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Dynakit ST70 - Unopened and Untouched?

It would. Heck, with a 4 amp 5v winding I'd be extremely tempted to run dual 5AR4s. Move the caps to the bottom, put the second socket where the can is. Even if modern rectifiers aren't quite as stout, I'd bet a parallel pair of the worst modern ones money can buy would handle more current than a single old one that costs several times as much.
 
gadget73 said:
It would. Heck, with a 4 amp 5v winding I'd be extremely tempted to run dual 5AR4s. Move the caps to the bottom, put the second socket where the can is. Even if modern rectifiers aren't quite as stout, I'd bet a parallel pair of the worst modern ones money can buy would handle more current than a single old one that costs several times as much.
Click to expand...
Dual rectifiers would be a definite improvement. With that power transformer I would be inclined to use two rectifiers if it were mine.
 
This is excellent information guys, it is greatly appreciated! I will order a new new 62 ohm choke and new capacitors. Seeing as this unit is already modified, why not take it to its full potential and add a second 5ar4!

However, I have no experience in performing these modifications, I am confident that under correct guidance I can do it. This being said, would anyone care to continue helping me along the way?

I have tested the tubes with an Eico 635 and the 5u4gb tested weak and one of 7199 didn't turn on at all. Otherwise they all tested around 70 after 1 to 2 minute warm-up.

Here are the tools I currently have at my disposal. I will aquire whatever tool or material is required to get the job done correctly.

Soldering iron (Amazon)+ 3rd hand
Tektronix Type 549 Oscilloscope
Hewlett Packard 524C + 525A frequency counter
Philmore Variac
Multimeter: MTP 2327
Eico 635 Tube Tester
 
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don't let my crazy ideas distract you too much. I tend to get distracted by possibilities.

Verify the bias supply works, don't even need tubes in the amp to do that. You should see somewhere around -40v at pin 5 of the EL34 sockets and it should change with the bias control. Maybe just change those light blue caps in the corner before even trying to turn it on. Set the bias control to the most-negative voltage at pin 5.

If all that is good, put the tubes in and bring it up slow on the variac. Even a weak 5U4 should give you enough to see if its got signs of life. Dead 7199 will make one channel not work but thats not terribly important just yet.
 
Gadget gives solid advice and I concur.
Bring it up and check it out as is.

Long term, plan on a second 5ar4. I'll even send you a free tube socket in the mail.
The previous owner took care of 90% of the task of making this a hotrod 70, which really performs. A sleeper so to speak. They did all of the heavy lifting (transformers aren't cheap) and you just get to put the icing on the cake.
 
Depends on voltage output level, either will restrict full power out potential. Should have really been re-transformered for silicon rectification with a heavier choke.
 
Oh boy... I would like a shot at this one.....
Congrads.. although not the holly grail... it still a unbelievable good amp if done the mods the right way... Yes "hot rodding" is the right word for it.

Please don not ever put Rectifier diodes in there instead of the rectifier tube(s)...It is the same as drinking a real good wine out of plastic cup instead of nice glass... It works but not the what the Tube God's/Allah's/Budah's/Burning-man intended. And there are also technical issues I do not want to get into now..

I think you did the right thing to Check the tubes in a Tester... Doing some ohms measurement on the transformers will give an indication about there condition..
After that... run those with a variac to start... and let them run an hour or two... No load... just run them...

New power supply caps (same value or very near on the UF. Higher voltage is okay, lower voltage results in loud hissing and white smelly smoke)...new "un-roasted" choke... and new bleed resistors...Clean/ deox the tube sockets and stick a new Rectifier... again let it run a few hours.... Supply transformer should get cozy warm now...check voltages should be higher then stated in the service manual...(there is no load on the running supply).

Shutdown.... and let it bleed...
Wiggle new matched power tubes in...Power up on a resistor speaker-load (dummy)...check voltages and keep an eye on the power tubes... red cheeks are not good.. that is the result of a to high Bias (to close to zero) and therefor to much current... Do not try to set the bias yet.... keep checking voltages and the cheeks while running it this way...

A nice way to check the MA running though a power tube is lifting the kathode from ground with a 1 ohm 3 watt resistor... you should measure about 35mv up to 45mv depending on High voltage over that resistor...

Shutdown if you are happy (or not) and correct if there is a problem, or stick the phase shifter tube in...Power up... let it heat up...

Well I think you got an idea by now how you COULD (not "must") get this beauty back up running flawless...
As many AK members..We would be glad to help you get it back up and running....that sweet Dynaco!
 
VintageDutch said:
Oh boy... I would like a shot at this one.....
Congrads.. although not the holly grail... it still a unbelievable good amp if done the mods the right way... Yes "hot rodding" is the right word for it.

