Current Driven Transfornerless Reverb Utilising 12AU7 for Drive and Recovery

[Helmholtz]

Can someone tell me how the dry signal path goes?

V2a and V2b are coupled via their cathodes. I.e. for the dry signal V2a operates as cathode follower driving V2a in grounded grid mode. Its similar to the cathode coupling in a LTPI.

[allyshake]

Kevin O'Connors (London Power) circuit using a push pull 12AT7 driver with 12AX7 Recovery. Accutronics 9 Series Tank (Medium Delay) That is seriously lovely.

I reached out to Kevin via his website direct email and without wanting to turn this thread into a which is better; Tube/Current driven vs OT/Voltage driven, contest because as we all know the ear is subjective when it comes to component preferences despite the science that says otherwise; he made the following observations:

Quote: Frankly, constant current drive for reverb is always going to sound bad which is why any tube reverb that sounds good is done in the standard voltage-drive format.

Quote: tube reverbs that do not use a transformer driver as a group produce not very well-liked reverb tones.

Quote: The best sounding tube reverbs ALL use two tubes AND use voltage drive via a transformer.

Quote: There is no need to drive a lot of power into the tank; rather, the level only needs to be sufficient to overcome noise and should itself be noise-free. It is highly desirable to have "excess" recovery gain as this assures a warm reverb tone.

Quote: you could retain the no-extra-tubes goal by adding a solid-state driver. This allows using a standard tank and the tube can be changed to a 12AX7 and used wholly on the recovery side. An alternative is to hybridise both the send and return sides to achieve lower z-out on the drive side and higher gain on the recovery. To do it all-tube requires adding another tube and a transformer.

Some interesting observations, however I have seen plenty of threads where people swear by the sound of their tube only reverb circuit.

Notwithstanding, his last comment is where I think I will get the most mileage in order to achieve my aim, but my theory knowledge comes up a little short at this point.

Option One: Compromise and use a solid state driver.

Option Two: hybridise both the send and return sides....I don't know how to do this from a circuit design perspective but I think this might??? achieve my aim of using one tube for the drive and recovery as in the original which is my ultimate goal and preferred solution.

Option Three revert to a Fender style which as he points out may require an extra tube and an OT, (my least favoured option.

Any thoughts on the above would be welcome.

[ric]

I reached out to Kevin via his website direct email and without wanting to turn this thread into a which is better; Tube/Current driven vs OT/Voltage driven, contest because as we all know the ear is subjective when it comes to component preferences despite the science that says otherwise; he made the following observations:

Quote: Frankly, constant current drive for reverb is always going to sound bad which is why any tube reverb that sounds good is done in the standard voltage-drive format.

Quote: tube reverbs that do not use a transformer driver as a group produce not very well-liked reverb tones.

Quote: The best sounding tube reverbs ALL use two tubes AND use voltage drive via a transformer.

Quote: There is no need to drive a lot of power into the tank; rather, the level only needs to be sufficient to overcome noise and should itself be noise-free. It is highly desirable to have "excess" recovery gain as this assures a warm reverb tone.

Quote: you could retain the no-extra-tubes goal by adding a solid-state driver. This allows using a standard tank and the tube can be changed to a 12AX7 and used wholly on the recovery side. An alternative is to hybridise both the send and return sides to achieve lower z-out on the drive side and higher gain on the recovery. To do it all-tube requires adding another tube and a transformer.

Some interesting observations, however I have seen plenty of threads where people swear by the sound of their tube only reverb circuit.

Notwithstanding, his last comment is where I think I will get the most mileage in order to achieve my aim, but my theory knowledge comes up a little short at this point.

Option One: Compromise and use a solid state driver.

Option Two: hybridise both the send and return sides....I don't know how to do this from a circuit design perspective but I think this might??? achieve my aim of using one tube for the drive and recovery as in the original which is my ultimate goal and preferred solution.

Option Three revert to a Fender style which as he points out may require an extra tube and an OT, (my least favoured option.

Any thoughts on the above would be welcome.

Maybe it's time to breadboard and see what you like the sound of.

If there's a local guitar shop available to you, you could see what's there, check schematics and compare the different types.

I recall the Ampeg Reverbrocket schematic on the web shows triode plate drive. Through a .02 cap out to the tank.

