Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on November 03, 2014, 01:58:22 am
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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.
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Ciao TIMBO
Without transformer (the last schematic uses a common transformer only as an inductor)
Franco
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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
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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://el34world.com/Forum/index.php?topic=17010.0;nowap)
http://www.guitargear.net.au/discussion/index.php?topic=30880.0 (http://www.guitargear.net.au/discussion/index.php?topic=30880.0)
Franco
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Thanks K, There seems to be a couple of options for the transformer, so I will proceed and give them a try. :icon_biggrin:
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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
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Ciao TIMBO
What happen if you bypass the 6BM8 triode and go directly from 12ax7 (V2a - pin 1) to 6BM8 (V1 - pin 3) ?
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Have you considered a switch between pin 2 and pin 7 of 12ax7 V4 ?
Franco
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Hi K, the amp isn't finished yet, just getting some things sorted before the board is installed. Thanks
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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