BF pro reverb and a 5U4?

smolder

Member
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14,294
A while back, based upon the doc's recommendation, I swapped out the 5AR4 with a 5U4. It added touch, sustain and a little compression while dropping the wattage and volume by a third - which I like.

I just bought a 66 Pro Reverb and wondered if it would do the same? It just so happens I have four or five NOS laying around.
 

SatelliteAmps

Member
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6,186
The only real issue is the filament current. A 5U4 draws more current. It should be fine. I would watch the power transformer for overheating during the first time running it.
 

zombiwoof

Supporting Member
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5,761
If you are worried about the filament current issue (which some say you should in an amp that has a stock 5AR4), try a 5V4. Similar to the 5U4 as far as voltage drop, and has about the same filament current as the 5AR4. I use one in my '65 Princeton Reverb (original) and it works great.

Al
 

Prairie Dawg

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1,932
If you are worried about the filament current issue (which some say you should in an amp that has a stock 5AR4), try a 5V4. Similar to the 5U4 as far as voltage drop, and has about the same filament current as the 5AR4. I use one in my '65 Princeton Reverb (original) and it works great.

Al
The filament current draw of a 5U4 is 3.0 a, where the 5AR4 is 1.9 a and the 5V4 is 2.0 a. However the voltage drop of the 5V4 is somewhere between the 5AR4 and the 5U4, i.e., less voltage drop and more headroom all other things being equal.

If a person wanted that 5U4 voltage drop without the increased filament draw a nice 5R4 might be just the ticket-of course, meaning not one of those Cetron doorknob tubes. The 5R4 was designed for high altitude performance in military radios
 
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1,059
I have had great success running a 5r4 in my super reverb. It prings the voltage down to 403 volts then I can run original tung-sol 5881's yummy!
 

RussB

low rent hobbyist
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11,174
GZ34 to a 5U4 dropped the volume by a third?

something just ain't right with that statement
 

smolder

Member
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14,294
GZ34 to a 5U4 dropped the volume by a third?

something just ain't right with that statement
Well yea... but I live in a condo. Every tried to play a Route 66 slightly overdriven with neighbors? It's a tricky proposition, even with an attenuator.
 

RussB

low rent hobbyist
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11,174
It doesn't mater where you live, swapping a GZ34 for a 5U4 won't change much of anything, and there's just no way the overall volume drops by 1/3...or 1/20th
 

slider313

Silver Supporting Member
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8,130
In my '66 Pro Reverb I do run a 5V4. I would not take a chance with a 5U4 and a vintage transformer.
 
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1,059
The filament current draw of a 5U4 is 3.0 a, where the 5AR4 is 1.9 a and the 5V4 is 2.0 a. However the voltage drop of the 5V4 is somewhere between the 5AR4 and the 5U4, i.e., less voltage drop and more headroom all other things being equal.

If a person wanted that 5U4 voltage drop without the increased filament draw a nice 5R4 might be just the ticket-of course, meaning not one of those Cetron doorknob tubes. The 5R4 was designed for high altitude performance in military radios

You dont like those cetron 5r4's ?
I thought they were pretty robust.:)
 

jcs

Member
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8,047
you would be lucky to drop the plate volts by more than 1 to 3% overall.

its not that drastic of a change (plate volts) going from 5ar4 to 5u4, heck its not a lot of change going to 5r4 either but it definitely feels and responds differently, no question (and thats whats important imo).

if you want to drop the plate volts a bunch get a variac and back the wall supply volts down to 105-110,,,, BIG difference!
 

slider313

Silver Supporting Member
Messages
8,130
In my experience there is a big difference going from a 5AR4 to a 5R4. The drop in plate voltage was in the 30 volt range going from 438v to 410v in my original '65 Deluxe Reverb.
 

doctorx

Member
Messages
4,453
When you rebias with a 5U4... where did you set the bias and what kind of plate voltage were you getting?
Sorry I don't remember, it's been a couple of years. I usually set the bias pretty hot in my amps, and the amp is working fine. There wasn't any volume drop, it just got smoother and a little more compressed when pushed, which I do a lot.
 

jcs

Member
Messages
8,047
yeah probably 6 to 8% plate volt drop for subbing a 5r4 for a 5ar4, the difference is substantial here.

maybe 2-5% drop for 5u4 subbing for 5ar4, but i've seen little change sometimes with my mullard 5ar4 going to various usa 5u4, keep in mind there are a lot of versions of 5u4 as well and i suspect they will vary a bit!
 

Dave C

Member
Messages
1,353
Since Pro Reverbs using either rectifier come with the same 125P5D PT so filament current shouldn't be a concern. I do agree however that the lower current draw of the GZ34 or 5r4 would be "easier" on a vintage xformer.
 




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