Problem setting bias

I want to run a peculiar situation I have with my system to see what you all think.

On my Manley amps (mid hi towers on my Infinity IRS Beta) I cannot set the bias with the speakers connected since the voltage at the tubes fluctuates constantly.

Manley had sent me a plug with a resistor (I think) to simulate a speaker load.

With that on, I get very stable voltages and can set the bias. So, the speakers are what is causing the fluctuation.

I have changed most capacitors on the crossovers but have not changed the resistors or inductors.

What could causing the changes in voltage at the power tubes?

Manley has had the amplifiers two times and they are perfectly on specification

I figure since it does not happen with the resistor plug it could be the resistors on the crossovers or perhaps the drivers.

Maybe @Paul, @Chris_Brunhaver or @bstadtherr with their experience with Inifinity and planar drivers could shed some light.

I appreciate your thoughts.

Since the IRS towers are only mids and highs, all of the drivers are capacitively coupled as part of the crossover. So unlike most speakers there is no DC load on the amplifier. It is just a guess but I am thinking that the Manley amps don’t like that. Maybe you can add a dummy resistor in parallel with the towers to maintain a DC load. You might have to experiment to see what the highest value that works is. I would start somewhere around 32 ohms as this would still be a very light load.

I’m not familiar with the Manley design, so I’m just going off what I know to be different about the IRS.



Hello @bstadtherr and THANKS!

To set it in parallel means to wire it how? (I dont want to screw up!)

What are Manley telling you about this?

They went through the amps and tested everything. They were very helpful and quick!

They did ask if it happened with other speakers. I have not tried with my “regular” speakers

Wherever it is convenient, add a resistor between the + and – terminals. Could be at the amp or at the speaker.


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Just a quick follow up and thanks. Installed a 30 ohm resistor as you suggested.

No more “hunting” from the tubes. Bias is very stable on each of the 4 tubes, cycling noise on speakers is gone and the cycling effluence of the tubes is now gone also.

I have been hunting after this for a couple of years now. THANKS MUCH @bstadtherr Bob for sharing your knowledge! :index_pointing_at_the_viewer:


Where else are you going to get that level of advice? Wow.

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My contact at Manley gave Kudos to Bob!