6GW8/ECL86 datasheet notes

[terminalgs]

I'm doing some research for a small ECL86/6GW8 build (basically a Magnatone M2/M4 sans vibrato/reverb).  In reading the datasheet ( https://frank.pocnet.net/sheets/010/e/ECL86.pdf ) I found interesting notes that I've not seen on datasheets before:


HUM

The hum level will be better than 60dB under the following conditions:
Input voltage minumum 10mV RMS for 50mW output.
Grid circuit impedance max 0.5Mohm @ 50Hz.
Cathode decoupling capacitor minimum 100uf.
Pin 4 connected to earth.



MICROPHONY

The triode section can be used without special precautions again microphonic
effect in circuits in which an output of 50mW is obtained at an input voltage
of not less than 4mV RMS.


Thoughts and comments about any of these are welcome!


All of these notes are interesting, but specifically regarding "Pin 4 connected to earth.":

Magnatone was one of the few amp makers that used the 6GW8, so I looked at what they did.  The schematics for the EL34 and 7189A amps (M10,M13,M15..) all have PT's with a center-tap for the heaters,  but the 6GW8 (M2,M4,411) amps specifically all ground one side of the heater wiring pair (I'm assuming pin 4..).  I'll add the engineers at Estey during this era were all licensed electrical engineers and seem to pay close attention to the datasheet specs. 


I know that grounding one side of the heater pair was a cheap way to reference the heaters to a common ground for lots of amps/radios,  especially early examples and/or inexpensive examples.  I'd dismiss the use of it in these Magnatones if not for the fact that the 6GW8 datasheet specifically mentions this and that the tube was an advanced design (1960 release).


All that said..,  Magnatone engineers didn't follow the 100uf cathode cap recommendation :-)


for convenient reference, sluckey's lil maggie build has the original M2 schematic ( http://sluckeyamps.com/lil_maggie/Magnatone_M2.pdf )

[sluckey]

All of these notes are interesting, but specifically regarding "Pin 4 connected to earth.":

Just a thought...Maybe that statement was written by someone that was unaware of the superior hum rejection obtained by using a heater winding with a CT connected to earth? That was happening about the time that many amp builders were swapping over from grounding one side to using the CT. Generally speaking, I always considered the grounding of one side of the filament string to the chassis as a cheap, convenient method of wiring the filaments. I never considered that as a hum reduction technique.

Hard to say about the Magnatone engineers. Maybe they were on to something. Maybe they were just being cheap on that low cost M2 amp. There were several other Magnatone 'student' level amps that had one side of the filament winding connected to ground. Or maybe they did read that datasheet and took it to heart. As an aside... did you notice on the M2 schematic that the node D filter cap was 0.1µF? Node D feeds three triodes. What were the engineers thinking when they did that?  When I built my M2 it would motorboat pretty badly at higher volumes and just generally sounded cheap at lower volumes.

BTW, my M2 was built from a Hammond AO-44 amp, same 6GW8s, almost identical poweramp schematic. Hammond used a CT on there heater winding. Those Hammond engineers must have missed that note in the Phillips data book. 

[terminalgs]

All of these notes are interesting, but specifically regarding "Pin 4 connected to earth.":

Just a thought...Maybe that statement was written by someone that was unaware of the superior hum rejection obtained by using a heater winding with a CT connected to earth?

I would have thought that as well but I figured that with the tube being developed in the late 50's and it being an advanced tube, that the engineers would be "all knowing" about such things.    The only other time I remember seeing such a statement was in the EF86 D.S. which points out a pin grounding is required for the internal shield.

As an aside... did you notice on the M2 schematic that the node D filter cap was 0.1µF? Node D feeds three triodes. What were the engineers thinking when they did that?  When I built my M2 it would motorboat pretty badly at higher volumes and just generally sounded cheap at lower volumes.

A lot of those Magnatones had a small cap at the end of the line.  The 213 had a .047 cap there, as did the 413/440/450.  The M10 & M15 1uf. But, the 280/480 had a 10uf cap there.   Certainly a way to save money?   


Regarding the sound of your build.  I'm certainly glad to know that.  My goal is to build a light weight, small combo that I won't mind carrying up and down stairs.  If I could get an bluecheck Ampeg Gemini to weigh in at half it's weight, I'd be happy.  I want to build it with as many parts that I have on hand as possible:  I've got a small chassis ( 4"x2.5"x17") that fits an old Airline 12" cabinet.  the 6GW8 is attractive  because I can do 2 sockets instead of 3  (plus I have a handful of NOS units),  but,, in the end, if it won't sound good.... well, it's certainly not worth it.     Maybe I'll punch the power tube sockets as octals with an extra noval.   If I do try the 6GW8's (with octal/noval plates) and it doesn't work, I can punt and go back to 6V6's.

BTW, my M2 was built from a Hammond AO-44 amp, same 6GW8s, almost identical poweramp schematic. Hammond used a CT on there heater winding. Those Hammond engineers must have missed that note in the Phillips data book. 

Indeed!  I imagine Hammond was in the habit of hiring talented EE's as well. 

[sluckey]

Hope I didn't mislead too much about the sound. The sound really came alive with a cathode bypass cap on the first two triodes and a 47µF filter at node D and a nice 12" speaker. Still sounds like a small amp with an 8" speaker though.

I always suspected Magnatone had some purpose for using such a small filter cap at the final node in some of their amps other than just saving a buck. I also replaced the 1µF in my M10A with a 22µF.

I'll be interested to see what you come up with.

[jojokeo]

Sluckey, after noticing several corner cutting discoveries it appears to be cost cutting techniques a la Fender to me? I completely agree with you (as usual ;))

Terminalgs, take a gander when you get a chance:
http://el34world.com/Forum/index.php?topic=22181.0