Vox's take on the Princeton Reverb?

[dwinstonwood]

I never bothered to give this schematic a close look until this morning. I guess you could think of it as a Vox EL84 PR.
Has anyone owned or played through one of these? I'm not interested in building one, just in knowing people's experiences with it.
Thanks!

[mresistor]

A long tail pair phase inverter kind of says it isn't a PR ... maybe a K biased deluxe ?


I repaired one once and played it briefly, I'm not really good at explaing how things sound, but that amp had a sweet tone to it. It had a 2x10 cab that needed extensive repair as someone had hacked it up weth a claw hammer to install a cheap 12" speaker.  I was just looking at the photos this morning, android look back from 5 years ago. It was one of the piggy back setups.

[dwinstonwood]

A long tail pair phase inverter kind of says it isn't a PR ... maybe a K biased deluxe ?

Yeah, LTP, Cathode biased, no NFB, and EL84's... very much a Vox.  So not really PR at all, except in wattage.
The two 10" thing is really cool. I'll read up on it some more.

[mresistor]

They called it a Vox AC10 Berkeley Super reverb 18 watt.,. https://www.voxshowroom.com/us/amp/berk.html

[Latole]

To me this Vox is not close to a Princeton Reverb

[Willabe]

I think that's 1 of the Vox's that was made in the US? Most were solid state. And were the big piggy back set ups with the metal frames.

IIRC, Gerald Weber, Kendrick amps, said they sounded pretty good.

[dwinstonwood]

Well, when I said "Vox's take on a PR," I meant it's a sub-20 watt, single-channel amp with treble, bass, reverb and tremolo. Not a clone, but a similarly equipped amp in the same power bracket with Vox DNA.   

IIRC, Gerald Weber, Kendrick amps, said they sounded pretty good.

That's good to know. I really like the way they look. Very Ready, Steady, Go! (the British TV show).

[Lectroid]

I think that's 1 of the Vox's that was made in the US? Most were solid state. And were the big piggy back set ups with the metal frames.

IIRC, Gerald Weber, Kendrick amps, said they sounded pretty good.

I had one of those monsters: the Vox Super Beatle. 4-12 Celestions and about 100W as I recall.  I played bass through it and it did that well enough for 1970 bar-band rock.  My lead quitarist as the time played it but he wasn't a fan of the SS sound.  He said he liked his Silvertone 1484 better.  Its best quality was that it had a regulated "E" tone generator that everyone could tune to.  Otherwise, a total pita to haul around.  But damn, I was young, and it looked so cool!  Worst part was that I traded in a 63 BandMaster to get it.  Ouch.

[dwinstonwood]

So, the V-3 Cambridge Reverb is even closer. It's a 1-10" combo, not a 2-10" piggyback:

https://el34world.com/charts/Schematics/files/Vox/Vox_cambridge_reverb_v3.pdf

https://www.voxshowroom.com/us/amp/camb.html

[bmccowan]

Checkout this Truetone lounge vid with the great Kenny Vaughn.
He's playing through a Cambridge and talks about the amp partway through.
But, the entire vid is worth a watch. In fact most everything with Kenny on YouTube is worth watching.
https://www.youtube.com/watch?v=sb4JKW4bs7M&t=2583s

[mresistor]

Kenny is way cool  ..   love him as a Superlative   

[bmccowan]

Yup - when Kenny and Marty get going its out of this world. Two Teles with B-benders - yikes!

[dwinstonwood]

Thanks for that video bmccowan. Made my lunchbreak even better.

And, to quote Mr. Vaughn from the video, "It's like a Princeton Reverb with a Vox tone."

[HotBluePlates]

I never bothered to give this schematic a close look until this morning. I guess you could think of it as a Vox EL84 PR. ...

Princeton, Berkeley, Cambridge.  All great names for a student model amplifier.

FWIW, I agree with you that it's a thinly-veiled copy of the Princeton Reverb.  The few differences are not super-relevant to me.

[dwinstonwood]

And, it probably wouldn't take much effort to convert a PR layout to the Cambridge schematic; the PR chassis and cabinet are readily available. Maybe I'll draw one up since I enjoy that part of the hobby.

Or, start here, and change the necessary parts: https://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf

[Merlin]

That 'Vox' amp was designed by the Thomas Organ company in the US, so it's not entirely surprising that it's basically a Fender amp with a Vox output stage bolted on!

[mresistor]

An observation - it lacks the third amplification stage the PR has right before the phase inverter which the cathodyne PI allows.  Sure it's only one channel like a PR.  Most all of the Fender amps have the same reverb/vibrato setup which doesn't necessarily make it "like" a PR.

[SEL49]

The Vox uses a LTP PI which has it's own gain so the overall gain of the Vox is still roughly equivalent to the gain of the PR with the gain stage plus the cathodyne.

[mresistor]

I suppose I agree that it's like a PR preamp mated to a different power amp.

[dwinstonwood]

I feel like it can be thought of in technical terms (phase inverters, biasing methods, power tubes, etc), and also as a product marketed to guitar players. From the point of view of a typical young guitarist in the mid-60's, it seemed to have all the features of a Fender PR. If it sold for less, that might have been as far as the customer investigated. That's not to say that guitarists in the mid-60's weren't up on the technical differences between, say, a 6G8-A and an AB763. Just that "knobs and effects" probably sold more amps to beginners back then.

[HotBluePlates]

An observation - it lacks the third amplification stage the PR has right before the phase inverter which the cathodyne PI allows.  ...

The Vox uses a LTP PI which has it's own gain so the overall gain of the Vox is still roughly equivalent to the gain of the PR with the gain stage plus the cathodyne.

Restating to make the similarity pointed out by SEL49 even more overt:

   A typical 12AX7 gain stage has a gain of about 60.

   A long-tail inverter delivers a gain to one output about half what it would do in "a typical gain stage": so a gain of about 30 to one output.

   A long-tail inverter obviously has 2 outputs, so we could say "the gain from one output to the other" is "30 + 30 = 60."  2 triodes deliver the gain 1 triode would normally provide.

   A split-load/cathodyne inverter provides no gain of its own.  It is composed of a single triode.

   Almost always, a split-load/cathodyne inverter has a "pre-gain stage" right before it, and the feedback loop (when present) is returned to this stage (rather than to the inverter itself, as with the long-tail pair).

   The "pre-gain stage" plus the split-load/cathodyne inverter now has "gain of 60" (pre-gain stage) plus "gain of zero" (inverter), matching the performance of the long-tail pair (at least in this manner).

   The "pre-gain stage" plus the split-load/cathodyne inverter also is composed of 2 triodes, making it comparable to the long-tail pair in this manner.


Hopefully that helps explain why I said, "The few differences are not super-relevant to me.". I probably should have said "troublesome" or "meaningful" instead of "relevant."

A difference that has very much more impact on the amp's sonics is the lack of a negative feedback loop around the power section in the Cambridge Reverb.