Vox Buckingham V112 rebuild (was Vox Buckingham questions)

[martin manning]

@stevem: Cool meter. I gotta get one of those!

RG, there are newer versions of that ATMega328 based component tester with improved functionality, including reporting of Ic0 for testing Ge transistors, and identification of low-voltage Zeners. Sold under the brand Aideepen, they are still around $20. A recent thread here focused on inductors revealed that they are not great at measuring large inductances, but they seem fine and reasonably accurate for everything else.

[R.G.]

Thanks, Martin! I'll give that a look. I'll have to FreeCAD up a 3d printable enclosure for it at some point.

[martin manning]

Thanks, Martin! I'll give that a look. I'll have to FreeCAD up a 3d printable enclosure for it at some point.

No need, here's a link: https://www.amazon.com/dp/B08YNB7K8G/re ... F9kZXRhaWw

[R.G.]

Thanks!

[seveneves]

Update:

I couldn't easily physically mount the toroidal power transformer on the Buckingham power amp chassis, so I went with the closest (electrically and physically) that I could find that would fit the chassis as is

I settled on the Hammond 166L42:



I thought Hammond offshored all these kinds of transformers to China but I was pleasantly surprised to see this one was/is still made in Canada (as they should be!).

Had to mount it diagonally. Barely fits the holes in the chassis, if it was any narrower, it would not have been wide enough to fit/bolt in.

Also used a tie-wrap to keep the winding wires from hitting the sharp-ish edges of the chassis cutouts.

Voltages are now at 31.5, +/- so 0.5V over the published spec. I'll keep the old transformer, maybe one day I'll take a go at rewinding it. Nothing to lose.

Next up: preamp rework.

[Stevem]

I save a fair amount of transformer cores also, be they power or output should I need to get one made into something I can't get anymore.

Here's a 200 watt Marshall major OT and a 1.5" tall roll of painters tape.

One day I will build something when I get this rewound.

Your far braver then I am to try rewinding stuff!

[seveneves]

I have the preamp redone. 100% working. Sounds pretty damn nice, actually! Pics later.

I do have a question (below): I purchased a Mod reverb tank to use with the Buckingham as it never came with one. I ordered a 4EB2C1B. I wasn't quite sure I needed this one with input impedance (600 ohm) but figured it would work anyway, maybe not as strong (have to turn up the level more)?

At least it had the input insulated and the output grounded, which is what I believe it needs (at least per the schematic).

However, when I plugged it in, switched to either the brilliant or normal channels and turned up the blend (reverb) control, the output got lower. And, no reverb signal... (turned up all the way)

I had a friend drop by with a spare tank (similar, Mod brand) he had lying around. 4BB2A1B. Grounded input and output. 150 ohm input impedance. Dang it if the reverb didn't work?

Trying to figure out if the tank I got is bad? I was under the impression that lower input impedance is harder to drive/possibly cause distortion? And that a higher input impedance would at least sound but may need to turn up the reverb level more?

[Stevem]

Your amp looks like it uses the same pan driver transformer as my TV reverb twin.

I have a list of a ton of reverb tank resistance test, but do not have the details of that one of mine

I will open it up later a see how it test out and report back to you.

That being said I can’t see why a tank being driven by a transformer even in a SS amp would need a tank with a input of 600 ohms.

[Stevem]

Ok I cracked open my Berkeley II for you.

The input resistance of my original pan is 178.9 ohms, the output is a tad less at 177.8.
Originally my guess is they where both meant to be close same within 600 to 800 ohms impedance wise.

Both jacks are grounded to the pan, but I did just find a about 10 ohms of resistance between the two that I guess is due to age of the crimp of the metal rivets .

I took advantage of having the tank out and soldered on a jumper and I polished the tarnishing off both jacks, tightened up the bight down on the tips and lubed up Deoxit.


My input cable to the tank is not shielded just like the schematic shows, so only the tanks output side is grounded to the main chassis.

My driver transformer is a 606-6-2F

It’s primary checks in at 54.2 ohms.
It’s secondary is 130 ohms.

