Substituting 5U4GB for GZ34 rectifier tube

[bluesguitar]

I have a 67 super reverb. I would like to achieve more sag and compression in the tone. I heard about switching the GZ34 rectifier tube and replacing it with a 5U4GB. However, I read warnings that this could stress the power transformer. I don't want to risk damage to this amp. Has anyone tried this mod? Is it safe? Any recommendations for achieving more sag and compression?

[David Root]

Don't use the 5U4G, it takes 3A heater current and the PT is only designed for the 2A that the GZ34/5AR4 needs.

Try the 5R4G, or the later and smaller 5R4GB. It only needs 2A and will give slightly more compression than the 5U4G.

In that tube I would recommend the Sylvania 5R4GYB with the close-spaced black plates, about 1/8" apart on the inside. The wider spaced ones are OK too, according to the Audio types, but the close spaced ones are said to be better.

I use RCA 5R4GY military coke bottles from 1944, two of them, in my corksniffer's low power tweed Twin and they are excellent. The Sylvania 5R4GYB from the '60s can be had for about $20.00 each NOS and the coke bottle '44s for nearer $30.00 NOS.

Either of these tubes will drop the plate and screen voltage about 30-40V below what the GZ34 gives, this will reduce power and headroom somewhat but the Super is a loud amp so it should work out pretty well.

[FYL]

Fender briefly used GZ34s in various amps, then switched to 5U4GBs. Your amps is probably an AB763, fitted with a 125PSD power transformer, which is designed for up to 3 A for the recto. The same xformer was used in the AB568 and later revisions of the Super Reverb with a 5U4GB.

So you can safely use a 5U4GB if you wish.

[John_P_WI]

Any recommendations for achieving more sag and compression?

There are numerous methods to achieve the sonic effect of sag. Simple approaches such as adding a power resistor between the filter cap and center tap of the output transformer, increasing the value of the output tube screen resistors etc.

More elaborate approaches as this: http://www.londonpower.com/catalog/prod ... ucts_id=81 can be used too, to adjust any amount of sag....

John

[Diablo1]

You can also use a Weber copper cap, which is just a solid state rectifier with a pair of matched sag resistors to mimic a 5U4GB. The benefit is that it doesn't draw any filament current from your PT.

http://www.webervst.com/ccap.html

[MCK]

What are the odds... I have a 67 SR as well and bought a RCA 5R4GB with the exact same plan in mind. One of these days I'll get around to popping it in but I have no idea how to bias this amp. Would there be a bias adjuster pot in there or do I need to solder in an appropriate bias resistor? I did not pull the chassis out yet so I don't know. Apologies for the newbie question. Thanks

[M Fowler]

I use the 5V4 to drop voltage it is 2A and fits inbetween the 5AR4 and 5U4G for voltage drop.

Mark

[rogb]

Good timing, this thread! I just ordered a 5U4GB for my AB763 build

The diode recto sounds clean and stiff at 450V and the Mullard GZ34 I had is barely 10 volts lower, so there is not much contrast between the 2, so I was looking for a lower B+ rectifier to add some "brown". Looked at them all, including the 5V4.

I called the helpful Watford Valves https://www.watfordvalves.com/home.asp for a chat and they said as my MM PT is speccd for 3A on the 5V AC, the 5U4GB should be OK. We'll see

[Zippy]

Good timing, this thread! I just ordered a 5U4GB for my AB763 build

The diode recto sounds clean and stiff at 450V...

Add some resistance in series and you'll take care of that. Calculate your power amp load (current) and the desired voltage drop and do the math to simulate the sag that you hope to get from the tube rectifier.

That's what the various Weber Copper Caps are all about.

[rogb]

Of course, I forgot about the Robben Ford mod, duh. I will try and calculate the resistor needed - thanks for the idea

EDIT: not really a RF mod as this subs a resistor for the choke, this is before the choke