Gibson EH-125

[ic-racer]

I have a broken Gibson EH-125 coming to go with a Gibson ES-150 I am repairing.

I'll try to draw up a schematic, because this particular EH-125 is not the one with a schematic available on the internet.

This is a point to point amp and someone has already been in there. Some of the original parts are gone, so I hope they made appropriate component replacements. Otherwise the intended original circuit could be lost forever.

[ic-racer]

The Gibson EH-150 is the amplifier that was sold with the ES-150 guitar, but I went with the EH-125 amplifier for a few reasons.
1) The EH-150 is kind of an oddball circuit.
2) The EH-125 is a simpler two-6V6 circuit
3) Both amps came housed in a similar curved top cabinet with removable back.
4) Both amps came with 12" speakers

The EH-125 just seems like it will be easier to repair and keep running due to the simpler design.

[ic-racer]

Gibson EH125 with 6SJ7 and 6J5 [img:949:750]http://img.photobucket.com/albums/v670/ ... 0EH125.jpg[/img]
Gibson EH150 (early version?)[img:440:400]http://img.photobucket.com/albums/v670/ ... /EH150.jpg[/img]

[ic-racer]

*********************************************
*****OUTDATED POST
*****ALL SPECULATION RESOLVED LATER IN THREAD
*********************************************



I did find, on this very forum, a posted picture of a similar EH-125 innards shot. Not much info in the related thread; maybe the guy gave up on the amp?
Anyway I analyzed the picture and put in some annotations. Of course, I'll be getting my own amp in the mail shortly, so I can resolve any of the ambiguous areas.

Of note: Notice the V1 (6SJ7) plate resistor. It looks to have an old-style code that is one thousand ohms different. In the current system it would be 10k, but that can't be right. Must be 100k, yes? In the picture it is clearly marked Brown-Black-Green.
[img768]http://img.photobucket.com/albums/v670/ ... %20try.jpg[/img]

The important thing to me is that this amp circuit is 100% repairable. No oddball components. Even a new power transformer is available. Tubes are 6SJ7 and 6J5 which are not hard to find.

[Stevem]

That resistor looks to be in fine shape so what does it test at?

All of these airovox caps are shot , trust me !

[ic-racer]

That resistor looks to be in fine shape so what does it test at?

I'd like to know. The amp guts shot I posted and annotated came from this thread; ; it is someone els's amp.

http://ampgarage.com/forum/viewtopic.php?t=25701

But, there is not much information about the amp. No mention of the resistor values in that thread.


My amp should be here this weekend. According to the seller, someone replaced "some resistors" and he does not have the originals and the amp does not work.

I don't know what I'll find in my amp, but one thing I was suspecting is finding a modern 10k resistor on the plate or some value that makes the amp distort too much.

[sluckey]

Most of the resistor values you annotated on the pic are incorrect...

RED-GRN-YEL = 250KΩ
BRN-BLK-YEL = 100KΩ
GRN-BLU-YEL = 560KΩ
GRN-BLK-GRN = 5MΩ
BRN-BLK-GRN = 1MΩ
ORN-BLK-RED = 3KΩ (correct)

[Malcolm Irving]

Brown Black Green in the old 3-band system would be 1M

https://www.easycalculation.com/physics ... r-code.php

Nice amps.

Edit: simulpost with Sluckey

[ic-racer]

1 meg plate resistor with 30k cathode? I have not seen a schematic for 6SJ7 with that, that I why I was questioning some older system where the last green band would be 10k multiplier.

The EH125 schematic with 6SQ7 tubes lists the plate resistor as "100M" and cathode resistor as "3M" using, I presume a nomenclature where M stands for what would today be annotated K.

One way it might work out is "100M" = Brown-Black-Green somehow?


Anyway, USPS shows my amp arriving tomorrow. Problem might be that resistor could have been one that was replaced during the prior 'repair.'

I did look up some Plate/Cathode combinations for other 6SJ7 amps for reference. None are 1Meg or 100Meg. No 30k cathode.

200k/1k
220k/1.2k
100k/910r
100k/1.5k
390k/0
270k/1k
62k/680r
100k/560r

[sluckey]

In the '50s mega-ohm was usually spelled out as MEG. K had not become standard yet so M meant thousand (like the Roman Numeral).

One way it might work out is "100M" = Brown-Black-Green somehow?

BRN-BLK-GRN has always been 1,000,000, 1 million, 1MEG, 1M (current standard). Your old 100M is really 100K, 100,000 ohms.

[rp]

All of these airovox caps are shot , trust me !

^ This. When you see wax caps (and bumblebees) don't even waste a minute of your time. Some old radio guys somehow manage to stick a new small MF inside them to keep the looks.

In that pict is that the ignition coil or the brake cylinder?

Keep posting, always interested in these ancient Gibsons.

[Malcolm Irving]

Maybe the person who put those resistors in was confused by those old schematics and put some crazy values in?

[ic-racer]

Thanks for the comments. The EH125 pictured above is the one i bought. Usually i wont buy an amp that has been tampered, but this one looks exceptionally clean.

[Firestorm]

Field coil speaker? That can be a bitch because different coils were used. OLD Gibson amps often had resistive voltage stabilisers (large Watt resistors to ground are the giveaway). And even models where the schematic doesn't show them, they often used them anyway. Force of habit? Repurposed chassis'? Gibson engineers changed their minds more often than anyone and although there is huge number of Gibson schematics out there, they cover less than half of what Gibson actually did. And you thought Fender was bad?

[ic-racer]

Just for the record, here is an EH-150 schematic that is available. I was reluctant to buy a non-working model like this due to the fact that it has four transformers and transformers are a component that can be extremely difficult to find adequate replacements. In this case there is a Power Transformer, a Choke, the speaker Field Coil, Phase Transformer and Output Transformer.
[img725]http://img.photobucket.com/albums/v670/ ... .pdf_1.png[/img]