GZ34 in 5U4GB Fender circuits

[tatittle]

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?

 

[tatittle]

I came across these points at fenderguru:

It takes very little effort to blackface the silverface PR models without the push-pull volume boost. If you have a blackface PR you should inspect the circuit and not just the tube chart to verify which model you have, either the AA1164, AA764 or AB1270. Sharp readers will see that the silverface AB1270 circuit is identical to the blackface AB1270. Otherwise, there are minor differences within all blackface and silverface circuits.

To our knowledge, the following changes were introduced in AB1270 and AA764 (compared to the pre-CBS AA1164):

The 5U4 rectifier tube was changed to a GZ34 in AA764. GZ34 has noticably less sag than a 5U4. So if you want the sag (compression effect) of a pre-CBS blackface PR, go down to 5U4.
The electrolytic cap providing dc voltage to tremolo circuit changed from 25?F/50V to 50?F/70V in AB1270 and 50?F/70V in AA764.
The reverb recovery circuit in AB1270 got a 2000pF highpass cap leading higher frequencies to ground. This makes the reverb slightly darker with less presence, if noticable at all. See the green circled cap to the left.

But also this quote at freeshell:

Installing the GZ34 in an amp with any other tubes mentioned above will increase voltages and reduce sag (if the amp draws enough current over the rectifier to cause it to sag). This may give a pleasing sound to you, but be warned: the voltage increase may cause problems. Parts that have been working together for 25 years sometimes don't react well to increasing voltage on them. Also, you don't want to install a GZ34/5AR4 in a Deluxe Reverb or Princeton Reverb that came stock with a 5U4GB. This will increase voltages and those 6V6's are already way beyond their limits in these amps!

 

[Rod Parsons]

I really love the soulful sound of a little 'sag'. Some so-called tech at a music retail store, apparently couldn't find out what was wrong with my Princeton Reverb and it remained in their repair shop so long that I went and asked for it back.. The repair guy, for some reason had put in a replacement solid state devise... That was way back in 1992. A band mate knows a lot about tube amps and immediately diagnosed it as a blown power transformer, and a couple resistors or something and fixed it. He left the solid state rectifier in and it blew a cheap Russian tube that the repair guy put in... I got some NOS USA Phillips power tubes and it worked fine When I joined up with this forum, I began to learn about rectifiers and tons of other stuff. I got a nos,[new- old-stock], 5U4 and ..WOW! then sound came alive.... The beautiful SAG was back.. Then I read that an old,[nos], 5R4 Rectifier would work... same voltages to the plates as the 'correct' 5U4, and it only used 2 amps of current instead of the 5U4, [which uses 3], thus putting less overall stress to the power transformer, which is already stressed when stock.. I put in nos 5R4's in my 1980 Princeton Reverb and my 1968 Bandmaster Reverb. They both sound fantastic... Cost for one 5R4 ,[that will last decades],...$11.00-$15.00..online... On computer, search NOS Tubes and select options for buying them. Try one??? ..

 

[tatittle]

Rod, I assume you mean 5AR4 when you type "5R4", right?

I am not sure though because 5AR4 NOS tubes are more expensive than the figure you quote. New ones are in that range though.

If it is a 5AR4 you are referring to, sounds like i is working for you...

Thanks!

 

[BobbyZ]

No he means 5R4. It ain't the same as a 5AR4.

 

[alnicopu]

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.

 

[Rod Parsons]

Rod, I assume you mean 5AR4 when you type "5R4", right?

I am not sure though because 5AR4 NOS tubes are more expensive than the figure you quote. New ones are in that range though.

If it is a 5AR4 you are referring to, sounds like i is working for you...

Thanks!

Yes it is the 5R4, not the 5AR4, which IS VERY EXPENSIVE! Actually an NOS 5U4 isn't very expensive either.. all the old American made tubes are good.. I have a 1969 Bandmaster Reverb amp that has a tube chart that calls for a GZ34/5AR4, [same tube].. That tube takes 2 amps of current. The amp had a 5U4 in it and I wasn't sure if I should be using that tube... it take 3 amps, and might blow the power transformer. So I asked "what should I do", here on TDPRI and a tech advised me to get a NOS 5R4 that gave the same performance as the 5U4, but using 1 amp less current in case the amp really did want a GZ34/5AR4 in there. Around the time my amp was made came a change from the 5AR4 to the 5U4. The 5R4 would work in both circuits and was cheap, and gave me a great sound with some sag.... I also put a 5R4 in my 1980 Princeton Reverb... Sounds better than ever, and runs cooler... 1 less amp...

