# Fender Bias Circuit



## ampaholic (Sep 19, 2006)

Hey folks, I'm having a Super Reverb issue that has caused me to dig into the bias circuit to check a few things out. One issue that I have (although it may not really cause a problem) is that I'm just barely creating enough negative voltage out of the circuit to bias my tubes. It's adequate but not a lot of wiggle room.
I've been doing some checks and actually have been doing a side by side comparison to my Pro Reverb to see what kind of difference there is. Here's what I'm finding.
With both the Super and the Pro I have about 50.5 VAC on the transformer side of the 470 Ohm resistor. On the other side of the resistor it drops to 48.3 VAC on the Super and 49.7 VAC on the Pro.
The big difference comes on the negative side of the diode where the Super is generating -55.8 VDC while the Pro is cranking out -63 Volts. When setting bias on the Super I almost have the 10K bias pot at it's minimum in order to get in a comfortable bias range (currently 35 ma with plate voltage of 440 Volts).
I guess my question is this; do you think the difference in negative bias voltage (55.8 versus 63) would be more likely due to a variance in the value of the 470 ohm resistor or perhaps a leaky diode? My instinct is the diode in that the AC voltage difference on the + side of the diode is less than 1-1/2 volts which I'm not sure would cause the 8 volt difference in DC volts?
Any thoughts would be appreciated!
PS: the bias cap has been changed in both amps with a 47 mfd/160 volt cap.


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## WCGill (Mar 27, 2009)

You have lots of negative voltage there, 35ma is too cold, yielding only 15 watts dissipation. I like 20-21 watts.


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## ampaholic (Sep 19, 2006)

WCGill said:


> You have lots of negative voltage there, 35ma is too cold, yielding only 15 watts dissipation. I like 20-21 watts.



Good point, I'm just a bit gun shy because when I cracked the amp open both the tubes were redplating. I quickly checked the bias voltage and it was at -29 with 90 ma running through the tubes!
I have no idea what caused this in that I readjusted and it has been stable since. Has anyone ever seen the pot itself go bad? I put a meter on it and it seems to run through the 10K sweep pretty smoothly but realistically a 50 year old pot can easily have a couple of dead spots that the digital meter doesn't pick up.
Because the bias cap is 10 years old I replaced it with a new Sprague 100 mfd/ 100 V just in case. The 470 ohm resistor has drifted pretty good (560 ohms) but otherwise seems stable. I've got some NOS Allen Bradley 470 ohm 2 watts so I might just change it.
Also, as mentioned I'm suspicious of the diode. I've got some 1N4007's and maybe should just change it.


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## WCGill (Mar 27, 2009)

Yes the pot could be bad. I've had new CTS pots faulty out of the box, one cost me a set of KT88's before I found out what was blowing fuses in the amp. Give it a cleaning and remeasure. Check the screen resistors also.


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## Wild Bill (May 3, 2006)

First off, who cares what is the actual bias voltage? All that's important is the idling current. We calculate the "sweet spot" as Pd / Vp * .7 = desired idle current 

Your voltages with both amps are very close. Don't forget, the pot resistance value has probably a +/- 20% tolerance. Also, what about the tolerance of the 27 k resistor?

I don't think there's anything wrong in either amp. Sometimes for whatever reason (usually not worth the time to figure out) there may not be enough adjustment range on the pot. So I just change the value of the 27k resistor a bit. 27k is actually a bit less common than 22k. Whatever works is fine!

Wild Bill/Busen Amps


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## ampaholic (Sep 19, 2006)

WCGill said:


> Yes the pot could be bad. I've had new CTS pots faulty out of the box, one cost me a set of KT88's before I found out what was blowing fuses in the amp. Give it a cleaning and remeasure. Check the screen resistors also.


Yeah, I'm suspicious of that pot. I've given it a shot of deoxit and I still get a couple of bumps on the meter when I turn it slowly.
Any recommendations on a good pot or just stick with the standard CTS?


