coltrane
electrobaroCKjazzer
hi there all,
the reason why i am writing again is that i just managed to purchase a good 'n ol' S500 from the first series (4-8-16 ohm taps Hinchley transformers, vents around the valves, 3 boards for the phono line and driver circuitry, 470k volume pot). it sounds pretty good, however, i want to get the most out of it. googled the net in the quest as to how but now seems to be pretty scarce info on this. earlier there were dedicated sites for modding the S500 but now they either do not exist or provide nothing just the idea of how worthwile the tweak would be. i found some thoughts at a site regarding modding - here they come:
"Here are some tips on how to tweek this amp - I've done this and a few other things, but you will not get the simple Dynaco phono stage to sound really great... If you have time and patience you can replace it with something slightly more exotic (like Allan Wright's) .
Two main options: a “basic” and an “extreme” one.
BASIC (costing about US $70 in passive components):
1. Replace the EL 34 cathode resistors with Resista type and remove the poor electrolytic capacitors connected in parallel to these resistors, whose function is to partially decouple them and, consequently, to “soften” clipping at high volume levels. The tweak will help reduce sound grain. Clipping will be sharper, but you won’t face the problem at all with sensitive speakers. If you don’t want to remove these capacitors, replace them with high grade audio types such as Elna Cerafine or Nichicon Muse. Just remove them
2. Remove the two input 0.22mF/400V ERO MKT 1813 capacitors. Their only function is to protect the circuit from possible residual AC (EU requirements?). Signal will be cleaner
3. The two generic 100mF/50V cathode electrolytic capacitors in the drive stage should be replaced with Elna Cerafine or Nichicon Muse types. You should extend the tweak to the phono stage, if you use it often. Go for Elna Cerafine if you can find them
4. Decoupling capacitors: the generic 33mF/350V capacitors decoupling the drive and final stages should be replaced with compact radial or snap-in 105°C low ESR types. If you go for 450V then it will sound a bit dark at first - you can also use 47mF caps here...
5. Input selectors: they are quite poor and should be replaced with better types (silver contacts), ensuring great precision for long periods. If the ones you have are good then skip this
6. Volume control: the original potentiometer is of poor quality: a standard ALPS will guarantee a long life with no “scratches”. A good stepped attenuator also work wonders, but good luck fitting it in the small chassis...
EXTREME (add further US $120):
7. Power supply: replace the generic bridge rectifiers with IR Hexfred HFA08TA60C (anode voltage) and Schottky SB 340 soft recovery diodes (driving and phono stage filaments); The Schotty SB340's make a big difference. The Hexfred rectifier can be skipped
8. Coupling capacitors: the AI 500 is equipped with very good ERO MKT 1813 capacitors, among the best sounding in the polyester series. Yet, their replacement with Jensen paper in oil capacitors (aluminium foil) will allow a significant improvement in detail, depth and imaging. The considerable dimensions of paper in oil capacitors make replacement complicated, as it implies a new design in the printed circuit of the stage. Its hard to fit Jensens in the chassis - go for some Russians - they're smaller, much cheaper and sound as nice or better
9. Replace Beyschlag metal film anodic resistors with 2W matching pairs Allen Bradley carbon film resistors. You can go for other brands of quality resistors here, but AB's are cheap and do the trick. Match them well though."
- and some more:
"The AI 500 was designed to use PCC88's. The problem is that the filament voltage for these is 7.6 Volts, not 6.3 as per ECC88, 6922, 6DJ8. If you want to drop 1.3 Volts at 0.3 Amps it means a 4.3 Ohm resistor per at least one Watt. A better way is to bridge rectify it, then a simple pi-filter - capacitor resistor capacitor (CRC) - will drop the voltage to 6.3 Volts; this supply will have less ripple. For the 500 use 200mF cap 47 Ohm power resistor 220mF capacitor.
I presume that the RC networks on the plate of the input tube are for stability. I am not keen on the shared cathode resistors, esp. in the output stage. Would be nice to see a slightly larger resistor on the inverting tube or an adjustment scheme to help in balance.
