Metropoulos Forum

I've lost you. The ts calculator shows a impedance of 38 that is correct for a cathode follower, no? Where have you come up with this 500 number for the PI and how would that affect the ts frequency??

look at the output of the treble pot. EDIT: R5 should not be 517k but 2-4M

I can see what you mean where the slightly higher value pots only give you an extra 1dB, however in LTSpice this translates into an extra 4V on the signal off the treble pot going into the PI. I'll double check when I get home tonight but that could be noticeable.

Quite tempted to replace my Alpha pots with CTS so I can pull them apart and increase the value.

I'm getting ca 3db extra gain in the calculator, but I am not sure that the calculator is right in saying that the tonestack input impedance is 1.3k (or something like that) on a marshall and 56k on a vox. If it is 56k on a top boost vox, then it is 100k on a standard marshall since they use similar cathode followers (although the cathode resistors vary from 56k to 100k)

An option is using 50k alpha pots with a 120k resistor in parallell and 500k pots with 1.2m resistors in parallel

I think that impedance is right for a cathode follower. Not sure how different it would be for a Vox with a 56K resistor though.

I want it to look stock though, I have to be difficult Roe .

I'm sure I read that you're a fan of the Alpha pots. Do you not find their taper to be a bit linear in places? I think they're basically two linear tapers put together and I can definitely hear this around 8-9. I also think they come on a bit quick which is why I can't clean up the amp by turning the volume down or why I can't have the bass above 1.

I made a new thread regarding my JTM50 as this topic is about comparing clones to original Marshalls really.

Still interested to know what makes them different. Is it just the pots? As a different pot value can affect a wide frequency spectrum (think thats the right term) then it makes sense that I noticed a difference when George played big open chords, but next to no difference when he was playing single notes.

I also wonder why Marshall used log pots for the Treble and Middle instead of Linear. Maybe someone screwed up an order and they had 100 pots to get through? Maybe they didn't know?

Alphas audio pot have a ca 30% taper, if I remember correctly. It is difficult to see small 120k/1.2M resistors in parallell to the pots

The ones I've measured are 15% taper I think? Then again, are there different Alpha pots?

I think even the one 1M3 volume potentiometer would be considerable in its effect on the cutoff frequency after v1, especially with any variance from stock values in the plates or coupling capacitors. With .0023, 101k, and 1M3 this would put your cut off frequency at 738.7hz; that's close to a 40hz lower difference from stock values! alternatively .0021 and 99k puts this well above 800hz.

As we all know, the guitar's dominant frequency range is smack dab in the midrange. A lot of cutting frequencies added here when mixed in with the whole sound.

yes, you might use a 2.7 or 3.3nf cap instead

Do the Mustard Caps have quite a wide tolerance then?

I have a bunch in front of me and the all are very tight tolerance, but some vary by about +-.001uf, or .0001uf, enough to make the possible noted addition of valuable midrange frequencies in a stock amplifier.

At .0021 it with stock pot and Plate it puts the corner at 834hz! Over a 100hz available swing on available frequencies in some of the most dominant guitar frequencies, especially for notes above the 12th fret. I think this stock variability, along with the tonestack eq curve, is where much of the amp dialing is to occur.

What about the .022uF Caps?

I was referring to both. I have found more measuring .023 than .021 in my stash. Most of mine measure somewhere between .022 and under .023. These in between values are just as significant when factoring exact frequency response.

The more you think about it the more you realize how modern part clones subtract these unintentional additional frequencies. An under spec setup, as what would inevitably occur often times with the modern parts I have found, say 925k pot with 99k Plate and .022uf and lower on the coupling gradually moves the corner up to as high as 841. Change the pot to say 890k, like I have seen before in part variance, now you are 845hz.

You are basically exponentially cutting out the fundamental frequencies below the high E string 17th fret if you are wondering where all your balls and gain went...

You could also think of it all in terms of specific notes instead of hz, at 738hz you are cutting below almost exactly at F#. At exact spec stock 784hz You are cutting off below almost exactly G. At 841hz you are cutting off pretty much below A, slightly above Aflat. Possibly helpful to consider when dialing the circuit for different styles and tunings of guitar.

whopperplate wrote:I have a bunch in front of me and the all are very tight tolerance, but some vary by about +-.001uf, or .0001uf,...

+ 1 on the tight tolerance