GZ37 rectifier and the Wadar amp

[Ian - G4JQT]

I've built one of Gerry Wells's Wadar Amplifier designs and have a query about the GZ37 rectifier valve it uses.

I am thinking of getting a spare set of valves, but the GZ37 is remarkably expensive. Cheap Chinese versions being only £20 ranging to £60 for better known makes.

After exchanging helpful emails with Gerald at Crowthorne Tubes, he confirmed that the GZ37 is rated for a reservoir capacitor of only 4uF. In the Wadar this is way above that at 32uF - in my version anyway.

I noticed during initial testing that there is a small (?) flashover inside the GZ37 if it is switched off for a few seconds then switched back on again. Never to be recommended, but inevitably happens very occasionally, and the small flash presumably caused by the large reservoir capacitor.

I'm surprised Gerry did this, but maybe he had lots of GZ37s at the museum and in practice it isn't a problem. Or maybe he used various rectifier valves depending on what he had, but the GZ37 was what ended up in the documentation I acquired.

The GZ34 (5AR4) seems to be more suitably rated for up to 60uF reservoir capacitor, but with similar current rating and valve base.

The Wadar never was designed to be an audiophiles dream, it is just very good. Therefore I'm not looking for the highest quality valves for this amp - just something that won't burn out the transformer. (Although I have 250mA fuses before and after the rectifier - just in case!)

So... replace GZ37 with a GZ34 (or equivalent) when the time comes? What do other Wardar owners have as the rectifier?

Thanks you.

Ian

[Radio Wrangler]

It sounds like you've answered your own questions.

If you are seeing visible distress in the GZ37, then I don't suppose it's going to last too long, so you do need to do something about it. Those max reservoir ratings were decided upon by sensible calculations and testing.

A GZ34 would be a suitable fix.

So would a pair of silicon diodes with some series resistors. As it wasn't an audiophile's dream, then their dreams of some sort of purity of being entirely thermionic do not apply and you have a freedom they deny themselves.

Guitarists have a valid point about the sound effects of thermionic rectifiers allowing HT rails to sag when they thrash the amplifier, but this doesn't apply to domestic reproduction. You don't have one amplifier per instrument, you have one amplifier per channel handling all instruments and if you thrash it, you get terrible intermod between the various instruments, you get exaggerated high frequency energy from the harmonics, and burned-out tweeters.

David

[trobbins]

I'd recommend adding series 1N4007's in each GZ37 anode - that may avoid PIV stress and arcing.

I had a look at the GZ37 datasheet and tried to compare it to the GZ34 - you may find something relevant depending on the exact use of the GZ37:
https://www.dalmura.com.au/static/Po...ube%20amps.pdf

[Radio Wrangler]

For use as rectifiers, I VERY STRONGLY advise against using 1N4007, 1N4008, 1N4009.

The high voltage end of this family are actually PIN diodes to get the voltage rating. This makes them extremely slow in switching, and in switching off, they do so with a fast snap, a while after the current through them has reversed. This makes lots of radio interference and also stresses transformers and components around them.

Back in the day when the 1N400x family were as good as you got, transformers were often festooned with ceramic capacitors to reduce the noise and the transients generated by the diodes. Doing product testing, you often found switch mode power supplies where the majority of the noise they generated came from the plain old 50Hz rectifiers.... Then you learned that 'linear' supplies could have the same trouble.

It sounds counter-intuitive, but using 'fast' or 'ultra fast' rectifier diode families greatly reduces radio interference.

Put in series with a GZ34 anode, the GZ34 is fast and will do the real switching, so it covers for the failings of the silicon, but this means the silicon doesn't de-stress the GZ34 quite as much as you might think. It'll work to a useful extent.

But if you use silicon rectifiers to do the whole job, then 1N400x ought to be avoided.

Pet niggle!

But the 1N400x are so embedded in folklore, they're the one everyone reaches for first.


David

[davebms]

hello all, the diode on the anode is on the internet from guitar people.
apparently the new production valves are not as strong as
some of the originals (so they say)

dave

[radioman]

Perhaps the UF4007 would be a better bet ?
Going back to the original post, how about a Cossor 53KU ?
- it's rated for 16uF max. reservoir capacitance.

Andy

[kalee20]

I don't like seeing valves in distress, they are living things after all and have feelings...

I've used valves above their reservoir capacitance with no problems. But it has to be done 'advisedly' with knowledge of what is the reason for the manufacturer's limit. After all, 1,000uF of capacitance - with 1 megohm in series - is not going to hurt. But reduce that 1 megohm to 0.1 ohm of wiring resistance and it will hurt. Somewhere in the middle, it's time to call a halt. (I have a circuit using an EZ80 with 200uF, way above limit, but the transformer driving it has effectively 500 ohms winding resistance and moreover is only 190 - 0 - 190V. So the EZ80 is happy.)

There is also a distinction between the repetitive current peaks which arise each half-cycle in a capacitor input filter (which, given some series resistance such as transformer winding resistance) are limited and may barely change from if you replace 32uF with 470uF, and the hot switch-on surge which occurs if you switch off and then switch on a few seconds later. In fact, umpteen thousand microfarads may actually REDUCE the hot switch-on surge, as this capacitance will discharge much less during the seconds the amplifier is switched off, so on re-switch-on it won't take such a big gulp of current (it will just take a smaller current for longer). Down side is that cold switch-on, it will take much longer to charge when the cathode is barely emitting, and that's not good either.

I think you have answered your own question really. Use the GZ34.

[GrimJosef]

Originally Posted by radioman

... Going back to the original post, how about a Cossor 53KU ...

