Lower Bias range

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Goonies
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Lower Bias range

Post by Goonies » Sat Mar 25, 2017 10:47 pm

Hi folks,

I Just finish a 12000 series super lead metro kit. This is my second build. My first it was OK. The second one I´ve got some voltage problems. The sound is OK, but the bias range pot is -40v to -50v without power tubes. Why is this so small range? it must be -30v to -50v. Using 25K pot, piher. Caps are ok and grounded. The maximum value for bias is 35mv with this range. I've checked diode direction, bias caps direction, ground and jumpers.

All the voltage from the preamp tubes and PI preamp it is OK. Voltage Range by 100W PDF metrokit.

Capacitors: 2x100uf FT main 2x33uf FT screen. 32+32Phase inverter filter. 33+33uF FT preamp. Sozo caps. All caps and resistors are Standard Super lead. JJ EL34

Output Transformer: 1202-132 Marstran
Power Transformer: T2562 Marstran and choke.
Power voltage AC: 115volts
Plate voltage DC: 465volts
Bias voltage AC : 100volts
Bias voltage after 27K: AC 80V
V7 and V6 Power Tube PIN 8: 35mV
V4 and V5 Power Tube PIN 8: 27volts (high voltage) !! I don´t understand why it has high voltage and the tube is working fine. swap tubes position does not change this
V4 to V7 PIN 5 with no EL34: -42volts

I ´ve found that the color wires for the secondary OT is wrong. The website said:
Orange - common
Gray - 16omhs
Green - 8omhs
Yellow - 4omhs

But I think this is wrong. I´ve put 40volts at primary and found:
Orange + gray : 3.8volts
Orange + green: 1.9volts
Orange + Yellow: 0.9 volts (very low value)

I think the correct wire diagram is (40 volts at primary gives a reading):
Gray = Common
Green = 4omhs 2 volts
Yellow = 8 omhs 2.9volts
Orange = 16omhs 3.9 volts

I've take a read on other OT classic tone C1998, and the voltage range is the same. Same thing, put 40 volts, and the secondary is the same voltage range.

First I´ve put the wrong(website wire diagram) Marstran OT wires and the sound was very high pith noisy if the orange(common by marstran) is grounded. The only wire that accept ground is the gray wire. All others come the high pith noisy sound.


Thanks!

Goonies
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Posts: 65
Joined: Sun Aug 17, 2014 7:09 pm
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Re: Lower Bias range

Post by Goonies » Sun Mar 26, 2017 12:09 pm

I´,ve solder again all the 1 omhs resistors from the 1 and 8 pin V4 to V7 and the voltage it is in mV to all tubes. But the range is very low. The bias might be cold.

thanks

danman
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Re: Lower Bias range

Post by danman » Sun Mar 26, 2017 1:24 pm

The set of power tubes that you are using may fall just outside of the range of your bias pot. You could try another set of tubes to see if they bias up better or you could change the resistor that is inline with the bias pot. Lowering the resistor value will help to raise the current draw mv of the tubes...raising the value will bring down the current draw. Just make small changes of 10-15k at a time.

Goonies
Senior Member
Posts: 65
Joined: Sun Aug 17, 2014 7:09 pm
Just the numbers in order: 13492

Re: Lower Bias range

Post by Goonies » Sun Mar 26, 2017 7:44 pm

danman,

Thanks!
I´ve changed the 56K bias to 33K and the bias is perfect now. The range is larger -30 to -42. I think 47K would be a perfect choice too.

Now I wonder, how to find the correct wire color for the OT. I´m worried about the colors. Will open a new post with this subject.

thanks

danman
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Joined: Tue Sep 10, 2013 9:09 pm
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Re: Lower Bias range

Post by danman » Mon Mar 27, 2017 4:52 pm

Your method of checking the OT seems good. I have had one PT with the wire colors marked incorrectly so it can happen. I would email the manufacturer for their suggestions on how to proceed.

Goonies
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Posts: 65
Joined: Sun Aug 17, 2014 7:09 pm
Just the numbers in order: 13492

Re: Lower Bias range

Post by Goonies » Mon Mar 27, 2017 9:11 pm

danman wrote:Your method of checking the OT seems good. I have had one PT with the wire colors marked incorrectly so it can happen. I would email the manufacturer for their suggestions on how to proceed.
Thanks for the answer. I´ve sent a email to Marstran and they told me to check the wire diagram at marstran website.

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