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Post subject: 64' Twin Reverb bias setting
Posted: Wed Oct 30, 2013 12:57 pm
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Aspiring Musician
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I realize that every amp has it's own calculated setting. I'm using a bias/plate voltage meter on 2 tubes (I don't have 4 probes). I also know that I need to do the inner 2 and/or the outer 2 because of the parallel design. I have it biased at 33 now with a plate voltage of 450. The tubes, JJ 6L6GC's are matched well. It seems every online bias calculator suggests a different setting. Should I be setting it by ear as long as I stay within a reasonable and safe setting, or should I set the bias pot to -52v, read the plate voltage, and then and set the bias to the calculated amount from that reading?

Just learning so please bear with me, I just want to learn it right.

Thanks for any help.


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Post subject: Re: 64' Twin Reverb bias setting
Posted: Wed Oct 30, 2013 1:19 pm
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Hi upnorth,

Don't worry about the negative grid voltage, just the Plate voltage and Plate (or Cathode) current. 450 volts at 33 mA is just under 15 watts (450V x .033A = 14.85W), still too cold. A good range to shoot for is 15 watts to 21 watts (50% to 70% of the tube's max Plate wattage rating), to your taste in that range. When changing the bias, the Plate voltage will change with change in current, so remeasure and recalculate with each adjustment.

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Post subject: Re: 64' Twin Reverb bias setting
Posted: Wed Oct 30, 2013 1:40 pm
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Thanks Shimmilou,

I get it. Keep adjusting, reading and doing the math until the wattage is good, or what the tube and your ear can be pleased with together. I need to check the specs on the JJ's.

Thanks again.


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Post subject: Re: 64' Twin Reverb bias setting
Posted: Wed Oct 30, 2013 1:49 pm
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Read this:

viewtopic.php?f=13&t=88475

JJ 6L6GCs are 30 watt tubes. That's the only spec you need to know.

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Post subject: Re: 64' Twin Reverb bias setting
Posted: Wed Oct 30, 2013 2:29 pm
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Thanks Bill,

I was concentrating too much on the cathode current alone, not the math with the plate voltage and the cathode current together. So all is well, in theory, if I stay between 50% (15w) and 70% (21w) with these tubes. The sweet spot for my ear needs to lie between those numbers for the good of the amp and more so the tubes. This would work on fixed as well (Changing resistors-caps) to optimize let's say, a single 6V6 in a Tweed Champ?

Thanks again.


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Post subject: Re: 64' Twin Reverb bias setting
Posted: Wed Oct 30, 2013 2:36 pm
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shimmilou wrote:
Hi upnorth,

Don't worry about the negative grid voltage, just the Plate voltage and Plate (or Cathode) current. 450 volts at 33 mA is just under 15 watts (450V x .033A = 14.85W), still too cold. A good range to shoot for is 15 watts to 21 watts (50% to 70% of the tube's max Plate wattage rating), to your taste in that range. When changing the bias, the Plate voltage will change with change in current, so remeasure and recalculate with each adjustment.


+1000


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Post subject: Re: 64' Twin Reverb bias setting
Posted: Wed Oct 30, 2013 2:58 pm
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So is it safe to say the tone would be warmer the higher the bias?


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Post subject: Re: 64' Twin Reverb bias setting
Posted: Wed Oct 30, 2013 3:40 pm
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upnorth wrote:
Thanks Bill,

I was concentrating too much on the cathode current alone, not the math with the plate voltage and the cathode current together. So all is well, in theory, if I stay between 50% (15w) and 70% (21w) with these tubes. The sweet spot for my ear needs to lie between those numbers for the good of the amp and more so the tubes. This would work on fixed as well (Changing resistors-caps) to optimize let's say, a single 6V6 in a Tweed Champ?

Thanks again.


Yes, but plate voltages are calculated differently with a Class A amp like a Tweed Champ.

See "Calculate Plate Dissipation In Cathode Biased Output Stage Based On Voltage And Resistance Readings" or "Calculate Plate Dissipation Based On Plate Voltage And Cathode Current Readings" (read the note) at http://www.webervst.com/tubes1/calcbias.htm.

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Post subject: Re: 64' Twin Reverb bias setting
Posted: Wed Oct 30, 2013 3:41 pm
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upnorth wrote:
So is it safe to say the tone would be warmer the higher the bias?


Yes.

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Post subject: Re: 64' Twin Reverb bias setting
Posted: Thu Oct 31, 2013 12:50 am
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upnorth wrote:
So is it safe to say the tone would be warmer the higher the bias?


If bias too cold , sound is thin , lack of punch . Good bias ; punch and warm sound ,tubes are in safe range . Too hot , not more warmer , distortion sometimes , short tube 's life .

Keep bias as cold you can if you like the sound.


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Post subject: Re: 64' Twin Reverb bias setting
Posted: Thu Oct 31, 2013 1:06 am
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upnorth wrote:
s. This would work on fixed as well (Changing resistors-caps) to optimize let's say, a single 6V6 in a Tweed Champ?

Thanks again.



In a class A like Champ amp , plate curent still the same at any power , any volume position with guitar plug in. That is why you can have a very hot bias to have a "great" tone . If the bias is at 90% , plate disipation , it will stay at 90 % at ANY volume.


In class AB ( your Twin ) plate dissipation / current must be at not more then 70 % at idle , no guitar plugged, When you crank the volume with guitar , this plate current may go to more than 150 % of tube plate dissipation at maximum volume.

That is why a colder bias is better for tubes life. But you may like the sound first. It is a matter of taste too.


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Post subject: Re: 64' Twin Reverb bias setting
Posted: Thu Oct 31, 2013 1:13 am
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stratele52 wrote:
That is why a colder bias is better for tubes life. But you may like the sound first. It is a matter of taste too.


+1

When you're running $30 bottles like EH or JJ you may be inclined to "derring-do" in the quest for tone.

OTOH, when $75 tubes such as vintage blackplate RCA's are at stake, you make be more circumspect regarding the concept of longevity.

Arjay

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Post subject: Re: 64' Twin Reverb bias setting
Posted: Thu Oct 31, 2013 1:48 am
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Retroverbial wrote:

When you're running $30 bottles like EH or JJ you may be inclined to "derring-do" in the quest for tone.

OTOH, when $75 tubes such as vintage blackplate RCA's are at stake, you make be more circumspect regarding the concept of longevity.

Arjay



+1


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Post subject: Re: 64' Twin Reverb bias setting
Posted: Thu Oct 31, 2013 4:17 am
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upnorth wrote:
...This would work on fixed as well (Changing resistors-caps) to optimize let's say, a single 6V6 in a Tweed Champ?

Thanks again.


To add to what the others have said, no need to worry about the bias in a single-ended amp like a Champ. The Champ, and others are "Cathode biased" and are pretty much self correcting, provided you use like tubes, ie 6V6 for another 6V6. Most preamp tubes are Cathode biased, and are plug and play with no adjustment needed, even when switching from a 12AX7 to a 5751, or 12AT7. "Fixed bias" is the type that needs a bias adjust with change of output tubes. The fixed bias just means that a negative voltage is applied to the grid to set the current through the tube. Some fixed bias amps are adjustable with a pot, and some have no adjustment pot, but a resistor that can be changed to adjust the bias. Your amp would be a fixed bias with an adjustment pot.

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Post subject: Re: 64' Twin Reverb bias setting
Posted: Thu Oct 31, 2013 7:41 am
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Got it. I've got the Twin at about 18.5 watts. It sounds as good I think, as new production tubes can. (What a difference with the right caps. I can't believe I did that)


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