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Old 20th December 2008, 04:21 AM   #1
packman is offline packman  Italy
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Default Gibson tube amp mod for JAZZ

Hi dear guys, i have that Gibson goldtone amp, class A 15W all valve amp that i want to mod for use it with 2 guitars: a vintage epiphone sheraton (335-like) and a washburn j6 that has now humbucking gibson classic 57
This had initially a Celestion V30 speaker, sounds a bit boxy and with few basses so have now upgraded with a jbl M-121 (switched internally for 8 ohm speakers)
The preamp saturate easily so when the guitar volume is at 8 have distorted sound, i want a clean sound with volume guitar at 10 and a warmer and smoother reverb because now seems a "rockabilly" reverb, what mods i have to do?
this is the scheme of the amp:

http://www.gibson.com/Files/schematics/GA15RV.pdf

(hope this section is good for tube instrument amps too)
thanks in advance!
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Old 20th December 2008, 10:19 AM   #2
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Hi,

Thank gods! A TUBE guitar amp question

Looks like a fair common circuit, only made for gain rather than Jazz cool-clean

I'd start by making R1 100K and C2 10 or 22uF. R15 would benefit from being 82K for better AC balance drive to the output tubes. C15 can be upped to 47n or 100n to "phatten" it up.

That's not even getting into the tonestack mods...



Cheers!
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Old 20th December 2008, 11:03 AM   #3
packman is offline packman  Italy
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Default nice

Hey Gregg, today seems my lucky day, your site have valve hifi design AND guitar valve amps, perfect!
Sure i will try your mods litterally, have to change R15 AND R16 accordingly?
Do you think the mod reduce the brittle of the reverb a bit ?
thanks
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Old 20th December 2008, 12:33 PM   #4
45 is offline 45  Italy
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Default Re: Gibson tube amp mod for JAZZ

Quote:
Originally posted by packman
Hi dear guys, i have that Gibson goldtone amp, class A 15W all valve amp that i want to mod for use it with 2 guitars: a vintage epiphone sheraton (335-like) and a washburn j6 that has now humbucking gibson classic 57
This had initially a Celestion V30 speaker, sounds a bit boxy and with few basses so have now upgraded with a jbl M-121 (switched internally for 8 ohm speakers)
The preamp saturate easily so when the guitar volume is at 8 have distorted sound, i want a clean sound with volume guitar at 10 and a warmer and smoother reverb because now seems a "rockabilly" reverb, what mods i have to do?
this is the scheme of the amp:

http://www.gibson.com/Files/schematics/GA15RV.pdf

(hope this section is good for tube instrument amps too)
thanks in advance!
A simple thing you could try, if you don't want distortion, is to replace the ECC83 valve in the differential stage with an ECC82 type.
It should work without any other modification!

Cheers,
45
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Old 20th December 2008, 01:05 PM   #5
packman is offline packman  Italy
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thanks 45, need to fix bias? i'm a tech but don't have experience with valves
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Old 20th December 2008, 01:21 PM   #6
45 is offline 45  Italy
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Quote:
Originally posted by packman
thanks 45, need to fix bias? i'm a tech but don't have experience with valves

No, you just have to take out the ECC83 and put in the ECC82.
These two valves are pin-to-pin identical and have the same filament supply requirements.

The voltage supply and the resistors in your differential stage should work for the ECC82 as well.

As a rule of thumb, you will get less saturation from the first ECC83 amplification stages + you will have lower gain from the ECC82, resulting in lower distortion.
I guess that with full open volume controls (both on your guitar and your amp) you will have just a crunchy sound.

Cheers,
45
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Old 20th December 2008, 01:22 PM   #7
45 is offline 45  Italy
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The overall result also depends very much on the output of your guitar.
Usually the humbucker pick up has more output than a single.

Cheers,
45
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Old 20th December 2008, 01:52 PM   #8
packman is offline packman  Italy
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I want to have a clean tone with the humbuckers with the volume of guitar at 10 and if i want to saturate add an external booster.
There are some preferred brands in valve like this 45 and Geek?
thanks
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Old 20th December 2008, 02:15 PM   #9
45 is offline 45  Italy
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Quote:
Originally posted by packman
I want to have a clean tone with the humbuckers with the volume of guitar at 10 and if i want to saturate add an external booster.
So, try to replace the second ECC83 with an ECC82.

Quote:
Originally posted by packman

There are some preferred brands in valve like this 45 and Geek?

thanks [/B]
IMO, in the first place just pick a good working tube!
I would buy a new tube: JJ ECC802S, for example.
Then if you get what you are looking for, you can make some experiments with various brands.

Cheers,
45
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Old 20th December 2008, 03:29 PM   #10
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Another recommendation is to replace C6 and R6 with a jumper wire. The other suggestions sound good, a 12AY7 instead of a 12AX7, larger bypass on the first stage cathode resistor, etc.

Another, but more complicated suggestion, is to replace the tone stack with the Baxandall circuit from the earlier Ampeg amplifiers.
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