5u4 vs GZ34 B+ in Princeton Reverb clone

Ronno25

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I've been using an old RCA 5u4 in my princeton reverb clone with a B+ of 390 and ~70% plate dissipation. Curious to find out what the amp would sound like with the called for B+ of about ~420 I swapped in an old Sylvania "big bottle" USA GZ34. The part I don't understand is why my B+ and bias ended up virtually unchanged. Well, actually my B+ dropped a few volts.

I'm running a classic tone champ PT in the amp, which is said to be underpowered for the job. Could this little PT be limiting the B+?

I also read that the Sylvania big bottle GZ34's give a bit more voltage drop than, say, a Mullard. But it seems odd to me that it would be effectively the same as a 5u4.

Edit: I noticed the mojotone schem for the PR states a b+ of 390. But others, including the original fender schem say 420. What gives?
 

Phrygian77

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That's going to depend on your transformer and the VA rating of the transformer (and of course, the tubes). You're not giving us enough info. A PR OT also drops a few more volts than a DR OT does. My PRs that were built with ClassicTone 630 VAC secondary 100mA transformers were between 412-422 VDC with a GZ34. My last PR built with an Edcor 650 VAC transformer was 432 VDC with a GZ34 and 410 VDC with a 5U4GB.
 

Wally

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I've been using an old RCA 5u4 in my princeton reverb clone with a B+ of 390 and ~70% plate dissipation. Curious to find out what the amp would sound like with the called for B+ of about ~420 I swapped in an old Sylvania "big bottle" USA GZ34. The part I don't understand is why my B+ and bias ended up virtually unchanged. Well, actually my B+ dropped a few volts.

I'm running a classic tone champ PT in the amp, which is said to be underpowered for the job. Could this little PT be limiting the B+?

I also read that the Sylvania big bottle GZ34's give a bit more voltage drop than, say, a Mullard. But it seems odd to me that it would be effectively the same as a 5u4.

Edit: I noticed the mojotone schem for the PR states a b+ of 390. But others, including the original fender schem say 420. What gives?

That would make me want to try a ‘known’ GZ34…..one for which I know the output is correct for a 5AR4/GZ34 rectifier. I have never seen a proper GZ34 that dropped more voltage than a 5U4.
 

Ronno25

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The PT is the classictone below: 650v at 70ma HT and 3A rectifier filament secondaries. The OPT is one I took from a vintage organ. It's sized roughly the same as a princeton reverb OPT and is about 8k at 8ohms and 8k at 4ohms (yes, it has two taps).

Good call, Wally. I do have a Mullard GZ34 that I can try in there. It isn't exactly "known" to me but rather it's in an amp that works and sounds good. The Sylvania rectifier I tried is new to me and I haven't seen it used in a working amp.
 

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Wally

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The PT is the classictone below: 650v at 70ma HT and 3A rectifier filament secondaries. The OPT is one I took from a vintage organ. It's sized roughly the same as a princeton reverb OPT and is about 8k at 8ohms and 8k at 4ohms (yes, it has two taps).

Good call, Wally. I do have a Mullard GZ34 that I can try in there. It isn't exactly "known" to me but rather it's in an amp that works and sounds good. The Sylvania rectifier I tried is new to me and I haven't seen it used in a working amp.

you could compare the two GZ34s in that other amp…voltages should be similar. This would eliminate any questions that might be of concern about this Prin Rev clone‘s functioning as this other amp is somewhat of a known entity.
 

Dacious

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With 70ma of current draw you're pulling B+ down because that's too little to support high draw. You could try running diodes which should push B+ up a bit.

Bias your tubes lower and your B+ will rise. That's exactly what a real BF/SF Princeton will do. B+ will fall and transformer warm up if you try to crank current up. BFs blow their power trans very easily.

You'll notice your trem works better too
 

King Fan

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I'm no pro, but this puzzles me a little. FWIW, I have a 70mA (though 660, not 650V) Mojo PT in my stock PR, and get B+ at 410-420 even on a *5V4* (which drops at least as much current as a 5U4). Is it the 10 extra HT volts, or is it something else in the circuit making the difference? Lessee, my favorite B+ calculator says 10V HT difference won't make much B+ difference, and even with a 5U4, predicts your 325V HT should get you B+ around 416V. Hmm, maybe despite their HT specs, my PT is hotter or yours is cooler than spec?

So is it the mA? I agree, of course, your PT may bog down under load, but measuring B+ at idle?

Hmm, your rescued OT? Could that make a difference?

