6L6 operation: plate v suppressor voltage

[centervolume]

Hello
I have been troubleshooting some old fender amps and studying schematics, taking DC voltage measurements etc. I've noticed the occasional error on old fender schematics from the late 50s but overall find them pretty reliable. One thing that struck me recently though was the uncertainty surrounding 6L6 power tube measurements. In some drawings the plates are 5-10 volts higher than the suppressors and in others the suppressor grids are higher.

It is my understanding that, based on theoretical operation, the plates are highest and the suppressors are a little under to help control the cloud of electrons from getting too far from the plate. So I figured the schematics showing higher grid voltage had an error. Then I starting finding this higher grid voltage in actual amp measurements and I'm wondering if this poses a threat to those power tubes, if I need to put in the 470 Ohm resistors found in later models or if this is not a problem.

I measured quiescent (at idle, standby off) voltage and then under operation to see if what I'm taking as the proper relationship (plate higher than suppressor) was achieved  when passing signal. These voltages did not fluctuate however, so maybe I'm overthinking things?   

[shooter]

maybe I'm overthinking things?

I had the same ? after building my 1st SE amp.  I can't spout the science, but I know it works just fine, up to a certain point, once B+ get big enough, G2 needs to be kept in check, otherwise you will shorten the tube life.  I care more about G2 current when I measure, so pretty much all my builds have some R so I can calc I, the exception is UL OT's.  In those builds I figure if Max plate is ok, G2 is probably ok

[92Volts]

The suppressors should be 0v (or at least roughly as low as the cathode), and 6L6s technically don't have suppressors. The thing that acts like a suppressor is connected to the cathode inside the tube, so you don't need to worry about it.

You're looking at the screen grids, which help the tube draw more current (and more consistent current) despite changes in plate voltage, allowing more power output.

You're right that it's normally lower than the plate but in these circuits the big-picture thing is it's roughly similar to the plate voltage, a couple volts more or less doesn't matter.

The actual reason it's lower is it draws power from the same source as the plate. Screen voltage is limited/reduced by a resistor, but the screen grid itself draws little power, meaning it's still very close to the original supply voltage.

The plate draws power through the output transformer. The output transformer isn't meant to have a lot of resistance, and isn't meant to drop the voltage down, but it has a little bit and there's a lot more current flowing through it. So under some circumstances the plate could end up lower due to that restriction.

[PRR]

Have you been reading about True Tetrodes?

Forget that. They have some uses in BIG rigs, but in our-size gear everything is PENTODES, not tetrodes.

("Beam Power Tetrode" is a red herring to dodge the patent on Power Pentode. It "is" a pentode, just that Grid 3 is not a grid.)

[centervolume]

I've been studying mostly the schematics and so yeah, I guess they do show the power tubes as composed of four elements - plate, cathode and then what appears to be 2 grids- the drawings lead me to consider it as a tetrode. Also It looks like I'm not using the correct terminology so that's a problem for me to straighten out. Ultimately I'm interested in the fact that one of these apparent grids is occasionally operating at a higher D.C.
Voltage than the plate and if this is a problem. Also, why did fender start using
The 470 ohm resistors on the 6l6s after about 1960 (the next circuit update after the late tweeds I am working on now)? Is this a simple update/mod I should consider if the situation I am Finding is a problem?

[sluckey]

It's not a problem to operate the screen grid at a slightly higher voltage than the plate. All those schematics you've been looking at support that.

Adding 470Ω screen resistors is a good idea. It provides a bit of protection for the output tubes. Fender began using them because it's a smart thing to do. Using them probably cut down on the number of output tube failures while under warranty.

I have three 6V6 amps that don't have screen resistors. I'm not concerned.

[centervolume]

Seems to harken back to the days when rca black plates were looked at more like lightbulbs than expensive audio gear - push'em till they blow them replace.

[jjasilli]

Screen voltage is a recurrent topic on this Forum.  If you search you should get lots of hits.

In practice, screen voltage can exceed plate voltage, at idle.  The reason is Ohms' Law applied to the typical B+ circuit:  screens and plates draw B+ from the same source, in parallel.  Yes there are dropping resistor(s) and/or a choke to the screens.  But plates draw current at idle, causing a B+ voltage drop in the plate circuit.  But at idle, screens draw about -0- current, so there's about -0- voltage drop in the screen circuit at idle despite the dropping resistors.  To get lower screen voltage at idle requires a more complex circuit.

[92Volts]

Keep in mind plate voltage changes with audio signal, by as much as +/- hundreds of volts and as often as thousands of times per second.

If the screen has a constant voltage (similar to the plate at idle), half of the time the plate will be higher and half of the time it will be lower. The plate being lower than the screen at any moment is not a major problem... if it were higher than the plate on average throughout the cycle, maybe that would be a problem, but a few volts just isn't relevant compared to everything else that's going on.

In true tetrodes it WAS a problem for the plate to be lower than the screen, not only at idle but any time during the cycle... meaning however low your plate voltage could drop as it changed with signal, the screen needed to be lower than that or you'd get distortion.

But pentodes and beam tetrodes like the 6L6 avoid that issue.

[HotBluePlates]

... One thing that struck me recently though was the uncertainty surrounding 6L6 power tube measurements. In some drawings the plates are 5-10 volts higher than the suppressors and in others the suppressor grids are higher. ...

Don't take the voltage figures on a Fender amp schematic as gospel; interpret them as "the voltage should be similar, within reason."

After a long time of only owning amps I built, I went nutty this year & bought a bunch of vintage amps.  I've got a '63 Deluxe (6G3 brownface), '64 Deluxe Reverb, '65 VibroChamp, '66 Pro Reverb and a '67 Super Reverb.  Using a variac to make sure they're fed a wall voltage appropriate to the era of amp, voltages are similar to the schematics but not necessarily exact.

I've found in some places the draftsman may have just "winged it" when it came to marking the voltage.  That is, they didn't appear to be probing an actual amp powered up & writing down the figures.  Or if they did, they rounded generously.  Not a big deal as long as you don't get bent out of shape when you measure 280v where they have 265v marked.