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1965 Deluxe Reverb rectifier current winding

Discussion in 'Amp Tech Center' started by Diverted, Feb 5, 2021.

  1. Diverted

    Diverted Tele-Meister

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    Hi,

    I've got a 1965 Fender AB763 deluxe reverb that is running high plate voltage due to today's wall voltage. At 460 plate voltage I'm biased at around 19ma, for 64 percent or so dissipation.
    I want to bring that plate voltage down as I don't want to shorten the life of these tubes (NOS GE 6V6GTs). I don't want to use a bucking transformer or variac.

    A lot of people would put JJ 6V6s in there as they can handle higher plate voltage. However I have a question:

    The power transformer is original Schumacher 025130.
    I have read contradictory stuff online about what kind of rectifiers the 5v winding can handle. I have seen some that say it's meant for a 2A GZ34, and others that say it regularly shipped in amps that used a 3A 5U4.

    If this is the case I would rather try swapping out the rectifier to give me around 430 on the plates.

    Can anyone point me to any definitive info online as to whether this transformer has the current capacity to handle a 3A rectifier safely?

    Thanks!
     
  2. timfred

    timfred Tele-Meister

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    I'm going to sidestep your question and suggest a 5R4 rectifier instead, which is what I run in my 1966 DR. It brought B+ down from 450V to just over 400V if I remember correctly.

    It's a 2 amp filament like the GZ34/5AR4, and the same 250ma output current. The ruggedized 5R4GYB versions were spec'ed for a 20uF first filter cap (DR has 32uF stock, so not too much higher) but I haven't had any problem with a plain 5R4G even though the datasheets say only 4uF for first filter cap.

    As a plus NOS 5R4 are super cheap.
     
    Last edited: Feb 6, 2021
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  3. Wally

    Wally Telefied Ad Free Member

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    What is the heater filament voltage?
     
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  4. Diverted

    Diverted Tele-Meister

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    5.8 at the moment.
     
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  5. BassKulcha

    BassKulcha TDPRI Member

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    Maybe an off-the-wall question (apologies in advance if this comes across as a thread hijack...)

    In the vintage tube hifi world (where I originally come from), a way to both slightly reduce higher wall voltages and also save tube life is to solder in a CL-80 or CL-90 inrush current limiter/thermistor before the fuse sees any AC. Is this done in the guitar amp world? Would it be useful in this instance?

    Here's an example of a CL-80 installed in a Pilot SA-232 stereo amplifier that I restored:

    index-5.jpg index-6.jpg
     
  6. SoK66

    SoK66 Tele-Afflicted

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    To keep the amp's designed in characteristics, rather than jacking with differnt rectifiers try a buck transformer to drop the wall voltage. (Buck transformers do the opposite of a regular power transformer. They LOWER voltage rather than increase it.) Most will give you something like a -6vac and -12vac drop. Here at my place that means dropping the normally 123-125vac at the wall to 117 - 119 vac, or on the -12vac setting, 111vac - 113vac. With the Tone Preserver I have been using for the last 15 years set at -6vac the B+ on my '66 BFDR runs between 425vdc - 430vdc, right in the pocket.

    There are a few commercial buck tranformers out there, the Brown Box, The Tone Preserver, etc. are dedicated to tube amps, affordabe and robust. Another option is just using a variac, but they can be bulky. You can also make one using a commercially available transformer and an enclosure. Here's a link:

    http://www.geofex.com/article_folders/vintvolt/vintvolt.htm
     
  7. BassKulcha

    BassKulcha TDPRI Member

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    I have a vintage Powerstat that does this for my gear that really does need to run at lower wall voltages. Built like a tank, works forever, pleasing to use (big bakelite-style knob) and fits in aesthetically with the gear I run. You can sometimes find them around $100-150...
     
  8. Wally

    Wally Telefied Ad Free Member

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    If that 5.8VAC is on the 6.3vac heater filament winding and not the 5vac rectifier filament winding, that is curious. One would expect the 6.3vac winding to read quite a bit higher.

    Re: Schumacher #025130. A 1965 Deluxe Reverb should not have a PT with this number in it. That model number was not introduced until 1966. Are you sure that PT is original, or could it be a replacement transformer with a 1975 date code instead of 1965?
    If this PT is from 1975, it was designed to use a 5U4 rectifier, and a GZ34 will yield very high voltages.
     
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  9. Dacious

    Dacious Poster Extraordinaire

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    You're about 5% away from spec. The schematic shows 5% tolerance. The variation is nothing. Bias voltage and current draw determine plate dissipation. If voltage is high drop bias as you have done.

    Don't obsess over b+.

    If you want to drop By+ use a 5V4. When you crank up bias it'll probably sound pretty much as before.
     
  10. Wally

    Wally Telefied Ad Free Member

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    imho, the B+ is about 10% high. If this 5.8vac is the 6.3vac winding, that is odd because it too should be higher than the schematic by about 10%. It makes more sense that the 5vac winding is reading 5.8vac.
    I do not think this is a original PT...or this is not a 1965DR....the devil is in the details, eh? After ascertaining what PT this is, the. One knows what rectifier the PT was designed to use....aGZ34 for the BF schemes or a 5U4 which was introduced in the AB868 schematic...August, 1968. And yes, maybe a 5V4 or even a 5R4 would be a better rectifier IF needed no matter which PT this is.
     
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