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FDP Forum / Amp Mods, Repairs, and Projects / Edumacate me on different rectifiers in a black face Deluxe Reverb


What It Was!

Fairly Unbalanced
Dec 10th, 2017 04:45 PM   Edit   Profile  

Supposedly they change the feel of the response when you go with different value tubes… I’m not qualified to bias the amp if it needs it, just trying to learn a little bit.


on the rhine

Dec 11th, 2017 03:07 PM   Edit   Profile  

Hi Dolemite!

The stock rectifier-tube is a GZ34 (or 5R4 if I have that right...).
You CAN use other tubes (5U4, or even 5Y3), but you have to check if the voltages are in line and - possibly - adjust the bias.

Personally, I would rather experiment with the pre-amp tubes - like, pulling the first tube of the unused channel (more gain to the used channel) or substitute the phase-inverter tube (last pre-amp tube) for a 12AX7 instead of a 12AT7 for more gain/breakup...

cheers - C.



Dec 12th, 2017 04:17 AM   Edit   Profile  

The stock rectifier is GZ34 / 5AR4, which is the least lossy rectifier type.
Assuming that your current GZ34 is functional, any other rectifier will reduce the HT and hence create a cooler bias condition, so can be swapped in without concern that the power tubes will run hot.
They will make the amp's high volume response more saggy and reduce the power output, the point being that you're not going to notice much difference unless you play loud.

The alternative rectifier types include 5V4, 5U4, 5R4 and 5Y3.
None of those are without potential issues but experience has shown that they are unlikely to cause an actual problem.
These are that a 5U4 will draw a bit more power from the amp's power transformer, and with the others, the amp's circuit may cause one or more of the tube's limiting values to be exceeded (reservoir cap value, input voltage).

Note that current production 5Y3 are thought not to comply with type characteristics, ie they are likely a similar Russian tube type that has been re-labelled, and they put out more voltage than a vintage USA made item. Given your application that isn't likely to be an issue, as they will almost certainly be saggier than a GZ34.


What It Was!

Fairly Unbalanced
Dec 12th, 2017 06:27 AM   Edit   Profile  

Thanks guys. Carsten, that's where I went first. Picked up a used Marsh BFDR and tried a few swaps.. sounds even sweeter now.

It appears that the bottom line on rectifier swaps is that the safe way to do it is to have our local amp tech measure the voltage of a potential change. Don't want to cook NOS power tubes or PT, they ain't cheap!

Contributing Member


All I need is one more guitar
Mar 20th, 2018 07:25 AM   Edit   Profile  

Carsten mentioned the idea of pulling a preamp tube from the Normal channel.
Is that V1 ?
Could I just leave it out permanently?



Mar 22nd, 2018 07:56 AM   Edit   Profile  

Yes, that's V1.
It won't cause a problem if it's permanently left out.

Steve Dallman

Merrill, Wisconsin

Age is just a number...mine is big
Mar 22nd, 2018 09:54 AM   Edit   Profile  

5AR4 is the usual tube. Fender switched to the 5U4 back when the supply of 5AR4's was dwindling. The power transformer was changed to adapt to the 5U4 voltage drop.

Use the right tube. If a 5AR4 is used in a Silver Face DR designed for the 5U4, the higher voltages may exceed the power handling of the filter caps.

A 5U4 in a blackface will lower voltages, put a greater load on the 5v power transformer supply, and you will use the "soft start" action of the 5AR4.

Pulling the V1 does kick up the Vibrato channel a little.

Another thing to try is replacing the 12AT7 phase inverter (the tube next to the power tubes) with the 12AX7 you just pulled from V1. It has higher gain, but it won't be louder. It will allow the PI to break up easier when the amp is pushed.

Much of the desired "power tube distortion" people love is the phase inverter breaking up, and that breakup is amplified by the power tubes.

A 12AX7 will break up easier than a 12AT7 in the PI slot.



Mar 22nd, 2018 02:46 PM   Edit   Profile  

Note that the defining point of clipping in a class AB1 amp occurs when the peak signal voltage (at the power tube g1) equals the bias voltage, and the power tube g1 begins to be forward biased, becoming a diode.
At that point, the g1 input impedance drops from infinity to ~2k and it clips off the top of the input wave.
There will be further signal swing available from the LTP, a fact revealed when a type 1 or 2 master volume is fitted and turned down some.

So, it may be seen that at clipping, neither the power tube or the LTP has reached their max potential output; as a dc coupled, low impedance driver could push more power from the power tubes into AB2 mode.
And the LTP could put out more signal into a less demanding load.

But given the constraints and simple design of an AB1 amp, g1 clipping defines its overdrive point.

(This message was last edited by pdf64 at 08:20 AM, Mar 23rd, 2018)

FDP Forum / Amp Mods, Repairs, and Projects / Edumacate me on different rectifiers in a black face Deluxe Reverb

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