ST-70. SS Diodes or Rectifier tube? Both?

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ST-70. SS Diodes or Rectifier tube? Both?

Postby jbeezy » Thu Jan 01, 2009 3:01 pm

If hybrid is the way, and excuse my ignorace, but wouldn't I solder in the diodes the same way regardless?
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Postby burnedfingers » Fri Jan 02, 2009 6:11 am

If you want to have the ability to use either SS diodes or a tube rectifier then I would suggest that you either buy a plug in SS rectifier or make one. This will allow you to plug in the SS rectifier when you wish. Since the voltage will be higher you will naturally have to rebias the output tubes. There are companies that make a ready to use SS rectifier or if you wish you can build your own. I use a base from a worn out octal tube. I break out the glass/base and solder in diodes. I test it to make sure it works properly and when I'm satisfied I fill it with a black silicone.
This type of modification allows either the use of the tube or plug in SS rectifier.

If you would like to take the strain off the 5AR4 rectifier you can solder in a diode from pin 3 to pin 4 and attach the AC to pin 3. You can solder in a diode from pin 5 to pin six and put the AC on pin 5. This will allow the diodes to rectify the AC to DC and the slow turn on of the 5AR4 will slowly ramp up the DC just like the tube would have done by itself. You still have the option of unplugging the 5AR4 and plugging in a SS rectifier.

I first saw this mod in a pair of Quicksilver KT88 Mono Block amplifiers that I purchased.I never had a problem keeping the two 5AR4 rectifiers alive in that one. Later I purchased a pair of 8417's and couldn't keep a pair of 5AR4's in longer than about 3 months. I made the modification and I have been doing it on every tube amp I own since then. It seems to take the strain off the 5AR4's and keep them running.

There are those that might argue about using a Weber cap in place of the 5AR4 and you certainly could do that. Their main arguement is going to be that the high voltage comes on instantly with the SS diode when used alone. Well, it does. But keep this in mind....tube equipment has been using rectifiers like the #80. 5U4, 5R4 and others in tube amplifiers for years and they have lived. One main thing to keep in mind on any tube equipment and that is NO SIGNAL until the unit has warmed up. Allowing signal is what causes cathode stripping.
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Postby jbeezy » Fri Jan 02, 2009 8:44 am

First of all, thank you.

So, the technique of soldering two pair of 1N4007 diodes onto the tube socket (twisted/soldered together cathode ends to pin 8 and the two free anode ends to pins 4 and 6) is only used in place of the tube? I don't mind doing this either but I'm afraid (to be perfectly honest) that I'd feel like I'm missing out on even the slightest tube glow. I'd love to keep the tube in and have no problem wasting power lighting the filament.

I am intrigued about making my own ss tube out of an old busted one though. Never done that.

-jb
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Postby burnedfingers » Fri Jan 02, 2009 9:25 am

Quote:
So, the technique of soldering two pair of 1N4007 diodes onto the tube socket (twisted/soldered together cathode ends to pin 8 and the two free anode ends to pins 4 and 6) is only used in place of the tube?

Correct

If you want to use diodes only then you can solder them as you mentioned.

Or

If you would like to use a plug in SS rectifier then you can make it as I mentioned or purchase one.

If you would like to have the hybrid then you can solder the diodes in as I mentioned and use your 5AR4.

I like to use diodes and 5AR4 as I mentioned. Best of both worlds.
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Postby jbeezy » Fri Jan 02, 2009 1:19 pm

Brilliant. Thank you so much. I'll go hybrid.

Does anyone think this means they are selling a matched quad of EL34s for $39

http://www.diyhifisupply.com/?q=node/130

-jb
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Postby TerrySmith » Fri Jan 02, 2009 2:54 pm

Those are Valvefart tubes, made in China. Make sure you fuse the cathode on these for when they go pyrotechnic! :parp: :hothead:
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Postby jbeezy » Fri Jan 02, 2009 3:16 pm

So, I put in the diodes as described for a hybrid full wave rectifier. I feel like the st-70's sound quality improved. But that could be bullshit like when you think your car goes faster after you buy new floor mats.

Anyway, the bias seemed odd as it was completely out of whack for the two channels and I can't get it to bias anywhere above 1.09.

Again, I'm sure I'm showing my ignorance.
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Postby burnedfingers » Fri Jan 02, 2009 4:24 pm

Have you changed your coupling caps?

Have you re built your bias circuit? Pulled the selentum (SP) rectifier and replaced with a 1N4007 diode? Replaced the bias caps?

Adding the diodes to the rectifier 5AR4 circuit has no effect on your biasing. It could be as simple as bad output tubes also.

Also, if it were me I would pass on the tubes from china.

Do a individual tube bias modification and buy some 6BG6GA's at $8 ea.
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Postby burnedfingers » Fri Jan 02, 2009 4:29 pm

So, I put in the diodes as described for a hybrid full wave rectifier. I feel like the st-70's sound quality improved. But that could be bullshit like when you think your car goes faster after you buy new floor mats.


Its kind of like purchasing high dollar capacitors. You spent the money and your mind tells you it sounds better because its XYZ brand.
Personally I felt the bass sounded a little tighter but then again it could have been the 1/2 bottle of wine I drank Yellow_Light_Colorz_PDT_01
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