ZL FX Loop question?

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rpurdue
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ZL FX Loop question?

Post by rpurdue » Sun Jan 13, 2019 11:30 am

Question....installing this on an amp based off of a 2204 circuit. In the instructions there are 2 resistors in series for the bias supply node (pictured) and you change one to compensate for the power draw from the loop On my amp though, there's only one resistor (22k) with similar value of the other 2 resistors. In this situation, should I just change that one resistor to maybe a 16k or so?
https://ibb.co/3yz1n5D
https://ibb.co/KV5q7Fr

danman
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Re: ZL FX Loop question?

Post by danman » Sun Jan 13, 2019 6:45 pm

The two resistors in series are the main dropping resistors for the power supply. Bias voltage is usually taken from one of the PT"s secondary leads and has it's own dropping resistor. Either way, yes you can adjust your 22k to give you the necessary voltage for the rest of the circuit. 16-18k would be a good place to start.

rpurdue
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Re: ZL FX Loop question?

Post by rpurdue » Sun Jan 13, 2019 7:35 pm

Thanks! I think I’ll just play around until I can get the bias value back to the pre loop value.

danman
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Re: ZL FX Loop question?

Post by danman » Sun Jan 13, 2019 9:41 pm

The 22k resistor in your amp's power supply comes after the power tubes plates so neither your bias voltage or your plate voltage (power tubes only) will be affected. From your pics, your bias voltage feed appears to be coming from the standby switch and connecting to the board at the large green resistor. This would be AC voltage from one of the PT's secondary wires that also connect at the switch. Just try to get all preamp voltages back to their original readings and you will be good to go.

rpurdue
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Re: ZL FX Loop question?

Post by rpurdue » Sun Jan 13, 2019 10:05 pm

Thanks! Is it still safe then to power the loop from the 22k resistor? Voltage reading was 306V

rpurdue
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Re: ZL FX Loop question?

Post by rpurdue » Sun Jan 13, 2019 11:11 pm

And sorry, I meant B+ voltage not bias

danman
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Re: ZL FX Loop question?

Post by danman » Mon Jan 14, 2019 7:34 pm

I would think that voltage would be fine. You could always add a dropping resistor in series with the supply to the loop if you need to bring down voltages. You could also add a new turret to the board and split the 22k into two 10k's if it help. Tube Depot has some nice little turrets that screw into the board for times that you can't lift the board to add a new turret.

rpurdue
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Re: ZL FX Loop question?

Post by rpurdue » Mon Jan 14, 2019 7:44 pm

Thanks Danman! Great idea. I have a bunch of 2W resistors though so i think instead of splitting them i'll just play around with single values in the 16k range to get the B+ close to where it was. As for the 306V for the board, i guess its safe to use the 10k dropping resistor value in the instructions for that voltage.

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