Using a Variac for starting an old tube set

[Foxs Mercantile]

On 5/17/2017 10:39 AM, tabbypurr@gmail.com wrote:
> It's not the ideal method but it does work.

And it's an absolute waste of time.
Any cap that needs to be reformed is going to fail.
Just replace them damn things and get on with it.

--
Jeff-1.0
wa6fwi
http://www.foxsmercantile.com

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[pfjw@aol.com]

That's just wrong. If reforming by running a complete set from a variac, wind the voltage up _very_ slowly until rectifier conduction just begins. At this point the rect won't pass much current, and any bad cap present will conduct, keeping the voltage on the lytics low. After a while the variac can be inched up _slightly._ Bear in mind that even a 2v increase on 240v mains increases rectifier conduction many times 1%, so very slowly does it.

It's not the ideal method but it does work.


NT

The thing about rectifiers is that they are go/no-go devices.
The thing about Variacs is that they are not current-limiting devices but voltage limiting devices.

Which means: When the rectifier "goes" the voltage that hits the caps is a function of the voltage from the transformer secondary to the transformer primary. In a Transformer-type radio. Or the minimum trigger voltage for the rectifier on an AA5 type.

The typical tube rectifier will operate somewhere between 75% to 80% of the rated filament voltage. In some few cases, as low as 70%.

The typical tube rectifier as in those devices under discussion operates at a filament votage of 5V for transformer-types and 50V for an AA5-type set.

B+ for the typical transformer-type set is about 350 VDC. About 100 VAC for an AA5. Caps are rated (usually) at 450V and 150V respectively. "Typical", not "EVERY".

Meaning that 245V is what the caps will see in the one case, and 70V for the other.

Again, not hardly a soft start. Nor worth bothering about. Replace the poor, silly things outright and be done with it.

Peter Wieck
Melrose Park, PA

 

On Wednesday, 17 May 2017 18:00:39 UTC+1, pf...@aol.com wrote:
NT:

That's just wrong. If reforming by running a complete set from a variac, wind the voltage up _very_ slowly until rectifier conduction just begins. At this point the rect won't pass much current, and any bad cap present will conduct, keeping the voltage on the lytics low. After a while the variac can be inched up _slightly._ Bear in mind that even a 2v increase on 240v mains increases rectifier conduction many times 1%, so very slowly does it.

It's not the ideal method but it does work.

The thing about rectifiers is that they are go/no-go devices.

wrong. bzzt.
As folks familiar with valves know that they will pass only small current when the heater is run well below rated voltage.


NT

 

[pfjw@aol.com]

On Wednesday, May 17, 2017 at 7:23:15 PM UTC-4, tabb...@gmail.com wrote:

> As folks familiar with valves know that they will pass only small current when the heater is run well below rated voltage.

a) Why is it that so many mistake *voltage* for *current*?
b) Please explain to me how a Variac delivering 84 VAC on a transformer-type device, and that being the minimum trigger voltage on say... a 5Y3, will deliver any less current than that rectifier is able to pass at that voltage?
c) Further, explain to me how 245 VDC constitutes any sort of soft-start for the on-board capacitors.

If one happens to have a metered Variac, one can observe the onset of B+. It does NOT hover when the rectifier starts to pass DC.

Peter Wieck
Melrose Park, PA

 

On Wednesday, May 17, 2017 at 10:49:22 PM UTC-4, pf...@aol.com wrote:

> b) Please explain to me how a Variac delivering 84 VAC on a transformer-type device, and that being the minimum trigger voltage on say... a 5Y3, will deliver any less current than that rectifier is able to pass at that voltage?

Because the filament voltage of the 5Y3 (for example) will be reduced a similar amount. Low filament voltage makes a weak tube. With low filament, the current capability of the rectifier will sag with a load causing a corresponding voltage drop.

Whether the reduced current capability of the sagging rectifier is enough to keep a leaky cap from trashing some other component is something I don't know or really care about - I replace all old paper caps if I should happen to be working on vintage stuff. I use my variac mostly for isolation but also for troubleshooting.

 

On Thursday, 18 May 2017 03:49:22 UTC+1, pf...@aol.com wrote:

On Wednesday, May 17, 2017 at 7:23:15 PM UTC-4, tabby wrote:

As folks familiar with valves know that they will pass only small current when the heater is run well below rated voltage.

a) Why is it that so many mistake *voltage* for *current*?
b) Please explain to me how a Variac delivering 84 VAC on a transformer-type device, and that being the minimum trigger voltage on say... a 5Y3, will deliver any less current than that rectifier is able to pass at that voltage?
c) Further, explain to me how 245 VDC constitutes any sort of soft-start for the on-board capacitors.

If one happens to have a metered Variac, one can observe the onset of B+. It does NOT hover when the rectifier starts to pass DC.

Peter Wieck
Melrose Park, PA

If you show at attempt to follow what I explained I'll gladly explain further.


NT

 

On Thursday, 18 May 2017 13:43:33 UTC+1, ohg...@gmail.com wrote:

On Wednesday, May 17, 2017 at 10:49:22 PM UTC-4, pf...@aol.com wrote:

b) Please explain to me how a Variac delivering 84 VAC on a transformer-type device, and that being the minimum trigger voltage on say... a 5Y3, will deliver any less current than that rectifier is able to pass at that voltage?

Because the filament voltage of the 5Y3 (for example) will be reduced a similar amount. Low filament voltage makes a weak tube. With low filament, the current capability of the rectifier will sag with a load causing a corresponding voltage drop.

it will of course drop to the voltage where the cap no longer leaks too badly, if circuit resistances are satisfied with current.

> Whether the reduced current capability of the sagging rectifier is enough to keep a leaky cap from trashing some other component is something I don't know or really care about

Nothing is getting trashed when the rectifier only delivers 10mA.
You can only reform lytics, not paper caps. Normally lytics go across the supply, often via a dropper, and paper caps go from anode to grid.

> - I replace all old paper caps if I should happen to be working on vintage stuff. I use my variac mostly for isolation but also for troubleshooting.

Memory a bit vague but I think rectifier conduction started somewhere just over 60v for a 1937 240v set.


NT