Dynaco ST70 Refurb - HELP, Dim Bulb Lighting up when GZ34 is installed

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Yamariv

Member
Hey Guys!

About a month ago I picked up a Dynaco ST70 off of Ebay sold to me in full working condition and it arrived safely. Prior to me starting the upgrades, replacing the caps, rewiring the 3 prong cord properly, installing new RCA jacks etc I plugged it in for a quick test. I installed the EL34's with no GZ34 and everything was fine. When I installed the GZ34 and powered it back on, after about 10 seconds my Dim bulb(300w) started to glow pretty bright, not white bright but a big orange glow.

Now, I hoped with all the cap changes and upgrades I was going to do, I may fluke out and solve the problem while I upgraded it and wen't ahead replacing parts. Here's all that I did:

- Replaced the 2 Bias grid Caps with 47uf /500v. Checked the resistors tied to them and they were in spec
- Replaced the 4 Cap Can with a new one from Dynakit (30/20/20/20), all 4 caps read great prior to install
- Replaced the 6 caps on the PCB board with new Zoso Caps
- Installed new RCA jacks on the front
- Wired the 3 prong power cord correctly, black going through the switch then to the fuse and onto the power transformer
- Reflowed all connections on EL34's and GZ34 sockets (just incase)
- Reflowed all connections on PCB that looked a little dry
- Changed the Rectifier to a Diode from the Selenium one (as part of later trouble shooting)

Before changing the speaker connectors out for upgraded banana plug style, I decided to plug in the amp and tubes and see what she does.

As I plugged it in it did the same thing it did prior to all the mods. All the EL34's and the 7199's light up normally. When I input the GZ34, it lights up normally but after about 10 seconds, my dim bulb starts to glow and I turn it off..I'm not a pro at diagnosing, and have no formal electronics training so go easy on me ;)Here's a list of things I know so far which I hope can help you pros point me in the direction of where the problem could be..

- All tubes check out perfect on my B&K 707 Refurbished and Calibrated tube tester
- I've tried two different GZ34's with the same results
- I've checked all the wiring and cannot for the life of me find any possible shorts or miss run wires
- I have absolutely no reading at the 1.56 Bias test point on the front of the amp.
- I have no large voltage at all on any of the EL34 socket pins
- I have no large voltage at the GZ34 pins when it is not installed
- I have no large voltage at the 4 Cap Cap at all
- I do have -70 ish Volts at the now replaced Bias Diode going to the bias caps. I have about -7 volts on the other end of the diode
- The bias caps are getting the minus voltage from the Diode.
- The 15.6 Ohm resistors under on the EL34 sockets test great as well do the resistors on the 4 Cap Can.

Any and all advice is appreciated, I've reached my troubleshooting knowledge limit unfortunately. I hope this is something simple..

Thanks!

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Have you plugged it in to the mains with speakers and a source ? The first thing to do
is adjust the bias according to the manual.
Next is to compare the voltages with the given in the manual.

Is there any anomalies here ?
 
\ As the rectifier begins to conduct it will draw more current when all is well. The problem is when all is not well, it will draw a lot more current. You won't get any HV without the rectifier, and with no HV there will be no current flow so nothing reads at the bias test points on the front.

Try this
Remove all of the tubes except the GZ34
see if the lamp gets bright after 5-10 seconds. if it does, it means there is a short somewhere in the HV supply. If it doesn't get a bunch brighter, the HV side is OK. If you have a small lamp, say 100w, that will work fine for this test. It will have some amount of glow just because of the draw of the rectifier tube, but you don't want to see it start getting bright once the rectifier begins to conduct. It might flash a little as the caps charge up but it shouldn't stay bright.

Since the lamp doesn't go to near full brightness, I'm going to guess you're OK.
 
gadget73 said:
\ As the rectifier begins to conduct it will draw more current when all is well. The problem is when all is not well, it will draw a lot more current. You won't get any HV without the rectifier, and with no HV there will be no current flow so nothing reads at the bias test points on the front.

Try this
Remove all of the tubes except the GZ34
see if the lamp gets bright after 5-10 seconds. if it does, it means there is a short somewhere in the HV supply. If it doesn't get a bunch brighter, the HV side is OK. If you have a small lamp, say 100w, that will work fine for this test. It will have some amount of glow just because of the draw of the rectifier tube, but you don't want to see it start getting bright once the rectifier begins to conduct. It might flash a little as the caps charge up but it shouldn't stay bright.

Since the lamp doesn't go to near full brightness, I'm going to guess you're OK.
Click to expand...

Ok interesting!.. So there's a chance this is normal behavior..

Dumb question, is there a way to test all this without a speaker load and source or is that the only way to do it. At the time I was testing it, I had no load on it, just the unit by itself in the garage.

Second, would this GZ34 only test work with my 300watt dim bulb or are you saying to switch it to 100w, I'm not sure I have a real 100w in the house anymore, eek? I'll try to try this later tonight, so if the bulb glows a little bit with my 300w bulb with the rectifier powered up that could be normal? If it does, do I keep it on and try to start testing voltages at the EL34's pins?
 
Think you are fine with the 300 Watt, you need to get the output on either a dummy load or speakers. A normal amp will dim the bulb. Hook it up and give it a listen, you may be surprised!
 
If there are no output tubes, nothing to worry about. If the output tubes are in you want it connected to a dummy load.

You could probably do this with a 60w if thats what you have. It won't pull all that much current with just a single tube in. If the lamp doesn't get bright you should get some 500v+ at the pins on the EL34 sockets. Don't leave it on very long, the voltages will be much higher than normal.
 
Yamariv said:
Ok interesting!.. So there's a chance this is normal behavior..

Dumb question, is there a way to test all this without a speaker load and source or is that the only way to do it. At the time I was testing it, I had no load on it, just the unit by itself in the garage.

