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Post by bdunard on Apr 29, 2008 15:37:13 GMT -7
My 6545 went south the other day in the studio. It just quit working. I called Dr. Z and they told me to try new power tubes and a 12AX7 before I send it in. So I am going to buy some power tubes and a 12AX7. I bias my Allen Amp that I built but I must say that it is very simple. I feel that I am capable of biasing this amp but I need a bit of help.
1. Which pin of the power tube should I use?
2. If I place one lead on the tube and the other on the chassis what should be my DC reading? I am using the tube rectifier. I have read anywhere between 430 and 480...
Is this the best way to bias the amp?
I will take any and all advice!
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Post by Jones'n for Tone on Apr 29, 2008 16:41:46 GMT -7
According to a well informed source.....
"set the bias to 32 mA / tube with say a Bias Rite adaptor"
It will cost a little for the adaptor, but it's much safer than poking around with the voltmeter.
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Post by Phil (aka Phil) on May 8, 2008 4:32:19 GMT -7
bdunard, I'm with Jones'n for Tone - I'm an experienced tube geek and I use the Weber Bias Rite when I bias amps. This method is not quite as precise as the "shunt method" Dr. Z describes on his web site, but it's a heck of a lot safer. The shunt method requires that you set your multimeter to the "amps" setting, which essentially makes it a straight wire. Touching the wrong thing when in this mode could lead to disaster for you or your amp. The questions you asked (#1 and #2) make me think that you're slightly confused- putting a voltmeter from a tube pin (I think the one you're thinking of is pin 3- the "plate") to ground will give you a voltage reading- which on the 6545 probably will be up near that 480V range you mentioned, and will vary based on your "wall voltage" from the outlet. However, voltage is only one part of the bias "equation", and the other variable we're looking for is to bias the amp is "cathode current". To get a "current" reading - which is what we are looking for to bias your amp using the "Bias Rite" tool or any other method which measures cathode current - you need to put your multimeter (set to the "amps" or, more precisely "milliamps" setting) in SERIES with the tube's cathode path to ground. You COULD unsolder the ground lead from the tube's cathode, but this is a complicated and unnecessary step since we have the Bias Rite type tools. That's how these tools work- when you plug them into the tube socket and plug the tube into them you are putting the ammeter in series with this connection , and it's a whole lot safer. Now, to determine the bias setting you need that plate voltage. You can either measure it from pin 3 to ground with your multimeter set to VOLTS or, better, get the Weber Bias tool with the switch that lets you read the plate voltage. Now you need to know your tube's maximum plate dissipation (power rating). The GT EL34LS, for example, is rated at 25 watts. Go to Myles Rose' site www.guitaramplifierblueprinting.com/index2.html and click on the red button in the upper right hand corner. Download and open the Excel file. Put your plate voltage and tube's maximum plate dissipation rating in the spreadsheet. Just as an example, for 480 plate volts on a 25W tube the recommended bias range will be between 31-36mA on the chart. I usually shoot for 65% ID (idle dissipation) which is about 34mA in this case. As always, if you are not totally comfortable with working on a high voltage tube amp, take it to an experienced tech. If you touch the wrong thing (and there are LOTS of them) inside that amp when you have the chassis out, you will be dead.
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Post by Paul (TRANE) on May 8, 2008 10:02:45 GMT -7
One quick clarification:
The GT-E34LS is a 30 Watt tube, per Myles.
Paul G.
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Post by Phil (aka Phil) on May 8, 2008 12:26:23 GMT -7
One quick clarification: The GT-E34LS is a 30 Watt tube, per Myles. Paul G. You're right Paul. I quickly typed all that up this morning at work in between actual "job related" functions. Good to see you're out there to correct me!
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Post by myles on May 8, 2008 13:21:24 GMT -7
My 6545 went south the other day in the studio. It just quit working. I called Dr. Z and they told me to try new power tubes and a 12AX7 before I send it in. So I am going to buy some power tubes and a 12AX7. I bias my Allen Amp that I built but I must say that it is very simple. I feel that I am capable of biasing this amp but I need a bit of help. 1. Which pin of the power tube should I use? 2. If I place one lead on the tube and the other on the chassis what should be my DC reading? I am using the tube rectifier. I have read anywhere between 430 and 480... Is this the best way to bias the amp? I will take any and all advice! Your voltage should be read off pin 3 ... the plate, and should not vary by 50 volts as you wrote. it should be quite stable. If not, you need a variac as you line voltage is very unstable. Make sure all the tubes are in the socket, the amp is hooked to a speaker or proper load, nothing is in the input jack and the volumes are all the way down. The ones of these I have set up have the bias set at 38mA if the plate voltage is 460. I set this the same whether the tube is a normal 25 watter or the 30 watt E34LS. Use a proper bias tool .... bias rite, bias probe, bias tool, etc.
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Post by Phil (aka Phil) on May 8, 2008 13:25:05 GMT -7
2. If I place one lead on the tube and the other on the chassis what should be my DC reading? I am using the tube rectifier. I have read anywhere between 430 and 480... Your voltage should be read off pin 3 ... the plate, and should not vary by 50 volts as you wrote. it should be quite stable. If not, you need a variac as you line voltage is very unstable. Myles, I took this to mean that bdunard "read" (as in on the Internet) that the voltage should be between 430 and 480V, not that he "read" that value on his own amp with a voltmeter... maybe I misunderstood....
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Post by Paul (TRANE) on May 8, 2008 14:33:58 GMT -7
Phil-et-min-Möm Ahhhh,..yes..."actual job related functions". I know how that goes. It has been a slow day at work here for me. Outside of an ion column blowing up all is well and peaceful. I like days like this. Time to head home.
Paul G.
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