Filmosound - Tone Knob - Tertiaries

[atmars]

http://s294.photobucket.com/albums/m...mosoundL-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 refer 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. One is I am under the impression this is a not a good tone circuit for guitar and is hard for me 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

[Enzo]

"Under the impression"??? It either sounds OK TO YOU or it doesn't. Whether or not someone else thinks it is not a good style of control is irrelevant. If you don't like what it does, then change it, but please don't change it because someone told you it was not cool.

As to the 5879 tube, this circuit doeosn't connect to that tube at all. I cant see how it would affect it in the slightest. This tone control is the feedback circuit for the power stage, and it feeds into the PI tube, not the 5879.

This tone control is in the NFB loop, so really instead of tone control, I'd have to say it was more a presence control. it has its own winding on the OT instead of tapping off a speaker winding. I'd guess that was because they were never sure just what speakers might get connected to the thing, so this way the feedback was consistent.

Yes, disconnecting that winding will certainly stop it from having any tone function. If the control has no effect on the sound with that winding disconnected, then I think you could just remove the control for space and leave the small parts. There is a 10k resistor between the cathode of the 12AX7 and this stuff, so the little caps there would have little if any effect on the tube response without the feedback signal.

[atmars]

Thanks for the info.

As far as the tone control goes, it's sounds fine, but I'm not sure I need a tone control at all, (especially if it is robbing me of gain and volume(it is)) but if I do , there are passive ones that I know much more about and it would be easier for me to tweak. I don't want to give the impression i am making changes on this amp out of peer pressure. Advice that this tone control is not optimal was but one contributing factor in trying to remove it. The primary reasons are gain, the ability to capture valuable real estate, and the elimination of variables so I can begin to build up after I tear down. Right now this amplifier sounds like it got a lot potential (at the very least as a learning tool) but is a big humming squawking mess right now. I need to change filter caps, but I am just know getting to the point where I can even see the connections under all the spaghetti (thanks to your help in large part). This thing is my first attempt at turning one kind of amplifier into another. If I would have known more I would have picked something a little less challenging, but I am making some real progress with the help of this forum.

Thanks

[Enzo]

So you get a volume boost when you disconnect that extra secondary winding then?\

Yes, an old Bogen would have been a lot more convenient to work in.

[atmars]

It's hard to make an accurate assessment, but I would say 15% more volume with more grit, but I guess that would make sense if the tone knob is really a negative feedback loop.

[Fred2202]

I have done a few filmosound amps with pretty good results, but I must say this is not the best candidate for conversion. I do not like the transformerless circuit. That being said, I would strip out the wiring between the 5879 and 6sl7Gt and wire it with more guitar amp like values. Try a 0.0022 or 0.0033 uf coupling cap between the 5879 and the first stage of the 6SL7GT. The 5879 pentode can pass a lot of bass and I try to cut it back early in the circuit with a small value coupling cap.
You can put a simple passive tone control in between the 5879 and the 6SL7GT. These tight chassis do not leave any room for squeezing in more pots. I would look at some of old F*nd*r schematics as some of these circuits show a single knob tone circuit that will work. The single tone control should not load the 5879 down too much. I would also put an SPST off/on switch for the negative feedback on the back of the amp chassis. That way you can try it with and without feedback to see how it sounds, or use it as a boost switch for a little more dirt and volume.

[atmars]

Thanks for the advice.

I realize shortly after I acquired this amp that is should have looked elsewhere for an amp to convert. However, I have learned quite a bit already so I've gotten my money's worth. Before I owned this Filmosound I didn't even realize transformerless designs existed, but I soon acquired a cheap isolation transformer, so that aspect is covered.

I have been removing unneeded components in order the understand the circuit and make room for repairs/mod. I have gotten enough stripped so I can get to the filter caps and replace them. (Side note: I have added new filter caps across the old ones from the middle +post to the one of the outside -post, but have not removed the old cap cans. Is this acceptable, or does leaving the old cap in the circuit cause noise?)

