S1-45 Bias PR1

THE NEW #1. Series One Amplifiers.
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ehalvorsen
Posts: 4
Joined: Tue Jan 24, 2017 9:17 pm
Location: Norway

Tue Jan 24, 2017 9:27 pm

Hi
I just bought a used Series One 45 (2010-model), which had bad tubes.
I am installing 2x new Mesa EL34 and all new preamp tubes.

What is the correct bias voltage in mV measured on CON1?

PR1 on the largest board seems to be the right pot for bias.
But on the preamp-board there is another PR1 pot. Is this the balance?

Can someone mail me the servicemanual?
espen.halvorsen@hotmail.com

I am an electrical engineer, and I want to do this myself.

GibBFG
Posts: 3
Joined: Wed Jan 25, 2017 1:54 am

Wed Jan 25, 2017 1:59 am

I am looking to do mine as well and was wondering the same. Please let me know how you make out.Good luck!

ehalvorsen
Posts: 4
Joined: Tue Jan 24, 2017 9:17 pm
Location: Norway

Thu Jan 26, 2017 8:28 pm

The recommended voltage which I got from Blackstar today is:

CON1: 50mV
Adjust with PR1 on the powersupplyboard.

The PR1 on the preampboard shall be just in the middle IF your new tubes are matched. Otherwise, adjust until hum is gone.

The Mesa EL34 sound amazing by the way :)

/Espen

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thephantum
Posts: 1160
Joined: Wed Jan 29, 2014 7:42 pm
Location: Virginia, United States

Thu Jan 26, 2017 11:56 pm

Keep in mind that 50 mA would be the theoretical, per tube, bias current based on a 350V plate voltage. You're actual plate voltage is not going to be exactly 350V. Depending on the tubes, the voltage will vary by +/-5%...that's a 35V end to end swing, which brings the swing in bias number between 47 mA - 53 mA. So 50 mA is the median and in the ballpark, but it's not precise.

Since that's also a per tube number, CON1 is most likely only giving you one side of the pair. Since you are reading it in mV, there is at least one resistor in there (Ohms Law), so it could be giving you half of the total as well.

Either way, I prefer to take measurements directly from the tubes. That allows me to get the exact plate voltage measurements as well as bias readings as the tubes "see" it...not across other components. Also remember that as you adjust bias, the plate voltage will change. So you need to repeat the measure, calculate, adjust pattern a few times until the amp is dialed in. Also, since matched sets can have differences that are within tolerance, it also lets you set that balance to get both tubes as close to each other as possible (as opposed to just eliminating audible hum).

If you thing the amp sounds good now, get it dialed in... :mrgreen:

GibBFG
Posts: 3
Joined: Wed Jan 25, 2017 1:54 am

Mon Jan 30, 2017 12:08 am

ehalvorsen wrote:The recommended voltage which I got from Blackstar today is:

CON1: 50mV
Adjust with PR1 on the powersupplyboard.

The PR1 on the preampboard shall be just in the middle IF your new tubes are matched. Otherwise, adjust until hum is gone.

The Mesa EL34 sound amazing by the way :)

/Espen
First off why are blackstar giving info to some and not others? They wouldn't tell me anything. Second, I just changed the tubes in mine last week and re-biased but from the info that I found (online and talking to an amp tech) that CON1 is a measurement of both valves together and that it should read 100mV. Through talking with my amp tech he agreed when I said this was too hot and that it should be nearer 80mV based on the plate voltage. The plate voltage on mine was 430V so I adjusted my bias to read 70mV which would (in my understanding) be 35mV per valve and under the 70% of the EL-34's plate dissipation. 0.7x25/Plate voltage. In my case the plate voltage was 430V as I said.

GibBFG
Posts: 3
Joined: Wed Jan 25, 2017 1:54 am

Mon Jan 30, 2017 12:19 am

thephantum wrote:Either way, I prefer to take measurements directly from the tubes. That allows me to get the exact plate voltage measurements as well as bias readings as the tubes "see" it...not across other components. Also remember that as you adjust bias, the plate voltage will change. So you need to repeat the measure, calculate, adjust pattern a few times until the amp is dialed in. Also, since matched sets can have differences that are within tolerance, it also lets you set that balance to get both tubes as close to each other as possible (as opposed to just eliminating audible hum).

If you thing the amp sounds good now, get it dialed in... :mrgreen:
@thephantum I measured the plate voltage from the tube socket and got 430V, so instead of using CON1 I should be able to get the mA from the tube socket as well right? Pin 5 if I'm correct? Just looking to be sure because this is conflicting info after the fact that I am finding out so if CON1 is infact one tube then I have mine set too hot at 70mV as measured on CON1. Before changing the tubes CON1 measured 47.7mV so this would have been closer based on what the OP said he was told from blackstar. But, that still would seem hot for a PV of 430V would it not?

ehalvorsen
Posts: 4
Joined: Tue Jan 24, 2017 9:17 pm
Location: Norway

Mon Jan 30, 2017 8:33 pm

This was the answer I got which I referred to:

Hi Espen,

With your Digital volt meter set to its 200mvDc range measure across CON 1 on the power valve pcb and adjust PR1 on the power supply PCB to get 50mv,

We bias at 25mv per valve,

Leave PR1 on the preamp valve pcb in its midway position,

If you need any further help please contact me,

Kind regards

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