# Fender Reverb II bias question



## mkaye (Jan 25, 2009)

i am working on this amp (original Fender output tubes) and i measure bias current of 136/129ma using the OT resistance & voltage drop.
the B+ is 462V, so a quick calculation indicates that 30W*2/462V=130ma
so the tubes are biased pretty close to 100% dissipation!!
it has an output tube balancing pot, which seems to swing the bias voltage back & forth to each pair, allowing you to balance, but not really adjust the bias point
do i change resistors to get the bias point closer to 70%?
i have 2 pair of matched Sovtek 5881's that the customer gave me and was thinking of dropping them in & see what difference i get & hope that their bias point is closer

mark

i also have 10V p-p on the 1st P/S stage - is this normal, or are the caps getting old?

i did a better measurement (i was measuring the voltage at the C/T and the plates and subtracting, but i was losing some accuracy)
i measured the difference between the C/T and plates and now get 80.6/44.2ma - much better, not biased properly, but in the correct range


----------



## Wild Bill (May 3, 2006)

mkaye said:


> i am working on this amp (original Fender output tubes) and i measure bias current of 136/129ma using the OT resistance & voltage drop.
> the B+ is 462V, so a quick calculation indicates that 30W*2/462V=130ma
> so the tubes are biased pretty close to 100% dissipation!!
> it has an output tube balancing pot, which seems to swing the bias voltage back & forth to each pair, allowing you to balance, but not really adjust the bias point
> ...


Mark, the voltage drop method means you measure the resistance from the CT to the plate on one tube and then from the CT to the plate of the other tube. The resistance will almost certainly not be the same. This is because of the way the windings are wound inside the transformer. The winding on the inside will have fewer turns but more inductance.

You measure the voltage drops at the same points.

I would expect to see 30/462 times 70% = 45 ma as the target for EACH tube! This is with no signal applied.

That 45 ma is an upper limit. 70% of the plate dissipation figure is considered the "sweet spot" for best tone vs. tube life. I doubt if the human ear would notice much difference from 60-70% or even a bit lower. 

It is important that both tubes are within a few ma. of idle current to each other. I wouldn't want a spread of more than 5 ma. This is because you can get a situation where the power load is not equal. One tube will be loafing and the other will be flogged beyond its ratings. Not a good idea!

The bias balance pot was a later idea in Fender amps. The engineers who took over after Fender sold the company to CBS seemed to be all hifi amp guys. Close balancing gives the least distortion.

Why on earth would someone want a hifi amp for an electric guitar? Certainly not if he wanted to sound like Van Halen.

Since virtually everyone buys their power tubes as matched pairs these days the balance circuit is kinda useless. Most folks will have the circuit changed to the traditional black face bias level arrangement, which is much more useful. Also much easier than continually dinking around changing resistors.

As for 10 volts PP of AC ripple at the 1st power supply node, so what? 10 volts out of 462 is a pretty good percentage. Push-pull circuits balance this ripple out anyway. Does the amp have excessive hum? If not, fuggetaboutit!:smile:

:food-smiley-004:


----------



## mkaye (Jan 25, 2009)

i measured the 5881WXT's in the circuit, a pair at a time, but measured all the tubes in the same socket (same bias & voltage)
#1 30.8ma
#2 34.3ma
#3 26.2ma
#4 29.4ma

#1 & #4 are within 4.5% - is that close enough for parallel tubes in a Twin?
i checked another matched pair of TAD's i had on hand & they were identical
should i keep 1&4 and recommend a new pair of 5881's or 2 new matched pairs?

mark

Wild Bill
just read your response after posting the current results
if 5ma difference is OK, then i could use 1&2, 3&4 as the pairs
what do you think?


----------



## Wild Bill (May 3, 2006)

mkaye said:


> i measured the 5881WXT's in the circuit, a pair at a time, but measured all the tubes in the same socket (same bias & voltage)
> #1 30.8ma
> #2 34.3ma
> #3 26.2ma
> ...


Mark, a Twin is so flippin' loud that you're never likely to crank the amp up to its limit anyway! So power sharing is not that big a deal with the tubes. 

