Pot specs for Carvin amp

[JohnH]

Runewalker and I have been discussing this old Carvin Valve-Master amp, which needs some new pots due to scratchiness which has not responded to cleaning. Here’s the schematic:

www.carvinmuseum.com/pdf/amps/ValveMaster.pdf

The questions we have are how to interpret the pot specifications. Take for example, the Channel 1 Tone controls:
• Volume 1 is P1: B100K
• Treble is P4::B1Meg
• MId is P3: 15A25K (this one is a puzzle)
• Bass is P2: B1Meg

I would have expected that the ‘B’ types are linear, its just that these pots in this type of circuit (Baxandall) I have only seen as log before. The Mid control, I’m guessing is 25k log, but second opinions please. What would the 15 prefix signify, in 15A25k?.

Elsewhere in the circuit, there are similar questions, for example, the drive control on Channel 2 ( Pot P5) is noted 5A500k. Is that a 500k log? What is the prefix 5?

The next issue is power rating. Physically, these pots are 16mm pcb mounted. Are any special power or voltage ratings needed for these pots, or will standard 16mm types from Mouser do? It seems to me that these pots do not have significant dc voltages across them, but I am not sure what power rating spec is normal.

Interestingly, the tone controls on this amp are not the usual B/M/T stack as on Fenders and Marshalls, but are based on a passive Baxandall or James design., but with the addition of a mid control. I tried putting these values into the ‘Tone Stack Calculator’, from Duncan Amps (a great free software gadget), and could not get a level response at mid settings with the stated values, but log pots for the bass and treble seemed better than linear ones.

with thanks
John

[Runewalker]

There are a couple of other issues with this amp (StageMaster version of the ValveMaster --- has twin 12s):

On the clean channel, the scratchy mid pot syndrome goes beyond what I have experienced in dirty pots. It makes a Pteradctl like screech and when moving it the scratchyness will 'notch' so that the amp becomes much louder, and almost overdriven, or sort of muffled and quiter.

It sounds sort of what this post at the Carvin forum describes:

www.carvin.com/carvinbbs/viewtopic.php?t=4361&highlight=replacement+pots

But not quite, in that it is not as dirty and gritty as the dirty channel.

Any thoughts?

Thanks for your help.

One other thing about the schematic, I believe this is version II of the VM which had a mid boost circuit and an additional presence control The amp I am working on has one presence contriol and no mid-boost, so the mids are not Push/Pulls.

(when working right the clean is quite articulate and sparkly especially through a set of JBLs I am experimenting with)

[UnklMickey]

johnh said:

...Interestingly, the tone controls on this amp are not the usual B/M/T stack as on Fenders and Marshalls, but are based on a passive Baxandall or James design., but with the addition of a mid control. I tried putting these values into the ‘Tone Stack Calculator’, from Duncan Amps (a great free software gadget), and could not get a level response at mid settings with the stated values, but log pots for the bass and treble seemed better than linear ones.

with thanks
John

hi John,

i can't help with those designations on the "mid" controls.

(also, i don't know exactly what those controls will do, but i'm fairly certain, calling them midrange controls would be a mis-nomer.)

in general, you can confirm or refute your assumptions on what A and B mean for the pots in this amp.

since your treble controls have capacitors on BOTH ends, dc resistance measurements will be unaffected by the other circuitry.

so set the treble control to it's mid-point and measure between the wiper and either end.

if it is a linear, the resistances will be approximately equal.

if audio taper there will be a huge disparity.

it wouldn't surprise me if you got a frequency response curve that was not level when the controls are at their mid-points.

Fender / Vox / Marshall controls have a big "scoop" in the middle, when their controls are set to the midpoints.

so, Carvin may have intentionally offset this circuit to have a similar scoop, rather than a flat response at the mid-point settings.

my guess is, it will still have a "hump" in the midrange (the one you can't get with a typical tonestack), when the T and B are set to minimum?



unk

[ChrisK]

I'm going to go out on a limb here since it's been some time since I looked into this stuff.

Typically, an "A" curve is audio (log) and a "B" curve is linear. the numbers before these letters may specify which curve variation within the letter catagory, for that manufacturer. There are different curve types since the pots are made with deposition layer thickness variations and tend to be somewhat piecewise approximations of an ideal curve.

