Volume pot pre V1

[shaun]

Hi All,
I'm sure there has been a debate about the value of installing a Volume pot on the input before V1. Could anyone point me to a thread, or alternately, offer pros versus cons?

I'm one of those people who likes to leave the guitar dimed, but that's not always possible with a Hi impedance input without hitting low headroom. Also, on an amp with 2 or more inputs, there is a Hi-Lo jump from often a 1M to ground to zero, or sometimes an in-between 470k.

I realize this is may be a simplistic topic, but it seems to me that a vol pot pre V1 gives the end user greater control. Just wondered why no one does it.

Thanks, and hope you're having a great day, whatever workbench you're huddled over.

[PRR]

1) What is the difference between a post-axe pot and a pre-amplifier pot? You are throwing-away signal at the same place in the signal chain. If you think "Oh, the guitar is dimed", you are fooling yourself.

2) What is the cost to TRY? You look like a dude with a pot supply, so maybe zero.

3) There were a few super-cheap commercial amps with the one pot at the front. Some work OK when PUSHED. When setting very quiet or idle, they hiss, even if turned-down.

[acheld]

++ what PRR says.   

Thinking of this in a slightly different way, one of the main tasks that your first gain stage has is to amplify a very small voltage signal into a much larger signal without allowing or injecting noise into the signal.   This larger (now amplified) signal then can be manipulated -- such as a volume pot, tone stack -- with a much higher signal to noise ratio.

If you try to do this stuff "pre-gain" stage, you lose a lot of signal compared to the noise floor, which stays constant (at best), and your signal to noise ratio is much lower -- and this persists through the remainder of your signal chain / gain stages.
 

[shaun]

1) What is the difference between a post-axe pot and a pre-amplifier pot? You are throwing-away signal at the same place in the signal chain. If you think "Oh, the guitar is dimed", you are fooling yourself.

Thanks for the info. Yes, I wondered about that (and boy, am I ever good at fooling myself - been married twice :). I was thinking the post-axe pot would possibly shape the way the pick-up functions because it interacts with the guitar tone pot, but I guess the pup winding is what it is, no matter where the guitar volume is set.

[shaun]

Thinking of this in a slightly different way, one of the main tasks that your first gain stage has is to amplify a very small voltage signal into a much larger signal without allowing or injecting noise into the signal.   This larger (now amplified) signal then can be manipulated -- such as a volume pot, tone stack -- with a much higher signal to noise ratio.

This makes sense, too. Thanks acheld.

[PRR]

I just ran into the schem for my cardboard Kent. Note the one pot out front ahead of all else.

No, I have not fired this up because it is a death-trap, and probably a fire-bomb too. But experience with related topologies tells me that, anything less than proper "line level" (already preamplified) sources, you will be fighting hiss.

[shaun]

Aha. Thank you PRR, the penny has officially dropped at my end. The line level concept - of course. Makes perfect sense. And that Kent...a firebug's delight?  Hey, if it sounds great, who cares about the house...

[traynor]

The WEM ER 15 has the volume before the first grid. Great amp by the way...

[Williamblake]

That WEM ER 15 has a rather unique phase inverter, hasn't it? Coupled at cathode and G2? Does this have a name? Thanks for posting this.

[sluckey]

It is an oddball but it's not a push/pull amp.

There is no phase inverter. V3 is a single ended amp driving half of the OT. The screen grid is connected to a B+ node and held at AC ground by C12 filter cap. V4 is identical. Both tubes share a common cathode resistor and are held at AC ground by the shared C22 bypass cap. V3 grid is driven by the channel 1 preamp and V4 is driven by the channel 2 preamp.

So, you basically have two identical single ended amps whose outputs are combined in the OT to drive a single speaker load.

This is so wrong. Sorry.

[Williamblake]

Thanks for the clarification. So 15 in WEM 15 probly was made up by the marketing department. But you could have maybe an output and a reverb driver without DC saturation using just one iron. A happy day for juggernauts this is.

Regarding the topic of this threat, there still is no answer to what a pre V1 volume control does what a guitar volume does not. Me thinks, an amp with just an on/off switch is a humble task to accomplish.

[sluckey]

After looking a bit more closely, I think my above explanation is completely wrong. Looks like V1A and V2A make up a fixed biasd LTP PI and V3 and V4 really are a typical push/pull amp.

[shaun]

Robinette, whose work holds considerable interest, has this design. There is something interesting going on with the pot before V1 - using it to divide voltage/attenuate signal, and also probably a high pass system. That input goes direct to a cold clipper stage, and having built one of those recently, they seem fairly sensitive and may benefit from the front-end control. Apparently, Trinity are selling this design.

[sluckey]

That's just the standard 2204 input arrangement. Input signal to the high gain lower jack, through V1A, upper jack, voltage divider, VOL pot to V1B.

The low gain upper jack simply bypasses V1A preamp.

[PRR]

V1A and V2A make up a fixed biasd LTP PI and V3 and V4 really are a typical push/pull amp.

Yes. It is just drawn funny, half upside down.

(lecture) When you read a schematic, think about the guy/gal who had to draw it. Should I put this part here? Over on the side? With this other related section? Or out in the clear with long connection lines? There's no rules. It should be readable but what is readable is a debatable point. And if I cram it all into the next 45 minutes I can still make Happy Hour at the tavern.....

[shaun]

That's just the standard 2204 input arrangement. Input signal to the high gain lower jack, through V1A, upper jack, voltage divider, VOL pot to V1B.

The low gain upper jack simply bypasses V1A preamp.

Ah yes. Silly me. Thank you for helping see that.