Carvin StageMaster (Vintage Series) II

[Runewalker]

Had an earlier thread on some strange things that were happening with a Carving 2X12 ValveMaster AKA StageMaster. Further research and communications with Hassebrock reveals this platform evolved into the current Vintage Series - Nomad, Bel Air, etc.

This is an update with an additional question.

The symptom that was odd was combined with a trashed Mid Tone pot. The Clean channel pot was irretrievably scratchy and when the strings were struck with authority a nasty transient surfaced that was louder and distorted, but not distorted in a pleasing way.

Finally purchased the pot from Carvin. It had a nomenclature which I think Unk thought he had snooped: 25A25K. But I just bought it from Carvin as it was too mysterious to go through guessing.

I opened it up, replaced the two Mid pots, and performed 3 of the Stage one Hassebrock mods:

Removed the diodes
changed 2 resistors to different values on the tone stack and
Performed the cathode bypass cap add-on

I did not convert the Presence control, as you also have to re-do the Cathode bypass, and I wanted to hear what the first one did.

Distortion in drive channel is now smooth, the tone is less muddy.

Interested next in the Master Vol and "Attitude" mods.

The question.

While the new mid pots cured the scratchiness on the old pot.... but....

There is still a nasty transient buzzy, electronic style distortion in the clean when the string is struck hard. It does not appear in gentle picking mode (am I even capable of that?)

So I thought I would start sub0ing in a new 12AX7 in each of the Channel one preamp tubes to see if that fixes the problem.

Any other ideas on what could cause that transient peak nastiness?

RW

[JohnH]

This might be one of those annoying problems which are a bit subtle and therefore hard to trace.

I'm posting the circuit that you pointed to last time:

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

Looking at Channel 1, there is input, tube V1b, tone controls, volume, tube V3b, then on to the power amp. How about a test to narrow the problem down?

If you turn the guitar volume down, and turn the clean channel amp volume up to compensate, does the buzz reduce? If so, then it maybe a problem in V1, because in this case, there is less signal getting to V1, but the same at V3. Similarly, does the treble control change the character of the buzz? - it is after V1 but before V3.

John

[UnklMickey]

hi Rune,

this is prolly gonna be a tough nut to crack

John's, advice seems pretty good.

the volume shift will give some good info, but the treble adjustment might not be as definite.

since you will be sending more highs to the latter stage, if that's where the problem is, that might also affect how it acts.

if the problem occurs in an earlier stage, it will sound different.

it might be difficult to be sure which, if the treble adjustment does change the character of the buzz.


you make it sound like the problem is isolated to the clean channel.

let's be sure.

adjust the volume of the dirty channel fairly high and adjust the drive control for the same volume that you had when the nastiness on the clean channel.

if you're now certain it's only the clean channel, that limits the possibilities.

sub%uting or swapping tubes is easy, so that's always an early place to check.


you might want to determine if the problem is mechanical.

determine the volume level required to cause the problem.

this is a combo amp?

disconnect the internal speaker, and connect to a cabinet.

cover the Carvin with some furniture pads to isolate it from the sound.

does that change the threshold of where it gets nasty?

if so, you may find that tapping in the right places will lead you to where the problem is.

of course we're talking tube amp here, so use something plastic to tap on things when you get inside, not your fingers or any other appendages.

it shares the same input jack, for both channels, so we can rule that out.

but on the relay, that switches between channels, the contacts are not common.

you could solder in a jumper between pins 4 and 6 of the relay to bypass the contacts.

also, contacts of tube sockets would be suspect, as well as poorly soldered joints.


if the problem is NOT mechanically affected, and the tube replacement doesn't fix it, i'd be more inclined to think capacitor.

you have 8 of them before the relay. you might spend days trying to figure out which one, or just replace them all.

at least the ones before or after the volume control, depending on the results of John's test.

i don't think that will break the bank.
i have seen resistors cause some weird distortions.

but it's rare, unless they have overheated and show physical damage.

the more likely candidates are R2,3,6,and7.


oh, and also, use your eyes.

i have seen some problems that we pretty obvious, if you paid attention.

the most interesting one was a bug that had crawled inside the chassis, and fried on the anode pin of a tube socket.

it didn't cause any problem, until the voltage swing was large enough to cause some arcing.

then it sounded reeeeeeeeeal bad.

well, that's my best deBUGging story. so good luck.


unk

[Runewalker]

Unk, that is actually a great little troubleshooting sequence. Thank you for all that effort.

After the tube replacement test I described at first, sub-ing out the phase inverter cleaned up the symptom. I now conclude it needs a new set of pres and powers.

The little mods worked nicely. The Carvin has a unique and rich clean channel. The drive channel has the typical indistinctness of multiple preamp gain stages. Pulling the diodes greatly sweetened the drive channel.

Now I have to re-program the modeler patched to accomdate the sonic footprint of this particular amp.

Lots of questions in the troubleshooting. Yes it is a combo.

But I will now plan for the tube job. However, I also think there are some tweaking opportunities in cap and resistor value selections. The first simple mods helped the drive channel immensely, but there is still too much mud.

Also, the sucker is LOUD. Put a Scholz PowerSoak on to open up the power tube side, but the PS colors the sound.

This whole concept of balance proves illusive.

Thanks to my friends Unk and John for the attention.

After the recent implosion, I feel compelled to behave a little better. Taht conversion should last until this afternoon.

RW