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## WCGill (Mar 27, 2009)

I'm only able to source 10k CTS pots that directly replace the Fender item. I think they'll be fine but a check before install would be in order.


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## ampaholic (Sep 19, 2006)

Wild Bill said:


> First off, who cares what is the actual bias voltage? All that's important is the idling current. We calculate the "sweet spot" as Pd / Vp * .7 = desired idle current
> Wild Bill/Busen Amps


Thanks Wild Bill. My question wasn't intended to suggest using bias voltage as a means to set power tube bias but rather was centred around whether or not I had a sufficient range of negative voltage to adequately bias the tubes.
Thanks for the advice all!


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## loudtubeamps (Feb 2, 2012)

Wild Bill said:


> First off, who cares what is the actual bias voltage? All that's important is the idling current. We calculate the "sweet spot" as Pd / Vp * .7 = desired idle current
> 
> With all due respect.
> I don't think you really meant "who cares what is the actual bias voltage?" did you Bill?
> ...


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## epis (Feb 25, 2012)

Hi Doug, I'm not trying to be smart ass, but it seems that You didn't get what Wild Bill said.His second sentence tells everything :" All that's important is the idling-BIAS current." So it doesn't matter if You measure -55.8V or -63V, because You have to measure bias current.Output tubes from different manufacturers and differently worn out will have a different bias current at same bias voltage. Cheers, Damir


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## loudtubeamps (Feb 2, 2012)

Hi EPIS..no worries. Your question and remarks are spot on.
I assume that anyone with a basic knowledge and desire to fiddle with,maintain or build tube amps will have a grip on output tube bias and plate current draw these days.All the info you need is just a click away, right? , never been easier.
My comments may not have been explicit enough ,my apologies.
When I see that a tech. question has been addressed by the likes of WCGILL, WILDBILL or others that take the time to respond, one can assume that the problem is well on the way to being solved. 
These guys obviously know what they are on about and my hats off to them for taking the time to share and to teach others.
The question that AMPAHOLIC posted (as I understood it) was more to do with sufficient bias voltage in order to stabilize the output stage rather than "how to measure bias current".
I was simply stating an obvious fact in response to the comment that the bias voltage does not matter.
WB is of course correct, it's not relavent for the purpose of setting or measuring the bias trim but that was not the initial concern. 
Simply a difference of interpretation of the initial question and response both from WILDBILL and susequent reply from AMPAHOLIC.
cheers, doug


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## epis (Feb 25, 2012)

Hi Doug, people new in tube electronics very often trust to much voltages marked on old diagrams. They don't consider the old components value drift, different mains voltages, different tubes. If you are in +- 20% limits, everything is fine. Cheers, Damir


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## loudtubeamps (Feb 2, 2012)

epis said:


> Hi Doug, people new in tube electronics very often trust to much voltages marked on old diagrams. They don't consider the old components value drift, different mains voltages, different tubes. If you are in +- 20% limits, everything is fine. Cheers, Damir


 ..........and that is why we have DMM's.
cheers, doug


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## epis (Feb 25, 2012)

Yup, with less than 1% tolerance hahaha


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## keeperofthegood (Apr 30, 2008)

ampaholic said:


> Hey folks,
> *[followed by a bunch of stuff]*
> PS: the bias cap has been changed in both amps with a 47 mfd/160 volt cap.



Is this the deal with the 470/3W resistor that has some notoriety of smoking?

And yes, you can go up in voltage on capacitors so long as the can size fits or they are not ceramics (ceramics value is voltage dependent), but you should not go down in voltage.


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## loudtubeamps (Feb 2, 2012)

Hi. Keep.
The 470Ω (or whatever the value required) to set the D.C.- voltage(-50 to -60) range is usually a 1 watter.
I have seen them in a 1/2 watt value as well. 
I can't remember having to replace one of these for a very long time,(in over 30 years) if ever.
I guess everyone has their own reasoning for capacitance and voltage ratings on the filter.
I like to replace as per stock value in a repair or just enough capacitance to get the job done and 20 volts or so over the D.C. - supply in a new build.


