Considerations when replacing GZ34 with 1N4007's

[Mr_bibbles]

Hey Y'all,

I normally wouldn't do something like this, but I have a very nicely made Lumley HIFI that has a goofy socket for it's rectifier; long story short, the pins of any rectifier are not long enough to fit into the tension sockets... At the very bottom of their travel they barely even touch the contacts, so even a spring retainer might not be reliable: When the amplifier is on, any tiny bump will disconnect a pin of the rectifier, making it far less than ideal. I tried looking underneath to see if I could tighten the socket pins, considered adding solder to the leads, etc etc, but there's simply no way to get this tube (or any others) to fit snugly.

So if I were to replace the gz34 with a hard wired full-wave 1N4007, I know the voltage would go up by about 50v so I would need to ensure that the B+ line is rated high enough, and that the filaments are off their own transformer taps so that they'd be unaffected.

My question is, am I missing some important variable here? Would this be safe assuming I don't exceed component values? If not, could I add a simple dropping resistor to pad the voltage down?

Thanks!

[nevetslab]

One thing that will occur making this change, is 5AR4/GZ34 rectifier tube has a 10 sec or so warmup, and the DC supplies slowly ramp up to nominal supply voltage, while the heater supply warms up the tubes. With the 1N4007 rectifiers in place of the rectifier tube, the HV will be applied immediately.

You might want to consider replacing the tube socket, or, if feasible, re-tension the tube sockets.

[Mr_bibbles]

One thing that will occur making this change, is 5AR4/GZ34 rectifier tube has a 10 sec or so warmup, and the DC supplies slowly ramp up to nominal supply voltage, while the heater supply warms up the tubes. With the 1N4007 rectifiers in place of the rectifier tube, the HV will be applied immediately.

You might want to consider replacing the tube socket, or, if feasible, re-tension the tube sockets.

Thanks Nevetslab, it's hard to explain but I tried describing it in my first post; the tube pins barely even REACH the sockets, much less fit snugly. There is no way to re-tension them. It has never worked while my customer had it. I wish I could take a picture to explain how funky this thing truly is, but a new phone is in the US mail service at the moment... My next move usually would be to replace the socket, but it's mounted in such a way that the non-standard chassis mount points function as part of the circuit's ground, so I would have to rewire half of the power supply to get a new standardized socket in there IF It would fit in the mount.

I never like altering a circuit when I could repair the existing one, but in this case it seems like the least aggressive course of action.

As for the warm up times, I saw someone in a different forum suggest adding a thermistor to help with the warmups.. the issue here is that B+ would hit the rest of the tubes before the heater can make them comfortable with the voltages?

[g1]

I guess it's a different socket than the ones for the 6SL7's ?
I had wondered if maybe there was one of those repair bases on the 5AR4, that are used when the locator pin breaks off.

[Pixel]

How about pushing a long lead octal socket into the existing socket?

[Enzo]

Looking at the picture. Let me ask, is the socket merely mounted below the chassis surface, AND your tube won't fit through the hole in the chassis? And that is why it won't mate the socket?

If so, what are the odds of removing whatever spacers there are and mounting the socket flush with the chassis surface so the tube doesn't have to fit in the hole.

[Mr_bibbles]

G1, it's definitely the 5AR4, and this socket is the only offender on the HiFi.

Pixel, that's a pretty great idea! Unfortunately since it's a HiFi all of the tubes are on display on a very nice finished chassis box... it might be a little unsightly, but I'll look around and see if I can find something that could work.

Enzo, the tube base fits flush with the socket base and not the chassis itself; it mates with the socket perfectly. THEN, while it's kissing the plastic, you can see the pins travel underneath and just barely even reach the actual socket pins. It's astounding.


Like you guys, my first thought was "There's no way this socket would work like this / I'm missing something / / etc etc " but after a thorough inspection (and a little laugh with the shop manager who took a peek) I'm considering the 1n4007 as the "least invasive" course of action. So let's say, for the sake of argument, the Space/Time Police have issued us a warning, and we are not allowed to install a new 5ar4 and are going to use 1n4007's to make a full wave rectifier. I'll try and get a picture going of the doom socket once my phone arrives, at least for the forum's entertainment.

SO

1. We know we'll have to test voltages to ensure no component is exposed to a voltage higher than it's rating

2. As per Nevetslab's post, the in rush "warm up" times for the B+ and other associated parts will be much faster. This might be totally fine since the rectifier is probably the most at risk for inrush damage, but a thermistor in series with the 5ar4 could help inrush times.

3. ????? Perhaps a dropping resistor to emulate the voltage drop over the 5ar4?


Perhaps I'm being overzealous here and a SS rectifier isn't going to be a big deal here, but I want to make sure first and foremost I couldn't damage this amp with some rookie mistake I'm missing.


