Univox Bias questions HIGH VOLTAGE!!!

[GuitarZombie]

Howdy Folks
New here, but everyone seems to be pretty helpful with each other and knowledgeable. Here's my question:
A Univox 1221. Replaced caps already. Been upgraded with 3 prong power cord by previous owner.
Schematic available here: http://www.univox.org/pics/schematics/1221.gif
See those high plate Volt's 660V. Half that on the screen grids.
I'm read 730V on the plates last night, 718V this morning. Maybe the tubes were warmer last night. ?
Anyhow, using the method of 70% of 6L6GC being 15.75 watts, dividing that by my plate volts: 15.75/730=.021... 21ma. Right?
There's already a 1w1ohm resistor built in to the circuit to read the cathodes (see schematic). Schematic also reads -33.5 for the set bias.1
I'm getting 84.6 on the cathode reading. Dividing that by 2, (because the 1ohm resistor is connected to the 2 tubes) you get 42ma. ?
According to my calculations (which I realize could be wrong, math's not my strongest trait), that's OVER 100% dissipation!

Further confusion, doing the same formula/method with the 660V on the schematic, the bias would be 23ma for 70%. Not the stated 33.5 it has on the schematic.
33.5 would be nearly 100% as well.

Is this method/formula whack? Is the OT primary impedance so high to let this be biased so (according to Aiken's "Final word on biasing") Amp doesn't sound bad. Actually sound quite good, with the exception of tremolo puttering in the verb. The 6L6's are not red plating at all, signal or not.

Thanks

[Enzo]

6L6GC is a 30 watt tube, so your 70% is 21 watts.

And make sure your mains wiring to the transformer is set for 115v and not the 100v.

And check your mains voltage. 660v B+ means for every volt the main is over 115v, the B+ will rise close to 6v. SO if your mains go up yo 125v, your B+ should be about 60v higher than spec.

[GuitarZombie]

6L6GC is a 30 watt tube, so your 70% is 21 watts.

And make sure your mains wiring to the transformer is set for 115v and not the 100v.

And check your mains voltage. 660v B+ means for every volt the main is over 115v, the B+ will rise close to 6v. SO if your mains go up yo 125v, your B+ should be about 60v higher than spec.

Enzo, just the man I was hoping to hear from.

My old RCA manual reads 6L6GC's at the 22.5, that's where I got that. But ok, 30 watts. There's still something not lining up here.

The schematic doesn't state what wire would be 100v & 115v on the PT, but it's the same color wired up as is on the schematic.
How do I go about confirming it and how to check my mains? Shove my meter probes in an outlet?

Is this basically just getting to the source/reason why the voltage on the plates is that much over the schem, but what about biasing this booger? I'm somewhat new to tinkering with my valve amps and boy did I pick a doozie to start with.

[Enzo]

Your schematic says the 115v wire is orange and the 100v wire is yellow.

Check the mains? SHove your meter into a wall outlet works. Or since the amp is already open on your bench - I assume - measure it where it comes into the amp.

I think you may be confused. -33.5 is the bias voltage applied to the grids. I don't spot ANY reference to idle current on the drawing. -33.5 should be taken as a ball park figure. You have to calculate the target current for a given plate voltage - as you did - the actual bias voltage is not the issue, you set it to result in the target current.

[GuitarZombie]

Your schematic says the 115v wire is orange and the 100v wire is yellow.

Check the mains? SHove your meter into a wall outlet works. Or since the amp is already open on your bench - I assume - measure it where it comes into the amp.

I think you may be confused. -33.5 is the bias voltage applied to the grids. I don't spot ANY reference to idle current on the drawing. -33.5 should be taken as a ball park figure. You have to calculate the target current for a given plate voltage - as you did - the actual bias voltage is not the issue, you set it to result in the target current.

Doh! Yep, there it is. 115 & 100. I was going off of memory when I replied that last time. But it is still wired as schematic. 115V. Wall supply check reads 124.4V.
Ok, so round-about 10V more out of the wall, times 6V as you said would give 60V + 660=my second reading of 718V. Makes sense.

Oh there's no question as to me being confused. I'm still wrapping my head around all this, to understand it all. But I just know that once I get past the roadblocks in my mind, I won't understand how I didn't understand what was so un-understandable before. Understand what I'm saying?

What it sounds like you're saying is I did something right (calculated the target current for the plates), but mistook it as my neg bias voltage. ? I'm having flashbacks of math class. "Correct formula, but not for this question."