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## loudtubeamps (Feb 2, 2012)

*bias cap stuff*

I dug up this bit of info...


"Those "claimed learned folks" can indeed ignore it because the filter capacitance is effectively in series with the coupling capacitor for purposes of frequency response, so the value doesn't have to be very large at all in conjunction with the bias feed resistor value. 

There will be very little difference in response as long as you are over 10x the size of the coupling capacitor, and virtually identical response if you are 100x the size of the coupling capacitor, so you can get by with as little as 1uF to 10uF if you have a 0.01uF to 0.1uF coupling capacitor and the usual 100K to 220K bias feed resistor. 10uF is usually chosen because 1uF will usually not give you enough ripple rejection, unless you use a very large series filter resistor.

There is no low frequency response benefit to going any higher than that, and, as you correctly pointed out, the risetime of the bias supply is very important, so you want to keep it small.

If you want to calculate the minimum size filter cap you need for frequency response purposes, use the formula: 1/(2*pi*R*C). The value for "C" would be the series combination of the coupling capacitor and the bias filter capacitor. 

For example, with a 0.022uF cap, a 220k bias feed resistor, and a 10uF bias filter cap, the effective value of "C" would be 0.022uF||10uF = 0.02195uF, giving a -3dB response of 32.96Hz. If you reduced the capacitor down to 1uF, the effective value of "C" would be 1uF||0.022uF = 0.02153uF, for a -3dB response of 33.61Hz. A 1Hz difference is negligible that far down below the guitar response. Since even a 1uF filter capacitor will give you sufficient frequency response, the selection of the filter cap value comes down to ripple and risetime.

The design of a bias supply is a tradeoff between adequate filtering, fast risetime, and a lower -3dB point higher than you would get with the RC filter formed by the coupling cap and bias feed resistor so it will have no impact on the tone.

Obviously, from a ripple filtering standpoint, you would want the largest possible filter capacitor, but that is at odds with the need for minimum risetime. In order to get adequate filtering using smallish capacitors for fast risetime, it is best to go with a multiple-section filter. As shown previously, frequency response concerns will be negligible for any commonly used value.

As far as the ripple rejection goes, you can calculate that, too. A 1-section RC lowpass filter has a -3dB point of, again, 1/(2*pi*R*C). So a typical 10K resistor in conjunction with a 10uF capacitor would yield a -3dB point of 1.59Hz. 

Now, you might think that a corner frequency of 1.59Hz would be fine when dealing with a 60Hz half-wave rectified ripple, but a single-pole RC filter only has a rolloff rate of -6dB/octave, or -20dB/decade, so the hum rejection would only be down -20dB at 15Hz, and only about -30dB at 60Hz. This is not enough ripple rejection to provide hum-free operation. If you are going to use a 1-section filter, you will have to increase your filter cap value or increase the value of the series resistor, or both, in order to obtain adequate hum rejection. This is why you will see values larger than 10uF in single-section filters.

A 2-section RC filter, on the other hand, has a rolloff rate of -12dB/octave, or -40dB per decade, so it will give you over 60dB of ripple rejection at 60Hz, which is usually good enough, especially in a push-pull amplifier, which will cancel out the bias supply hum due to the differential nature of the amplifier and the fact the that hum is common-mode, meaning the same phase on each input of the difference amplifier.

For even better hum performance, you can go with a full-wave bias supply rectifier, which will shift the hum frequency up to 120Hz, relaxing the filtering requirements."


Randall Aiken
__________________
http://www.aikenamps.com


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## Wild Bill (May 3, 2006)

Randall Aiken is one of my heroes but I think his words are being misapplied here. He starts off by pointing out that the values used already in the typical bias circuit, including the Fender one under discussion are more than adequate for the job. He points out that if you are really anal about any ripple in the bias supply you might want to go to a much larger value of filter but that would have a rise time problem.

