Mesa Boogie Tremoverb Bias MOD

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NippleWrestler
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Mesa Boogie Tremoverb Bias MOD

Post by NippleWrestler »

This is here for posterity as this information relating directly to the Tremoverb is impossible to find online so I worked it out for myself.

Usual caveats apply with tube amps: don't mess around in them if you're unsure. Discharge the capacitors before you poke around with a soldering iron. Don't touch anything you need to etc etc. If you're at all dubious then do not proceed, it isn't worth shocking yourself over (potentially lethally).

The Mesa Tremoverb, like all Mesa amps, is fixed bias. This bias is set through 2 resistors for either what Mesa dubs 6L6 or EL34 bias. The 6L6 bias mode uses a 22k, and the EL34 mode just brings in a 15k in parallel with that 22k, making the total resistance around 9k. We're going to put the bias on a pot for a range of 7.5k to 57.5k.

The schematic (stock) looks like this:

Image

The new bias schematic looks like this:

Image

New bias circuit allowing a total bias range of 7.5k (hotter than stock) to 57.5k (colder than stock). You can lower the 7.5k to something else but I wouldn't go lower than 5k.

This resistor works as a stopper which sets the minimum resistance value available, and also prevents amp death if the pot dies as the resistor will still be in place.

Image

B50k pot with 7.5k resistor in series (heatshrunk).

Note how the left lug is jumpered to the middle lug then a red wire is connected off of the centre lug.

Also note there is a black wire coming from the other end end of the 7.5k (1 watt) resistor which goes to where the 22k resistor used to live on the PCB. You should have 2 wires coming off of the pot.

Image

Disconnect red and black switch wires and heat shrink the ends. This takes the switch and the 15k resistor out of the circuit. For future reference the RED goes to TOP RIGHT, and BLACK to TOP MIDDLE.

Image

That stock 22k lives back here by the filter caps and power tubes. Desolder the 22k and install the RED and BLACK wires from the pot in its place.

Image

The 22k used to be here, by the horizontal 47uf and large orange caps.

Image

I removed the SLAVE jack and installed the pot in its place with a black MXR style knob I had. Upon testing in SPONGY and TUBE RECTIFIER mode I was able to record a sweep of 348v - 399v plate voltage. It works, allowing a broader range of tubes to be used.

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Re: Mesa Boogie Tremoverb Bias MOD

Post by JHorner »

NippleWrestler wrote: Sat Nov 20, 2021 2:38 pm also prevents amp death if the pot dies
Clarry replaced the resistor on my Mesa with a preset pot.

Now you're making me nervous.

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Re: Mesa Boogie Tremoverb Bias MOD

Post by NippleWrestler »

JHorner wrote: Sun Nov 21, 2021 5:28 pm
NippleWrestler wrote: Sat Nov 20, 2021 2:38 pm also prevents amp death if the pot dies
Clarry replaced the resistor on my Mesa with a preset pot.

Now you're making me nervous.
A preset pot is pretty much a resistor though.

But if that pot dies then the bias just goes to ground and that's bad. If the pot doesn't die you've got nothing to worry about but why take the risk for a 5c part. If you'd like a 1 watt 7.5k I have 9 spare.

Clarry worked on this Mesa too. He really fucked it up. The Rockshop fixed it in the end as they're the licensed Mesa techs for NZ.

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Re: Mesa Boogie Tremoverb Bias MOD

Post by murky »

Why jumper the two lugs on the pot? Incase the wiper dies/shorts?

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Re: Mesa Boogie Tremoverb Bias MOD

Post by NippleWrestler »

murky wrote: Sun Nov 21, 2021 5:59 pm Why jumper the two lugs on the pot? Incase the wiper dies/shorts?
Because that's how you wire a pot as avariable resistor, see the Rat tone control. Same deal.

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Re: Mesa Boogie Tremoverb Bias MOD

Post by RectifiedAmps »

That’s a great mod but aren’t you a bit nervous about having the bias adjust so accessible on the back of the amp? A recessed fender-style 50k bias pot might be safer but I’m probably just paranoid.

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Re: Mesa Boogie Tremoverb Bias MOD

Post by NippleWrestler »

RectifiedAmps wrote: Sun Nov 21, 2021 6:50 pm That’s a great mod but aren’t you a bit nervous about having the bias adjust so accessible on the back of the amp? A recessed fender-style 50k bias pot might be safer but I’m probably just paranoid.
On my own amp? Not a bit. It hardly leaves the house and is under a big metal bar that all rectos have. Plus, even if it does get knocked the range of pv is well within safety. These tubes can take over 450v all day long and they can't get above 400v on the settings I use (stock is around 365v).

But at the end of the day it's a pot on wires. It can go anywhere. 30 seconds with a step bit and there you go.

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Re: Mesa Boogie Tremoverb Bias MOD

Post by murky »

NippleWrestler wrote: Sun Nov 21, 2021 6:01 pm
murky wrote: Sun Nov 21, 2021 5:59 pm Why jumper the two lugs on the pot? Incase the wiper dies/shorts?
Because that's how you wire a pot as avariable resistor, see the Rat tone control. Same deal.
You could just wire to only the wiper lug though right?

Not being a dick, just trying to work it through in my head.

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Re: Mesa Boogie Tremoverb Bias MOD

Post by Jay »

murky wrote: Mon Nov 22, 2021 8:34 pm
NippleWrestler wrote: Sun Nov 21, 2021 6:01 pm
murky wrote: Sun Nov 21, 2021 5:59 pm Why jumper the two lugs on the pot? Incase the wiper dies/shorts?
Because that's how you wire a pot as avariable resistor, see the Rat tone control. Same deal.
You could just wire to only the wiper lug though right?

Not being a dick, just trying to work it through in my head.
Yes
When faced with quality, I recognise it every time.

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Re: Mesa Boogie Tremoverb Bias MOD

Post by NippleWrestler »

murky wrote: Mon Nov 22, 2021 8:34 pm
NippleWrestler wrote: Sun Nov 21, 2021 6:01 pm
murky wrote: Sun Nov 21, 2021 5:59 pm Why jumper the two lugs on the pot? Incase the wiper dies/shorts?
Because that's how you wire a pot as avariable resistor, see the Rat tone control. Same deal.
You could just wire to only the wiper lug though right?

Not being a dick, just trying to work it through in my head.
No worries. Yeah you can do that for sure, that's how the tubescreamsers gain pot is wired, but I don't like leaving things unconnected.

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Re: Mesa Boogie Tremoverb Bias MOD

Post by Optical »

Sometimes the wiper can disconnect from the resistor element inside the pot. Wiring then together is best practice if the wiper floating would cause a bad time

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