Vox Buckingham V112 rebuild (was Vox Buckingham questions)

[R.G.]

Please do. I won't see the actual hard copy for a few more days, and I've learned that until you have the hard copy in your hands, you really don't know.

I also appreciate comments where the final thing, even though correct, is just plain confusing. So let me know what you think as you go.

[seveneves]

So let me know what you think as you go.

Will do.

Also FYI, I just obtained a (Super?) Beatle power amp. May need that supplement too...

BTW, is it true that I can "tame" the output of a Beatle power amp but running through a higher ohm cabinet? i.e., with an 8-ohm load, it will output no louder than a Buckingham (30 watts)? I thought I read that somewhere on one of your pages.

[R.G.]

Congratulations!

I hope it's a V1141 model. The V14/V114 and V1143 models are in much the condition that the V12/V112 was - tech data ready, text and descriptions needing a good solid edit.

It is true that the Beatle (and in fact all of this line) put out lower power to higher impedance speakers.
All of the Viscount, Buckingham, Guardsman and Beatle models in all their versions use a +/- 31V power supply for their output stage. The output stage is voltage limited; it simply outside that range, and that limits the output power to about 30Wrms into an 8 ohm load. The current capability of the output amplifier is changed for each model, to allow more total power into lower speaker impedances. That's how they got away with using the same preamps and effects but providing multiple different output powers; only the output power chassis really changed.
The Buckingham and Viscount use a pair of 16 ohm speakers in parallel to present an 8 ohm load to their output and get 30W.
The Guardsman uses a pair of 8 ohm speakers in parallel to get 60W, 30W from each speaker.
The Beatle models use four 8 ohm speakers in parallel to get 120W, again 30W from each speaker.
You could in fact get 15W by loading any one of these with 16 ohms, ~7 to 8 by using a a 32 ohm speaker, etc. Tube amps don't work this way,

If you got the Beatle cabinet, first congratulations, they're hard to find, being huge and heavy. Second, if it has the horn inside it, disconnect the wires leading to the horn. The horn makes that cab sound as hard edged as shattering glass. Third, if there is a hole where the horn used to be, cover up the hole from the inside with a bit of plywood screwed on. It gets better bass response that way. It took me some years to find a Beatle cab, and even more years to score a trolley without paying a fortune.

[seveneves]

I hope it's a V1141 model. The V14/V114 and V1143 models are in much the condition that the V12/V112 was - tech data ready, text and descriptions needing a good solid edit.

To clarify, it's just the Beatle power amp. No preamp, no head cabinet, no speaker cabinet...

I was thinking it might be good to have the Beatle power amp to use with the Buckingham preamp, in case I wanted a little more output. Maybe not 100-120 watts but, say, 60.

You could in fact get 15W by loading any one of these with 16 ohms, ~7 to 8 by using a a 32 ohm speaker, etc. Tube amps don't work this way

I like this and that's why I got the Beatle power amp from the same person who sold me the Buckingham head: I felt that I could always get the Beatle to get the amp "softer" by loading it with a higher impedance. But I also felt that using the Buckingham power amp to get the amp louder wasn't going to happen due to the small driver transformer, small power transformer and the lack of the right type/number of output transistors. In fact, if I were to run the Buckingham power amp into a 2-ohm load, wouldn't this fry it up good?

[R.G.]

I like this and that's why I got the Beatle power amp from the same person who sold me the Buckingham head: I felt that I could always get the Beatle to get the amp "softer" by loading it with a higher impedance. But I also felt that using the Buckingham power amp to get the amp louder wasn't going to happen due to the small driver transformer, small power transformer and the lack of the right type/number of output transistors. In fact, if I were to run the Buckingham power amp into a 2-ohm load, wouldn't this fry it up good?

You're right about the Buckingham not working at higher powers. The differences in the power chassis as you go from low power to high power are bigger power transformers, bigger rectifiers, bigger filter caps, bigger driver transformer, more and more powerful output transistors - that is, exactly what you suspected. Yep, trying to get the Buckingham to run a 2 ohm load would kill at least something in the power supply or power amp. Good intuition.

