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Post subject: early breakup
Posted: Wed Feb 09, 2011 6:16 pm
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just curious when changing from a tube with a 4-7 breakup to one with a1-3 breakup what would be best to set the bias at in a bdri. i know that fender sets them from the factory to 60mV and i know how to properly bias an amp but i've never went to a tube with an early breakup.is it best to stop with the 60mV or use something else. am i going to see 60mV at a lower range on the bias adjustment or later. has anyone ever done this? just so you know i am using 30w tubes. i don't care about opinions about tubes i just need some solid info.


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Post subject: Re: early breakup
Posted: Wed Feb 09, 2011 8:35 pm
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Hi blackstrat71,

You can start with the 60 mV setting and play the amp and see how it sounds, but that is pretty cold. You should measure the Plate voltage and record it, maybe around 440 volts or more. The B+ is roughly the Plate voltage. Then, take the mV reading at the test point which can be read as mA and is the Cathode current for both tubes. Divide the Cathode current in half and subtract 5 mA, then multiply that number by the Plate voltage to get the idle wattage of one tube. Example, a reading of 60 mV at the test point is 60 mA Cathode current for both tubes. 60 mA divided by two, is 30 mA, minus 5 is 25 mA. 25 mA Plate current at 440 Plate volts equals 11 watts idle dissipation, and is considered quite cold. At 440 Plate volts, a good setting for the bias would be between 80 mV and 100 mV for a 30 watt tube. Use your actual Plate voltage to know where to set your bias. Usually, the bias is best at 50% to 70% of the tubes rating, or between 15 and 21 watts idle. You will probably be good at around 85 or 90 mV, depending on your Plate voltage. :idea:

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Post subject: Re: early breakup
Posted: Wed Feb 09, 2011 8:41 pm
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Shimmy, I've gone away from those "rated" GT power tubes. Was wondering though, do you bias them to the same idle dissipation (watts), regardless of "rating?" I used to fine-tune or tweak the bias with these rated tubes--- to get the desired results. But, that was back in the days when they had number ratings.

Thanks!


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Post subject: Re: early breakup
Posted: Wed Feb 09, 2011 9:26 pm
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Hi BMW2002Ti,

I usually try to set mine at about 60% of the tube's rating for whatever tubes that I use. The exception for me is my BJr with the EL84s, I have them set closer to 70%. I prefer the red label GTs myself, which are the high 8-10 rating, late breakup. With the early breakup rated tubes, they will breakup earlier anyway, so no need to bias them hotter if you ask me. I have become more flexible on my settings, thanks to members here on this forum, so I could see tweaking the bias a little to taste, but I like to stay in the 50% to 70% range, typically in the middle at about 60%, for all tubes. :idea:

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Post subject: Re: early breakup
Posted: Thu Feb 10, 2011 3:40 am
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great info. that realy helps with biasing. i like to set mine around 60% because i don't want to kill'em. the tubes have a label on the box that has Ip 45 on it. i don't know much about tube labeling so i take it that means idle current sence I represents current(amps). so when i change them i could go as high as 90mV on the bias(45x2).is that correct.


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Post subject: Re: early breakup
Posted: Thu Feb 10, 2011 7:43 am
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blackstrat71,

Ip would be Plate current, I is current, p is Plate, read as "I sub P". Sounds like the number on the box is the actual Plate current that the tube was tested with. Remember that the Plate current is lower than the Cathode current (Cathode current for both output tubes is the test point reading), so to get 45 mA of Plate current, add 5, then multilply by two, that would be about 100 mV at the test point. 2 x (45 + 5) = 100.

It is important to know what your Plate voltage is, to know where the idle wattage really is. Those that tested the tubes at 45 mA could have used 480 volts or 400 volts on the Plate, we just don't know. So, 45 mA (100 mV at test point) at 440 Plate volts is about 20 watts idle, which is closer to 70% of the 30 watt tube. If your Plate voltage is higher, then the idle will be higher with the same test point reading. If your Plate voltage is close to 440, then 90 mV at the test point, would be about 60 % idle for the 30 watt tube (that's 40 mA Plate current for each tube). :idea:

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Post subject: Re: early breakup
Posted: Thu Feb 10, 2011 11:11 am
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thanks, this info goes a long way.


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Post subject: Re: early breakup
Posted: Sat Feb 12, 2011 8:05 am
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Shimmy, Thanks for your response.

My experience is if the plate volts are on the higher side and the idle current through the cathode is lower (to get, say, 22 watts of dissipation per 6L6GC) = slightly better headroom. Later crunch.

When plate volts are lower and idle current is higher (to get same 22 watt per 6L6GC) = earlier compression, less headroom.

Is that your experience?


Thanks!


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Post subject: Re: early breakup
Posted: Sat Feb 12, 2011 8:44 am
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Hmmmm, that does seem to be the case BMW2002Ti. The amps that have lower Plate voltages do deem to be the ones that breakup earlier (think class A). I could believe that higher current and lower Plate voltage would give earlier breakup, all other things being equal. Definitely an interesting observation, I'll have to give it some more consideration. :)

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Post subject: Re: early breakup
Posted: Sat Feb 12, 2011 11:30 am
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i would truly love to know the voltage on the plate. whitch pin (with refrence to ground) will give me this reading? there is a lot of "educated guessing" on line as to the plate voltage in the blues deluxe reissue. i have read everything from 406v to 440v that is one heck of a spread. any insight will be appriciated.


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Post subject: Re: early breakup
Posted: Sat Feb 12, 2011 1:22 pm
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Pin 3 for a 6L6, 6V6 and EL34, pin 7 on EL84, and the voltage will depend on the amp that they are in. My HRDlx is about 430V on the Plate, while my HRDvl is closer to 485V, both use 6L6 output tubes, so the voltage can vary a lot from amp to amp. Be careful with a probe around the tube pins and socket connection points, very easy to accidentally short two points together with a meter lead. I check the Plate voltage on one of the output transformer primary leads, where they connect to the circuit board, much safer. :idea:

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Post subject: Re: early breakup
Posted: Sat Feb 12, 2011 3:16 pm
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awesome thanks shimmilou you da man thats what i've been waiting to see. yeh i think the x-fmr secondary will be safer to check 'cause those tube pins are close together.


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Post subject: Re: early breakup
Posted: Sat Feb 12, 2011 5:57 pm
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OT primary, not secondary. :idea: :)

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Post subject: Re: early breakup
Posted: Sat Feb 12, 2011 7:45 pm
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output x-fmr primairy, thats right because the secondary goes to the speaker(s).correct?i had a brain fart. i do electrical work and we deal with a lot of secondary voltage.had to remember the amp has two x-fmrs. main or primairy and output, both have their own voltages(in and out).thanks.


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Post subject: Re: early breakup
Posted: Sat Feb 12, 2011 8:21 pm
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Yes, secondary of OT to speaker. There is a tendency to think secondary when output is mentioned, I catch myself doing the same thing sometimes. :lol: Generally you can call the other transformer the "power transformer". :)

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