Running the Crack without a volume control

So, I've built my Crack and I'm loving it. I have the Speedball upgrade built and waiting to install after I get some listening hours on the basic unit, but my thoughts are turning back to this question I raised before I ordered the kit.

Rather than upgrade my Crack too much, I think I'm going to order a Carackatwoa and upgrade that, as according to everything I've read, an upgraded Crack will never better an upgraded C2A and I want to stick with OTL. That said, I still want to do away with a volume pot in the amp.

Listening tests in my main system, using the previously mention TVC as a stepped attenuator between source and amp has shown a night and day difference in sound quality in favour of the TVC as volume control - as one would expect given the price of a good TVC vs a carbon track potentiometer. So, I plan to change the pot on the crack and have it as a portable headphone amp, while the C2A will always be used in the main system with the TVC.

Now, thanks for your patience. The question is, will it be sufficient to just drop a 100k resister from the grid of the input tube to ground and the signal straight to the grid. That's effectively what's happening when I run the Crack-pot (lol) wide open, so my thinking is that this would be fine.

But... In many similar tube amps schematics, I see a 1k resistor in the signal path between signal and grid (part of a low pass filter?) and a 1m resister to ground, often as well as a 100k-250k pot where one is present. Can anyone tell me why this might be necessary or desirable. I understand that the 1m resistor is giving the grid of the input tube a reference to ground, presumably in the event that the tube should fail, but isn't the 100k resistor (or pot carbon track) also providing this? Why do some amps have both?

All answers and thoughts gratefully appreciated.

-- Steve
 
Just FYI we tested and rejected TVCs when we developed the stepped attenuator used in the TwoQuiet and our other premium kits. Not saying you won't like TVCs. Just saying it will sound different than what we decided we liked best.
 
Thanks Paul/Doc.

I understand that a simple wire change on the pot will effectively hardwire the signal to the grid, bypassing the pot. The question was more about the presence of a commonly seen setup in tube amp schematics, using a 100k pot and/or the other two resistors mentioned. I just wondered why circuits are often designed with those additional resistors and what they might do (or not) for a circuit such as this.

TVC vs stepped resistor attenuation is another matter altogether really, and I'm sure results vary greatly depending on the quality of the transformers used, as it must be with the quality and matching of resistors. It's undoubtedly cheaper to build a good resistor based attenuator than a good transformer based one. I have a very good TVC - the Music First Audio Baby Reference - and for my setup, it's the better option for me vs any other kind of volume control as it sounds great and is also my preamp for input switching.

Just curious about the use of the other resistors before the input grid on amps of similar design.
 
CrackMeUp said:
But... In many similar tube amps schematics, I see a 1k resistor in the signal path between signal and grid (part of a low pass filter?) and a 1m resister to ground, often as well as a 100k-250k pot where one is present. Can anyone tell me why this might be necessary or desirable.
The 1K resistor in series with the grid is a stopper and we haven't found them to be particularly necessary with a 12AU7.  The extra grid to ground resistor keeps the tube happy if your pot fails.  You could certainly add these resistors from each grid to the center post of the 9 pin socket, but if the pot wiper fails you'll get no sound and not really a whole lot else.  In some other circuits, loss of a pot wiper can cause significant damage that's definitely worth preventing!
 
We used Stevens and Billington TX102s. Essentially the same TVC you are listening to.
 
Paul Birkeland said:
The 1K resistor in series with the grid is a stopper and we haven't found them to be particularly necessary with a 12AU7.  The extra grid to ground resistor keeps the tube happy if your pot fails.  You could certainly add these resistors from each grid to the center post of the 9 pin socket, but if the pot wiper fails you'll get no sound and not really a whole lot else.  In some other circuits, loss of a pot wiper can cause significant damage that's definitely worth preventing!

Thanks very much Paul. That's just what I wanted to know. So the 1m is a kind of backup to save damage in the event of tube failure and/or pot wiper failure. Got it and much appreciated.


 
Doc B. said:
We used Stevens and Billington TX102s. Essentially the same TVC you are listening to.

Yes, much the same, but the RX63 I believe in the reference models. The main point for me in using what I have is that (apart from sounding very good indeed in my setup) iThe TVC preamp is part of the system the Crack or eventually the C2A will join, and there's no point adding more resistors to the signal path, pot or stepped resistor attenuator. Thank you for the information though.

--Steve
 
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