The capacitor values in both the treble and bass sections are not related by the usual ratio, which is 10:1. This would indicate to me that linear pots may well have been used.

This tone control topology is much better than the Fender one, but still suffers from noise. An active implementation of this topology does substantially reduce noise, and does use linear potentiometers.


Again, I'm guess'membering since I can't even remember which city holds my 1980 National Linear Applications Handbook with the most excellent "floobydust" section.

Do you know the pot vendors name?

[UnklMickey]

hey all,

i've discovered why the weird designation (15A25k) on the "mid" pots.

look at revision (K) in the legend!



unk

[JohnH]

OK, I guess the 15A25k was a bit of sloppy drafting, for a revsion from 25k to 15k. Good luck finding a 15k pot from Radio shack! When I ran the clean tone circuit through 'Tone stack calculator', it did give a mid hump, with treble and bass at zero, but this turns to level by about 0.5, and a big mid scoop everywhere else. I reckon that the mid control is not a critical value, because it is only connected to two terminals, hence a larger pot can do everything a smaller one can do. It might be good to have a 50k pot, to get more range - it wouldnt lose any tone that a 15k pot or 25k pot can make.

There is one of the questions left that is important and we really need some advice from those who, unlike me, have tinkered with tube amps. Its to do with the power rating on the pots, which are 16mm. I would think that standard ones are fine, but wouldnt want to get that wrong in a tube amp if it is important. Opinions are politely requested!

John

[Runewalker]

With a little more research and contact with a Carvin 'expert' I have discovered more about this amp.

There were 2 versions of the Valve Master. The one I have has 1 presence control, and no push'pull boost circuit on the mids. The VM-II has 2 presence controls and a boost circuit on the mids with more circuitry changes. Subsequently we have been looking at the wrong diagram. Whupps.

The correct one is correct because it is under a different mark. The VM I have was turned into the Bel Air/Nomad Vintage Series.

www.carvinmuseum.com/pdf/amps/112%20Nomad%20212%20Bel%20Air%20REV-E1.pdf

Carvin has the parts for the Bel Air that fit this VM, so that will not answer the technical issues raised here, but will source the parts.

There are a number of mods that are relatively easy to do that will hopefully improve this amp. The clean circuit is pretty nice. The drive circuit gets muddy and square-wavy. Like Marshall in the 80's and even now, the circuit contains clipping diodes, fine for a stompbox, but not a tube amp. So D1-4 have gotta go.

The guy on the web with the most material on these amps is named Hassebrock and he has worked through several mods to bring out the inherent potential in the drive circuit.

[UnklMickey]

runewalker said:

...There are a number of mods that are relatively easy to do that will hopefully improve this amp. The clean circuit is pretty nice. The drive circuit gets muddy and square-wavy. ...

Rune,

this is completly off the cuff, but......

i agree the diodes need to go.

you might not have enough gain to get sufficient distortion in the tubes to replace the cheezy distortion you got from the diodes.

this design doesn't have the cathode bypass caps on the second and third stages like the other design does. you might need to add them.

also the 22k resistor linking the bass and treble sections in the dirty channel seems too small.

this might cause some of the muddiness, and make the controls more interactive with each other.

you might even try changing all the values in the dirty channel's tone controls to match what is in the clean channel.



as i said this is all off-the-cuff. so take these thoughts with a liberal dose of salt.


unk

[sumgai]

John,

When working with low-level audio signals, we don't worry about power, as power is normally defined (amps x volts = watts). The very term "low-level" refers to the voltage being amplified from perhaps 10's of millivolts to maybe 10 volts, perhaps a bit more. But in those same preamp stages, the current is never amplified along with the voltage, it would be lucky to exceed 40 or perhaps 50ma, up until the driver stage, which is usually also the phase inverter.

As it happens, the material found inside of even a small pot is quite a bit more than you'd find inside a standard 1/2 watt carbon resistor. Ergo, there should be no "power" concerns.

Usually when a variable resistor (a pot) is meant to handle higher voltages or current, it will be labeled as such with a power (or wattage) rating. For most manufacturers, that rating usually starts at 1 watt. They'll be bigger units too, due to the fact that current, voltage and resistance go hand-in-hand to generate heat!

