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71' Marshall Standby Switch / Hot side

ptrickamp222

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My new 71 needs the bias wire moved to the hot side of the standby. Do I just move the wire going to the diode to the 220K then to diode????Move it by itself or move the top 2 Blue one's / one of the bias leads and swap them with the red leads on the bottom. Looking in from the top of the chassis with the tranny's down. The standby has 2 blue on top and 2 red on the middle of the switch. One of the blue wire's is the bias lead. The standby is a 6 pole with the top and middle only being used. Is the middle pole's all hot???? And I swap the top to middle leads??? I looked at my 72' which is wired right but is different than this one. Cannot copy. I will double check with the volt meter with power on. I just want to know if I swap the red with the blue leads????Thanks!!
 

ptrickamp222

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I just looked at the amp and schematic and the blue leads go the big silver cans before the bias and the red leads go to the PT. Do I just swap the red and blue leads so the blue is in the middle which I am sure is hot????? will double check when I get the new bias pot from Metro. Also do you recommend to change the bias diode also???? What part number 10 something. Also what resister to change the 47K to get a higher current reading from max 25mA??? I need at least 35 to 40 mA. Thanks!!
 

gutpile

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try changing the 47k to a 33K... if still not enough try a 27K... you need a lower resistor value... good luck!

Mike
 

ptrickamp222

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try changing the 47k to a 33K... if still not enough try a 27K... you need a lower resistor value... good luck!

Mike

Thanks I just did the formula and came up with around 38k. 33k sounds good, Thanks. also the 27K waiting would not be bad.....:salude
 

Hamilton

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try changing the 47k to a 33K... if still not enough try a 27K... you need a lower resistor value... good luck!

Mike

Mike, that is backwards. But I also had it backwards on the other thread...now it's ok. He needs more plate current, therefore he needs less grid voltage, therefore he needs higher resistance to the feed. uuugghhh I hate getting old.

Now, to moving the bias feed wire, turn the amp on but leave it in standby. Take your voltmeter and probe to see which lugs are hot on the SB switch - top or bottom. Then turn amp off and wait for caps to drain.

Then remove the bias feed wire from the box rectifier and solder it to your hot lug on the SB switch.

The diode is a 4001.
 
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Wannatone

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Mike, that is backwards. But I also had it backwards on the other thread...now it's ok. He needs more plate current, therefore he needs less grid voltage, therefore he needs higher resistance to the feed. uuugghhh I hate getting old.

Take it this way. The bias pot in series with the 47k resistor is part of a voltage divider. The other part of the divider consists of a 15k resistor and another 27k (up to 150k something, depending on amp model / year). The control grids are connected to the common point of both voltage divider parts. When you INCREASE the pot/47k value, you get more negative voltage out of the circuit (less plate current) and vice versa.

With the other part of the voltage divider, it's exactly the opposite. When you INCREASE the 15k or 27k (or something, depends on amp model and year), you get less negative grid voltage, thus more plate current.
 

Hamilton

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As I said on the other thread I was stupidly thinking the 47K was hooked to the diode. Sorry for messing this whole thing up....
 

ptrickamp222

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Thanks for the reply's. I just ordered from Metro. Bias pot, 33k, 27k, diode, 1k 5watt screen's and 5.6k for bias. Will change the 47K to 33K that attaches to the bias pot to hopefully raise the current. The most I got was 25mA at 417Vdc on the plates! And that was with my second set of Mullard's out of my 72". I ordered the 27k just incase I need to lower voltage more. Right now it's at -40Vdc. Way to high. It's hard to believe the head has been played like this since new..........When I checked the tubes they were at 17mA using the shunt method!!!!!! That is one cold amp setting:hmm
 

gutpile

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17ma???:pop bbbrrrr!!! that couldn't have sounded right!! glad to see you got it worked out!:salude keep us posted on the results!

Mike
 

ptrickamp222

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Take it this way. The bias pot in series with the 47k resistor is part of a voltage divider. The other part of the divider consists of a 15k resistor and another 27k (up to 150k something, depending on amp model / year). The control grids are connected to the common point of both voltage divider parts. When you INCREASE the pot/47k value, you get more negative voltage out of the circuit (less plate current) and vice versa.

With the other part of the voltage divider, it's exactly the opposite. When you INCREASE the 15k or 27k (or something, depends on amp model and year), you get less negative grid voltage, thus more plate current.

I am getting mixed answers. I am thinking of changing the 47K to the bias pot to 33K to get more current. Is this wrong???? Should I change the 15K to 27K then???? The bias goes to a 220k to diode to 15K bias lead to 47k to bias pot 25K. Which resistors do I change to increase current.....the 47K or the 15K??????? Thanks!!!
 

gutpile

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change the 47K to 33K and you should be good to go! If you still run out of room (too cold with bias pot maxed out), change the 33K to a 27K... 33K should do the trick IMO!
 

ptrickamp222

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I just ordered the parts this morning from metro as you suggested. resistors, pot,diode, screen resistors and pin 5 bias resistors to since I was ordering that way I have it all if needed. Thanks for the answer! I keep getting mixed answers from different sources. But Metro sales said I am on the right track. I have a question on their site also. And emailed them. Some say to change the 15K to alter and all kinds of different answers. I am using ohm's law and that works with what you said. But this is a voltage divider and it's been years since electronic's. I figured I am not the only one to not get the right bias range on a plexi circuit. How did they bias these things.....in the artic!!!!
 

ptrickamp222

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Aug 22, 2007
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This is correct. I would just have started with 39k. But, in your case 33k would work, too.

Thanks for the input, I maybe should have bought one. I have a box and bag full of old resistors, may find one if needed. I just wanted to use new. The old ones gather moisture after many years.
 
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