Vox Buckingham V112 rebuild (was Vox Buckingham questions)

[Stevem]

RG, what would happen in a circuit like this had developed some noisy resistors?

Where not passing audio so where not going to hear the Bacon frying sound, but what could such a issue cause?

Besides dealing with this type of issue in used gear I also all too many times when QCing new tube or SS Mic's at work have had noisy resistors.

Most times they are values above 47k, but in some parts of a circuit I have had bad ones even as low as 1.5k, and many times it takes 10 seconds for this noise to reach its peak .

Maybe this is just a shot in the dark, but I was wondering if such a issue could be a factor here.

[R.G.]

Hmmmm.
Not much, I think. The reason is the big difference in signal levels from the noise level of a resistor to the running levels of the oscillator. Even a really noisy resistor - not intermittently conducting, just noisy - doesn't make volts of noise. The proper oscillation sine waves are on the order of two to five volts peak.

There is an abstract reason that noisy resistors could actually aid faster oscillator start up. The gain of a phase shift oscillator has to be carefully balanced to only a hair more than the attenuation of the feedback path. If it's any less than the attenuation of the feedback path, there isn't enough gain to sustain an oscillation, and it won't start; even if whacked with a transient, the oscillation fades out. This is the basis of the old-school electronic drums circuits. If it's exactly enough, any oscillation will be sustained forever, but it won't start. If it's a hair more than the feedback attenuation, it will start up and oscillate, but the "sine" waveform gets progressively more distorted the more extra gain there is. In effect, the clip level of whatever distortion exists amounts to a crude compression, and it self-adjusts down to a sine-ish waveform at some level of distortion.

The tremolo-kill switching in fact stops and starts the trem oscillator by cutting and releasing respectively y the gain of Q206.

This happens with all RC oscillations in one way or another. In fact Hewlett-Packard as a company was started on the basis of using an incandescent bulb's variable resistance as a soft gain-adjusting element in a Wien-bridge oscillator. It's the basis of the HP 200 low distortion sine wave oscillator, and the idea spawned the company.

In simulating an R-C oscillator I often find the circuit is entered correctly but won't start. It has the right running gain, but not enough excess to start, and the simulator starts all components at either 0.0000 volts or at DC equilibrium. Adding a transient generator or a noisy resistor adds enough tickling to get them to start reliably. So modestly noisy resistors in the oscillator circuit around Q206 might help a fast startup.

[seveneves]

Long-awaited update on the Buckingham. For those of you who care...

With the help of a well-known [Thomas Organ] Vox guru, I finally got the tremolo working without the thump. But... I really didn't do anything! Let me explain.

I was asked to scope the input and output of the four-diode modulator. Upon doing so, the thumping stopped! Not sure I can explain what the scope probe did (or didn't do?) but it seemed there was some kind of discontinuity that was "made continuous" with the introduction of the scope probe. Anyone else, feel free to educate me here.

My rebuild was validated because I was thinking that I may have made an error in the replacement of the components, despite being very careful during the rebuild of the circuit board and that part of the circuit specifically. I spent a lot of time looking at that part of the board looking for an error in my work. Doesn't really matter now, I have a thump-free trem. Pretty unique sounding, TBH. Moreover I now have an essentially "new" old Vox Buckingham that I'll likely (hopefully?) never have to worry about again (I'm 53)? Maybe filter caps a decade or two from now? I'll worry about it if/when it happens that they need replacing.

With the electronics done, I then concentrated on spiffing up the rather tatty head cab. Being that this is an early Thomas Organ Vox Buckingham (V112), it still has the plywood head cabinet. MUCH better than the particle board stuff that you see with most of these...

Here's a few before pics:











You can see how much the grill (green threads) has faded. 57 years of UV exposure will likely do that.



Otherwise, just needed a good cleaning, really. After which, I put in some new gold piping:



Then the rest of the cosmetics (grill piping, logos, feet, corners, handle, etc.). Fortunately, North Coast Music still carries most/all(?) of the Thomas Organ Vox parts and are 100% perfect DNA repros of the originals. BTW, if any of you are hesitating to restore your Vox (Thomas Organ or other) with NCM parts, don't wait. I have it on good authority that Gary is winding down the business and looking to retire quite soon. And I don't see anyone realistically picking up the business.

At this point the cabinet is done:







Now it's just a matter of assembly:







There was a crack on the back panel. VERY common on these Voxes:



Glued (inserted with a 18 gauge needle and pumped in with a syringe) and clamped overnight:



Complete:







For many of you, what I am about to say next is going to sound counterintuitive, but... believe me when I tell you that this amp RIPS! "Chewy", chimey as all get out, can be bold/authoritative, NOT brittle, overdrives relatively easy/low on the volume in a very warm/tube-like way. Is it the best amp ever made? No. Does it sound good/great? You bet. Very useable tones. Vintage or modern. But don't take my (or R.G.'s) word for it. As they say, if you know, you know!

Speaking of R.G., I can't thank him enough for his praising and support of these amps. He's pretty much the only reason why I took on something like this. R.G. has done A LOT of analysis wrt these amps. I hope more people can keep these amps going based on all the good work he's done in promoting them and their repair. Although they used to be almost ubiquitous (and, as a consequence, cheap) on the used market in the pre-internet days, they are slowly becoming quite hard to find. I suspect a lot of them were dumped/trashed over the years. Such a shame.

This particular amp was nearly at death's door when I got it. Unloved for DECADES, it sat on a shelf in a shop and had a great potential of being straight-out DUMPED. I so love resurrecting amps from the DEAD!

So the hope is this thread also provides some inspiration/motivation for those similarly looking to resurrecting these out there now needing some love. It CAN be done (having rebuilt essentially the whole amp at this point), so I can tell you it's NOT REALLY THAT BAD! What you need more than anything, IMO, is a LOT of patience and time: you're unlikely to turn it around over a weekend, especially if it's your first time. But those qualities will get you VERY FAR. Planning also is very important as things don't seem on the surface very straight-forward as far as sequence of work goes. Spending a little time thinking, assessing/reassessing as you work pays off, I find. In the end, it's very rewarding and satisfying because the tone of these are really TERRIFIC! Of course, IMO!

Although R.G.'s website is a GREAT resource, these two books (note model specific on the right) are very highly recommended, as well. That is, if you're serious about repairing, maintaining and owning these amps:

[Stevem]

Nice job on cleaning all that off, I now how much tooth brush scrubbing that takes since when I got my reverb twin it was near 4 hours of work with hot soapy water.

[seveneves]

Nice job on cleaning all that off, I now how much tooth brush scrubbing that takes since when I got my reverb twin it was near 4 hours of work with hot soapy water.

On the head cab restoration and it's cosmetics, as a whole, I spent a whole weekend working casually from start to finish. I'm quite meticulous. Got every square centimeter as good as I could. The head was never cleaner in over 50 years, I'd wager.

I cleaned/shined/rehab'ed as much of this head as was possible. Cosmetically and electronically.

[seveneves]

Spent a little time today cleaning up the footswitch:



Note that the early amps (V11x) didn't have the large footswitch. They had the two button and the single button - yes, TWO footswitches.

Trem and reverb (1/4" TRS):



MRB (RCA/phono plug):





The amp didn't come with its footswitches when I got it. Besides, I didn't want two footswitches, so I got a Berkeley II/III footswitch to use instead. I'll wire up an adapter (6-pin DIN to 1/4" TRS and RCA/phono plugs) so I don't have to modify the wiring/connector of the Berkeley footswitch.