JMI era Vox AC30 layouts for AC30/4, AC30/6 and Integrated TB version

[Digsby]

Hey all,

I've been learning DIYLC for a few days and have put together layouts for three versions of the JMI AC30s. I know that this is a well-trodden path (I have tried to acknowledge previous drawings and versions) but I could not find a DIYLC version of the Integrated TB versions, for example, or the power section of the AC30/4 (which is the same as the AC30/6 but with different component numbers).

These are a FIRST ATTEMPT so likely to contain errors and omissions because my eyesight (and brain, frankly) isn't what it used to be. Some variations (notably the Cathode resistor and the treble/bass versions) are currently not shown and the routing of some of the wiring is simplified to make the connections easier to read (much of it is under the turret boards). Colour of wires also varies between chassis.

The two AC30/6 layouts are drawn partly from several chassis that I have owned but the AC30/4 is not. Heaters are not earthed on the AC30/4 layout - I believe they were probably grounded at the Pre-Amp ground next to V1 but haven't been able to confirm this.

Please let me know if you find errors so that I can update and post revisions and DO NOT assume this is correct (at least, yet).

[Willabe]

You should also make a schematic to be able to double check your layout for yourself and also for when you build the amp.

You really need both the schematic and the layout drawing.

And use this with the schematic and layout for double checking each other and the build;

https://el34world.com/Forum/index.php?topic=17701.msg178630#msg178630

Your drawings do look good, but still need a schmo to verify. (And then time, which I'm sorry I don't have.)

[Willabe]

Most guys find the 'cut' control across the power tube grid wires pretty much useless on Vox AC 15/30 amps or any other amp for that matter.

If you really think you need a MV use a different type. 

If you set up the power tubes in 2 pairs, with a KR and K bypass cap each pair, then you could pull 2 of the power tubs for 1/2 power.

[Willabe]

And now is the time to figure out your grounding before you build it.

I would not use a cap can for the B+ filter caps. Many here/do with good results, but it makes grounding harder. It's explained in the link from Merlin.

https://www.valvewizard.co.uk/Grounding.pdf

[Willabe]

More on grounding if you need it;

https://www.aikenamps.com/index.php/grounding

And all kinds of info in here;

https://robrobinette.com/Amp_Stuff.htm

[Digsby]

Thanks for the (good) advice.

I should have said that I have schematics for all the variants above (and more) and am using those and that I'm not planning to build these atm. This is mainly for documentation and to aid service and restoration of damaged amps. I'll make these available as downloads from my website if and when I think they're accurate enough just as a service to other guys working on these amps.

I usually make some mods to the ground scheme (not shown in the layouts) and, if I do build one or more of these will certainly improve that(notably the heaters). I like Vox's 'top cut' control btw :-) - it is a late-in-circuit tone control, not a MV.

I already found, and noted, a couple of errors but will wait for any other updates before replacing these with corrected versions

- Grid resistors are on the wrong pins of the EL84 (doh)
- There's a missing link from C19-R42

[Willabe]

I like Vox's 'top cut' control btw :-) - it is a late-in-circuit tone control, not a MV.

My mistake.

[Digsby]

No worries - your advice is very welcome and the two links on grounding are really clear and useful, thanks.

It's not top of my priorities atm but I probably will build an AC30/4 eventually as I have the transformers and most of the parts. When I do, I will definitely incorporate some improvements on the historic layout such as:

- Optimise the grounding scheme and maybe move the pre-amp ground to a better location;
- Ground the heaters with a virtual CT on the EL84 cathodes;
- Better control on the Vib/Trem circuit with pots rather than switches.

[Digsby]

Updated with various bits of tidying up, notes, wire colours, labels, details etc. plus:

- EL84s now have grid resistors on the correct pins (!)
- Choke is correctly labelled
- Missing link on power supply board (C19-R42) added