Tell me what to do. I'm basically wanting to see how these tubes are currently biased with that sky-high plate voltage. I was thumbing through the net and have seen other Uni's of the same time frame with the exact same brand power tubes. Toshibas. Not a common 6L6 to come across. Have to assume they are the originals. Still seem to be operating good. Not in a rush to change them. (Though I would fancy hearing a pair of EL34's in there). Just want to see where they're running and if I need to tweak them. How to tweak them. Where to throw in a pot. Etc. If I stumble across some cheap Tung-sol's in a flea market or grandma's attic, I would like to be able to slap them in and bias her up to see if I like them vs. paying a tech when they might not appeal to me. Then have to pay to put my old one's back.
Plus, I've read that the newer crops of tubes wouldn't be able to take that plate voltage. So I need to know how to go about not frying the limited oldies out there.

[Gtr_tech]

Wow. You better be using *good* output tubes in that thing with super high plate supply. Something like a Philips 7581 would be all I'd expect to run reliably. The JJ 6L6 may handle it, but I haven't tested them at that kinda voltage.

I'd recommend seperate 470r screen resistors while yer in there. Keeps the output tubes from interacting and causing weird oscillations.

[Jazz P Bass]

I installed Sovtek 6L6WXT's in one of these beasts.
They work fine.
It is not the voltage so much as the current.
I would bias them as cold as you can go.
Just hot enough to remove the crossover distortion.
Forget all of that 70% crap.
And yeah, separate 470 screen resistors.
If you want to play it cool, I would recommend that you install a fuse on one of the heater wires.
10 amp should do it.

[GuitarZombie]

I installed Sovtek 6L6WXT's in one of these beasts.
They work fine.
It is not the voltage so much as the current.
I would bias them as cold as you can go.
Just hot enough to remove the crossover distortion.
Forget all of that 70% crap.
And yeah, separate 470 screen resistors.
If you want to play it cool, I would recommend that you install a fuse on one of the heater wires.
10 amp should do it.

Hey Thanks for the info fellas. I appriciate it.
Ok, 2 for screen resistors. A fuse on there is something I've never heard before.
There's no pot for adjusting. "Fixed" Fixed. Read somewhere to replace the 10K going to ground with a 5K and 10K in series to give me 5k-15k in adjustment range. ? Might have been on this forum. I think it was even for a different model Uni, the precursor to the 1221. The bias circuit was identical though.

[GuitarZombie]

6L6GC is a 30 watt tube, so your 70% is 21 watts.

And make sure your mains wiring to the transformer is set for 115v and not the 100v.

And check your mains voltage. 660v B+ means for every volt the main is over 115v, the B+ will rise close to 6v. SO if your mains go up yo 125v, your B+ should be about 60v higher than spec.

Boy I feel like a dingdong. Confused is right. So looking at the schematic again last night (and probably just getting more comfortable with all of this) my stupidity slapped me in the face. "Just measure what its at off the grid ". Doh
So right now it's set at -34.4v. Close to schematic.

Will adding the screen resistors tweek that a bit? If so, can someone please explain. Cathode current is the sum of plate and screen current?

Thanks for the help and patience with my ignorance.

[Gtr_tech]

I don't really worry about screen current with beam tubes. They don't draw near as much screen I as regular pentodes a'la EL34 do.

The screen resistors provide a bit of isolation between the screens, so we're more worried about that than limiting screen current. Old USA mfgr tubes would tolerate the lack of screen resistors better than the new production "wannabe" tubes will. Any amp I work on that doesn't have screen resistors, I will add them without question. Same with output tube control grid stoppers. Cheap insurance....

[Gtr_tech]

And like JazzP said, you'll likely have good luck with the sovwreck 5881/6L6WGC types. They're cheap and durable, and tend to sound more like actual 6L6s when run at elevated plate voltage. The 6L6WXT I can't comment on as I've only used those in Fenders and other std issue amps. But I've always liked those. Nice old time chime to them, and decent power output.

Sovtek 5881WXT Power Vacuum Tube

I sure wish they'd quit labelling those as a 5881. I'm getting tired of customers looking at me like I'm out of my mind when I tell them the reissue 5881 tubes they bought to replace the sov "5881" won't work in their amp.

[GuitarZombie]

And like JazzP said, you'll likely have good luck with the sovwreck 5881/6L6WGC types. They're cheap and durable, and tend to sound more like actual 6L6s when run at elevated plate voltage. The 6L6WXT I can't comment on as I've only used those in Fenders and other std issue amps. But I've always liked those. Nice old time chime to them, and decent power output.

