Mid 70's VOX AC50 pre amp question

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Mid 70's VOX AC50 pre amp question

Postby bobster on Sat Sep 09, 2017 2:08 pm

Hi there , sorry to trouble you all with a question about my amp.
I was reading about the dallas arbiter made VSL mid 70s hand wired AC50's on this site.http://www.voxac50.org.uk/ac50_vsl.htm
I bought one recently and it is very well made and in good condition.

My question is , can i put a 12AU7 /Ecc82 in the V1 position to get a feel for the lower gain 60s amp sounds.?

The reason i ask is that it appears that the VSL had all 4 pre amps as 12ax7's stock and i am wondering if the bias would be out if i put in an Ecc82 ?
It may be that i would have to change the value of a few resistors to compensate?
Any advice would be much appreciated !
[ the schematic is on the above link btw ]
many thanks
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Re: Mid 70's VOX AC50 pre amp question

Postby Vortexion on Sat Sep 09, 2017 8:23 pm

Well the good news is that you'll do no harm to anything at all if you simply swap out the ECC83 for an ECC82 and see if you like the resulting sound/behaviour you get.

I've compared the schematics for the mid-1960s AC50 and the 1976 VSL version and, as far as I can see, if you wanted to go the whole hog and change component values too, the only ones you'd have to change are the cathode resistors. The one coming off pin 3 should be changed from 2.7k to 4.7k and the one off pin 8 changed from 2.7k to 1.5k All the other components (anode load resistors, cathode bypass caps, coupling caps etc.) appear to be the same values or so close as not to matter.

In other words, suck it and see. :wink:
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Re: Mid 70's VOX AC50 pre amp question

Postby bobster on Sun Sep 10, 2017 1:54 am

thanks Vort , thats a good plan.
I figured there would be no major issues firing one in but its good to know the the accepted values for the '82 resistors should i wish to make a permanent change ,
cheers :urawk:
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Re: Mid 70's VOX AC50 pre amp question

Postby Vortexion on Sun Sep 10, 2017 8:28 am

A naughty thought for you (and maybe not one you'd be keen to go with because it means making an irreversible mod to a vintage amp and thus affect resale value), but here goes:

If you found that you liked both the existing ECC83 setup and the correctly-biased ECC82 and couldn't decide which to settle with, you could fit a DPDT switch that would toggle between the different cathode resistor values. Pretty simple to wire up, but would involve drilling a hole somewhere in the chassis.

Also, if you did fit such a switch, you'd have to remember to turn both your input volume controls down before throwing it, 'cos otherwise it would make one hell of a pop! :mrgreen:
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Re: Mid 70's VOX AC50 pre amp question

Postby bobster on Wed Sep 13, 2017 9:25 pm

oft ! cool idea but i think i'll leave the old girl unmolested.
i popped in a mazda ecc82 today and gave it a whirl and even with the slightly off biasing resistors it sounds very nice and pedals can still get me the required dirt when needed.
will try it at gig this weekend with full pedal board as cant do much in the house with neighbours small kids etc
:cheers:
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Re: Mid 70's VOX AC50 pre amp question

Postby pdf64 on Sun Sep 17, 2017 8:36 am

Bear in mind that 1/2 watt plate resistors may overdissipate if the bias is inadequate (cathode resistor value too small).
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Re: Mid 70's VOX AC50 pre amp question

Postby bobster on Sun Oct 08, 2017 10:04 pm

Thanks Peter , i didn't realise that could be an issue.
I ended up putting the 12ax7 back in as there didn't seem to be much difference in sound other than a reduction in spl....
:cheers:
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Re: Mid 70's VOX AC50 pre amp question

Postby gusfinley on Tue Oct 10, 2017 5:34 am

Another option is to use the 12DW7 tube. It's half a 12AX7, Half a 12AU7.

This way, one channel has the 1/5 less gain, while the other is stock. (You can buy JJ 12DW7's that put the 12Au7 section in the position you want it in).

This especially works well where the 12AX7 preamp sections of the tube are cascaded where the theoretical amplification factor would normally be 10,000 (100 x 100 ) but would only be 400 (20 x 20 ) with a 12Au7 in that position. The 12DW7 gives you a theoretical amplification factor of 2,000 (20 x 100 ).

This was a lifesaver for my Marshall DSL401 in the V2 position. This kept the clean channel nice and chimey, but dropped the distortion channel by 1/5 - more into classic rock territory than modern rock. I could actually find that point on the gain knob where it became fizzy, instead of always being fizzy.

12DW7. Now you know.... and knowing is half of the battle.
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Re: Mid 70's VOX AC50 pre amp question

Postby bobster on Fri Oct 13, 2017 6:33 pm

cool ! thanks Gus
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