Beyond that, there's nothing left to tell. That's my story, and I'm sticking to it! ;D


sumgai

[UnklMickey]

sounds like a good story to me.

although the small signals are often riding on hundreds of volts (D.C.), the DC is almost always blocked by a capacitor before the pot.



unk

[sumgai]

unk,

sounds like a good story to me.

although the small signals are often riding on hundreds of volts (D.C.), the DC is almost always blocked by a capacitor before the pot.


unk

Well, that is a large offset, to be sure. As you surmise, proper design philosophy dictates that blocking capacitors be mounted as close to the loading device as practical (in this case, the tube's plate). However, you do raise a point in the realm of safety.

Should a renegade DC voltage suddenly appear on a pot's resistive element, and the pot is not made for dissipating the heat thus generated (250 volts x 0.050 ma = 12.5 watts!), one could expect a pretty stinky meltdown to occur, probably in about the same time that it took me to write this. The casing may remain intact, but it's a sure bet that the resistive element will have violated the Environmental Protection Agency's Clean Air Act guidelines!

It has been my experience that protecting against that possibility would cost grossly more than simply fixing the damage, should the need arise. Could you see every control in nearly every tube amp being made to withstand 15 or 20 watts? Like Neo said, "Whoa!"

John, the story stands, as edited by unklmickey! ;D


sumgai

[Runewalker]

So what I am getting out of these geek riffs (said with love and affection of course) is just use the same type pots (with PCB pins instead of solder lugs) as you would in guitar harnesses and "fo'getaboutit."

Here is the link to Hassebrock's mod site for Nomads/Bel Airs/Vintage and now VM-Is:

www.geocities.com/rlhassebrock/nomaddioderemoval.html

I believe his Stage I mods go in the direction suggested by Unk, then the Stage II does a combination of component value shifts towards both the clean channel's best aspects, and in the direction of old style Marshalls. I will at least do the Stage I mods. Not really much detail on the Stage II stuff other than a general description.

One question I have for this group if you look at the mods: I have a Master Vol on my 70's Twin brain, that is basically a crap-o=meter control. It sounds horrible unless either bypassed or run full out. So Hassebrock has a Master Vol mod. Is it more of the Twin crap-o-control or really something of merit. He puts it post Phase inverter.

If of merit, it would be worthwhile as this amp is LOUD.... even for the big sky of Texas.

Finally the 'Attitude' control looks intriguing, especially if it has any splashover to personal attitude adjustment.

[UnklMickey]

runewalker said:

...Is it more of the Twin crap-o-control or really something of merit. He puts it post Phase inverter....

...Finally the 'Attitude' control looks intriguing, especially if it has any splashover to personal attitude adjustment.

hi Rune,

the fact that he's putting it after the PI gives it a much better chance of being useful. but no guarantees.

the attitude control just dials back the amount of negative feedback.

Negative Feedback helps keep the sound pristine, even when the power tubes or PI begin to distort.

so the attitude control will be REAL handy.

it also will act as a volume control (well ..... sorta).

when you dial back the NF, the volume of course, increases.


that guy seems be pretty sharp.

and he has been experimenting / tweaking values in places that i would have guessed needed attention.

so it looks to me, like this is all good stuff.




unk

[JohnH]

Heres my thought on the Overdrive channel:

Reading about these Carvins, one comment seems to be that the OD channel is 'muddy'. Setting aside the diodes (to be removed and replaced with a suitable pull-down resistor), there seems to be no significant tone shaping in the signal anywhere in the first three stages.

My playing around with JFET overdrives leads me to think that the best OD sounds come from emphasising treble and de-emphasing bass, early in the chain, before distortion. Then the treble gets boosted until it spits and sizzles, and the bass does not make a loud farting noise.

In the Carvin, the tone shaping is all after the three preamp tubes, by which time the die is cast, the sound character of the preamp is set and all the tone controls can do is tweak it.

A Cap or two, could make this adjustment, EG a 39nF to 82nF cap across R10 might make the second stage effectively into a treble booster - with a consequent kick in the pants.

