Vox Royal Gaurdsman - normal channel low volume?

[mrm0to]

I've been working on a Royal Gaurdsman (the 1133 model) for a little while and am wondering if the Normal channel is supposed to be significantly quieter than the Brilliant? Right now, it is. I've recapped it and replaced a handful of the preamp BJT's that were shot. The voltages are good - not right on w/ the schematic, but close (for instance, the 20V source is actually 18V).

Thanks for any help/insight.

-Dave

[R.G.]

I've been working on a Royal Gaurdsman (the 1133 model) for a little while and am wondering if the Normal channel is supposed to be significantly quieter than the Brilliant?

No, it's not.

I would suspect the mixer/limiter input for that channel if you have recapped already.

I just recapped and three-wired a Cambridge Reverb a couple of nights ago.

[mrm0to]

The DC voltages in the mixer looked okay, so I began tracing a test signal from the input and found it was actually *lower* after the first stage (at the collector of the PNP; this is the version w/ the JFETs, too) than the input level I was testing with (200mV). Then...

Well, at that point the supply voltage was ~17.8 V (vs 20 on the schematic), the collector was at 7 and the base very close to the supply. This seemed like the JFET was doing its job so I pulled out the PNP to test and, sure enough, one of the junctions measured open (just doing a simple DMM check). Replaced (after checking the new one). Fired it back up and still the same situation. I stopped working on it for a little while. When I came back, I fired it up and started checking the voltages at the filter caps for sanity before moving on and somehow discharged the cap at the 24V source. Nice spark.

So, one step forward, two steps back. That node is reading 11V now and it's too late to work on this more tonight.

[R.G.]

... This seemed like the JFET was doing its job so I pulled out the PNP to test and, sure enough, one of the junctions measured open (just doing a simple DMM check).

These amps often led hard lives. The PNP having an open junction means to me that the amp has had an ugly input at some time in its working life. That could have damaged not only the PNP but also the other stuff around it.

I like to remanufacture the input stages on these amps where there is any question about the input stage. It's only nine resistors and five caps and the only PITA is the usual one of getting to the solder side of the PCB without breaking off more wires. If it were me, I'd get metal film resistors (for low excess noise) and replace the entire JFET/PNP section there.

IF you haven't already, you might want to read my stuff on maintaining Thomas Organ Vox amps at geofex.com. I've composed a list of commercial part number replacements for the TV transistors as well as some tips on bulletproofing the amps.

In this preamp, at least replace the two 68Ks and 1M with metal film to lower the input noise.

I recently did a set of PCBs to replace the *entire* insides of the Royal Guardsman and Beatle amps with the same circuits but modern parts and get rid of the yards of wiring. You can seea picture of that board set here. The idea was to mount each subsection on the back of the pots it services and eliminate 90% of the wiring with short wires directly to the pots. The reason I mention this is that I replaced the dropping resistor and filter cap power supply sections with linear regulators on the PCBs. For instance "17V" becomes 18V out of a 78L18 regulator in a TO-92 package, so there is also none of the stringing power supply wires from here to there. The regulators are cheaper than the large value filter caps that had to be used in the R-C power supply in the originals, and do a better job.

Obviously, I'm a little bit nuts about these amps and willing to bore all the way to the bare metal on them.

[52 Bill]

I recently did a set of PCBs to replace the *entire* insides of the Royal Guardsman and Beatle amps with the same circuits but modern parts and get rid of the yards of wiring.
...Obviously, I'm a little bit nuts about these amps and willing to bore all the way to the bare metal on them.

R.G. You are truly a sick man! Did you ever finish the Royal Guardsman driver transformer project?

[R.G.]

R.G. You are truly a sick man!

Thank you.

Did you ever finish the Royal Guardsman driver transformer project?

I got it to the place where it got preempted. I have the "core donor" transformer in hand, and the winding data, but haven't ordered the magnet wire.

I have, just this morning, done the initial debug on the Brilliant channel, Normal channel, Bass channel, and mixer/limiter boards. True to form, I made a couple of stupid errors in the schemo or layout, but so far they're easily fixable. The distortion works, although it's a bit hard edged and may need tamed down a bit. I made minor changes to the power supply stuff, using small TO-92 regulators on each board to give the necessary power supply voltages instead of the power resistors and big caps. Actually turned out cheaper, as well as smaller and less wasted heat/power.

Part of this mess is that I did a fits-same-screwholes PCB for the power amp too, and so when I get down to there, I ... have... to go wind a driver transformer.

What kicked all of this off is that I came into possession of a complete set of Beatle head sheet metal, completely empty. I think it had been used for cannibalization, and I got the leftover bones. Right now, it looks like it's going to make noise again.

Remember my mantra - a monstrous mind is a toy forever.

[52 Bill]

And I'll bet that these new boards will reduce the hum and noise levels as well.

[mrm0to]

If it were me, I'd get metal film resistors (for low excess noise) and replace the entire JFET/PNP section there.

IF you haven't already, you might want to read my stuff on maintaining Thomas Organ Vox amps at geofex.com. I've composed a list of commercial part number replacements for the TV transistors as well as some tips on bulletproofing the amps.

I hadn't occurred to me, but I bet rebuilding will take considerably less time than evaluating each of those components. The owner is in no hurry, which helps . I've bookmarked your Thomas Vox pages; in fact if I hadn't already seen them I probably wouldn't have offered to take it in. Knock on wood - I don't find them particularly difficult to work on, so long as you're patient and deliberate. I just turned the preamp board up a little and strung it to the front of the chassis. Stays in place and is easy to get to both sides.

I'll let you know know what happens next... I probably won't get to dig in again until Wednesday.

[mrm0to]

Whoo! This thing lives and sings. I was sick for pretty much all of April. One last thing: I need to install a new XLR out jack (the owner got it w/out a back panel and an XLR female just hardwired in). Do I join 1 & 2 (to ground) or 1 & 3? There's an older message from Graydean where he indicates joining 1 & 2 to sleeve for a phono adapter, but this guy has the old cabinet w/ XLR input.

Thanks.