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Old 25th February 2006, 02:43 PM   #1
Trout is offline Trout  United States
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Default Schematic opinion needed (guitar amp)

Wow, I have not posted in a while, But here goes.

Basically, I have built several various guitar amps the past few months/year, Mostly vintage fender based. There are of course things that I have felt did not work correctly or I did not like the sound of.

So,
I have played around the last couple days re-drawing the original Bandmaster 5E7 as follows,
#1Removed the feedback, and presence control.
#2 Added a more conventional Bass Treble & Mid-range instead of the original Bass/Treble setup that worked rather poorly in the original.

#3 I split the 1st 12AX7 cathodes to provide a cleaner clean channel & a more overdriven channel instead of the original both the same setup.
#4 Added a master volume to allow pushing the pre-amp stages without having to blow the walls down to get good overdriven tones.
Tenative #5 Switch to adjustable bias instead of a cathode resistor & Bypass cap.

I was Hoping maybe if someone had time they could review the schematic and check for possible problems.

Schematic Update

I have the chassis prepared and ready to do the final wiring
Trout
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Old 27th February 2006, 05:41 PM   #2
Trout is offline Trout  United States
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I Must be blessed lol,

I just built the nicest 35W signal generator I think I have ever seen.
I really must have botched what appeared to be a really nice build. (in my eyes)

I am getting very little signal from the input/guitar, If I raise the volumes to just were I just hear the guitar , I get very a strong audible tone that sounds like a Theremin device in frequency.

I have just sat here 3 hours re-tracing my wiring and have turned up nothing except an error of a cap value on the schematic . The .002 cap on the treble pot is actually supposed to read .0002 which I do have correct in the build.

I'll put up pictures later, All things considered, There are very few differences in the current version Vs the original I built and they work great.

Original Build

Latest Build

I guess I have a lot of questions about routing wires and what Is and what Is Not considered signal path.

Trout
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Old 27th February 2006, 07:16 PM   #3
Jax is offline Jax  Sweden
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Dunno what could be wrong with it other than perhaps routing. Pictures would be valuable in the search for the squealing. Things I can think of is how the grounds are done and if any wire near the output is routed close to an input.

You could compare it to mine which is not that unlike your design.

My guitar amp

This amp works just fine, at least with good EL34's that can withstand 440V on the screens.
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Old 28th February 2006, 01:37 AM   #4
Trout is offline Trout  United States
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Im clueless,
I spent 5 hours checking everything over and over, I finally resorted back to the original schematic.
"BUT" as with the original, The ONLY way it will work is with the feedback from the speaker disconnected. With the connection, It squeels and or farts depending on control settings, without it, It sounds GREAT. I have no idea on how to optimize the circuit for the componets (transformers) to cure this. They are supposed to be drop in replacements, But I do not see how they possibly could be. I have built 4 of these, They all do the same thing.

So, anyway, Its working now, Sounds great, Just the mystery of the feedback lingeres.
Trout
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Old 28th February 2006, 02:35 PM   #5
Jax is offline Jax  Sweden
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Quote:
Originally posted by Trout
"BUT" as with the original, The ONLY way it will work is with the feedback from the speaker disconnected.
!

Try reversing the phase on the feedback. Easiest by switching feedback and ground on the transformer secondary.
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Old 28th February 2006, 03:02 PM   #6
Trout is offline Trout  United States
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Ya Know,

I have been wondering about the transformer polarity for some time.

The OPT wiring diagram shows BLK as common and marked with a dot & Grn as 4 0hm. The primary has a polarity indicater on the drawing, Red of course is the CT, Brown is marked with a dot And Blue is unmarked.

I have no true idea which tube to put the Brown or Blue wire to so, I have it hooked up as follows,
Blue to upper EL34 in schematic & Brown to the lower.

I have previously tried reversing the primary with no success, But never the secondary.

Guess its worth a try!
Trout
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Old 28th February 2006, 03:52 PM   #7
Jax is offline Jax  Sweden
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Try it out. Reversing the primary leads has the same effect so the problem with the squealing could be something else. The phase is correct when the amp gain is reduced with feedback connected.

I saw in the original circuit that the master volume is inside the feedback loop. That is normally not good. Moving the pot to the grid of the previous tube will fix that.

One other thing to try is to add a small capacitor in parallel with the feedback resistor. Say some 47pF as a ballpark.
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Old 28th February 2006, 11:00 PM   #8
Trout is offline Trout  United States
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One thing I have noticed, Based on the original circuit. The noise is not limited to just higher frequencies. At certain control settings, Rather than a high freq, There is a loud low freq tone rumble/farting/fluttering type sound with the feedback active.

I can get either a squeel, or the rumbly thing using different Tone / Volume / Presence pot combinations

The Master Volume setup is not that uncommon though agreed less conventional. The " CLAIM " is that locating it there offers less tone deterioration.

Without the feedback attached, The presence control does offer some effect, But not nearly as much as expected. More or less it makes the .1/630V film cap into an adjustable bypass cap?

Perhaps some experimentation with the value might also offer interesting results.

As it sits, Without the feedack attached it sounds very loud and wonderfully crisp. But I can not help to think more could be had.
Trout
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Old 2nd March 2006, 02:35 AM   #9
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Default motorboating?

perhaps another stage of B+ decoupling for the preamp tubes would help? (I notice some Fender schems have it).
Pete McK
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