Volume box for HRD - anyone added a capacitor?

Philtre

I'm putting together a volume box for a HRD to put in the loop to tame the output volume. Basically it's an input and output jack wired via a 100K pot in an enclosure (see http://stinkfoot.se/archives/2159).

I heard that adding 100pF cap across two lugs on the pot (treble bleed) will brighten the sound, which might be handy for my Victory V40. Anyone added a cap? Did you hear a difference?

Danny1969

High frequencies pass more easily through a cap than low frequencies, the equation being Xc (Reactance)  1 / (2 x Pi x F x C ) where Xc is found in Ohms, F = frequency and C the value of the capacitor. So a 100N has a Reactance of around 1.5K Ohm at a frequency of 1K (treble) but a much larger 15 ish K Ohms at 100Hz (bass)  .........this is basically how we use caps to pass or shunt high frequencies to ground so yeah you can use a treble bleed cap in the loop but to be honest a lack of treble isn't generally a problem on an HRD, lack of bass maybe for some !

Note we say the Reactance in Xc rather than the resistance because a capacitors opposition to current depends on the frequency of the current  .... a resistors opposition to current isn't bothered by the frequency 

Philtre

@Danny1969 Thanks for the info. As I said, I was thinking of using it also in the loop of a Victory V40 which is a dark amp.

A side issue - I've been experimenting by inserting a volume pedal in the loop of the HRD. Now that I've taken it out of the loop I notice that there is guitar signal coming through on the clean channel when the amp's volume control is at zero (well, "1"). Never had that before.

Danny1969

Ah sorry missed the bit about the V40, yeah it might help but the high frequencies need to be present in the first place, basically google high pass filter which in the simplest terms is just one cap and one resistor 

Yeah I've seen that zero volume bleed happen on a Blues Deluxe, I assume the pot doesn't get down to zero Ohms on the wiper due to bad contact of the wiper on the taper or corrosion but I expect @ICBM knows exactly what causes it  

Philtre

Thanks.

Volume seems to have bleed between 1 and 1.5 now. "Proper" volume kicks in at 1.5. Never noticed that before. Seems like a half-dead spot on the pot.

ICBM

Danny1969 said:

Ah sorry missed the bit about the V40, yeah it might help but the high frequencies need to be present in the first place, basically google high pass filter which in the simplest terms is just one cap and one resistor 

Yeah I've seen that zero volume bleed happen on a Blues Deluxe, I assume the pot doesn't get down to zero Ohms on the wiper due to bad contact of the wiper on the taper or corrosion but I expect @ICBM knows exactly what causes it  

It can be that, but also sometimes signal bypasses the pot via stray coupling somewhere, usually through the power supply chain. If that happens from one stage to the next which are out of phase, you quite often get some sound when the pot is at 0, then a dip at about 1, then louder as normal after that.

I agree that a treble pass cap might not work that well with the V40 - the one I heard seemed to have virtually no top-end at all, as if it had all been deliberately rolled off.

Philtre

ICBM said:

It can be that, but also sometimes signal bypasses the pot via stray coupling somewhere, usually through the power supply chain. If that happens from one stage to the next which are out of phase, you quite often get some sound when the pot is at 0, then a dip at about 1, then louder as normal after that.

I was getting some (fuzzy) sound at zero (actually "1" on a HRD), then a dip then normal. I swapped out v1 and it's OK now.

(One day I'll source some reliable 12AX7s ;-) )

ICBM

Philtre said:

I was getting some (fuzzy) sound at zero (actually "1" on a HRD), then a dip then normal. I swapped out v1 and it's OK now.

That was stray coupling between the two halves of the valve itself.

Philtre

ICBM said:

Philtre said:

I was getting some (fuzzy) sound at zero (actually "1" on a HRD), then a dip then normal. I swapped out v1 and it's OK now.

That was stray coupling between the two halves of the valve itself.

Thanks once again for the info. I appreciate it. What is "stray coupling"?

ICBM

Philtre said:

What is "stray coupling"?

Some signal from one part of a circuit is getting into another part that it’s not directly connected to.

There are various different ways, including capacitance between closely-arranged wiring, unwanted resistance in a common ground connection, imperfect filtering in the power supply, etc. This one must have been in the valve itself since changing it fixed it.

I’m not sure what the exact mechanism in the faulty valve was, but the result was that a tiny bit of the signal from the first gain stage got into the second. When the volume control which is between the two is set just-so they more or less cancel out, because they're out of phase. Below that point the spurious signal is audible (because the genuine one is turned right down), and above it the genuine one dominates.

Hope that makes some sense!

Philtre

@ICBM Thanks for the detailed reply, I appreciate it. :-) Yes, it makes sense, particularly as the volume control is between the two gain stages in v1.

Philtre

Here's the volume box for the HRD. Didn't bother with a treble bleed cap. If anything, I'd say there's some bass loss using it.

Philtre

A question for @Danny1969 or @ICBM

In this YT video Brian Wampler shows how to make the box. At 15:14 he explains about adding a 100pF cap across lugs 2 and 3 of the pot. This effectively puts the cap across the input and output connections as per this layout:

I tried it but it doesn't seem to make any difference. Is it correct?

ICBM

Given the low impedance of the send and return, and especially if you used quite a low-value pot, it may make very little audible difference. You could try a larger cap if you want it to.

But there really is no need, the HRD loop is very well-buffered and doesn't suffer from any significant tone loss, probably even if you use miles of cable.

Philtre

@ICBM Thanks. I tried a 220pF and it seems to be a tad brighter. But then I might be imagining it. Won't do any harm. ;-)

SteveF

Must get round to making one of these - The lion tamer I bought from @Danny1969 is excellent, but it does mean I can't use my effects loop. 

ICBM

SteveF said:

Must get round to making one of these - The lion tamer I bought from @Danny1969 is excellent, but it does mean I can't use my effects loop. 

Do you use the drive channel on the amp? If not there's very little point in using the FX loop. Only asking since almost no-one seems to use it .

SteveF

The drive channel?  God no, it's awful!   

I was just thinking of using it for ease of switching my board between amps. It's wired in pretty tight at the moment and getting to the input of the first pedal in the FX loop section of my board is a pain without lifting the velcro.  I should buy a board with more space really! 

SteveF

Mine is a Blues Deluxe btw. Apart from the tweed, what’s the real difference? 

ICBM

SteveF said:

The drive channel?  God no, it's awful!

Thought so . I actually like it a lot .

SteveF said:

Mine is a Blues Deluxe btw. Apart from the tweed, what’s the real difference? 

Quite a bit. The drive channel isn't a separate channel, it's a boosted mode on the clean channel - lower gain, in fact it barely breaks up unless it's maxed - and the clean channel itself is slightly 'hairier'. The whole amp is voiced a bit differently too, a little looser and slightly more scooped.

If you've got the Blues you're really missing a trick by not using the drive channel, even if you don't really use it for more dirt - it's ideal as a solo boost. But in that case it still doesn't need the loop to put the FX in, since it won't have much more gain than the clean channel.

SteveF

Thanks @ICBM I'll have to explore the drive channel a bit more then.  I found it quite... I'm not sure of the right word... maybe nasal is the best description when I tried it, but to be fair I haven't played with it a huge amount.  I tend to use the clean channel with a pedal for some gain.  I love the sound of the clean channel.  For more gainy stuff I tend to use my Sheriff at the moment.