GZ34 in 5U4GB Fender circuits

muchxs

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Whole front part of this (zombie) thread is about Princeton Reverbs. Fender changed from the 022772 PT to the 01--- whatever. A few early units for the 125P1B but they're mostly the 022772 "Champ" transformer. They made the change in 1970. Princeton Reverbs up to 1970 usually have the 022772 unless it mellted and was changed. The quick way to melt a 022772 is to run a 5U4G rectifier. It will still take a while. The transformer will run barbecue hot.

Aftermarket "022772" transformers are more or less faithful to the specifications of the original. We want the ones that are less faithful top the original design. We want 5V / 3A and high voltage as low as we can get it. Stock is around 330v-0-330v. There are aftermarket versions intended to run EL84 tubes. Start low and jack the high voltage with a GZ34. You amp will thank you.

See... 6V6 tubes were designed to operate on 300v max. 6V6GTA tubes will withstand much more. Still, they sound stressed to me up and over 400v. Maybe it's just that I know I'm cutting the life expectancy of my tubes in half.


On to the Deluxe Reverb. '60s versions use Fender's AB763 circuit. Every other AB763 amp uses 6L6 tubes. It's as if Fender designed the '60s line, ran out of time with the sale to CBS and found themselves very light on "6V6" amps.

Stock Deluxe Reverb transformers will likely run 5U4G rectifiers or GZ34s. I really don't like how high the high voltage goes. Never have. Good old RCA 6V6GTA tubes as used in 1965 will live. I wouldn't trust EH or Tung-Sol reissues in there. It's not like we have a lot of good choices here...

Actually, we do. Go off the charts. The tube charts that is. Use a 5V4 rectifier. It drops voltage more like a 5U4G but it has a 2 amp filament like a GZ34.

I have a sleeve of Sylvania "GZ34s" that are likely 5V4s. They look just like 'em.

I never liked the high voltage situation in Deluxe Reverbs so I fixed it. I went in with an associate. We had Classictone wind a bunch of Deluxe Reverb transformers with tapped high voltage secondaries. We had 'em wound with 2 amp filament windings. The idea was to keep the transformer as light as we could. If I had to do it over I might pay the weight penalty and use a 3 amp filament. I know better than to use a 5U4G with that transformer. Still, it's a question that comes up all the time.
 

schmee

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Those amps often run way over the voltage 6V6's should tolerate. So a GZ34 is adding to that problem. If you use JJ 6V6's you should be OK. Otherwise you risk blowing out the output tranny. Dont ask how I know.... OK, I'll tell you anyway... A set of NOS RCA Black Plates took out the tranny in my BFDR in less than 3 hours use. It runs about 465 plate voltage. JJ's have been fine for a decade. Both my old BFDR and BFD run up there and sound lucious though so I've been reluctant to lower the voltage.
 

zombiwoof

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Whole front part of this (zombie) thread is about Princeton Reverbs. Fender changed from the 022772 PT to the 01--- whatever. A few early units for the 125P1B but they're mostly the 022772 "Champ" transformer. They made the change in 1970. Princeton Reverbs up to 1970 usually have the 022772 unless it mellted and was changed. The quick way to melt a 022772 is to run a 5U4G rectifier. It will still take a while. The transformer will run barbecue hot.

Aftermarket "022772" transformers are more or less faithful to the specifications of the original. We want the ones that are less faithful top the original design. We want 5V / 3A and high voltage as low as we can get it. Stock is around 330v-0-330v. There are aftermarket versions intended to run EL84 tubes. Start low and jack the high voltage with a GZ34. You amp will thank you.

See... 6V6 tubes were designed to operate on 300v max. 6V6GTA tubes will withstand much more. Still, they sound stressed to me up and over 400v. Maybe it's just that I know I'm cutting the life expectancy of my tubes in half.


On to the Deluxe Reverb. '60s versions use Fender's AB763 circuit. Every other AB763 amp uses 6L6 tubes. It's as if Fender designed the '60s line, ran out of time with the sale to CBS and found themselves very light on "6V6" amps.

Stock Deluxe Reverb transformers will likely run 5U4G rectifiers or GZ34s. I really don't like how high the high voltage goes. Never have. Good old RCA 6V6GTA tubes as used in 1965 will live. I wouldn't trust EH or Tung-Sol reissues in there. It's not like we have a lot of good choices here...

Actually, we do. Go off the charts. The tube charts that is. Use a 5V4 rectifier. It drops voltage more like a 5U4G but it has a 2 amp filament like a GZ34.

I have a sleeve of Sylvania "GZ34s" that are likely 5V4s. They look just like 'em.

I never liked the high voltage situation in Deluxe Reverbs so I fixed it. I went in with an associate. We had Classictone wind a bunch of Deluxe Reverb transformers with tapped high voltage secondaries. We had 'em wound with 2 amp filament windings. The idea was to keep the transformer as light as we could. If I had to do it over I might pay the weight penalty and use a 3 amp filament. I know better than to use a 5U4G with that transformer. Still, it's a question that comes up all the time.
I put a 5V4 in my vintage '65 PR to bring down the voltages a bit with the modern wall voltage, the good thing is that they are not too expensive even for name brand tubes. I had tried a 5R4 but that dropped things a bit too much, more like a 5Y3 does, so I put that one away for another day. I've never measured how much the 5V4 drops the voltage, but I would suspect it does what it's meant to.
Al
 

Krackle

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Thanks man..yes. The amp is in for a recap right now and I asked the tech who's doing it to take some measurements and sort out whether the Fender is being correct..it could well have been built
based on the earlier specs of the circuit..we'll see.
 

