Yet Another Vox Buckingham

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Yet Another Vox Buckingham

Postby Frank_NH on Thu Jun 09, 2016 4:11 am

Hello all,

I'm new here, Frank from New Hampshire. I signed up since this seemed to be the forum for all things related to Thomas-Vox amps. :D

Anyhow, I recently acquired a Vox Buckingham head. I got this amp eBay for a fair price, and I'd been wanting one for a while after seeing pictures of the Beatles using the Super Beatles on stage in 1966. I've read up extensively on the amp's design and features (many thanks to R.G Keen and North Coast Music), and have the main schematic for the Buckingham.

This one is serial number 1048742, which I believe makes it a 1967 model. And as you can see, it's had a rough life. All of the plastic corners and logos are gone, two vents were nailed (!) to the cab, the metal is ding and dented, and the tolex ripped in places. The back of the unit was cracked and the previous owner put in some makeshift brackets to keep it together. Despite this, the tolex overall is in pretty good shape as is the grill cloth. I plan to fix all of these cosmetic issues in due course, after giving the electronics an overhaul.

I've played the amp through a 1x12 speaker, and I was blown away by the sound! The normal channel output is low (probably a electro capacitor or two), but the brilliant and bass channels are fine (and VERY loud), as are the effects. I had lots of fun with the MRB effect - it really gives overdriven leads a unique sound, like that in the song "Fixing a Hole" on the Beatles' Sergeant Pepper album. And the Reverb is just awesome! In fact, I ran my tube screamer into the Buckingham's Brilliant channel at half gain, turned up the reverb, and I just nailed the sound of Magic Sam's "West Side Soul" with my Telecaster. I was uncanny. Full up the reverb reminds me of the drippy reverb on 60's surf record. This would be a great surf amp.

I'm going to test more pedals out in the future, but the amp seems to work well with overdrives. Unfortunately, I don't have a footswitch, but I'll rig something up so I can test the built-in distortion/fuzz and tremolo.

I plan to work on the electronics first as part of an eventual full restoration. I've built a tube amp and have built tons of pedals, so I'm comfortable doing the electronics overhaul (and I'll try not to break any wires :shock: ). I also plan to take tons of pictures of the inside as I proceed, so if anyone would like to see those I can post them here.

cheers,
Frank

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Re: Yet Another Vox Buckingham

Postby Frank_NH on Sun Jun 12, 2016 2:46 am

Here it is on my workbench. I have it disassembled now and am documenting all of the internals as I go with numerous pictures from my iPhone. As I mentioned, there are a lot of cosmetic issues that will be fixed in due course.

Some interesting information I've obtained thus far:

* Though there are many missing screws, it appears that no one has touched the preamp or poweramp circuit boards. The unit has the original capacitors (those silver things + all of those black electrolytics + big cans in the power supply).
* The reverb pan indicates that it was manufactured by Gibbs Manufacturing and Research Corporation, apparently farmed out by Hammond to them in the mid sixties. The reverb definitely has that deep, Fender-y sound to it, and I suppose this helps explain why. The reverb pan itself is in great shape.
* Removing the chassis was fairly easy. Just needed to be mindful of the connectors. To get the 11/32" hex nuts off, I was able to use a 9mm deep socket from Home Depot with an extension on my ratchet wrench.
* All of the small signal preamp transistors are the 5044-2 type, with the yellow-green dots on top. They appear to be Fairchild based on the markings.

I'll be replacing the capacitors once I get some suitable ones ordered, and I'll be **very** caerful given the mess of wires that these amps are known for. :shock:

Let me know if anyone wants to see some pictures of the insides of this this amp.

Frank_NH

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Re: Yet Another Vox Buckingham

Postby R.G. on Sun Jun 12, 2016 2:35 pm

Nice find. As mentioned in other posts here, I'm a little bit nutzo over this family or amps.

If you haven't found it already, there are reference materials on geofex.com. This includes a list of electrolytic cap values and transistor types and substituations.

If you're diving into it yourself, make a paper/cardboard prop to be able to bend the main board(s) up once, do all your work and testing and then put it back down when you're done. The fewer bends the better.

The silver caps are film and foil types, not electro. They don't need replacing - or at least I've never seen a bad one. The electros rot with age.

The electros on the main PCB are mostly axial packages. This package is about 2-5 times the cost of the radial package these days. It's better to use radial caps for the ones on the PCB and bend the leads so they fit the original spacing. Much easier to find radials and much less expensive.