I changed the coupling caps to Auricaps, and the cathode electrolytics to Blackgates. The sound is truly remarkable. I've compared it to amps in the $3-$5000 range with outstanding results. Before the changes this amps weaknesses were its mushy bass and its inability to remain articulate during complex passages. No longer. Now the bass is tight and textured, imaging is holographic, and the amp is infinitely more musical. My recommendation for somebody who doesn't have the cash to pay for the Borderpatrol(amazing!) is to change the cathode electrolytics to Blackgates."
i wonder what your opinion on this above series of mods is and, also, please advise as to how to triode connect the el34s as i have found no explanation on that.
your input will be much appreciated
the reason why i am writing again is that i just managed to purchase a good 'n ol' S500 from the first series (4-8-16 ohm taps Hinchley transformers, vents around the valves, 3 boards for the phono line and driver circuitry, 470k volume pot). it sounds pretty good, however, i want to get the most out of it. googled the net in the quest as to how but now seems to be pretty scarce info on this. earlier there were dedicated sites for modding the S500 but now they either do not exist or provide nothing just the idea of how worthwile the tweak would be. i found some thoughts at a site regarding modding - here they come:
"Here are some tips on how to tweek this amp - I've done this and a few other things, but you will not get the simple Dynaco phono stage to sound really great... If you have time and patience you can replace it with something slightly more exotic (like Allan Wright's) .
Two main options: a “basic” and an “extreme” one.
BASIC (costing about US $70 in passive components):
1. Replace the EL 34 cathode resistors with Resista type and remove the poor electrolytic capacitors connected in parallel to these resistors, whose function is to partially decouple them and, consequently, to “soften” clipping at high volume levels. The tweak will help reduce sound grain. Clipping will be sharper, but you won’t face the problem at all with sensitive speakers. If you don’t want to remove these capacitors, replace them with high grade audio types such as Elna Cerafine or Nichicon Muse. Just remove them
2. Remove the two input 0.22mF/400V ERO MKT 1813 capacitors. Their only function is to protect the circuit from possible residual AC (EU requirements?). Signal will be cleaner
3. The two generic 100mF/50V cathode electrolytic capacitors in the drive stage should be replaced with Elna Cerafine or Nichicon Muse types. You should extend the tweak to the phono stage, if you use it often. Go for Elna Cerafine if you can find them
4. Decoupling capacitors: the generic 33mF/350V capacitors decoupling the drive and final stages should be replaced with compact radial or snap-in 105°C low ESR types. If you go for 450V then it will sound a bit dark at first - you can also use 47mF caps here...
5. Input selectors: they are quite poor and should be replaced with better types (silver contacts), ensuring great precision for long periods. If the ones you have are good then skip this
6. Volume control: the original potentiometer is of poor quality: a standard ALPS will guarantee a long life with no “scratches”. A good stepped attenuator also work wonders, but good luck fitting it in the small chassis...
EXTREME (add further US $120):
7. Power supply: replace the generic bridge rectifiers with IR Hexfred HFA08TA60C (anode voltage) and Schottky SB 340 soft recovery diodes (driving and phono stage filaments); The Schotty SB340's make a big difference. The Hexfred rectifier can be skipped
8. Coupling capacitors: the AI 500 is equipped with very good ERO MKT 1813 capacitors, among the best sounding in the polyester series. Yet, their replacement with Jensen paper in oil capacitors (aluminium foil) will allow a significant improvement in detail, depth and imaging. The considerable dimensions of paper in oil capacitors make replacement complicated, as it implies a new design in the printed circuit of the stage. Its hard to fit Jensens in the chassis - go for some Russians - they're smaller, much cheaper and sound as nice or better
9. Replace Beyschlag metal film anodic resistors with 2W matching pairs Allen Bradley carbon film resistors. You can go for other brands of quality resistors here, but AB's are cheap and do the trick. Match them well though."
- and some more:
"The AI 500 was designed to use PCC88's. The problem is that the filament voltage for these is 7.6 Volts, not 6.3 as per ECC88, 6922, 6DJ8. If you want to drop 1.3 Volts at 0.3 Amps it means a 4.3 Ohm resistor per at least one Watt. A better way is to bridge rectify it, then a simple pi-filter - capacitor resistor capacitor (CRC) - will drop the voltage to 6.3 Volts; this supply will have less ripple. For the 500 use 200mF cap 47 Ohm power resistor 220mF capacitor.
I presume that the RC networks on the plate of the input tube are for stability. I am not keen on the shared cathode resistors, esp. in the output stage. Would be nice to see a slightly larger resistor on the inverting tube or an adjustment scheme to help in balance.
I changed the coupling caps to Auricaps, and the cathode electrolytics to Blackgates. The sound is truly remarkable. I've compared it to amps in the $3-$5000 range with outstanding results. Before the changes this amps weaknesses were its mushy bass and its inability to remain articulate during complex passages. No longer. Now the bass is tight and textured, imaging is holographic, and the amp is infinitely more musical. My recommendation for somebody who doesn't have the cash to pay for the Borderpatrol(amazing!) is to change the cathode electrolytics to Blackgates."
i wonder what your opinion on this above series of mods is and, also, please advise as to how to triode connect the el34s as i have found no explanation on that.
your input will be much appreciated
