Bias tweaker or "touch"

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Andy Le Blanc
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Location: central Maine

Bias tweaker or "touch"

Post by Andy Le Blanc »

There are many ways to apply tremolo in an amp.
One way is to modulate the grids of the power tubes.
But the trade off becomes bias stability of the power tubes.
With this in mind I was looking for a trem that was less impactfull.
I found this intensity control in some old magnatones with cathode bias

Its self explanatory way to get the output of a LFO to modulate the power tube grids.

But I noticed that in the absence of the LFO signal the bias current
measured over a 1ohm at the test point was varied with the
47k and 220k setting a high and low bias currents.

For the amp that I was noodling with this was pertfect.
It gave a variable cold to hot adjustable control.
A bias based "feel" control for cathode bias.
lazymaryamps
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jjman
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Re: Bias tweaker or "touch"

Post by jjman »

I'm not understanding why that adjuster would affect the idle current with the LFO off. Maybe some DC is leaking thru the LFO coupler?
If it says "Vintage" on it, -it isn't.
Andy Le Blanc
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Location: central Maine

Re: Bias tweaker or "touch"

Post by Andy Le Blanc »

Confirmed... And it works.

The cathode bias sets the minimum or cold condition.
And a positive fixed voltage is applied at the grids to
raise the bias to a hot setting.

So instead of LFO signal varying the bias condition
It becomes a user contol. Anti bias.

This happens as an amp ages as well. Old amp tone for those
with that type of trem.

Arguments against such a control?
lazymaryamps
shane
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Re: Bias tweaker or "touch"

Post by shane »

Yes should work fine. There's more than one way to do it too.
Have a look at this amp I designed for a company some years ago. The circuit turned one half of the push pull off while adjusting the bias closer to class A at the same time.
You could skip one half of the pot and just keep the bias bit.
5W PUSH-PULL AMP JPEG.jpg
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dorrisant
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Re: Bias tweaker or "touch"

Post by dorrisant »

Andy, I understand the concept of the cathode bias setting the low bias point, but can't you just wiggle the bias on a preamp tube instead? I have been looking for a solution that doesn't involve the power tubes.

Shane, interesting schems there. I would love to try that out. What primary impedance for the OT would you suggest?
"Education is what you're left with after you have forgotten what you have learned" - Enzo
shane
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Joined: Fri Jan 14, 2005 2:18 pm
Location: birmingham UK

Re: Bias tweaker or "touch"

Post by shane »

Can't recall exactly what I used then, it's been a few years. But I think it was around 15-18k a-a. The HT was about 300v on the output valves.
I originally built them without the bias change circuit and used 1k cathode resistors, but kept the pot that turns one half of the push pull drive.

It was a great sounding little amp and we sold quite a few.
Don't know if the company still trades but I'm sure they would claim intellectual property on it, but hey if you wanna try one why not. :D
tubeswell
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Re: Bias tweaker or "touch"

Post by tubeswell »

dorrisant wrote: Tue Sep 12, 2017 12:09 pm...can't you just wiggle the bias on a preamp tube instead?
Yup 55 Tweed tremolux, VibroChamp, Marshall 18Wer, a few Voxs and numerous other amps besides
He who dies with the most tubes... wins
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