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Post subject: Mesa Bias- yes,I'am at the right forum
Posted: Sun Oct 24, 2010 2:01 pm
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What is the proper way to bias a Mesa tripple Rectifier ?

These amp amp have 2 switches at the back to change voltage. Here they are:

First switch ; Position BOLD ; more voltage on the primary of power transformer
Position SPONGY ; less voltage on the primary of power transformer
Second switch ; Position :Silicon rectifer; more DC voltage . Position :tubes rectifier; less DC voltage.

The only schematis avalaible is the double rectifier. Power supply is the same, with the bias
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I do bias with the more voltage ( position Bold and Silicon rectifier) at 70% power dissipation on each EL34 (39 milliamps at 448 volts DC)

If I change any position of these 2 switches the bias is set very cold, because voltage drop.
This is what I had:

Spongy and Tubes rectifier : 22 milliamp at 384 volts DC = very cold
Bold and tubes rectifier : 25 milliamps at 422 volts DC = cold

Have you any idea to do a better bias ?


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Posted: Sun Oct 24, 2010 5:18 pm
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Mesa/Boogie amps are not made to be biased. Replace the tubes with the recommended Mesa tubes and you're all set.

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Posted: Sun Oct 24, 2010 6:20 pm
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Stratele, do you have a link to the schematic ? I've worked on the 2:90 power amp, but not the dual or triple recto's. Logic would dictate a coordination between the plate and bias voltages when switching. The bias supply must have some kind of multi-tap capability. Unless the tubes are cathode biased. Art

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Posted: Mon Oct 25, 2010 2:01 am
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http://www.schematicx.com/schematic/mes ... schematic/

This is a link to see the Mesa schematic. This is a Double Rectifier. As i say , power supply/ bias circuit is the same as Tripple Rectifier (witch schematic is not avalaible).

This amp is grid bias. I made bias to the highest amp voltage , for safety; Bold and Silicon rectifier switches positions.
Hollis Prince : I built 2 pots for ajustable bias , one pot fot 6L6 and another one for EL34, these work fine.

I think thre are not better solution but I would apreciate your opinions
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Posted: Mon Oct 25, 2010 2:13 am
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Image

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Posted: Mon Oct 25, 2010 6:03 am
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On the Power Supply schematics, you can see the BOLD / SPONGY switch on the primary of power transformer.

Also on DC side, the Silicon Diodes / Tubes rectifier switch

I do bias on max voltage avalaible position BOLD and Silicon Diodes ,for safety.

JJ EL34 : 39 milliamps at 448 volts DC , 70 % power dissipation


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Posted: Mon Oct 25, 2010 1:18 pm
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Stratele52,

as far as there are no compensation, you're are on the right way for bias compromise. Hot for high voltage solidstate as worse case and cold for low voltage valve rectifiers oposite worse case.
Good idea to make different pots for EL34 and 6L6. :)
An other idea could be to use multicontact switches, which allow to modify the negative bias voltage by adding or shortcuting one or two resistors.

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Last edited by Tissan on Mon Oct 25, 2010 1:29 pm, edited 1 time in total.

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Post subject: Mesa bias
Posted: Mon Oct 25, 2010 1:27 pm
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Hi stratele52,

Looks like a nice mod that you have there. :D Can you show how you hooked it up, where in the circuit? So, I guess that you used the existing bias select switch, and just removed two resistors and replaced with the pots and circuit board? I would run the 6L6s at least 29-33 mA plate current range, at 422 Plate volts. I'd say that the EL34s are just right. What kind of output tubes are you using? 8)

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Posted: Mon Oct 25, 2010 1:56 pm
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This all the circuit in the mesa Boogie triple rectifier. My customer want the cheapest good tubes like JJ. I suggest Winge C but there are twice the price. I ordered at Tube Store. He need 6 power tubes EL34 and 3 rectifier tubes

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Posted: Mon Oct 25, 2010 3:51 pm
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This picture show you two sketches. In the upper, the original bias circuit and in the lower my new bias circuit with two pots.
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Posted: Mon Oct 25, 2010 5:50 pm
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That looks very good. I think that the two 25K pots in series with the bias resistors are good idea. Nice work! I might have just replaced the 15K and 22K with the pots instead, but yours is also nice. What kind of range of bias voltage do you have with that setup, does it work well? Maybe you can make and sell kits for Mesa amps! We have Billm, Fromel, Fargen and now maybe stratele52 mods.....? :)

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Posted: Mon Oct 25, 2010 8:34 pm
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Yes I agree with Shimmilou. Stratele, your work is very good and the drawings are excellent. And I also agree with Tissan, the bias is a compromise, bummer ! I thought there would have been some type of compensation to accomodate the power shifts. I have two questions: What brand are those bias pots ? And what is the wall outlet voltage in Canada ? I always thought it was 220vac. Art

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Posted: Tue Oct 26, 2010 2:44 am
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Thank's everybody for your good words.
Shimiliou:
I kept the 15 K and 25 K in circuit for safety, in case somebody turn a bias pot to 0 ohms . If this "appened " you'll have no negative bias voltage and burn the tubes.

Yes it work well, (If was not, I won' t talk about here ah,ah !). The more negative voltage I could have are -57 volts for 6L6 position and -50 volts for EL34 position and I can go lower to bias tubes very hot.

aclempoppi;
Pots are Allen Bradley, multiturn cernet, I found at an electronic surplus store. In Canada we have, like your country, 120 volts AC at wall outlets.
Job in the Mesa is finish , I'll give the amp today to my custommer.

Thank's for your help


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Posted: Tue Oct 26, 2010 6:03 am
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I vould suggest to put a 1µ film cap on the bias (220K and mid point of 25K).

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Posted: Tue Oct 26, 2010 7:09 am
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What will this 1mfd will do ?


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