Top-boost switch not working on Vox JMI Conqueror

[fazeka]

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:



A 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?

[Dave]

Tube amp tweaks and repairs.


Dave

[fazeka]

Oops. Apologies.

[PRR]

> top boost in these amps works by shorting R46 (1k)...

R46 is just anti-pop. "Boost" is a 1u cap across the 51 ohm emitter resistor. Has that cap been replaced? The switch can be checked by unsoldering it, then soldering a jumper; does it top-boost less or more? I'd poke the Ohms of the 51r just to be sure but it seems unlikely to fail (unless bad brand or damp life).

[fazeka]

> top boost in these amps works by shorting R46 (1k)...

R46 is just anti-pop.

Ah, OK. Thanks for that PRR.

"Boost" is a 1u cap across the 51 ohm emitter resistor. Has that cap been replaced?

C25 (1uF) has been replaced, yes.

The switch can be checked by unsoldering it, then soldering a jumper; does it top-boost less or more? I'd poke the Ohms of the 51r just to be sure but it seems unlikely to fail (unless bad brand or damp life).

I already jumpered but to no avail.

R.G. Keen has simulated the circuit and found that "[JMI] put the boost point way too high in frequency". I guess it could have been a factory error, though both this amp and the schematic had C25 = 1uF. Anyway, he had me parallel a 4.7uF cap with C25 and... it works! 

For more info: https://ampgarage.com/forum/viewtopic.php?f=6&t=36824

[PRR]

> R.G. Keen has

Ah, I was too lazy (tired) to do math. 1uFd against 51r is at the top of many guitar speakers (and old ears), may be too subtle to hear. 4.7uFd puts us right in guitar midband, 700Hz. "Top" to me implies higher than 700Hz, but now you have options.

[fazeka]

> R.G. Keen has

Ah, I was too lazy (tired) to do math.

PRR, you don't have to explain! Thank you for all of your help! You and R.G. have forgotten more than I'll ever know!

1uFd against 51r is at the top of many ... (...old ears), may be too subtle to hear.

Unfortunately, I resemble that remark!