Second, would this GZ34 only test work with my 300watt dim bulb or are you saying to switch it to 100w, I'm not sure I have a real 100w in the house anymore, eek? I'll try to try this later tonight, so if the bulb glows a little bit with my 300w bulb with the rectifier powered up that could be normal? If it does, do I keep it on and try to start testing voltages at the EL34's pins?
Click to expand...
Nm, you MUST have a load, prefferably speakers but a power resistor will do(but then you don't know
what happens)
 
Ruidoso Bill said:
Think you are fine with the 300 Watt, you need to get the output on either a dummy load or speakers. A normal amp will dim the bulb. Hook it up and give it a listen, you may be surprised!
Click to expand...

Ok, will do! I just got worried cause the last time I saw the bulb light up dimly was my first Tube Tester that had a bad transformer in it. I saw the bulb dimly light up again with the ST70 and had flashbacks! Lol

I'm out of town this weekend unfortunately but will test her asap when I return. I'll start her with the GZ34 only first then if all goes well, I'll hook it up to some speakers and see what she does. :rolleyes:

I'll definitely keep you guys posted as soon as I get to it, thanks for all the help so far :beerchug:
 
In addition you failed to to the most basic test BEFORE starting to replace stuff : connect and listen, noting any problems. Now if you encounter a problem afterwords you do not know if you have done
a mistake or if this was a problem before you started.
 
UPDATE: Got around to testing the high voltage tonight with only the GZ34 in the amp and the EL34's removed. Good news, I had 500+ Volts DC at the GZ34, the El34 sockets and the Can Capacitors. Only left the amp on for 10 seconds at a time while I checked the readings and the 300 watt dim bulb did not light up or glow at all which is awesome.(I have some 100w bulbs coming for later tests..) Now for the hookup to speakers, bias and to hopefully hear this thing play music another night if all goes well!

Question: I purchased the Speaker Banana Plug upgrade kit from Dynakit and installed it tonight. Everything went smooth but I'm not sure where to reconnect the wires coming from pins #12 and #13 on the PCB which used to connect to the 16ohm speaker output. The Dynakit eliminates the 16ohm tap which gets soldered to a single terminal strip which eliminates it. Do I connect that wire to the terminal strip with the no longer used 16ohm tap or does this wire now have to go onto another live tap like the 4 or 8ohm speaker out?

Thanks for all your help guys, very much appreciated!
 
You should have a common ground and then just select between the 4Ω or 8Ω connections depending on your speakers. Dunno if this is what you mean? Some pics would help.
 
Feedback is to be connected to the 16 ohm output from transformer.
When the output post is removed one has to install another soldering post inside.
 
^^ this, the original connection to the 16 ohm tap needs to stay connected to the 16 ohm wire from the transformer.
 
Hey Guys!

So just to be sure we're talking about the same thing I took a pic this morning. In the first pic you'll see the new speaker jack setup which doesn't have enough room for a 16ohm output anymore so they supply a terminal lug which I've installed on the left side. I've soldered the 16ohm wire from the transformer to the lug but am not sure what to do with the extra black wire coming from the PCB board that was originally wired to the 16ohm tap. Does it get soldered to the terminal lug with the old 16ohm tap like I've done or does it go to a live tap like the 4 or 8ohm posts now? I've also attached the schematic to show the wire I'm talking about, it's on the very far left and goes to the #12 of the PCB board. There is an identical wire coming from #13 of the PCB for the other channel. Thanks!

Dynaco.jpg Dynaco1.jpg
 
You did it right. The black wire takes a small portion of the output waveform and injects it into the cathode of the first tube. The result is a reduction in gain, and a reduction in hatmonic distortion. With the right network in place the designer of the amp can tune for the limitations of his other parts. Opinions will vary, but that wire (the global feedback) is everything. Without it you will never approach full power and you certainly will not get low distortion.

Then again, dont let me tell you what to do.
Snip that feedback out and have a listen, you wont break anything by doing so.
 
trainbuftony said:
You did it right. The black wire takes a small portion of the output waveform and injects it into the cathode of the first tube. The result is a reduction in gain, and a reduction in hatmonic distortion. With the right network in place the designer of the amp can tune for the limitations of his other parts. Opinions will vary, but that wire (the global feedback) is everything. Without it you will never approach full power and you certainly will not get low distortion.

Then again, dont let me tell you what to do.
Snip that feedback out and have a listen, you wont break anything by doing so.
Click to expand...

Awesome, great to know it's right, Thanks! :beerchug: I kept thinking the black wire was taken out of play by putting it on the terminal lug but I guess it's still in play which makes more sense. Cool, thanks again!
 
Follow the wire. It should come off the 16 ohm tap. One end of that winding is grounded. It then goez through a resistor in paralell with a small cap. Then it should go to a voltage divider in the cathode circuit of the first tube, where it has one more resistor to pass through to get back to ground, which makes a complete circuit. Make sense?
 
trainbuftony said:
Follow the wire. It should come off the 16 ohm tap. One end of that winding is grounded. It then goez through a resistor in paralell with a small cap. Then it should go to a voltage divider in the cathode circuit of the first tube, where it has one more resistor to pass through to get back to ground, which makes a complete circuit. Make sense?
Click to expand...

Oh yeah, makes sense for sure. I just meant for some reason I was thinking I was grounding the one end by connecting it to the new terminal lug with the 16ohm tap..just a brain fart. The way it's done now, the black wire is still functional, very cool. Thanks again!
 
You'll make full power without the feedback, and it will do it very easily. The amp will be very sensitive. It will also have much higher distortion and worse frequency response, along with higher output impedance so it will be affected more by the speaker load.

Tl;dr, it will sound bad.
 
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