Re Mojo's scheme, I know for a long time they sneaked the Stokes mod into their layout. I don't use that mod -- even Mr. Stokes didn't after a while -- so I don't know: Does it affect measured B+? Does your amp use it?
 

Ronno25

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My amp is wired stock except for a nfb on/off switch and a high-cut pot. Sooner or later I'll pop a known GZ34 in there and will let you guys know what happens to the B+.
 

MickM

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2L man

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Some transformer "math" :) When output is rated 70mA and it is used full wave CT rectified and filtered it is possible to take ~100mA filtered DC out of it and transformer should not "burn". This is because secondary halfs deliver current alternately and current multiplier is sqr2
 

SnidelyWhiplash

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My Mojotone Princeton Reverb clone measures 426VDC with a GZ34. It's strange that the OP would measure quite a bit less with one. What meter was used?
 

King Fan

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The weird deal here is the OP got the same (or slightly lower?) B+ when he changed from the 5U4 to the GZ34. If I'm thinking straight, I'm not sure simple meter error or even wall voltage would account for that.

In fact, I'm not sure what *could*. Bad tube, maybe. Hoping for more data, @Ronno25 . You said you're gonna try other rectifier tubes 'sooner or later'? Have you measured your HT voltages, and for that matter your 5 and 6.3 VAC taps? You've got hundreds of us PR guys on the edge of our seats... :)
 

schmee

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No change from 5U4 to the GZ34 is quite possible. Some old GZ34's are actually 5V4's remarked. A "USA" GZ34 may just be a remarked 5V4. They did that some as the 5V4 can overlap with the 5AR4 specs and qualify as either.
You mention using a 5U4, is it that or a 5U4GB? They are different spec tubes if I remember correctly...? Or do I have that wrong?
I was told once by the old gentleman I knew, (former 42 year electronic store owner with a warehouse full of tubes) that all real 5AR4's are imports, even if they have USA marking.... Is this true? I know many people tried to find certain 5V4's cheap because they were actually Mullard G.B. GZ34's that had been remarked.
 

Ronno25

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So I plugged in a known Mullard GZ34 and the results are in:

B+ shot up to 419 with plate dissipation at about 70%.

So the tube is the culprit. So perhaps as @schmee stated above these are actually 5v4's? But don't 5v4's have only slightly less voltage drop than GZ34's? I'm getting the same drop as a 5u4GB.

I have three of these USA Sylvania GZ34 tubes, I tested them all, and they all produced basically the same B+, which is comparable to my 5u4GB.

I then tested a real known 5v4GA and voltages came up about 5 to 10 vdc from the 5u4GB to a B+ of about 400 with 70% dissipation.

So what in the world are these USA Sylvania GZ34 tubes that I have? signal-2022-04-21-183246_001.jpeg signal-2022-04-21-183246_002.jpeg
 
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schmee

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So I plugged in a known Mullard GZ34 and the results are in:
B+ shot up to 419 with plate dissipation at about 70%.
So the tube is the culprit. So perhaps as @schmee stated above these are actually 5v4's? But don't 5v4's have only slightly less voltage drop than GZ34's? I'm getting the same drop as a 5u4GB.
I have three of these USA Sylvania GZ34 tubes, I tested them all, and they all produced basically the same B+, which is comparable to my 5u4GB.
I then tested a real known 5v4GA and voltages came up about 5 to 10 vdc from the 5u4GB to a B+ of about 400 with 70% dissipation.
So what in the world are these USA Sylvania GZ34 tubes that I have? View attachment 976058 View attachment 976059
Not sure what they are, I think I have one of those. The bottles are bigger than the usual smaller size Mullard bottle.
I got the 5V4/GZ34 thing backwards in the post above. There were GZ34's marked as 5V4's not vice versa. So people used to look for them because 5V4's were a bargain price wise and GZ34's were $150!
 

peteb

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So what in the world are these USA Sylvania GZ34 tubes

you may not ever know.

I would take the AC voltage going to the rectifier, divide by the square root of two, I believe, to get the theoretical amount of DC voltage available, then subtract the DC voltage you have, to find out how much DC voltage your rectifier is dropping.

then, instead of saying I have this type of rectifier tube, you can say I have a rectifier tube that drops this much voltage, behaving like this type of rectifier tube.
 

Ronno25

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@peteb good tip! I was also hoping to verify if this tube has an independently heated cathode as well as its current draw.

The b+ on the reservoir cap doesn’t spike at power up with this tube. I guess that means it’s independently heated.
 
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