Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => EZ Board postings => Topic started by: Geezer on June 27, 2005, 07:29:25 pm
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eddiejg
Senior tube assistant
Posts: 237
(3/1/04 11:49 am)
Trainwreck Modified Cross-Line Master Volume
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Over the weekend I got a chance to try the master volume used in the above amp & it worked very well with the AA1164 I just built.
I also tried it with a Matchless Spitfire I built with less exciting results.
It seems to cut the treble in that circuit.
I'm curious if anyone can explain what the .22uF cap does.
Also, can anyone explain why a 270K resistor across the 1 meg pot terminals is used?
Why not just use a 250K pot?
I'm hoping someone can give me some insight into how this works so I can use more than trial & error to try to modify the circuit for better results with the Spitfire.
Thanks.
-Ed
eddiejg
Senior tube assistant
Posts: 239
(3/2/04 4:10 pm)
Trainwreck Modified Cross-Line Master Volume
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Let me try to stir up a little more interest / response on this.
Let me ask 1 question & make it a more general question.
In the circuit I'm referring to, there is a resistor across the 2 terminals of the pot, wired as a variable resistor.
I've done this with guitar wiring sometimes because I've read that a 500K volume pot with a 180K resistor across 2 terminals makes the pickup "see" ~330K.
I don't understand the theory behind this.
In the amp circuit, it's a 1meg pot with a 270K resistor, & the measured resistance across the 2 terminals goes from 0 to ~220K.
Any insights?
Thanks.
-Ed
dazco6550
Junior tube assistant
Posts: 30
(3/2/04 4:50 pm)
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I think you have confused the matchless "cut control" with the master. Both are wired exactly the same except that the cut control uses a cap in series with the pot. W/othe cap it would become a master volume that shouldn't affect highs an
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W/othe cap it would become a master volume that shouldn't affect highs any more than one would expect from turning the volume down.
HotBluePlates
I only work on Fender's
Posts: 696
(3/2/04 4:55 pm)
Cross-line MV
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Unless so large a cap were used that it affected all frequencines, and was used for DC blocking to prevent the control from sounding scratchy.
eddiejg
Senior tube assistant
Posts: 240
(3/2/04 5:06 pm)
Cross-line MV
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Dazco-
The Spitfire MV I'm referring to has only the pot wired between the 220K resistors & only affects volume (exactly like you said).
The Trainwreck has a .22uF cap inline with the pot & a 270K resistor across the 2 pot terminals.
HBP-
Is a .22uF sufficiently large enough to affect all frequencies?
And what is the deal with using a 270K resistor across the terminals?
Any different that using a 250K pot?
Thanks guys.
-Ed
HotBluePlates
I only work on Fender's
Posts: 699
(3/2/04 5:24 pm)
Cross-line MV
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Whether .22uF is large enough depends on the other circuit values in that part of the amp. What I mean is what RC time constant is formed between the cap and the grid leak resistors.
The only reason I can think of for using the resistor across the pot is for approximating a smoother taper. I have seen such connections, where the resistor was referred to as a "law-faking resistor", meaning logarithmic or audio law. Maybe using a 1M and resistor has a smoother, slower taper to the control.
There are other ways of doing a cross-line MV that are simpler. Where did you copy the circuit from? Maybe there are specific reason for using that particular circuit and resistor/pot values in that amp.
eddiejg
Senior tube assistant
Posts: 241
(3/2/04 11:00 pm)
Cro
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eddiejg
Senior tube assistant
Posts: 241
(3/2/04 11:00 pm)
Cross-line MV
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HBP-
I think you are on to the reason for the resistor (and the reason for my experimentation & questions).
A simple cross-line like in the Matchless Spitfire (just a 1 meg pot) is simple & effective, but it lacks a smooth taper- 1/4 turn is nearly full volume, after that it's a little more grind & not much more volume.
The modified version I got from the Trainwreck Express (version A5h), and it smooths out the taper.
The modified circuit works very well in the AA1164, which also sports a split-load driver, but doesn't work as well in the Spitfire with a long-tail pair.
At the same time, a simple pot cross-line with a smaller value pot (250K or 500K) also helps smooth out the taper.
Is there an advantage to using the larger value pot & combining it with the resistor (as opposed to just a pot)?
-Ed