My best informed guess as to the original tank would be
4FB( 2 if mid time delay) A1D, so 4FB2A1D

[R.G.]

Congratulations! Getting the preamp working is about 90% of the work.

Weird that a new tank from Mod didn't do anything at all. It's unusual, but sometimes new stuff is defective. Stevem is on the right track - check the resistance of both input and output coils. It could be that one of them is open, or shorted. Can you still return it? I think you have full justification if another tank just plugs in and returns a signal and the new one doesn't work.

Reverb tanks need to be driven with a certain power, as they're moving a similar spring mass in all of them. The impedance at the input is more a reflection of the tradeoff of voltage and current in that power = voltage times current. Lower input impedance means you need more current but less voltage, and vice versa. The output impedance is a measure of how much voltage-times-current the output coil puts out. In general, the higher the output impedance, the higher the output signal voltage, as most reverb returns don't need much current. As a result of this trade off of current and voltage, most good tanks will give you some reverb signal, just maybe not enough to balance correctly in the returned mix.

Vox chose to use a reverb mixing setup that lets you choose a mix on the reverb control that lets you pan all the way from 100% dry to 100% wet. This means that if there is no reverb return, the reverb control is effectively another volume control, as you found.

[seveneves]

Congratulations! Getting the preamp working is about 90% of the work.

Yes, and it was a LOT of work as you know R.G.. In the end, I did replace 97-98% of all the components in the preamp. The only original components are the inductors (MRB and the "Tone-X" channel), the three transistors in the trem circuit, the two transistors in the "Bulldog" limiter and the two reverb send transistors (before the reverb tank). There was also one 100 ohm resistor in between the bass and treble pots on the brilliant channel that I left, as well. It was just too inaccessible to get to. The pots, switches and jacks are also original. They are not noisy and are working fine.

I did this as I didn't want to put in new transistors in the preamps, the mixer and the reverb recovery just to then have a cap or resistor go south. I do NOT want to go back into this preamp again if I can help it!

Weird that a new tank from Mod didn't do anything at all. It's unusual, but sometimes new stuff is defective. Stevem is on the right track - check the resistance of both input and output coils. It
could be that one of them is open, or shorted. Can you still return it? I think you have full justification if another tank just plugs in and returns a signal and the new one doesn't work.

Well, to be clear: the new Mod tank with the 600 ohm input impedance and input insulated did not work. My buddy's spare Mod tank with a 150 ohm input impedance and a grounded input DID work. Still trying to understand that because as Stevem says, a "4F" tank (1475 input) should work...? So why would a 150 ohm tank work and not a 600 ohm tank work if a 1475 ohm tank works? Do you think the tank's input needs to be grounded (despite the schem indicating insulated tank input)? Maybe that's why the 600 ohm tank doesn't work?

Another question: will I be damaging the reverb send circuit (specifically Q201 and Q202 transistors) using a lower input impedance tank like this 150 ohm one? As a reminder, this is an early Buckingham (V112).

Reverb tanks need to be driven with a certain power, as they're moving a similar spring mass in all of them. The impedance at the input is more a reflection of the tradeoff of voltage and current in that power = voltage times current. Lower input impedance means you need more current but less voltage, and vice versa. The output impedance is a measure of how much voltage-times-current the output coil puts out. In general, the higher the output impedance, the higher the output signal voltage, as most reverb returns don't need much current. As a result of this trade off of current and voltage, most good tanks will give you some reverb signal, just maybe not enough to balance correctly in the returned mix.

Yes, that's what I was suspecting with the 600 ohm input tank... Some kind of sound.

Vox chose to use a reverb mixing setup that lets you choose a mix on the reverb control that lets you pan all the way from 100% dry to 100% wet. This means that if there is no reverb return, the reverb control is effectively another volume control, as you found.

Yes, I noticed this when I tried it with the one tank that worked. Seemed a markedly different type of reverb experience compared to, say, a blackface Fender, which my ear is MUCH more accustomed to.

