AC15TBX - Cage and Reverb

[JammyDodger]

Hey All,

I just picked up a sweet 90's era VOXAC15TBX. I have a couple of questions maybe the group can help with.

It has a 'cage' that fits over all of the tubes. The bodies of the tubes are right against the sides of the cage. Is this thing really necessary? Other than the fact that it may shield the preamp tubes a bit, it really seems overkill and makes it impossible to change the tubes without removing the entire chassis.

Secondly, the reverb barely works. The seller warned me of this. I know at best the Vox reverb was not good. This is not the issue, it's just hardly there, even on '10'.
I've tested the RCA cables and they are good. If I bang the reverb pan it makes noise so I believe that the recovery circuit is good. That would leave only the input stage and transformer but it dosen't seem like much can go wrong there. Any thoughts?

[dorrisant]

Measure the resistance at the reverb tank input and report.

[Reeltarded]

Turn the reverb about 50% and touch the return.

[martin manning]

It has a 'cage' that fits over all of the tubes. The bodies of the tubes are right against the sides of the cage. Is this thing really necessary? Other than the fact that it may shield the preamp tubes a bit, it really seems overkill and makes it impossible to change the tubes without removing the entire chassis.

I think you are ok running without that, a least for testing and trial purposes. It may or may not provide any useful shielding; one way to find out...

Secondly, the reverb barely works. ... Any thoughts?

Reverb diagnostic procedure recently posted here by a highly respected professional: https://ampgarage.com/forum/viewtopic.p ... 74#p453974

[JammyDodger]

The reverb measures 75 ohms (DC resistance) at input.

If I turn the reverb all the way up I just hear the slightest amount, at halfway up, nothing. If I shake the tank, I hear the reverb string 'thunder'

Cheers, Mike

[JammyDodger]

Very nice trouble shooting tips..... At this point I think that it must be the the reverb transformer.


Plug RCA cable into return jack, turn up volume, touch tip. Do you have buzz ? If the return works, plug in the output of the pan. Shake pan. If you get clank, output of pan is good. Switch pan wires, shake pan. If you get clank (reduced volume), then input of pan works. (you are listening to the input end of the pan) Plug test speaker into reverb output (pan input connection), put signal into amp and see if reverb output has a signal. If it does, run a jumper from send to return. If raising reverb volume increases amp level a lot, then reverb circuits work properly. Likely bad pan. If you have no send signal, it could be a bad driver tube (12AT7) or cable or reverb transformer. If you have no return signal, could be bad tube, cable or something on board. Not much else to check beyond that...

[Stevem]

I think you have the wrong reverb pan in that.
For a amp that is driving the input side of the pan by the output of a driver transformer 75 ohms is way too high of a resistance reading.

A bet if you get a replacement pan for a amp like a Fender super reverb , twin reverb or bandmaster reverb you will have the issue solved.

Does anyone you know have one of these amps?

You can place them back to back and it out in less then 3 minutes!

[JammyDodger]

I have tried it with another reverb pan, it was a BS3EB3C1B, Same result unfortunately.

[JammyDodger]

Long delay in responding I know......

It looks like the reverb transformer is bad. It is Vox # 30A-25. Does anyone know if there is a cross reference to this?

I can't seem to find anything on line.

Cheers, Mike

[pompeiisneaks]

This is possibly the right one: https://www.partsisparts.net/catalog/vo ... tbx-ac15tb BUT it's discontinued, you may contact the site and see if they are just 'gone' or what. It looks like a dagnall d2477 from the photo? Maybe you can contact them about the specs to get one or the like?

https://www.dagnall.co.uk/transformers/pcb.html


~Phil

[JammyDodger]

Thanks so much for the replies.

I did get a response back from Mercury Magnets. They have a equivalent part, it's just not on their web site.
It was originally made by Drake.

Cheers, Mike