Non top boost AC-30 in for repair.

[Reeltarded]

lil 10ks on both PI grids to reject just that much air above the fire

[Mark]

Thanks for the advise, I did try using 1K grid stoppers on my 18 watt build years ago.

[Mark]

Checking out the amp before boxing it up.The amp was still noisier than I like due to the P.I. stage and ythe bleed through is still about as I stuck to the original 22uF shared Cathode cap.

472E75EC-4C0A-4A1A-A654-E51F33562BD6.jpeg

No one really had a clue about earthing issues or the bleed through worldwide. The AC-30 is a popular amp, but it isn’t the platform for modifications than Marshall and Fender are, and people are more accepting of what it is. The Shere Sound stuff looks better than the Vox amps which is kind of funny in a way.

94FA47EA-3475-4827-8E83-8EDFE58D3EC3.jpeg

These are a few observations on the journey with this amp. Nothing should be interrupted as being critical, just thinking aloud after it’s finished.

[pdf64]

Well done, kudos for your patience and doggedness! As enjoyable as it’s been, I’m sure you’re glad to finally button it up though
I guess that to do 0V arrangement properly, the vib/trem channel’s 0V wiring should be made separate to that of the normal/brights’.
Even if that helped though, the top boost version might need yet further separation, for that of the normal/bright channels.
So a master plan 0V system mod that worked for all might not be feasible.
The AC30’s overall layout, with the speaker output only a few inches from the inputs, makes crude attempts at hot rodding prone to failure. Plus there wasn’t much spare room in the preamp chassis for extra stuff. Even just the extra gain etc of the top boost models tends to result in them teetering on the verge of instability. AC50 tried to address that with that by the addition of a screening plate for the input jacks, but whilst that helped, the issue remained, especially for the versions with ECC83 (rather than ECC82) input stages.
On the other hand, I recall an article in a UK electronics mag in the early 80s, which dealt with modernising an AC30; the power amp was put into fixed bias, and extra preamp gain was achieved by putting a BC109 in the cathode return of an input stage, with the cathode bias resistor also serving as the collector load for the transistor. Quite a neat trick
I’ve actually spent some time searching for that article, on the internet and in the British Library archive, but in vain

[Reeltarded]

sorry for the side trip.. so, the ac50 and the 100... you ever known anyone DYING for either?

NOPE!!

what a kooky and unfortunate design on both of those with a typical outcome, then the ac30 is kooky and fantastic.kind of like the maseratti powered citroen of amps.. goofy af and SOUNDS great!

[pdf64]

Whoa there, I beg to differ
AC50 make an amazing bass amp, about the best bass tone I've ever heard. An absolutely massive OT, beefier stack than any 100 watter, probably helps with that, but I suspect that their open loop operation is also a significant factor.
After rebuilding the ones a friend has bought, I take them to a band session for a workout, through my 4x12. Whichever version, they all sounded fantastic.
He runs a local studio and uses them (he's rationalised down to a valve rectified MkII now) as the house bass rig.
See the 1st exhaustive set of photos on this page https://www.voxac50.org.uk/ac50_large_03.htm

[pompeiisneaks]

sorry for the side trip.. so, the ac50 and the 100... you ever known anyone DYING for either?

NOPE!!

what a kooky and unfortunate design on both of those with a typical outcome, then the ac30 is kooky and fantastic.kind of like the maseratti powered citroen of amps.. goofy af and SOUNDS great!

I diy built an AC100 myself, loved the tone, but didn't realize my PT was dying. I kept having inconsistent results from it. eventually gutted it to build sluckey's dual marshall thinking the AC100 circuit was to blame and then started having issues yet again. Upon replacing the PT the amps been golden ever since.

The beatles used the 50 and 100 for a long time.

[Reeltarded]

Whoa there, I beg to differ
AC50 make an amazing bass amp, about the best bass tone I've ever heard. An absolutely massive OT, beefier stack than any 100 watter, probably helps with that, but I suspect that their open loop operation is also a significant factor.

They arent ac30 replacements and they catch fire almost as bad as the 100s!

My point is the 50 was supposed to supercede the 30 and it just did not.

[Reeltarded]

sorry for the side trip.. so, the ac50 and the 100... you ever known anyone DYING for either?

NOPE!!

what a kooky and unfortunate design on both of those with a typical outcome, then the ac30 is kooky and fantastic.kind of like the maseratti powered citroen of amps.. goofy af and SOUNDS great!

The beatles used the 50 and 100 for a long time.

Unsure of that... could not have been too long because they were playing Fenders almost immediately after playing super beatles?

They did not make many 100s and working amps are rare because volunteer fire departments. heh

[Mark]

Thanks for all the advice, I do appreciate you taking time to help me working on this amp. I did look under every rock trying to work out a fix for this amp.

I’m a little surprised that over the years the earthing of these new and vintage amps hasn’t been improved.
The things that stuck me as strange are the centre tap of the secondary and the main filter cap not being earthed in the same spot. The power supply earth for the trem/vib stage is weird too as this is earthed with the main filter cap or on an anodised clip holding the cap in place. The P.I. and preamp filters being 8uF is weird too as the trem/vib stage gets a 33uF filter cap. I suspect 16uF caps could easily replace the 8uF caps with no sonic difference, as I used a 16uF cap and the tone was the same with the amp being played clean.
The preamp earth buss bar is strange as there are two serial lines which are in series. You would think the Brilliant/normal channel would have its own node as would the trem/vib stage, but it doesn’t. There are parts of the trem/vib stage on both earth buss which are earthed on a valve socket screw rather than returning to the filter cap earth.

I did find that the brilliant and normal channel cathode earths have to be in the same spot roughly as hum will result. I found this out by splitting the cathodes and earthing one to negative end I’d the filter cap and the other remained where it was.

The noise in the non top boost AC-30 I worked on was in the P.I. stage. I had encountered this before twice when building a Rocket clone. With Jelle Welagen’s advice, the issue was a conductive circuit card which was fixed with a G10 board, the amp was very quiet after this issue had been rectified. I did try mounting the components directly onto the socket to eliminate the circuit card as a possibility, but the noise continued unless I earthed the input to the P.I. stage.

This is pretty much my condensed finding. Again, thanks for all your help along the way.

[pdf64]

...AC50...

They arent ac30 replacements and they catch fire almost as bad as the 100s!
...

Yikes, I've not come across that, but I've only dealt with large box MkIIs and IIIs; which versions have the fire issue, and what exactly catches fire, what's the root cause?

[Reeltarded]

now you make me look through years of pics i have a funny story!

I think the pics are on a phone I can't charge for now. When I sold my amp pile in.. 2013(?) along with them went a MkII era 50 and a similar 100. Both had evidence of previous fire, the 50 was two made into one. I think it starts in the bias.. when I was like 17 a guy was playing a 50 that melted idling.

There may be pics here because I know I posted the amp room pics.. umm let's look.