Phono Frustration

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RhythMick
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#31 Re: Phono Frustration

Post by RhythMick »

I'd be very interested to hear what you think of separate vs shared cathodes
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#32 Re: Phono Frustration

Post by RhythMick »

Steve, just looking at your shared cathode resistor for the 5687 triodes. What B+ voltage are you running them at and what bias and plate current are you expecting ?

The datasheet for the 5687 shows a wide range of operating conditions. With 180v on the plate, -7v bias with 25mA (each triode) would indicate a shared cathode of about 140R, so not a million miles away from your 180R. Separate would therefore be around 280R. 1W or greater for safety (I use a minimum factor of 4).
Capture.PNG
I wanted to check my own understanding and calculation for the separate cathode bypass caps - I hope you don't mind me running through the calculation for review. With the LL1660 giving primary inductance of 290H (each primary) and assuming the above operating point, with an assumed mu of 17, aiming for a -3db frequency corner of 5Hz, you'd need around 165uF bypass if I have the calculation right.

Inductive Ra = 2 * pi * F * L = 9111R
Plate Resistance from datasheet = 2000R
Rk = 280R

Rk || (Ra + rp) / (mu + 1) = 193R

Bypass = 1000000 / (2 * pi * F * R) = 165uF

Very happy for that to be checked and corrected. I'm still wrapping my head around a lot of this stuff.

If it's right then 280R with 100uF cap would give a frequency corner of around 8Hz.

I don't like using electrolytics, though I will readily admit it's only from reading other people's views and not from my own experimentation. I've used Mundorf Tubecaps in mine and love the sound, but the 26s have an Rk around 1K8 so need a much lower bypass cap. I'm using 30uF for a calculated corner of 6Hz. My 71As will have Rk of 2K2 and 20uF for a corner around 8Hz - perhaps I should take that opportunity to experiment with electrolytics and see if I hear the difference.
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izzy wizzy
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#33 Re: Phono Frustration

Post by izzy wizzy »

Thanks for that. I seem to remember cathode bypasses not only bypass the resistor but have to deal with the cathode impedance as well. Is that what you did with the calcs? I've stayed away from them cap bypasses ....well, forever. Hence history of using battery bias and diff circuits. And I'm a lazy theorist coz I use a sim. Bung stuff in and press button.

This circuit started off dc coupled but then realised the stages were compromising each other in terms of op points. So recently changed it to cap coupled and liked it better or was it I could set it up better so I felt better. I dunno.

Then the limitation was the psu in terms of current. It could only supply 100mA so roughly 20mA ea D3a and 15mA per 5687. D3a I think is a bit low in reality and the 5687s aren't biasing where they should or where I remember so....

I need to check this all over. Maybe the 5687s are tired. They've been in there 10 years. I now have a 200mA capable supply so can try a lot more things hence the rebuild. I'm limited to 300V HT due to the first cap in the psu box being a bg wkz and it runs close to its 350V rating. Can't remember what the actual 5687 HT is after the dropper from 300V.

So it's a movable feast now where it was more rigid in its limitations/compromises previously. Many think the 5687 gets going around 18mA. I also want to try the E810F a truly mental pentode as I have some and it's the same pinout. But other things first like your suggestion.

Cheers,
Stephen
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#34 Re: Phono Frustration

Post by RhythMick »

I have LT Spice and can use it for components it has loaded, but I haven't been able to master adding models to it. Maybe it needs a concerted effort and some good tutorials ...
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#35 Re: Phono Frustration

Post by RhythMick »

Could you measure the voltage drop across the 180R ?

Also consider putting 10R in each cathode to provide a convenient measurement point.
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izzy wizzy
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#36 Re: Phono Frustration

Post by izzy wizzy »

RhythMick wrote: Tue Mar 05, 2019 10:46 am Could you measure the voltage drop across the 180R ?

Also consider putting 10R in each cathode to provide a convenient measurement point.
It's about 5.7V from memory. The 10R would do that but I wouldn't want the extra resistance in there and the tx can handle the imbalance. Unless there's another good reason for it?

Cheers,
Stephen
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izzy wizzy
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#37 Re: Phono Frustration

Post by izzy wizzy »

RhythMick wrote: Tue Mar 05, 2019 10:38 am I have LT Spice and can use it for components it has loaded, but I haven't been able to master adding models to it. Maybe it needs a concerted effort and some good tutorials ...
There's some good valve models that operate from drop down menus over on the intact audio site. My pooter just bombed so had to install from scratch and it is dead easy. I could point you to the bits that can make this happen weeded out from the stuff around it if you're interested?

