Bias resistors

[daz]

Someone who is quite knowledgeable told me that the typical 220k bias resistors in most EL34 amps should be 100k. And that the specs for EL34's specifies that and that the likely reason they are still 220k is that back in the day marshall copied the bassman and just never changed it because it became a sort of accepted part of the mojo. because this person is so knowledgeable i tend to believe that, but then again it just seems odd that especially in this day of boutique amps where being the same is the opposite of what they are looking to do, that this practice would remain unless there is something advantageous about it. Can anyone shed some light on this subject? I DID try it and it seemed there was no real difference till i turned it up, then it seemed smoother and less harsh.

[guitician]

Tung-Sol's data sheet for the 5881 states that .1M ohm as the max. grid resistance. And the Fender Bassman used 220k anyway. I guess that it's really not that important. If the output tubes are driven hard into clipping, causing grid current to flow, you will have to take that into account. They played pretty clean in the fifties.

[Jazz P Bass]

EL 34 bias feed resistors

Here is a quote from The Tone Lounge Lizard.
Quote:
At point #1 we see the bias-feed resistors. Usually these are 220K for EL34 output tubes, and 150K for 6550 output tubes. I lower both resistors, regardless of tube type, to 100K. This keeps the tubes on a 'shorter leash', and does promote long life. The theory behind this move is that the chance for the output tubes to draw grid current at high volumes is reduced.
Link:Tales From The Tone Lounge; The Ultimate JCM800?

[Steve Conner]

It's true! Every EL34 datasheet says 100k max, but Marshall used 220k.

The value of the bias resistor is a compromise. Higher values are easier for the PI to drive, but they make older tubes vulnerable to thermal runaway.

Grid leakage tends to increase as the tube goes through its life, and once it gets beyond a certain level, that drops a voltage across the bias resistor, causing the tube to bias itself hotter. As the tube's guts get hotter, the leakage current increases, so it's a vicious circle that continues until the tube melts or the fuse blows. Once a tube starts doing this, you have to replace it, so lower bias resistors will give you more useful tube life.

This is "reverse grid current", a different thing to the kind of grid current that flows when the amp is driven into clipping.

[guitician]

Svetlana's EL34 has 200k as max. control grid resistance. But I guess if your amp doesnt need maximum PI drive, then 100k would do just fine. This would also shift the LF rolloff, and you may need to increase the size of you coupling caps.

[MWJB]

"It's true! Every EL34 datasheet says 100k max, but Marshall used 220k." As do 6L6 data sheets for fixed bias applications.

Also note that different values are often specified for fixed bias and cathode bias applications...some builders follow this example others just use the same value as fixed bias amps, irrespective od bias mode. E.g. you might see cathode biased amps with 470K grid load resistors, but very rarely will you see fixed bias amps with much more than 220K.

Many boutique builders are building different models based on the same core group of amps...just trying to build the optimum incarnation in their eyes...this is simply not an area where straying from conventional wisdom yeilds significant gains. I have used 470K in fixed bias 6L6/6550/KT88 amps...it works the tubes harder & is more aggressive sounding...220K is smoother with better fidelity. If you're not having reliability problems with 220K & EL34 then all is good, but if you have some expensive NOS and want peace of mind, then go with 100K. Sturdy EL34 like the JJE34L don't seem to mind 220K in fixed bias.

[daz]

Thanks, but i still don't understand why the vast majority use 220k if 100k makes for longer tube like. Unless you guys are saying that a 100k may not drive the tubes hard enough....or maybe because a lot of amp manufacturers sell tubes too.

[MWJB]

Most tube manufacturers use current production tubes, obviously they don't have a problem with them failing under warranty, otherwise they'd revise their designs. Yes, they sell tubes, but they don't make the tubes, so they have no control whether you buy the replacements from themselves or from another retailer...it wouldn't do them any favours to sell amps that eat tubes within warranty period.

On the other hand if you ordered an amp & expressed a desire to run your expensive collection of NOS EL34, a boutique builder might have their own views on this and adjust the bias feed resistors as necessary.

If your amp sounds like 100K drives the tubes hard enough, then it does. If you're happy with the tone at this vallue, I don't see any reason to change.

It's easy to make tubes last a long time, back off the voltage, back off the plate current...whether the amp sounds any good with tubes that will last 15yrs of hard gigging is another matter. A couple of thousand hours of gigging use is reasonable enough. Maximum power tube longevity, by itself, is not really the aim of the amp builder...think of it like tyres, would you want the tyres that lasted 5 times longer than anyone elses at the expense of grip, or would you settle for acceptable part life and good/benchmark grip?

