EL34 VS: 6L6/5881

[n2qei]

Anyone have a thought on weather it is ok to run 6L6/5881 mod tubes in replace of the EL34's in a Eico 730 modulator? For the hell of it, I stuck a pair of EL34's in an AF68, and they seem to work ok with 22.0 volts of Bias. But what about 6L6's in a 730 modulator? Seem s like the 6l6's would have more meat over the EL34's. Just concerned about the bias and plate voltage on the 6L6's in the Eico.

[w4bfs]

You have already addressed the bias question .. the load impedance is close enuff to not be a worry ... the screen dropping resistor (if present) should be investigated to ensure that adequate screen current can flow which can affect production of outpoot before distortion sets in ....beware of the kink...73 John

[Bill, KD0HG]

What's the plate voltage in that Eico modulator?
IIRC, 5881s have a much lower max rating than EL34s do..

On edit @ home-

Checked my GE Essential Characteristics book..The 6CA7/EL34 is good for 800 V Ep, and 425 V, 8 watts G2, 90 watts out, P-P class AB1.The 5881's max ratings are 360 V Ep, and 270 V., 3 watts G2, 26.5 watts out, Class AB1.

You might be better off with a 6L6GC...Max Ep is 500V and max output is 55 watts AB1.

Even better a 6550:..Huge reserve outpoot power in AB1, maybe 100 watts, and good for 800 V on the plate, same as the EL34.

The 6CA7/EL34 is a far more capable toob than the 5881 or 6L6. I think it would be a bad swap. Better off with the 6550, 6L6GC (only) or even the KT-88. lots of Russian replicas for sale at reasonable prices. I've has a quad of Svetlana 6550s in my homebrew stereo at 600 Volts for the last 5 years and they're still like new.

[n2qei]

so you think 6550's or 6L6GC'S Would be better? would it be a direct drop in? I know I can get the EL34's but wanted to try something more readilly available. I  believe plate voltage is 460 bias -36v

[Bill, KD0HG]

so you think 6550's or 6L6GC'S Would be better? would it be a direct drop in? I know I can get the EL34's but wanted to try something more readilly available. I  believe plate voltage is 460 bias -36v

The 6L6GCs would be a very good match. I looked up the schematic for your modulator on the BAMA website and it says plate is 460 and screen is 445. -40 to -45 would be correct for bias, so you'd crank the bias pot R29 for the correct idling current, about 100 Ma. Idling current can be measured across the 10 ohm cathode resistors R25 and R26, in the cathodes of the output tubes. Use  Ohm's Law and you want 50 mils per tube. That would be about 1/2 volt from each cathode to chassis ground. Use the bias balance pot R19 as needed for equal currents in both tubes.

A pair of 6L6GCs will give you around 50 watts of audio, the rating of the modulator. Forget about using the 5881s/6L6/6L6G/6L6GA/1614, they're just going to be too wussy.

I have always wanted an Eico 720 & 730 setup, what a beautiful rig.

[AF9J]

What's the plate voltage in that Eico modulator?
IIRC, 5881s have a much lower max rating than EL34s do..

On edit @ home-

Checked my GE Essential Characteristics book..The 6CA7/EL34 is good for 800 V Ep, and 425 V, 8 watts G2, 90 watts out, P-P class AB1.The 5881's max ratings are 360 V Ep, and 270 V., 3 watts G2, 26.5 watts out, Class AB1.

You might be better off with a 6L6GC...Max Ep is 500V and max output is 55 watts AB1.

Even better a 6550:..Huge reserve outpoot power in AB1, maybe 100 watts, and good for 800 V on the plate, same as the EL34.

The 6CA7/EL34 is a far more capable toob than the 5881 or 6L6. I think it would be a bad swap. Better off with the 6550, 6L6GC (only) or even the KT-88. lots of Russian replicas for sale at reasonable prices. I've has a quad of Svetlana 6550s in my homebrew stereo at 600 Volts for the last 5 years and they're still like new.

Bill,

Are you sure about that 26W push-pull rating at AB1 for 5881s?  I used twin 6L6 powered guitar amps for years.  They were all good for anywhere from 40-60W output, even without distortion.

73,
Ellen - AF9J

[Bill, KD0HG]

Ellen: Indeed, a pair of 6L6s will easily do around 50 watts output in class AB2 (with a bit of grid current and a few watts of drive/transformer driver), but not even close in AB1. The 6L6/5881 is rated for 360 plate volts max in AB1 and 26 watts max out per pair. The Eico 730 we're discussing is a pure class AB1 design with around 450 plate volts, so the 6L6GC is the bottle to use.

The 6L6GC is a considerably heftier bottle. 30 vs. 19 plate dissipation watts..Will do 50+ watts output in voltage-drive AB1 with fixed bias and 450 plate volts. .

[n2qei]

Hey Bill,

Thanks for all the great input, But what about the the 6550's, could they be a plug n play type of tube? what changes would i need to do? And keep in mind, I'm Not a very good engineer with this stuff. I did the audio mod thats on this site, along with some extra timtron additions, sounds pretty good. just wanted to try something different, so whst do you think about the 6550 idea?

pete..

I'm a huge fan of the 6550. It uses a bit more heater current than the 6L6s, but it ought to work fine in that modulator with lots of reserve power. I don't have my tube manuals here at the office, but I'll check out the curves and biasing this evening to verify that you won't have any problems. Stay tuned.

[w4bfs]

nice going Bill ... the 6550 needs a bit more fil current than the others ... if the power tranny has the reserve and also remember to check fil voltage anyway ... the old line voltage was 110 V

ps  I think the confusion about 5881 started with the Sovtek 5881 which is essentially a 6l6gc 600V tube

[AF9J]

Ellen: Indeed, a pair of 6L6s will easily do around 50 watts output in class AB2 (with a bit of grid current and a few watts of drive/transformer driver), but not even close in AB1. The 6L6/5881 is rated for 360 plate volts max in AB1 and 26 watts max out per pair. The Eico 730 we're discussing is a pure class AB1 design with around 450 plate volts, so the 6L6GC is the bottle to use.