More modern solid state amp designs include protection circuits to shut things down as the loads get out of hand. On these amps, about all you can do is fuse the output and/or the power supply leading to it. And as all engineers know, power transistors always give their lives to protect the fuses.

[seveneves]

Just a follow-up on this thread, it's been over 2 months. Had a good excuse, had a nice early tube Berkeley I needed to recap.

I just got through rebuilding the Buckingham power supply/power amp:



and powered it up with the light bulb limiter, as per R.G.'s fine publication . Unfortunately, found out that the power transformer is shorted or otherwise bad because it's drawing excessive current.

I have a toroidal (https://www.ebay.com/itm/372903358279) that I swapped in and it seems to work despite measuring 28.5VDC instead of 31VDC as per the schematic. I plugged in the original preamp subassembly but at first there was no sound. Ultimately, after a few minutes, I heard the amp speak again after gawd knows how many years! It sounds pretty nice, actually!

I'm thinking it should be possible to have the original power transformer rewound, right? Anyone know any good rewinders? Does anyone think it would be cost prohibitive to do so? Of course, it's not a "collectible" amp but the transformer has a footprint that I am having trouble finding a replacement for with the specs the amp needs.

[teemuk]

More modern solid state amp designs include protection circuits to shut things down as the loads get out of hand. On these amps, about all you can do is fuse the output and/or the power supply leading to it. And as all engineers know, power transistors always give their lives to protect the fuses.

Speaking about that, these things have those weird speaker jacks and plugs because due to mechanical structure of a standard phone jack and plug they briefly short circuit the output everytime you plug them in or out. And as these amps have no short circuit protection of even rudimentary kind they would, well... fail. So Thomas Organ solved that with an interconnect that can not short circuit.

And Sawa Jacobson had the nerve to brag that these "...were built like a battleship. Nothing but an exploding bomb would make them fail."

The Jennings power amp design from the same era actually featured "current feedback" for crude short circuit protection, which would start to limit output voltage if output current increased too high and would simultanously provide a low series resistance for the load. Today an almost similar circuit is used for "tube emulation" because it also replicates the higher damping factor. The side effect of "current feedback" is that output power does not increase or decrease in a linear ratio with the load impedance (so in many cases that rule of thumb is inapplicable to SS guitar amps). Anyway, Warwick Electronics / Thomas basically copied the Jennings preamp but for some reason decided to retain their own power amp design so we have this.

[R.G.]

Dang, you do nice work!! Looks like new.

Unfortunately, found out that the power transformer is shorted or otherwise bad because it's drawing excessive current.

That's a heart breaker.
Just checking - did you test with the secondaries >completely< open and the primary only connected to the line cord? I'm hoping that there is a sneaky load somewhere making it look bad.

I have a toroidal (https://www.ebay.com/itm/372903358279) that I swapped in and it seems to work despite measuring 28.5VDC instead of 31VDC as per the schematic. I plugged in the original preamp subassembly but at first there was no sound. Ultimately, after a few minutes, I heard the amp speak again after gawd knows how many years! It sounds pretty nice, actually!

GREAT!
The "after a few minutes" whispers in my ear that there are leaky filter caps in the preamp assembly and these managed to re-form a little over time.

I'm thinking it should be possible to have the original power transformer rewound, right? Anyone know any good rewinders? Does anyone think it would be cost prohibitive to do so? Of course, it's not a "collectible" amp but the transformer has a footprint that I am having trouble finding a replacement for with the specs the amp needs.

It's completely feasible to have it re-wound, in the technical sense. It's a plain-vanilla power transformer inside, no funny winding tricks needed. I don't know about winding services or the cost of re-winding though.

[seveneves]

Dang, you do nice work!! Looks like new.

Thank you.

Unfortunately, found out that the power transformer is shorted or otherwise bad because it's drawing excessive current.

