Reverb Insertion after CF driven tonestack. How?

[pnadora]

Hallo Guys,
I am planning a EF86 amp right now and I am wondering where and how to insert the Reverb at the moment. I am using a Cathode Follower to drive the Tonestack/Volume control after the EF86 and want to insert a spring reverb now. I am using parts from an old broken Hammond Organ and copied the Reverb circuit from from that layout only replacing the 6FQ7 reverb driver tube with on half of a 12AU7. (The other half is the CF).
I noticed that Fender uses a 3,3M resistor bypassed by a capacitor between the reverbs send and return point, but since the Reverb comes directly after the tonestack in my build I dont think that using such high values are a good idea. In the 5879 "Blues amp" schematic I found on this board a simle one tube Reverb comes directly after the Tonestack using only a 150k resistor to prevent feedback.

Now my question: How do I decide what value of a resistor is good for my build?
On the picture you can see my plan so far with the probably wrong 3,3M/capacitor system going into a mixer 12AX7 and a paraphase Inverter. Any help ist highly welcome.

[tubenit]

I am not familiar with the power amp & phase invertor (?) in your schematic.

Given that I am not familiar,  this is what I would do with those tubes.  I attached a SCH schematic for you.

I am not sure a half of a 12AU7 would drive the reverb enough?  It might, but I just don't know?   And you could use a dwell pot of 1M instead
of the 1M resistor

with respect, Tubenit

[tubenit]

IF you need/want to stick with your original topology, then I would simply use a mosfet CF and free the other half of the 12AU7 as a reverb driver tube.  I've used a 5879 with a mosfet CF and it worked fine.

with respect, Tubenit

[sluckey]

Now my question: How do I decide what value of a resistor is good for my build?

Experimentally. Start with 3.3M, but be prepared to change it until you are satisfied. Lower values will increase the ratio of dry to wet signal. If you go too low you will lose your reverb.

That PI is called a paraphrase inverter.

[HotBluePlates]

The Fender reverb circuit you're copying also has a 220kΩ from the grid of 12AX7 1b to ground.

[pnadora]

Ok guys first thanks for the comments!!!!  I´ll answer to all of you in a row to keep it readable :)

Tubenit: Thanks so much for the layouts, that really helps! The idea for using one half of the 12AU7 to drive the reverb comes from the organ schematic of the organ I am building this amp out of (which used one half of a 6FQ7). I do not have much information about the reverb driver transformer so I thought staying close to the original layout will be the safest way. Instead of using a mosfet CF I would probably prefer to change the type of phase inverter to a cathodyne one and use a 12AX7 as a CF in the bassman configuration. The phase Inverter I was planning is a paraphase inverter. I watched a video on youtube by "uncle doug" about phase inverters and that one was the one I found easiest to understand. It is also what analog outfitters are using in their sarge amp (the one that started the whole organ to guitar amp conversion idea). The power-amp layout is the one of the Ampeg reverberrocket that uses the same output tubes that were in the broken organ.

Sluckey: Thats probably what I am going to do :)

HotBluePlates: I missed that one, thank you!

I attached the schematic of the organ I am using. English is not my first language so pls excuse my mistakes in writing and phrasing :)

[PRR]

> the schematic of the organ I am using

That might be a fine guitar amp with very little mods. It IS a musical instrument amp, and was sold on "fine tone". And you KNOW it works (or once worked), which saves a lot of time re-building and then discovering your mistakes.

Replace the Expression Pedal with a tone stack. Remove R613. Try disconnecting the wire from output to R729. Change reverb return level from switch to pot.

[pnadora]

True it would probably make a good guitar amp. But it was noisy and I think some of the resistors and capacitors are gone bad. Also I am a beginner as far as tube amp building is concerned and it is easier for me to start with a new build because that way I know the function of every componet and part of the circuit. Also as far as safety is concerned I feel more confident when I know exactly what parts of the circuit are high voltage and what capacitors I have to discharge before I stick my hands in the enclosure. The chassis of the organ was pretty full and I want to make it easier fo rme by having a pretty neat and clean layout.

[sluckey]

Which old Hammond chassis is this? Several people on the forum have experience with a variety of the old Hammonds.

[pnadora]

It is from an K-100 organ. Its a "hybrid"-organ where only the amplifier is tube, so it is ideal for me since it does not have much value for the organ lovers and I do not feel so bad not repairing it but rebuilding it into a guitar amp. I think the number of the chassis is AO-68.

[pnadora]

I have a follow up question regarding the one tube reverb, that you have suggested. I realised that the reverb transformer in the organ is not working and so I want to follow your suggestions instead of just leaving the organ circuit alone at that point.

I want to use one 12AU7 or 6FQ7  triode as the reverb driver and a 12AX7 for recovery as this would fit the tube layout of the amp. Would I still use a standard fender-style reverb transformer?

I am also designing the power supply at the moment and have problems to figure out the current draw of the reverb driver triode. On the 5879 blues amp schematic the voltage of pin3 (cathode) of the reverb driver is 1.45V with a 1.5k bias resistor. So the current with a 12AX7 is around 1mA if my thinking is right. The problem is that I do not know how to transfer that to a 12AU7 and draw a load line for that case (since the load is a transformer). Also using this method the current of the 6FQ7 in the original organ schematic works out to be around 14mA what seems to be super high and makes me wonder if I have an error in my approach to start with.

Again any help is highly appreciated :)