EL34 vs 6CA7

[Lennart]

Hi,
going to replace the tubes in a Music Man 410HD. Original power tubes are 6CA7. Now there are EL34s.
I assume there are some differences between the two. Would it be a good idea to go back to 6CA7?

[bigsbybender]

The differences are nearly negligible. The 6CA7 was a North American built version of the Euro EL34.... If you find an original Sylvania 6CA7 you will find that it is built more like an American "Beam Tetrode" (like 6V6, 6L6) as opposed to a pure Pentode (EL84, EL34).

The differences in usage are near non-existent and you can drop one to replace the other. I strongly suspect that in new production that there is less of a difference. The difference being tube selection of a production run as opposed to making a tube slightly different.

I'd just go with the EL34's.... in this day and age they are the same tube.

j.

[Lennart]

OK, thank you, Mr B!

[Tone Junkie]

Where as yes they kind of sound the same in a nut shell I can tell the differance between both jj and EH 6ca7 as compared to each companies el34 offerings. Im a tube roller I waste money to see what sounds good to my ear. There is a differance just like there kt77 sounds differant and there all interchangable.
   Its kind of up in the air because just as the last person said they even sound differant between batches of tubes. 
Doug has some good ones here also.
Thanks Bill

[PRR]

In the MusicMan.... any tube which will stand the stress should work the same.

Most circuits drive the tubes and they do what they want.

The MusicMan drives transistors which FORCE the tubes to do exactly what the transistors command.

MusicMan beat the stuffing out of those tubes. Use any modern HIGH-quality "EL-34" from a reputable vendor (like Hoffman). Hi-Fi tubes and Banjo World tubes may not take the beating.

[Lennart]

But there is still a need for bias adjustment?

[Jennings]

Yes...you'll need to check the bias when chaning the output tubes of any fixed bias amp and make sure it's adjusted to within the tube's comfortable operating specs.  Unless it's a cathode bias amp that regulates itself (although it's often a good idea to check those from time to time to ensure all's well), which this I'm presuming isn't.

[PRR]

There is a bias adjust on some MusicMans, but it does not need to be changed when the tubes are changed, even to another type.

[Lennart]

Yes PRR. That´s what I thought. Pin 8 is connected to the emitter of a transistor. But what seems strange to me is that the pin 5, grid, is not connected to output signal from preamp, but to a zenerdiod. Output signal from preamp is on the base of the transistor mentioned.

[PRR]

> Pin 8 is connected to the emitter

Collector.

> pin 5, grid, is not connected to output signal from preamp, but to a zenerdiod

The tube is run "grounded grid". Actually the grid DC voltage is up positive so that the transistor can sit underneath and still have voltage to work with. However the tube _current_ is exactly equal to transistor current. And transistor current is strictly controlled by transistor bias (or DC NFB in some models) and emitter resistor.

[Lennart]

Then I wonder what the purpose of the tubes is. For the sound? For the weight?

[phsyconoodler]

EL34's and 6CA7's may work the same,but usually the sonic differences are large.6CA7's do not breakup as early as EL34's and 6CA7's usually are fatter sounding.This is from experience using both NOS versions in amps I've built.I can't speak for modern 6CA& versions but the NOS versions sound quite different.
  Musicman likely chose 6CA7's over EL34's for just that reason.They distort less.

[Lennart]

> Pin 8 is connected to the emitter

Collector.

> pin 5, grid, is not connected to output signal from preamp, but to a zenerdiod

The tube is run "grounded grid". Actually the grid DC voltage is up positive so that the transistor can sit underneath and still have voltage to work with. However the tube _current_ is exactly equal to transistor current. And transistor current is strictly controlled by transistor bias (or DC NFB in some models) and emitter resistor.

Maybe I was wrong about what I said. If you look at this page, you see two schematics. I studied the firs one, and thought the second one was the same, with some changes.
 http://www.el34world.com/charts/Schematics/files/musicman/musicman_2475-130_&_2275-130.pdf

[HotBluePlates]

EL34's and 6CA7's may work the same,but usually the sonic differences are large.6CA7's do not breakup as early as EL34's and 6CA7's usually are fatter sounding.This is from experience using both NOS versions in amps I've built.I can't speak for modern 6CA& versions but the NOS versions sound quite different.
  Musicman likely chose 6CA7's over EL34's for just that reason.They distort less.

Sonic differences don't matter in this case. As PRR said, the tube can only do exactly what the transistor allows it to do.

MusicMan likely used the 6CA7 because it was the domestically produced tube, and therefore cheaper. That's the same reason Marshall used the EL34 (6550 don't count, cause the American distributor put that in).

[PRR]

> you see two schematics

That's right, there are several MusicMan plans.

In that PDF, the one on page 3 has a "conventional" output stage. Good old cathodyne and "fixed" negative bias on power tube grids. As it says, adjust RV-2 "-35V" so that you get 1/2 Volt across the 10 ohm cathode resistors (100mA in four tubes). Tube "flavor" may matter like any conventional amp.

In the plan on page 2, TR1 adjusts 0.6V to 1.2V. This is divided by four 470 resistors. Q1/Q2 drop 0.6V base-emitter. This leaves 0.0V-0.6V across the 3.9 ohm emitter resistors. You probably trim for 0.2V across the 3.9 ohms. Transistor current is set ONLY by these parts plus the audio signal from IC-8. This transistor current flow through the collector to the cathodes of the tubes. Tube cathode current is _exactly_ equal to the transistor current. The transistor collector will rise or fall as far as necessary so that tubes' grid-cathode voltage allows that current to pass. "Grounded grid" is the buzz-word.

On third thought: plate (OT) current is not quite cathode current; some flows to screen. Different tube-constructions and/or low plate voltage peaks give different plate/screen division ratios. So different tubes might have slightly different sound. However these may not be the same-differents as EL34/6CA7s working in conventional grounded cathode.

> what the purpose of the tubes is.

MusicMan was Leo Fender after he sold out to CBS. Fender Company had rushed to transistor amps, and they all burned up; meanwhile CBS's tube amps were going strong. Peavey also had burn-up in his first transistor amps, but he was dogged enough to find and cure the problems. Still, when these amps were made, the safe bet was tubes for high-power HIGH-abuse stage amplifiers. Recall that Woodstock was mostly powered with Mac sweep-tube 300+ amps. I was making beer money with some 300W Bogen 8417 amps retired from film-motor duty. In my day-job I was maintaining Dyna 35-60 amps both canned and bottled; the bottle-amps gave little trouble while the newfangled ones rotated through my repair bench.

So tubes were the better bet, but costly and heavy. Also the soft old tweed tone was going out of style with the hard Sunns and the better (working) transistor amps leading the way to new tones. The grounded-grid MusicMan attempts to make the most of the bottles with HIGH 700 Volts and the very strict cathode-drive. While special-contract 6550 could be rated 700V, stock 6550 only claim 600V; Philips claimed EL34 was good for 800V and the 6CA7 claimed the same.

[Lennart]

PRR, obviously you have deep knowledge in this. Nice! Could you possibly explain why there is two different schematics? Both say chassis 2275 & 2475.