel, and the other between eyelets 7 & 8 for the Left channel.
4. Install two 1 meg 1/4 watt resistors in the positions originally occupied by the two 470K resistors in the original build.
This procedure effectively places an appropriate coupling cap in series with each input of the ST-70, so that any low level DC that might be part of the input signal is blocked, and can no longer induce any LF instability.

 

[valco]

Thanks. The quad cap reads perfect voltages.

 

[s-petersen]

i would check the input cap and resistor connections, also the Mono / stereo switch, try switching to mono to see if anything changes

 

[valco]

Dah! Thank you all! Found it. It was that red wire from the transformer that connects to the quadcap. If I wiggle it, the sound comes back. Resoldered and its all good now.

 

[valco]

Yes! TEAM DYNACO!
You guys are the best. Thanks!

Also built a bucking transformer. That in combo with the CL90, the amp is now running at its correct voltages and a much cooler temperature too. I’m done messing with this amp now.

Except maybe some new power tubes....

 

[valco]

The ST70 is totally stock values. I rebuilt the entire thing with new PCBs, caps, resistors. Using original transformers.

Paired with a rebuilt to stock PAS-2. Except I removed the tone controls.

I built my own tone controls using the LM1036 IC. Which are removable with the flip of a switch. They do a great job and don’t mess up the sound at all. I use them to just bump up the bass and treble a bit.

Turntable is a Rek-o-Kut L34 Rodine Jr w/ an ESL S-1000 tonearm and a Shure M3D.

All into a pair of recapped/redoped KLH Model 6 speakers.

I really enjoy the sound. Its wonderful on older recordings like vintage jazz and classical stuff. Some modern stuff needs a little bit of help to my ears to reach a bit lower and higher. That’s why I have the tone controls. No extreme adjustments. They really open up some modern recordings.

I’m gonna order some KT-77s pretty soon to try out.

Amazingly, this entire system was given to me by a friend of a friend for free. The original owner (the father) passed away. I promised the family it would be in good hands, and that I would do some restoring to keep it running far into the future.

 

[valco]

What’s your setup, petercapo? Do you use the stock driver, or something else?

I know those VTA boards are popular. I’ve never heard them myself.

 

[gadget73]

I'd still love to see a full shoot-out of the various Dyna driver boards as compared to the stock circuit. The results might be interesting.

 

[stanko]

I'm with you.

I'd still love to see a full shoot-out of the various Dyna driver boards as compared to the stock circuit. The results might be interesting.

 

[gadget73]

I don't mean so much a subjective listening shoot-out, but a test gear report. I get that data isn't all of the story but its the only objective means of comparing things. I'm just curious what exactly the test gear has to say changes when a different topology driver is installed.

Subjective tests are impossible to prove anything with. Ask 10 people, get 10 answers, and they are all correct since its just an opinion.

 

[c_dk]

What would be very interesting is a series of bench tests collaborating the perceived sound differences between various tubes used in a chosen reference am or preamp.

A $200.00 vintage masterpiece, that receives all the accolades, compared to a mundane $10.00 current production tube should be different enough.

There has to be some sort of measurable in circuit difference.....correlating the measured differences to the gushing verbiage could be tough.....

 

[primosounds]

What would be very interesting is a series of bench tests collaborating the perceived sound differences between various tubes used in a chosen reference am or preamp.

A $200.00 vintage masterpiece, that receives all the accolades, compared to a mundane $10.00 current production tube should be different enough.

There has to be some sort of measurable in circuit difference.....correlating the measured differences to the gushing verbiage could be tough.....

The price of a vintage or rare tube is not really based on sound but rather on the perception and reputation or outright rarity of such a tube. So, doing a comparison would be nice but getting all the tubes that need to have the same test results including inherent noise level would be no small matter.

 

[c_dk]

I am also sure that there are many bottles of rare vintage wine that now contain vinegar....until you open one how do you know?

It is easy to make a sound pleasurable to a individual.....just push the right frequency distribution buttons to please a individual and you can make something sound good to them, but maybe not to someone else.

Creating a stereo image is more complicated but still has to be a interaction of those two audio streams we call L and R.

Has anyone made even a simple basic comparison of the frequency and noise characteristic of various tubes?

I have a mostly stock 3x sitting around that has been loaned out a few times and pronounced as sounding great. With such a simple few tube circuit it should make a decent 12x test mule.

Probably time to start a new thread.

 

[valco]

Wow, came here to update on the power tubes and saw all these new posts adding to the discussion. Not sure why audiokarma stops letting me know there are new responses.

I'm sure the different driver boards sound great, as so many people enjoy them, but one of the main reasons I stuck with rebuilding the original circuit, besides the fact that I was really enjoying the sound of the stock unit, was because I could not find any actual details on these new driver boards. I just kept reading or being told that "they sound better."

Anyway, I'd put in the quad of JJ KT77s and wow am I happy! They did exactly what I read and hoped they would do. The treble is improved so much that I turned off the tone control unit I built. I can add or remove the controls with a flip of a switch to go back and forth. And they are certainly not needed anymore. The treble has opened up right to where I wanted it yet still sound so smooth. Bass and midrange are great too.