[jrfrond]

Another valid reason for reducing the value of these resistors is to lessen the chance of blocking distortion, or driving the power tube grids positive. There are two ways around this, and the other one is reducing the coupling cap value. Both of these reduce the RC time constant so that the coupling caps never fully charge, because once they do, a positive voltage develops at the grid. Admittedly, this is really only an issue on amps that are played full up, but A LOT of amps are played this way, and it can get real ugly!

Two things to note are, 1) Reducing the resistor value also reduces the signal swing to the power tubes a bit, via virtual grounding through the bias supply, or true grounding if cathode-biased, 2) This also reduces low-frequency response a bit too. Whether you lower the cap value or resistor value, as long as the RC is lowered, so is the LF response.

[tubeswell]

Of course it doesn't have to be 100k or 220k (or 470k). It can be 180k, 150k or 120k etc, in which case it becomes more a matter of how you like the sound of it (than what the grid current is doing to your tube longevity).

[daz]

The reason i was interested in the first place is because when i tried it it sounded smoother. With most amps it seems in the transition from low volume to stage volume theres a place where all of a sudden the tone loses that smoothness it has at low volumes. With the 100k's it seems like that doesn't happen near as much. Made me wonder why anyone would use 220. And the amp doesn't seem any lower in volume. I suppose what it come down to is if an amp's pre isn't very hot like mine, the level going into the PI is lower and therefore 220's would sound the same as 100's do in mine.

[oskar]

Another point with this question , .
If the datasheet say 220k or 100k of max G1 resistance , (this data is for one tube only) Why all popular amps of 100w and two tubes(or more) in paralell each side of push&pull , run at the max value of G1 resistance for one tube only?? ...It should be the Half of the max G1 resistance , if the objetive is to run the two tubes at the same level of safe bias drift due to thermal runaway, becouse the G1 leakage currents are doubled .
It's a error repeated by tradition.

Sorry for my english , I hope you can understand

Oskar.

[daz]

Now i know the reason for the wide use of 220k !!! Last nite i did a gig and since i've been making changes to the amp i fear going to a gig only to find something i did screwed the sound up somehow. So i bring a soldering gun and a several common resistors. I set up and when i turned the amp up to sound check, at 2:00 on the master it wasn't loud at all ! At first i was thinking oh shite, somethings wrong. Then it him me ! I didn't have more than one value of each common resistor, but i pulled the chassis and added a 100k in series with one of the bias resistors, and i had a 220k that i swapped with the other. Instant volume ! I know you guys said it reduced drive to the PA, but i had no idea it was that drastic. I can't tell at home because here it turn it up just to where the master starts to allow sound thru and it sounds like Hiroshima. Good thing i thought to take that soldering gear and parts too, because not only that, but my 18 watt that i use as a effects return wouldn't power up and i had to pull that chassis too! It was a wire that came loose on the power switch. I'm telling you, the audience got more entertainment than they expected for last nite. By the way, do you think there would be any kind of effect of the bias resistors being slightly different in value like that? (200k/220k)

[Steve Conner]

You know you're hardcore when:

You're at a gig, everyone else has a beer or a joint in their hand, and you have a soldering iron.

I wouldn't have expected the change from 220k to 100k to lose THAT much power. Unless your PI was barely putting out enough drive to begin with?

For testing amps at home, some kind of power soak/dummy load device really helps. You can crank the amp up full and listen to the result on headphones. It won't sound the same as using a speaker, but you can check for any obvious problems.

[daz]

You're probably right steve. I told this to the person that suggested 100k's and was told the same thing you just said. So maybe it wasn't that because there WAS another thing that caused low volume that night and i figured that out. It was so frantic and my mind was spinning trying to get things working b4 we had to start, so i may have been confused, but heres what happened. I have a jack on the back of my main amp that runs a wire to the output lug of my master volume. I have a small rack that houses a few things including a relay and pot connected to a jack which i cable to the jack on the amp to have a footswitchable volume change. Well, when i set up i had some of the cables wrong and the one to the master on the amp was connected to the wrong place which apparently had a low enough ground reference to cut the volume way down. However, i could swear that happened B4 i determined the amp was lower than normal because it was way way low. And i DID figure that out. But in any case the point is maybe somehow in the frantic confusion it wasn't the bias resistors causing it. I am going to parallel some 220's with the 220's in there now on a switch so i can a/b them and not only see if theres a notable volume change but also a tonal diff.