Silvertone Organ amp to guitar amp conversion project (more changes)

[sluckey]

It's not necessary. It shunts high frequencies to ground. The amp should sound brighter without it.

[1blueheron]

Thanks Sluckey!  Will remove tomorrow and see how it sounds.

[PRR]

C6 is effectively *across* the OT primary. Hard to see because the path also runs through C7A C7B, but these are relatively huge so don't get in the way.

500pFd against an assumed 5,000 Ohm load computes as 66kHz. Where did you get this value?

Other notes:

R8 is more Volume than Tone.

R15 is another Volume but the backward connection cuts a LOT of bass when not full-up. (In a similar Tweed amp, C5 would be 10X bigger).

[1blueheron]

C6 is effectively *across* the OT primary. Hard to see because the path also runs through C7A C7B, but these are relatively huge so don't get in the way.

500pFd against an assumed 5,000 Ohm load computes as 66kHz. Where did you get this value?

Other notes:

R8 is more Volume than Tone.

R15 is another Volume but the backward connection cuts a LOT of bass when not full-up. (In a similar Tweed amp, C5 would be 10X bigger).

C6 value is what was written on factory Schematic.  I will verify real value.  (Edit, looking at parts list it is listed as "500 mmfd/1000V")

R8 "tone" was a copy/paste mistake on my part.  See corrected.

Does the drawing below show MV now in a forward connection vs the backwards connection that was stock?

[1blueheron]

Removed C6.
Changed C5 to .05uf
Changed orientaion of MV.

Considerable improvement was made.  Very bell like chimey qualities on clean. Plenty of high frequency passing now.  Good lows, Great harmonics. Great sensitivity. Turn up the gain a little and it really gets a nice grindy overdrive. Its different.  Fender like cleans, Voxish OD.
 
What's not so great...  When you turn the preamp gain past 12 o'clock it goes into a high frequency oscillation and starts squeaking and squealing like a stuck piglet.  With some guitars its worse than others.  Weird thing is this can be mitigated to an extent by turning the MV up.  Turn the MV all the way to WOT and it gets a rythmic heartbeat almost like the trem is on but it isn't.
It could maybe use a little bit of a scoop out of the midrange. 

Calling it quits for tonite. Will continue exploring and tomorrow try to solve the oscillation issue.

[1blueheron]

Made a few more mods to the Silvertone.

Changed C5 to a .082uf
Installed "bright cap"  100pf

Still had a squeal/oscillation if I turned up gain and it was farty sounding.

So I figured I had too much gain...

Swapped V2 out with a 12AT7.   No more squealing! No more flatulence.   My son has been playing it almost non stop since this change and he likes it a lot. 

Strange thing somewhere in the changes, I did something that caused the vibrato to quit working.  He doesn't use it so not a big deal but not sure what I did.  Thinking about removing vibrato circuit and using that knob position to add negative feedback control? 

Thoughts?

[1blueheron]

I have continued to work on refining and voicing this little amp.

The 12AT7 seemed and improvement but still lacked a little. Tried removing negative feedback to see what effect that would have.  Didn't seem to imporve the situation.

After closer examination/study, I noticed my plate voltages were off a bit from what they are supposed to be.   The dropping resistors,  had drifted high.  Replaced the dropping resistor.  With new plate voltages and the 12AT7 in place, sound was improved a great deal.  Yesterday played with it a bit more.   I disconnected the Vibrato circuit.  Installed a 12AU7 to see how it would sound.  Wasn't impressed very clean but very bland and dull. 

Found a 12BH7 and looked it up.  Specs seemed reasonable so I gave it a try.  I am liking this tube.  Nice range of tone.   Now it has me wondering if I should play around with plate voltages a bit more.  I can push the amp into distortion by turning the gain up, but as it begins to distort, it gets a little hollow sounding.  Not sure how to describe it or what the cause is?  Could this be caused by preamp tube running out of steam too early?

Now that the plate voltages are up closer to where they were supposed to be, thinking of also revisiting the previously mentioned bypass cap mods. Perhaps they will sound better now.

If you can't tell, I'm having fun learning

[Joel Rutledge]

Hi! I know it is a couple years past when this thread ended, but I was wondering if you had a schematic of the final results?  Thanks!
-Joel R

[AmberB]

I'm bumping this thread because I was just given a chassis from one of these Sears 157.10000 organs.  I was doing a google search and came across this thread.  I'm wondering how the amp came out and what the finished schematic looked like.  That would be helpful before I start on converting this little chassis to a guitar amp.
Whoever took this chassis out of the organ just cut all the wires going to the rest of the parts of the organ, so I'm still trying to figure out what was hooked up to those wires.  This thread did give me some information about that, so it will be a bit easier to figure out the circuits.