Vox AC15 (TBX) general Q's

[pdf64]

It should be feasible to identify if gain is being lost somewhere along the signal path without a scope, as long as the meter is reasonable. Need some sort of signal generator though, so it would very beneficial to get some tone generator software / app on a device with headphone etc output (eg pc / smartphone) which you can rig up a cable to plug into the amp's input jack.
Looking generate a sinewave in the range 100Hz to 1kHz (lower is better for regular meters, which have a limited bandwidth) of 100mV to 500mVrms.

[g1]

Hi g1 no this is not the C1.. its the "TB" basically you know those decent korg ac30 "TB" reissues circa mid 90's? this is the ac15 version.

Then this layout should be correct:

[Sea Chief]

Yes g1 that's like mine.. except.. mine has a 5Y3 rectifier.

[pdf64]

Yes, the 2 that I’ve worked on had 5Y3.
The chassis holes need opening out a couple mm in order to accept many modern tubes.

[Sea Chief]

I think that gz34 is a typo.

Thankfully my two el84 are old nos GE el84 (I think nos, they have "...phonic" written on a slant across them/ usa) so slim, & a jan philips usa made 5y3.

So do you recall trying the input volume @ max on them?

[g1]

The latter part of this vid is what I would expect with preamp volume full up.

[Sea Chief]

Hi g1.. yes I saw this clip prior to buying it.

With my preamp vol up full, the -1st- part of the vid is what I get. It tallies spot-on me with you, that the ammount of OD I'd expect would be like the latter part too. Hence my thread.

I don't suppose you can ID the latter to be preamp or 'proper amp OD' from this clip-? It does sound a bit 'fizzy/ so-so' suggestive of preamp to me, perhaps. Turning the MV up to ~3/4 to get a similar OD 'saturation', Id expect alot richer tone & sustain.. but it is a poor recording. And I can't do that here. Shame I can't see the dials.

[pdf64]

...So do you recall trying the input volume @ max on them?

I will have done, if only to check all was ok. If I had encountered to low gain issue you describe, I would have investigated.
Some amps are like that, the DrZ EZG50 comes to mind as having insufficient preamp gain to get any useful degree of overdrive. But the AC15TB should be able to get a reasonable degree of crunch.

[Sea Chief]

pdf one thing I do notice, is the input placcy jacks 1 & 2.. sound the same level: no typical attenuation drop between them: is this typical for vox amps? I recognise similar 68k/68k and 1M resistors I'm used to seeing on fenders around here on the schematic, so should I expect a similar vol drop between them?

My thinking is if 1 isn't as loud as it should be, I might be robbed of a bit of OD as a result.

[pdf64]

Yes, Input 1 should be 1M input resistance, input 2 134k and 6dB quieter. What input resistance do they measure? ie plug a cable in and measure the resistance between tip and screen.

[Sea Chief]

So measure @ the guitar end of the lead, unplugging gtr?

[Sea Chief]

Yes, Input 1 should be 1M input resistance, input 2 134k and 6dB quieter. What input resistance do they measure? ie plug a cable in and measure the resistance between tip and screen.

Actually 1st is 990k - 1050M, 2nd input 1M120 or so. Odd.

[pdf64]

So the shorting contact of the 1 jack is faulty. That will result in the inputs picking up hum when nothing is plugged in, and will prevent the 6dB attenuation of the 2 input.
So both inputs are equally loud, rather than equally quiet.

[Sea Chief]

Aha understood- damn I thought I might have solved the OD 'problem'!