11 Gauge

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I put a 5V4 in my vintage '65 PR to bring down the voltages a bit with the modern wall voltage, the good thing is that they are not too expensive even for name brand tubes.

I put the Weber Copper Cap equivalent of a 5V4 (their WV4) in a '79 DR that had an obnoxiously high B+, and it got things just right.
 

SoK66

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Reviving this relevant thread..hope that's ok..

I have a 1969-70 Deluxe Reverb..The Rectifier that was in it is an RCA 5U4GB...however, in oppostion to some of the info in this thread...the sticker on the inside of the cabinet calls for
a GZ34..So..it's basically a 1970 Silverface DR..all original with '69 PT and OT..choke n reverb driver are 1970..

Do i follow the sticker?? Sometimes..quite bit in these transition periods..they seem to be not accurate..

Beginning late '67 CBS Fender wasn't at all concerned with the accuracy of the tube charts and just used up what they had before ordering more. The new ones arrived maybe '71 (?), omitted the circuit code and showed the correct 5U4GB rectifier. On Princeton and Deluxe Reverbs the 5U4GBs started to show up in '69 as is the case with yours. As noted, only way to know for certain what you should have for a rectifier is to measure B+ & plate voltage, BUT be certain the line voltage going into the amp is around 117vac, not today's 122 - 125vac.

Differing stories about why Fender switched to the 5U4, one was they were concerned about the supply of GZ34s drying up, another was they were reacting to increasing line voltage as the grid employed more & more efficiencies. That last one seems odd, because as the power transformers in the amps changed around '70 - '71 even with a 5U4GB the B+ will measure way high compared to the silverface era schematic values.
 

MrCoolGuy

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You will get a slower ramping up of voltages with the gz34 but you need to rebias due to the higher voltages. I put a 5u4gb in my sfpr and the voltages immediately jumped up to over 510v then settled down to around 430-440. Put in the gz34/5ar4 and they slowly rose to about 460 (a little high). I haven't done it to this one yet but the last one I had that did that, I added a 1k resistor (I think it was 1k) before the first filter cap to knock it down to about 430. Your gonna want to check your filament voltages too with the change to make sure they aren't too high.
Old thread... I know, but you put a 1k resistor between the rectifier tube and the first filter cap and reduced 30v! ?
Explain this to me.
 

2L man

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Current cause voltage loss to resistor. Current calculates Voltage divided by Resistance so in that case current is 0,03A = 30mA.

When rectifier tube is too efficient like GZ34 proper place to install voltage dropper resistors is rectifier anodes one on each anode. Two about 200 ohm resistors are OK to make GZ34 act like 5U4.
 
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schmee

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I stuck a GZ34 in my 1972 Princeton Reverb (I think #AB1270) which calls for a 5U4GB rectifier tube. I like the tighter tone with less sag.

I have read that the GZ34 rectifier increases voltages though, and that this can be trouble particularly in the already strained 6V6 output tubes of Princeton and Deluxe Reverb Fenders. My questions are:

What am I risking by doing this? Just extra noise, shorter tube life, etc. --
or component failure?

Also, what is the difference between the 5U4GB rectified PR (AB1170?) and the GZ34 rectified Blackface circuit (AA764?)? What difference makes it riskier to use the GZ34 in the 5U4GB circuit?

I had my AB1270 PR brought up to speed/maintained a few months ago so it should be in good shape electrically...I should have "blackfaced" it then, but I forgot frankly. I had heard that the smaller Silverface and Blackface Fenders are quite similar and thought it relatively unimportant.

What changes do I need to make to the AB1270 circuit to maximize the tone with the GZ34, as well as make it reliable long term?
I've found in the real world there is little voltage increase from a 5U4GB to a GZ34. Often not as much as published, 5 volts maybe. The 5U4GB draws a lot more amperage though. But yeah, the older PR's and DR's run very high in voltage. If you blow a power tube it can take out your output tranny. Dont ask how I know! Since that occurred I only use JJ 6V6's. An RCA blackplate took out my tranny.

I've heard the later PR power trannies were more robust to deal with the 5U4GB's. Not sure though. PR trannies were always marginal to begin with.
 

schmee

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I've found in the real world there is little voltage increase from a 5U4GB to a GZ34. Often not as much as published, 5 volts maybe. The 5U4GB draws a lot more amperage though. But yeah, the older PR's and DR's run very high in voltage. If you blow a power tube it can take out your output tranny. Dont ask how I know! Since that occurred I only use JJ 6V6's. An RCA blackplate took out my tranny.

I've heard the later PR power trannies were more robust to deal with the 5U4GB's. Not sure though. PR trannies were always marginal to begin with.
Well crap, it's a Zombie thread! 🤣
 

SoK66

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If you run a vintage 5U4GB Fender at today's line voltage and then throw in a GZ34 substitution you're flirting with really high B+/plate voltage. Knock the line voltage back with a buck transformer to 115-117 VAC and you might be ok. Your guideline for the B+ sweet spot is the heater voltage. Whatever gets you closest to 6.3VAC is the ticket. Rectifiers have no influence on heater voltage, you have to adjust between the wall and the amp.
 




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