Improved AC-30 layout in stock chassis?

[Mark]

I was wondering if anyone knows if there is an improved layout for the AC-30’s using the vintage chassis.

I worked on one a year ago and thought the earthing was pretty ordinary in that it snakes it’s way around various circuits rather than having Star earth points. The amp was noisier than it needed to be. I did try to reduce the noise, but it was too great a task.

When I look at the post top boost layout it’s the previous layout with the top boost valve added at the end. I personally thought this was a bit odd.

I look forward to your replies and opinions.

[Colossal]

Hi Mark,

I think a more optimized layout would depend on the number of circuits included, channels, reverb, trem, etc. But yes, there is a lot of room for improvement and certainly a better ground scheme.

[Mark]

Thanks for your reply.

Improving the stock amp, is my intention. There are moves such as using a pull pot to improve the trem/vib layout where two switches were used. The extra pot location can be used for a LarMar PPIMV.

[pdf64]

The layout for the Weber kit looks a bit more logical https://www.tedweber.com/6v30-c-kt/

So that might help with devising a better 0V common arrangement.
I use pull push pots for both vib/trem effect control panel switch positions. Speed becomes a variable control, with its pull switch turning the oscillator on (saves pulling the footswitch out).
The vib / trem switch becomes a depth pot, with its switch selecting vib or trem.
I’ve got a long delayed back burner project of an AC30TB, but re-arranged as 2 channel, with both channels mixed and routed via the TB and vib/trem circuits. It seems a logical development that never happened (no point in changing a successful design I suppose).
I’m prepared for it to be a hissy / hummy / squealing disaster, hence I’ve been thinking about the layout too

[Mark]

There are some nice touches in this layout. I like the revised valve layout. The earth layout for the preamp side is a little sketchy.

I’m not sure that I like the cab cap. The 8uF caps in power supply is a bit questionable to me. I have had 16uF in that position and I can’t say I heard a difference when the amp was played clean.

There is even an option to have a AC-15 style power supply.

[pdf64]

... The earth layout for the preamp side is a little sketchy..

Yes, I don’t think it’s an example of best practice. I get the impression that there’s a Fender style brass or copper plate behind the control panel that may be seen as a low impedance common star point for all the preamp.

[Mark]

The other thing I wish to do to my build is to elevate the heaters to all audio valves.

I encountered an issue of crosstalk. I found it was due to the first shared cathode having a 25uF cathode bypass cap. In raising it to 100uF the crosstalk disappeared.

[srx430]

Hey there!
I'm a bit late to join the discussion, but someone might stumble over this at later times.

Anyway. I have worked on a couple of vintage AC30s. There's two things that one might overlook:
a) Tighten all the chassis screws. You might wanna losen them and "rub" them on the chassis to improve contact. The VOX chassis are made of two pieces of aluminium, hold together by zinc plated screws. There's lots of oxidation going on. Taking care of the contact between the chassis parts and bolts can sometimes quieten the amp down significantly.
b) the power switch is badly placed. There's bleedover from the AC voltage into the "Cut" pot. So, there's hum introduced right into the output stage and there's no way for you to get rid of it. I added a shield once which helped a little bit, but wasn't able to cure it.

Hope that helps

[220or240]

Check u tube videos from Psionic audio. He has several changes for eliminating ground loops and hum. One is in the power amp/HT power section. Move the ground for the cap can to the same ground point as the power transformer. Need about a 6" wire. Makes a difference.

[Roe]

you can rebuild the grounding scheme. but I prefer a marshall style chassis, which has the following benefits:
- easier access for service
- less weight and cost
- easier layout.

I have build ac30s both ways, but prefer a mid sized alu chassis, not the steel and alu combo