Question about prevalent 6L6GC Screen Grid Voltages

[Steve Conner]

Yeah, that sounds about right, except I'd shoot a good deal lower than 30 plate watts. The Bass 400+ has 12 power tubes, and they'd be dissipating 360 watts between them. Even with a fan, that's going to get toasty!

You can probably run it up to 60mA for short periods to hear how it sounds, but for the sake of reliability and tube life, I'd suggest 40 or less.

MWJB is a long time forum member who knows what he's talking about. Think of it as free advice rather than a smackdown, if it helps.

[Enzo]

So I was trying to find an explanation for the drastic tonal differences of three amps that use almost the exact same compliment of tubes/tone circuits.

pretty much all the amps we discuss use the same tube complement. And the tone stack is just one small part of the circuit. The amp sends a signal to the stack, a signal already having its own characteristic. The tone stack can only work with what it is given. Then the result is fed through the rest of the amp.

What is different? Circuit configuration, component values, transformers, speakers, layout, etc.

And a personal note, just me, maybe no one else feels this way, and it is not like it is a huge deal. I find it confusing to hear constructions such as:

increased bias voltages closer to zero

I recall Popular Electronics used to state things that way 30-40 years ago, for a while, and it grated. They would say something like reduce the voltage from -90 to -110 volts. I am sorry, but the voltage was increased, it just happens to be negative.

I find it a lot clearer to say you decreased the voltage, or increased the tube current. Increasing a voltage towards zero is self-contradictory. The very fact of having to include "towards zero" means the term is unclear on its own. When I hear someone say they "increased the bias," I don;t know what they mean. I have to figure it out from their context.

[apehead]

I was looking at the supposed R103 (this schematic sucks) to be the one to replace with a pot. As far as the "smackdown", I seriously do appreciate the advice, but you have to admit the delivery was a bit, shall we say, "curmudgeonly". As far as my terminology when referring to increasing/decreasing control grid bias voltages, I had a tough time not confusing myself! I was trying to keep it linear as in "decreasing the voltage" meant making it more negative, and "increasing" it meant make it more positive, regardless of the voltages relation to zero volts.

[Steve Conner]

Isn't the Peavey Butcher a guitar amp? You'll find that guitar amps are voiced with less bass output and more treble boost than bass amps. Typically this is done by making some of the coupling capacitors between stages smaller, and adding "bright caps" to the volume controls.

So to decrease the "thundering bass output" I'd look at swapping a few coupling caps for smaller ones. Well personally I wouldn't, because if I had a 300 watt tube amp I'd want some rumble.

However, IIRC, the Bass 400+ had a fair amount of tone controls and even graphic EQ. Can't you get the tone you want by playing with that stuff? Is it still too heavy even with the bass all the way down and the treble fully up? Can't you get your "Punch" by dialling in a midrange boost on the graphic?

To avoid the positive/negative/increase/decrease confusion, I like to talk about bias only in terms of "hotter/colder". -54V is colder than -35V.

[MWJB]

Apologies Apehead if the perception of my post was that I was issuing a smackdown, not the intention. However I do feel it's worth making a clear distinction as to whether your amp is poorly designed, or well designed and suffering a malfunction.

It's also important to realise (this is not just for your benefit, but also others that may read this thread) that data sheets & schematics are just bits of paper. They don't make any sound, they don't belch smoke & flames when things aren't right...that's why it's important to actually measure what is going on in the amp in question. Yes, they are useful, but voltages & values may vary from what you have in print and things still might be running satisfactorily.

Believe me, I'm all for tweaking amps & circuits if I feel they can be improved & indeed your personal preference might not coincide with Mesa's design philosophy, but from the info that you supplied, there does not appear to be an inherant design problem.

Before you start modding, make sure the amp is running properly to begin with, your idea to install a bias pot is a good one. Follow Steve's suggestion re. plate curent per tube.

[Enzo]

I think most times it is better to just be direct and make a clear point rather than worrying if it is stated gentle enough.

My -60v supply gave me a shock. I better decrease it to -100 for safety.


OK OK I'll stop.

[soundmasterg]

I think most times it is better to just be direct and make a clear point rather than worrying if it is stated gentle enough.

My -60v supply gave me a shock. I better decrease it to -100 for safety.


OK OK I'll stop.

Haha.

I'm with you on this Enzo.....its easier to understand if decreasing means going down in numbers and vice versa....

Greg

[apehead]

Yes, the butcher is a guitar amp. Come to think of it, almost every amp I've ever owned/played through regularly has been a guitar amp, excluding my first (Fender sidekick Bass 50) and second (a friend's Fender BXR). So maybe it's just the color that bass amps are intended to have that I don't really care for. Also, I use flatwounds with passive pickups, and something tells me that this amp (produced in the late 80's to early 90's) was designed with at least roundwounds, probably actives, and most definitely radical explorer or exagerated P-bass shaped instruments in mind. I guess my setup relies more heavily on the amp to enhance the top end. The tone control on the amp is decent, but the EQ is thoroughly lame, and I just bypass it. Even with the bass knob to 0 and the treble to 10, the amp lacks clarity and definition, only with no bass either. I think I will do a little schematic comparison with the Silverface and Butcher, and change some coupling cap values.

