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

[Silvertone Jockey]

I have a Western Electric 100f amp that was originally used for a telephone speaker. It's in excellent stock condition, already amplifies a guitar signal as is, and even has a stock 6" Jensen special design speaker. It has a 600 ohm input and makes a nice bedroom level practice amp as is, but it could use a bit more bite, and lows and highs are also lacking.

I originally bought it for an investment (it's worth about 4-6 times what I paid for it currently) But my wife has become attached to it so it's a keeper now. If I'm going to keep it, it's gonna have to give up the tone in return, so any suggestions as to mods that would help it to be more ax friendly are greatly appreciated. I've attached the schematic for perusal by those more experienced than I, and will follow up with more pictures of it's wooden cabinet coolness in upcoming posts. Thanks in advance for any ideas on this somewhat different amp than the usual fare here.

[Enzo]

Telephone system response is purposely limited in range. If i recall right it is 300Hz to 3kHz. In my shop I use 100Hz a lot as a test signal - it is so much easier to listen to than 1000Hz. I can crank it to ear shattering levels while I am on the phone, and the person at the other end will hear nothing. So it makes sense the amplifier is oriented towards a limited range signal.

First, this thing is dangerous - it is wired directly to the mains, no power transformer. You need to add an isolation transformer to be safe.

Find out where the limitations are. Connect the amp to a different speaker, that special design might be limited range for phone use. See how that sounds.

Once you have the isolation transformer, try putting a signal into the volume control directly, disconnect the input transformer, it too may be limited in range. Find out.

As to the circuit, maybe ditch that cap across the OT primary for some high end relief. And adding cathode bypass caps to the two tubes might be interesting. And raising the value of the one coupling cap might improve bottom end, but be careful: amps that are designed not to have bottom end may suffer when forced to do so.

[Silvertone Jockey]

Thanks Enzo. I am well aware of the power wiring safety issue, I am somewhat new to modding audio circuits but I have built more than 20 RF linear amps up to 8 kilowatts (5 KV+ on the plate!) from scratch, and repaired at least a hundred more, but I do appreciate your concern for mine and others well fare.

As to the speaker it already has the tone, the amp just needs some break up to be half way to sweet, and any more frequency response is a plus. I appreciate your input thanks again

[Enzo]

My concern is not over the voltage, but the exposure to it on the chassis. Many old record player amps and similar things are considered for conversion to guitar amp, but it is the transformerless part that kills, not the voltage. We call those "hot chassis" circuits. I bet those linears did not have that 5kv on the chassis. As it sits, it will have 120v on the chassis when the power plug is oriented one way. It may be coming through a cap, but nonetheless uncomfortable to grab.

Try the cathode bypass caps, they will increase the stage gain.

[Silvertone Jockey]

Here is a pic of the amp in question

[Enzo]

Cool.

Your photo shows two controls, and I see only volume control on schematic. is the other knob a rotary power switch maybe?

[Silvertone Jockey]

Yup power switch. Worry not I already have an isolation xfmr or 2 in my parts inventory so I will play it safe, and you are right those linear chassis' were always connected to a serious ground system before ever being powered up.

[bluto]

I have two of those myself and have converted one to guitar use...but as was said adding an isolation transformer is a must for safety, otherwise you can have 120 VAC on your strings which can and has been fatal. For only a few more $$$ you could buy a repro Fender reverb unit power transformer (from Weber and others) and then build a 5C1 Fender Champ from that chassis, much better sound and waaay less hum in my experience also safe if you build it right with a 3 prong cord, etc.

[trobbins]

Perhaps use a switched input 1/4" socket, so that the guitar input goes to a 1M grid leak and 10k grid stopper. That assumes circuitry 0V is safety earthed. Ground the speaker. See if star grounding is achievable. Add an AC input fuse.

Bandwidth could be increased by increasing 1nF coupling cap, and lowering output transformer primary 40nF bypass cap.

Cap bypassing for gain, as Enzo pointed out.

[potatofarmer]

If gain is lacking, split up the 6SL7 into two gain stages, put the volume control between them.

You could use a concentric pot to add a tweed princeton tone control - Adam's Amplifiers: Tone Stacks - or if you have room to move the power switch you could install the tone knob there.

I'd probably also add small grid stoppers and a screen grid stopper.

[Silvertone Jockey]

I'm getting ready to do this project, and the plan is to make the changes suggested to the coupling and bypass cap sizes (and or removal) along with the cathode caps. While I'm in there I'm replacing the can filter cap with new small electrolytic's.

I noticed the schematic shows that runs on 105-125v ac or dc, 400ma which by my thinking running it on dc would be the hot ticket. I bet it would be totally silent with the gain cranked even.

So I'm going to hack two 1/2 amp wall wort's for the transformers one 9 and one 12 volt, wire them back to back 9 driving 12 into a bridge rectifier + 60uf cap and bleeder, switched and fused for DC supply.

I acquired a Boss digital multi track recorder that has some very cool amp models built in and 600 ohm balanced outs, so I'm going to experiment both with driving the input transformer with it being as the impedance is an exact match along with Enzo's direct in suggestion. I'm trying to keep it as close to original as possible so I won't be removing the can capacitor or the input xformer.

Anything I'm missing here?

[trobbins]

So I'm going to hack two 1/2 amp wall wort's for the transformers one 9 and one 12 volt, wire them back to back 9 driving 12 into a bridge rectifier + 60uf cap and bleeder, switched and fused for DC supply.

The DC supply is going to need something like a 120VDC at 400mA capability - that's 50W. Trying to do that with back-to-back would need transformers of at least 100VA, and even then the back to back will sag the output. That means hacking apart 115V - 12VAC 8A rated transformer based supplies - is that what you have?

[Silvertone Jockey]

I knew I was missing something..........Ohm's law. So much for that plan. I thought I had isolation transformers but I must have sold them years ago on ebay. Back on the back burner with this one for now. Thanks for pointing out my flawed thinking. That's why I ask these questions, and I really do appreciate it

[Enzo]

Also, running the amp on DC will ONLY eliminate hum from power supply ripple IN THE AMP ITSELF. Ripple from your external DC supply will still be there. Also any hum that might have come from tube heaters would not be present.

Other sources of hum would not be affected, like grounding hum, picked up radiated hum, environmental hum, etc.

Your wall wart idea also won't work because the 1/2A rating is for the output of the wart. a 12v 0.5A wart means it will draw 0.05A on its primary. Reversing that, you might try t get 1/2A through the 12v secondary, but that would only provide 120v at 0.05A.

[Silvertone Jockey]

Also, running the amp on DC will ONLY eliminate hum from power supply ripple IN THE AMP ITSELF. Ripple from your external DC supply will still be there. Also any hum that might have come from tube heaters would not be present.

Other sources of hum would not be affected, like grounding hum, picked up radiated hum, environmental hum, etc.

Your wall wart idea also won't work because the 1/2A rating is for the output of the wart. a 12v 0.5A wart means it will draw 0.05A on its primary. Reversing that, you might try t get 1/2A through the 12v secondary, but that would only provide 120v at 0.05A.

Yes trobbins pointed out my lack of Ohms law calcs on my not so brilliant PS scheme but thanks for the reply.

I have another question, looking at the schematic the only place that there is a chassis ground in the circuit is the death cap. That being the case is there a reason that I can't put a 3 wire grounded power cord on it if I keep the input transformer? that is ground the chassis and keep the death cap on the white lead (neutral) of the cord for the circuit ground?