Major Modification of Ibanez Valbee "Tube amp"

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Kalle
Posts: 180
Joined: Sat Apr 03, 2010 7:22 pm

Major Modification of Ibanez Valbee "Tube amp"

Post by Kalle »

Hi Guys
If you remember, I have had a Ibanez Valbee guitar amp for a while.
The Valbee is a small guitar "Class A" tube amp :
Output power: 5W@4 ohm, Speaker: 6.5" 4 ohm Speaker, Tubes: 1 6L6GC & 1 2AX7[dual triode].
The "block diagram" looks like this:Image
The power tube amp design is very similar to the power amp design in a classic Fender Champ.
But the whole pre-amp an tone control parts are designed with Operational Amplifiers (JRC 4558).
But I have never been satisfied with distorsion sound in the Boost channel as the distorsion sound was made with "clipping diodes" (as in a "Tube Screamer" pedal) and not by an overdrived tube stage. An early modification I did was just to removing the clipping diodes. This made it possible to overdrive the tube stage in the boost channel and get a "tube distorsion" sound. But I was still not satisfied with the distorsion sound and decided to go to the bottom with the problem. After "reverse engineering" the amp and making a schematic diagram of the Pre-Amp I found the "culprit"..
The tube stage in the Boost channel was not a classic pre-amp tube gain stage, but a "cathode follower"(no gain, but good current driving capability) only used for driving the "clipping diodes". I decided to change the "cathode follower" to a classic pre-amp tube stage that could be overdrived in a normal way.
After studying the design of several classic Fender amps, I copied the design of a typical pre-amp stage.
The most critical part was setting the gain levels before and after the tube stage, making it possible to overdrive the tube hard (but not to hard) and avoiding that the IC´s in the tone control stage (after the tube stage) was overdrived.
The work included rewiring of printed circuits boards, changing and moving components/component values ( took several attemps) until I finally was satisfied.
Now I have an amp with a really good boost channel featuring a classic overdriven tube amp stage, that can go from clean sound to hard metal distorsion (and everything in between). The clean channel is not modified. I have also modified the 6L6 tube stage with a bias current sensing resistor and a potentiometer for bias current control.
/Kalle (the electronic engineer)
Tanglewood TW28STE (Shadow P7 EQ system) steelstring Dreadnought
Yamaha RBX 270 bass guitar with Laney DB150 amp.
Yamaha RGX 320 electric guitar with Egnator Tweaker tube amp.
http://www.soundclick.com/kalleinsweden
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VikingBlues
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Joined: Sat Apr 03, 2010 7:44 pm

Re: Major Modification of Ibanez Valbee "Tube amp"

Post by VikingBlues »

Shows what you can do if you have the knowledge / skills and are prepared to work at it. Sadly I have none of these virtues. :big_smile:

Good work to get an amp to sound the way you need it Kalle. :thumbsup:

I've seen threads from people who have VibroChamps who talk about changing things around in these sort of ways - bypassing voicings etc and moving towards having a more "real" tube amp set up. Seems a great set of skills to have, but I'm useless at learning new things these days - got my work cut out remembering how to do things I've already learned - must be down to being an :oldie:
An improv a day keeps the demons at bay!
MichaelRobinson
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Joined: Mon Oct 03, 2011 10:20 pm

Re: Major Modification of Ibanez Valbee "Tube amp"

Post by MichaelRobinson »

Well Kalle................I know, if I recall right, that you is educated engineer in electronics and know what you do.
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Kalle
Posts: 180
Joined: Sat Apr 03, 2010 7:22 pm

Re: Major Modification of Ibanez Valbee "Tube amp"

Post by Kalle »

Hi Michael
Yes, I was educated as an electronic engineer in the late sixties, in the transistor era , before IC´s come around.
And was working with analog electronic designs during the seventies and eigthies, when every electronic engineer built there own HiFi equipment.
Designing with tubes was already a dead knowledge in these times.
Now when many guitarist/musicians is talking about the fantastic sound from "point to point built Tube amps", I decided to learn more about tubes and tube amp designs. And the secrets of good tube amp designs are not difficult to reveal with a basic analog electronic knowledge.
Most guitar amps today are based on the early Fender Champ, Deluxe, Twin and Bassman designs (all from the late fifties).
Marshall copied these Fender amp designs and tweaked them for increased output power. And the same thing did Vox , Laney etc. etc.
/Kalle
Tanglewood TW28STE (Shadow P7 EQ system) steelstring Dreadnought
Yamaha RBX 270 bass guitar with Laney DB150 amp.
Yamaha RGX 320 electric guitar with Egnator Tweaker tube amp.
http://www.soundclick.com/kalleinsweden
chewbakka
Posts: 1
Joined: Sun Nov 16, 2014 9:26 pm

Re: Major Modification of Ibanez Valbee "Tube amp"

Post by chewbakka »

I have now finished modding the Valbee and I want to thank Kalle again for his help.
The mod for the pre-amp worked great - it sounds so much better now and the Boost is now a logical extension of the clean channel.
If I want diode clipping – I can still use a pedal in front.
What I like best in the new boost channel is the responsiveness to the input from the guitar - great dynamics.
I have also installed a jack to plug in an external speaker and the amp sounds a lot better with a bigger speaker.
I have also installed a switchable attenuator down to 1 Watt and even 0.2 Watt to allow for late night practicing without upsetting the neighbors and that works fine too. It cuts the highs a little, but is still good enough for practice.
So thank you again, the mod was a great success.
expansywny
Posts: 1
Joined: Mon Jan 12, 2015 11:57 am

Re: Major Modification of Ibanez Valbee "Tube amp"

Post by expansywny »

Hi Kalle

I want to follow Your path but i'm not engineer. I know how to keep soldering, also build a few effect pedals.
I've already changed original speaker in valbee to jensen alnico 8'' and replace 6l6, but I think that with Your mods it can play better.

Do You have any photos or schemas of Your mods?

Kuba
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Kalle
Posts: 180
Joined: Sat Apr 03, 2010 7:22 pm

Re: Major Modification of Ibanez Valbee "Tube amp"

Post by Kalle »

Hi Kuba
You have a PM.
/Kalle
Tanglewood TW28STE (Shadow P7 EQ system) steelstring Dreadnought
Yamaha RBX 270 bass guitar with Laney DB150 amp.
Yamaha RGX 320 electric guitar with Egnator Tweaker tube amp.
http://www.soundclick.com/kalleinsweden
Nik
Posts: 1
Joined: Mon Mar 09, 2015 9:22 am

Re: Major Modification of Ibanez Valbee "Tube amp"

Post by Nik »

Hi Kalle,

I understand the concept of your mod and i would like to do this for my Valbee, too. Your sound samples proves that you did a great job on this.
It would be nice if you can send me any photos and/or schemas of your mod.

Thanks,
Nik
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Kalle
Posts: 180
Joined: Sat Apr 03, 2010 7:22 pm

Re: Major Modification of Ibanez Valbee "Tube amp"

Post by Kalle »

Hi Nik
You have a PM.
Check also this
viewtopic.php?f=5&t=3300
What later happend with the Valbee...
/Kalle
Tanglewood TW28STE (Shadow P7 EQ system) steelstring Dreadnought
Yamaha RBX 270 bass guitar with Laney DB150 amp.
Yamaha RGX 320 electric guitar with Egnator Tweaker tube amp.
http://www.soundclick.com/kalleinsweden
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