Please post your results from whatever you decide. What didn't work and why is a learning opportunity too. (Voice of experience)

[Leo_Gnardo]

Quote: tube reverbs that do not use a transformer driver as a group produce not very well-liked reverb tones.

Quote: There is no need to drive a lot of power into the tank; rather, the level only needs to be sufficient to overcome noise and should itself be noise-free. It is highly desirable to have "excess" recovery gain as this assures a warm reverb tone.

I disagree with the first quoted quote. Plenty of folks love the reverb in 60's Ampegs, lush & smooth, driven off tube plate sans transformer.

For direct driven reverb circuits, it's a good idea to add a bleeder resistor maybe 10K or so on the tank side of the capacitor that delivers the signal. First, so you don't get a shock from the cap's "mirror charge" if you're working on the amp and the lead isn't plugged into the tank. And secondly so that charge isn't presented as a current burst to the tank transducer when you do plug in the tank, it could fuse the coil wire and wreck the tank.

Second quoted quote: agreed! IMHO a single 12AU7 triode or similar sized driver is plenty enough to supply signal to the tank.

[allyshake]

I Second quoted quote: agreed! IMHO a single 12AU7 triode or similar sized driver is plenty enough to supply signal to the tank.

Leo does that mean I could potentially use just one half of my 12AU7 (V3a in my circuit) OR am I getting confused here and that method would still necessitate using both triodes of the 12AU7 in parallel?

I recall you suggested using the 12DW7/7247. Could I thus use one tube, half for drive and half for recovery without an OT?

If so; this represents my ideal solution, however most circuits I have found use both triodes of a tube for the drive, eg Ampeg Gemini, Traynor YGM-3, Magnatone M10A, and Silvertone 1484.

Everyone please forgive my absolute ignorance. I have been playing around with this project on and off for a very long time and I have read a great deal and watched all of Uncle Doug's Youtube vids, (https://www.youtube.com/user/Stratosaurus1/videos) but perhaps its time to go back to square one, buy and read Merlins 2nd Edition book http://www.valvewizard.co.uk, try and get a better basic understanding of amps and their design and lastly start with a project that represents a tried and true design like a small Fenderesq or Marshall clone of which there are many available to buy as a kit.

Living on a veterans disability pension does have its limitations though, so my heart is set on building the old Vox, but the reverb part seems to be something that might be a bridge too far?

Lastly I really appreciate all the input and guidance and this forum certainly seems to be one of the best out there for the knowledge sharing and practical advice freely given out.

[ric]

If the parts to clone the Vox are unobtainable you may need to make a second choice for reverb. That could work out.

My choice is to send signal out of the amp that is NOT coming from the high volt plates and is low impedance. Thus the Gibson GA15 idea with the values adjusted to what sounds good to me.

I mention the Reverbrocket as an example of trannyless volt drive. Leo points out the potential issues.

[Leo_Gnardo]

I recall you suggested using the 12DW7/7247. Could I thus use one tube, half for drive and half for recovery without an OT

TBH I must issue a disclaimer: I haven't done this myself to prove whether it can be done. However, I think 12DW7/7247/ECC832 a perfectly good solution. There have been discussions on MEF also on The Amp Garage concerning "one tube reverb" should you care to research. Alternately a 6GH8 or 6U8 combo pentode + triode is worthy of consideration.

[allyshake]

There have been discussions on MEF....concerning "one tube reverb" should you care to research.

Thanks Leo...this thread may well provide some useful guidance...https://music-electronics-forum.com/...ad.php?t=22699

and here, looks like Swart use the 12DW7 for reverb in some of their amps: https://el34world.com/Forum/index.php?topic=21991.0

[allyshake]

Quick update: I have been told that the circuit closely resembles that of an Ampeg Reverberocket, model R-12-R-B which gives me the best lead on selecting a reverb tank that will potentially work. See Ampeg Reverberocket, model R-12-Rb

Link to schematic which more closely relates to solving the question of a suitable tube/circuit selection for single cap followed reverb tank https://www.dropbox.com/s/pskro8w6vl...matic.jpg?dl=0

[allyshake]

Re the link to the original schematic I posted. I have been told that it violates the copyright of the book it was published in about Vox amps so have removed the link, sorry. However the Ampeg circuit I have linked to is very similar and is widely available on the web.