Marcel's scratch build AC15 based clone

[dave.king1]

It doesn't appear to be an issue here but thought I'd drop this for future reference.

Back in the day I had a Lennard GB150 and later on a Dan Armstrong 100W, both had discrete components mounted on poorly mounted rather long tag strips, the flexing in the strips caused by the vibration of transport, bumping in and out and very loud playing caused the tails on the components to break regularly ( mainly the larger caps )

Wonderful sounding amps but once the problems started they became too unreliable.

Just something to keep in mind when mounting long boards

[Marcel]

I know what you are suggesting Dave however I have my doubts it will ever be a problem here.

My long boards are at best about 12cm and held at each end so the there is the potential for flex in the centre. But at 15W and with the tubes so exposed on the top and unless it is a dire emergency I doubt this amp will ever hit the road or see a gig. It will most probably see out its days in someone's bedroom or if it's lucky get to live the life of Riley hooked up to a 1x12 in the air conditioned comfort of a small recording studio.

But one never knows. I'll check it out and might put a third stay under each of the long boards.

[dave.king1]

I couldn't see a problem with it either Marcel, just putting it out there for when/if folks go bigger

[Marcel]

Layout - layout - layout....

A huge headache has developed. My brain is hurting...

I wired the input jacks and started installing components around V1...all good.. tight...but all good.

Hmm... The HT on V1 has gotta come from somewhere!... Hmmm... Vox in their wisdom use separate HT drop down resistors and HT filter caps for each stage whereas Marshall cascade the HT on each stage with the highest on V3 and dropping down in voltage to V2 and with the lowest HT voltage on V1. So in a Marshal (and most other brands) it is a chain but in a Vox (and Hohner) it is a star arrangement.

I've got all the needed caps... no problem....
Where to put the caps and various dropping resistors so the are reasonably close to their respective tubes and I don't have dozens of wires carrying top end HT all over the place.....In the tiny little space that is my amp ...Big headachey problem.!!!...

[Simon Barden]

Can't you modify the circuit so that it does cascade?

[DrNomis_44]

It should be relatively easy to mod the HT power supply so that it cascades like in a Marshall amp, doing that may also come with some benefits too, for example getting rid of hum noise from the HT power supply in the first gain-stage (which is probably why Marshall chose to do it that way), incidentally, Fender have the HT power supply cascading too.

[Marcel]

It would be all too easy to change to a cascade HT supply. And in some aspects a cascading HT is a better way to do things as any failure (burn out) of any of the supply components will effectively kill the V1 tube thus stopping the amp from trying to do anything....

BUT ...

Part of the charm of a VOX amp is how it breaks up with increasing volume/gain.... and I think the HT arrangement is integral to that break up behaviour... So therefore in my build the only significant changes to the HT config will be the use of a Marshall 18W power transformer (which isn't very significant as the HT voltage and currents are very close), and solid state rectifiers instead of a tube rectifier.

[Marcel]

Ever had one of those days? The sorta days where one of the kids calls for help at 3:30am? ... "..and Dad, can you bring the big trailer ?"...

So, this evening it is a trip down memory lane YouTube entertainment style from a time before computers, VCR's and iPods.... from when and how I was first introduced to the magic we work with today...

https://www.youtube.com/watch?v=ryCr4Y7ypLA

[dave.king1]

Reminds me of Electronics 3 in first year at college, damn that was a long time ago
( a certain long haired band from Liverpool toured Australia that year )

[Marcel]

One of the smaller issues I have is as V4a will be the tremolo oscillator and as this build will not have a reverb circuit thus at the moment V4b has nil purpose or job to do. Normally in a VOX AC series amp V4b would be the reverb recovery amp and a V5 tube the reverb drive amp. So what to do with the spare V4b?

Anyone have any useful suggestions?

As a side line, in my searching I have uncovered a number of useful and informative web pages.
https://robrobinette.com/How_the_Mar...M800_Works.htm is a bit technical however gives excellent explanations of why thing are as they are in Marshall designs and helps with understanding other tube amp circuit designs, and to some substantial extent about tube amps in general.