After I check the voltages, I was thinking of changing the wiring of the 5879 to make it a traditional preamp tube as you have suggested. I noticed it is used as such in some G*bs*n designs.

As far as voltages are concerned, I haven't checked any yet, but I know the voltage coming out of the socket here is about 124V. I believe this the case in many communities. Is there a common method to reduce overall voltages to to bring older gear closer to the 117 it was designed for?

[Fred2202]

I would leave the wiring on the 5879 pentode alone for the time being. It will probably sound OK as it is. The wiring I was refering to was the wiring coming off the plate of the 5879 and leading to the grid of the 6SL7GT (first triode section). This is where I would put the small coupling cap and simple tone control circuit.
If you are going to replace the filter caps you need to disconnect them from the circuit. If you bridge them as you described and they were leaky to begin with they will still leak, but not quite as badly.
I have made more room under filmosound chassis by clipping out the wiring for the eye tube and oscillator tube and getting rid of the oscillator coil. I even pulled the sockets for these tubes out of the chassis to make room for terminal strips to mount and wire the new filter capacitors. These little chassis are cramped and any extra room you can gain helps.

[atmars]

I have made more room under filmosound chassis by clipping out the wiring for the eye tube and oscillator tube and getting rid of the oscillator coil. I even pulled the sockets for these tubes out of the chassis to make room for terminal strips to mount and wire the new filter capacitors. These little chassis are cramped and any extra room you can gain helps.

I have gotten rid of the eye tube, oscillator tube components except the tube itself and the heater connections which are still running. Cramped is an understatement.

If you are going to replace the filter caps you need to disconnect them from the circuit. If you bridge them as you described and they were leaky to begin with they will still leak, but not quite as badly.

How do I go about this? The caps in this version have one central positive and three negative tabs that double as mounting tabs. If I clip the positive tab loose (--with my new cap already bridged--), will that take the old cap can of the circuit and leave all the other connections functioning ?

I would leave the wiring on the 5879 pentode alone for the time being. It will probably sound OK as it is. The wiring I was referring to was the wiring coming off the plate of the 5879 and leading to the grid of the 6SL7GT (first triode section).

I will try this. The 5879 was part of the eye tube circuit, right? Couldn't it be configured more like a conventional preamp tube in a guitar amp circuit and have more effect on the circuit (add more gain) or at least be easier to tweak by using other schematics as a reference ?

Thanks

[atmars]

How do I go about this? The caps in this version have one central positive and three negative tabs that double as mounting tabs. If I clip the positive tab loose (--with my new cap already bridged--), will that take the old cap can of the circuit and leave all the other connections functioning ?

I actually did this and everything seems to be ok. The hum was not reduced significantly, but it did seem to add punch.

[Fred2202]

All of the can type electrolytic caps I have seen have one negative connection which is usually connected to the metal cover of the can and to the chassis when the can is mounted. This is the common negative ground for all the caps in the metal can. The other tabs towards the center of the can are the individual positive connections for the caps in the can. There can be from one to five of these tabs depending upon how many caps are in the can. If you just clip the connections, the new capacitors you installed are going to be loose. These carry high voltage and must be secured. If they short to another component or the chassis it could be very dangerous. That is why I used terminal strips to mount and usually used a zip tie to hold them securely. It is very important to make sure the electrolytic caps are wired correctly. If you mix the positive and negative terminals up they tend to blow up like firecrackers and throw off shrapnel and liquids.
You do not need the eye tube as it is not intended for amplifying audio, at least not in this circuit. I would pull it and run your guitar input jack directly to the grid of the 5879 thru a 68K resistor. The resistor suppresses AM radio station signals.

[atmars]

Thanks for the advice.

Before I acted I identified the cap cans as containing just one cap with the positive terminal at the center and three negatives at the edges (verified with a continuity tester). I did one at a time and powered up with the amp shielded in case of reversed polarity and explosions. The connections are free floating but just for test purposes, they will be secured to terminal strips asap.

