Akai Roberts Single ended tube amp AC wiring

Thanks so much. Your schematic differs from the one I have. The value of C9 = .01, Cathode cap is C10, cathode resistor is R12, R11=500k. On the second side of the the 12A R16 is cathode resistor at 3K and C12 is the cathode cap. C9 and R13 connect (R13 is 250k). So it is largely similar except for the value of C9. I have two sets of these and 3 of the 4 mono amps work. I have compared the working with the non-working one and replaced with like values. I think I need to just go to each and every solder joint and every component in the signal path.

More to do but thanks.
 
In going through point to point comparing the full-functioning right-side amp with the partial-functioning amp I noted that R15 differs. In the working amp it has a 100k fixed resistor and in the bad amp a 50k resistor. The schematic calls for 50k but here the 100k amp is the one working.
 
Well, I changed out the 50k resistor for a 100k resistor and it made no difference. Amp is low-volume, distorted at output. However, it is clean if I tap into the preamp out and go to the other line in on the working amp.

I am missing something for sure. Bummed in the NW.

James
 
Hi, I apparently have two schematics for the Akai that are largely similar. One has voltages for the left amp in a couple of places. Using that, I get 119v at the plate for the 12AD7 just before C9 instead of 100v. I get 219v on the far side of R13 (250k). I get 146v off of the second plate for the 12AD77 whre it should be 125v. Those seem abnormally high. I am getting 1.2 and 1.2 volts at the cathodes and 6.5v at the heaters. The values of the resistors all check out per spec. Suggestions? My set works but it is low output and a bit distorted now. All coupling caps have been replaced and a couple of resistors too.
 
The voltages on the 12AD7 are within 20% of the schematic, barely, and I would not suspect them to be the cause of the distortion. Especially since the cathode voltages are good. Your heater voltage is a little high, indicating the wall voltage is probably more that 117vac. Again, not a problem but if you can use a variac so that the heaters read 6.3, then check the preamp voltages, you should see that they are even closer to spec.

You said in the previous post that the preamp is clean when you jump it to the other channel, do this should confirm that all is fine with the 12AD7. It sounds like your problem occurs after that. I think you should continue to use the good channel to your advantage to try to identify where things are going bad. Have you swapped power tubes to see if the distortion follows the tube? Are the plate, screen and cathode voltages the same from one power tube to the other? What happens when you swap the plate wires (effectively swapping output transformers? Are your VU meters connected? You can disconnect them for testing purposes. Do both OT secondaries have the 8 ohm resistor in-circuit?

You see where I am going....keep on investigating, focusing on the power amps and the differences from one side to the other.
 
Thanks RWood. I have been carefully comparing the good channel to the bad one and am finding no discrepancies in wiring. I have switched all tubes from the good channel to the bad one and it makes no difference. In following the the troubleshooting guide included in the service manual one thing to check is the choke coil at L3. I do not have an inductance tester but am taking the amp to a tech friend who has one.

The VU's do not work on the bad channel but work beautifully on the good one. I will check on switching the power tubes out and then rechecking the voltages. Both OT secondaries have the 8 ohm resistor in circuit. I will keep plugging away.

Fortunately, I have two of these Akai RtoR sets and have three of four amps working great. This one though escapes me.
 
Interesting and maybe not a coincidence that the VU meter is not working. It takes the signal from the OT secondary, as I recall. Try it with the meter disconnected. Just a thought.
 
At last, I found some time to dig in. It was a bad SW1 switch. I finally put it on an oscilloscope and traced the signal path. It stopped at the OT output. The only thing between that and the speaker out was the switch. I bypassed the switch and it works and sounds great. Thanks all for your patience. Lesson learned: trace all wiring first all the way to the end.
 
Hello all,

I am still working on some final stuff. I cut the original box down to size, cut off the top where the speaker screen was and re-purposed it as the top and glued it on the top.

The box had an ugly grey thin stock woven fabric glued to the exterior of it. The paint I used is a "Hammered" style surface so after you spray it on the surface randomly raises and gives it a texture. By spraying the existing fabric, the paint filled the little gaps in the fabric and ended up looking quite natural. The screen on the face of each amp were originally an off-white. Not a great color even when clean but these were stained by who knows what - whatever it was would not come off. So, I spray painted them too to match the rest. (When powered up you can still see the tubes glow). I still need to add a better back to it for safety. I will also add an IEC three-prong power connection soon with fuse.

I will use this to drive efficient open-baffle speakers I have. Sounds quite good actually.

This has been fun for sure and very educational. Thanks to all who responded with your excellent suggestions and observations.

Listening to Keb Mo' at the moment.

Have a great day.

James
 

Attachments

  • AkaiTopShot.jpg
    AkaiTopShot.jpg
    87.9 KB · Views: 42
  • AkaiAngleShot.jpg
    AkaiAngleShot.jpg
    74.8 KB · Views: 45
Back
Top Bottom