Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: WillieBoy on November 26, 2010, 11:33:49 am
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Looking at the layout, and comparing to my Deluxe Reverb RI schematic, I see that on the Hoffman version there are a pair of 1 Ohm resistors prior to connecting Pins 1-8 to the power tubes. What do these resistors do? I also looked at the AB763 for the twin reverb and the super and don't see them.
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This is a convienent and safe way to measure cathode current and set grid bias. By Ohms Law current = volts / resistance and the final
simplification 1 = 1/1. By measuring the voltage drop across that 1 ohm resistor you will know the grid current you are making by changing the bias. That is the voltage drop in mv infers the same number of grid current ma's. Leo Fender did not use them because resistors cost money and preferred that repair technicians measure grid current in what ever way they want.
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By measuring the voltage drop across that 1 ohm resistor you will know the grid current
Plate + screen currents actually. If there's grid current flowing then the grid is positive, and either the tube is defective or it's working in AB2. Grid voltage should always be negative, with no current flowing except in extreme situations.
Leo Fender did not use them because resistors cost money and preferred that repair technicians measure grid current in what ever way they want.
Fender specified a bias voltage, to be measured at the junction of the bias mixers/grid returns. And nothing for a number of amps wich were fixed fixed bias - Mesa and others still do the same.
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Hoffman offers a kit for a bias checker which allows you to measure the cathode
plate current directly:
http://www.hoffmanamps.com/MyStore/catalog/parts9.htm
scroll down to the bottom for parts & info
Sometimes pin 1 is used as a tie point because neither the 6V6 nor the 6L6 use pin 1. Note that EL-34s DO use pin 1 for the suppressor grid. However, tying that to ground is fine so no problem there either.
You might want to consult the newer version of Hoffman's layout even though it's only the board:http://www.el34world.com/Hoffman/images/AB763New.gif
Cheers,
Chip
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So Ala the article about rebuilding the DRRI by Mikey Mann, you could take wires off of these and measure bias using test posts?
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So Ala the article about rebuilding the DRRI by Mikey Mann, you could take wires off of these and measure bias using test posts?
Don't know about that article.
3 test jacks, 1 for each power tube cathode & 3rd for ground
pin 8 goes to a jack, tied to ground jack by 1 ohm (or 10 ohm) resistor
ground jack is, well, grounded :wink:
I still like Hoffman's bias checker a whole lot better. Measures only plate current for one thing.
Cheers,
Chip
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Hoffman offers a kit for a bias checker which allows you to measure the plate current directly:
It measures cathode current actually, which is a combo of screen and plate.
Screen current is very tiny compared to plate current, so it's mostly all plate current you are reading
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Sorry about the misinformation relating to Hoffman's bias checker. It's just so easy to use.
Chip
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The wires are 2 foot long in the kits now