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Old 24th Apr 2023, 12:40 pm   #1
E93AFAN
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Default Using Solid State Rectifiers To Replace Valves - Inrush Damage?

As a spring project (although it still feels like winter here in Wiltshire) I am about to make the Mullard 7 watt stereo amplifier (4 x ECL82, 1 x ECC83, 1 x EZ81), I made one back in the 60's and although not "Hi-Fi" it acquits itself pretty well.

I have all the components at hand other than a suitable mains transformer with a 6.3 volts @ 1 amp winding for the EZ81 and a separate winding @ 4 amps for the other valves, I do however have a transformer with a 4 amp winding and a 160ma 250v winding.

I considered using a silicon rectifier to replace the EZ81, but am worried that since this creates an immediate rush of unweighted HT this may push the valves beyond their safe operating voltage until the heaters are warmed up, with resultant damage. An alternate solution (again to hand) is to add a 1 amp heater transformer for the EZ81, although this is not my first preference I shall go down this route rather than using the more complex solution of "soft start" as described by other contributors here if the silicon route may cause damage.

Does anyone have a view on whether damage is likely if a solid state rectifier is used, I would welcome the wisdom of anyone who has practical experience of replacing valves with silicon in these circumstances.

Many thanks as always for your patience, kindness and support.
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Old 24th Apr 2023, 12:52 pm   #2
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

The HT will rise until the valves start to draw current, but it shouldn't be enough to cause any damage. You just need to ensure that all the caps are adequately rated.

If you're really concerned about this, you could always delay the HT by 60 seconds while the heaters warm up. You could do this manually with a switch, or build a little delay circuit with a couple of transistors and a relay. This isn't very complex.
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Old 24th Apr 2023, 1:09 pm   #3
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

Those amplifier valves have a limit of 300v on anodes and screen grid.
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Old 24th Apr 2023, 1:25 pm   #4
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

ECL82, max. Vao & Vg2o = 550 Volts.

ECC83, max.Vao = 550 Volts.

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Old 24th Apr 2023, 2:58 pm   #5
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

Even using a valve rectifier, you can't be sure that its heater won't bring the rectifier to conducting temperature before the other valves, although it should reduce the period of high HT voltage. Surely the whole circuit has to be designed to allow for a period of higher-than-normal HT voltage.
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Old 24th Apr 2023, 3:07 pm   #6
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

Hi.
The EZ81 with its indirectly heated cathode is quite slow and as the other valves start to conduct then the HT is rising so it should not rise disproportionately. The issue is with directly heated cathodes as in filamentary valves like the 5U4 etc that are virtually instant. I'd personally stick with the EZ81 which is the simplest solution.
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Old 24th Apr 2023, 3:21 pm   #7
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

In the world of guitar amps, plug in SS rectifiers are available as drop in replacements for GZ34 rectifiers and I'm not aware of firstly, any mods needed, and secondly that any dangers have been advised or experienced.

https://www.hotroxuk.com/tad-solid-s...ier-rt515.html

Edit: There's a solid state drop in for the EZ81 by Groove Tubes: https://reverb.com/item/2004528-groo...-free-shipping
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Old 24th Apr 2023, 3:42 pm   #8
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

When faced with the choice of replacing a valve rectifier with silicon rectifiers, I've always been far more concerned about the strain briefly raised HT could put on ageing reservoir and smoothing capacitors.
Once, when asked by father-in-law to do a quick running-repair to a 1940s radio with a failed octal rectifier, I removed the octal base from the old valve and used it as plug to connect a B9A valveholder for an EZ81 - using 5 bits of 2.5mm wire from stripped mains cable as connectors and "mounting pillars" for the B9A holder. A bit of a lash-up, but surprisingly rigid and completely effective, and since FiL didn't spend a whole lot of time gazing at the innards of the set, he didn't care!
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Last edited by Boulevardier; 24th Apr 2023 at 3:52 pm.
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Old 24th Apr 2023, 4:05 pm   #9
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

I'd be inclined to try the electrolytics with the full voltage for 15 seconds or so, while watching the current.

If it doesn't rise unduly in that time, the SS rectifiers will be good to go. Future switch-ons will only get less and less of an issue as the electrolytics form themselves for the briefly higher voltage.

Quote:
Originally Posted by Boulevardier View Post
Once, when asked by father-in-law to do a quick running-repair to a 1940s radio with a failed octal rectifier, I removed the octal base from the old valve and used it as plug to connect a B9A valveholder for an EZ81 - using 5 bits of 2.5mm wire from stripped mains cable as connectors and "mounting pillars" for the B9A holder. A bit of a lash-up, but surprisingly rigid and completely effective, and since FiL didn't spend a whole lot of time gazing at the innards of the set, he didn't care!
Presumably a 6X5 (EZ35) with 6.3V heater?
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Old 24th Apr 2023, 4:24 pm   #10
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

Quote:
Originally Posted by kalee20 View Post

Quote:
Originally Posted by Boulevardier View Post
Once, when asked by father-in-law to do a quick running-repair to a 1940s radio with a failed octal rectifier, I removed the octal base from the old valve and used it as plug to connect a B9A valveholder for an EZ81 - using 5 bits of 2.5mm wire from stripped mains cable as connectors and "mounting pillars" for the B9A holder. A bit of a lash-up, but surprisingly rigid and completely effective, and since FiL didn't spend a whole lot of time gazing at the innards of the set, he didn't care!
Presumably a 6X5 (EZ35) with 6.3V heater?