The OP thought that GZ37s were expensive. Heaven forbid you encourage him to look at Cossor 53KUs .

Cheers,

GJ

[trobbins]

David, I would suggest you are incorrectly seeing the operation of the 1N4007 when used in series with a valve diode. The valve diode 'slowly' turns off with mains rectification, as its plate resistance increases significantly as plate voltage falls towards zero, causing the series current at diode turn-off to be very soft and slow and in no way causing the ss diode to exhibit reverse recovery.

[Radio Wrangler]

I trust my understanding in this case, the valve turns off faster than the 1N4007 and the 1N4007 doesn't get into its reverse current snap recovery mode, so RFI isn't bad. Then later as the reverse voltage has built up, the 1N4007 reduces the PIV the valve sees. The slowness of the 1N4007 acts to limit this effect, but it'll still be useful.

I was more worried that the OP may go for silicon rectification without a valve. In which case a 1N4007 is a bad choice. A UF4007 would be a lot better.

For amplifiers not being driven to the point where rectifier cathode emission lets the HT sag as part of the performance, solid state rectifiers are fine, and an awful lot cheaper than GZ34s. He did say it wasn't an audiophile amp.

David

[turretslug]

Originally Posted by trobbins

I had a look at the GZ37 datasheet and tried to compare it to the GZ34 - you may find something relevant depending on the exact use of the GZ37.

Quite- some folk get quite dogmatic about the quoted maximum reservoir capacitor ratings associated with particular thermionic rectifiers, presumably on the grounds of a limited context mention elsewhere on the 'net, without stopping to consider that this can be a very elastic qualification with considered conditions that might be very different between rectifier types depending on the expected use of a particular type. Without consideration of both detailed valve type characteristics and particular operating conditions, "maximum reservoir capacitance" may be very nearly meaningless.

[GrimJosef]

Originally Posted by Radio Wrangler

... For amplifiers not being driven to the point where rectifier cathode emission lets the HT sag as part of the performance, solid state rectifiers are fine, and an awful lot cheaper than GZ34s ...

Just as long as all the capacitors can withstand the switch-on overvoltage transient which doesn't go away until the valves have warmed up. This is easy to achieve if the amp's being designed from scratch. But if you're modifying an existing amp then it needs checking. As does the circuit's general overvoltage tolerance, given the negligible forward voltage drop in solid-state rectifiers.

Cheers,

GJ

[PJL]

A GZ37 is definitely not going to have any trouble with voltages.

There is a similar GZ33 which tolerates a larger reservoir capacitor and it would be interesting to understand why and the intended purpose of the GZ37.

[G6Tanuki]

If you'd be happy to go semiconductor [if only so you could put your existing valve-rectifier somewhere safe for the future and in the meantime get on enjoying your amp without fretting] - in the 1960s there were plug-in semiconductor rectifier assemblies produced.

"International Rectifier" did a series of these - they were basically a 'tall' octal-plug with silicon diodes encased within.

Similarly, there was a series of "Rectifier subassemblies" which featured an octal-plug base with an upstand that housed sets of 1960s rectifiers [BY100 type] along with their associated and needed-at-the-time parallel-resistors/capacitors, all housed in a spun-aluminium case that looked a lot like an electrolytic-capacitor-of-the-time. I came across loads of these in Pye F27-type two-way-radio base-stations, where they were typically powering a QQV06/40A VHF double-tetrode that was delivering 60 Watts of RF output, day-in day-out. Never saw one fail!

[ms660]

Regarding International Rectifier, here's a link to International Rectifier replacement rectifiers, it includes info for surge limiter calculation and power dissipation of the said:

https://www.kevinchant.com/uploads/7...placements.pdf

Lawrence.

[GrimJosef]

I don't know if IR still make those devices but Ted Weber does something very similar. His are aimed mostly at guitar amp owners, who want the sag that a valve rectifier's substantial Vf under load can give - here for example https://www.tedweber.com/wu4gb. He also does a basic unit (the WS1, with negligible sag) which can have a thermistor included to give you a bit of a soft start.

Cheers,

GJ

[Heatercathodeshort]

The rectifier in my WADAR amp is a 5U4G [U52] J.

[Cobaltblue]

Re-opened at Ian's request

Cheers

Mike T

[Ian - G4JQT]

Had a minor mishap with my Wadar amp recently. It was switched off and suddenly switched back on after running for about five hours. Blue flash from the Cossor GZ37 and a blown 1A mains fuse. The 250mA HT fuses on the secondary before and after the rectifier did not blow. (Not yet tested the flashed GZ37 to see if it survived. It probably did.)

I had a similar thing happen a year or two back with the GZ34 which is higher rated regarding reservoir capacitors.

Therefore I'm minded to add 1N4007 diodes in series with each of the rectifier anodes as shown in post #5 with a replacement GZ34, although some posts here advise against this.

Nevertheless, if I add these diodes is there any benefit in putting say 0.01 uF disc ceramic capacitors in parallel with each diode, or from diode cathodes to earth, or between cathodes as there is some reservation regarding adding silicon diodes. Most hi-fi amps use silicon diodes in the power supplies so I don't understand the problem, or are the reservations regarding mixing silicon and thermionic rectifiers?

On a general note, my Wadar amp gets daily use for at least two or three hours a day, and although can't be described as top-end hi-fi it is more than adequate for the TV sound.

The only modification I made to the original design was to add a 'hum-dinger' balancing pot across 6.3V heater line to make a noticeable hum reduction.

Thanks and regards,

Ian

[jjl]

There's a thread on the GZ37 on here from January 2022. There does seem to be some doubt about what the rationale for developing the GZ37 was.

John