Replace all the bigger caps in the "basement" under the PCB, on terminal strips, too.

The transistors are 2N2925, mostly. House-numbered for Thomas. If you need replacements for them, they have the ECB order pinout. The 2SC1815 type is a drop-in for these, and works well. The 2SC1815 can be hard to find. The 2N5088 works in the circuit too, but has EBC pinout order, so you have to bend leads to the correct order. The 2N5088 is about $0.05 each; I designed them into the replacement PCB so transistors would be easier to find.

At least one of those transistors is a metal case TO-5 style, perhaps with a copper heat sink on it. It's the reverb transformer driver. Since the reverb is working, don't mess with it. If it dies, you'll have to use a TO-220 plastic pack. 2SC2073 works, but needs a small heat sink.

Yell if you need more tips.
Best regards,

R.G.
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Re: Yet Another Vox Buckingham

Postby Frank_NH on Sun Jun 12, 2016 7:10 pm

Thanks R.G. I think I've read all of your extensive online material about the TV amps. They've been a great resource. Thanks!

Here's the preamp insides in all it's glory. The blotchy appearance of the PCB is perhaps due to the original manufacturing process and/or aging. It seems stable enough, but I haven't lifted the PCB on the hinge just yet and will wait on the capacitors to arrive. Thanks for the heads up on the "silver" caps - I won't touch those unless I have to. My plan right now is:

(1) Replace all the electros (substituting film caps for the 1 uF and 2 uF) in the preamp box. Test it out to make sure it works.
(2) Replace the two prong power cord with a grounded cord. I will have some questions as I proceed with that, as I don't want to make anything unsafe. (I've read a few online article on how to do this for a Fender Amp).
(3) Replace the big caps in the power supply. There didn't appear to be any issues with the power when I played through the amp, but better safe than sorry.
(4) Repair all of the cosmetic issues (mainly buying and installing replacement parts and cleaning/repairing the tolex and piping). One important task will be to make a replacement back piece and ditch the broken one.

When the head's done, I'll think about either building or buying a 2x12 cab. Lots of fun with this project! :D

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Re: Yet Another Vox Buckingham

Postby R.G. on Sun Jun 12, 2016 7:34 pm

Yeah, the blotchy appearance is because the liquid soldering flux they used was thoroughly cleaned. Perhaps not cleaned at all. :shock:

As a tech note on replacing caps, you can cheat on the axial, lying-down caps by
(1) noting **carefully** which way round positive is; some of them are non-polar/bipolar (NP) and will have "+" marks on both ends.
(2) clipping off the leads to the cap on the top/component side, and removing the corpse, leaving the leads
(3) gently forming the remaining leads into hooks to hold the new cap's leads
(4) delicately soldering the pre-formed new cap leads into the hooks.

This puts off the ugly moment when you'll have to raise/bend the board up, and lets you test your work after each cap.

As a bit of advanced work, it turns out that their grounding setup is tricky. I have a Royal Guardsman open on the bench now (and have for about two months, as I have snippets of time to work on it) and I'm trying to do a definitive grounding setup. It's not easy to get this all to be both three-wire and hum free. I haven't yet trapped the problem in the wild. So do check before you start that.
Best regards,

R.G.
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Re: Yet Another Vox Buckingham

Postby Frank_NH on Mon Jun 13, 2016 5:32 pm

Thanks again R.G.

As I was examining the PCB to prepare to order the capacitors, I noticed that C210, C222, C223, C224 are 1 uF NP and not 10 uF polar as shown in the Vox 1141 schematic. The last three are the input coupling caps for the mixer. This must be a transitional change to the original circuit, since later versions appear to have those at 1 uF.

The image below is the poweramp board, with the power transistors mounted on the big metal plates on the rear. One interesting thing I found here was the 1K resistor (circled). I can't seem to find it on the schematic. Looks like it's connected to a (shielded?) ground wire?

Still deciding what to do about replacing the big can 2500 uF capacitors. The originals look good, but they're 50 years old, and so probably need to go.

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Re: Yet Another Vox Buckingham

Postby R.G. on Mon Jun 13, 2016 10:05 pm

Frank_NH wrote:As I was examining the PCB to prepare to order the capacitors, I noticed that C210, C222, C223, C224 are 1 uF NP and not 10 uF polar as shown in the Vox 1141 schematic. The last three are the input coupling caps for the mixer. This must be a transitional change to the original circuit, since later versions appear to have those at 1 uF.

