Vintage tube gear and Variac

[cason]

How many of you always power your vintage tube gear through a variac? Currently, I always bring my Fisher 500C up slowly to 112 volts with a variac and operate it at that voltage. The owners manual calls for a supply voltage of 105 to 120 volts. Am I being overly cautious?
Thanks,
Bob

 

[godzilla]

I'm not sure bringing it up through a variac everytime is neccessary. If you're worried about electric fluctuation you might want to try one of these(http://www.big-georges.com/index.asp?PageAction=VIEWPROD&ProdID=5911). They work nice...keeps everything running nice and smooth without any surges or fluctuations...I even use it for my antique slot cars...those little motors really show you how much it keeps everything running smooth...before I got it I could tell how the cars surged up and down and now they don't. They also help save your gear when you have brown outs and power outages.

 

[Tom Bavis]

I'd say it's TOO cautious... I put an inrush limiter in vintage amps and receivers, and don't worry about it after the initial power-up. I ALWAYS power up old amps for the first time with a variac (and inline power meter) - it has a 2A fuse. I do have a bigger one (5A), but haven't needed it yet...

 

[dshoaf]

If you've not had your tube gear's power supply components updated, then I'd be wanting to use some sort of way to control the inrush at startup. Tom's advice to use varistors, like the 60s vintage McIntosh amps did, is a good idea. Be aware, though, that you're still only buying time before a failure in this case.

If the power supply has new caps and is in working order, then I'd be less inclined to use a variac at all. I still consider a variac as a piece of test gear - if you're having to use one on a daily basis, there's something else wrong somewhere.

As for the voltage specs, yes nominal AC voltages back in the late 50s/60s were lowert than they are today. If the unit's been updated as above, this shouldn't cause a problem if the work was done correctly.

Cheers,

David

 

[motorstereo]

I always power up my tube gear with a variac. Right now I'm using a sansui 1000a because it generates much less heat in the summer than my mac 110 and stromberg monoblocks. I feel it's worth the extra time to bring them up slowly and possibly add some life to the aged components. They all have modern caps but some of the tubes are originals as are the transformers. I also run them at 115 volts. Who knows if adds any life to the units or not but with the scarcity and replacement cost of the older units I figure it surely can't hurt.

 

[jon_s]

I always run my gear on a Variac, to keep heater and plate voltages reasonable, and to be nice to the power supply electrolytics. I do not do a "slow start" by cranking it up each time, just flick the switch. 115V is a good safe bet, but I do run my Craftsmen 500as at 110V to avoid plate glow on the Chinese KT66s. My Citation gets 115V. Dynaco SCA35 just goes into the wall.

 

[jcmjrt]

Since the gear wasn't designed for 120+V, I use a variac to put it at 115V. I don't bring it up slowly everytime...I just use it to limit the V. Of course, with a "new" amp, I bring it up very slowly and keep an eye on it.

 

[cason]

Thanks for the respnses. My 500C has had Fisher Doc upgrades by a good tech, so no problems there. Most of my tubes are original (or at least Fisher replacements). My thinking, like Motorstereo, jon s, and jcmjrt, was to limit voltage and be kind to the tubes and transformers. The variac routine is really no inconvience. I find (at least until I get new [old] speakers) that I'm using the Fisher only two or three times a week. The Marantz gets more playing time because it mates better with the ARs. Again, Thanks for your thoughts.
Bob

 

[currituckco]

I'd say it's TOO cautious... I put an inrush limiter in vintage amps and receivers, and don't worry about it after the initial power-up. I ALWAYS power up old amps for the first time with a variac (and inline power meter) - it has a 2A fuse. I do have a bigger one (5A), but haven't needed it yet...

Hi Tom,

Could you describe the process of adding an inrush limiter to an amp, including what values of parts you use?

Thanks!

 

[ab0ez]

Definately too cautious. The primary purposes of a variac are for slowly bringing up an amp that hasn't been energized for a very long time, or troubleshooting a troublesome amp, and finally for reforming caps.

 

[motorstereo]

Definately too cautious. The primary purposes of a variac are for slowly bringing up an amp that hasn't been energized for a very long time, or troubleshooting a troublesome amp, and finally for reforming caps.

Too cautious with vintage tube gear? I don't think there is such a thing. Yes prolonging the life of our tube gear isn't the intended purpose of the variac. I figure I have the variac sitting there idle why not use it to be kind to the old gear.

 

[dshoaf]

Hi Tom,

Could you describe the process of adding an inrush limiter to an amp, including what values of parts you use?

Thanks!