PS Happy Fourth! I hope everyone is enjoying time with their family and friends today.

[Mick Bailey]

I would wire in a different socket. Did you take any measurements and compare the socket fitted to (say) a Belton?

[Mr_bibbles]

Thanks for the reply, Mick. I would also wire in a different socket normally, but it's not at all like a normal octal you'd think of like I was saying above, and replacing the socket would result in rewiring half of the PSU so this seems like a better solution... I've never ever seen a socket like this before, and It's a matter of PIN DEPTH not tension. I don't know how to explain without photos, but so far pretty much everyone except Nevetslab is more interested in the goofy socket than my question! So let me just say...

I know this is an abnormal approach to a poorly fitting tube, and have considered the normal remedies like tensioning and socket replacement.

...Seriously!

You just have to take my word on it. I'll try and post photos afterward.

Any advice on the real nitty gritty of solid state rectification's global effects? That's really what I was hoping to learn, especially any indirect consequences. Thanks!

[Enzo]

I can't help but fear we are missing part of the puzzle, since it seems so unlikely anyone would design an amp physically so that only a very specific specialized part was required

Photos of the offending socket would be SOOO helpful. Don't you have a friend, wife, neighbor, child, girlfriend, golfing buddy or SOMEONE who could snap a couple photos?

And just to be clear, your socket...if we could take JUST th actual socket part of the socket assembly out in our hand, and the tube shoved all the way to the hilt, the pins STILL wouldn't reach?

[Mr_bibbles]

Yes, yes I know it sounds like I'm crazy! I am not in the shop over the weekend and I will definitely get pictures when I can. It's always possible I have overlooked something, Especially when encountering something new so I will triple check everything.

The customer found this second hand and it has never once worked...

So lets say for now that using a tube in this socket is a heathen activity! A paganistic abomination that must be avoided at the cost of angering the sun god!

THEORETICALLY, if our ONLY option was to install a SS rectifier in the amp (which, again, I have never done and will probably never do again if possible)... what would we be watching out for? For the purposes of our own edification only, of course!...

*looks nervously at sun god statue*

[Mick Bailey]

Being a SET amp you don't need to worry too much about sag, but the over-voltage that a straight SS rectifier gives needs addressing. Measuring the current draw should enable you to calculate an appropriate resistor, but this will produce additional heat inside the enclosure. Another way is to use a MOSFET dropper, but again the excess voltage is dissipated as heat - though in most cases the chassis can be used as a heatsink. Another possibility is to use a Zener. The slow startup is more difficult to emulate because the warm-up time of a GZ34 is much longer than many thermistors. Some browsing of spec sheets needed. You may need to pick your diodes to prevent switching noise - maybe some ultra fast devices.

[Helmholtz]

Replacing the GZ34 with SS diodes should increase B+ by less than 20V. MIght not be a problem at all.

If you want to drop the extra voltage, please post B+ and 300B cathode voltage for advice.

[Mr_bibbles]

Being a SET amp you don't need to worry too much about sag, but the over-voltage that a straight SS rectifier gives needs addressing. Measuring the current draw should enable you to calculate an appropriate resistor, but this will produce additional heat inside the enclosure. Another way is to use a MOSFET dropper, but again the excess voltage is dissipated as heat - though in most cases the chassis can be used as a heatsink. Another possibility is to use a Zener. The slow startup is more difficult to emulate because the warm-up time of a GZ34 is much longer than many thermistors. Some browsing of spec sheets needed. You may need to pick your diodes to prevent switching noise - maybe some ultra fast devices.

Thanks so much Mick this is exactly the kind of thing I was asking for! I'll read into all of these options. As for using a resistor, the enclosure has PLENTY of space and is hand wired PTP so dissipation shouldn't be an issue, I usually over-rate my droppers when repairing too.

Replacing the GZ34 with SS diodes should increase B+ by less than 20V. Might not be a problem at all.

If you want to drop the extra voltage, please post B+ and 300B cathode voltage for advice.

Thanks Heimholtz! That's pretty much what I was thinking, but I wanted to look at it from all angles since in case of potential collateral damage. I'll do a bunch of comparative tests from before and after if I do decide to go this route and if anything seems rotten I'll definitely check in.

All hail the sun god,

Mr_Bibbles

[Helmholtz]

Regarding sag:

It's a class A amp. Class A means constant power consumption at all signal levels - so no dynamic sag. Sag isn't desirable for HiFi amps anyway as it reduces dynamics.

Dropping the extra voltage will produce some heat - but not more than the GZ34 did.

Diode switching noise will most probably be no problem because of the 2-step choke filtering.

Fast voltage ramp-up might show as a turn-on thump. Maybe not.