Machine Gun Amps
The Rectifier Tube

Replacing the rectifier tube is a good way to get a different sound from your amp. Going from one type to another (such as 5U4GB to GZ34) will give you different tones and different sag characteristics. However, some things must be considered when changing rectifier tubes. A tube that gives lower voltage may not be able to handle the current of a higher power amp. The tubes that give higher voltage may cause your amp to fail. Read on for info on what you should and shouldn't do as far as rectifier tubes are concerned. Most of the information here is geared toward Fenders, but the basic knowledge can be applied to other amps.

5Y3GT 5V4GT 5U4GB GZ34 GZ37 Solid State plug in

5Y3GT   NOS (JAN Philips/Sylvania)
This rectifier tube is found in smaller amps, most notably the Fender Champ. It's also in the Tweed Deluxe. This is a directly heated dual diode. It has a pretty big voltage drop, and has the most sag when pushed. This tube should probably not be used in anything other than what it came in where Fenders are concerned. Putting it in a 6L6 amp will give it a very short life and possibly cause it to arc, not good! I don't understand why Gerald Weber recommends using these in tweed Bassmans and the like. I've seen them arc under light loads in 2 6L6 amps. Leave these in the Champs and whatever else they came stock in. Filament draw is 2 amps. I normally carry a few of these, let me know what you are looking for!

5V4GT   NOS (various brands)
This is a rectifier tube that Fender never used to my knowledge. It is an indirectly heated dual diode. Indirectly heated means that it ramps up slower (it takes longer for voltage to pass through it because it heats up slower) which may increase tube life in amps without a standby switch. Tubes need a chance to warm up the cathode before high voltage is applied to the plates. The slower warm-up of an indirectly heated rectifier can help with this if you don't have a standby switch. If you do have a standby switch, then make sure you let the amp warm up at least 30 seconds before going to full power. It saves tubes! Don't just flip both switches on! Anyways, the 5V4 is a cool little tube. It increases voltages, so there's a chance that your amp might give you problems after installing this. These are cool for tweed Champs, tweed Deluxes and brown Princetons. Just be warned: the increased voltage may be enough to cause a marginal component to fail with the new voltage level. Filament draw is 2 amps. I have a few of these in stock. Let me know what you need and I'll see if I have it!

5U4GB NOS (various brands)
This is a tube that came commonly in older Fender tweeds and silverface amps. Silverface amps often specified this tube in the schematic, even though you normally find GZ34/5AR4's in the early ones. This is because the tube chart erroneously calls for the GZ34/5AR4. The 5U4GB is a directly heated dual diode. It has less voltage drop than the 5Y3 but more than the 5V4 and GZ34/5AR4. Filament draw is 3 amps. Installing this tube into an amp that calls for a GZ34/5AR4 MAY blow the power transformer. I've seen enough of these in blackface amps that originally came with GZ34/5AR4s to know that the 5U4GB will work most of the time. As a matter of fact, I have not directly seen one of these blow a blackface Fender power transformer. HOWEVER, if your power transformer is already weak then the extra load that this tube puts on it may put it over the edge. Don't say I didn't warn you. I make it no secret that I don't like the way this tube affects tone in blackface and silverface amps. I sometimes have a few, let me know if you need one. If you have a silverface Deluxe Reverb, then use this tube like the tube chart calls for. The GZ34/5AR4 may cause minor noise related problems or even cause it to fail due to increased voltages. Fender recommends this tube for use in the tweed reissue Bassman. I like the GZ34/5AR4 in those a lot better.

GZ34 (5AR4)   Chinese GZ34
This is the tube that came in almost all blackface amps with a rectifier tube, some brown amps that came with a rectifier tube, and the tweed Bassman and Twin. They were usually branded as Mullards or Amperexes (but made by Mullard). It is an indirectly heated dual diode. This tube gives the least sag of all rectifier tubes mentioned here. They give the highest B+ of any tube mentioned here. Filament draw is 1.9 amps. NOS examples of this tube are very expensive. Mullards go commonly for $50 - $60 a piece (or more). At this price, the Chinese version is a viable option. This tube is my favorite rectifier tube in blackface and silverface amps. They are almost solid state in their response. 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! This tube works nice in the reissue Fender Bassman. It gives a bit of sag where the stock solid state rectifier doesn't. If you like headroom and punch, then leave the solid state rectifier in. This tube also comes stock in the reissue Marshall JTM45 and Bluesbreaker.

GZ37/CV378 NOS Mullard
This is a British military rectifier tube, mostly made by Mullard as far as I know. Not to be confused with the GZ34. It's a neat looking tube for sure! It's very tall and has an ST shaped bottle (it has a shoulder). It has a neat brown base too. The plates are spot welded and sutured as well. It drops voltage a bit less than a 5U4GB. The 5U4GB dropped 5 more volts in my Super Reverb (430 plate volts for 5U4GB vs. 435-436 for the GZ37). The filament draw is 3 amps. This would make an ideal substitute for a 5U4GB if you have the space in your amp (these are maybe an inch taller than a regular 5U4GB). Once again, don't install it in an amp that the filament tap isn't up to it.

Solid State Rectifier
This is a plug in replacement for a tube rectifier. It's made with silicon diodes, similar to what the blackface Twin Reverb, Bandmaster, Bassman, Showman and others came stock with. Installing this will give more power, and no sag (none associated with the rectifier anyway). Many people like these in their blackface/silverface amps. They make the amp louder, tighter, and more responsive to pick attack. Notes jump more. Now on to the warning part: This thing may cause problems in your amp! The solid state rectifier increases voltages in the amp that may cause weak and old components to fail. An example of this: I put one of these in an amp for a friend. After a week or so the amp started popping really loud, like at speaker blowing levels. It turned out to be a bad cathode bypass cap. The increased stress the solid state rectifier put on the amp probably caused this component to fail. It failed at a gig, and wasn't a pleasant experience for the customers at the bar! I like the way an amp sounds with a solid state rectifier installed quite a bit. The tightness and punch are cool. It's worth a try for sure. The solid state rectifier should never be installed in a stock blackface/silverface Deluxe Reverb, Princeton Reverb, Champ or Vibro-Champ. Also, it is important that you get your amp rebiased if you are going to the solid state rectifier from a tube one.

What is Filament Draw?
The filament in a tube is the part that heats it up and gets the electron party going. Different tubes have different current draws. As far as filament draw is concerned, it is always OK to go to a tube with a lower filament draw. BUT, don't forget the other things that a particular tube will do! Going to a tube that has a higher filament draw is sometimes OK. BUT, there's no guarantee that putting the extra load on the power transformer isn't going to blow it! As a general rule, I always keep the filament draw on the rectifier tap where it came stock or lower. Reissue Bassmans are good for either the 5U4GB or the GZ34/5AR4.

Other things to consider when installing a different rectifier tube.
When you change the rectifier tube in an amp, it changes the voltages in an amp. This will affect bias. When installing a different rectifier tube, especially from one that gives a greater drop to one that has a lower drop (ex: 5U4GB to GZ34/5AR4), it will cause the amp to be biased differently. This needs to be checked/adjusted when you change this tube. Any time you change the rectifier tube to a different type, you should have the amp biased. Some techs may call what I'm explaining here hogwash or just plain too conservative, but I like amps to work properly so I stand behind this. Old amps that have worked for many many years don't always like to get things changed around on them. Sometimes they will let you know in short order that they didn't like it!

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Last updated 11/19/00