I also noticed a hum when switching the brilliant or the normal channels to reverb. I presume this is normal? Could this hum be improved somehow? Maybe new transistors for Q201 and Q202? I say this because the hiss/noise/hum with the reverb switched out is nearly non-existent! I am spoiled with the rebuild of the preamp. I wasn't expecting it to sound so quiet! Kudos to you R.G. for your years of analysis/work with these Thomas SS amps to help owners like me get the hiss/hum/noise to be as quiet as possible: the input transistors and mixer transistors replacement is the least anyone can do to reduce hiss in these amps!

[martin manning]

I purchased a Mod reverb tank to use with the Buckingham as it never came with one. I ordered a 4EB2C1B.
... At least it had the input insulated and the output grounded, which is what I believe it needs (at least per the schematic).
... And, no reverb signal... (turned up all the way).
... I had a friend drop by with a spare tank ... 4BB2A1B. Grounded input and output. Dang it if the reverb didn't work?

I wonder if you are missing a ground for the input transducer with the "ungrounded input" tank?

[Stevem]

From the chart that I have a input coil said to be 600 to 800 ohms impedance should resistance test at 58 ohms, is this the case with your new tank?

In terms of the hum you say you have and I assume your testing with things still apart than note this.
If the output side of the tank is too close to the power transformer it will pick up the 60 hz buzz from that .

The more you open up the reverb return section, the louder the buzz.

When you therefore mount the tank back in the amp the output side can not be on the same side as the PT.

[seveneves]

I wonder if you are missing a ground for the input transducer with the "ungrounded input" tank?

Yes, could be. Not sure if this is pertinent or I if I am interpreting the grounding/lack thereof with the inputs but when I looked at the inside of each tank (on the other side of the input jack), they appear to have the same small PCB configured/wired the same on either. I was expecting the grounded input tank to look different (i.e., have a jumper or a bridge soldered) than the insulated input tank. I'll confirm tonight.

[seveneves]

From the chart that I have a input coil said to be 600 to 800 ohms impedance should resistance test at 58 ohms, is this the case with your new tank?

I should have done this last night. I will confirm tonight.

In terms of the hum you say you have and I assume your testing with things still apart than note this.

Yes.

If the output side of the tank is too close to the power transformer it will pick up the 60 hz buzz from that .

The more you open up the reverb return section, the louder the buzz.

When you therefore mount the tank back in the amp the output side can not be on the same side as the PT.

Not sure if you're aware with the construction of these particular "big head" solid-state Thomas Organ Vox heads (i.e., Buckingham/Royal Guardsman/Beatle) but I am very restricted as to how/where the subassemblies are mounted. It's pretty tight in there and things have to go in a specific place. With the head cabinet empty, the power amp chassis goes in first (from the rear) and slides all the way in past the reverb tank mounts (there are little "notches" cut into the power amp chassis to clear these mounts, that's how tight it is in there!) right up to the front grill cloth baffle. Then the reverb tank goes behind it and butts up right along the power amp. It's rear edge is about an inch away from the cabinet edge; again, it's tight. Lastly, the preamp chassis/module goes in (above the reverb tank) and fills in 95% of the remaining space. The reverb tank has the open side facing down with the phono jacks facing the rear (because the front side of the tank is butted right up against the power amp) and there's no room for the input and output cables to go other than where they go to now, which is alongside and into the jacks at the rear of the tank.

All that said, the output (which I believe is more sensitive than the input, IIRC) of the reverb tank is about as far away as is possible from the power transformer. Maybe it's not far *enough* away but that is its inherent design. It's not a terribly loud hum but it's noticeable when switching from/to the OFF position... and with the input transistors changed and the amp sounding almost as quiet as a church mouse p1ssing on a cotton ball , it IS slightly distracting. Beggars can't be choosers?

So now I'm thinking if it's humming now in reverb mode during testing with the tank sitting a little further away than the power transformer than when installed, I guess the hum's not going to get any better when all of these assemblies are put together proper, nice and snug within the head cab.