750 volts too much for 6l6GC plates? (need big PA advice)

[imaradiostar]

I'm trying to be cheap and cut corners- not sure where to do so. The goal is to build a rack mounted stereo amp for monitor use- it's out of necessity, not the desire to create some .01% distortion hi-fi amp. I want loud with decent headroom and the ability to take peaks without blowing up. I have an assortment of 16 and 8 ohm wedges I'll be driving with it. I'm trying to use what I have laying around.

I have large output tf's that are 4k to 8, 4 and 2 ohms. Logic says I can use them as 8k:16,8,4 as well.

I have a big pile of 6l6gc's.

I have a number of power transformers of varying kinds- some various recycled things with lots of taps, a silverface twin pt, etc.

The original plan was to use an Antek Inc pt (4t550) connected with 120v across the 240 windings to give me 550 volts center tapped so I'd have around 750 volts for the plates and half of that for the screens. I'd use the outputs at 8k and two el34's per side to make 80-100 watts per channel. Pretty simple and easy to set up, no excessive drive or bias requirements.

This means buying 4 el34's and an antek PT- about $200 by the time I pay for everything and have it shipped.

I'm trying to use what I have so I can spend less and build it immediately or at least a lot sooner. I figured I could use the 6l6's with an antek PT and just pretend they're el34's- would they be happy with an 8k load, 750 Vplate and 375 Vg2?

Would I be better off using the twin PT and stacking an additional supply (random other PT) so my plate voltage will be between 100 and 300 volts higher?

I know that Ampeg and others ran 600+ volts on 7027a's without any trouble and kept the screen voltage quite a bit lower. I have a lot of random PT's I could use- from old o-scopes and equipment of different kinds.

One large PT has secondaries of 148, 148, and 165 volts with an additional 432vct winding. The DCR of the lower voltage windings is around 6 ohms and the 165 winding DCR is around 9- seems like they could support some pretty serious load with bridge rectifiers. This would give me around 400v for screens and a little over 600 for the plates.

Another thought was to take a matched pair of solid state 60vct PT's and run them in series and reversed with a pair of doublers stacked so they'd make about 650 volts DC with a screen node at half that-60vct secondaries used as primaries, in series so that I'd have two very stout 120vac primaries being used as secondaries- with doublers each tf would make around 325 volts which I could stack in series.

I'll have to post specs of some other stuff I have laying around but the idea is I can combine various transformers to get the appropriate voltages.

What would you do?

thanks!

jamie

[Old Tele man]

...look into the RF-version of the 6L6, the 6BG6A with top anode connection and rated at 700Vp:
http://tubedata.milbert.com/sheets/127/6/6BG6GA.pdf

[imaradiostar]

ah yes- they're both 807 derivatives, right? Aren't there theater amps making ridiculous power with the 6bg6 and something like 1000 volts on the plates? Maybe I'm thinking of something else.

So...what plate impedance would you run assuming you've got 600 to 800 volts on the plates and 300-400 on the screens- 4k or 8k?

I was planning to use a plain old LTPI for simplicity but I'm wondering if this calls for something akin to the marshall major PI setup. I have a ton of old 6sn7's I could use as drivers.

jamie

[Steve Conner]

Just hook it up and go. :P

One pair of 6L6s should work well at 8k, two pairs at 4k. IMO the only difference to EL34s in your application, is that the 6L6s might need smaller screen resistors or a bit more screen voltage than 375V to make the power. But 375V with 470 ohm screen resistors would be a reasonable starting point.

At a random guesstimate, you should get 70W from one pair or 140 from two.

[Chuck H]

As Steve said.

I was involved in a post here not long ago where a guy had some 750 Vp on a pair of 6L6's and reduced screen volts. This was in a factory built amp. He had some red plate and bias issues and I argued that the fix was to reduce Vp and was subsequently poo poo'd by Enzo and Steve who said it could be made to work. And eventually it did work. You mentioned that you have a bunch of 6L6's. Are they GC types (max Vp 500) or some other type. At that Vp I wouldn't try anything like a GB or 5881 (max VP 360) type of 6L6. To use the tubes you have at that voltage I think they would need to be GC type or some other special designation 6L6 capable of high plate volts. Many are not.

JM2C

Chuck

[imaradiostar]

Thanks for the replies. I have some sovtek 5881's and 6l6's (identical innards) and some other random 6l6gc's with different looking insides. I guess I'll have to see what can take the power and what can't.

I'm leaning toward 600 volts on the plates and 400 on the screens using an existing PT from an oscilloscope- it's the same ei lam size as a twin PT but about .5 to .75 taller stack. I'm hoping this'll do well with two 6l6's per side.