What he means by that is that too large a value will actually take seconds to initially charge up. This means that until it is fully charged your bias voltage is far too low, causing your tubes to draw current like mad! I have found many hifi audiophile techs make this mistake. I actually was given a capacitor replacement kit for a Harmon Kardon Citation II power amplifier with this boner! There is a tendency out there to bump up the filter capacitance a LOT, thinking it improves the response. I guess they all think the original design engineers were idiots! Anyhow, usually it does no harm with the main filters but in this kit the seller had done the same with the bias filter. It would have taken possibly 15-20 seconds to reach full charge! This would have been extremely hard on the output tubes during warmup. Obviously, I did NOT use this value!

I don't understand why Randall goes on about multi-section filters in the bias circuit at all! You only get ripple across a filter cap if you draw significant current. Otherwise the cap simply charges to the peak value of the ripple wave and stays there! It cannot drop in voltage during the slumps in the cycle because there is no load current drawing it down! Since bias supplies are simply voltage sources and any current draw would need an incredibly expensive meter to even measure any ripple will never be passed onto those output grids!

Especially when virtually all guitar amps are running Class A or AB1. Some very special designs run AB2, where a small amount of grid current IS drawn! With those amps I could understand a bit more complicated bias supply but such amps are exceedingly rare! In all my years of working on amps (and I'm OLD!) I have seen only TWO!

So in the words of Tony Soprano - fuggedaboutit!

Wild Bill/Busen Amps


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## WCGill (Mar 27, 2009)

Rise time probably isn't an issue because most amps have a standby that interrupts the B+ from hitting the power tubes until the amp is warmed up. The bias voltage, whether from a secondary leg or a dedicated supply is there as soon as the AC is turned on and the cap charges, usually before the standby is flipped. Even a cathode rectifier with slow voltage rise time delays the B+ long enough for your bias to be effective. That being said, there is no reason to use large values of capacitance for the bias filter, especially with many of the smaller and older amps with no standby and filamentary or solid state rectifiers because correctly-sized caps will charge before the power tube cathodes are hot. I don't think there's any relationship between grid current and bias supply ripple, but I'd like to find out.


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## ampaholic (Sep 19, 2006)

If I had something really intelligent to add now would be a great time to throw it in. Instead I'll just say thanks everyone. (I think!)


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## loudtubeamps (Feb 2, 2012)

ampaholic said:


> If I had something really intelligent to add now would be a great time to throw it in. Instead I'll just say thanks everyone. (I think!)


 Hey there ampaholic.
I assume by your earlier comments that you have found the source of your problem and have your output section under control now??
Was it the pot or something else?
Cheers, d.


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## ampaholic (Sep 19, 2006)

loudtubeamps said:


> Was it the pot or something else? Cheers, d.


Well, I've set bias and it seems pretty stable. Running things a bit hotter per advice in previous posts but I'm still well within a 70% dissipation.

I'm still a bit concerned about the pot but didn't have one on hand so I've got one on the way.

I was actually troubleshooting another problem when I got sidetracked by the bias thing. I've had a persistent noise in the amp that I can't track down. The best way to describe it is a "whooshing" noise when I play single notes and the amp is turned up a bit. Not crazy high volumes but at 3 or 4 on the volume, which in a Super Reverb is plenty loud. It's not a ghost note that would suggest a filtering problem but almost like a little blast of air after the note. Not describing it well but that's the best I can do!
I've tried tubes obviously, coupling caps, grid resistors, etc but it's still there. Next step is to put a scope on it and see if I can narrow it down to a specific stage although I've always suspected the issue is in the output stage in that when it makes the noise I can see a flash in one power tube.
Lots of fun!


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## loudtubeamps (Feb 2, 2012)

Is your amp an older point to point or a re-issue?