I would use 1 & 4 on one side and 2 & 3 on the other. This would average to about 60 ma idle current on each side. Your worst case spread is 26-34 ma. That's 8 ma but overall I don't think it will matter. 

Overall they will be a bit on the cold side but hey, it's a Twin! We're talking clean and loud here. It's not that critical and the tubes will show better life.

You could rewire the bias pot to adjust level instead, then set it while measuring the hottest tube. That way all of the tubes will not exceed the maximum.

It's important to remember that we are dealing with devices that will operate just fine over a RANGE of conditions, with little or no differences in overall output and performance. Trying to get fussy and talk about fractions of a ma serves no purpose. 

It's like the old line about measuring with a micrometer and cutting with an axe!:smile:

I would worry far more about not exceeding the overall limit of the idle current than the differences you have quoted.

If you were talking aout a 50 watt Marshall with only one pair of tubes that is likely to always run near its limit then maybe things would be a bit more critical.

BTW, didn't you start off referring to a Rev II? Wasn't that a 6V6 amp?

:food-smiley-004:


----------



## mkaye (Jan 25, 2009)

thanks for the quick reply
yup, a Twin Reverb II w/4 6L6's (schematic agrees)

i grouped them as 1&2, 3&4 and used the balance pot to get them within 1ma
so ~62ma for each pair, running cool @~50% dissipation

time to give a listen & see if the hum is still there

1 of the original 6L6's was definitely a lot different than it's mate

mark


----------



## Wild Bill (May 3, 2006)

mkaye said:


> thanks for the quick reply
> yup, a Twin Reverb II w/4 6L6's (schematic agrees)
> 
> i grouped them as 1&2, 3&4 and used the balance pot to get them within 1ma
> ...


Ah so! I was thinking of the Princeton Reverb II. I had one once. Great amp!


----------



## mkaye (Jan 25, 2009)

still got a buzzing that i can't track down
if i pull V5 it stops (V1/2 had no effect)
the voltage at P/S node C & beyond has a waveform that dances all over my scope on DC and 20ms/div
i tried another 20uf cap in parallel with the 1 at node C, but no change
running out of troubleshooting ideas

mark


----------



## Wild Bill (May 3, 2006)

mkaye said:


> still got a buzzing that i can't track down
> if i pull V5 it stops (V1/2 had no effect)
> the voltage at P/S node C & beyond has a waveform that dances all over my scope on DC and 20ms/div
> i tried another 20uf cap in parallel with the 1 at node C, but no change
> ...


Sounds like you have an oscillation! I would check the voltages at the plates of all the preamp tubes. If a stage is oscillating it will be drawing more current than it should, which should show up as a higher than normal drop across the plate load resistor. It would also likely show a higher than normal voltage at the cathode, across the typical Fender 1K5 cathode resistors, or whatever value.

I'd also check the voltages around V5, the Phase Inverter. 

Your scope should show you any oscillation waveform at the plate of the offending stage but if it's strong enough it can ride through the P/S nodes to affect the other stages.

Isn't servicing fun?kkjuw

:food-smiley-004:


----------



## mkaye (Jan 25, 2009)

pulled all the tubes & broke the P/S at the 2nd node, so i have a few resistors & caps in the circuit - still have a dancing waveform all over the scope
could it be 1 of the diodes in the full-wave bridge acting up?

mark


----------



## Wild Bill (May 3, 2006)

mkaye said:


> pulled all the tubes & broke the P/S at the 2nd node, so i have a few resistors & caps in the circuit - still have a dancing waveform all over the scope
> could it be 1 of the diodes in the full-wave bridge acting up?
> 
> mark


If that were true, it would be impossible for you to have 462 vdc B+!

What do you mean by a 'dancing waveform'? That sounds to me like you have no ground return on your scope probe and the probe/lead is picking up just stray EMI.

Can you describe the waveform better? Is it a sine wave, or like rectified pulses, or what?