So, if the data sheet is not to held as gospel in some regards, than how should we determine the actual real world physical limits of the hardware? When the amp is already built, it's easy enought to measure, but what about designing from scratch? Just keep pushing until something does melt, or smoke, or catch fire, or blow a fuse? Seems to me like there would be a better way. I just assumed (wrongly) that's what the datasheets were there for, so you didn't have to hook a tube up to a Van Der Graaf Generator until you saw lightining in the bottle to determine what the max plate volts can be.

Now for the goofy stuff. Thanks for the apology, but my feelings weren't hurt by what you said, hence the irreverant "" emoticon after my initial "smackdown" comment. I guess I kind of have to expect a few old cranks on a forum that specializes in a technology that peaked in '69! "I keed, I keed..." (feeble attempt at expressing Triumph the Insult Comedy Dog's accent in a text format). And let me tell you, Enzo's endorsement doesn't help against the "curmudgeon" label, that's for sure!

Oh yeah two can play at this game

"I increased the supply to +5VDC, then decreased it back to 0VDC, than increased it again to -10VDC."

[pdf64]

'The tone control on the amp is decent, but the EQ is thoroughly lame, and I just bypass it. Even with the bass knob to 0 and the treble to 10, the amp lacks clarity and definition, only with no bass either. I think I will do a little schematic comparison with the Silverface and Butcher, and change some coupling cap values.'
From what you say, Steve's idea of a bright cap on the vol control is what's needed, as that should give you clarity and definition. They sit between the hot track end and wiper, value somewhere between 47 to 470pF, and act to give a top end boost which reduces as the volume control is turned up. Amps can sound muffled/dull to some people if the bright cap is too small or there isn't one.
So I would try that before reducing the values of coupling caps, as that will cut bass without neccessarily improving clarity.
Peter.

[Steve Conner]

I didn't realise you used flatwounds. If you're looking for top-end, it seems to me like the sensible thing would be to go back to roundwounds and change them often.

If you have other reasons to use flatwounds, sure try the bright caps, or make yourself a treble booster pedal ala Dallas Rangemaster etc.

About tube datasheets: What's "gospel" is the designs that have proven reliable over the years, and that means the classic Fender and Marshall amps. These are actually more of a de facto standard than the tube datasheets themselves: no maker of reissue tubes would bring out a 6L6GC that didn't work in a Twin, or an EL34 that didn't work in a Plexi. (In theory...)

[BackwardsBoB]

Comparing your schematic with the tube datasheet, the "increased" (more negative) bias voltage goes along with the higher screen voltage. Their absolute values need to track almost linearly.

[apehead]

Here is the schematic for the Pre-amp and FX loop/Eq, since the topic is shifting gears a little. If I understand Bright cap placement correctly, than there are some active in this circuit when the channel pots are pulled out (they always are) and it does help out a bit, but I had an interesting empirical observation that seems validated by looking at the mesa compared to a silverface. About a year ago, I had a silverface twin reverb, and a silverface quad reverb at home. When A/B'ed, the quad was always bassier, and to my ears, a little muddier, than the twin. I'd always chalked it up to different speaker config., different tubes, and the "quirky" nature of older amps. About two weeks ago, I set out to repair a phantom bad connection in the quad, and when looking at the schematic, noticed a discrepancy I had never noticed. At some point, the push-pull pot had been removed from the quad (I already knew that) and it had been re-wired to be permanently in "gain" mode (a stupid move if you have ever heard the gain mode of a fender silverface, but judging by the soldering/wiring job, this guy was no Tesla), and more interesting to this topic, the original pot was center tapped and a 120pF was connected to it as a bright cap. This amp had a regular three post pot in it, and the bright cap had been removed since there were no accomodations for its re-attatchment. Unfortunately, I've sold the twin, and I haven't looked for a four post pot to restore the bright cap, so at this point, it's still just a theory. Although comparing the circuit of the silverface and the mesa between the second pre stage and the phase inverter, there are quite a few smaller caps in the fender. I think I'll try a 240pF across the master volume in the mesa, and changing the .1uF C233(?) coupling cap to a .047uF. Does this seem like a logical step? Maybe just one at a time?

[apehead]

One you go flat, you never go back!

[Steve Conner]

Well, I have two basses just now, one with flatwounds and one with roundwounds, and I find the rounds more versatile because of the extra high end they give. Flatwounds give a kind of muffled, thumpy sound, especially when they get old. If I want that sound with roundwounds, I can always turn the tone control on the bass down.

Think of Jaco and how much midrange boost he had to use.

[Enzo]

And then there are - or at least were - the coated strings. I haven;t seen them in a long time, but I used to know a cat who played them. Like round wounds dipped in rubber coating.