That's a heart breaker.

You're telling me!

Just checking - did you test with the secondaries >completely< open and the primary only connected to the line cord? I'm hoping that there is a sneaky load somewhere making it look bad.

Yes, I essentially disconnected it while it was still in the amp. Primaries, then secondaries. Tested it with the CT on the secondary still connected to ground, didn't think that mattered with the secondaries disconnected from the rectifiers? Let me know if it should have been disconnected, I can retest if necessary...

I have a toroidal (https://www.ebay.com/itm/372903358279) that I swapped in and it seems to work despite measuring 28.5VDC instead of 31VDC as per the schematic. I plugged in the original preamp subassembly but at first there was no sound. Ultimately, after a few minutes, I heard the amp speak again after gawd knows how many years! It sounds pretty nice, actually!

GREAT!
The "after a few minutes" whispers in my ear that there are leaky filter caps in the preamp assembly and these managed to re-form a little over time.

Yes, this is what I was suspecting but wasn't sure. Glad you confirmed it! I can't tell you how glad I was to hear it sound... it's a big milestone for me in rebuilding this amp as it's state was unknown. Literally sat on a shelf in pieces in a shop, probably a decade or three... Although, I have to be honest... I am NOT looking forward to diving under that circuit board to replace those electrolytics waiting for me at the bottom...

I'm thinking it should be possible to have the original power transformer rewound, right? Anyone know any good rewinders? Does anyone think it would be cost prohibitive to do so? Of course, it's not a "collectible" amp but the transformer has a footprint that I am having trouble finding a replacement for with the specs the amp needs.

It's completely feasible to have it re-wound, in the technical sense. It's a plain-vanilla power transformer inside, no funny winding tricks needed. I don't know about winding services or the cost of re-winding though.

Good to know! I was hoping it was something exotic, like the Thomas driver transformer... I know Mercury Magnetics could probably rewind it but I suspect they're going to come in higher $-wise than the typical rewinder. Do you think anyone who's capable will be able to bring it back to the same stock voltages it had on the secondary?

[R.G.]

Good to know! I was hoping it was something exotic, like the Thomas driver transformer... I know Mercury Magnetics could probably rewind it but I suspect they're going to come in higher $-wise than the typical rewinder. Do you think anyone who's capable will be able to bring it back to the same stock voltages it had on the secondary?

I googled "transformer rewinding service" and Mercury did come up, but so did many other places. Even an alternator rewinder has the capability, although I might not trust such an outfit with AC safety isolation.

[seveneves]

Typo in the last few sentences of my previous post:

I was hoping it was something *THAT WASN'T* exotic, like the Thomas driver transformer.

[Stevem]

It might be of interest a opinion I have interns of another reason why these output stages tend to blow up .

I believe very high frequency oscillation may be why and the reason for this is the crazy long wires going to the output transistors and also there unequal length.

I am talking 60 kHz and up, so your not going to hear it.

The cure I have found is to even out the impedance of the wire harness feeding these two or 4 output devices.

Adding a 10 ohm 1/2 watt right on the Base lead of each output seems to do the trick nicely.

Yes, you will drop off a small amount of output power doing this, but the stability is far better.

[seveneves]

The cure I have found is to even out the impedance of the wire harness feeding these two or 4 output devices.

Adding a 10 ohm 1/2 watt right on the Base lead of each output seems to do the trick nicely.

Interesting Steve. I wonder what R.G. thinks.

If I got it right, I'm inserting that resistor in-between the leads (coming from underneath the chassis) and the Base pin on the output transistor socket (circled in red)?



Any DISADVANTAGE to doing this (other than reduced output power)?

[Stevem]

Yes, correct.
No disadvantage other then what I have posted.

Just like we do for filiment wires I would twist those wires up some going into the PI transformer and also out to each TO3.

[Stevem]

Other then getting your PT rewound there’s no off the shelf PT I can find that will work for you other then a Toroid, but using such would be a nice upgrade.