 

[BiggerJohn]

I run a GZ34 in my early SF Princeton Reverb. Tube chart calls for 5U4. Voltages are still within reason plus it's louder.

BTW, a 5R4 will have more sag than a 5U4.

 

[SoK66]

A GZ34/5AR4 generates about 20 - 25 vdc more B+ voltage than a 5U4. This may not be harmful, but you need to know the upper plate voltage threshold of the power tubes you're running to ensure you don't go past a sensible level when properly biased. Where PRs are concerned, most of the blackface amps ran a 5U4. Only the earliest ones have a GZ34. The post 68 SF amps are all basically the same as their BF predecessors up until the pull-boost. Not much to "blackface".

 

[Inventour]

I have some old stock 6V6GTA tubes I been saving for years, I have one set of these in a 1965 Deluxe Reverb and they seem like the best tubes to handle the load. They say 420 volts is not uncommon on the old DRs when pushed. Out of 32 6V6GTA tubes I got 9 matched pairs, the rest I use for singles. After burning the tubes-in for a while you can see the dual flash marks on the bottom of these tubes. I have a Hobbs hour meter in the amp to keep track of the total time on the power tubes, it's inside the chassis and I will pull it to check in the near future. If your building a new amp you can add an extra punch hole (same as the Mains Cord) and install the hour meter in the rear of the amp and keep track of your total time on the power tubes. I just hook up the meter and leave them inside the chassis, so I don't drill holes in the old amps. You can also run the 2 wires down the strain-relief hole and plastic cable-tie the meter to the power cord in the rear of the amp.

http://rover.ebay.com/rover/1/711-5...0001&campid=5338148343&icep_item=140736571848

6V6GT = 12 watt max plate dissipation (Single Getter/Flash)
6V6GTA = 14 watt max plate dissipation w/controlled warm-up time) (Dual Getter/Flash)

 

[vibrasonic]

Nice stash of tube there Inventour.

 

[muchxs]

The Central Scrutinizer makes an appearance:

I came across these points at fenderguru:

It takes very little effort to blackface the silverface PR models without the push-pull volume boost. If you have a blackface PR you should inspect the circuit and not just the tube chart to verify which model you have, either the AA1164, AA764 or AB1270. Sharp readers will see that the silverface AB1270 circuit is identical to the blackface AB1270. Otherwise, there are minor differences within all blackface and silverface circuits.!

AA1164 is a Princeton Reverb.

AA764 is a Champ.

SF Princeton Reverb schematic is labled B1270

There is no blackface AB1270.


I tend to mix 'em up when I'm talkin' about 'em but I don't mix 'em up when I'm workin' on 'em.



.

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.

Which means stick a 525 volt CE cap can in there pronto or say "bye, bye" to the 450v stock Mallory.


I'll say this again: Fender came out with a nice new line of amplifiers in '63 and '64. Then Leo sold the company.


The Princeton Reverb (Class AB1) uses the same wimpy power transformer as the Champ (Class A). It's around a 70ma transformer which is inadequate for 2x 6V6s. Voltage is too high, current is too low. A transformer like that will have vicious sag under load. Oh, kewl. We like sag!

Oh, not cool. Transformer is overloaded from the get-go. Overloaded transformers run hot. The same transformers fail in Champs which no one seems to mention. I see SF Champs all the time with cooked PTs. It's only powering one 6V6 in a Champ. There are two in a Princeton. More 12A- tubes in a Princeton but they don't draw much.


Bottom line: Supply voltage is too high in BF / SF Princetons. I expect someone at CBS era Fender realized that and spec'ed a 5U4G instead if a 5AR4. Forwards thinking, too. 5AR4a were just about extnict by 1970.


Yeah it works. It's not a good idea.

 

[BiggerJohn]

Supply voltage in my drip edge Princeton Reverb is just fine, about 425 volts on the plates. I'm running the old coin-base GE 6V6GTs. Biased at about 21 mA, they sound great! No red stripe on the plates and no glow coming from the screens.

 

[telex76]

The previous owner of my SFDR included a note telling what his tech had done to the amp. Among other things it said; "My tech put in a GZ34/5AR4 to stiffen voltage".

It's hard to believe a tech would do such a thing. When I got the amp and took meter readings the plate voltage was 490. I put a 5U4GB in like the tube chart called for
And plate voltage dropped to 430, still on the high side, but not too bad.

 

[alnicopu]

The Central Scrutinizer makes an appearance:



AA1164 is a Princeton Reverb.

