I recently came cross a box of 6sq7 tubes and would like to put an eh125 type amp together but was wondering about the old schematics,usually "M" stands for megohm,the tube lineup is a pair of 6sq7 tubes feeding a 6j5 then on out to a pair of 6v6's,I'm assumong "m" does not mean meg but means kilo here right???
http://www.gibson.com/Files/schematics/EH-125 Amp.pdf
http://www.gibson.com/Files/schematics/EH-125 Amp.pdf
I'm going to agree with you on this.The exceptions are the 6V6 cathode resistor which would be 200 ohms and the B+ feed resistor that's probably 500 ohms. It's very bizarre the way they designated those values.
Meg is Meg, M is k, M. is Ohm (as in 200M. = 200 Ohms for the 6V6 cathode resistor) in these old drawings.
I stand (sit?) corrected. Weird. I knew there were some odd symbols for capacitance but never ran into this before.
I'm sure you are right. "M" means "kilo ohms". There is no way the amp would work otherwise. Plus you'd have one heck of a time finding 400 megohm resisters
In the one case where a 2Meg resister make sense "Meg" is spelled out
This amp was never intended to be over driven. Looks to be from a time when people wanted a cleaner lower volume sound. But it would be interesting to hear what that kind of phase splitter sounds like when driven to clipping. I just can't "picture the sound in my head" Only one phase would clip. After you build us do let us know
I've built one of the EH150 circuits, 6L6G instead of 6V6G.
There were four versions of the EH150 schematic. I built the last one, the Mark 4, which was released in 1941. I used 400V on the plates so it is much louder with more headroom than the original (270V) but one day I'll change it out to 270V for that browned out Plexi tone.
It uses three 6SQ7, a 6N7 phase inverter and a pair of 6L6GA instead of the original 6V6GT. I used a 5V4G rectifier. I have the schematic & parts list if you're interested.
All four version schematics are on the web, and IIRC in the 1969 Gibson Service Manual which used to be on schematic heaven.
I did not use a field coil speaker, you cannot overdrive those, well not for very long anyway.
There were four versions of the EH150 schematic. I built the last one, the Mark 4, which was released in 1941. I used 400V on the plates so it is much louder with more headroom than the original (270V) but one day I'll change it out to 270V for that browned out Plexi tone.
It uses three 6SQ7, a 6N7 phase inverter and a pair of 6L6GA instead of the original 6V6GT. I used a 5V4G rectifier. I have the schematic & parts list if you're interested.
All four version schematics are on the web, and IIRC in the 1969 Gibson Service Manual which used to be on schematic heaven.
I did not use a field coil speaker, you cannot overdrive those, well not for very long anyway.
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FWIW the schematic has *very* detailed instructions explaining how *not* to overdrive it.
It also helps explain why the very poor (technically) 1/4" plug became the dominant MI connector: the telephone plug was the *only* standardized audio connector which could be found *anywhere*, even the smallest town (which had a telephone service, of course).
It also helps explain why the very poor (technically) 1/4" plug became the dominant MI connector: the telephone plug was the *only* standardized audio connector which could be found *anywhere*, even the smallest town (which had a telephone service, of course).
awesome,a parts list would be great,I'd probably opt for the pair of 6L6 if I had any,I have lots of 6v6 so thats what I'll use,I love the 6sq7 for the sound and the price,I'll more than likely end up using a 6sn7 as the inverter since again its what I have on hand,but the parts list would be great for sure
I've built quite a few MI amps and the problem I have with the 1/4" phone connection is not the plug but the jack. They don't build them like they used to and I had about a 30% rejection rate on Switchcraft jacks, never mind the no-name stuff.
I still use Switchcraft but only the Mil Spec jacks (made in USA) type C12X with plated silver contacts and decent spring steel. The non-MilSpec stuff is no doubt made in C***a and doesn't cut it any more.
From what I've seen of RCA phono connectors that's not a great design either!
AP72, e-mail me at fleghorn@shaw.ca and I'll send you what I have on the EH150. If you want to use up four of those 6V6 you have lots of, why not use the EH150 circuit but replace the 2x6L6 with 4x6V6, the 4K primary of the EH150 OT is then right for two pairs of 6V6 and you get 30W just like 2x6L6, but with a little more attitude I think.
For 400V in old 6V6 you'll probably want to go with the later GTA or GTB type if you have them. Sylvania GTAs can handle it. The RCAs from the '40s are the best tone to my ears but they won't like MI duty at 400V unless you bias them on the cold side and then they won't sound as nice I think.
I still use Switchcraft but only the Mil Spec jacks (made in USA) type C12X with plated silver contacts and decent spring steel. The non-MilSpec stuff is no doubt made in C***a and doesn't cut it any more.
From what I've seen of RCA phono connectors that's not a great design either!
AP72, e-mail me at fleghorn@shaw.ca and I'll send you what I have on the EH150. If you want to use up four of those 6V6 you have lots of, why not use the EH150 circuit but replace the 2x6L6 with 4x6V6, the 4K primary of the EH150 OT is then right for two pairs of 6V6 and you get 30W just like 2x6L6, but with a little more attitude I think.
For 400V in old 6V6 you'll probably want to go with the later GTA or GTB type if you have them. Sylvania GTAs can handle it. The RCAs from the '40s are the best tone to my ears but they won't like MI duty at 400V unless you bias them on the cold side and then they won't sound as nice I think.
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