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 Post subject: Question On Gibson EH-185 Guitar Amplifier
PostPosted: Nov Sat 18, 2006 5:59 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 129
Location: Roslyn Heights
Hi all I've been working on getting this amp back to life again.
I have a question as the a .00075mfd cap in the shown schematic.
What is its fuction in the circuit? If I used a 00025mfd or 001mfd what would be the differance I don't have a .00075mfd at the moment.
Also there is a 10uf cap before the 100K resisitor going to the 1 meg pot.
If the cap was 20uf would it change the tone of the amp, if so what other 10uf caps would change the tone in their locations.
thanks for any advise.
I ask these questions on the 10uf caps because the ones that were there were 40uf and 20uf yet the schematic calls for the shown 10uf caps.
Also is there a sub tube for the 6SQ7 tube that I can use, would a 6SN7 work just for testing
regards
jim



http://www.harpamps.com/gibson/pg_0030.jpg


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 Post subject: Re: Question On Gibson EH-185 Guitar Amplifier
PostPosted: Nov Sat 18, 2006 2:21 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 564
Location: Wisconsin
jsel84 wrote:
Hi all I've been working on getting this amp back to life again.
I have a question as the a .00075mfd cap in the shown schematic.
What is its fuction in the circuit? If I used a 00025mfd or 001mfd what would be the differance I don't have a .00075mfd at the moment.


It's for frequency shaping. Probably to keep the amplifier from oscillating. You can use .001mfd.

Quote:
Also there is a 10uf cap before the 100K resisitor going to the 1 meg pot.
If the cap was 20uf would it change the tone of the amp, if so what other 10uf caps would change the tone in their locations.
thanks for any advise.


It might, but not much. It's to decouple the plate supply between the various stages.

Quote:
I ask these questions on the 10uf caps because the ones that were there were 40uf and 20uf yet the schematic calls for the shown 10uf caps.


Then someone was in there before you. You can use 20uF.

Quote:
Also is there a sub tube for the 6SQ7 tube that I can use, would a 6SN7 work just for testing
regards
jim


No, it's a totally different tube. I'm surprised they used it, actually, since it has a twin diode in it too for use as a detector/amplifier in radio sets.

Tom


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 Post subject:
PostPosted: Nov Sat 18, 2006 4:16 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 129
Location: Roslyn Heights
Thanks Tom for the info, and the fast reply.
would 40uf be too high for the 5AU4 rec tube?
Could I use a 12SQ7 tube just for testing as shown they used 6SQ7 peramp tubes
Thanks
jim


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 Post subject:
PostPosted: Nov Sun 19, 2006 2:29 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 564
Location: Wisconsin
jsel84 wrote:
Thanks Tom for the info, and the fast reply.
would 40uf be too high for the 5AU4 rec tube?


40uF is fine. That would be the max size, by the way.

Quote:
Could I use a 12SQ7 tube just for testing as shown they used 6SQ7 peramp tubes
Thanks
jim


No, because then you would have to rewire the socket. Just get the right tube.

Tom


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 Post subject:
PostPosted: Nov Sun 19, 2006 7:56 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 129
Location: Roslyn Heights
Thanks Tom for the advise
Jim


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