Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Other Topics => Topic started by: DummyLoad on August 17, 2011, 01:09:45 am
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don't think i'd pay the asking price - even if it is "new". i overhauled a MA-25 a while back - nice sounding amp.
--DL
http://cgi.ebay.com/MASCO-MA-35N-VINTAGE-TUBE-HARP-AMP-WESTERN-ELECTRIC-/220832642945?_trksid=p4340.m185&_trkparms=algo%3DSIC.OPJS%26its%3DI%26itu%3DUA%26otn%3D5%26pmod%3D260832769523%26ps%3D63%26clkid%3D2118242203729584681 (http://cgi.ebay.com/MASCO-MA-35N-VINTAGE-TUBE-HARP-AMP-WESTERN-ELECTRIC-/220832642945?_trksid=p4340.m185&_trkparms=algo%3DSIC.OPJS%26its%3DI%26itu%3DUA%26otn%3D5%26pmod%3D260832769523%26ps%3D63%26clkid%3D2118242203729584681)
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Wow, look at those old 6L6 bottles! Too cool. I remember my elementary school had one of those in the cafeteria. I wonder how much of this good old stuff ended up in the trash...
Jim
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OK, so how does this OP stage work?
CFs tied to OP tube grids, with a choke tied to where the OP tube grid return R's would normally be wired.
Negative bias to the 6SN7's (driver?) grids? So how are the OP tubes biased, by the choke?
Can an off the shelf choke be found that would work for a new build?
I wonder if this would be a good sounding slide amp of this amp?
Little Walter's sound had a lot of compression/sustain to it, but who knows if he really played through this amp. I grew up in Chgo. and hung/played around some of the blues clubs. I've never seen or even heard of one of these amps as being used by any of the old blues harp players. Not that I was there in the 50's.
DL, what did you think of the tone of this amp? Was it creamy, dirty, clean, a lot of head room, squishy?
http://www.el34world.com/charts/Schematics/files/masco/Masco_MA-35.pdf (http://www.el34world.com/charts/Schematics/files/masco/Masco_MA-35.pdf)
Thanks, Brad :think1:
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I went back and looked at the schemo again and I see I missed that the choke has a CT were the -bias is feed to the OP tubes grids.
But why use a CT'ed coil to feed the OP tubes bias?
Thanks, Brad :dontknow:
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6L6G can only do 27 Watts in mode AB1. Limited by the 270V max on G2.
In AB2 you can do 31W-47W.
When you try to run AB2 with C-R coupled grids, it just biases-down and blocks.
With a _LOW_ resistance grid current return (choke), AB2 works fine.
However as the grids go positive their impedance falls from 100Meg to 1K. You need a driver which does not sag too bad in 1K load. A cathode follower (for each grid) is almost enough. But this (and a Bogen beast) couple two rich cathode followers in a push-pull winding for double the current into the positive grid.
The lack of NFB is odd. In this price-class (this was a big-bucks amp!) even in this time (1947?) you would expect NFB so that the per-speaker output did not change much for two to eight speakers connected (it is too big for any one speaker of the day). Perhaps 1942?
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Thank you PRR for the explanation.
I guess this type of OP stage went by the way side as bigger/better versions of OP tubes came along?
Thanks, Brad :icon_biggrin:
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DL, what did you think of the tone of this amp? Was it creamy, dirty, clean, a lot of head room, squishy?
brad, i had the MA25N - AB1 and a completely different circuit. it has very smooth overdrive, it's kind of difficult to play clean since it did not have much input headroom, so i'd have to say it was more compressed than dirty.
the unit i acquired was stored for years in a humid environment (barn) and had quite a few resistors way out of specification. a cap job, and replacing nearly every power rail and anode load resistors, we had a decent sounding guitar amp. that amp sold us on the 6J7 tone for a front end. yes it has a grid cap, but damn it's a sweet sounding tube. sold the amp to a friend - he still plays it.
attached the schemo of what we ended up building - we made as few mods as possible to the original ckt. coupling caps, bypass cap, 1/4inch jacks etc.
--DL