variac damage

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variac damage

Postby nyazzip » Mon Apr 21, 2008 10:16 pm

i recall reading about people with antique radios, starting them up slowly with a variac, and actually being able to eventually "re-form" the bad capacitors to functional operation...don't know if this is true but i suppose it could be.
anyway i have been playing around running my 5 watt epiphone valve junior at low voltages(it makes sound with as little as 40-50 volts!!!) to get warm fuzzy distortion. but it occurred to me that maybe enough of this low voltage operation might "reverse-form" the capacitors, so they eventually won't work properly at normal voltages...? do you think its a bad idea to run a single ended valve amp at low voltages for long periods of time? can this damage tubes as well?
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Postby Cygnus X1 » Mon Apr 21, 2008 11:41 pm

Someone more knowledgable will be able to answer your question more clearly, but I have heard enough to say DONT DO IT!
The variac is great for some things, I have formed some old caps in a pinch,
but I never run it on an amp that has tubes in it. I just use it to slowly power up after changing power filter caps. Running the amp below 105 VAC is getting into dangerous territory.
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Postby EWBrown » Tue Apr 22, 2008 9:08 am

I remember from doing product safety testing that for the 100-120VAC voltage range, that 90VAC was the minumum and 132 VAC was the maximum. And that was for more modern SS equipment., using regulated switching power supplies. Yellow_Light_Colorz_PDT_11

I'd SWAG that going down to 90VAC isn't harmful, but going down to 40-50 volts is just asking for problems,. I wouldn't go above the local line voltage, either, as that would overstress things real quickly.

I'd suggest, if you like that "fuzzy" sound, try a pin-compatible rectifier tube with higher forward voltage drop (or use a run down tired old rectifier on its last legs) Yellow_Light_Colorz_PDT_05

/ed B in NH
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Postby nyazzip » Tue Apr 22, 2008 7:18 pm

i appreciate the heck out of the responses, but can you tell me why it might be a bad idea to run low voltage? just wondering what component i need to fret over first. at 50 volts the tubes are basically unlit. the in/out trannies remain cool to the touch, if that means anything. i haven't been dumb enough to run the st35 at low voltage, as it cost 5x what my VJ amp did. that, plus, i soldered it, not some chinese children(wink). plus, the goal there is high fidelitity, unlike the guitar schtick
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Postby Cygnus X1 » Wed Apr 23, 2008 12:20 am

Cathode stripping can occur if the heater isn't doing its job.
When the tube heaters see less than 6 volts, they aren't heating the tubes sufficiently, even though there may still be just enough to operate the tube.
Variacs are sometimes used in vintage, or vintage style guitar amps to alter the tonal character. But dropping to less than 100 volts is generally considered to be not healthy for the componenets. Sorry, I don't know all the details, it's kind of like common knowledge, but I can't tell you exactly why.
Such as, a power brown out can cuase as much damage as over-voltage to power supplies. And, the personal danger involves floating grounds, and the
ground point can shift to unexpected places. Such as yourself.
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Postby nyazzip » Sun Apr 27, 2008 2:32 pm

....but i have read that cathode stripping does not apply at voltages below 10,000(ionizing energies). the floating ground sounds a bit spooky though
i was thinking more about the transformers and caps
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Postby Cygnus X1 » Wed Apr 30, 2008 4:14 pm

[quote="EWBrown"
I'd suggest, if you like that "fuzzy" sound, try a pin-compatible rectifier tube with higher forward voltage drop (or use a run down tired old rectifier on its last legs) Yellow_Light_Colorz_PDT_05

/ed B in NH[/quote]

That is a viable option.
I use different recto's in my 67 Deluxe Reverb to get different sounds.
BUT...it was designed to use the GZ34. Which means it can safely go both ways, a little up, a little down. A 5Y3 will work, and it makes it distort warmly, but it turns to mud fast.
A 5U4GB also works, and makes it "brown", almost tweed sounding.
The GZ34 makes it bright and happy.
Just watch you current draws, and make sure you moniter the health of your power transformer when playing with recto options.
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Postby wyatt » Wed Apr 30, 2008 11:37 pm

Cygnus X1 wrote:
EWBrown wrote:I'd suggest, if you like that "fuzzy" sound, try a pin-compatible rectifier tube with higher forward voltage drop (or use a run down tired old rectifier on its last legs) Yellow_Light_Colorz_PDT_05

/ed B in NH


That is a viable option.
I use different recto's in my 67 Deluxe Reverb to get different sounds.
BUT...it was designed to use the GZ34. Which means it can safely go both ways, a little up, a little down. A 5Y3 will work, and it makes it distort warmly, but it turns to mud fast.
A 5U4GB also works, and makes it "brown", almost tweed sounding.
The GZ34 makes it bright and happy.
Just watch you current draws, and make sure you moniter the health of your power transformer when playing with recto options.


Don't ever put that 5U4GB back in to that amp.

Careful what you use in some Fender amps.

A 5U4G is a 3A rectifier tube and will put an hefty strain on the BFDR's "barely big enough as is" power transformer. Try a 5V4G instead.

When SF Deluxe Reverbs switched to 5U4G rectifier tubes in the late '60's, they also put a beefier power transformer to supply the extra current to the 5V tap.

In turn, the new SF power supply also supplied more VDC to the plates to compensate for the bigger voltage drop of the 5U4G. So, if you were to try and run a 5AR4/GZ34 in a SFDR built for a 5U4G, you'll push the plate voltage beyond the limits of most NOS and current made 6V6GT's.

So, run of thumb...in BFDR's never use a 5U4G or a SS plug-in and never try and run 6L6's. In SFDR's, never use a 5AR4/GZ34 or SS plug-in unless switching to 6L6's (the SF power tranny can handle 6L6's if the rectifier tube is switched to SS, removing the current draw of the rectifier filament).

Now, there are also WberVST copper caps, which are SS rectifiers which mimic tubes. Since they aren't real tubes, they have no current draw and they are available in any number (or custom user-defined) amounts of voltage drop and sag. For these, you only need to watch plate votlage (you still wouldn't want to use a WZ34 in a SFDR with 6V6GT's).
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Postby Kagliostro » Thu May 01, 2008 5:58 pm

Hi :)

May be that here u can find someting of interesting, like power scaler and post phase inverterter master:

http://www.el34world.com/Forum/yabb2/Ya ... 209421/8#8

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Variac Use

Postby HiFiGuy » Wed Feb 17, 2010 2:12 pm

If you have an amp with a neg bias voltage you should NEVER run them lower than stated values. Doing so will eat your output tubes. If you must run them lower its safer to do it with an amplifier that has the power tubes biased via a cathode resistor. You need to verify that it uses NO neg bias voltage on grid 1.
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Postby Cygnus X1 » Wed Feb 17, 2010 2:26 pm

I have grown and am more educated about the opinion that I posted.

I am running the GZ34, (really no reason to justify changing it anyhow).

Thanks for clearing that up for the original poster
and I sincerely hope I didn't blow anyone's amp up in the
process!

;)
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