Akai Roberts Single ended tube amp AC wiring

[Omegaman]

These amps have not been recapped
No TrackerX mods have been performed
The amps are all original



Ok, first I cleaned the amps

I added a AC power cords and 1 amp fuses

Here is how I wired Roberts 770x (Akai M8) AC

The right amp already has an AC power cord


Lets walk through the working unit wiring:
Start at the label PLUG. That is the two pronged power cord
One side is neutral and one side is hot

One side (neutral) goes up and connects under that brown heat shield tube to the blue wire from the power switch on/off on the front of the amp

The on/off switch has a green wire on the other end of the switch (blue in, green out)
The green wire runs from the switch to the transformer far end connection marked GND or 0V
This provides on/off control thru the neutral

The other wire from the plug goes to the 117 of the transformer

That's how the amps is to be wired

What I did is wire the LEFT amp for AC like the RIGHT amp


The other amp that does not have a power cord
This amp was the control amp for the tape deck

Look at the image labeled LEFT PT AS BUILT

On this, the main control amp, the AC is also controlled by the tape deck "tape end" switch which shuts down the whole unit when the tape is over. That is a blue wire from the BIG connector plug that connects to the blue wire (under the brown heat shield tube) that goes to the ON/OFF switch on front. This blue BIG connector wire will be replaced by the neutral side of the new power cord.

The green wire from the on/off switch (blue in/green out) ORIGINALLY RUNS from the switch to the 117v OF THE TRANSFORMER

MOVE THE GREEN WIRE (from the switch) FROM THE 117v TO THE 0v TAP OF THE TRANSFORMER.

Attach the HOT side of the new power cord to the 117v of the transformer

Dealing with the big connector on the left amp was easy



I left the AC socket in the LEFT amp so that the RIGHT amp could plug in and both amps could be powered by the LEFT amp on/off

I also preserved the connection to keep the STEREOPHONES operational.

Slapped a couple of cases together and there it is, instant cool, Single-Ended Pentode mono block stereo

 

[Kim G]

Very nice and thanks for the pictures. I have one of these in the project to do pile. Someday I will get to it, thanks for the inspiration and jog to the memory. Are the fans required or just for an extra bit of protection?

 

[Omegaman]

The fans aren't really needed as there is a screened vent on top
I just like them and use fans wherever possible

12vdc computer fan with 6vdc 200ma power supply
I should make cases for the fans

 

[x3workshop]

Thanks for this. I have a pair far down in the queue.

 

[jmccanna]

Hi, I have the good fortune to be working on a pair of these right now. The pictures you provided are so helpful. In mine, while still in the case, the left amp worked if I plugged a line level source into the pickup jack but the right would not. All the tubes light up fine. So, I pulled the amps to do some troubleshooting after reading TrackerX's guide. Interestingly, I noticed that the transformer is wired in at 100 volts rather than 117v.

I have only tested the right amp out of the box so far because it already has a power plug. I get no output though. This occurs irrespective of where I plug the source in. I have slid the record slider full left and removed the spring. The lights come on and the tubes glow but I get nothing. I have tried inserting the source signal in the line signal jack and the pickup jack. I have hooked a speaker into the speaker jack and the headphone jack and get nothing. I have switched out the EL84 with a known working one and get nothing.

I will read your post above to figure out how to wire the left amp but would welcome suggestions on what I should try next for the right amp.

TXOne is the right amp transformer and TX2 is the left amp transformer.

Thanks a ton.

James

 

[johnblue]

my first foray into tube Amps, there's so much you can do with these as a building block. I made really high quality microphone amps with mine, and the SE power Amps are bonus. my first pair lives in our recording studio.

I'm thinking of getting another set soon and dailing them in for a hi-fi setup at home.

 

[RWood]

Interestingly, I noticed that the transformer is wired in at 100 volts rather than 117v.

That's how they all start out when they were mounted in their original cabinets. They got 100VAC from the main power transformer of the tape recorder.

Now that you have removed the amps from the tape recorder you need to unsolder that primary wire from the 100V tap and move it to the 117V tap. (Unless you happen to live in Japan). Otherwise, don't power up the amp until you have fixed the PT voltage. Only then should you proceed to any further troubleshooting.

 

[jmccanna]

my first foray into tube Amps, there's so much you can do with these as a building block. I made really high quality microphone amps with mine, and the SE power Amps are bonus. my first pair lives in our recording studio.

I'm thinking of getting another set soon and dailing them in for a hi-fi setup at home.

Thanks. I am debating my options. First choice would be a hi-fi set of monos but the microphone pre is intriguing to for my studio. Hmm....... Choices.

 

[jmccanna]

That's how they all start out when they were mounted in their original cabinets. They got 100VAC from the main power transformer of the tape recorder.

