Layout and lead dress is a huge tone factor! I've built quite a few of these ODS conversions like this one in different chassis types and PCB styles including a Tweed and several 12X12 Hammond mini's in both PCB and turret boards and IMO none of them sounded very good (including my little mini D-lator) or sound anything like a real ODS.(more like an old Boogie) It seemed like the smaller the chassis I built them in the worse they got. Same with the PCB amps! which drastically alters the gain structure of the amp. The sound I ended with was either far too gainey,aggressive,harsh and harmonically speaking a mess
After hours of tweaking modifications and a whole host of bandaids I finally succumb to the fact that if you pack everything into a small box and alter the layout and lead dress things begin to interact and not in a good way. IMO the ODS design needs some distance between certain components if you want to capture the sound of the original design. One rather small example of this being Dumble would run the plate and cathode wires on V1 and 2 on opposing triodes in parallel and as many builders here including myself say there is an audible difference. This has been well documented here at the Garage.
BTW. Mullard also recommends using pins 678 as the input for improved hum rejection. IIRC.
Anyway Glad you got it sorted out
ODS_124049.jpg
Tony
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" The psychics on my bench is the same as Dumble'"
Incredible Tony, you are right! Literally on the first page of the Mullard ecc83 data sheet it says pins 6, 7 and 8 are favourable for hum rejection. Never really noticed the ’upside down’ mounting of the preamp sockets in the layouts, but it’s the case in all of them. The devil (and HAD) is in the details.
Some data sheets mention this heater hum difference between sections and some do not, so it may depend upon the internal construction used by a particular manufacturer.
I’ve only ever seen the ‘lower hum in the 678 section’ note in Phillips ecc83 info, never in any 12AX7 or even more pertinently, 7025 info.
So it may be a quirk of the particular arrangement / construction Phillips used.
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I’d guess less interaction between the amp and the power supply wire running to the first preamp plate resistors. RF can be picked up, as can other signals and be fed back to the first triodes. Wires remain tiny antennas with small inductance and capacitance wrt other wires. In op amp design you usually put a snubber cap right at the supply pin(s) of the op amp itself to prevent that. Otherwise you could create a feedback loop with the power supply and the pcb traces to the op amp afaik. If that is what Tony means, could you prevent this in a Dumblesque amp with a 100nF snubber on the preamp board? Between plate resistors connection to the PS and ground? And if so, might this influence the sound of the amp? (Like a zobel or snubber network for the secondary leakage inductance of the power transformer might influence things down the line) Had might have placed his components where they are out of convenience or because it sounds best that way. You never really know that with him imho.
pdf64 wrote: ↑Wed Dec 11, 2019 5:51 pm
I’ve only ever seen the ‘lower hum in the 678 section’ note in Phillips ecc83 info, never in any 12AX7 or even more pertinently, 7025 info.
So it may be a quirk of the particular arrangement / construction Phillips used.
I haven't seen it myself, but I have seen many of the fender schematics and some others I think that specified using 6,7,8 for the first input stage, instead of 1,2,3 and I always wondered why. This seems to make sense. As I noted earlier, logically having two out of phase ac heaters on either side of your pins should induce the same colliding potential for noise but out of phase, therefore knocking themselves out. Whereas 1,2,3 has a big gap between pin 1 and 9 therefore less ability to cancel hum.
Not 100% sure of that, but that makes sense to me.
rootz wrote: ↑Wed Dec 11, 2019 6:06 pm
I’d guess less interaction between the amp and the power supply wire running to the first preamp plate resistors. RF can be picked up, as can other signals and be fed back to the first triodes. Wires remain tiny antennas with small inductance and capacitance wrt other wires. In op amp design you usually put a snubber cap right at the supply pin(s) of the op amp itself to prevent that. Otherwise you could create a feedback loop with the power supply and the pcb traces to the op amp afaik. If that is what Tony means, could you prevent this in a Dumblesque amp with a 100nF snubber on the preamp board? Between plate resistors connection to the PS and ground? And if so, might this influence the sound of the amp? (Like a zobel or snubber network for the secondary leakage inductance of the power transformer might influence things down the line) Had might have placed his components where they are out of convenience or because it sounds best that way. You never really know that with him imho.
You got it It also never hurts to keep your ground wires short as possible at that end of the chassis star along with the input stage. Dumble has also been known to use a can in the middle of the chassis like 102 closer to the preamp end of the chassis. You are right we don't know for sure if HAD positioned his caps for this reason and his later Precision power supply does not use this method. I do however remember reading some info a while back that Dumble was rather picky about where he spaced positioned and grounded his caps (EMSR) . So I am sure he was well aware of what we are talking about
rootz wrote: ↑Wed Dec 11, 2019 6:06 pmIf that is what Tony means, could you prevent this in a Dumblesque amp with a 100nF snubber on the preamp board? Between plate resistors connection to the PS and ground? And if so, might this influence the sound of the amp?
Easy enough to try that out, but a well constructed ODS already has a very low noise floor. Since the later power supply design eliminated the remote preamp cap, HAD must have decided that it wasn't worth the trip.
So then at some point, the question becomes? At what point does the noise floor reach acceptable audible levels (subjective) however you still have the overall sound quality and purity of signal to contend with and how this may affect the overall sound of the amp including harmonic content?. Capacitor orientation/ resistor noise/lead dress being a few other examples.
Tony
" The psychics on my bench is the same as Dumble'"
It also never hurts to keep your ground wires short as possible at that end of the chassis star along with the input stage. Dumble has also been known to use a can in the middle of the chassis like 102 closer to the preamp end of the chassis. You are right we don't know for sure if HAD positioned his caps for this reason and his later Precision power supply does not use this method. I do however remember reading some info a while back that Dumble was rather picky about where he spaced positioned and grounded his caps (EMSR) . So I am sure he was well aware of what we are talking about