EL34/6L6GC switchable bias question

[tedmich]

looking at the Peavey JSX/XXX switchable bias circuit I see that its a simple voltage divider with a switch (cleverly using both poles of a DPDT to double up the typical 28-30VDC contact rating) to put the VR19 pot as either series or parallel.

My value table works out like this

forget the negative sign on the percentages!

Does this look accurate for the 24VAC going into a voltage tripler? The 6L6GC setting looks like it would cover all the voltages available to the EL34 setting and go way too cold too, feeding -72v at the extreme setting. I was lead to believe this control gives the stock -55v for 6L6GC setting and when switched to the EL34 setting its at -42v with the pot VR19 in the same position... what am I missing?

[Gingertube]

I think you looked and saw what you expected to see. There is no "series / parallel" switching.

In the EL34 position there is an extra voltage dropping resistor in the chain, in the 6L6 position that extra resistor is shorted which puts the full -72 on the top of the bias adjustment pot.

You may get -42V and -55V for the EL34 and 6L6 switch positions respectively (with the same pot adjustment position) but I wouldn't bet on it.

I don't see this a a short comming, you change the output tubes you re-bias whether you are using the same tube type or not.

Cheers,
Ian

[Enzo]

They provide an adjustment and test points. Why would we assume the switch automatically sets both voltages? The switch sets up either the 6L6 range or the EL34 range, you still need to set it for each.



Just to nitpick... Putting two switches in parallel will not increase their voltage handling, it would double their current handling, though that is not an issue. I'd suspect it was doubled to increase the reliability of the connection. Less likely to get dirty.

[tedmich]

Thnx for the insights! I wonder about the switches utility, especially because very few switches are rated above 30VDC

[Enzo]

Too late to worry now. Look at all those standby switches on Fenders over the last 60 years, none of them are rated for the B+ they switch. But no one told the switches, so they work anyway. This is a bias circuit, there is almost zero current, nothing will spark the contacts.

[NateS]

The paranoia might be justified since failure of the switch is pretty mutch guaranteed to exercise your protection systems. On a related note, has anyone encountered one of these switchable amps where they bothered scaling the signal and not just the bias to fit the tube type selected? I don't recall hearing of one, but I might not have been paying attention.

[Tooboob]

I haven't seen an amp like that. I have seen dual-bias amps that run pairs of EL34s and 6L6s or EL84s and 6V6s, and the designer expects them to make an equal contribution.

I have a little pickup I bought new. For the test drive, I only went maybe a mile on city streets. The gas pedal was set fairly touchy, and it seemed like it had a lot of power. It's under 100 HP, but the touchy gas gave the effect. I think it may have been designed that way intentionally (might be paranoia). I sometimes think that the marketing at amp companies forces a large output change near "0". Lots of store won't let you crank it, so you get the illusion that if "2" is loud, "10" must be killer.

Signal levels in audio are just a suggestion, and I keep finding people that can't comprehend more than one volume control. You've all seen threads about "should I turn up the channel and turn down the master, or turn up the master and turn down the channel?" There is a difference, but I keep hearing "I like to keep the master high to cook the output tubes".

My wife absolutely can't deal with three volumes on the TV - cable box, AV receiver, and TV. Sometimes you need three remote controls to turn it back up after she turns it down so that you can find the early stage she set near zero volume. I actually changed it so that only the AV receiver could control volume.

You could make a bias circuit that meets the needs of both EL34 and 6L6 without a switch, but if you're running EL34s hot, you change to 6L6s, turn on the amp, and go to adjust the bias, the smoke could escape before you get the screwdriver in the pot. I think the switch is a nod to this potential faux pas.

It amazes me how users react to touchy controls. You'd think that if they couldn't turn a knob slowly and smoothly, guitar playing would be out of the question. I like a hair of spring reverb, and it's hard to get there on my Carvins, but I've never whipped out a part catalog and soldering iron for it.

A master volume in particular needs a bit of headroom, since you don't know the guitar output or where the channel volume is set. If these problems didn't exist, and the volume controls only covered the intended output range, the early amps wouldn't have been able to saturate the power stage and make "that noise", and we'd probably be playing SS amps with DSPs for lots of wacky time-based distortion.

With some amps, nothing comes out until you get to "2". It's something to check for when picking a pot.

[Enzo]

has anyone encountered one of these switchable amps where they bothered scaling the signal and not just the bias

I don't think they will bother. This is not an adjustment you would do on the fly like say channel switching. So you flip the switch to get proper bias range, but after that, you will be turning knobs to get whatever tone you want so getting real fancy with signal levels is moot.

[NateS]

I was thinking of EL34's vs 6550's, but I also had it in my head that 6550's required significantly higher input swings than they do.

[Enzo]

OK, but is that difference more than can be covered by simply turning the master from 6 to 7? Or some similar solution?

[NateS]

Where I'm coming from - I've been looking at AB2 for a little while and was actually a bit more concerned about the difference between master from 10 to 14. If you have big enough grid stoppers that they'll never conduct, then hammer away. But otherwise I think it's probably a good idea to scale things so you really CAN'T drive it past whatever level of torture you deem acceptable. 6L6's have sturdier grids than EL34's, and require a higher drive voltage to boot. It's probably best just not to allow grid current at all if you're planning on switching between these two types, but I could see damaging EL34's if you dropped them into a circuit designed for 6L6 AB-2 and only rebiased.

[frus]

Aren't we talking about conventional guitar amps here, the ones that have a 12AX7 or 12AT7 as a PI tube, that can never drive the power tube grids positive?

[NateS]

The grid stopper is what keeps the grid from going positive, not the tube driving it.

[Tooboob]

How does that work? a source into the grid stopper (if the grid isn't driven positive) only sees the grid stopper and the bias resistor to the bias supply, forming a divier at the grid.

1. Is this always enough to keep the grid below the cathode (usually ground or near it for AB)?
2. Is the grid stopper usually chosen for this behavior as one of it's criteria or its main criterium?, Does it just work out that way?
3. Why are folks buffering the power tube grids with FETs if a large enough grid stopper solves the problem?
4. ls there an advantage to driving the grid positive? Does the power tube sink ia higher current past that point?

[NateS]

Normaly the grid draws no current. If it begins drawing current, the voltage drop across the grid stopper lowers the grid voltage. The grid leak and grid stopper resistors dont form a divider because theyre in parallel, one to bias and the other to the grid, which isnt drawing any current. Buffering the grids actually is at odds with the effect of the grid stopper, as it provides vastly increased current drive capability, but it still cant overcome ohms law. You typically would run no or minimal grid stoppers if youre buffering them.

As for advantages, AB2 is mostly about reaming the living daylights out of the power tubes. It can squeeze a few more watts too, but I doubt any of us care about that. It's just another sound.