Top-boost switch not working on Vox JMI Conqueror

[seveneves]

So I have this ca. '67 JMI (UK) Vox Conqueror solid-state head, mk 1 version (i.e., OS/136 schematic).

I've just had it gone over, recapped as necessary. Don't think anything was done wrong as the rest of the amp works fine but the top boost switch apparently has no effect... ?!? Couldn't determine/forgot to check if the top boost worked before the recap as it still sported original electrolytics prior to that and so was not fully operational (hence the recapping!).

Apparently, top boost in these amps works by shorting R46 (1k)... and the switch has been tested/works, as does C25 (1uF). FWIW, another well-regarded Vox tech says his experience is similar with one Defiant he encountered...

Pre-amp schematics for reference. Photocopy of an original:



An older, slightly cleaned up version

Normal channel in question:



Presuming it's not a factory/schematic error, outside of checking the switch, how do I go about troubleshooting and ultimately resolving this?

[maxkracht]

If you had someone else recap it, bring it back and have them fix it all the way. If you’re doing it yourself, how close are your voltages to the ones on the schematic? Have any resistors drifted? Do you have another cap to sub in? Anything 1uf or more should give a noticeable boost. Can skip the switch, just tack it over that resistor to test. Does the transistor test ok or has it become a resistor? You can measure diode junctions to get a better idea.

[R.G.]

I did what I should have done earlier and stuck it into a simulator to see what happens.

Near as I can tell, they put the boost point way too high in frequency. Try paralleling that cap (C25, 1uF) with a bigger cap. 4.7uF should be noticeable, 10uF makes a huge change, may be too much. As is, the boost frequency is up at several kHz and may not be noticeable. Using a bigger cap lowers the start of the boost.

So yeah, working as designed, but designed to not what we'd like to have.

Other insights from digging into the sim:
That preamp in the non-boost switch setting is flat up to about a MHz (!) with a bit of a peak out there. That's way more than it needs. Paralleling a 3.3nF cap or something close cuts the response back to still over the audio band, flat to about 50kHz.

[maxkracht]

Sorry, I was under the impression that the switch worked originally.

[seveneves]

Try paralleling that cap (C25, 1uF) with a bigger cap. 4.7uF should be noticeable, 10uF makes a huge change, may be too much. As is, the boost frequency is up at several kHz and may not be noticeable. Using a bigger cap lowers the start of the boost.

So yeah, working as designed, but designed to not what we'd like to have.

Thanks R.G., that's it! I paralleled 4uF and yes, that what I'd expected! I daren't try 10uF, the resulting top boost might rip my head off.

Other insights from digging into the sim:
That preamp in the non-boost switch setting is flat up to about a MHz (!) with a bit of a peak out there. That's way more than it needs.

Wow. Had to be an oversight on JMI engineering's part? I checked in with a few owners I know of and they said they can hear a difference with the top boost on, wonder if there was a change in production for a bigger cap at some (later?) point?

Paralleling a 3.3nF cap or something close cuts the response back to still over the audio band, flat to about 50kHz.

Sorry, where would one parallel that 3n3 cap?

[R.G.]

Wow. Had to be an oversight on JMI engineering's part? I checked in with a few owners I know of and they said they can hear a difference with the top boost on, wonder if there was a change in production for a bigger cap at some (later?) point?

It's hard to say. Maybe it was a flat mistake or omission. It's like the chefs say, you have to keep tasting your stuff as you cook.

Paralleling a 3.3nF cap or something close cuts the response back to still over the audio band, flat to about 50kHz.

Sorry, where would one parallel that 3n3 cap?

Oooops. Across the 820 ohm collector resistor of the second transistor. Should produce no audible effect unless you have 50kHz ears.

[martin manning]

Dunno if you guys have seen the White Pedal, but it's the Clipping/MRB section out of the Conqueror/Super Beatle distortion channel. It's a unique, maybe even iconic sound. Cloned here: https://ampgarage.com/forum/viewtopic.p ... 42#p399742

[R.G.]

I hadn't seen that. Makes sense that it would be a useful pedal in its own right.

I got off on a kick a few years ago of laying out new PCBs for the preamps of the various Vox amps. In that mess was the MK3 Supreme/Conqueror/Defiant preamp. I did the whole preamp, channels, effects and all. But that's much more an amp front end than a pedal.

[seveneves]

Dunno if you guys have seen the White Pedal, but it's the Clipping/MRB section out of the Conqueror/Super Beatle distortion channel.

Yup, saw that. Kinda cool. There's also this which was made about a decade before the Jext Telez version. But I had a chance to get a real amp as I love my Thomas Organ Buckingham and I was curious/intrigued as to how the Brits copied the Americans who copied the Brits...

[martin manning]

Interesting, I didn't know about the MMBox. Looks like it retains the switchable distortion and MRB functions where the White has them both permanently on. I rarely see a Vox amp

[seveneves]

So, just a follow up on this. It seems that R.G. really nailed it. Again!

I did some sleuthing and went to look at pictures of other similar amp internals.

It's easy to find internal shots of these amps online. 99% of people take pictures from the same angle (i.e., from the bottom edge of the preamp chassis).

However, the top of the normal preamp channel board is right up on the underside of the backs of the controls and most don't bother with that angle when they're taking pictures.

I found two Defiants (the Conqueror, Defiant and Supreme all share the same preamp) that give a clue as to their values of C25.

This one shows that there's another cap "piggybacking" in parallel across C25. Values are not seen but presumably the bigger one is 1uF:



This second Defiant clearly shows C25 with a value of 2.x:



So again, thanks to R.G. for simulating the normal channel preamp and helping me get this top boost working!

And maybe/hopefully it may help others now or in the future with theirs.

[Dennis Perusse]

Hello all,


Don't forget, over at Vox Showroom he is letting people download a digitally cleaned up copy of many vox schematics. Here is the Url for it. http://voxshowroom.com/uk/amp/schematics_uk.html

Fantastic job with the solution at hand. Great stuff.

Dennis


ps Hey, RG, check your junk folder in your email. Some of my emails may have ended up there again.

[R.G.]

Yep, junk folder again. I wonder if my mail server is doing something odd with your emails, because you're on my contact list.

Email is getting weirder. I found that I cannot send email to anyone with a gmail address because gmail bounces any email without a special "I'm not spam" certification in the header, and my hosting/email servers refuse to add the certification step. Grrrr....

[seveneves]

Oooops. Across the 820 ohm collector resistor of the second transistor. Should produce no audible effect unless you have 50kHz ears.

Just so happened to re-read this... would it make sense to add in that cap anyway? Is the amp oscillating in some way without it? I hear about amps oscillating up in the 100s of kHz and how power gets wasted (generates more heat). Doesn't seem too good...?

[R.G.]

Just so happened to re-read this... would it make sense to add in that cap anyway? Is the amp oscillating in some way without it? I hear about amps oscillating up in the 100s of kHz and how power gets wasted (generates more heat). Doesn't seem too good...?

It's one of those things that are not strictly necessary, but can prevent unwanted things, sometimes, maybe. Audio preamps like this don't need to respond much above audio, and the excess bandwidth can add noise and support RF oscillation. Not always, but it can happen. One tenet of low-noise design is to not provide more bandwidth than your signal really needs so you attenuate noise outside the frequency band you really care about.
Some amps do oscillate at ultrasonic or RF, and that does eat power, overheat things, and can sometimes cause audible interference as the US/RF gets aliased back down into audio.
The Conqueror probably works fine without it. I like it because it's good design. You might try with and without it to see if it makes any difference to the sound. If not ... leave it in.