vox ac 30 tb6 standby not working, light good, new switch. Idears?.

[english dave]

The standby side of amp not working, was intermittent, light works, I put in new standby switch, still no sound, amp was woking great before it quit, help needed. English dave.

[Jazz P Bass]

Vox AC30 top boost

I do not know exactly which AC30 top boost amp you have.
If you have a rectifier tube diode, it may be bad.
Or it's fuse.
You really need to do some diagnostics.
Here is link to the schematics.
Link: VOX | Vintage Circuit Diagrams

[english dave]

Thanks for your input jazz p bass, I think its the rectifier tube, I have a 1997 tb6 ac30, and have been using the standby switch, I understand that,s a bad thing, I did notice the rectifier didn,t light up, every thing else looks good, all the other tubes are lighted, all fuses are good, the other thing is, Ive only had the amp a few weeks and its been working just fine, from now on I will leave the standby on . Thanks again, if its not the rectifier Ill be needing more help. english dave.

[Robert M. Martinelli]

I have a 1997 tb6 ac30, and have been using the standby switch, I understand that,s a bad thing.....from now on I will leave the standby on

Hi Dave,
the amp is 13 years old, I think you should heed JPbass' advice.... it could just be that you simply have either a blown fuse (F1-3A) or that the rectifier valve's heater needs some rest,and it has decided to "go south" for a long vacation

Using the standby switch is not to be considered "bad practice" IMHO, the cathodes do have a certain amount of thermal inertia, so, leaving the standby switch on at all times results in a higher "surge" HT voltage when powering on the amp (because the cathodes are still cold, so there's no current drawn from the HT, and the "no-load" voltage is obviously higher) and using the standby switch could/should help to prolong the valves' life expectancy.

As a side note, remember that in an AC30, the four power valves (EL84/6BQ5) are cathode biased at an "insane" level (some 125% of the max plate Pd) so their life expectancy cannot be very long (especially with modern valves). If the amp has never been overhauled, maybe it's time to throw in a set of new valves.

Hope this helps

Best regards

Bob

[english dave]

Thanks bob, Ive checked all fuses (5). they were good Ive got a new rectifier tube on the way plus Ive installed 4 new EL84 tubes I,ll let everyone know if that fixes the problem. One side note, the amp looks like brand new. English dave

[pdf64]

I seem to remember reading that some (Rose Morris?) AC30s were built with the GZ34 wired incorrectly, the B+ being taken from pin 2 rather than pin 8, thereby routing B+ current via the rectifier heater. The reservoir cap charge up current when switching the standby switch to 'play mode' could blow the rectifier heater open. But as english dave's AC30 was built by Marshall (?), that won't be the problem here. Pete.

[english dave]

Changing the rectifier tube seems to have fixed the problem, english dave.