Matchless Tornado Build with Reverb And Tremolo

[mebond11]

I finished a Matchless Tornado tremolo build, with a one-tube reverb circuit from this forum, and I thought I would share a couple pics of the finished product.  I used the Mojotone undrilled chassis, a Hoffman turret board, and Mojotone head cabinet.

The amp works great...I pick up a little bit of hum from the PT into the reverb tank on maximum due to proximity inside the head cabinet, but it isn’t bad.  Other than that the amp is quiet with nice tone.

The one problem I am having is that the reverb is weak to my ears.   With the controls at max I would call it only adequate, equivalent to a digital reverb pedal on only 3 or 4.   The reverb works otherwise as expected, and if I jostle the tank I get the Devil’s own cacophony crashing out of the speaker.  I already switched out the reverb pot for a 2M.

I haven’t taken it apart to take photos of the innards, but can do so; I checked the tube voltages previously but can do so again.  I am attaching a copy of the layout I used (sans power supply and heaters); and the reverb schematic.

I was wondering if I have an impedance matching issue in the circuit.   I used a fender style reverb tranny, with 29K ohms primary input impedance and 8 ohm output impedance.   I used a Revisit reverb tank (the only one I could find for the open side up orientation) with 8 ohms input impedance and 2250 ohms output impedance.

The plate on the first half of the reverb 12AX7 is connected through the reverb tranny, which had a measured DC resistance of a little over 1K, and using a plate resistance of 62.5K for a 12AX7, I get an output impedance of about 1.5K from the triode stage, but I must be doing this calculation wrong because it doesn’t make sense compared to the input impedance of the tranny.   Similarly, if the input impedance of the next triode stage is equal to the 220K grid leak resistor, that is significantly different that the 2250 output of the reverb tank.

I am obviously all messed up in these calcs.

Here is a link to the reverb tranny:

https://www.mojotone.com/amp-parts/Reverb-Drivers-and-Interstage-Transformers/Reverb-Driver

Would love any suggestions to boost this reverb...thank you all for reading!

[mebond11]

Sorry layout attached.

[vampwizzard]

Looks like your pdf attachment didnt upload correctly. Could you reload it?

Also: looks great!!!

[mebond11]

Thanks!   I uploaded a jpeg of the layout.   the Mojotone aluminum chassis and cabinet was really easy to work with.

[mebond11]

Also here’s a link to the Tornado schematic into which I inserted the tube reverb circuit, post volume/tone controls.

https://www.thetubestore.com/lib/thetubestore/schematics/Matchless/Matchless-Tornado-Schematic.pdf

[DummyLoad]

The one problem I am having is that the reverb is weak to my ears...

one tube reverb are a waste of time if you want a deep reverberation effect. i have not been able to make that circuit work to my satisfaction. obviously, that is a subjective statement. if you are looking to get super/twin/6G15 reverb depth, then you're following the wrong path to that end.

there is a caveat to the above, in that i was able to make a 6GW8 and 6BM8 plans work fairly well, however, they both were subject parasitic oscillation - the 6BM8 less so. 


--pete

[PRR]

> doesn’t make sense compared to the input impedance of the tranny.

The 2250r of the pickup is an inductance and is specced at 800 or 1kHz. So it could be 20k at the top of the audio band.

The usual convention for voltage transfer is to make the load ~~10X the source. So at the top of the audio band we might want 200k load.

When we add the 500pFd cap we get a resonance near 11kHz. We ask what resistance damps this; not much? But this high up on these not-thin lams, maybe iron losses keep it from ringing bad.

*Driving* a tank is more empirical. Instead of lo-Z drives hi-Z, we naturally get a "flat" response with a hi-Z source.

However your core complaint may be as DL says: a 12AX7 will "make reverb" but won't "MAKE REVERB!" If your salt-shaker goes all year between refills it may be OK. If you gave up on shakers and apply reverb with a shovel, the 12AX7 is a tiny trowel. (You would think you could drive small and gain-up after the tank; usually the hiss is bad.)

[mebond11]

> doesn’t make sense compared to the input impedance of the tranny.

The 2250r of the pickup is an inductance and is specced at 800 or 1kHz. So it could be 20k at the top of the audio band.

The usual convention for voltage transfer is to make the load ~~10X the source. So at the top of the audio band we might want 200k load.

When we add the 500pFd cap we get a resonance near 11kHz. We ask what resistance damps this; not much? But this high up on these not-thin lams, maybe iron losses keep it from ringing bad.

*Driving* a tank is more empirical. Instead of lo-Z drives hi-Z, we naturally get a "flat" response with a hi-Z source.

However your core complaint may be as DL says: a 12AX7 will "make reverb" but won't "MAKE REVERB!" If your salt-shaker goes all year between refills it may be OK. If you gave up on shakers and apply reverb with a shovel, the 12AX7 is a tiny trowel. (You would think you could drive small and gain-up after the tank; usually the hiss is bad.)

Thanks so much...how do I calculate properly the Z-out of the input stage, with the transformer serving as the plate load?  Do you think my product selections should be changed or are they well suited?  If in the future I chain together the two triodes of a 12ax7 will that boost the gain sufficiently or does that require a lot of reconfiguration downstream?  Thank you.

[ac427v]

I think the reverb transformer is fine. You "just" need more signal driving the tank and more amplification after the reverb recovery stage. You can get more by changing the circuit. It still might not be enough and it might not sound exactly like a Tornado. I don't have the theoretical and math skills to predict the outcome so I spend time experimenting.
Ideas to try:
Change the 100k mix resistor to 470k to push the tank driver harder.Set the "dwell" control to max or replace with a 1Meg resistor to ground to push the tank driver harder.
Change the phase inverter to Plexi resistor values to make up the gain lost by the 470k mix resistor.
Use a 12DW7 tube instead of the 12AX7. I have never done this but theoretically the 12AU7 section of this dual triode would drive the tank harder if you get the right cathode resistor value. It would suck more amperage from the power supply so you need to have a cool running PT before trying this.

[Jatchley]

I was told when building the 1 tube reverb circuit to play with the 100k mixing resistor value for level tweaking. Don’t know if that could help.

Edit: sorry just saw that was recommended above.

[Bieworm]

I'm no expert, but wouldn't a mosfet be a good solution to make it a full reverb circuit ?
Anyway, I did the 1 tube at first but used 2 different tubes to keep the option open for full BF reverb. Afterwards I doubled the triodes of the pre, but didn't notice a lot of difference