AC10 SRT BUILD SCHEMATIC ACCURACY

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AC10 SRT BUILD SCHEMATIC ACCURACY

Postby Zapp on Sat Jul 30, 2016 4:21 am

I would like to build a striped down version of this amp minus reverb and vibrato using el34 pp power section but not sure if this schematic is correct . Has anyone compared this schematic to an actual amp ?Going by the schematic V2b has the plate resistor after the coupling cap(something i have not seen before) wouldn't that put 100s of VDC on the grid of the gain stage before the phase inverter?Any help appreciated and thanks in advance!

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Re: AC10 SRT BUILD SCHEMATIC ACCURACY

Postby Dr Death on Fri Mar 17, 2017 5:57 pm

I thought I would add-on to this post even though we are now some 15-months on from the member’s question having verified the schematic OS/71 as accurate. This was due to having acquired one of these head versions with the copper control panel.
It took a determined look through the pages of the brilliant book “JMI – The VOX Years” to establish this was indeed a rare amp and one I have had the good fortune to come across, which is now singing and dancing like never before.
What threw me was the lack of info including a missing S/N plate. I finally found in the JMI book however, a page explain Tom Jennings need to produce a Fender styled amp having the sloping front control panel, and there, sure enough was the one I had, albeit the head version. Knowing the model and most importantly the SRT tag to it, I was able to glean the necessary information to bring mine back to some semblance of its former glory. As many VOX lovers know the circuitry is not that easy to follow inside the amp and I decided to track-through the whole thing, culminating in a positional layout with no values attached. I then compared my layout with the OS/71 schematic in terms of connectivity, component values and reference (R1, C1, etc.), which proved to me this circuit was literally identical to the one used in the pyramid shaped SRT head.
In a nutshell I have done the following to the amp, which is now ready to sell-on.
1. The cabinet had no front panel, so I made one up with the small VOX badge and brown fret cloth.
2. The rear panel (not knowing what an original looks like) now has a jack socket for the [fabulous] Vibrato, a repro Serial number badge and a speaker plate housing both Speakon and jack socket outlets for 8 and 15 ohm impedances.
3. There was no reverb fitted, but the shielded wires were there. I toyed with the idea of making up a reverb tray using vintage pick-ups I have that I have successfully used in the VOX Solid-State amps such as a Supreme head. My final decision was to incorporate a Reverb Driver transformer and Type 4 reverb tray. This only required minor modification to the circuit, in-so-far as removing two components, and like the Vibrato now sings out loud when turned up on the Reverb control knob.
4. There are two multi-tap filter cans and the internal one returned excellent ESR readings, whilst the externally mounted one (50+50) did not and of all makes was a HUNTS. This was replaced with a high quality can.
5. Someone prior to me had replaced most of the capacitors with quality Polyproplyne ones (a wise move if they had been HUNTS ones). For my sins, I only replaced a couple of resistors, which fell outside a respectable tolerance.
6. All valves tested perfect and consist of Mullard and Pinnacle types.
So that in a nutshell is it. For studio work this amp would be ideal having literally no background noise worth mentioning.
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Re: AC10 SRT BUILD SCHEMATIC ACCURACY

Postby voxjunior on Sun Mar 19, 2017 1:23 pm

Check out this video which may have some useful insights:

https://www.youtube.com/watch?v=alBO_yEn5o4
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Re: AC10 SRT BUILD SCHEMATIC ACCURACY

Postby Dr Death on Sun Mar 19, 2017 1:49 pm

Thanks voxjunior, I had come across these two chaps who rate these amps very highly. They're cuppa's are the stars me-thinks. Thanks again.
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Re: AC10 SRT BUILD SCHEMATIC ACCURACY

Postby voxjunior on Sun Mar 19, 2017 9:33 pm

It's rare to see 3 of these heads at once, especially a copper panel one!
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Re: AC10 SRT BUILD SCHEMATIC ACCURACY

Postby Dr Death on Mon Mar 20, 2017 9:30 am

The copper ones are the most rarest since only a few were made p, well at least that is what is stated in the magnificent VOX book that I mentioned in my original post. The Grey panels came in when Vox formally introduced the AC10SRT with its odd trapeziodal head.
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Re: AC10 SRT BUILD SCHEMATIC ACCURACY

Postby Dr Death on Mon Mar 20, 2017 10:56 am

This is my finished AC10 SRT amp head

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Re: AC10 SRT BUILD SCHEMATIC ACCURACY

Postby lion on Tue Mar 21, 2017 8:08 am

Thanks for sharing your findings.

From the pic your head cab seems much higher than I would have thought needed - it's not by any chance a cut down combo cab is it?

There's a pic of the combo version in Jim Elyea's book. Have you chosen not to add white piping round your front panel, would be the finishing touch IMO.

Any change of some pics of the back and innards 8)

Thanks again for sharing.

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Re: AC10 SRT BUILD SCHEMATIC ACCURACY

Postby Dr Death on Tue Mar 21, 2017 8:50 am

I looked long and hard at the internal construction of the cab and IMHO concluded it was not cut-down. Below are some more images that show the component side as well.

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Re: AC10 SRT BUILD SCHEMATIC ACCURACY

Postby fazeka on Tue Mar 21, 2017 7:42 pm

Something else to consider: if Tom Jennings and crew were truly trying to produce a Fender-styled amp having the sloping front control panel, it makes sense to have the height that this head cabinet has. Otherwise, yes, maybe it didn't really need to be as high as it is.
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Re: AC10 SRT BUILD SCHEMATIC ACCURACY

Postby voxjunior on Wed Mar 22, 2017 2:41 am

Very nice amp and restoration!

Maybe this is one model Vox should consider reissuing as a HW since it looks like Korg is building some limited run 60th anniversary HW AC15 and AC30 amps in the UK:

https://youtu.be/0oFbm0dHYEs
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Re: AC10 SRT BUILD SCHEMATIC ACCURACY

Postby Dr Death on Wed Mar 22, 2017 8:03 am

Interesting link on the 60th Anniv. models "voxjunior". What I find is a shame is that there are too many variants of the AC range being produced. If you just take the AC30, there are limited ed. colours, HW versions, speaker variants (C2 and CX2) and so it goes on and on. My opinion would be for a more focused offering across the range.
As for this AC10, there are not many circuits like this that can produce a sound so free of background hum and noise and I would (along with everyone else who loves the sound) like to see a re-issue hit the streets.
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Re: AC10 SRT BUILD SCHEMATIC ACCURACY

Postby grod915 on Fri Jun 16, 2017 9:21 pm

Having come across this thread, I was surprised that no one actually answered the original question of schematic accuracy. It is not accurate. According to a schematic included in Jim Elyea's book(schem. #8), Zapp was correct in pointing out that the high voltage from R7(100K) had to connect to the left(Plate side) of capacitor C7, not to the right. Also, there is a ground connection between R29(470k) and R30(100k) that has been omitted and the wiper for VR4 should go to ground, not to the R12(22k) connection. Jim's schematic also shows the inputs as non-switching types with the 1meg resistor going from the right of R2(68k) to ground. I think that covers all the differences between the two schematics.
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Re: AC10 SRT BUILD SCHEMATIC ACCURACY

Postby pullshocks on Mon Jun 19, 2017 6:29 am

Thanks for posting all this info. The schematic does not list the power transformer secondary voltage. Does anyone have voltage readings?

Thank you
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Re: AC10 SRT BUILD SCHEMATIC ACCURACY

Postby pullshocks on Sun Jun 25, 2017 4:27 pm

DrDeath, do you have any voltage readings from your amp, especially on the power tubes?

Thanks
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