Now that you have removed the amps from the tape recorder you need to unsolder that primary wire from the 100V tap and move it to the 117V tap. (Unless you happen to live in Japan). Otherwise, don't power up the amp until you have fixed the PT voltage. Only then should you proceed to any further troubleshooting.

Thank RWood. Too late on the powering up thing. I bought it off of a guy who got it at an estate sale and he said he had tested it and it played audio as late as last week. So, before checking it fully (oops) I got it home and wanted to confirm what he told me. Then, I pulled the amps. I know I know I know. . . . . . . . . .

Based on what I see I need only move the one wire for troubleshooting and then configure the rest on the right amp. I am not a transformer expert so I have a question. Would the 100v power tap mean it won't work? If so, the left amp is tapped the same way but did work?

 

[RWood]

Would the 100v power tap mean it won't work?

No, it means that unless it is being supplied 100vac, it is getting the wrong voltage. I don't know where you live but around here I get 120V out of the wall........ so that means if I were to plug it in here all the voltages would be approx. 20% more than they should be. Filaments (three separate circuits) and the HT (high voltage) to the rectifier will all be high. Being the robust tube amp that it is, it would probably/possibly/for-a-time work under those conditions. It just would not be happy about it and the already aged capacitors might flip their lids and ka-pow, now the transformers are shorted.

So.....I just wanted to make sure you correct this before moving on to trouble-shooting. And yes, it is as simple as moving the one wire on each transformer.

Let us know how you make out!

 

[jmccanna]

No, it means that unless it is being supplied 100vac, it is getting the wrong voltage. I don't know where you live but around here I get 120V out of the wall........ so that means if I were to plug it in here all the voltages would be approx. 20% more than they should be. Filaments (three separate circuits) and the HT (high voltage) to the rectifier will all be high. Being the robust tube amp that it is, it would probably/possibly/for-a-time work under those conditions. It just would not be happy about it and the already aged capacitors might flip their lids and ka-pow, now the transformers are shorted.

So.....I just wanted to make sure you correct this before moving on to trouble-shooting. And yes, it is as simple as moving the one wire on each transformer.

Let us know how you make out!

Thanks. I enjoy learning new things. I live in Kingston, WA and get 120 out of the wall. All the tubes light up on the right side but I am getting zero output. I am assuming the capacitors will need replacing but it seems like I should get some output even if it is static and such.

I am still learning about transformer testing. How do I test the output transformers? I will Google it as well but thought I would ask.

Thanks again,

James

 

[jmccanna]

No, it means that unless it is being supplied 100vac, it is getting the wrong voltage. I don't know where you live but around here I get 120V out of the wall........ so that means if I were to plug it in here all the voltages would be approx. 20% more than they should be. Filaments (three separate circuits) and the HT (high voltage) to the rectifier will all be high. Being the robust tube amp that it is, it would probably/possibly/for-a-time work under those conditions. It just would not be happy about it and the already aged capacitors might flip their lids and ka-pow, now the transformers are shorted.

So.....I just wanted to make sure you correct this before moving on to trouble-shooting. And yes, it is as simple as moving the one wire on each transformer.

Let us know how you make out!

I forgot to mention that the right amp (non-working) has only one wire soldered to the 100v tap whereas the left amp has 3. I will move all taps soldered to the 100v tap to the 117v tap.

Thanks again. What I was able to get out of the working amp sounded pretty good so I am hopeful.

James

 

[Omegaman]

I think the other wires are to feed power to the plug socket so each amp can be on/offed from the MAIN amp

Did you remove the tubes from the amps and put them back in?

tube placement is different for each amp

 

[jmccanna]

Hi,

I will double-check the tube placement. I have not changed it but that does not mean someone else in here before put them in the right place. I have the schematic and pictures so I will check that too. Wouldn't that be nice if it is that easy?

 

[jmccanna]

Hello OmegaMan,

In your picture above of the right amp I cannot quite tell the position of the 6bq5. On mine going from the left the 6bq5 is third-most but mounted in the fourth socket and there is a smaller tube to its right. The middle socket is empty/open.

 

[Omegaman]

Here is the best picture I have of the two amps tube lineup


EL84 and 6X4 are swapped and one emptry socket between EL84 and 12ax7 tube

 

[Selmerdave]

That is so weird...

 

[jmccanna]

Here is the best picture I have of the two amps tube lineup

View attachment 966743
EL84 and 6X4 are swapped and one emptry socket between EL84 and 12ax7 tube

Hi, the bottom one is the right side it would appear. My tubes are in that configuration. But the one to the left of the EL84 is a 12AU7 not 12AX7.

 

[jmccanna]

The left amp has a Telefunken tube that looks like the 12AU7 in the right amp with numbers on it - b8204701 - FCC83

 

[jmccanna]

My left side amp - which worked on initial test - has the same set up as your does.