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Old 12th September 2003, 09:03 PM   #11
SY is offline SY  United States
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Yeah, if you want to change over to EL34s or the like, you'll need a lot more screen voltage. My ST-70-based pentode amp uses about 480V on the screens. Curcio's used 390. Audio Research typically used about 400. That's not necessary for high-perveance tubes like sweeps; in fact, most of them have pretty low max screen voltage ratings.
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Old 12th September 2003, 10:24 PM   #12
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Been following some of these threads and will attemp to make my first post here.

I think you can use EL34s if you up the screens to 450-500v as SY said. This should keep your bias voltages within the range already built into the PS. Your cathode resistors will probably show slightly higher voltage (.1V or so) because you'll be pulling a little more idle current, but this won't matter.

It may be that the 700V is close to the max for EL34s, I can't quite remember this number.

And the power will go down. Since the max Ik for the EL34 is around 150mA, your power will go down below 100W but might max out above 70W. Parallel a couple of EL34s and you've got it made.

Also, your volume will have less range because you won't need to supply as much grid voltage to max the power. That is, there is probably too much gain in the front end for EL34s in this configuration.

I am curious to know from the other far better tube jockeys than I, why the follower on the first stage? It seems like you could drive the phase splitter directly from the first stage no matter what tube you were using.
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Old 13th September 2003, 12:04 AM   #13
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Should have thought a little more before I hit submit.

I quickly looked on the hammond site to see what the primary impedance might be for those transformers. I guessed at about 2k ct based on the transformers that were listed at 120W.

So, with EL34 and this low of a load, the power should drop to around 40W.
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Old 14th September 2003, 05:43 AM   #14
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Well, looks like the EL34 has a max plate voltage of 800v so I think I'll be alright running class B 700v. For screen, I'll shoot for 400v, Bias V will end up close to what I've got now.

What's odd is this pair of Heathkit 55w W-7A monoblocks I've got here. They've got 495v on the plates and 498v! on the screens. (which exceeds my databook max values) Sure sounds sweet though.

Below is my regulated 150v screen supply- Not having much experience with regulation ckts, what's my best bet of altering this for 400v output at the cathode? It's fed 700v on the plate. Tube is 6BM8. Would toying with the feedback resistors R35 and R37 do? Ignore everything after R38, it's impressed DC for the AC filaments. I know that R39 is a simple load to keep the tube in its conducting region.

Thanks!
Cory
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Old 14th September 2003, 09:22 AM   #15
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Hi,

Quote:
Tube is 6BM8. Would toying with the feedback resistors R35 and R37 do? Ignore everything after R38, it's impressed DC for the AC filaments. I know that R39 is a simple load to keep the tube in its conducting region.
Basically, yes...making R37 partially adjustable would give you some range to toy with.

Better would be a complete redesign of the regulator altogether IMHO.

Cheers,
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Old 14th September 2003, 05:52 PM   #16
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Say, cadman, would you be kind enough to supply the component values for the regulator.

I don't think that adjusting R35 and R37 will do the trick, because the regulator is also dependent on where the screen is set on the 6bm8.
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Old 14th September 2003, 07:30 PM   #17
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Cadman, without knowing what your component values are now, if you're willing to simply change component values, this regulator should produce ~480V with a total idle current of about 6mA not including the EL34 screen current.

If you want a variable supply, replace the 165k resistor with a 150K resistor in series with a 50k pot. This will adjust from about 453V to about 500V.

Click the image to open in full size.

This was a quick redraw, but I believe it will work correctly.
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Old 15th September 2003, 02:10 AM   #18
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One other thought is to try the EL509 MKII. These are octal based EL509s. If the problem is noval pin outs, this might solve it.

I only suggest this because you're going to lose a tremendous amount of power if you use EL34s. If the octal EL509s work, you can avoid rewiring the screen supply and you'll have to replace the novals with octals in either case.

You can check out the MKIIs at Tubebuider. MHO.
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Old 15th September 2003, 06:57 PM   #19
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Default BACK TO SQUARE ONE....

Hi,

Quote:
One other thought is to try the EL509 MKII. These are octal based EL509s. If the problem is noval pin outs, this might solve it.
Not many people actually know this but the KT90 is nothing more than a rebased EL509 (EI).

The EL509 MkII is the same thing but they're not allow to call it a KT90...

Quote:
I only suggest this because you're going to lose a tremendous amount of power if you use EL34s.
While I like the way the EL509s sound, I've never been a great fan of the European EL34s, the U.S. 6CA7s usually sounding much better in the bass department, YMMV.

Cheers,
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Old 15th September 2003, 09:29 PM   #20
PRR is offline PRR  United States
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Alex- good to see you here.

Cadman --

> the quality of available EL509s. Arc overs, open filaments, imbalances and cracked base glass

Get better 509s? Open-filament, if you are not doing something very odd, is so unlikely that the vendor should take it back.

Arc-over.... welll, running 700V B+, arc-over is a risk. There are reasons why hi-fi amps rarely run much over 500V B+. Some of the PA amps using higher voltages were known as tube-killers. Even when the nominal plate voltage rating allows 700V, transformer action, clipping, reactive loads, and other oddness give much more trouble at higher voltage.

I am trying to figure what you expect to get here. 700V B+, fix-bias, and apparently a 1.9KΩ plate-to-plate impedance: that roughs out to something like 360 Watts RMS! That is a fork of a lot of power to expect from any pair of octal or noval tubes. Even if you do not actually ask 300++Watts of it, the idle voltage and impedance puts a lot of strain on the tubes.

There are few small (not Transmitter) tubes that will stand 700V. Oddly the original EL34 was rated 800V, though some folks think that's a bit high, and the plate power rating won't allow large current or power. With a perfectly regulated plate supply and a precision resistor load, the EL34 claimed 100 Watts RMS; but those conditions are absurd except maybe in some specific radio transmitter.

The 6550 and the 8417 will usually stand 700V, though this is over their rating. Available 8417 are expensive and fragile; while I loved and lived by 8417 in a past life, I would no longer use one. 6550 and kin are available and mostly quite robust. A pair of 6550 working at 600+ volts will give up to 100 Watts, but the load resistance should be more like 4K or 6K, not 2K. Forced to use a 2K transformer, I would think about a quad of 6550, though if your chassis is built there may not be space.

6L6GC is rated 450V Plate. We know that guitarists often run them at far higher voltage, and some as high as 700V idle (sagging to much lower on power-chords). You could find out what brand and batch works good in high-power guitar amps, then spark-test a few to find some that live with 700V on their plates. Looking into a 2K load, I really think you need four or six 6L6GC to handle the current.

If you can lose the 700V B+ supply: still the 2K transformer winding tends to mandate more than a pair of tubes. Few tubes have large-signal effective resistance much less than 2K, and the 2K P-P winding presents 500Ω to each side. With triodes, you would want Rp to be around 250 ohms; pentodes do better for their size but still there are no standard pentodes that fat.

Quick answer: put an 8Ω speaker on the "4Ω" winding so the effective load is 4K. Bass response and power will suffer a little compared to the tranny specs, but it will work. Run 6550 at 700V B+ and about 300V on the screens. Bias the G1 at about -40V (I don't have a 6550 sheet handy). The cathode current must be held to 50mA or less at idle (a wee bit more if you want to push the ability of an original TungSol-made 6550). It will run far up on the B side of Class AB, often not the happiest sound. Without heavy feedback it will jump and bark as the gain changes with signal level. With heavy feedback: why run such high voltage and low impedance to get the same sound as any other high feedback amp?
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