Rogue 88 amp NFB relay question

[cytovette]

I am working on a Rogue 88 amp for a friend and am wondering why the NFB circuit is controlled by a relay. Every amp I have worked on before always had a direct connection from the output to the input cathode. To me it looks like when the relay is engaged the output is shunted to ground.
The red marks on the attached schematic are my edits for clarity.

 

[cytovette]

A friend just told me that it is to stop the thump at turn on.

 

[sparkie]

A friend just told me that it is to stop the thump at turn on.

Yes, its a "mute". It should have its own loading resistors for the transformer. Its the only one you can do w/o going to more expensive industrial relays.
What I would do is use a Potter & Brumfield T92 and lift the screen resistors on the output tubes for the mute or lift the 500V

 

[cytovette]

Someone did a bunch of shoddy diy mods, including bypassing the output transformer center tap fuses. It over heated and took out a lot of components. Now the input tubes are overdriving and failing.

 

[sparkie]

Someone did a bunch of shoddy diy mods, including bypassing the output transformer center tap fuses. It over heated and took out a lot of components. Now the input tubes are overdriving and failing.

Output transformer center tap fuses?
Never seen that one in an output transformer.
I would just pull the tubes, and throw away the rest and start new with a tube rectifier and good transformers and with a case that isn't going to cook the electronics. I really don't know how junk amps like this get great reviews. The transformers look like hammonds so its only going o be so good but I probably wouldn't trust warranty them on a unit..

 

[cytovette]

I was getting low output with distortion, I suspected something in the NFB circuit, which is why I asked the (silly?) question.

My electronics training was almost 50 years ago and I changed careers a few years after passing the aCET etc. I do my own build and repairs and also for friends on occasion. So my knowledge is somewhat sketchy at time.

Yeah there are fuses in line with the OPT center taps and B+.

BTW the schematic is for the Magnum 88 amp and this is the standard 88, so there are minor discrepancies.

I pulled the relay and the distortion and signal level came back to normal, I am getting some popping and cracking though.

 

[sparkie]

I pulled the relay and the distortion and signal level came back to normal, I am getting some popping and cracking though.

now you need to see what is causing it. I would measure your 500V and see if its too high.

But popping and crackling could be a bad tube as well. Besides other things like bad sockets.

 

[cytovette]

The popping was due to an intermittent ground contact on the left RCA input jack. Someone replaced the original jacks with Cardas, which are slightly larger. He didn't drill out the holes so the shoulder washers could fit properly and fix the gap around it from the chassis. I could see the popping on the sinewave so I traced it back to the input.

Sine and square waves both look good now.

I am thinking the distortion was the relay and/or the 555 timing chip that controls it is/are bad. Gonna replace both.

 

[Reagan]

I am working on a Rogue 88 amp for a friend and am wondering why the NFB circuit is controlled by a relay. Every amp I have worked on before always had a direct connection from the output to the input cathode. To me it looks like when the relay is engaged the output is shunted to ground.
The red marks on the attached schematic are my edits for clarity.

View attachment 121407

Did you replace any of the caps? My Rogue Audio 88 has a slight hum. Rogue Audio suggested changing the coupling caps may help. I would like to make a few changes. Any recommendations on cap and/or resistor changes would be much appreciated.

 

[cytovette]

Did you replace any of the caps? My Rogue Audio 88 has a slight hum. Rogue Audio suggested changing the coupling caps may help. I would like to make a few changes. Any recommendations on cap and/or resistor changes would be much appreciated.

Yes I replaced a lot of caps because of the overheating. I used 0.1uF Solen Fastcaps for the first coupling stage and 2.7uF Solen Fastcaps for the second stage. The owner is very pleased with the sound now. I replaced all of the capacitors in the power supply as they either already failed or were failing. I didn't change any resistors. If the hum is in both channels I would suspect a power supply issue. Here is a video I made of the repair I did:

 

[Reagan]

Thank you! I will watch the video.