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Old 2nd July 2011, 06:48 PM   #1
sansuii is offline sansuii  Germany
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Join Date: Jul 2011
Location: Braunschweig
Default Bass Guitar Amp with GU81 M

Hi folks,

i have build a 1100 watt strong and 110 Lbs heavy guitaramp.

For schematic and more details see:


If you have questions let me know...

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Old 3rd July 2011, 03:39 AM   #2
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Location: Pittsburgh, crumbling wasteland
Very impressive! Any reason why you used a 300b to drive the GU81?
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Old 3rd July 2011, 08:51 AM   #3
sansuii is offline sansuii  Germany
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Join Date: Jul 2011
Location: Braunschweig
Originally Posted by astouffer View Post
Very impressive! Any reason why you used a 300b to drive the GU81?
In the beginning i use EL34 in triode connection, but by switching on the amp some times it gives a arc, flash, short, inside of the EL34. The reason is that at the Anode there is 330V an on the Katode there is 750V that means 1080V between Anode and Katode in the moment of switching on, when there is no current... This is no Problem for a 300B but some types of EL34 (Chinese) can not handle it.

I use a katodefollower because the GU81M takes at G1 by -10V around 10mA grindcurrent ...
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Old 8th January 2012, 09:32 AM   #4
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With the author's permission, I would like to open this thread to get some insight on the possibility of replicating this amp:
To adequately do so I really need to know impedance and characteristics of the final and interstage transformer.
The author has been very very kind in providing me with some info:

The amp works with these conditions:

Ug1: -158V
Ug2: +600V
Ug3: 0V
Ua: +1600V
Ia: 150mA
Ra: 2500Ohm

The interstage transformer has a gear ratio of 1:2. and an ultralinear tap of 50%.

The output transformer has a prim. 5kOhm and two 20% tap for negative feedback, also and sec. 8Ohm/4Ohm.

Unfortunately I do not posess simulation software nor would I trust myself in designing such a complex design.

If anyone is able to help me out in replicating this design I would be very grateful!
Also if it were possible to eliminate the interstage transformer (or at leasr find an equivalent of commercial production) it would simplify the work greatly.

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