That's correct. Leave them at 1uF. Some of my motivation for writing up the tech work on the Vox amps is little things like this.

The image below is the poweramp board, with the power transistors mounted on the big metal plates on the rear. One interesting thing I found here was the 1K resistor (circled). I can't seem to find it on the schematic. Looks like it's connected to a (shielded?) ground wire?

Another of those little transitional things. The 1K is placed across the input to the power amp (that's what that shielded cable is; it plugs into the preamp chassis) and keeps the power amp sane in the event that the plug comes loose from the preamp.

Still deciding what to do about replacing the big can 2500 uF capacitors. The originals look good, but they're 50 years old, and so probably need to go.

Up to you. One strategy is to remove them (which is difficult!) and hollow them out, inserting a modern cap inside the shells for appearances. Another is to replace them with modern snap-in capacitors in capacitor clamps that you place so the terminals project through the existing holes. Those caps are about 1 3/8" diameter, and 35mm is a standard size of modern snap in caps. 35mm = 1.378", versus 1.375" for 1 3/8", so they're essentially the same diameter, and fit. The modern 35mm ones will be cheap, and can be had in values far larger than 2500uF, cutting your power supply ripple a lot.

A lot depends on your mechanical/hacking abilities on the chassis. The old caps are held to the chassis with metal tabs, stuck into slots on the chassis, then twisted, and then soldered to the chassis. The thermal mass of the chassis is so large that you either need one of those big Weller soldering guns to get enough heat in the tab to get the solder loose, or you need to strip the chassis so you can get a propane torch powered soldering tip in there to get enough heat. I designed a PCB to mount two modern caps and a modern FWB rectifier on standoffs in the same position, but the problem of removing the old caps remains.
Best regards,

R.G.
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Re: Yet Another Vox Buckingham

Postby Frank_NH on Tue Jun 21, 2016 3:31 am

An update. I'm currently in the middle of recapping the Buckingham. I've gotten into the belly of the beast and have finished replacing the capacitors in "the trunk" (that's the area below the main PCB which contains a plethora of components for the power amp and power supply, along with wires to various controls). Along the way, I'm comparing my unit with the published Vox 1121 schematic. Here are some anomalies I've found.

* The 100 ohm resistor (circled) apparently connects the relay to the footswitch. This is not on the schematic.
* The 330 ohm resistor on the left is marked at 300 ohm in the schematic (not a big deal since there are no 300 ohm resistors...are there?).
* I discovered a 220 pF ceramic disk capacitor soldered ** below the board ** which connects the base and collector of Q202 in the Reverb module. Maybe that was done after the fact to correct some high frequency oscillation issues?

It's now on to the main board. I have the unit **carefully** propped open with some strategically placed sticks and foam, so as not to disturb the wiring too much. So far, no wire breaks (that I know of). More later... :D

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Re: Yet Another Vox Buckingham

Postby R.G. on Tue Jun 21, 2016 3:52 pm

Frank_NH wrote:An update. I'm currently in the middle of recapping the Buckingham. I've gotten into the belly of the beast and have finished replacing the capacitors in "the trunk" (that's the area below the main PCB which contains a plethora of components for the power amp and power supply, along with wires to various controls).

Yer a brave man, Frank. (said in Scots-Irish-English)
Along the way, I'm comparing my unit with the published Vox 1121 schematic. Here are some anomalies I've found.
* The 100 ohm resistor (circled) apparently connects the relay to the footswitch. This is not on the schematic.
* The 330 ohm resistor on the left is marked at 300 ohm in the schematic (not a big deal since there are no 300 ohm resistors...are there?).
* I discovered a 220 pF ceramic disk capacitor soldered ** below the board ** which connects the base and collector of Q202 in the Reverb module. Maybe that was done after the fact to correct some high frequency oscillation issues?

Back in the 1950s and 1960s, printing service literature was very, very difficult. Engineers drew up the schematics and mechanical drawings and made handwritten lists. Typists (there were people whose entire career was typing at a manual typewriter, odd as that seems now) typed it up, engineers corrected and reviewed lists, then all of it went to the layout/print house, which did a prototype printing ("galley proof") which was checked by engineers, techs, and now-very-bored typists, and when as done as anyone could stand, it was committed to print and the entire print run was done. The documentation could easily take as long to prepare as the manufacturing run. Note that this was all by hand; no computers were used, as they did not exist in a form useful for this work.

