Roberts 770X R2R Mono Blocks

[s-petersen]

It shouldn't hurt anything, but over time they will degrade IIRC from running lower than designed voltage.I have to look at the schematic again to see , but I think they are the cathode bypass caps.
Edit: After looking at the schematic, there are 3 in each amp, they are cathode bypass caps
I am not sure what it would do to the frequency response on the preamp tubes, there may be a chance of low frequency oscillation, if your speakers behave strangely "breathe" you may have to change back

 

[chetgallery]

This is an old thread but I have to try. I got a set of Akai M-8 amps and I have the mods done (just using line in for audio)and a full recap and they sound amazing but I have a maybe silly question. After all was done I always check voltages on the tubes. Left amp has 17-18 VDC on the 6x4 tube on pins 3 and 4. Right amp only has 5.7 VDC on those pins. Is this a concern? AC volts are right on at 6.3 VAC on on both amps of the 6x4. Hope someone is out there…

 

[s-petersen]

This is an old thread but I have to try. I got a set of Akai M-8 amps and I have the mods done (just using line in for audio)and a full recap and they sound amazing but I have a maybe silly question. After all was done I always check voltages on the tubes. Left amp has 17-18 VDC on the 6x4 tube on pins 3 and 4. Right amp only has 5.7 VDC on those pins. Is this a concern? AC volts are right on at 6.3 VAC on on both amps of the 6x4. Hope someone is out there…

Try switching 6X4's I believe it may be heater cathode leakage

 

[chetgallery]

Try switching 6X4's I believe it may be heater cathode leakage

I have already tried that. The problem doesn’t move with the tube. The schematic I have only shows voltages out of the transformer and in to the 6x4 tube. I also looked at the Roberts schematic for voltages with no luck. I have the Akai schematic.

 

[s-petersen]

When I looked at the Akai schematic, the filament wiring looks isolated from the rest if the circuit. Maybe try a 10K resistor to ground from the filament and see if it is just leakage in the transformer

 

[chetgallery]

When I looked at the Akai schematic, the filament wiring looks isolated from the rest if the circuit. Maybe try a 10K resistor to ground from the filament and see if it is just leakage in the transformer

I will give it a try and share the results. Turns out it was the right amp. Have not tried the 10k yet because I noticed that on the left amp there was a daisy chain between tubes coming from the transformer 0 volts to the center of the sockets. Just want to confirm that this isn’t a problem. Looks like they are just solder points, no center pins on 6x4 tubes. If that is not related, can I use the 10k right from the transformer to chassis ground or should I go to 0 volt tap on the transformer?
Put a 10k resister to ground and have 0 volts dc. Tried both taps on the transformer with the same results. A game of cat and mouse for me.

 

[chetgallery]

When I looked at the Akai schematic, the filament wiring looks isolated from the rest if the circuit. Maybe try a 10K resistor to ground from the filament and see if it is just leakage in the transformer

I did find DC on the transformer taps. I will maybe use the other 6.3 volt tap if I do not find any DC there since I am not using the 6267 tube. Thanks for pointing me toward the transformer on this.

 

[chetgallery]

After a night of listening and monitoring voltages, S Petersen was right on with his assessment. That is why I posted. To me it is a cat and mouse game. To him it’s just experience.Thank you for the direction and knowledge. To anyone who is diving into one of these Akai M-8 mono blocks, I had DC Voltage that was way too high off of the transformer to the 6x4 tube heaters and I ended up using the taps for the 6.3 VAC that feeds the 6bq5 and 12AD7 (blue wires) tubes. Big THANKS to Mr Petersen. Worth the time to do the Mods on these. Rates right up with a Pioneer SX 34 integrated amp.

 

[s-petersen]

Your welcome!