John

[UnklMickey]

John,

that might work, but decimating the lows is not always the best approach.

that can sometimes lead to "fizziness".

i bought a couple of the Pignose G40v amps before Pignose got greedy.

they sounded like $#!+ out of the box.

nothing even resembling a bottom end.

they had a partially bypassed cathode resistor.

one of many things i did, was to un-bypass the cathode resistor.

this amp had WAY too much gain available anyway.

that cured a lot of ills.

where and how the "fartiness" is occuring is key to solving the problem.

if it's related to intermodulation distortion, (and the difference frequencies are the culprit) occuring later, then the approach you suggested could work, but i'd be concerned cost might be high in terms of tone shaping.

sometimes minor adjustments in the size of coupling caps is all that's needed to move the LF roll-off frequency up..........just enough to get rid of the "gas", without losing the "breath".

distorting a bright, harmonically rich signal is not always the best tone.

ask E.C.

for single notes, one tried and true method is to get rid of everything but the fundamental, and generate harmonics (preferably even) through distortion.

there are a number of different approaches. none totally bad. none completely wonderful.

i'm certain we can agree on 86ing the diodes.

lots of little things, can make a big difference.

you have to resist the temptation to attack the problem from a single direction.

or at a single stage.

fortunately this problem seems to be localized to the dirty channel, so any changes in that section won't affect the clean channel.



unk

[Runewalker]

Heres how Hassebrock addresses the cathode bypass caps:

members.dslextreme.com/users/rlhasse/CarvinMods/NomadCathResBypass.pdf

and how he addresses the issues raised by John and Unk on frequency emphasis in the tone stack:

members.dslextreme.com/users/rlhasse/CarvinMods/NomadToneStackMod.pdf

It has the Tone Stack Calc graph on before and after resistor subs, although he has since changed the values of the subs to:

100k for R15 & 47k for R16.

[Runewalker]

unklmickey said:

distorting a bright, harmonically rich signal is not always the best tone.

ask E.C.

Eric, if you keep wanting to sound like you did with Cream, then dump the mid boosted strat, and go back to your LP and 335. Really, man.

That mid-boosted strat sounds so fake.

[Runewalker]

"...fortunately this problem seems to be localized to the dirty channel, so any changes in that section won't affect the clean channel.... Unk

The clean channel on these amps is pretty remarkable, rich and articulate with a roundness that is hard to find. Better than my Twin Brain.

There is a minor issue there however. You have to be very carefull with the treble control. There is a upper strata of ice-pickiness that could use some component adjustments. I've shared that with Hassebrock and he concurred, but his efforts have been targeted to fixing the drive channel issues.

Right now I attack it with and EQ, but it needs another solution.

[UnklMickey]

runewalker said:

...Eric, if you keep wanting to sound like you did with Cream, then dump the mid boosted strat, and go back to your LP and 335. Really, man.

That mid-boosted strat sounds so fake.

they say long exposure to high volume levels can lead to high frequency hearing loss.

E.C. played real loud, for real long.

so, maybe Eric's HF loss extends all the way down into the mids.

and he needs the mid-boost just to make it sound "normal" to him.


LSH...



unk

[UnklMickey]

johnh said:

...a 39nF to 82nF cap across R10 might make the second stage effectively into a treble booster - with a consequent kick in the pants.

John

hi John,

while i still have reservations about doing this, exactly as you suggested (the increase in gain at the higher frequencies will probably be too dramatic.), it did get me to thinkin'.

i have a couple thoughts on similar possibilities.

bypassing the cathode fully, but with a somewhat smaller than normal capacitor. if the cap is selected so that the "knee" occurs at about 80Hz, any IM generated difference frequecies below 80Hz will be reduced.

another way to do things would be to use a small cap in series with a resistor.

putting that in parallel with the cathode resistor might add just a bit of sparkle, without being too in-your-face obvious.

of couse much of this depends on what how much gain is needed, and where, to get some distortion, now that the diodes are history.



unk

[JohnH]

Yep, those caps I noted are quite small in relation to that resistor, giving a knee that just boosts the treble above the range 400 to 800 Hz (depending on value). A resistor as you suggest can tame the top end a bit. Post distortion, the tone controls can then tke the edge off - to restore smoothness. Variations on this theme bring forth many colours of smooth overdrive

J

J