Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TubeGeek on September 28, 2013, 07:06:57 pm
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This amp came to me for a tune-up. I'm pretty sure this is the oldest amp I have had the pleasure to work on.
The owner stated it was a 1939. Looking on the internet for a few mins and I'd agree. Any body know for sure how to date these?
It uses 1x 6C8, 1x 6C5, 1x 6N7, 2x 6V6 and an 80 rectifier.
Somebody has serviced it previously. The filter caps are definitely not original. I'm trying to sort out just exactly what this is, kinda looks like a combination of two eh-100 schematics.
http://www.el34world.com/charts/Schematics/files/gibson/EH-100.pdf (http://www.el34world.com/charts/Schematics/files/gibson/EH-100.pdf)
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Any body know for sure how to date these?
Date codes on the speaker and pots.
The EH-100 seems to be the amp for the "Electric Hawaiian"-100 lap steel, which made its first appearance in the 1936 Gibson catalog.
Yours seems to be later and a bit more refined than this 1936 example (http://www.offsetguitars.com/forums/viewtopic.php?f=31&t=22346&start=0), but earlier than the balanced-looking layout (http://www.relic-tronics.com/images/gibsonEH-09.jpg) of this 1940's EH-100 (http://www.relic-tronics.com/images/gibsonEH-12.jpg).
The October 1940 Gibson catalog (http://fox-guitars.com/Gibson_Guitar_Catalogs.html) has a picture of the EH-100 (http://fox-guitars.com/images/1940_whats_new_es-300_c432.jpg) with the spaced out tube layout, making it probable that your amp is from 1936-1939.
Speaker/pot codes would probably lock up the date.
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3 pre tubes right under the input jacks and 6 tubes total jammed up close together.
Looks like they got away from building amps that way pretty quickly?
Still very cool.
Brad :icon_biggrin:
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GEEEZ, are those tubes close together or what? They must have been paranoid about the field coil influencing the tubes! Or perhaps the speaker magnet itself physically impinging on the tubes. Wouldn't want all those 'trons packed up on one side of the tube, you know.
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But in 1939 (or whenever) they still had lots to learn. Unlike us, who know ever' dam' thang. :laugh:
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the clustering on the tubes looks like it was to give clearance for the speaker (see NPB's linked photo). that was also pretty common in radios of the time to jam them all together to make the chassis real estate as small as possible.
what is the 6N7 used for? a PI? its hard to tell in the picture. There are two pieces of iron to the right in the chassis, one ends in "04", so maybe that's the L-6904 choke? and the other is the interstage T-8790? or is the 6N7 used as a a PI?
in the schematic listed, notice all the enormous resistor values. I'd be interested to see your voltage readings.
cool amp, good pics. thanks for posting.
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in the schematic listed, notice all the enormous resistor values.
All those resistor values such as 100MΩ are not megohms. They are kilo-ohms. An actual mega-ohm resistor is listed as MEGΩ.
There are a bunch of errors on the schematic. No center tap for the secondary of the interstage transformer, no ground connection for the output tube cathodes, no center tap for the primary of the OT, no B+ connection for the OT. There may be more, but that's enough to make me draw my own schematic.
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The linked schematic uses an interstage transformer for a PI. Another schematic I have has the 6N7, and instead of the twin-triode 6C8, a single triode (double diode, diodes unused) 6SQ7. This does appear to be somewhere between the two. Both have a speaker field coil, but the one with the transformer-PI adds L-6904. The OT have different numbers, as does the PT.
Now, I would wager, this one uses the 6N7 as the PI, although it may be a little hot driving the 6V6 bottles. (I've seen it in a circuit driving 6L6 bottles). Iron was getting more expensive, and the valve/tube was cheaper than an interstage/PI implemented via a tranny (T-8790).
what is the 6N7 used for? a PI? its hard to tell in the picture. There are two pieces of iron to the right in the chassis, one ends in "04", so maybe that's the L-6904 choke? and the other is the interstage T-8790? or is the 6N7 used as a a PI?
3 pre tubes right under the input jacks and 6 tubes total jammed up close together.
Correction: two pre tubes (6C8 and 6C5) and one PI (6N7) the other three tubes are the (2)6V6 and (1)80 (later known, IIRC, as the 5U4 with a different base)
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later known, IIRC, as the 5U4 with a different base
The 5Y3 was the octal equivalent of the type 80. The 5U4 has higher ratings.
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later known, IIRC, as the 5U4 with a different base
The 5Y3 was the octal equivalent of the type 80. The 5U4 has higher ratings.
That makes sense. the 80 had a 2A filament.
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I had a Gibson that was older than that one, with just a black covered cab....fixed it up & gave it away to one of my sons. It was quite primitive...one screw through the bottom held the chassis in the cab. Couldn't find a schematic for it though !
I do also have an EH150 7 string steel guitar from 1938, according to the Gibson site & s/n. It plays & sounds good..
The tweed cabs cabs came later than mine was.....that's cool little amp in your pic...