Gibson tube amp mod for JAZZ

[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!

 

[Geek]

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!

 

[packman]

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

 

[45]

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

 

[packman]

thanks 45, need to fix bias? i'm a tech but don't have experience with valves

 

[45]

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

 

[45]

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

 

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

 

[45]

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.

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

 

[guitar88]

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.

 

[packman]

you guys give me a lot of great suggestions, great.

for guitar88: 12AY7? ECC82 are not the equivalent of the 12AU7 (have read searching in ebay)?

for Geek: because here (in my city) is not simple fo find the right material (film resistence, good caps for audio and the JJ valve etc) have you that components in stock?
thanks

 

[guitar88]

Of the 12AX7, 12AT7, 12AY7 and 12AU7 the 12AU7 is the lowest gain. The 12AY7 I like better for its sonic quality in guitar amplifiers. It is still a lot lower gain than the 12AX7.

 

[Geek]

Re: nice

Hi,

Sure i will try your mods litterally, have to change R15 AND R16 accordingly?

Nope, just R15. The reason for that is in a long-tailed phase splitter, one side inherently has lower gain than the other. The different resistors is standard practise by manu companies to equalize the gain.

But I see someone already mentioned trying a different tube first. Try an ECC81.

Do you think the mod reduce the brittle of the reverb a bit ?
thanks

Hard to say... I don't like "sand-state" reverb sections, so tended to avoid them.

for Geek: because here (in my city) is not simple fo find the right material (film resistence, good caps for audio and the JJ valve etc) have you that components in stock?
thanks

I have some stuff in stock, but don't ship to Italy as the postal loss is too high. Even registered mail goes missing

Cheers!

 

[packman]

for guitar88: "The 12AY7 I like better for its sonic quality"
ok!

for Geek: "I have some stuff in stock, but don't ship to Italy as the postal loss is too high. Even registered mail goes missing "

Really? Can be canadian-to-italy posts because i have bought a lot of items in US and never had problems (mainly with USPS).

Thanks anyway and you guys have a Great Christmas!

 

[mod_evil]

I'm brazilian and I make some modifications on Guitar Amplifiers like Marshall and Fender.

I think you MAY test 12at7 and 12au7 in this amplifier before starting the mods.

I use here when I don't need much distortion 12au7. But some friends use 12ay7 too and them liked these two tubes.

Best regards.

 

[45]

you guys give me a lot of great suggestions, great.

for guitar88: 12AY7? ECC82 are not the equivalent of the 12AU7 (have read searching in ebay)?

thanks

12AY7 has no european equivalent in my knowledge.
Other american equivalts of 12AY7 are 6072, 6072A.

12AU7 is the american equivalent of ECC82.

ECC82-12AU7 exist in several versions: E82CC, ECC802S, 12AU7A, 5814 and some other less common.

However 12AY7, 12AU7 and 12AX7 all have the same pin-out and filament supply requirements.

Cheers!

 

[packman]

[I think you MAY test 12at7 and 12au7 in this amplifier before starting the mods.
I use here when I don't need much distortion 12au7. But some friends use 12ay7 too and them liked these two tubes.
[/B]

Thanks! yesterday have made the first 2 mods, bypass R6 & C6
and put C2= 10u, the sound mellowed a bit, before make other mods in the circuit i want to try the change of the 12AX7 with
AU7 and AY7 (have to bought them), what is the valve yiu suggest to change, the first (input) or the second?

12AY7 has no european equivalent in my knowledge.
Other american equivalts of 12AY7 are 6072, 6072A.

12AU7 is the american equivalent of ECC82.

ECC82-12AU7 exist in several versions: E82CC, ECC802S, 12AU7A, 5814 and some other less common.

However 12AY7, 12AU7 and 12AX7 all have the same pin-out and filament supply requirements.

Cheers!

Thanks a lot
After your advises guys i googled and discovered an interesting program to calculate the gain with the various ECC81/2/3

http://www.audiocostruzioni.com/a_d/idee/tube-math-di-antonio/tube-math.htm

then this site is very useful too:

http://thetubestore.com/gainfactor.html

here compare the gain of the varoius type and compare various brand too!:

http://thetubestore.com/12ax7review.html

 

[45]

Packman, kwowing the gain of devices is useful as line of guide.

However a guitar amp has to produce a sound and not re-produce with the best possible fidelity a recorded sound.

So, I think the best way is : "plug them in and listen to what comes out!"


Cheers,
45

 

[45]

An important question to me.

If you amp is in good condition, I think it is not a good idea to heavily modify it because it could loose all its value.

Just replacing the tube will not compromise the historic value of the amp as you can always fully restore its original configuration.

Cheers,
45

 

[VictoriaGuy]

An important question to me.

If you amp is in good condition, I think it is not a good idea to heavily modify it because it could loose all its value.

Just replacing the tube will not compromise the historic value of the amp as you can always fully restore its original configuration.

Cheers,
45

It's sounds like it is a re-issue amp, so one needn't be too concerned about historic value in that case.
One does need to be careful, though, since most of the reissues are built on pcb and don't always hold up well to a lot of soldering and swapping of components.
Cheers
John