Sovtek 5881WXT Power Vacuum Tube

I sure wish they'd quit labelling those as a 5881. I'm getting tired of customers looking at me like I'm out of my mind when I tell them the reissue 5881 tubes they bought to replace the sov "5881" won't work in their amp.

Let me throw this one at youz.

Swapping the 6L6's with EL34's or KT66's.
I realize making sure pin layout is proper. Not too hard. (Though why is it required that some of the resistors be right on the pins? I've read that on some, say if it's using an unused pin as a mounting point, and you need that pin for your swapped in tube, to keep it on the required pin and use heat wrap to help insolate it. But keep it there.?)
I know the heater draw must be looked at. If all else fails, separate filament transformer.
And I know the bias supply may need to be tweaked to accommodate the new tube, and/or both tubes if possible making swapping back and forth easy. Tube swinging. An "open-tube relationship". Kinky.
But what I'm currently hung up on is the OT primary impedance. This badboy is rocking 10K. Seems higher than what I've seen used in more common amps. But so is the plate voltage on this booger. Is that higher impedance another factor in the way-over the limit plate voltage? And how would this play on El34's and
KT66s?

Plus is there any other things that I missed that are needed to look at in a swap?

Fanx

[GuitarZombie]

Let me throw this one at youz.

Swapping the 6L6's with EL34's or KT66's.
I realize making sure pin layout is proper. Not too hard. (Though why is it required that some of the resistors be right on the pins? I've read that on some, say if it's using an unused pin as a mounting point, and you need that pin for your swapped in tube, to keep it on the required pin and use heat wrap to help insolate it. But keep it there.?)
I know the heater draw must be looked at. If all else fails, separate filament transformer.
And I know the bias supply may need to be tweaked to accommodate the new tube, and/or both tubes if possible making swapping back and forth easy. Tube swinging. An "open-tube relationship". Kinky.
But what I'm currently hung up on is the OT primary impedance. This badboy is rocking 10K. Seems higher than what I've seen used in more common amps. But so is the plate voltage on this booger. Is that higher impedance another factor in the way-over the limit plate voltage? And how would this play on El34's and
KT66s?

Plus is there any other things that I missed that are needed to look at in a swap?

Fanx

Bump & new inquiry.

I picked up some Winged C el34's on the cheap.
Popped a wire on pin 1 to pin 8. Have already installed 1k1w screen r's.
Had to yank the 3 preamp tubes that deal with the 'verb and tremolo. Down to the v1 12ax7, the 6an8 pphase inverter, and the el34's.
Slapped them in, read the heater current on standby (4.1-ish, good). Flipped the switch, took readings with load and no signal. Plates = 732v, screen = 363v, cathode current across the manufacturer installed 1ohm (power tubes are sharing the 1) = 62.5ma, bias is still -34.3v. No red plating at idle. No red plating with signal. PT is running cool. All good there.
Doesn't sound as full as the 6L6gc's. But I'm not sure they're at the optimum bias sonically.

Online bias calculators say I'm running at 100%. I'm thinking the screens (being much lower than Vp) and primary z (being 10K) are making this not so but I need clarification on this. (Though I have been running a 8ohm speaker in the 16ohm load, halving the Primary z to 5K).

From Aiken "Last word on biasing"
"In addition, the "70% rule" falls apart as you use very high plate voltages, unless the primary impedance is increased accordingly. In some cases, if the voltage is high enough, there is no bias setting that will result in safe operation without exceeding the maximum plate dissipation of the tube. For example, in a 100W EL34 amp with 480V on the plates and a 1.7K primay impedance, there is no bias setting that will keep the output tubes from redplating at some point in the power curve, even if you bias the amp at 0mA! If you double the primary impedance to 3.4K, however, the amplifier will operate fine at all power levels, and you can bias it all the way up to 52mA without ever exceeding the plate dissipation at any point in the operating curves. This may sound odd, because the bias point is right at 100%, but it works because of the very high plate impedance load, which never allows the tube dissipation to increase above the idle level. It also may not sound as good, because the screen voltage should ideally be decreased so the loadline intersects the "knee" of the curves if not, the nonlinearity increases drastically. The bottom line is that you have to take into account not only the plate voltage and plate current, but also the primary impedance to find out the safe bias area."

Too, I'm not as familiar with the tonal qualities of EL34's vs. 6L6gc's and maybe this is just the difference in them. But I would like to rule out the weak biasing 1st and attempt to understand the role of primary impedance on biasing a bit better. Most info I find online just talks on the sonics of differing primary z and doesn't go deep (or dumb it down enough) into the biasing role it has. This has to be a factor in the ability of these Vp on 6L6's, don't it?

Thanks again fellas