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Old 30th November 2006, 09:26 PM   #1
diyAudio Member
Join Date: Nov 2006
Default Modding Roberts/Akai mono block tube amps

I have just received a pair of these amps that have already been worked on by an extremely competent retired audio engineer. At this time they are just very quiet mono block amps but I want to add a balanced mic input to each and a balanced output as well. I've purchased all of the necessary components for both amp:

Input side
2 - octal bases (input)
2 - JBL plug-in input transformers (600 ohm/50Kohm)

Output side
2 - XLRM
2 - 500 Ohm resistors
2 - Jensen output transformers JT-11-DMCF
2 - Alps Blue Velvet 100K poteniometers (L-pad)

Shielded wire.

Can anyone give me any pointers to look out for when I start this project?

The present input is a 1/4" unbalanced jack socket (high Z), the output is also an unbalanced high-level line output via a 1/4" jack socket, way too hot for a DAW, A/D converter or mixer to accept.
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Old 1st December 2006, 05:30 PM   #2
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Join Date: Nov 2006
Default Re: Roberts/Akai tube mods

This is my first posting on this site, can anyone help?
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Old 1st December 2006, 06:14 PM   #3
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Alain Dupont's Avatar
Join Date: Sep 2003
Location: Paris - France

Well to be able to help we would need the # model of these Akai amplifiers....

And a link or a schematic would be a must have!


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Old 1st December 2006, 07:34 PM   #4
wboyd is offline wboyd  United States
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Join Date: Aug 2004
Location: Louisville, KY
Sounds like and interesting project....

I currently have 3 pairs of these very nice sounding mono blocks. Only one pair has been modified/updated (really updated-new caps etc). I have never really thought about balanced in/out on these, but it does sound interesting....

And just to see if we are on the same page....what models do you have. I have a 2 pairs of M-8's and 1 pair of the M-5 or 6 model...I think.

Let me know...


"I am always doing that which I cannot do, in order that I may learn how to do it"
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Old 2nd December 2006, 07:50 PM   #5
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Join Date: Nov 2006
Default Re: Roberts/Akai mono blocks

I'll check the units for model number on monday since they are in my office.
Hit a small snag as components began to arrive. Didn't realize that the JBL potted input transformers were 9-pin and I had bought 8-pin bases from I searched all afternoon on the net friday but couldn't locate a compatible base. The 9-pin tube bases don't look as if they will fit, the pin spacing seems different and the center hole doesn't have room for the orientation tab. If anyone has suggestions to solve this i would welcome them, I really dont want to strap them down with tie wraps and solder on the 4 pins that I will need to use. Failing this i may have to buy another pair of input transformers with 600ohm primaries and 15 Kohm secondaries. If you have any suggestions and sources on those too i would be very grateful..this is starting to get much more expensive than I had originally planned
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