http://s294.photobucket.com/albums/mm109/atmars/?action=view¤t=filmosoundL-2.jpg
I have been doing some work on changing a filmosound film amplifier to a guitar amp ( with a lot of help from forum members). What I have done recently is remove some unneeded components from the chassis. If you check to link above you can see the before and after schematic. This has been a big relief as it makes the the circuit easier to understand and makes it possible to work inside a chassis that was near impossible before.
My question concerns the tone knob. I would like to remove the tone knob from this circuit for a couple of reasons. On is i am under the impression this is a not a good tone circuit for guitar and is hard for to understand with my limited abilities. I would like to have no tone knob and use the space for something else or try a simple passive tone circuit. this tone circuit is attached to a set of tertiary wires on the output transformer. In another forum a member told me i could disconnect these and without ill effect and dispense with the tone circuit. I tried this and the 5879 tube was destroyed. i returned to original wiring and a new tube. Maybe the dead tube was a coincidence, but I am reluctant to try this experiment again without more input from experts. What i have done recently is placed a switch between the left between side of the tone stack (on the schematic) and the black-red tertiary. This seems to take the tone knob out of the circuit with no apparent ill effect and increases the gain and volume of the overall signal. Does this seem ok and if so what else can I strip out to make room?
Any insight would be appreciated as i don't really understand what these tertiaries are for other than the tone circuit is an integral part.
Thanks
I have been doing some work on changing a filmosound film amplifier to a guitar amp ( with a lot of help from forum members). What I have done recently is remove some unneeded components from the chassis. If you check to link above you can see the before and after schematic. This has been a big relief as it makes the the circuit easier to understand and makes it possible to work inside a chassis that was near impossible before.
My question concerns the tone knob. I would like to remove the tone knob from this circuit for a couple of reasons. On is i am under the impression this is a not a good tone circuit for guitar and is hard for to understand with my limited abilities. I would like to have no tone knob and use the space for something else or try a simple passive tone circuit. this tone circuit is attached to a set of tertiary wires on the output transformer. In another forum a member told me i could disconnect these and without ill effect and dispense with the tone circuit. I tried this and the 5879 tube was destroyed. i returned to original wiring and a new tube. Maybe the dead tube was a coincidence, but I am reluctant to try this experiment again without more input from experts. What i have done recently is placed a switch between the left between side of the tone stack (on the schematic) and the black-red tertiary. This seems to take the tone knob out of the circuit with no apparent ill effect and increases the gain and volume of the overall signal. Does this seem ok and if so what else can I strip out to make room?
Any insight would be appreciated as i don't really understand what these tertiaries are for other than the tone circuit is an integral part.
Thanks
Hi,
First of all, this amp does not have a power transformer; it is connected directly to the line (wall) voltage source. THIS IS VERY DANGEROUS! The potential for electrocution is fairly high, considering part of the amps circuitry (the guitar) is wrapped around your neck when in use. At the very least use an isolation transformer.
As for your tone control problem, the "tertiary" winding is actually the feedback winding of the transformer. This winding provides negative global feedback to the circuit. The tone control is just part of the NFB circuit; so I would consider it an active tone control.
You should be able to take it out with no ill effects. When removing it, just disconnect the 10 meg resistor from the phase inverter, this will take the feedback loop out of the circuit.
As for the pentode being destroyed, probably just a coincedence, as the feedback circuit doesn't even go to that tube. When working on the amplifier the first time, did you remove the tubes before starting work? Don't do work on the amp with the tubes still in as all the banging around will destroy the tubes.
Hope this helps... Daniel
First of all, this amp does not have a power transformer; it is connected directly to the line (wall) voltage source. THIS IS VERY DANGEROUS! The potential for electrocution is fairly high, considering part of the amps circuitry (the guitar) is wrapped around your neck when in use. At the very least use an isolation transformer.
As for your tone control problem, the "tertiary" winding is actually the feedback winding of the transformer. This winding provides negative global feedback to the circuit. The tone control is just part of the NFB circuit; so I would consider it an active tone control.
You should be able to take it out with no ill effects. When removing it, just disconnect the 10 meg resistor from the phase inverter, this will take the feedback loop out of the circuit.
As for the pentode being destroyed, probably just a coincedence, as the feedback circuit doesn't even go to that tube. When working on the amplifier the first time, did you remove the tubes before starting work? Don't do work on the amp with the tubes still in as all the banging around will destroy the tubes.
Hope this helps... Daniel
DOES have isolation transformer
Thanks - that does help. I'll give it a try and post my results.
And, yes, although I didn't include the information in this post (should have), I do have an isolation transformer in front of this amplifier. It was one of the first things I inquired about on this site.
Thanks again.
Thanks - that does help. I'll give it a try and post my results.
And, yes, although I didn't include the information in this post (should have), I do have an isolation transformer in front of this amplifier. It was one of the first things I inquired about on this site.
Thanks again.
Just so I am clear, when you say 10 meg from the phase inverter, I am not exactly sure what you are referring to. It's my understanding that this schematic is old enough that everything is labeled m when it really means K. So if we are talking about a 10k resistor, are we talking about the one just below the 6SL7 connected to the tone pot?
Also, does the black-red of the feedback windings act as a ground reference for the cathodes of the tubes? If this is cut an capped off how do these parts of the circuit function? Sorry for my ignorance - my previous experience is with single ended simple circuits, so I am pretty foggy on how this crazy circuit functions.
Thanks
Yes, the 10K resistor; 10 meg seemed too high, I just assumed that 10m meant 10 meg,
The red-black wire of the transformer is grounded, the red-yellow (or black-yellow, I forgot) is the NFB lead. NFB always is referenced to ground. This winding has nothing to do with a ground reference for the cathodes of the output tubes. The output tubes in this amp are self-biased (36 ohm(?) resistor and capacitor in parallel with said resistor). This boosts the cathodes to +12.5 volts with reference to the output tubes' grids which are at gound potential.
This is a real "cheapie" amp; all the short cuts were taken to reduce costs in builing it. The nice thing though, is that these cheapie amps usually sound great when a guitar is put through them.
I would change out all the electrolytic caps in this amp, as they are all probably bad by now.
Daniel
The red-black wire of the transformer is grounded, the red-yellow (or black-yellow, I forgot) is the NFB lead. NFB always is referenced to ground. This winding has nothing to do with a ground reference for the cathodes of the output tubes. The output tubes in this amp are self-biased (36 ohm(?) resistor and capacitor in parallel with said resistor). This boosts the cathodes to +12.5 volts with reference to the output tubes' grids which are at gound potential.
This is a real "cheapie" amp; all the short cuts were taken to reduce costs in builing it. The nice thing though, is that these cheapie amps usually sound great when a guitar is put through them.
I would change out all the electrolytic caps in this amp, as they are all probably bad by now.
Daniel
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