# Blackfacing a late 70s Deluxe Reverb



## starjag (Jan 30, 2008)

I have a late 70s Deluxe Reverb for sale in the forum. The amp was in need of maintenance recently and I asked the tech to "blackface" it.

I do not understand the technical details, but a person asked in the FS thread if the "blackfacing" work included changing the rectifier tube from the 5U4GB used in the silver face series to the GZ34 tube used in the blackface series.

Schematics...
AB763 schematic here.
Pull-Boost schematic here.

I was curious about this, so I asked the tech about it. He said that "both a silver face and blackface run the following for power supply. Blackface is a gz34, the transformer voltage is 330 volts AC, an the main B+ is 420 volts DC. A silver face uses a 5U4, the transformer is 330 Volts AC, and the main B+ is 420 volts DC. Both of these rectifier tubes are interchangeable with no true audible differences, and no changes to the filtered B+ rail."

Again, I do not understand the technical details, but I do trust this tech. Curious to hear opinions about the rectifier tube issue when "blackfacing" a silverface amp. Cheers!


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## nonreverb (Sep 19, 2006)

The 5U4G has the most voltage drop across it. You'll get 20 - 30 volts more B+ with a GZ34(5AR4) depending on the ac voltage at the power transformer. It also sags less than a 5U4. The amps output will be a little stiffer with the extra voltage. Since the transformers were basically the same from the Blackface to the Silverface (to mid 1970's) they're interchangeable. It's prudent to check the B+ caps whenever doing this mod on a Silverface just in case.
The same cannot be said for other type amps using a GZ34. The filament current requirements for a 5U4 are substantially higher than the GZ34 and can overload the filament windings in the transformer causing them to fail.



starjag said:


> I have a late 70s Deluxe Reverb for sale in the forum. The amp was in need of maintenance recently and I asked the tech to "blackface" it.
> 
> I do not understand the technical details, but a person asked in the FS thread if the "blackfacing" work included changing the rectifier tube from the 5U4GB used in the silver face series to the GZ34 tube used in the blackface series.
> 
> ...


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## StevieMac (Mar 4, 2006)

*starjag*, I removed my questions from yous FS post...sorry if it created any confusion. Regarding the technical concerns, even if a GZ34 has been added and the plate voltages had bumped up accordingly (i.e. +20-30 volts), I see that there are JJ power tubes installed and my understanding is they're good for nearly 500 volts, so no real worries then in that regard. Other 6V6 tubes running at 440-450 volts might be a concern however. If it's running the original 5U4GB, *nonreverb* has explained how that would be different (sag, envelope of attack, etc) from an original BF amp running the GZ34. 

Anyway, thanks for making the effort to clear things up and, again, I apologize if my questions in the FS thread caused any problems for you. Feel free to simply repost the amp and I'll keep any technical questions out of it. Cheers.


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## starjag (Jan 30, 2008)

All good info guys, thanks! It is actually a great asset that many of you have so much experience with vintage amps.

The good people from Mack Amps have posted an informative note about sag.

I seldom play at high volumes, so I am not really sure I understand what sag really means from a player's perspective.

Any ideas of youtube videos that demonstrate sag using the exact same amp with different rectifier tubes? That would be cool.


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## WCGill (Mar 27, 2009)

Sag at high volumes will manifest itself as a momentary drop of volume on hard transients at worst as the power supply caps recharge. This effect is more noticeable if the reservoir cap is undersized. I build a dual switchable tube rectifier amp and notice that with the lower voltage and higher impedance 5Y3, the amp is warmer, slightly slower and the bottom isn't quite as defined.


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## nonreverb (Sep 19, 2006)

While we're on the subject on 5Y3's...I just had an AC30 in foe repair with a blown rectifier.Turns out the last tech installed a 5Y3 instead of a GZ34!!Unbelieveably, it lasted several months before succumbing.


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## WCGill (Mar 27, 2009)

nonreverb said:


> Unbelieveably, it lasted several months before succumbing.


Wow, they don't make 'em like that anymore. Some new production GZ34's don't do much better.


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