Please don not ever put Rectifier diodes in there instead of the rectifier tube(s)...It is the same as drinking a real good wine out of plastic cup instead of nice glass... It works but not the what the Tube God's/Allah's/Budah's/Burning-man intended. And there are also technical issues I do not want to get into now..

I think you did the right thing to Check the tubes in a Tester... Doing some ohms measurement on the transformers will give an indication about there condition..
After that... run those with a variac to start... and let them run an hour or two... No load... just run them...

New power supply caps (same value or very near on the UF. Higher voltage is okay, lower voltage results in loud hissing and white smelly smoke)...new "un-roasted" choke... and new bleed resistors...Clean/ deox the tube sockets and stick a new Rectifier... again let it run a few hours.... Supply transformer should get cozy warm now...check voltages should be higher then stated in the service manual...(there is no load on the running supply).

Shutdown.... and let it bleed...
Wiggle new matched power tubes in...Power up on a resistor speaker-load (dummy)...check voltages and keep an eye on the power tubes... red cheeks are not good.. that is the result of a to high Bias (to close to zero) and therefor to much current... Do not try to set the bias yet.... keep checking voltages and the cheeks while running it this way...

A nice way to check the MA running though a power tube is lifting the kathode from ground with a 1 ohm 3 watt resistor... you should measure about 35mv up to 45mv depending on High voltage over that resistor...

Shutdown if you are happy (or not) and correct if there is a problem, or stick the phase shifter tube in...Power up... let it heat up...

Well I think you got an idea by now how you COULD (not "must") get this beauty back up running flawless...
As many AK members..We would be glad to help you get it back up and running....that sweet Dynaco!
Click to expand...
I really appreciate the help, thank you! I have a spare 5ar4 on hand, should I swap it for the weak 5u4gb before warming it up on the variac?
 
You get the opportunity handed to you to avoid the light AB1 bias point, and treat the xf1 screen grids more gently with that 300-0-300 plate TX. Don't waste it... :)
cheers,
Douglas
 
Check the fuse..so no damage is done if things go bad. Consider running 6550s, using diodes & one of those can caps that have 80uf-40-40-40 550volt or more uf... HK citation II & McIntosh used diodes. Its not a sin...
 
I just wanted to check with you all before proceeding, this is what I have done:

-ST-70 is plugged into Variac
-ALL tubes removed from circuit
-Shop speakers (sacrificial) connected to ST-70

I will bring up the variac to 25 volts and then measure the voltage at pin-5. Once warmed up, I will then adjust the bias to see what happens to the voltage.

If this performs as it should, I power down the amplifier and let it rest for an hour or so, then place the tubes back into the circuit, connect preamp/source and bring it back up slowly on the variac and then try it with a signal?

I apologize for the "hand-holding", but I would rather be safe than sorry!
 
Warm up what exactly? Theres no tubes in it.
Fire it up and make sure your grid voltage is negative and changes with the bias pot.
If it is, stick some tubes in it, see what they do.
You should have negative grid voltage as soon as current flows in the primary.
Nothing needs to warm up to make that happen...
By all means go as slowly as you need to, in order to be safe, but nothing needs to warm up to verify bias voltage.
 
trainbuftony said:
Warm up what exactly? Theres no tubes in it.
Fire it up and make sure your grid voltage is negative and changes with the bias pot.
If it is, stick some tubes in it, see what they do.
You should have negative grid voltage as soon as current flows in the primary.
Nothing needs to warm up to make that happen...
By all means go as slowly as you need to, in order to be safe, but nothing needs to warm up to verify bias voltage.
Click to expand...
That makes perfect sense lol, I guess I was simply warming up my courage.
 
I plugged in the amp, no tubes but with a pair of speakers connected. Set the MM to 200V range and measured pin-5, positive probe to pin-5, negative to chasis. It reads the same on both sides and ranges from 12.5 to 18.5 when I adjust the bias control.
 
Leviasean15 said:
I plugged in the amp, no tubes but with a pair of speakers connected. Set the MM to 200V range and measured pin-5, positive probe to pin-5, negative to chasis. It reads the same on both sides and ranges from 12.5 to 18.5 when I adjust the bias control.
Click to expand...
What's your line voltage when these measurements were taken?
Also
12.5 to 18.5 or -12.5 to -18.5?
You need negative voltage there, and IIRC, under operation you're gonna need more (negative voltage) than that.
 
I have the ST-70 plugged into 120v wall outlet, no preamp connected. This is how I currently have the MM set.
 

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Your leads are plugged in backwards but yeah that's 18v, 18v with backwards leads makes -18 volts
Coffee is hittin hard this morning, mind is over alert. Sharp as a tack.
Normally I'd totally rebuild the bias supply before I realize my meter is backwards.
 
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