AA764 is a Champ.

SF Princeton Reverb schematic is labled B1270

There is no blackface AB1270.


I tend to mix 'em up when I'm talkin' about 'em but I don't mix 'em up when I'm workin' on 'em.



.


Which means stick a 525 volt CE cap can in there pronto or say "bye, bye" to the 450v stock Mallory.


I'll say this again: Fender came out with a nice new line of amplifiers in '63 and '64. Then Leo sold the company.


The Princeton Reverb (Class AB1) uses the same wimpy power transformer as the Champ (Class A). It's around a 70ma transformer which is inadequate for 2x 6V6s. Voltage is too high, current is too low. A transformer like that will have vicious sag under load. Oh, kewl. We like sag!

Oh, not cool. Transformer is overloaded from the get-go. Overloaded transformers run hot. The same transformers fail in Champs which no one seems to mention. I see SF Champs all the time with cooked PTs. It's only powering one 6V6 in a Champ. There are two in a Princeton. More 12A- tubes in a Princeton but they don't draw much.


Bottom line: Supply voltage is too high in BF / SF Princetons. I expect someone at CBS era Fender realized that and spec'ed a 5U4G instead if a 5AR4. Forwards thinking, too. 5AR4a were just about extnict by 1970.


Yeah it works. It's not a good idea.

What do you consider the proper remedy? Different transformer or just higher voltage filter caps and a 5u4g. Would it also be a better idea to replace the ot as well?

Thanks in advance. I would rather fix a design flaw than leave it stock, smoke a pt., repair it and it not be stock anymore by default.

 

[Rod Parsons]

The best thing you could do is put the 5U4 in there and don't worry about it... They are not expensive $15.00, for a good new old stock one. Brent Jesse,[online], has them. Also you could use a 5R4 substitute, that uses only 2 amps current, compared to the 3 amps for the 5U4. The 5R4 works, they are real cheap, they will last a long, long time... I am using them... they sound great... 1 less amp in current means your power trans runs cooler.. By the way [The 5AR4 is NOT the same tube as the 5R4]. Good luck.

 

[muchxs]

What do you consider the proper remedy? Different transformer or just higher voltage filter caps and a 5u4g. Would it also be a better idea to replace the ot as well?

Thanks in advance. I would rather fix a design flaw than leave it stock, smoke a pt., repair it and it not be stock anymore by default.

Hindsight is 20 / 20. Fender didn't have the benefit of hindsight in 1964. I'm seein' BF / SF Champ transformers that have melted their guts out running one 6V6. It's only through the magic of Class AB1 that we can get away with runnin' two. Princeton Reverb uses the same transformer.

I'd just as soon change it. Stick the original in a baggie. I don't care about "untouched solder joints", I can solder as well as the people who used to work at Fender. Maybe better 'cuz I take my time.

Then down the road when I'm dead and gone someone can stick a 5AR4 in there as per AA1164 and have no problems. The 525v CE cap can will probably last longer than I will.

While I'm in there I'll add a BillM designed Allen TO20 output transformer.

For those who don't want to mod your amp... don't.

490 volts with a 5AR4 in a silverface... 'nuff said.



.

 

[SoK66]

Back in the day we used to see so many PRs with smoked PTs coming back for warranty repairs that I stopped ordering them from Fender and walked the customers up to DRs instead. The reduced B+ of the 5U4s helped a bit, but the revised PTs post-'70 in the SFPRs seemed to be OK. My Franken Princeton is a '79 SFPR PB chassis, PT & OT with BF board, OS blue moldeds, etc. With a 12w 6V6GT set it runs 436vdc+ on the plates biased at 16 ma.

 

[Krackle]

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..

 

[BobbyZ]

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..

CBS wasn't real fussy with those charts and used up a lot of old tube charts after they started changing circuits in 1968. I suspect that's the case here.
While they were still making blackfaces they did it too. My 67 BFDR has a Deluxe non reverb chart and there's a bunch of Princeton Reverbs around 66-67 with AA764 charts instead of AA1164. The circuit dates break down like this AA764 is August 1964 and AA1164 is November 1964.

Anyway on an amp as late as your 1970 you definitely want to know the voltages the 6v6s are seeing before stuffing a GZ34 in there. Because the voltage will go up and it's probably very high already.
That's another little thing. They changed power transformers here and there but never showed that on the schmatics and layouts.
That'll bite someone that's stuffing a 5u4 in blackface amp that came with a GZ34. The voltage goes down but the current for the tube goes from 2 to 3 amps, a significant increase.