DIY AC30 build

[la.minute.off]

Hi guys, I managed to build myself an AC30 clone.
The amp is running but i have a permanent problem...
General informations : amp is based on initial vox schematics and i just kept normal and brilliant channels (no trem anymore)
So i have 1 GZ34, 3 ECC83 and 4 EL84 in that amp.
The only variation to the schem are :
=> higher cathode R to run it a bit cooler (68 vs 50 R)
=> bright cap mod
=> master volume added
=> 2 resistances added for heater balance

Now the problem => I have a 150 Hz buzz... (please notice i live in europe so mains is 50 Hz not 60)
A lot of people complain about 50 or 100 Hz buzz but mine is 3rd harmonic...
I tried a lot of things to get rid of it but nothing worked (different grouding schemes to avoid ground loop, using shielded cable at some critical points, changing all tubes, moving cables to find to eventuel radiations...)

Does anybody have an idea about what can cause a 150 Hz hum or 180 Hz is mains is 60 Hz ???

Last comment => my power tannny is vibrating and i got the feeling it is the same freq... => power tranny problem ??? if yes, how to check it ???

Thanks in advance

[soundguruman]

Well, you see, the schematics are generally incorrect.
Vox never provided schematics that showed the actual circuit of an AC30. The schematic was close, but left out many important details.
And the reissue is of course, nothing like the real one.
Yes, Vox schematics were a secret. I have drawn my own when I had the amp in front of me, of an AC 120.

Now if you were to draw a schematic of what you built, we might be able to see whats up with it.

Like for example, mismatched output tubes will cause a heck of a buzz. Put in two, then four, then go from there. Eliminate a bad one or a weak one.
it's a pretty touchy subject to balance an output with that many tubes.

[Roe]

vox themselves have published many old schematics on their webpage. the circuit is well know. I'd take a look at grounding and filtering

[la.minute.off]

Thanks for replies.
I actually have schematics of my amp (based on those you can find on vox website as said by Roe).
The file format is pdf, how can I post them so you can have a look at it ?
(besides this, if you know how to integrate pictures, i could post some...)

For the directions you mentionned in your posts
=> power tubes : I already tried to switch tubes in the different sockets and 150Hz still present - I also put a complete other set of power tubes with no change
=> grounding scheme : tried a lot of variations, brought some small improvements but did not change the world
=> filtering : already tried to increase values staying in what the GZ34 can handle, brought small improvements (similar to grounding)

I have the possibility to do some testing with an oscilloscope in order to find the source of it... I will do this this weekend
I will also try to unmount the main transformer and solder longer wires to it in order to get it farther away from the board, tubes... I am quite surprised by the vibrations caused by this tranny, i also have a jcm800 that doesn't vibrate at all, and its tranny is big !!! I wonder if it is not a problem with the main transformer ?!?

Just that you have a better understanding of the problem : the amp sounds very good, very responsive, so I don't think I made a major mistake building it... I normally play with master at 40%, and 150Hz buzz is present on both channels. Buzz volume increases if I increase the channel levels, and when I am plugged in brilliant channel, I can find a position on normal channel volume which decreases the buzz but it is still present (in fact, both channels seem to interact a bit).

@ Roe => I've seen pictures of some nice builds you did (Marshall and Vox), cool...

[la.minute.off]

Links on images and schematics

http://img849.imageshack.us/img849/7270/ac30vtz.jpg" onclick="window.open(this.href);return false;
http://img844.imageshack.us/img844/259/ac30vtz.png" onclick="window.open(this.href);return false;

[soundguruman]

Like I said Col. Vox kept a lot of secrets.
If you compare the published schematics to the actual amplifier, you will find a lot of differences.

[la.minute.off]

And the story goes on...
I made some improvements this week end.
I found that I had a grouding issue on all my input jacks => there was a steel part of the jacks that made a loop on chassis...I solved this issue and had clear improvement.
There is still hum, but less than before, and it becomes only loud when volume is turned up at a level where I need some ear plugs... But there is still...

Several comments : i only kept normal and brilliant channels, but both channels seem to interact. For a given position of brilliant volume, i can find a position on normal volume where the hum is reduced. Is this normal and linked to ac30 conception ??? The way i set up bass and treeble also affects hum.
More generally, is hum normal on ac30 ???

Thanks for answers

[soundguruman]

Hi guys, I managed to build myself an AC30 clone.
The amp is running but i have a permanent problem...
General informations : amp is based on initial vox schematics and i just kept normal and brilliant channels (no trem anymore)
So i have 1 GZ34, 3 ECC83 and 4 EL84 in that amp.
The only variation to the schem are :
=> higher cathode R to run it a bit cooler (68 vs 50 R)
=> bright cap mod
=> master volume added
=> 2 resistances added for heater balance

Now the problem => I have a 150 Hz buzz... (please notice i live in europe so mains is 50 Hz not 60)
A lot of people complain about 50 or 100 Hz buzz but mine is 3rd harmonic...
I tried a lot of things to get rid of it but nothing worked (different grouding schemes to avoid ground loop, using shielded cable at some critical points, changing all tubes, moving cables to find to eventuel radiations...)

Does anybody have an idea about what can cause a 150 Hz hum or 180 Hz is mains is 60 Hz ???

Last comment => my power tannny is vibrating and i got the feeling it is the same freq... => power tranny problem ??? if yes, how to check it ???

Thanks in advance

Please e mail me directly
soundguruman@yahoo.com
and I will be happy to have a look at it.

[ermac999]

Hi all,
My experience of loud hum on an AC30 is from my 60's original. I have though isolated the source area, it is in the two ECC83 EL83 's in my case that are the preamp section. The ones prior to the push pull splitting EL83, it came to light when I put a standby switch in, it turned off the heater 6V and the hum went silent. I haven't looked at it much in the past twenty some odd years, but intend sorting it soon. Also I assume you are using PLASTIC or electrically isolated input jacks as mine had when I got it, I messed about with others and prob introduced the hum there, but replacement plastic ones were very rare in the seventies, everyone trying to be the newest and the best ! LMAO We can get them now though, Ha HA. Oh and somewhere in my extensive clutter I have a circuit diagram that was supplied to me by VOX directly. Back then,If you asked the owner of a company very nicely by letter and phoned, help was offered. Maybe I was just very lucky, you just never know.