I guess I'll run them at 8k.

Any suggestions on PI design? Simpler is better! I was toying with using a 6sn7 ltpi with a 6sn7 or 12ax7 out front for additional gain if needed. I could probably copy the PI from a high power fender like the 100 watt twin or similar amp- no need for super high grid drive when it's only ab1 and I'm only driving a pair of tubes.

jamie

[Enzo]

I wonder this: if this is to drive monitor speakers and not for guitar, why are you making this heavy thing? Why not buy a used solid state power amp - many fine ones available for very few dollars - and use your tube stuff to build some nice guitar amps later?

[jbrew73]

can someone explain to me why you would want the tubes running at say an 8k load instead of the "standard" 4k load.

[imaradiostar]

I wonder this: if this is to drive monitor speakers and not for guitar, why are you making this heavy thing? Why not buy a used solid state power amp - many fine ones available for very few dollars - and use your tube stuff to build some nice guitar amps later?

Point me to the cheap ones! I've looked and haven't been satisfied with the kits and prebuilts I've found. Anything solid state and affordable that I've seen thus far hits a brick wall in power output at a relatively low point. A large tube amp will sound musical well after a comparable wattage solid state amp starts adding ugly harmonic content.

All the solid state stuff I've seen thus far is overly expensive and I don't necessarily trust it to hold up. I have a number of large power transformers for solid state amps but I don't want to make pcb's...too much hassle with my current "in between shops" bench and workspace.

About plate impedances- at lower voltages (under 400) the 6L6 does well with 4k loads in ab1. As voltage goes up the tube can work more efficiently with a slightly higher load. Somewhere out there in internet land their are charts for various tubes showing the distortion and power output at a given operating point. Fender stuck with 4k pretty militantly though the numbers say around 6k would be a lot happier in most of their circuits. EL34's call for 11k loads with 800 on the plates and 400 on the screens. Mainly I was hoping that a 6l6 (7027a) would work well with the higher load like an EL34.

jamie

[Enzo]

For example
Music Go Round

[imaradiostar]

and use your tube stuff to build some nice guitar amps later?

That's a great question- I do have a giant 2.5k:4&8 tf that'd make a fabulous bass amp with the aforementioned o-scope PT.

I'll take a look at the link you posted, thanks!

jamie

[imaradiostar]

Wait- isn't it better to have nice tube PA amps and nice tube guitar amps?

I have quite a few big transformers collecting dust- I think it's better to build something than nothing? Frankly the church youth group worship band I've been helping needs a better monitor amp by month's end and no budget- I can help them so I think I will. I've also got an old yamaha console and a few other things I'm setting them up with so they can have a better worship experience with less sonic trouble.

jamie

[guitardude57]

I would use your tube stuff for guitar/bass amp things.
You can find all kinds of very reliable used conventional (Pro Audio) stereo power amps for your monitors.

Check out htics.com .......they list as a third party seller of Sound company and other individuals' sound equipment for real decent prices.
Heck for 5-600 bucks you could probably find several QSC or Crest amps, that would work for you. A Crest CA6 will give you 600 WPC clean. 500 is usual asking price.
hope this helps......Mike

[unparalleled]

ah yes- they're both 807 derivatives, right? Aren't there theater amps making ridiculous power with the 6bg6 and something like 1000 volts on the plates? Maybe I'm thinking of something else.

So...what plate impedance would you run assuming you've got 600 to 800 volts on the plates and 300-400 on the screens- 4k or 8k?

I was planning to use a plain old LTPI for simplicity but I'm wondering if this calls for something akin to the marshall major PI setup. I have a ton of old 6sn7's I could use as drivers.

jamie

GOOD STUFF HERE: ( If you are thinking Marshall Major)
Have a look around the other circuits and stuff.
http://ozvalveamps.elands.com/holden/schematc.gif

[Enzo]

Wait- isn't it better to have nice tube PA amps and nice tube guitar amps?

Well, not in my world, maybe in yours. The whole point of tube guitar amps is what they add to the sound. The whole point of PA amps is that they don;t add anything to the sound, they just faithfully amplify. We want a PA amp to amplify the quiet stuff the same as the loud stuff, as opposed to the delightful compression/sag effect the tubes give us.

My 300 watt tube amp will weight 85-100 pounds. My similarly powered PA solid state amp will weigh a fraction of that. And I can buy some old perfectly fine SS PA amp for about the price of a set of power tubes for an SVT sized amp.

If you want to build something and donate it, then I can hardly argue with that price. But consider the entire budget, even after you pitch in those transformers.