Really can't remember - 50 years ago. It might well have been a UU6 - so actually Mazda octal (albeit slight overkill for Ia).
Mike
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Old 24th Apr 2023, 4:54 pm   #11
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

Quote:
Originally Posted by Boulevardier View Post
Quote:
Originally Posted by kalee20 View Post

Quote:
Originally Posted by Boulevardier View Post
Once, when asked by father-in-law to do a quick running-repair to a 1940s radio with a failed octal rectifier, I removed the octal base from the old valve and used it as plug to connect a B9A valveholder for an EZ81 - using 5 bits of 2.5mm wire from stripped mains cable as connectors and "mounting pillars" for the B9A holder. A bit of a lash-up, but surprisingly rigid and completely effective, and since FiL didn't spend a whole lot of time gazing at the innards of the set, he didn't care!
Presumably a 6X5 (EZ35) with 6.3V heater?

Really can't remember - 50 years ago. It might well have been a UU6 - so actually Mazda octal (albeit slight overkill for Ia).
Mike

No, couldn't have been UU6 with its 4V heaters. So no, I can't remember now - possibly EZ35. But, whatever, obviously the valvebase would have fitted its original socket!
Mike
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Old 24th Apr 2023, 7:18 pm   #12
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

Assuming all other parts [electrolytics!] are healthy, I'd happily substitute a solid-state rectifier for a valve.

In times-past, the US International Rectifier Corporation produced a range of solid-state rectifier assemblies built into octal valve-bases, intended as replacements for valve rectifiers; I used these with success in the past, they generated less heat in two ways: firstly they had much lower forward-voltage-drop, and secondly by removing the filament-heating power this also reduced heat and let the mains transformer run cooler because there was less filament-power needed.

Previous thread here: https://www.vintage-radio.net/forum/...ad.php?t=65904

If you do go the semiconductor route, use modern fast-turn-off versions [UF series] rather than the obsolete and potentially-noise-generating 1N-types.
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Old 24th Apr 2023, 9:50 pm   #13
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

An approach I've used in the past is to put some zeners and a resistor across the HT carefully chosen so that they will only conduct during that period while the valves warm up and the HT would otherwise be higher than in the normal operation. As the valves start to conduct, the HT falls to a value below the zeners' threshold.

I've also used zeners in HT circuits to get the "right voltage" to feed a voltage stabiliser tube. The temperature coefficient of the zeners is rather high to allow them to be used without the gas stabiliser.

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Old 25th Apr 2023, 1:48 pm   #14
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

Solid state rectifiers will produce a higher HT voltage which will mean higher ECL82 anode currents and possible overheating and thermal runaway. This can be overcome by a transformer with a lower HT voltage winding or some changes to the circuit resistance values. You can use Duncans PSU designer to workout what winding is needed to create the correct HT voltage.
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Old 25th Apr 2023, 3:54 pm   #15
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

Quote:
Originally Posted by PJL View Post
Solid state rectifiers will produce a higher HT voltage which will mean higher ECL82 anode currents and possible overheating and thermal runaway. This can be overcome by a transformer with a lower HT voltage winding or some changes to the circuit resistance values. You can use Duncans PSU designer to workout what winding is needed to create the correct HT voltage.
I believe that 'drop in' replacement rectifiers ie the commercially available ones with valve bases have circuitry to account for the increased efficiency. Not 100% sure on that but it's in the depths of my memory from when I was using them.
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Old 25th Apr 2023, 4:58 pm   #16
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

Thank you for your support and help, all very much appreciated.

I'll go for broke and use silicon to start with, but check the Mullard voltage data supplied with the circuit against actual voltages in operation to assess if over voltage looks like causing a problem. If it looks like a problem then it's out with the silicon and in with an EZ81.

I wish you all a happy day and thanks again.
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Old 25th Apr 2023, 5:01 pm   #17
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

Expensive communications receivers such as the American version of the Racal RA17 have used semiconductor rectifiers.
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Old 26th Apr 2023, 9:14 am   #18
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

Quote:
Originally Posted by stevehertz View Post
In the world of guitar amps, plug in SS rectifiers are available as drop in replacements for GZ34 rectifiers and I'm not aware of firstly, any mods needed, and secondly that any dangers have been advised or experienced.

https://www.hotroxuk.com/tad-solid-s...ier-rt515.html

Edit: There's a solid state drop in for the EZ81 by Groove Tubes: https://reverb.com/item/2004528-groo...-free-shipping

Hi
Have come across these a few times and in three instances have had to replace the reservoir cap and smoother, even adding a series surge limiter the HT went way above the capacitor surge volts. A bad idea indeed, also the running HT was higher the additional surge limiter at least brought this back to the correct voltage.
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Old 26th Apr 2023, 10:09 am   #19
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

I'd taken it for granted that E93FAN's original post had included added resistance in series with the silicon diodes, so that the 'running' HT voltage is the same as with the EZ81, and it was only the voltage surge at switch-on that was the issue.

But looking back, I can't see any mention of this... maybe an unjustified assumption? (If so, it's simple to fix, but be sure to use adequately-rated resistors).
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Old 27th Apr 2023, 4:09 pm   #20
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Default Re: Using Solid State Rectifiers To Replace Valves - Inrush Damage?

KALEE20 - Thank you, yes I was intending to calculate the value of a chunky resistor (or two) to reduce the voltage to that expected from the EZ81, you were correct that my only concern was the application of unloaded HT at switch on.
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