The 6L6GC is a considerably heftier bottle. 30 vs. 19 plate dissipation watts..Will do 50+ watts output in voltage-drive AB1 with fixed bias and 450 plate volts. .

Hmmm,

Good point Bill.  I didn;t think about that.

73,
Ellen - AF9J

[John K5PRO]

6550 is a sweet big tube, lotta cathode current allowed, but you better watch that you don't burn up the Eico power transformer trying to light the filaments on a pair.

[The Slab Bacon]

Like some of the others have said, 6550s are great tubes for AB1 as well!! Just tickle the grids with a few volts of audio and watch the munky swing!! Like others have said, the only disadvantage is the extra filament current needed to light them. I believe these tubes were designed for class A and AB1 operation without any grid current.

I have used several pairs of them in rangers and other transmitters and gotten gobs and gobs of audio with just a simple 6C4 phase inverter to drive them!!

They are great bottles for audio, but beware of the "kink" in the plate curve. They will definately benefit immensely from a little feedback wrapped around them. It is almost essential.

                                                               The Slab Bacon

[WBear2GCR]

It's not perfectly clear what the max power out is going to be for a given tube and a given transformer's input impedance.

The other problem in the situation where it's an RF tube that's being modulated is that the impedance of the plate (the load of the secondary of the modualtor) can vary a bit depending on how the transmitter is loaded - meaning the match changes, which changes the loadlines too.

Anyhow, a good example is in the Dyna MkII and MkIII mono amplifiers. There is nil visible difference between the two. However one was designed to work with EL-34/6CA7 tubes and the later one (iirc) 6550 type tubes (maybe 6L6GC). If you put the 6L6GC into the MkII amps, you will not make anywhere near the spec'd full snot output at all.

Dunno if this works out the exactly same way for the typical plate modulated rig, but it's something to beware of, especially if you make a similar swap and can't seem to get full tilt boogie out. 

                    _-_-Wbear2GCR

[Steve - WB3HUZ]

Like some of the others have said, 6550s are great tubes for AB1 as well!!

The 6CA7s in the Eico 730 are running AB1.

[flintstone mop]

That Eico modulator was supposed to be as close to Hi-Fi as you can get for a modulator. And connecting to a 6146 RF final is what it was designed for. You can then feed a Linear to get you to legal limit.

Fred

[Steve - WB3HUZ]

Yep. It has a very similar topology as the Eico hi-fi amps including two preamp stages, a phase-splitter, RC couped/fed AB1 output tubes, negative feedback and adjustable  bias. They just made the coupling caps smaller and put in a stupid clipper/filter stage. If you change out a few of the caps and bypass the clipper/filter stage, you have a nice hi-fi modulator.


http://www.amwindow.org/tech/htm/eico730.htm

[Tim WA1HnyLR]

OK FINE!!! lets dispel some old wives tales here.
The 6550 will produce the greatest  outpoot for a given plate voltage. Electrically a 6550  is equivalent to apair pf 6L6 tubes in parallel .They pull TWICE the phillament current. By the way I have used 5881 s at 600+ volts without any flashovers.  6550s have one big problem. Over a period of time of use they tend to get leaky. The grid circut resistance should not exceed 68K . I have had them run away. I can tell many stories about 6550 tubes. I had a homebrew audio amplifier thatr I built based on the 50 watt 7027 amp in the RCA receiving tube manual. I used 150 K grid leak resistors. The power supply cosisted of a Triad 800Vct@ 500MA power transformer. A real brute. I used a pair of GZ34 rectifiers in parallel and oil condenshers (cap inpoot) power supply. I used a cross laminated CVM3 as an outpoot transformer to drive the pair of 4-1000 tubes in the modulator in my 4X1 rig. Being that this setup was in the experimental mode I did not have the amplifier /audio driver wired into the main power breaker for the transmitter.After a session of morning radio I shut down and went out to have breakfast with a friend of mine. When I returned home the transmitter room was fill with lingering smoke and the acrid stench of burning transformer. The 15 amp circut breaker for the outlet I had plugged the amplifier into had tripped. Both GZ 34 tubes were shorted. One 6550s envelope was blackened one side of the CVM3 was burned open. What a mess. The power transformer miraculously did not short even though it was burnt. The one thing I left out of the equasion was a fuse of the proper value in line to the primary. As a matter of fact. In a service bulletin for the Gatesbc5p5kw AM broadcsat transmitter which used 4 6550 s push pull parallel as an audio driver recommended useing 6ca7/el34 in place of the 6550s . 6550s are a great tube but 'ya gotta keep them on a short leash. Also check out the 8417.A tube with similar power outpoot capabilities as the 6550 but higher gain. Less biass required and similar outpoot. I have a pair in a Ranger I did up.They have been very stable in their operation.
Tim WA1HnyLR

[Opcom]

If space allows, an 807 has a good voltage rating.

Also consider some octal based sweep tubes. They are fairly cheap because the audiophools don't use them:

6BQ6 (If=1.2A, 110mA current, 11W diss.) -maybe a little light.
6BG6 (If=0.9A, 110mA current, 20W dissipation)
6DQ6 (If=1.2A, 115mA, 18W diss.)

[w4bfs]

interesting discussion ...  most sweep tubes will work but would require screen grid changes ... lower operating voltage limit for most tubes ... the 6bg6 is a 6l6 with a plate cap and is an excellent choice if you have the room ... Tim, what is your read for correct load impedance for a 6550 for a given set of conditions?