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Post subject: 69 Vibrolux Reverb tube questions
Posted: Wed Sep 11, 2019 8:48 am
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I recently got a '69 drip edge Vibrolux Reverb, looks like an AB568 circuit. I've changed the electrolytic caps but otherwise it's completely stock. Sounds pretty great but is very clean compared to my friend's 72 VR that he had blackfaced, so I've been debating doing the same thing as I think his sounds better.

In researching this, I've read some stuff about tube differences that I don't understand. The first thing is the difference between the 5U4 and the GZ34 rectifier. I get that the GZ34 outputs higher voltage and has less sag, but everything I read on the internet says this gives more headroom and a cleaner sound. My understanding is that higher supply voltage results in more voltage gain in each stage, which multiply, so that should result in quicker distortion. Plus the output transformer and speakers would be hit harder, which would give more compression and break things up sooner.

The second thing is the difference between 5881s and 6L6GCs, with the difference being a max plate dissipation of 23 W and 30 W respectively. Everything I read says the 5881 breaks up sooner because it is a "lower power tube". But plate dissipation and output power are not the same thing, and being able to bias the 6L6GCs 7 W higher than the 5881 should should result in a hotter, more distorted sound. Hotter bias = less headroom before saturation = more distortion. Am I wrong?


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Post subject: Re: 69 Vibrolux Reverb tube questions
Posted: Wed Sep 11, 2019 9:03 am
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One of the more significant differences between the AB568 and AA964 circuits is the power-tube plate voltage -- it's 440 VDC for the former, 410 VDC in the latter. Likely that's whats keeping your '69 a little cleaner than your buddy's '72.

Arjay

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Post subject: Re: 69 Vibrolux Reverb tube questions
Posted: Wed Sep 11, 2019 9:42 am
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How does the AB568 with the 5U4 get higher plate voltages than the AA964 with the GZ34?


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Post subject: Re: 69 Vibrolux Reverb tube questions
Posted: Wed Sep 11, 2019 9:56 am
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You'd have to ask Leo -- he designed the original circuit. I'm merely reading the specs from both schematics.

And you've got the output data for those two rectifier tubes bassackwards -- the GZ34 is outputting 410VDC in the AA964 revision while the 5U4 is supplying 465 VDC in the AB568 (and the AA270) circuit.

Arjay

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