Vermilion Cliffs - An Overdrive Reverb Design

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Based on the ODR #60 schematic, but with some unique personal changes.

[*]Major changes are AB2 output stage, fed by a 6CG7 rather than 12BH7, this can provide more current for the power tube grids to draw, and has improved headroom. Now AB1 w/ 12BH7 driver stage

Reverb send and receive methods are new, and sit somewhere theorywise between SSS #002 and ODR #60.
[*]Reverb driver is a 6AQ8, has better headroom than a 12AT7. Reverted back to 12AT7, no longer using LTP as reverb mixer

The overdrive portion is standard, with the exception of a diode pair to stop bias excursion (which can be excluded). I've built this portion of the circuit into a standalone amptop box and it sounds incredibly good.

[*]I have not designed the power supply yet. It will be run of the mill Precision, but have a slightly more robust bias supply to support some of the offset tricks in the preamp. PSU added

10/31/24 -
Bias supply adjusted, using that first trimmer to dump 300V was too risky.
Grounding switch has a new mode added.
Series 220uF power caps have been replaced with single 100uF (you can double up on these in parallel for a stiffer supply). The balance resistors create more heat that we don't need, it's just better in the long-run to source quality 500V capacitors instead of using two 250V rated pieces.
Relay supply has been changed to a full bridge rectifier, half wave as before would only give 3.15V.

11/01/24 -
Bias supply improved.
6L6 grid stoppers increased.
Important resistor wattages added.
12BH7 power tap now is created from the screen supply, preventing preamp sag.

11/02/24 -
More resistor wattage additions / changes.
Reverb return capacitor reduced to 50n to avoid an imbalance between dry and wet signal impedance.
LNFB resistor values adjusted.
Bias trimmers now operate to ground, so their failure mode creates a large negative voltage, preventing tube burnout.
Resistor added across Stand-By switch, this is to keep the bias voltage from leaking into the positive power rail from the 12BH7 during startup and standby.

11/05/24 -
"Accent" filters recalculated.

11/11/24 -
Mid Boost switch antipop resistor corrected.
Jazz/Rock is now a center-off switch to add PAB capability,
Grid stoppers added to reverb mixer and reverb cathode follower.
Reverb mixer feedback increased.


Comments, criticism, suggestions and questions welcome.

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What's the loadline?

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Screenshot_20241021_201455_Brave.jpg

Right at the dissipation line, ready to swing +5V.
It could be pushed further by raising +B and +g2, but that would come at the expense of useful tube life I suppose.

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For those interested, I've updated the schem.
Still no PSU drawn, I'm having a hard time deciding what to do for the bias supply. As designed this will need a stiff -70V. Instead of using the usual Fender PT tap method, I may use a seperate transformer...

Output is now AB1 ala-Bassman with small value differences. Driver tube is now a 12BH7 and reverb driver is a 12AT7. I figured using more generic tubes would be beneficial here.
Reverb send and return has changed. I discovered that although "reverb has no phase", the truth is everything has a phase when you're putting it through a LTP. Injecting the reverb into the cathode, while it seemed smart, would wind up leaving the signal uninverted on both sides, meaning you won't hear it. So I had to move things around, and think I came up with a good compromise that doesn't take up another 12AX7.

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Following this. Nice work

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Power supply has been designed and added to the schem, I opted for a Twin Reverb PT and a Bassman choke. Bias supply is full HT, the PT bias tap wouldn't supply enough voltage unless put through a doubler, and I didn't want to risk not being able to supply sufficient amperage.

Some voltages around the 12BH7 have been adjusted and the bias circuit has been recalculated.

I've added an index page with calculated idle plate currents and component suggestions. One of these choices included forgoing Orange Drop 225Ps because they're now made in China by CDE, my recommended replacements are BC MKT1813s. Any Qs or suggestions feel free.

Hoping somebody could chime in about my choice to capacitor couple the relay supply to the heater voltage, in my mind it will reduce noise and provide some isolation, but it may also be pointless?

Next step will be designing the PCBs for the PSU and relay support circuit.

EDIT: Just so everyone is aware, a flub has come to my attention. The 12BH7 grids should be -76V (21 volts lower than the -55 on the cathode). Not only did I mislabel it on the schem, I used that typo to calculate the bias trim resistors, so I've got to redo that math.

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Schematic has been corrected.
I decided to lower the 12BH7 B+ a tad, because we have to factor in the -55V on the cathode. 430V plate to cathode was pushing it too much, now it will be 375V.
Some math mistakes were fixed in the power supply (I must've not eaten enough Wheaties yesterday).

I'm also attaching a page from Analog Devices' history of opamps. It briefly discusses the negative voltage cathode bias that I used for the LTP and cathodyne reverb splitter.

[Colossal]

One of these choices included forgoing Orange Drop 225Ps because they're now made in China by CDE, my recommended replacements are BC MKT1813s.

Thanks for the heads-up on the 225Ps. I won't be using those anymore either. Nice work on your design.

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I've once again updated the schem, and barring anyone pointing out errors or improvements, I think it is the last revision.

I plan to start designing PSU and relay support boards, and I think for the main board I may do what TheToneGeek did with his Ultraphonix Bandmaster project. I have some blue garolite lying around too if I just decide to do it the old way.

I'd like this to be a combo with either a 12" or 15" JBL E series. What are the chances I could cram this all in a Princeton or Pro sized chassis?

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Relay power board is drawn up. Very primitive, but it'll work.
Main PSU is next

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PSU board drawn up. Some renders of both boards attached.

Currently having a hard time finding a quality SSS002 chassis for this (need to fit 7 preamp tubes), has anyone used Modulus before? He offers a nice laser cut ODS chassis, and I wonder if he'll do a custom 23" special.

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Relay boards redesigned.

I realized having the relays on the same board as the +5 would make for long signal runs, potentially risking some parasitic noise.
Really beginning to understand HAD's choices now

Supply now uses a smaller rectifier too.
If anyone would like the etching diagrams let me know.

Nik at Ceriatone is fabricating an aluminum SS002/ODR60 style chassis for this, will post pics when I receive it.