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RE: Modding Papworth M100

Alan, hi. 1. use a Rel TFT for C1. C1 may be useful in blocking DC from the 12AX7 grids. The 100K pot probably causes more harm to the audio than the input cap. My first choice would be Teflon & tin for C1. You could use 1K as a grid stopper in series with C1 but R1 sets the op points for 12AX7 the first triode unit. You have changed the op points by changing the value of R1. 3. C5 as Auricap? Ok, I guess. Not my first choice space allowing. 4. C10,11 look to be 0.47uF on schematic. Did you change the value to 0.68uF? Here RTX are fine. I would not bypass. If budget allows I would strongly suggest Rel Cap TFT Teflon & tin for C10,11. They have the least coloration of any cap I have heard. 5. I don't see R6,7 as 2M2. I see R15,16 as 2M2. C6,7 0.022 as RTX is fine. Better is TFT. 6. Bypassing power supply electrolytics is applying wishful thinking. Better to select low ESR etc caps in the first place. Some Panasonic and some Philips are in this category. Also, Black Gate and Cerafine. The practice of running a wire for ground and not a circuit trace is evidence that the builders/designers of M100 see a wire as better than a trace in this regard. You can do the same where re-work of board has lifted traces. Just run wire. I suggest 22AWG. 7. C12,13,14,15 bypassing is wishful thinking again. Low ESR etc more important. Again, select best electrolytics here including Black Gate & Cerafine; wet polypropylene like Obligato from DIY HI FI Supply, ASC etc; dry poly like Solen. There are other choices from Mundorf, Jantzen, and Jensen to consider. Make a list and do some research. 8. I would skip bypassing. Quality of the cap is more important. Planning mods: the 12BH7 is running at 100% if you have 5mA in each triode. There are lots a choices of good 12BH7 at prices that are not crazy. I would not look to re-engineer your amp for 6H30 at this time. I think there are other things you can do - like better R throughout the amp. If most of the R are metal film or metal oxide you can do better. Everyone has an opinion on this but I tend to favor: precision carbon films and precision wire wounds. As the M100 have 'borrowed' from the McIntosh Unity Coupled circuit it would be wise to keep in mind that each side of the push pull arrangement ideally should be in perfect balance. All R related to phase splitter/driver and output tubes should be matched to 1% or better. Oh, it looks like there is a mistake on the schematic. Bias input (B) is OK on the upper output tubes as shown but the B on the lower tubes shows it running to output tube cathodes. The upper goes to control grids which is correct. Is this not a mistake? Anyway, have fun and Happy Listening.


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