# Fixed Bias Circuit Conversion?



## ampaholic (Sep 19, 2006)

Hi folks, I hope everyone is keeping safe.
With nothing but time on my hands I've tackled a project that has been hanging over my head for years. It involves converting an old Spartan mono tube hifi chassis into a guitar amplifier. The Spartan circuit ran a pair of 6V6 power tubes, a 5Y3 rectifier and a variety of 6AU6 type tubes.
I've decided to go for a Fender Harvard 5F10 design which uses a single 6AT6 and a 12AX7.
I'm using parts that I have on hand and as a result I don't have a turret board so I'm working in true point-to-point fashion. I've got a real hornets nest going and if this thing doesn't work I'm not sure I'll ever be able to figure out where I went wrong!
Here's the issue I'm working through:
The power transformer does not have a separate tap for bias in that the 6V6's were cathode biased. The 5F10 circuit is fixed bias and does have a transformer bias tap.
So, I'm thinking I have 2 options. First, I could convert the 5F10 circuit to cathode bias. I have a tube amp book by Dan Torres that describes the process of converting a Princeton Reverb to cathode bias.
The second option (I think) is to do what other Fender circuits have used by hooking into the secondary winding of the power transformer that goes to the rectifier plate.
I'm not sure which is better, or which is easier. I think I'd lean towards keeping the fixed bias circuit and hooking into the rectifier plate.
If I did that I assume that the 6.8K dropping resistor in the bias circuit would need to be increased. In the Princeton circuit it is 100K. Since I don't really know what I have for voltages I'm kind of shooting in the dark.
Here's the top side of the chassis. I'd be afraid to show you the other side! 
Any thoughts or advice?


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## Lincoln (Jun 2, 2008)

check out either a Fender 6G2 Princeton circuit or a A1164 Princeton Reverb. They both use a wire run directly to the high voltage output to operate the bias. Copy their bias board. I think it's about -35 volts bias that you're shooting for.


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## dtsaudio (Apr 15, 2009)

What Lincoln says is going to get you pretty close. You won't really know until you get the power supply up and running. Don't try it with the output tubes installed in case the bias voltage is too low. 
Once you know how much bias voltage you have, you can move forward. -35V will be plenty to proceed.
I would also advise putting two 1 ohm resistors (one on each cathode of the output tubes) to give you a measuring point to set bias correctly.


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## ampaholic (Sep 19, 2006)

Lincoln said:


> check out either a Fender 6G2 Princeton circuit or a A1164 Princeton Reverb. They both use a wire run directly to the high voltage output to operate the bias. Copy their bias board. I think it's about -35 volts bias that you're shooting for.





dtsaudio said:


> What Lincoln says is going to get you pretty close. You won't really know until you get the power supply up and running. Don't try it with the output tubes installed in case the bias voltage is too low.
> Once you know how much bias voltage you have, you can move forward. -35V will be plenty to proceed.
> I would also advise putting two 1 ohm resistors (one on each cathode of the output tubes) to give you a measuring point to set bias correctly.


Thanks Lincoln and dts.
A couple more questions.
Should the 100K resistor be higher wattage?
Would a 1N5408 diode work? I'm hoping so because that's all I have!


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## dtsaudio (Apr 15, 2009)

A half watt resistor will be fine. There's very little current there. The 1N5408 is massive overkill, but will work fine.


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## Lincoln (Jun 2, 2008)




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## ampaholic (Sep 19, 2006)

OK, thanks again guys. I'll have to dig around and see what I have for a pot that will work.


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## dtsaudio (Apr 15, 2009)

You don't need a pot that big. Anything from 10k to 100k will work.
That 250k pot would go to a bias wiggle tremolo circuit.


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## jb welder (Sep 14, 2010)

Also, where the pot would go to the trem circuit, you must have something. It won't do much as shown.
Typical fender non-trem would have a 10K pot there, and the open end would go to a 10K resistor and that 10K resistor would be grounded. (AB763)
The 27K resistor would be omitted.


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## ampaholic (Sep 19, 2006)

Well, I've got good news and bad news. Good news is that the bias circuit is working well (thanks dts and jb welder!). Further good news is that I can power my project up and not get any smoke!
Bad news is that I can't get any sound. Here's what I've got going on:
I'm pretty confident that power supply and output section are working ok. I've got a very light hum out of the speaker and when I do a back to front voltage check I can hear noise out of the speaker when I touch the meter probe to components but I only get the noise back to the grid of the second half of the 12AX7. (see circuit below).
I may well have other problems but trying to solve them one at a time it seems as if the front half of the 12AX7 is dead. My B+ is 237 Volts (running the amp at 110VAC). I don't have any voltage drop across the 100K resistor going from B+ to the Plate (Pin 1) of the 12AX7. Typically I think this points to an open cathode? Voltage at the cathode (Pin 3) is 0 V. I've quadruple checked connections, component values and of course swapped out tubes.
The second half of the 12AX7 seems to be OK. Plate voltage (pin 6) is 192 V versus B+ of 237V, cathode voltage is 47 V ( bit higher than the schematic's 44V but I think still in range) and I get a nice satisfying click out of the speaker when I touch either side of the .02 coupling capacitor (pin 1 to 7) with the meter probe.
For what it's worth the voltages on the 6AT6 are in normal range. 142 V on the plate and 1.4 V on the cathode.
Any suggestions on why the first half of the 12AX7 appears dead?


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## jb welder (Sep 14, 2010)

Pull the tube and do a resistance check from pin3 (tube side of socket) to chassis. Should be 1K5.
(assuming you have seen heater lit on both sides of that 12AX)


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## ampaholic (Sep 19, 2006)

jb welder said:


> Pull the tube and do a resistance check from pin3 (tube side of socket) to chassis. Should be 1K5.
> (assuming you have seen heater lit on both sides of that 12AX)


Man oh man, I'm feeling pretty stupid right now! Without even looking at the amp I know it's the heater. I just casually looked and could see a heater on so never thought to check both sides.
I'll cut myself some slack though because the layout doesn't show pins 4 & 5 connected. Still should have known! 
Off to the workbench....


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## ampaholic (Sep 19, 2006)

Bingo! It works. Sounds like crap... but it works!


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