Grid Bias or Cathode Bias

[duderduderini]

Hi Again
I have searched threads and am wondering pros and cons of grid biasing a PSE of 6550s vs Cathode Biasing. My Understanding is that grid leak resistors change in either setup as do coupling caps. You also have a bit more freedom with b+ in that you have more available in grid bias scenario but the thought of Bias supply voltage failure sounds scary Can anyone recommend a circuit/implementation (Bias winding from transformer or voltage divider from b+) for grid bias that can be adjusted and a good operating point for two 6550's in a pse configuration.
Does one sound different to the other?
Many thanks
Nick

 

[el156]

Hi Again
I have searched threads and am wondering pros and cons of grid biasing a PSE of 6550s vs Cathode Biasing. My Understanding is that grid leak resistors change in either setup as do coupling caps. You also have a bit more freedom with b+ in that you have more available in grid bias scenario but the thought of Bias supply voltage failure sounds scary Can anyone recommend a circuit/implementation (Bias winding from transformer or voltage divider from b+) for grid bias that can be adjusted and a good operating point for two 6550's in a pse configuration.
Does one sound different to the other?
Many thanks
Nick

HI !
I think if you prefer grid biasing and to make sure it doesnt fail, you must add in serial with the tube supply, a fuse of about 160ma/tube. But i prefer cathode bias,wich makes an automatic ajustment of tube current. I tested both and didnt notice a big difference in sound quality, and i always use a decoupling capacitor in paralell with cathode resistor to avoid feedback,because it sounds better without it!
But it is only my opinion! some people think different!
Good Luck

 

[Eli Duttman]

When you get beyond the needs of "12" W. types and 7591s, cathode bias can become problematic due to I²R heating in the cathode resistor.

A well sized (wattage wise) 10 Ω metal film resistor under each cathode, when employing "fixed" bias, serves both as an "idle" current test point and a fuse. If the bias supply fails, the resistor burns out, before tube damage occurs.

An interesting option that has much (IMO) going for it is combination bias. Stand the cathode on an unbypassed 100 Ω resistor and apply a negative voltage to the control grid. The cathode resistor provides a convenient test point, part of the total bias voltage, stabilizes the operating point, and provides a small amount of distortion control via degeneration (local current NFB).

 

[sstulken80]

A well sized (wattage wise) 10 Ω metal film resistor under each cathode, when employing "fixed" bias, serves both as an "idle" current test point and a fuse. If the bias supply fails, the resistor burns out, before tube damage occurs.

An interesting option that has much (IMO) going for it is combination bias. Stand the cathode on an unbypassed 100 Ω resistor and apply a negative voltage to the control grid. The cathode resistor provides a convenient test point, part of the total bias voltage, stabilizes the operating point, and provides a small amount of distortion control via degeneration (local current NFB).

+1. "Mixed bias" gets a bad rep from certain CBS-era Fender amps, but done properly, it should allow more stability without sacrificing very much power. Depending on how you size the cathode resistors, you get a little "tube balancing" in the deal, too.

- Scott

 

[duderduderini]

Whats the most reliable methode of providing bisa voltage

Hi Eli
Thanks for the replies ( i mean to thank all of you)
I see a lot of different ways to supply the bias voltage. transformer tap, suitably rectified etc... lm317 with lots of components (that could fail?). Where could i look to find as bulletproof a design for grid biasing (along with partial cathode biasing)
Thanks again to all of you

 

[chrish]

The power amp schematics at AX84 have grid bias with the power supply taken from the B+ with a very simple circuit. Look here at one of the PP amp to see AX84.com - The Cooperative Tube Guitar Amp Project

 

[rifraf50]

safe biasing

hi,the way I go about grid biasing is to find the resistance value needed which sounds nice to your ears but safe to your tubes ,then sub the resistor out for a pot......I do it this way so that if for whatever reason the wiper lifts of the track it then goes to max resistance.....amp may sound cold but safe....an easy fix

if for example you need 82k for the right sound then use a 100k quality linear pot,or if you want less swing use a 68k resistor and a 25k pot in series

this is something that needs to be done right and worth testing and checking to ensure faultless operation

 

[exeric]

I've noted this before so I won't push it too hard. I'd use standard cap bypassed cathode bias but add a source follower mosfet stage to drive the stage. Any power loss that one might be worried about by using such a method over fixed bias will be more than made up by the AB2 class of drive you are getting from the source follower. For sure there won't be any net power loss then and the cathode resistor still provides current stability.

That is, unless your source follower goes whacko!

 

[rifraf50]

an error

hi,the way I go about grid biasing is to find the resistance value needed which sounds nice to your ears but safe to your tubes ,then sub the resistor out for a pot......I do it this way so that if for whatever reason the wiper lifts of the track it then goes to max resistance.....amp may sound cold but safe....an easy fix

if for example you need 82k for the right sound then use a 100k quality linear pot,or if you want less swing use a 68k resistor and a 25k pot in series

this is something that needs to be done right and worth testing and checking to ensure faultless operation

did I say max resistance!!! I mean min resistance.....let those negatives fly baby and put out the fire!!!

I dig what your saying exeric........but I like simple.

have you heard about the its Christmas time mod where you tie pin1 to pin 8 with lets say a .045uf cap on el34's to tighten the sound up...I think its more a hifi thing but wondering if anyone has done it to a guitar amp?

 

[Blitz]

If I would try this 10 or 100 ohm mixed bias: which wattages would you propose for the resitor to use it as a fuse ? Lets assume 50mA through a el34 as an example.

So, 0,05a*0,5v...0,025 watt ... not sure to find such a small resistor

With 100 ohm...0.05a*5v=0.250 watt

So, is the fuse aspect adressed with 0.6 watt 100ohm resistor ?

 

[rifraf50]

which resistor

If I would try this 10 or 100 ohm mixed bias: which wattages would you propose for the resitor to use it as a fuse ? Lets assume 50mA through a el34 as an example.

So, 0,05a*0,5v...0,025 watt ... not sure to find such a small resistor

With 100 ohm...0.05a*5v=0.250 watt

So, is the fuse aspect adressed with 0.6 watt 100ohm resistor ?

start small and see what happens....keep your finger on the standby switch ,put in some old tubes, disconnect negative voltage ,watcth your tubes closely and get ready to shut her down if the resistor does not blow...that's what I would do, but I have not considered dual bias before.....start with .25 watt first and see how that goes....from my perspective as a gigging guitarist last thing I would want is a amp out during a show...ugh!!

cathode bias = nice sound not the most sound,fixed bias = most sound not always nice sound ...just my opinion

 

[troposnet]

my experience with 6550s is that, somewhere along the line when they do age a little, just one of the 4 tubes would start to red plate and precisely at that moment, you will find out that being in the cathode bias mode, you are unable to resolve it.

again, it would quickly seem (and factual at that) fixed bias is the solution and definitely a must for tube circuitry as you can now calibrate the bias point for each of the tubes. it would also put out more power in cases as the voltage between anode and cathode is larger now that you convert the old cathode bias circuit (the voltage drop across cathode resistor now appears between anode to cathode in addition to B+).

sound wise, the cathode bias would seem to have deeper, slower bass with a more laid back soundstage as versus the fixed bias which has faster, less depth bass with a more forward staging sound. thats purely my observation given all other equipment are equal.