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Old 19th May 2009, 10:23 PM   #1
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Default Some help/advice with a guitar amp layout.

Hi, i am currently working towards building my first guitar amp (my first attempt at anything electronic in fact) and i was hoping that some of you could help me with my layout. I've had a lot of help from one of your members (leadbelly) who has kindly answered my numerous rookie questions about tube amps and helped me get to where i am at the moment.

Here is the schematic that i'm working from, adapted from a Vox AC15 with a few modifications:

Click the image to open in full size.

And here is my attempt at a wiring layout:

Click the image to open in full size.

Would anyone mind giving my layout a look over and see if i have made any mistakes?

This is only my first attempt so please feel free to tell me if my routing is total garbage.

Thanks!
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Old 20th May 2009, 08:37 AM   #2
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The AC15 is actually a moderately tough project - not least because it relies on the output transformer of the the day (expensive remakes are available though).

It also relies on the Mullard EF86 (modern copies are rarely good enough - too microphonic).

The EZ81 rectifier is a table radio model with inadequate current delivery really - shows up as weak bass.


But if you have the right transformers, and you really want the VOX sound, stock up on some Mullards!

As to your layout, beware the separation of the output trafo from the input valve EF86. Magnetic coupling could give feedback screams and poor sound. Try to keep the EF86 away from any magnetic material, and minimize the distance to the input socket. Mount it on bolts decoupled from the chassis by rubber grommets, to minimize vibration.

If it were me, I'd move the EF86/ECC83 near to the controls, and spread out the EL84s - These get very HOT, and you want this heat to be vented as efficiently as possible.

Modern preamp valves hum more than old Mullards, so either increase the EF86 cathode cap to 220uF/16V and apply 90V to the heaters (from a resistor divider - search the archives here for elevated heater voltage or some such)

OR rectify the heater voltage and apply 6.1Vdc to the preamp valves.

If you are not commited to a VOX remake, just get the Maplin output transfomer N91CC. Use UF4007 rectifiers in a bridge to generate about 320V. Samwha 68uF/400V capacitors from Rapid Electronics. Connect it all up like a Marshall 18W or any of the other circuits that use ECC83 to drive EL84s (in PENTODE mode).

These sound really splendid, huge fun, really versatile - and very cheap!

Have fun! (I'm out till Friday if you have questions on what I said here)
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Old 20th May 2009, 08:45 AM   #3
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By the way, if you really want AC15 sound you must run pentode mode for the EL84s - connect the 100 Ohm Rs from pin 9 to the HT supply, rather than the anodes. You're running triode mode at the moment, which gives less power, and NOT the Vox sound!
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Old 20th May 2009, 11:01 AM   #4
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Hi, thanks very much for your reply!

I should have said in my original post that my schematic is adapted from the 'new' Vox AC15 handwired reissue, so there were a few deviations from the original designs before i started my own modifications. I have played these amps a few times and totally fell in love with the EF86 channel, that's what got me started on this project.

I can understand what you are saying about the bass response with the EZ81, the new Vox amps are certainly not overly strong in the bass department, but i was happy overall with their tone. Is there any issue with reliability with these valves or is it purely a tone issue?

I have my transformers ordered already, they are not clones of the Vox designs (i did contemplate some Mercury Magnetics transformers but the price of importing was too high to justify) and i'm not worried about nailing the Vox tone exactly, i just want to get in that general ballpark and modify and experiment with the sound once the amp is up and running. I expect this will take me a long time since i am a total novice, but that's half the fun of it!

I forgot to mention about the triode configuration, i will mostly be using the amp for some home recording and possibly a few small gigs, so Triode will be useful for getting some mild overdrive at lower levels. The transformer i have ordered is going to have UL taps, so i intend to add a switch for Triode/UL/Pentode operation. I'm afraid i just haven't got around to updating my drawings!

Thanks for the advice on valve placement, i was aware that the EF86 valves have a tendancy go microphonic from mechanical vibration but i didn't realise the proximity to the OT would have a negative effect. To be honest, i think i'll probably stick to new production JJ valves as the Mullards seem to be getting more and more expensive. I've set aside enough money to buy 4-5 EF86's so hopefully i'll get at least one that has good characteristics. I will keep an eye out for some NOS in future though and maybe i'll get lucky. I'm not sure what EF86's Vox are using for the reissue, i think they are Sovtek or EHX (both made by New Sensor?).

Regarding the EL84's, i wasn't aware that they got extremely hot, i'm intending to leave the amp as an open design so hopefully that will help with the heat. I'll definately space them out anyway if i move the EF86 to the front of the chassis though.

Thanks again for your help, i'll get to work on a new layout and post it up when i'm done. If you or anyone else notice any mistakes in my wiring in the mean time please let me know!

Cheers!
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Old 20th May 2009, 11:08 AM   #5
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Something else to watch:

The grid (g1) resistors on EF 86 and EL84 (often needed on ECC83 too) are 'stoppers' - they stop oscillation at high frequency developing.

To work properly, they should be mounted right on the valve socket (maybe 3-5mm of lead).

In general, try to keep the wiring of grids (and cathodes) very short, or noise pickup and instability will result. Mounting cathode components on the socket is a better plan too.
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Old 20th May 2009, 11:27 AM   #6
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Quote:
Originally posted by Rod Coleman
Something else to watch:

The grid (g1) resistors on EF 86 and EL84 (often needed on ECC83 too) are 'stoppers' - they stop oscillation at high frequency developing.

To work properly, they should be mounted right on the valve socket (maybe 3-5mm of lead).

In general, try to keep the wiring of grids (and cathodes) very short, or noise pickup and instability will result. Mounting cathode components on the socket is a better plan too.
Aha, thanks, i wasn't sure about this. I basically just layed out as many components on the turret board as i could, because it helped simplify things in my own head for my first attempt. I'll move these to the sockets in my next layout.
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