Variable Bias for Garnet Amplifiers

JC Maillet (c) 2008

Notes posted here are for information sharing purposes only. 

WARNING: Tube amps may contain voltages that are lethal and only qualified service technicians should have access

to the insides of  these consciousness altering machines.


When I met Gar the issue of Biasing came up several times.  Gar's idea was that since tubes he was getting from Mullard were on average so bang-on bias wise that he felt there was no need to provide Bias pots like most Fender and Marshall amplifiers had in their circuits. He mentioned that some problems occured through the years from people wrongly modifying his amps in this area - leading to serious damage to tubes and circuitry.  It is quite understandable that Gar would not want to encourage people to seriously screw up their Garnet amplifier by accident.

In his book "The How and Why of Guitar Tubes Amps, as Gar Sees It" (available from Gar presents his official Garnet Bias circuit, which I've reproduced below.  In fact, this is the circuit that is commonly seen in almost all Garnet tube amps, the component values DO NOT change to reflect use of differing power tubes in the output stage.  Instead, Gar carefully chose power transformer voltages to match the tube biasing requirements via this circuit - pretty cool eh ?!  The important thing to notice is how the AC voltage coming off the HV windings is divided by the 120k (2w) and 33k resistors before getting rectified - quite unlike other amplifier designs.

In his book, Gar suggests setting the divider resistors (presumably unloaded) so that it brings the AC voltage down to around 70vac and once loaded and rectified will produce an appropriate bias voltage to the tubes.  Gar suggests adding a resistor in parallel with the already present loading resistor R1 (82k usually) to adjust bias.  Part of the problem here is that if the amp is already running too hot for a certain brand of tubes this tweak will not help things.  The other problem is that if someone were to tweak the bottom resistor in the AC divider and then add a pot of some kind at the output of the loading circuit it is possible that the filter cap would exceed its voltage rating by too much and then blow - leading to power tube melt. Gar told me this is was his greatest fear with techies mucking with the circuit.  Of course, if your Garnet amp biases your power tubes just fine then why mod it ?!

Fair enough !

In case you're wondering, what's a 64v  cap doing in a circuit that calls for a 70vac feed voltage anyway ?  Well, at the time Electrolytic caps were generally designed to handle twice their rated voltage and some circuit designers relied on this.  Even though, I guess Gar realized at some point this wasn't such a hot idea and some time in the 70's he switched to a 100uF/150v filter cap.



Schematic of early Garnet Bias Voltage Circuit, R1 was typically 82k


Going Variable


There might be times when, say, you're going through a bunch of quality power tubes using Bias grades (numbered by re-sellers) and you find there's no point trying to decide which Bias Grade sounds best in the amp without carefully adjusting the bias for each set.  Last time I did that in a SessionMan there wasn't even one pair within the whole numbering span that biased to my liking - some were not too far off, but nothing bang on.  That's where you would find a need for variable/adjustable bias - now every set in the grading ladder can be biased in.  Another major reason would be to extend the useful life of the power tubes through what I call periodic re-biasing - something not possible with non-adjustable biasing. All this doesn't just apply to non-NOS tubes and Garnet amps either, it's a generally useful thing to have variable bias for that purpose alone.

To avoid the complications mentioned above and to be safe, I like to  converted to a circuit that has appeared in several other vacuum tube amplifiers and shown itself reliable.  In a nutshell: 

(i) The bottom divider resistor is removed, (ii) the filter cap is beefed up to 100volt rating and (iii) the 82k loading resistor is replaced by a 50k trimmer, similar in principle to the early Marshall 50 watters and Fender Tremolux 6G9-A circuits.  Just remember, there's no "safety" resistor on this trimmer-only mod. Yes, it's a little cavalier !


 Garnet Bias Voltage Circuit Made Variable


Early-era Garnet Bias board


Green arrows point to the ground connections (black wires) -> leave them intact when soldering !

The Red arrows point to the HT feeds to the circuit - do not modify either !

**The Yellow wire feeds the Bias voltage to the power tube sockets**


Later-era Garnet Bias board


Garnet Bias Circuit Mod

warning: if you don't have much experience don't do this on your own !!!




(i) Carefully replace the cap - observing polarity ...  (ii) remove both resistors aside except for the larger 2-watt 120k resistor (red red yellow) ... (iii) replace the bottom 33k resistor by a 50k trimmer (this will require solidly attaching extenders on the outside legs) ... (iv) the yellow Bias feed wire goes to the sweeper on the trimmer ... make sure all connections are secure and you're done soldering ... apply a glob of silicon between the yellow wire and chassis near the attachment point ... again, Green arrows in the picture denotes ground connections and need be left intact.

With no power tubes in their sockets turn the amplifier on, once the bias circuit is charged up make sure that you measure an acceptable range of negative voltages at pin 5 of all power tube sockets ... -54 volts to -29 volts would be very good in a standard circuit but in this case, with no safety resistor, the trimmer will allow you to raise the Bias Voltage all the way to zero volts and fry your power tubes along the way when the amp is live - so of course be careful when biasing and make a note of it for the future (I use green and red markes on the trimmer itself) ... make sure you use a high-quality trimmer that can handle at least 1watt of heat, don't use the tiny stuff for this ... when you're confident things are doing what they're supposed to - ie., enough negative voltage up to zero - turn the bias control for full negative voltage, shut off the amp, stick power tubes in the sockets and now you're ready to proceed to the actual biasing part ...

If you have any questions don't hesitate to ask !

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