VOX AC30 1971 Birch Stolec Reverb Tank Problem

[Helmholtz]

I'll play around with a high to low impedance transformer connected to the capacitor and then to the input of the reverb unit and see if this works.

I think the main problem is that a standard tank with magnetic transducers has lower output than a piezo tank and will probably require a real recovery stage, i.e.an additional tube.

[pdf64]

Could consider an Ampeg V4 type arrangement, using a 12DW7 in V6 https://el34world.com/charts/Schemat..._vt22_revg.pdf
The 'U' section as the driver, the X section as recovery. Maybe even an ECL86 or 82, with a power pentode to drive the tank. Both either direct from anode via a blocking cap, or using a transformer drive.
If more gain is needed for the driver, could using a silicon device powered from the driver cathode, along the lines of the Vox V125 https://el34world.com/charts/Schemat...x_v125lead.pdf
Obviously it's a lot more tricky to do such re-arrangements with a pcb, especially one that's old, maybe fragile, and has the valve sockets mounted to it. Though having twisted pair wiring (rather than pcb tracks) for the heaters might help.
Other options include re-arranging it for an fx loop, using Lou Rossi's solution for the Belton digital reverb https://www.luigiretro.com/

[g1]

Could consider an Ampeg V4 type arrangement, using a 12DW7 in V6 https://el34world.com/charts/Schemat..._vt22_revg.pdf
The 'U' section as the driver, the X section as recovery. Maybe even an ECL86 or 82, with a power pentode to drive the tank. Both either direct from anode via a blocking cap, or using a transformer drive.

Maybe I'm taking it too literally, but that Ampeg in the link uses 6CG7 driving the tank.

[pdf64]

Maybe I'm taking it too literally, but that Ampeg in the link uses 6CG7 driving the tank.

oh eck, hope I wasn’t too far off, but not familiar with 6CG7; checking the characteristics, they seemed close to those of a 12AU7.

[g1]

oh eck, hope I wasn’t too far off, but not familiar with 6CG7; checking the characteristics, they seemed close to those of a 12AU7.

I think the 6CG7 is good for roughly twice the power. Traynor YGM3 uses a similar circuit with a higher impedance 'FB' series tank, but drives with 2 halves of 12AX7 paralleled. That seems to be the minimum for adequate drive I think.
Most of the 'one tube' reverb circuits seem to come up short and I don't think there are any real world mass produced examples that really cut it as far as good reverb tone.
I think Chuck H commented about this recently, maybe he can elaborate.

[Chuck H]

Ah! Such a clumsy arrangement. Changing to a transducer tank actually creates more problems. Being capacitor driven will have lower output than transformer driven. Maybe 4mV or 5mV output from the tank. Even if you used something like a 12dw7 and used the ax7 side for a recovery stage you still have only have about a quarter of a volt of reverb going into the PI. IMHO that's insufficient. That's why Fender (for example) used a "mixer" stage to re-amp the padded dry signal and further amplify the already recovered and re-amped reverb signal. This requirement of high and fast amplification is the primary reason why transducer reverb circuits tend to be a bit noisy. Thanks for the vote of confidence g1. I'm good, but not that good. This doen't even consider the rewiring on that ancient board with it's anomalous connectors and board mounted sockets. I don'r imagine the OP is up for the trouble of such a retrofit as adding a tube, transformer or both to make this a reality. I think it might be easier to build a separate board that achieves a low voltage power supply and use transistors for some of the heavy lifting here. Or, as already proposed, rewire the reverb circuit as an effects loop so that an outboard reverb (or anything else you might want) can be plugged into it.

Really anything that can be done with the circuit as it is, using available tanks will be a disappointing compromise. I expect the original circuit was also like that anyway.

Any solution will require some retrofitting. I think the effects loop is the most practical solution. Second would be to design and add a low voltage supply/amplifier on a floating board.

JM2C

[vintagekiki]

How to make a DIY Spring Reverb

https://www.youtube.com/watch?v=TJLvuAYu6U0

[martin.sadler]

I have just acquired the same model last week and I'm planning to install a reverb tank. Looking at the schematic provided by Colin Hinz (for which many thanks) you could use the existing circuit to drive a high-impedance 'G'-type Accutronic tank and use one of the "Normal" channel inputs as the recovery stage (I'm assuming you'd be using the "Brilliant" or "Vib/Trem"??!! for your guitar). The "Normal" volume control would then act as the reverb level.

I did get a bit confused at first - looking at the underside of the PCB I assumed that it runs V1 - V7 left to right but looking closely at the PCB tracks, the 7th valve seems to be wired as a standard gain stage directly coupled to an cathode follower - pin 1 is directly connected to pin7 - which suggests this is V5a+b on Colin's diagram. I need to spend a bit more time checking this out.

Did Colin also do the schematic for the power amp section, too? Not really important it's it's pretty simple but if it's available it would be nice to have.

[pdf64]

Good idea, that would be a way forward. That V3b tank driver circuit may still benefit from a bit of a rejig. Consider a jj ECC823, then the high current section can drive the tank a lot harder, given a tweak to the plate, cathode resistors and coupling cap value.

[martin.sadler]

Good idea, that would be a way forward. That V3b tank driver circuit may still benefit from a bit of a rejig. Consider a jj ECC823, then the high current section can drive the tank a lot harder, given a tweak to the plate, cathode resistors and coupling cap value.

I wasn't aware of that combination - thanks. Yes, this would most likely be the best solution. I had thought of using an ECC82 but this looks better. BTW, re the 6CG7 mentioned in other posts, it has a PaMax of 4w per section or 5.6w total. It's similar to an ECC82 but NOT pin compatible.

I'm thinking of adding a jack socket for the reverb return, attached to the underside of the board that carries the amp chassis. I can then use a short patch-lead to plug it into the normal channel. I was thinking of setting up some clever switching arrangement using a switching jack socket but on reflection I don't think it's necessary - just disable the switch that allows the Normal channel to go via the reverb - although it might be interesting to see what happens if you did that! In fact, my amp has had all the push-pull switched pots changed for standard pots but it came with the old pots so I could re-instate them.

[martin.sadler]

Ok, I've now got a look at the component side and I can see that Colin Hinz is absolutely spot on with his valve numbering - what a weird way of doing it - V1, V6, V2, V4, V3, V7, V5! I read that Birch-Stolec had this unusual reverb drive and recovery scheme because they were too mean to pay Hammond/Accutronic the licensee for their reverb tanks - yet they still populated the board with all the components, including the valve, for their non-reverb version, but not the "Rev Depth" control - has a blanking plug on the control panel! I've ordered a tank from Banzai Music in Germany. I can't find a UK/European source for an ECC823 - ECC832, yes, but that's the wrong way round! There is one on a Polish site but the price is quoted in Zlotys and there is no (apparent) English option so I'm stymied. I suppose I'll just have to get one from the US and pay the import duty, etc. Blimey - tax and shipping is more than the cost of the valve! Oh well...