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Looking for comments in my first 300B project


2005-08-10 10:38 pm
I high value the contents posted on this forum and over the years it has been very helpful as I learn the art of building tube gear. I currently have a pair of Dynaco MKIIIs which I rebuilt from scratch about a year ago using the original circuit design and am happy with the result. Now, I'm ready to build my next project which is a 300B SET amp.

The circuit I have my eyes on is from a 2001 Japanese tube magazine, unfortunately, I don't understand Japanese so basically, I've been using it as a reference design and comparing it against what I could find from other people's 300B SET design on the net and comments made on this forum and making notes for myself along the way so that hopefully I can improve it for my own project...

I would like to solicit opinions from my fellow members about this design so that hopefully others can find merits with this design and build one for themselves...

For starters, the following is what I like to improve from the present design:
1] modify the HUM circuitry using 100 ohm 2W pots for minimum hum
2] Introduce B+ delay circuit with in-rush current thermistors as outlined in Pete Millet's site for a prolonged tube life
3] decouple 6.3 AC heater from the B++ circuit. I'm not sure what was the idea behind tying the two together.
4] I plan to use polypropylene coupling cap in prepamp stage one (C101) and paper in oil in pre-amp stage two (C103)

Once I'm done building this amp in phase 1, I plan on revisiting this design and further fine tune it perhaps using other pre-amp tubes.

Thanks for reading.



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Sorry to sound like I'm joining in on bashing the design, but...

Can anybody think of a reason why an SRPP as the first stage - driving the grid of a 6J5 - is beneficial? Or is this just a Japanese fashion statement?

My personal favorite (you've all heard it before) would be a single-stage pentode. D3A or 12HG7 (or C3m or [insert favorite high Gm pentode here])...



2005-08-10 10:38 pm

I guess west meet east design concepts didn't go too well here ;)

In my research about the 6j5 is basically a single-triode 6sn7. Like I said it earlier, this is just part 1 of this project and I'll most likely plan for phase 2 with a different SRPP configuration based on some of the suggestions made here and do the A/B thing.

dsavitsk: If I understood your comment correctly about

3] decouple 6.3 AC heater from the B++ circuit. I'm not sure what was the idea behind tying the two together.

Are you saying I will need it for heater bias supply right?

You've got to love this DIY audio thing, based on the comments made so far I'm seeing the road to get to audio summit is going to take some time to get there ;)