60's Vox Kensington Bass: This chump is stumped

[Luthier Vandros]

I've met my match. I've got a transistory Vox bass amp from the 60's on the ol' bench and am getting no guitar signal to pass through the output found two poop caps feeding Q1/2 bases. The amp passes an injected 1khz sine with authority and ease, but will not pass a guitar signal. Any hot tips for this weary electron wrangler?

TIA

 

[GByxbee]

I've met my match. I've got a transistory Vox bass amp from the 60's on the ol' bench and am getting no guitar signal to pass through the output found two poop caps feeding Q1/2 bases. The amp passes an injected 1khz sine with authority and ease, but will not pass a guitar signal. Any hot tips for this weary electron wrangler?

TIA

According to the schematic, there is no cap at the base of Q2. Everybody, including me, will tell you it must be the guitar or guitar cable. I'd be interested in knowing what the amplitude of your 1Khz signal is as well as it's DC offset. The amp should be able to process a 20mVrms signal with a 0V DC offset. You should be able to put your AC meter at the end of the guitar cable, strum the guitar at a good volume and get a reading on the meter to tell you your guitar and cable are putting out. Seems to me I got about 50mv out of my guitar, full volume with a "E" chord.

 

[Jon Snell]

This is an extremely simple amplifier with a simple pre-amp tone section.
1/ Inject a signal on the wiper of the vol control. Output or not?
2/ If there is output, work backwards. If no output work backwards from the output.
3/ If C3 is leaky, there will be no gain.
4/ Have you got 2v DC on the base of Q1 with no signal input?
5/ Does the Collector of Q2 sit at around half HT; 9volts ish?
6/ Are the input jacks, that are over 60years old, OK?

 

[Luthier Vandros]

Pardon the friendly fire, Guys. I sortet it out and forgot to update you. The owner brought the amp over as the bare chassis —and once I realized that q2 wasn’t grounded without the g-tuner connected, asked for the cab, which houses the off-board portion of the system. Without the molex connectors joined, q2 is essentially off.

With respect to my test rig, it was uncalibrated and sending 10.5vac into the input, bypassing q2. It’s now calibrated at roughly line level.

BTW - if you’re onto the schematic, Q2’s diagrammed EBC pin out doesn’t reflect reality.

cheers!