Magna-Vox (Inspierd by Vox AC10 SRT Build)

[65greg]

Hey all,
So I too got inspired to try out a Vox AC10 SRT amp as shown on the Doug and Pat show so I built up the closest thing I could out of the parts I had around.

I started with an old Magnavox amp I had picked up off of the local Craigslist for $60.00 and used the parts to build a SRT (Stripped of Reverb and Tremolo) version. I read all of the posts I could find and got some good help here from earl2 to help me understand the errors on the SRT schematic and strip out the reverb and Tremolo. Thanks earl2!

The donor amp had all of its tubes with it which was parallel 5Y3 rectifiers, 4x6V6, 1x12AX7, and 1x6AV6

The output transformer on the Magnavox was quite a mismatch for an 8 Ohm load and 4x 6V6 so I used one of the taps for my 8 Ohm cab and wound up with around a 12.8K primary / 8 Ohm secondary and currently running 2x6V6 so I’m mismatched by around 60% (considering 2x6V6 want to see 8K)
I also used the 6AV6 as my 1st gain stage but that was a mistake as even though it’s exactly like ½ of a 12AX7 I could not find a non-microphonic 6AV6 so I changed it over to a 9 pin socket and am now running a 5751 or a 12AY7 for the 1st stage. I find 12aX7 sounds ok there too but has too much gain to get a useful range out of the volume control.

Sound wise I like it! I have never built or used any amp with cathodyne phase inverter before and I dig the early and chewy breakup. I took it to band practice and even though it seemed really loud through my 2X12 cab at home it barely kept up in a band setting. It would get loud enough but was totally out of steam and too distorted if I tried to go louder.
Since the chassis is already wired for the other pair of 6V6 my plan is to install a Vox AC30 output tranny and add the other pair of output tubes.

Comments and Questions:
1 – Since I’m using 6V6 instead of EL84, I’m thinking since the 6V6 require more drive voltage I must be getting a bit more overdrive (distortion) from the gain stages and phase inverter than I would using the EL84 (pushing them to an equal volume level). Anyone care to chime in on this?

2 – The original schematic has the output tube plates and grids connected to the 2nd filter cap after the 1K resistor. Anyone know if the original amp is really wired that way? I tried tying the plates to the 1st filter cap as in a normal amp and yes, voltages went up and things got less compressed but I’m wondering if the “sag” resistor is part of the SRT sound. I thought it sounded ok wired either way. I wound up cutting the resistor to 500 Ohms.

[65greg]

IMG_20190217_204447.jpg

Magna-Vox Schematic.jpg

https://www.youtube.com/watch?v=2q93ejtR52I

[earl2]

Yes the original has that first dropping resistor before the plates. I`ve seen gut shots of three or four of them and they were all like that. Gets very very hot though, anything less than 10watts tends to catch fire !!
If you`re using 6v6 you would be better off with a 250R cathode bias resistor instead of 130R .

[65greg]

Thanks for confirming the plates are hooked up after the 1K resistor.
I am using a 250 Ohm for the bias resistor but forgot to note it on my schematic...