6BM8 reverb circuit

[TIMBO]

Hi guys, I have another old Fi Sonic amp that has room on the PCB that uses a 6BM8 and 12AX7 in its circuit. There seems to be a problem getting a reverb transformer to use in this circuit.
I have not got a tank for circuit so if there is a way to use this circuit with a off the shelf transformer and tank, that would be great.

[kagliostro]

Ciao TIMBO


Without transformer (the last schematic uses a common transformer only as an inductor)


Franco

[TIMBO]

Thanks K, These old amp are a bugger when sourcing parts.
It would be simple job just to put in a 1 tube reverb as I have a 022921 reverb tranny and a short tank to match.
As I said the board has the 6BM8 circuit and this I would love to use.

This is what I am thinking, it uses 1/2 12ax7 as a normal input stage and the other as a recovery stage, so that will stay.
If I replace the 6BM8 with another 12ax7 I could wire it as the schem bottom left.
If at some stage a transformer and tank becomes available to suit the 6BM8, the 9 pin socket will be already in place.

The schem bottom right is a just a thought, as it is a similar to the 6BM8 wiring, could this be an option to another way of doing a reverb driver circuit.Thanks

[kagliostro]

Ciao TIMBO

do you remember this old treads ? May be there is something that it is worth remembering

http://el34world.com/Forum/index.php?topic=17010.0;nowap


http://www.guitargear.net.au/discussion/index.php?topic=30880.0

Franco

[TIMBO]

Thanks K, There seems to be a couple of options for the transformer, so I will proceed and give them a try. 

[TIMBO]

Hi guys, This part of the project is slowly coming together.
The circuit seems to work fine with plenty of reverb but only at a certain part of the reverb pot rotation.                                            Some of the guys here have suggested to put the pot at the recovery end.

Having build some reverb circuits and having a DWELL as well as a LEVEL gave more control over the reverb.
I not really wanting another pot on the back of the amp, so would adding a SET AND FORGET pot something that would help control the distortion that happens when the reverb pot is turned up. Thanks

[kagliostro]

Ciao TIMBO

What happen if you bypass the 6BM8 triode and go directly from 12ax7 (V2a - pin 1) to 6BM8 (V1 - pin 3) ?

--

Have you considered a switch between pin 2 and pin 7 of 12ax7 V4 ?

Franco

[TIMBO]

Hi K, the amp isn't finished yet, just getting some things sorted before the board is installed. Thanks

[TIMBO]

Hi guys, I'm back on the job to get this sorted.
It seems to be working with plenty of reverb, but when turning up the 500k pot I get a sizzling sound in the tank and turning the pot up more I get arching across the socket pins 6&7 of the 6BM8.
There is plenty of reverb form 0-3 and from 4-5 the fireworks start.
Usually with 1 tube reverb a DWELL and LEVEL pot is used, so I set the 500k pot to about 3 and added 100k pot to the input of the recovery stage (V2b) this gave more control to the amount of reverb and seem to work very well, NO arching.
I lack the nuts and bolts to why this arching is occurring.

I do know that the other Fi Sonics the reverb goes to s###t when the pot is turned up but I'm not getting any arching that I know of.

Could I replace the 500k pot with a voltage divider or trim pot and just use the 100k pot to set the amount of reverb.
I didn't want to leave it they way it was and cause damage. Thanks