The 18Wer's Tone controls

loverocker · Post by **loverocker** » Fri 06/20/03 1:08 pm

I'm very poor on theory, although I can follow most of the Marshall-style schematics I've looked at, but I can't work out what's going on with the 18Wer's Tone and Volume controls - esp that 100K between the two pots on the non-trem channel?

Can anyone here give me an idiot's guide to what they do? I was wondering why they aren't just wired up like a guitar's Volume and Tone pots? Must be a good reason...

roadking · Post by **roadking** » Fri 06/20/03 6:40 pm

It's been a long time since I did any real bench work, but I'll take a stab at it. Without the 100k resistor, if the tone and volume pots were both at
their minimums (i.e. 0 ohms) there would be no load to develop input to the next stage (V2). I don't have much experience with tubes, but that what I would say if there were transistors there instead. How did I do?

Post by **markh** » Fri 06/20/03 8:25 pm

At the one extreme, the .01u is connected directly to ground, and all treble (and a bit of mid) frequencies will be shunted to ground at that point - "bassy" tone.

At the other extreme, there's 500K at the bottom, so virtually no treble is lost.

If this were the end of the story, the tone control would still work, but the second setting would be louder, because a significant portion of the signal is lost in the first (bassy) setting.

By connecting the 100k to the other end, there is a 100K paralleled with the volume pot in the second setting, attentuating the signal a bit, and evening out the end to end (tone setting) volume level.

That's my guess, anyway!

--mark

loverocker · Post by **loverocker** » Sat 06/21/03 5:03 am

Thanks chaps.

Roadking - I'm a bit confused (it's easily done!). With the Volume at minimum we don't want any signal into V2A, no?

markh - that makes sense, except I'm still confused about what that .005uF cap feeding the input to the Volume pot is up to?

It's all a bit academic at the moment, because I'll just copy the 18W circuit exactly to start with, but it'd be nice to understand what's going on for later tweaks/etc.

Post by **markh** » Sat 06/21/03 10:04 am

loverocker wrote: markh - that makes sense, except I'm still confused about what that .005uF cap feeding the input to the Volume pot is up to?

Well, something needs to keep the 200 volts of DC from the previous stage off of the volume control.

--mark

loverocker · Post by **loverocker** » Sat 06/21/03 2:14 pm

Oh I see - I didn't realise that. I saw the .022uF that follows it and assumed that that blocking the DC from the PI was all that was needed. Info filed away for future reference - thanks.

Post by **markh** » Mon 06/23/03 4:29 am

The .022 keeps the (nonzero) PI grid voltage off of the volume pot, and at the same time keeps the volume setting from affecting the bias.

The .005, being so small WRT the 500K volume will attenuate the lows a bit, so could be thought of a part of the tone shaping network.

--mark

loverocker · Post by **loverocker** » Mon 06/23/03 6:39 am

Thanks Mark - this stuff is slowly beginning to make sense now.

For guys who've biult/got 18Wers with both channels, which Tone control is more effective?

loverocker · Post by **loverocker** » Mon 06/23/03 11:30 am

Thanks Paul - I think I might try that one first. I presume the other channel has a different Tone control because it was intended for mic input?

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The 18Wer's Tone controls

The 18Wer's Tone controls