It change bias tension

JCM Range, 800s, 900s, 2000/DSL, 2000/TSL

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Roger
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It change bias tension

Postby Roger » 06 Sep 2016, 13:22

Hi guys :howdy

I have a TSL 100 that I change all boards recently. In this days i have been making some home recordings and I want to comment that if you push the output mute the bias tension change. Normally i adjust about 88mV both sides. When you push the output mute the voltage on one side is 82mV and the other side 94mV. Also this happens with the old boards. If you want adjust the bias with the outpute mute in on its imposible to do. Can this be worring? :think
The amps works perfectly with or without ouput mute.

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Re: It change bias tension

Postby Roger » 07 Sep 2016, 02:20

More information: :howdy
I have tried with other power tubes. Does the same. When you press the output mute button the bias voltage change. I have tried adjusting both sides at 78mV or 82mV. It do the same. The best voltage setting is to leave in 86mV, then one side stays 90mV and the other 83mV. If you press the VPR button or any other button don't change the voltage. :think :think

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Re: It change bias tension

Postby Roger » 17 Mar 2017, 13:15

The solution is use a perfect matched quad of power valves. Some sellers say his valves are matched and it's not true. With good power valves the bias tension not change.

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Re: It change bias tension

Postby Anitoli » 17 Mar 2017, 18:12

As long as the difference between the pairs isn't more than around 8mA, you wont see an increase in noise and it really won't affect the amps operation.

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Re: It change bias tension

Postby Roger » 18 Mar 2017, 02:46

Anitoli wrote:As long as the difference between the pairs isn't more than around 8mA, you wont see an increase in noise and it really won't affect the amps operation.
The amplifier does not make any noise and works perfectly with mismatched valves but when pressing the button output mute the bias voltage is changed. You may have a 10mA difference between each side. I don't have a tube tester In this situation would be fine have one.

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Re: It change bias tension

Postby Anitoli » 18 Mar 2017, 05:45

Not accounting for what appears to be a typo in the Marshall schematics, all that power amp mute switch does is kill the signal after the PI. I don't see any reason why the bias would fluctuate with that switch engaged. It would be no different than turning the mv to zero. You state in your Sig that the amp is modded, do you know what was done to it?

Look for CON 12:

http://drtube.com/schematics/marshall/T ... 20iss4.pdf

http://drtube.com/schematics/marshall/T ... 20iss7.pdf

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Re: It change bias tension

Postby Roger » 18 Mar 2017, 07:44

https://www.google.es/url?sa=t&source=w ... Eq_9NXktfQ

I made the mods myself. It try almost all the modifications that appear in the link with different versions of plates that I have of this amp. I have three different versions of almost all the plates. Now it works with the latest version. Basically values ​​of some components have been changed and values ​​have been placed closer to the DSL has. Some components have also been added and removed. I add 6H choke filter and some switchs that connect more components.
The bias changes value instantly when you press the button, it does not fluctuate. It also change instantly when you release it. Now it stays totally fixed and it does not matter how it is adjusted the bias
Also change the valves of the preamp, maybe the answer is in V4.
I'll take a look on the schematics.

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Re: It change bias tension

Postby Roger » 18 Mar 2017, 10:42

I have so many plates of this amplifier not because I have had many TSL, I only have one (1998). But I found on eBay a bargain from a TSL chassis with clean / crunch / lead and reverb plates. This amplifier of 2002. In fact recently my reverb plate of 1998 ceased to function and this 2002 works well. In 2010 I changed the motherboard to the new motherboard. And then I got the kit of new clean / crunch / lead plates only compatible with the new motherboard. I also have a completely new motherboard thanks to the low efficiency and disorganization they sometimes have in Thomann. I ask for the kit and they bring me a new motherboard. After thousands of phone calls, thousands of emails and thousands of photos give me the reason and send me the kit.
I currently have installed the new clean / crunch / lead plates and the new motherboard I bought in 2010.
The mods that are currently in my TSL100 are the following.
Motherboard: TR3 disassembled. R71 replaced with 6H choke 200mA. C18 changed to 220pf - closer to DSL value (470p) instead of TSL value (100p), bleeds more high frequencies off after initial distortion stage (V1AA) in crunch and lead channels. Added toggle to switch C16 from 2.2n to 6.6n (thicker bass on both drive channels). Added toggle to switch R31 from 1K (stock) to 500R (a bit of extra gain on all channels, more asymmetric for rawer breakup). Added toggle to switch 390pf cap in parallel with R1 (plate bypass), cuts high frequencies over 8.5kHz after distortion, gently. Affects all channels.

Lead Panel:Add 82K resistor in series with C8, C7 changed to 500p.
47pf mica cap added across VR2 (channel volume) - first-order lowpass filter bleeding off treble frequencies above ~7kHz.

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