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## ampaholic (Sep 19, 2006)

'68 Drip Edge AB763


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## WCGill (Mar 27, 2009)

You say you've tried tubes so I'm assuming you've changed the outputs. If the flash is always on the same socket, then it's obviously in the amp and I'm sure you've figured this one out if switching tubes yields the same result. I'd be trying screen resistors if you haven't already.


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## loudtubeamps (Feb 2, 2012)

What WC said....
As well,have a good look at the output tube socket bases.
If they are not clean and shiny or you see any kind of markings between pins 3 and 4 you may have some carbon arching going on.
also..check the condition of the 1k5 resistors connected to pin 5 for fractured/cracked or loose connections.


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## loudtubeamps (Feb 2, 2012)

I forgot the obvious other half :check the bottom of the tube base (between pins 3/4) for signs of carbon arching as well.
cheers d


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## WCGill (Mar 27, 2009)

Yup, like he said, sockets too.


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## ampaholic (Sep 19, 2006)

Thanks guys, this has been a perplexing problem. Obviously tubes was the first thought but reversing them didn't change anything. For the hell of it I replaced them anyway. Same noise and blue flash in the same side.
I bought 25 NOS Allen Bradley 2 watt 470 ohm resistors and matched a pair of them up. I've also replaced the 1K5 resistors and even went so far as to offset the resistor because I read somewhere that to prevent noise the lead on the tube side of this resistor needs to be shorter than the lead on the other end. In other words the resistor is hugging the grid pin quite closely.
I've cleaned and re-tensioned the socket pins more than once and have given the sockets a pretty good examination but perhaps I'll get the magnifying glass out and be more thorough.
I'm not handy with a scope but I do have a Tek digital scope. Think it might pick up the noise?


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## loudtubeamps (Feb 2, 2012)

If you're getting flashing going on in the output tube, it's pretty much a high voltage problem.
Your carbon trace may be difficult to see.
Usually it's pretty obvious and looks like a burn or scorch mark or a dulling of the socket base material.
You could try a didgi multi meter set on Ω ohms and with the amp unplugged(of course) and the caps drained, put the meter probes on the tube socket base near pins 3 and 4 and move them around slowly and look for _any readings_. 
The proximity of the 1k5 resistor would not be causing the problem you are describing.
It will be, in all likelyhood something fairly simple even though it doesn't seem that way now.
If the tubes, sockets, 470Ω resistors are known to be OK for sure ,then the next place to look would have to be the output transformer.
Unless the amp has been modded or some other info that you have missed??

Just a shot in the dark....check the flex leads on your speaker for wear or fraying... they will arch if they are becoming fatigued... and can cause some pretty weird symptoms in an amp.
Like I said....a shot in the dark. 
Try to reproduce the tube flash using an extension cab with your combo speaker disconnected.If it still happens then you know it's not vibration related.


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## WCGill (Mar 27, 2009)

Try reversing the plate leads. If it's the same location, not your OPT. You will have to unsolder your feedback resistor for this operation.


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## loudtubeamps (Feb 2, 2012)

also... .be sure you have a solid ground connection from chassis to Pin 8.
and as mentioned...has this amp had any mods to the output section? diode strings etc.


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## ampaholic (Sep 19, 2006)

Guys, thanks so much for the help!
I've given the tube sockets a pretty thorough exam and didn't see any signs of cracks, burns, etc. They really look like new. All connections are solid. Once again I re-tensioned the socket pins and took a small needle file and scraped out the inside surface of each pin.
The only odd thing that I noticed is that on one tube (the side giving the problem) there was a slight discolouration on pin 6. At first glance it looked like an intensely hot burn mark that was a grey ash colour. It wiped right off and I gave pin 6 on the socket a really good look and didn't see any sign of arcing.
Coincidentally I played the amp for 30 minutes afterward and couldn't get it to make the noise. That has happened before though so I'm not jumping for joy yet!


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