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Old 27th October 2006, 03:50 PM   #1
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Default need tube rectifier info

I have an older tube amp that needs to be adapted to modern 120vac and am just trying to gather some ideas on how to do it. A lot of it is just common sense such as putting voltage dropping resistors inline with the heater elements. I think what I might be a bit stuck on is getting some more voltage drop from the tube rectifiers. GZ34's are in place now and probably still work, but am thinking I need to perhaps use another type, such as the GZ33. Does anyone know if the 33 has more drop? It does have a higher heater current requirement that I think the xfmr just might be able to handle with everything considered, and not cause overload or anything harmful. I'm not sure of levels here as I don't recall what line voltages were back in the 50's. Was it 110 or 115vac? I did run a quick noload test using a variac at 110, 115, and 120 and the output needless to say varied enough to be an issue. Any thoughts here or sage advice? I have no experience in tube rectifiers and have Googled and searched on here till I'm tired of it, but I think a rectifier with sufficient drop will put me back in the running. My setup does use dual GZ34s. Thanks! Btw, I got this amp at a garage sale with a blown EL34 and what looks like p.s. cap damage, maybe due to the amp being used with 120ac, but that's just a guess too.
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Old 27th October 2006, 04:00 PM   #2
kevinkr is offline kevinkr  United States
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You need to tell us what this amplifier you are talking about actually is. Note that at this point in time cap failures are to be expected regardless of line voltage - electrolytics don't last forever.

A bad EL34 could have been caused by a failed coupling capacitor, common with the impregnated paper types used in the 1950's and 1960's. A bad EL34 could also be caused by many thousands of hours of operation - ie it just wore out and died.

In most cases we just replace the existing electrolytics with ones that will survive at the slightly higher resulting voltages, usually nothing more is required.

No load testing tells you nothing more than that your transformer is good, without knowing its regulation you have no idea how much the voltage will actually sag when loaded. IT could drop as much as 15% - 20% depending on the winding resistances and core losses.

Most tubes are rated for +/-5% minimum of nominal filament voltage, and only in rare cases will it be (significantly) over the limit in most vintage gear designed for 115 - 117V operation, 110V units may be a little higher.

No need to overcomplicate. In extreme cases a small bucking transformer on the incoming ac can be used to reduce the supply voltage. I have never encountered a case where this was actually necessary.

The GZ33 is not currently in production afaik, and you can expect to pay a lot for vintage ones. Not necessary IMO..

Line voltage varies all over the place depending on time of day and your location. Where I live the voltage is a pretty steady 118V most of the time, sagging slightly below this during dinner hours. (Quincy MA) When I lived in Stow it varied between 128 and 122V until I complained to the utility about the excessive voltage which they happily addressed.

You haven't told us your line voltage,.

Note also that cheap dvms may have ac voltage accuracies of 3% or worse so depending on your meter you may not even accurately know the line voltage in your locale.
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Old 27th October 2006, 05:50 PM   #3
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You could always add an inrush-current limiter in series with the primary. Depending on the value you choose, it will drop a few volts and have the benefit of a softer start-up.
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Old 27th October 2006, 09:04 PM   #4
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Inrush current limiters are a good idea, and if you are concerned about the effect on the sound or the several volts they may drop you can short it with a relay shortly after power is applied. This is good for saving tube filaments, switch contacts, and of course fuses!
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Old 27th October 2006, 10:54 PM   #5
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Hi all,
your idea of replacing the rectifier is not so bad to me indeed (through it neer appeared to me !), the GZ34 is a rectifier tube with one of the most little voltage drops, it shares its pinout with a lot of commons rectifier tubes (so...with more voltage drop). this as Kevin says depends of your circuit. How much mA@whatV ?
Inrush current limiters are always a good idea with today's AC (are they are good for the filaments too (& switches: I never thought about that either, thx again).
this have to be tried.But before you should test cap leakage !!!
Don't be scary to much on AC voltage, I think that back in those times the line regulation was fluctuating a lot & circuit were made (as tubes) for a 20% tolerance.
PS: I always change or reform electrolytics in vintage gear ( <1% leakage, it depends of bleeders resistors also)
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Old 27th October 2006, 11:15 PM   #6
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In addition to the other points already made about the GZ33, keep its 3 A. heater draw in mind. The 5AR4/GZ34 draws only 2 A. of heater current.

If ratings are not exceeded, the 5V4/GZ32 could be an answer. The forward drop in the 5V4 is somewhat higher than that in the 5AR4. Like the 5AR4, the 5V4 has a cathode sleeve and exhibits a slow rise of B+.

If you REALLY want to shed some Volts, look into 5R4s. The 5R4 exhibits a 67 V. forward drop at the max. allowed 250 mA. Also, the 5R4 filament draw is 2 A., like the 5AR4. Unlike the 5AR4, the 5R4 is directly heated and turns on quickly.
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Old 28th October 2006, 02:51 PM   #7
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Default oops!

Boy, do I have egg on my face. I looked again for the voltage requirements label I didn't see the first time around and it says 117vac. It's on the bench in an awkward position stripped down so I had to rubberneck to see it, but it's there. It's a pleasant surprise, nothing that I am used to in this hobby where every inch must be fought for. Anyway, I still appreciate all the input and like to have good alternatives going into battle. I landed a matched quad set of Sovtek EL34s a few years ago from some guy selling surplus and just snagged an Electronics Concepts Unlytic 60uf custom cap that should fit right in with a little doing. Getting an electrolytic seemed like such an unexciting way to go. Hopefully something will get up and going when it comes in and maybe not much need be done outside of what's stock. I don't like replacing parts either if not needed since that's just a waste. The amp is a 50w/channel Sargent Rayment SR 5100 built like a tank,..and I'm trying my best not to be too stupid with it.....lol. I just kind of figured an amp built this long ago might have different votages but I guess I stand corrected. I'm not used to making mistakes and at one point thought I never did, but I was mistaken..
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