----------



## mkaye (Jan 25, 2009)

i have the scope set for 20ms/div (a long time)
as the trace goes across the screen i will see short dips or peaks that go off the top or bottom of the screen that are about 1 division wide, not a sine wave at all, these do not occur at regular intervals (that i can tell anyway)
when i 1st power up the amp there is low grumbling that lasts for about 30 seconds, but doesn't correlate with what i am seeing on the scope
just giving as much info as i can in case you can come up with a theory

mark


----------



## mkaye (Jan 25, 2009)

i am looking at the B+ with the amp in standby & see a sawtooth with an equal peak every 16ms, i see another peak 1/2 way in between, but it is only 1/2 height
this waveform is superimposed on a longer wave with the random spikes in either direction
i normally see equal peaks every 8ms
i think i have a diode or bypass capacitor problem??

mark


----------



## Wild Bill (May 3, 2006)

mkaye said:


> i am looking at the B+ with the amp in standby & see a sawtooth with an equal peak every 16ms, i see another peak 1/2 way in between, but it is only 1/2 height
> this waveform is superimposed on a longer wave with the random spikes in either direction
> i normally see equal peaks every 8ms
> i think i have a diode or bypass capacitor problem??
> ...


This sounds normal to me! You won't ever see a flat waveform at the initial P/S nodes, especially the 1st one. Ripple is expressed in a percentage of the total voltage. Out of 462 volts you aren't seeing much.

Besides, that node sees hum balanced out from a push-pull tube arrangement.

You have to consider that there is more than one source of hum and more than one KIND of hum!

Hum from the power supply is actually ripple. It comes from the rectified pulses of the AC HV source. It is easy to recognize since it has a frequency of 120 hz, which is twice as high as that of stray AC pickup in the gain stages of the amp.

If your 'hum' was coming from insufficient filtering then with the PI pulled you would still hear the hum from just the output tubes. That doesn't seem to be the case.

Hum in the preamp stages comes from many sources, like wiring pickup, a tube going wonky, filament AC hum getting into the grids or cathodes and a host of others. Since this is a printed circuit board amp we can rule out wiring, unless someone has changed things, particularly screwing around with the ground points which is a GREAT way to wind up with hum! There is a tendency for those not sufficiently learned to assume that ground in an amp is like the frame ground in a car and you can ground to any bare metal you find. The technical term for this approach is 'hum generating'.

I would try a different tube in the PI position. Then I would check all the voltages to make sure that you don't have one side or the other screwed up. Both triodes in the PI must work correctly to balance out hum.

How much hum are we talking, anyway? Few tube amps will ever be as hum-free as a solid state amp but they should be quiet enough that you don't hear anything until the volume is turned up at least halfway and always you never notice the hum when you're playing.

BTW, you told me the time involved on your waveforms but nothing about the amplitude. Are those pulses in mv or in hundreds of volts? It makes a difference!

Still having fun?:smile:

:food-smiley-004:


----------



## mkaye (Jan 25, 2009)

i thought i was chasing my tail - the voltages are pretty low 
they look worse when you have the scope on AC and crank the v/div down to see what you've got
i put it all back together & played it yesterday & thought i was done, but the buzz is a little more than i would expect & is worse with the lead channel engaged
you're right that i don't notice it while playing, but with the guitar plugged in and the volume up it is very noticeable & it does change with the V/C
on the scope i tried other probes and checked the P/S on a Marshal i was working on & i don't see this type of waveform
it is very hard to describe 
i see the P/S ripple and this is superimposed on a waveform that is 4-5x larger with a period in the 60-80ms with sharp random pulses that are at least 10x larger and maybe 10ms wide
this may be a red herring & have nothing to do with the buzz
i am going to go thru the amp a stage at a time again & see if it is something else

mark

more detail of what i am seeing
looking at the 2nd P/S node
50mv/div vertical
50ms/div horizontal
the P/S ripple is .2 div high
the random signal is 1-2 div high with peaks & valleys that go off the screen
period seems to be ~50ms, so 20Hz
so some sort of low frequency oscillation, but it is on every node of the P/S and i can't seem to find the source of it


----------