I'm not Tom but I'll share what I usually do. There's a couple of approaches:

- On the AC side of the transformer: You can use a varistor to help control the inrush of current on startup. If the tube amp had selenium rectifiers that were replaced with silicon diodes during restoration, then this can be used to add some resistance in line to reduce the operating voltages. For instance, I used a CL-80 on an MX-110 on the AC side of the power transformer to reduce the line voltage once the full-wave bridge rectifiers have been replaced. I note that Audio Classics does the same thing to their restored MX-110s, too. Mac did this with its old MC-225/240/275s series as well. By this age, they're usually cooked and may fall apart if they're original.

Benefits: This allows the whole amp, incuding the filaments, bias supply and plate voltages to come up to operating specs slower than without the varistor.

- Another approach is to put 2 varistors on the secondary side of the high voltage windings of the power transformer. This is Jim McShane's recommended approach with his HK Citation II updates. You benefit from this one by allowing the bias circuit and the filaments to come up to operating spec ahead of the plate voltage. Check his site for more details.

Both of these approaches are valid but depends on the individual circuits involved and the intended design requirements.

A word of warning: These things will get hot so you have to find a place where they can dissipate this heat without cooking other wiring or components. This is often a problem with tightly packed amps like the old Fishers and Scott receivers and integrated amps. Still they're worthwhile.

Hope that helps.

Cheers,

David

 

[currituckco]

Thanks for the post, David! Very helpful!

Is there a difference between varistors and thermistors? I've seen people mention using thermistors for this use too. Can you explain how they are rated, and what the proper rating for this application is? You mentioned using a CL-80 - is this also what Jim McShane uses two of on the secondary side of the power transformer? And how are the two used, in series between the transformer and the first filter cap section? Or before diodes? I don't know how the citation II power supply is laid out. I'm wondering for Dynaco MK4 amps and a Pilot 245A integrated, both of which are GZ34 tube rectified. I used 1n4007 diodes in the filament supply of the Pilot as well.

thanks for the info!!

EDIT - I see CL-80 current limiting thermistors at Mouser - 3A 47ohm - are these similar to what you use?

 

[onemug]

I use a variac on a couple of Marantz units that have ocilloscopes. These scopes are getting expensive and hard to find, so my thinking is to give it 5 to 10 seconds instead of that initial nano hit. Also, I was advised that these older units are a little happier at 115v and I usually have 120 to 122 coming out of the wall.

What are the thoughts on variacs with 60's solid state that has voltage regulation? I would think having a 5 second inrush would be better than the nano but not to spend a lot of time below 100v.

 

[dshoaf]

Is there a difference between varistors and thermistors?

Here's a pretty decent explanation of a Thermistor: http://en.wikipedia.org/wiki/Thermistor

And the Varistor: http://en.wikipedia.org/wiki/Varistor

You mentioned using a CL-80 - is this also what Jim McShane uses two of on the secondary side of the power transformer? And how are the two used, in series between the transformer and the first filter cap section? Or before diodes? I don't know how the citation II power supply is laid out.

Jim's power supply update, uses CL-144s. These are about 150 ohms when hot. Don't hold me to that as I'm working from memory, here. For the high voltage we're dealing with in a Citation II, this works well. They are installed in series with both leads of the secondary side, high-voltage winding of the power transformer - and before the diodes. This way, the high voltage comes up after the filaments and, more importantly, the bias voltage on the output tubes.

See here for an image of the CitII power supply schematic: http://pages.prodigy.net/jimmcshane/mods.htm along about the middle of the page.

I'm wondering for Dynaco MK4 amps and a Pilot 245A integrated, both of which are GZ34 tube rectified. I used 1n4007 diodes in the filament supply of the Pilot as well.

No need for an inrush limiter on tube rectifier power supplies - the function is built into the tube as it heats up naturally! The 1N4007 diodes in the filaments of the Pilot is ok since they'll come up ahead of the high voltage.

thanks for the info!!

You're welcome!

EDIT - I see CL-80 current limiting thermistors at Mouser - 3A 47ohm - are these similar to what you use?

I've used the CL-80s on my MX-110s as I just had them in stock. The Mac tube amps use a CL-90 and they're _still_ available from Mac mainly because the current MC275s use them, too.

Cheers,

David

 

[ab0ez]

If your vintage gear is in good operating condition I really don't see the need to run a variac in-line with it. But, as always, this is just my opinion and your mileage may vary.

 

[currituckco]

No need for an inrush limiter on tube rectifier power supplies - the function is built into the tube as it heats up naturally! The 1N4007 diodes in the filaments of the Pilot is ok since they'll come up ahead of the high voltage.

Hi David, thanks for the great info. I still may add an inrush limiter, as people have described situations in which the rectifier tube heats up before the output tubes leading to a momentary spike (around 575v IIRC) in voltage on the filter caps. As the new cans I'm putting in the Mark IVs are rated to 525 (like original Dyna mallorys) I'll probably sleep better with something in there to smooth out that inrush.