Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: scstill on August 20, 2025, 11:11:58 am
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I recently bought a Cordovox Amplifier and Tone Generator. I thought this would be a great project for converting to a 212 guitar amp with a 212 extension cabinet. Much of the cabinet mods have been done and now it is time to dive into the circuit design.
Goal to keep as "Vintage Cordovox" as possible converting to a low noise, clean sounding guitar amp. Following the Cordovox design, the guitar amp will be a 7591 push pull with a 12ax7 Cathodyne Phase Inverter and several 12ax7 (or 12au7) preamp stages.
I plan to use this thread to seek expert opinions on the approach and to understand the Cordovox design quirks. I have posted the design history here https://stillampd.com/cordovox
Original Cordovox schematics are in the link
Below is the Cordovox Conversion schematic, with an attempt to keep as much of the Cordovox design as possible, it is modified to meet layout and guitar amp design. There are three separate chassis': Power amp, Control Panel and PreAmp. BTW - there is only one 12ax7 in the preamp (Left side) other socket is empty
Would appreciate any comments on the design which is mostly Orig Cordovox. In particular:
1) Power Supply rectifier and reservoir cap design from Cordovox.
1b) is the voltage handling on the two res caps equal to twice the rating of the individuals
2) All the Cordovox design at the Speaker output. Will be running at 8ohm and 4 ohm with extension
3) Orig Cordovox Super small coupling cap at preamp 2
4) Lack of by pass cap on both preamps
5) Input grid cap on Preamp 1
6) Another conversion added thermistors on the input ac to control surge because the PT in Cordovox was driving 66 tubes that are no longer present https://noisemikers.com/cordovox-accordion-amp-conversion-with-schematics/
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Watching, with interest. I've got one that I stalled on, due to not being able to get the rosette-head screws out to swap the baffles and not being able to find my Dremel cut-off wheels at the time.
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Don't cut them off.
This worked for me...
To remove the baffles, the rosette screws that hold the black tolex reinforcement cabinet trim need to be loosened and tapped flush with the inner surface, I did not need to remove them. Loosen and remove the rosette nut, If the rosette spins with the nut you will need to hold the rosette with a pliers. I was able to tap the screw with the nut from the inside creating a bit of space to grab the rosette. Careful not to scratch the rosette, but if you do a dab of black paint will help.
I also waxed all the screws and bolts to make them easier for next time.
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I think it's just a voltage doubler circuit, isn't it?
The power supply should be fine with lower rated caps, although nowadays you can find caps that can handle the full voltage.
https://www.electricalvolt.com/voltage-doubler/
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I think it's just a voltage doubler circuit, isn't it
Explains why the original Cordovox design used 150uf 250v capacitors
The doubled voltage is ~420vdc, with half across each cap, 250 is adequate for Cordovox design.
I am using 450v caps so plenty of reserve...
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On item #4-Lack of by pass caps on pre-amp cathodes. If you got room, you could add a by pass cap switchable with a SPST switch. That way you could return it to stock setup at the flip of a switch
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On item #4-Lack of by pass caps on pre-amp cathodes. If you got room, you could add a by pass cap switchable with a SPST switch. That way you could return it to stock setup at the flip of a switch
not much room in the preamp chassis but I like the idea.
Will build as is and then will figure a plan to fit bypass and a bigger coupler.
I am excited to hear how this as built Cordovox sounds with guitar input.
It will live in my mancave which is 10x10ish, can you image 412 in that space
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Yelp 4-12's in a 10x10 space---whoa! That's the same size as my Jam Room. I use a SE EL84 amp through a 10" speaker cab with the volume set on about 2. My hearing is really bad! So now I really have to watch the volume level. I play through a clean amp and use pedals to get my dirt. Easier to control the volume that way. So take care of your ears(hearing)---you only get one set:>)
I'll be following this thread with great interest
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All Electrolytics replaced and grounding cleaned up so that there are only 3 stars (Cordovox had grounds in several places).
G1 - the Power Supply and Power Amp
G2 - the Preamp, Preamp Power Supply and Control Panel
G3 - the Speaker and OT (only because it is at the other end of the Power Amp Chassis)
These are all labeled with purple tape in the figure.
Questions:
1) should I keep the ground connection between the upper and lower chassis? this is the bare wire connecting pin 8 and pin 3 on the 9pin connector to the left of the AC cable. It essentially connects G1 and G2, which I'm thinking might be a noise source.
2) am I ok keeping G3? or should it be starred into G1?
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I've been looking at your power amp chassis and schematic. Usually when I start a new amp from scratch, I have the majority of the amp grounded on single 1/4" Bolt/nut/terminal located in the area of the input jack that provides grounding runs all the way through to the 2nd power node e-cap and the power amp section grounded to a PT lug like this 5E3 variation example 1st Pix below. All the ground wires are green except the heaters are dark green cloth type. With the 5E3's simple circuit the grounding is pretty easy to see on the Pix which has worked good for me on new builds.
But with this conversion with all the different chassis, the grounding scheme is kind of hard to change with this old style P to P. I converted a General Electric PA20 Hi-Fi amp that was two chassis for pre-amp and power amp connected by an umbilical cord (Like yours) and I just left the grounding scheme as is. It turned out quite enough to keep me happy. The schematic is below. I would recommend trying it as is and see how it goes. If you ran into some humm issues that was obviously grounding, I would then look at rearranging what I could that might help. Best idea I can come up with??
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I posted about one I purchased a while back and I am glad you posted this ....I found a site where a guy converted one and put up a schematic of what he did.....pretty good info and I saved the schematic ....I've put the project on hold as there are a few in the q ahead of it ....would you like me to post the schematic (Nevermind)
I missed the schematic you posted it looks better than the one I had