Western Electric 100F amp, suggested mods for guitar usage?

[Enzo]

We shouldn't dance around it, we should bite the bullet and put an isolation transformer where the 120vAC comes in. Then the cap is no longer a death cap, or you could just ground the circuit common.

If all you do is install a grounded power cord, your entire circuit is still hot to the AC mains. The fact yu have the input transformer doesn't change that.


As to the amp itself, I don't know that that input transformers is doing you any favors, especially at 600 ohms. As an experiment, try disconnecting the BL-WH wire from the volume control and putting your guitar input there. Just like any other guitar amp. See how that sounds. I don't know this amp, but if it was made for telephone use, remember the phone has a range of like 300Hz to 3kHz.

If I were going to make it more guitar friendly, I might look at the dual triode input stage. The triodes are in parallel. It would not be hard to wire them separate, one feeding the other. That would give you more gain through the amp. ALso a cathode bypass cap would as well.

And certainly experiment with speakers.

[bluto]

This is how I modified my 100F amp, made a sort of 5C1 Champ using an external speaker....but was not thrilled with the result, lotsa hum. A reverb unit transformer costs only slightly more than an isolation transformer and yields much better results than the series filament design in my experience.

[Silvertone Jockey]

This is how I modified my 100F amp, made a sort of 5C1 Champ using an external speaker....but was not thrilled with the result, lotsa hum. A reverb unit transformer costs only slightly more than an isolation transformer and yields much better results than the series filament design in my experience.

[ATTACH=CONFIG]33713[/ATTACH]

The half wave rectifier you have in your design might be the cause of your hum. No amount a capacitance will be able to smooth out the ripple in that.

Speaking of half wave rectification, I just removed the chassis on mine. I was thinking that the slight hum my unit exhibited was due to a failing can cap. I wonder if the fact that it had a 25L6 plugged into the 25Z6 rectifier slot might have something to do with it? It actually worked like that! Unbelievable, except I have to believe it because it did

edit: Looking at the tube pin outs, with a 25L6 plugged into the rectifier socket, the filaments and the plate and the cathode are the same as one half of a 25Z6. The grid was hooked up as the other plate and the screen was hooked up as the other cathode, so it makes sense that it still worked although I'm sure that the grid of that tube melted down very quickly if not immediately

[trobbins]

If you integrate an isolation transformer, it may be worthwhile using a capacitor input doubler to generate B+. That form of doubler would retain one end of the HT winding as a ground reference, to assist V1 heater hum. B+ would obviously be higher, so some thought of bias conditions is required. If you had a small choke to provide an LC filter after the main rectifier-filter, then the doubler capacitors could use relatively low values of capacitance, and B+ can be lowered without resistor dissipation, and ripple would be much much better.

[Silvertone Jockey]

Among those into tube audio (hi-fi type stuff and tubes in general) Western Electric is like the holy grail of the tube universe, especially over in the Asian countries. In reality modding this amp is a big no-no, but the wife said we're keeping it otherwise I wouldn't be touching it. She likes the way it looks as in the knick knack factor.

That being said I'm just going to put the correct rectifier in it, do a cap job if it still hums (I checked the can cap all seems well and it's not leaking) and change the coupling cap by snipping one lead of the original and adding the new one along side leaving the original in place. Then add the cathode cap and cut one side of the OT bypass cap loose and use the 600 ohm out of my Boss recorder with amp modeling to drive the 600 ohm input of the amp. This way I'm leaving it almost original

That way if she ever changes her mind I can still sell it with negligible value loss to some uber Western Electric fan guitar player type, by advertising that it has been modded just for guitar, along with the external isolation transformer that I'm going to obtain. That's the plan anyway

I'm going to have to disguise the isolation transformer some how or my plans for a living room amp that blends in will be for naught

Maybe I can just hide it behind the couch or something

[Silvertone Jockey]

Another question

It would be really cool if I could re-purpose the Western Electric brand OT transformer bypass cap as the larger coupling cap, espeicially because it's an original part withe the holy grail name on it, but going from .001 to .047, whats the down side, or what might I anticipate from that drastic of a change in value?

[guitician]

It's all in da bass...that's what you'll get, more low frequencies which, for phone use, they didn't want. There is the voltage rating to check too.

[eschertron]

It would be really cool if I could re-purpose the Western Electric brand OT transformer bypass cap as the larger coupling cap, espeicially because it's an original part withe the holy grail name on it, but going from .001 to .047, whats the down side, or what might I anticipate from that drastic of a change in value?

clip a .047 alongside the existing .001 coupling cap and evaluate it for yourself. Then decide if you want to swap in the WE part.

[bluto]

Maybe...but the original design with the tube rectifier is also half wave. If you remove the original speaker there is room inside for the isolation transformer, that is how I did it, wired a jack for an external speaker.

The half wave rectifier you have in your design might be the cause of your hum. No amount a capacitance will be able to smooth out the ripple in that.

Speaking of half wave rectification, I just removed the chassis on mine. I was thinking that the slight hum my unit exhibited was due to a failing can cap. I wonder if the fact that it had a 25L6 plugged into the 25Z6 rectifier slot might have something to do with it? It actually worked like that! Unbelievable, except I have to believe it because it did

edit: Looking at the tube pin outs, with a 25L6 plugged into the rectifier socket, the filaments and the plate and the cathode are the same as one half of a 25Z6. The grid was hooked up as the other plate and the screen was hooked up as the other cathode, so it makes sense that it still worked although I'm sure that the grid of that tube melted down very quickly if not immediately