So when a tweak had to be made so the stuff coming off the manufacturing line actually worked in the field, it was simply made and some notes were made for the manufacturing line. The service literature was **NOT** reprinted. In severe cases, field service notes were made and sent to authorized service centers. I managed to discover a fair number of these service bulletins and roll them in the "Vox Owner's Safety Net" and technical supplements.

So yes, your intuition is right - those were probably on-the-fly changes to get the stuff to work more reliably. Er, predictably. :D

It's now on to the main board. I have the unit **carefully** propped open with some strategically placed sticks and foam, so as not to disturb the wiring too much. So far, no wire breaks (that I know of). More later... :D

That's the spirit. Raise the board exactly once, fix everything in the "basement" while it's up. Don't forget to schpritz and clean the switches and pots accessible down there before deciding you're done. You don't ever want to do this again!

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Re: Yet Another Vox Buckingham

Postby Frank_NH on Tue Jun 21, 2016 5:35 pm

"Yer a brave man, Frank. (said in Scots-Irish-English)"

Yeah - I get it from my Dad, who was a electronics tech for Northrop for 40 years. I remember we used to get $10 broken televisions from Radio Shack, and Dad would find the bad part and have it running in no time. Got a lot of TVs and radios that way! :D

It also occurred to me that the Thomas Organ Co. was probably one of the few companies that could have realized Sava Jacobsen's advanced (for the time) amp designs. If you look at a typical organ (see below an example of a Hammond Organ), it's also a complex maze of hardware, circuit boards and wires, wires, wires! The assemblers were probably used to that style of construction and had the patience to make it work for the guitar amp line.

By the way, I've also ordered a bunch of parts from North Coast Music to start the rehab of the cabinet (tolex, feet, logo, piping, etc.). I'll post some pictures of that as it progresses. I need to make a new back panel and replace the 1/4" speaker jack with a Switchcraft C3M XLR style jack. Hopefully it will look NOS (not brand new) when finished, as it will be difficult to completely repair all the dents in the edges of the control panel.

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Re: Yet Another Vox Buckingham

Postby R.G. on Tue Jun 21, 2016 6:23 pm

Frank_NH wrote:Hopefully it will look NOS (not brand new) when finished, as it will be difficult to completely repair all the dents in the edges of the control panel.

That's a problem, OK. I have a scheme in mind that will replace/fix that control panel.

The panel is 0.02" aluminum sheet, punched and then glued on. I have removed them intact by **very** gentle prying and dribbling mineral spirits into the crack to slowly dissolve the adhesive without ruining the paint. Get a little too aggressive with the prying and you bend the aluminum.

But once it's removed, you could cut and bend a new 020 panel to get a good corner, then paint and silkscreen it. It rapidly becomes a labor of love. Or slavery and madness. :lol:

Another thought is to find a small black plastic extrusion that would simply cover up the dinged corner.
Best regards,

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Re: Yet Another Vox Buckingham

Postby Frank_NH on Fri Jun 24, 2016 8:04 pm

Another update (for those interested in the insides of these amps). You always have to look **under** everything to make sure you've gotten all of the electrolytics. :wink: Here's a 500 uF cap under the power amp / power supply chassis, with a cute little wire holder for the cap.

The two power transistors are in good shape and are marked Delco 86-5083-2 6617. These are germanium and are quite impossible to find replacements (Delco DTG-110) for if they ever burn out (hopefully not).

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Re: Yet Another Vox Buckingham

Postby R.G. on Fri Jun 24, 2016 8:27 pm

Frank_NH wrote:Another update (for those interested in the insides of these amps). You always have to look **under** everything to make sure you've gotten all of the electrolytics. :wink: Here's a 500 uF cap under the power amp / power supply chassis, with a cute little wire holder for the cap.

They were very sneaky that way!

The two power transistors are in good shape and are marked Delco 86-5083-2 6617. These are germanium and are quite impossible to find replacements (Delco DTG-110) for if they ever burn out (hopefully not).

As it turns out, there is a way to recover. The germanium parts are available at very, very high prices, from an outfit named "Germanium Power Devices" - or they were, once. They make ... new... germanium for mil-spec replacements.

But you can sub in a silicon PNP by changing two bias resistor values on the power amp board.

You can also put in **silicon NPN** devices by changing the transistors, the two bias resistors, two power supply wires, and two driver transformer wires.

I dug fairly deeply into the old designs and turned this up. It's outlined in the tech supplement I wrote for the Buckingham amps.
Best regards,

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