Using a Variac for start-up on my VTA ST 70

So I have a Superior Electric Variac that I use to fire up my stereo. I have the 2 diode mod on the rectifier.
I start from no voltage and slowly turn up the Variac till I get to 118vac, that is the voltage where the heaters are at 6.3.
My questions are:
Is it the surge of power coming in that blows the rectifier?
Does the rectifier blow while in use?
If I’m using the Variac every time the voltage surge isn’t there because I’m turning it up slowly from 0 to 118 is like 8-10 seconds. Vs flipping the switch and instantly having 118.

Also I have read that if you place a jumper between pin 4 and 6 on the rectifier it doubles the current capacity of the rectifier. Is anyone doing this and is it safe?
Thanks Mark

That sounds like a lot of trouble. I wired in a CL-80 inrush current limiter on my ST-70 and measurements confirm it ramps the voltage up rapidly, but not instantly. You just need to understand that a rapid OFF and back ON cycle will defeat it as it hasn’t had enough time to cool. I use a latching GFCI to preclude that from happening in case of an intermittent mains power interruption. If you are going to manually power down then back up, just wait a minute or two before powering back on to allow the current limiter to cool back down.

I can’t address the rectifier jumper question but I’ll be interested to hear if anyone has tried that or knows if/how it would work.

Jeff

I’m not turning it off and then back on. If I turn it off it’s off till the next time I use it. Which I is typically plenty of time to cool off. The reason I’m using a Variac is because my mains voltage is 122-125 which is high. I was wondering about the in rush of voltage/current. I’m gonna use a Variac no matter what because of the high mains. If the Variac will keep the voltage/current tame while turning it on I would like to not use the diodes and let the tube do it. Hence my other question was “can the rectifier blow while in use without the diodes”. I’ve read the rectifier is working really hard.

I'm not a tech but I've been reading this forum for about 4-5 years.
I've never heard of any problems running the VTA ST 70 with the 5AR4. I have heard some people 's opinions that the VTA ST 120 is running kind of close to the limit on the 5AR4.
As was stated above, you just don't want the amp to quickly cycle on/off.
I also use the latching GFCI...

https://www.amazon.com/gp/product/B000XVG72G/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

...in combination with a bucking transformer to keep my voltage down around 116 or so.
No problems for years (knock on wood).

Thanks for the link Dave. I’ll get one of those surge protectors. Still want to know more on the jumper and who is using the diodes and on which amp?
Thanks

The instructions that came with my circuit specialists 5A variable transformer say not to use with a GFCI protector. That's when I stopped using my GFCI. I use this "variac" w/ my ST120. I occasionally take a voltage reading (don't trust it's built-in meter) from the variac output and adjust to about 117, plug in my amp and go. My wall current ranges from 119-122.9

I used a TungSol 5AR4 w/ no problems. I now use a Weber WS-1t as I have an auto-bias board in my amp. It works fine and has a built in delay. My KT120's bias at .49

I did the yellow sheet diode mod. From what I understand, it's to keep current from flowing backwards from the caps into the rectifier if the amp is short cycled. In normal circumstances, the caps dissipate over a short time.

There are diodes in the solid state rectifier but I think that's to turn ac into dc. Someone wiser than I please chime in.

I got most of what I know by searching on this forum.

I've never heard anything about that jumper wire. Sounds shaky to me.

Years ago I asked a similar question about the diode mod and the GFCI.
Roy Mottram answered that if you use the GFCI you don't need to do the diode mod as the diode mod is to keep the rectifier tube from quickly cycling and that's what the GFCI is doing.

I guess you could use both the diode mod and the GFCI if you wanted to. I can't see any advantage or disadvantage to that but remember I'm not a tech.

Here's the sticky that goes into the diode mod in excruciating detail.

https://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-yellow-sheet-diode-mod

Here's the exchange between Roy and myself in that thread:

I've re-read this thread and it seems that the advantage of the diode mod is to protect the 5AR4 in case of power on/off blips.
I have a VTA ST-70 and I use a Sylvania 5AR4 and the entire system in plugged into one of the Skizo recommended non-latching GFI gizmos. It kicks out when there is a power interruption and I have to manually reset it.
So, am I right in thinking I don't have to worry about this diode mod?

Hi Dave,
yes, the diode mod is for protection, it doesn't do much of anything else. Seems that with your Skizo mod you're likely accomplishing the same protection.
But for most people 40c worth of diodes is cheap protection for a $20 rectiifier, let alone any other tubes downstream . . .

(I'm using a bucking transformer, not a Variac.)

May I chime in?

First off, don't jumper pins 4 and 6 together without the diode mod or there will be quick release of magic smoke with the power transformer!  At the very least you will blow the mains fuse and the transformer may end up damaged, and you will experience a very bad day.

If the 5AR4's pins are jumped together you are splitting hairs between a whole rectifier or two halves.  What I mean is the current flows through one diode and one half of the rectifier on each half cycle of the AC wave.  With the pins shorted, full wave rectification is now handled by the solid state diodes only, which means they must be capable of high reverse voltage, so at a minimum each diode should be a 1N4007/UF4007 and will be very close to their maximum reverse breakdown voltage.  The rectifier will have both plates combined but that doesn't increase current capacity of the rectifier.  In my opinion this is not a good idea.  The diode mod is extra insurance that the rectifier will be protected, and well worth the cost and labor to install them.

Tony

I should have mentioned that I, too have high mains, so I also use a bucking transformer for my ST70 that provides 116-118. I also did the diode mod. All cheap insurance.

The bucker combined with the CL-80 sorta soft starts my ST70. Be aware that the CL-80 is rated at 3 amps, so for any other amp that may pull more juice, you may need a different IRCL.

Jeff

I would advice NOT reconnecting the rectifier unless one knows what to do. Wrong connections might fry the power transformer.
Using GZ34 ( any brand) will generally work fine in any st70-type amp.