17-11-2015, 05:22 PM
There has been a real mystery that involves the AF grid coupling to the DH77 grid. It was a minor point so far as the overall picture is concerned but I decided to start from the output tube stages and onwards till I get to the Mixer stage.
The mystery came to be when I was carefully testing the DH77 grid bias. According to the manual I think it's R9 which builds up a D.C. voltage in the volume stage (R9 being the potentiometer resistance up to 500 K.) According to Chas Miller, this should always be checked since he found over time many repairers had been using 5 K pots by mistake. Mine was a new unit just shy of 500 K but I wanted to make sure I could trace the variable resistance to the grid, while moving the volume shaft. Of course, as is known, you find it's pretty close to zero in the near-off position and, at full volume, it goes up to 500 K.
So, I notice a small and worn-looking capacitor that goes directly to the grid of the DH77. Well, I looked at the schematic and at first I thought it had to be the AF coupling cap. Now, I'm not so sure at all. There is in fact a Sprague cap that goes to the same grid via the other shoddy cap. The former also looks far more like a picofarad capacitor due to being a fair bit smaller than the bypass caps I removed. The latter is rated accordingly for the AF coupling. So, I tested the variable resistance across the Sprague and this naturally is in circuit with any bias on the grid. Conclusion? There is no way to get access with the soldering iron direct to the grid so I left well alone. Just have to wonder whether there is some sort of a filter cap that's been left out of the circuit diagram or did they use 2 caps to make up a total value??
Circuits diagrams can be tricky I find in cases where a tube pin connects to chassis. In reality you see a resistor that seems to emerge from a pin but in the schematic it only shows as a remote connection to chassis and far removed from any tube.
Anode voltage on the output tube seems fine.
I already have suspicions my Mixer Tube is underperforming but that will remain to be seen...
The mystery came to be when I was carefully testing the DH77 grid bias. According to the manual I think it's R9 which builds up a D.C. voltage in the volume stage (R9 being the potentiometer resistance up to 500 K.) According to Chas Miller, this should always be checked since he found over time many repairers had been using 5 K pots by mistake. Mine was a new unit just shy of 500 K but I wanted to make sure I could trace the variable resistance to the grid, while moving the volume shaft. Of course, as is known, you find it's pretty close to zero in the near-off position and, at full volume, it goes up to 500 K.
So, I notice a small and worn-looking capacitor that goes directly to the grid of the DH77. Well, I looked at the schematic and at first I thought it had to be the AF coupling cap. Now, I'm not so sure at all. There is in fact a Sprague cap that goes to the same grid via the other shoddy cap. The former also looks far more like a picofarad capacitor due to being a fair bit smaller than the bypass caps I removed. The latter is rated accordingly for the AF coupling. So, I tested the variable resistance across the Sprague and this naturally is in circuit with any bias on the grid. Conclusion? There is no way to get access with the soldering iron direct to the grid so I left well alone. Just have to wonder whether there is some sort of a filter cap that's been left out of the circuit diagram or did they use 2 caps to make up a total value??
Circuits diagrams can be tricky I find in cases where a tube pin connects to chassis. In reality you see a resistor that seems to emerge from a pin but in the schematic it only shows as a remote connection to chassis and far removed from any tube.
Anode voltage on the output tube seems fine.
I already have suspicions my Mixer Tube is underperforming but that will remain to be seen...