Now that I have eliminated bad filter caps, I have a question about power up (which I might start a thread about, depending on the answer). When I power up the hum increases in volume to a point (about where guitar signal starts to be heard from the speakers), then there is a pop and the level of guitar signal and hum is reduced. This happens at every power up. It is like something is charging and then releases the charge. I thought it might be the old filter caps, but now they are out of the circuit. Any ideas?

[Enzo]

More lokely you are missing a ground reference for some spot in the circuit, for example a grid resistor that should go to ground but doesn't quite get there. To find that sort of thing, I would turn the amp on, let it come up to the stable condition, and start taking voltage readings at each tube pin. My first suspicion is a grid, so I would look at each grid to see if there is some DC sitting there that doesn';t belong.

[atmars]

http://s294.photobucket.com/albums/m...=1216270678587

A follow up for those who helped lead me out of the woods:

Major improvements are as follows: I had intermittent squeal problem. I had replaced the original 200k pot with a 500k which was all I had at the moment. 250k fixed this issue.

Reversed isolation transformer taking the measured voltage without a load from 130v to 117v, which is what the amp says it wants to see. Big improvement in quality of sound.

I would pull it and run your guitar input jack directly to the grid of the 5879 thru a 68K resistor. The resistor suppresses AM radio station signals.

I had planned to do this after I had solved my other issues. Used a 10k instead of 68k. Enormous amount of improvement. If I had known, I'd have done it first. Signal got much stronger and brighter as I expected, but hum virtually disappeared, which was unexpected. Overall this took the project from a D- to a B+ or better. the link above has the old schematic and the newly revised recent version. Any input would be appreciated. I think the next step will adjusting values of the coupling caps and cathode bypass caps.

Thanks

[freepressright]

Good results in stock form

I recently snagged a B&H 185 and harvested the amplifier. It's mint and sports an array of 25L6GTs, a 6SL7Gt and a 5879. I had to "build" my own speaker cable and power cable for it in order to satisfy my dying urge to test it, but I was all smiles at the results!

It has a very retro, late 60s early 70s tone. At low volumes, it stays nice and clean. At high volumes, it sings with warm, smooth distortion. I used my Marantz PMD660 digital recorder to snag some sound bytes. I used a Benson Audio Labs mic about a foot or so from the cabinet, out of fear I would throw too hot of a signal into the recorder.

This little 10-watt amplifier powered my Laney 4X12 (HH Equipped) like it was no one's business. It was loud and very pleasant. Using my Fender Stratocaster with Saga HO pickups, I haphazardly threw together some audio samples.

Keep in mind this amp is BONE STOCK with the original tubes and caps.

CLEAN TONE - STRAT - VOLUME AT 12:00

http://www.zshare.net/audio/5088299888fa8ea2/

SLIGHT OVERDRIVE - STRAT - VOLUME 75%

http://www.zshare.net/audio/508830890c0471dc/

FULL VOLUME - OVERDRIVE - STRAT

http://www.zshare.net/audio/508831403d888985/

BLUES LICKS - STRAT - NECK POSITION

http://www.zshare.net/audio/508832409d72a7af/

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But how would this amp deal with a high-gain application? I employed the use of my Behringer VT999, 12AX7-driven Vintage Tube Monster Pedal with the BitMo modification. I grabbed my Les Paul and went for an AC/DC-like tone, with slightly higher gain.

With the volume of the B&H amp at about 50 percent of the way up, and the VT999 configured in the following way:

GAIN PHASE SWITCH - Center
GAIN: 5
VOLUME: 6
MID: 4
TREBLE: 5/6 ish
BASS: 6

Here's the clip: http://www.zshare.net/audio/5088340799137de3/

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I wish I would have put the mic closer to the cab to do a better job capturing the essence of low, but overall I'm pleased with this little 10-watt dreamtoy.

Your thoughts?