audio-talk

izzy wizzy wrote: ↑Mon Aug 23, 2021 9:40 pm
High gm triodes all come with the penalty of high miller around 200pF then with cable capacitance >100pF you are well up there and over what most MM will take.

Why is capacitance less of a concern for MC carts ? Asking for a friend...

Only meant the link for the first post by SY. The rest ... well fairly typical DIYAudio thread. I got up to page 18. The only thing I gleaned was using a pentode to reduce miller for MM use was a good idea as noise levels would still be good with a d3a but was too advanced to consider?!? Oh well .....

For me the choice of options isn't something to be anxious about, it's why I do DIY and why I like learning so much. I plan to try... - D3a pentode single RIAA (per Izzy) - D3a triode DC into 2nd gain stage - both with single RIAA and split ... all with LTP into LL1660PP output. I don't really care ...

You'll be short of your gain requirement d3a triode/5687 but thre are other options as Mark says. d3a pentode/5687 50dB gain Something to consider, mentioned before. These hi gm valves as triodes have huge miller which for a MM could be an issue? From what I understand/read, miller is usually in pr...

I built 8 in a marathon session at the weekend. I found the production line method to work better. Do the smd cassia on all boards, do the cc resistors, etc.

I have the required number of boards unmade - I might make them with terminal blocks underneath the board for easy access. Or not make them with terminal blocks and solder to the pads. Could also mount the trimmer on the other side. However IME, it's super hard to remove once it's in. Andrew has lo...

I wouldn't install heatsink lower than the caps. I did install some of mine end on but it was in the prototype. The final layout has them board down horizontal. Didn't use terminals to start but have them for the final as it's assembled in modules. Ultimately it depends on how hot things get and ho...

IslandPink wrote: ↑Thu Aug 05, 2021 10:32 pm It's probably capacitor life that'll suffer first.

Interesting. So do most people install them PCB down ?

It's difficult to design for every eventuality, and I have seen heat damage to some of the PCBs in hot chassis, but I haven't yet had reports of dead regs. Some of these must be close to 9 years of service, I don't think I'd disappointed with that, nothing lasts for ever. Sorry Andrew I must have n...

In my builds using these boards I've always mounted the boards so that the fins point upwards in normal playing use, so that the heat rises away from the transistors. The disadvantage of that is that the terminal blocks are inaccessible especially when the amp is upside down being worked on. I'd eve...

I happen to have some carbon comp 820R handy so I could use those.

That would put the cut off frequency just shy of 2MHz...?

I've never calculated grid stoppers. For d3a I use 680R Allan Bradley carbon. Probably could use lower, down to 330R maybe. For some reason I settled on 680R. 220 to 330 for most other less hot valves. Thanks. On a related question, why carbon comp for grid stoppers? Is it because they are less ind...

Nick wrote: ↑Mon Aug 02, 2021 10:47 pm Why would you want to set the -3dB at 20kHz, the goal is to prevent unwanted HF oscillation. There is some distance between 20kHz and RF.

Fair point, I was following Merlin's approach. What would be a sensible cut off?

Reading Merlin Blencowe's website and book about grid stopper value calculation. This page is written primarily with regard to guitar amps, but I think the principles apply to hifi ? http://www.valvewizard.co.uk/gridstopper.html As always I find his explanations clear, but if I look at the D3a in tr...