Then I think it's best to play around with someone else's model to see how it works and then use that as a guide to get going. Model something you know next and then start to branch out when it makes sense. It's the only way I know of getting RIAA EQ accurate for one thing. Plus you can generally mess around without spending anything. There's people out there that are way more knowledgable than me on this.

Cheers,
Stephen
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#38 Re: Phono Frustration

Post by RhythMick »

izzy wizzy wrote: Tue Mar 05, 2019 1:29 pm
RhythMick wrote: Tue Mar 05, 2019 10:38 am I have LT Spice and can use it for components it has loaded, but I haven't been able to master adding models to it. Maybe it needs a concerted effort and some good tutorials ...
There's some good valve models that operate from drop down menus over on the intact audio site. My pooter just bombed so had to install from scratch and it is dead easy. I could point you to the bits that can make this happen weeded out from the stuff around it if you're interested?

Then I think it's best to play around with someone else's model to see how it works and then use that as a guide to get going. Model something you know next and then start to branch out when it makes sense. It's the only way I know of getting RIAA EQ accurate for one thing. Plus you can generally mess around without spending anything. There's people out there that are way more knowledgable than me on this.

Cheers,
Stephen
Yeah pointers would be welcomed
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IslandPink
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#39 Re: Phono Frustration

Post by IslandPink »

We've got discussions on LTSpice on the 'Stacks' section, but it would be nice for someone to ask the simple questions about the valve models again, because I got stuck :D
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RhythMick
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#40 Re: Phono Frustration

Post by RhythMick »

IslandPink wrote: Tue Mar 05, 2019 6:08 pm We've got discussions on LTSpice on the 'Stacks' section, but it would be nice for someone to ask the simple questions about the valve models again, because I got stuck :D
Oh I'm VERY good at asking the simple questions....
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izzy wizzy
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#41 Re: Phono Frustration

Post by izzy wizzy »

IslandPink wrote: Tue Mar 05, 2019 6:08 pm We've got discussions on LTSpice on the 'Stacks' section, but it would be nice for someone to ask the simple questions about the valve models again, because I got stuck :D
Yes I saw when I revisited that thread, that you got stuck. I think the instructions were a little all over the place. When I get a mo, I'll start a new thread on how to get going easily. Hopefully soon while it's still fresh in my head from having just done it from scratch.

Cheers,
Stephen
RhythMick
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#42 Re: Phono Frustration

Post by RhythMick »

izzy wizzy wrote: Tue Mar 05, 2019 6:20 pm
IslandPink wrote: Tue Mar 05, 2019 6:08 pm We've got discussions on LTSpice on the 'Stacks' section, but it would be nice for someone to ask the simple questions about the valve models again, because I got stuck :D
Yes I saw when I revisited that thread, that you got stuck. I think the instructions were a little all over the place. When I get a mo, I'll start a new thread on how to get going easily. Hopefully soon while it's still fresh in my head from having just done it from scratch.

Cheers,
Stephen
For me I found using ltspice with its installed components to be a doddle, but it's what to do when you need something that's not there. Simple stuff like the right diode. Bridge rectifier. And of course how to install a valve and use it.
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izzy wizzy
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#43 Re: Phono Frustration

Post by izzy wizzy »

RhythMick wrote: Tue Mar 05, 2019 6:24 pm For me I found using ltspice with its installed components to be a doddle, but it's what to do when you need something that's not there. Simple stuff like the right diode. Bridge rectifier. And of course how to install a valve and use it.
Hopefully if I do it right, you should be able to do it especially the valve bit. Adding other models should be fairly easy. There's someone around here that is good at that but can't remember who.

Cheers,
Stephen
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pre65
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#44 Re: Phono Frustration

Post by pre65 »

izzy wizzy wrote: Tue Mar 05, 2019 6:29 pm There's someone around here that is good at that but can't remember who.

Cheers,
Stephen
Mike H ? :)
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Mike H
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#45 Re: Phono Frustration

Post by Mike H »

Not done anything like that for ages tho - coincidentlly I was recently looking at some valve models I'd been using and thought how the f---- did I even do that .. :D

They're basically the Norman Koren models anyway, just tweaked a bit.
 
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