Hi Chris, thanks for looking at this.
I am not hung up on originality. The metal cases is bent in places, and a bit of trim is awol. There is a switch 'stereo/mono/channel (one channel to both outputs)' and I had considered wiring that so it was 'stereo/mono/direct', with the 'direct' setting skipping the balance and tone controls. Perhaps I should focus on that setting in the first incarnation, skip the tone stack for now, and get the power stage working more sweetly, then add a phono stage.
From what you said I could keep the old phono stage which is a solid enough performer. I think that would be good, then I can be faithful to the schematic, with some tweaks along the way, and I can get a feel for the character of the amplifier.
I have tried to keep all the Suzuki (grey) and Pioneer resistors (green). They look like they are wire wound, but I was told on another forum they are very early metal film. I have tested some of them and they are generally 'in the ballpark', but large tolerances, and maybe it is just more sensible to use new passives all the way. It takes away a lot of uncertainty when fault finding. There were lots of Suzuki PIO caps which I assumed were an audiophiles dream before I investigated them; most had stains of leaking oil at one end.
Would you retain the passives for originality, or is it a false economy?
Cheers, Richard
I am not hung up on originality. The metal cases is bent in places, and a bit of trim is awol. There is a switch 'stereo/mono/channel (one channel to both outputs)' and I had considered wiring that so it was 'stereo/mono/direct', with the 'direct' setting skipping the balance and tone controls. Perhaps I should focus on that setting in the first incarnation, skip the tone stack for now, and get the power stage working more sweetly, then add a phono stage.
From what you said I could keep the old phono stage which is a solid enough performer. I think that would be good, then I can be faithful to the schematic, with some tweaks along the way, and I can get a feel for the character of the amplifier.
I have tried to keep all the Suzuki (grey) and Pioneer resistors (green). They look like they are wire wound, but I was told on another forum they are very early metal film. I have tested some of them and they are generally 'in the ballpark', but large tolerances, and maybe it is just more sensible to use new passives all the way. It takes away a lot of uncertainty when fault finding. There were lots of Suzuki PIO caps which I assumed were an audiophiles dream before I investigated them; most had stains of leaking oil at one end.
Would you retain the passives for originality, or is it a false economy?
Cheers, Richard
I'm with you in thinking that they're wirewound resistors, but am certainly not an expert on the topic. In that era standard resistor tolerance was 20% and 1% resistors were hand-wound and values written on them with India ink, by hand. Today, we're spoiled rotten. I wouldn't change them, but that's personal.
If you wanted to keep the tone controls, but with a Baxandall anode follower config, maybe you could switch it in and out with the repurposed Mode switch. Baxandall tone controls need to be fed from a low impedance source, and also need linear taper pots (which must accept existing knobs), so it's tricky. Volume and balance controls need to be downstream of the Baxandall tone control - just as well; that's a good place for them.
All good fortune,
Chris
If you wanted to keep the tone controls, but with a Baxandall anode follower config, maybe you could switch it in and out with the repurposed Mode switch. Baxandall tone controls need to be fed from a low impedance source, and also need linear taper pots (which must accept existing knobs), so it's tricky. Volume and balance controls need to be downstream of the Baxandall tone control - just as well; that's a good place for them.
All good fortune,
Chris
If it were me I'd replace the resistors with 1% metal film resistors just to ensure you're not dealing with something like a noisy resistor in the early stages of the amp which indeed can cause noise to be heard.
Here's one problem. As is if you plug in a more modern lower impedance source (pretty much anything solid state) to the aux input the stock tone controls may not work properly which might be why the amp doesn't sound quite right. To check if that is a problem insert a 100K resistor in series with the aux input and see if the amp sounds better.
Also I see several things that can be done to improve the sound quality of the amp. Will take some time to go through it all so that will have to wait until later tonight when I have more time to do some calculations.
I may either update the schematic or draw a new one for the parts of the schematic that need to be changed to reflect new component values.
Here's one problem. As is if you plug in a more modern lower impedance source (pretty much anything solid state) to the aux input the stock tone controls may not work properly which might be why the amp doesn't sound quite right. To check if that is a problem insert a 100K resistor in series with the aux input and see if the amp sounds better.
Also I see several things that can be done to improve the sound quality of the amp. Will take some time to go through it all so that will have to wait until later tonight when I have more time to do some calculations.
I may either update the schematic or draw a new one for the parts of the schematic that need to be changed to reflect new component values.
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You're welcome.
For the supply voltage difference you can either get a step down transformer or maybe you can find a transformer supplier over there that makes a 240 volt transformer with the correct secondary voltages and currents that will fit in place of the stock transformer.
Got a bit busy and haven't looked at the schematic much, but will do so tomorrow.
Most of it is some basic changes such as upping the value of cathode bypass caps and upping the value of audio coupling caps.
That will make the most difference.
Going fixed bias will also make a difference as it eliminates the cathode bypass cap and cathode resistor for the output tubes. Plus you can bias the tubes cooler if so desired to make them last longer and it may get you a watt or two more out of the amp given the plate voltage will be higher by whatever the cathode voltage is given the cathodes will be going through a 1 ohm resistor to ground (can measure cathode current with a multimeter set to DC volts and read directly in mA) instead of the cathode bias resistor.
Now before doing any mods whatsoever I suggest taking measurements of the amp in its current state such as feeding a signal in from an AF generator and using an oscilloscope and a good quality DMM (with the amp connected to a resistor of the amp's rated impedance) to get a set of measurements as to the output power and gain at frequencies from 20Hz - 20kHz., I usually do that at these frequencies. 20, 40, 60, 80, 100, 200, 400, 600, 800, 1k, 2k, 4k, 6k, 8k, 10k, 12k, 14k, 16k, 18k, 20k.
Once you do get the measurements done, then do the mods and re-measure the amp and compare the measurements to see what if any improvement there is.
Also having a good set of measurements will help you if something isn't right after the mod as you can say before doing that I got this. Let me check my work and verify it is correct.
For the supply voltage difference you can either get a step down transformer or maybe you can find a transformer supplier over there that makes a 240 volt transformer with the correct secondary voltages and currents that will fit in place of the stock transformer.
Got a bit busy and haven't looked at the schematic much, but will do so tomorrow.
Most of it is some basic changes such as upping the value of cathode bypass caps and upping the value of audio coupling caps.
That will make the most difference.
Going fixed bias will also make a difference as it eliminates the cathode bypass cap and cathode resistor for the output tubes. Plus you can bias the tubes cooler if so desired to make them last longer and it may get you a watt or two more out of the amp given the plate voltage will be higher by whatever the cathode voltage is given the cathodes will be going through a 1 ohm resistor to ground (can measure cathode current with a multimeter set to DC volts and read directly in mA) instead of the cathode bias resistor.
Now before doing any mods whatsoever I suggest taking measurements of the amp in its current state such as feeding a signal in from an AF generator and using an oscilloscope and a good quality DMM (with the amp connected to a resistor of the amp's rated impedance) to get a set of measurements as to the output power and gain at frequencies from 20Hz - 20kHz., I usually do that at these frequencies. 20, 40, 60, 80, 100, 200, 400, 600, 800, 1k, 2k, 4k, 6k, 8k, 10k, 12k, 14k, 16k, 18k, 20k.
Once you do get the measurements done, then do the mods and re-measure the amp and compare the measurements to see what if any improvement there is.
Also having a good set of measurements will help you if something isn't right after the mod as you can say before doing that I got this. Let me check my work and verify it is correct.