Another insane valve for phonos

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IslandPink
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#1 Another insane valve for phonos

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#2 Re: Another insane valve for phonos

Post by JamesD »

It's interesting but I have always been suspicious of valves biased by grid leak current as I don't see how they cope well with the sort of transients that scratches and stuck dirt in the grooves can generate so I have avoided them...

Now-a-days a Gm stage with proper biasing looks a better bet to me - higher gain and lower distortion all in one go :-)

But I'd love to hear and compare the two stages and see which works best.... one day - maybe...

Also want to try the DHTRob design using a low Gm microphone pentode with the RIAA correction as part of the anode load... very interesting design that he recommends... https://www.dhtrob.com/projecten/phono_cf50_ac2.php I'd use a 6BR7 to start with...
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#3 Re: Another insane valve for phonos

Post by izzy wizzy »

I've been very interested in shunt phono EQ for a while. The overload thing you refer to seemed to be an issue for dhtrob but he says he fixed it but not how.

I remember on intact when discussing dave's shunt phono that Stephie mentioned there could be overload issues with such a design in the HF.

Low rp valves make shunt EQ quite difficult as the caps get huge.

DHP front end is fascinating. Design reminds me of the trafomatic phono. Hagerman had something similar too but neither of these are DH. I tried shunt EQ once with D3a but it didn't work well and didn't pursue it. Maybe another day.
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#4 Re: Another insane valve for phonos

Post by JamesD »

I thought I would use the 6BR7 first to ease my way into this design avoiding the DHT complications and, hopefully, making the overload problem easier to manage... we'll see

I've used shunt Eq before with medium (Cg3) and high (EL802) Gm pentodes successfully but then I've used them in transconductance amp configuration with the shunt Eq across the load resistor and into a high input impedance buffer - pentode CF. That was nice but a little too hifi for my taste and the 3Cg was nicer than the EL802 s maybe a lower transconductance pentode would have been better...

so many options to try - next phono I want to build is around the 6F12p but I also have an all ss design (CFPs) that I really need to try...

I like to design a lot of headroom into my phonos - silly amounts really but I have seen huge spikes off MC cartridges from scratches in the vinyl so I hope I'll avoid the hf overload problem in practise...
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#5 Re: Another insane valve for phonos

Post by izzy wizzy »

I'm with you on the headroom thing. The 6F12P looks interesting. I'd want to try it as pentode EQ triode so "cheap" all in one phono but I'd need pentode spice netlist to sim. I see so far Ale only has it for triode. Not keen as very high input capacitance which I've moved away from. Must post my "what I found in the shed phono" thead. Look forward to development of yours.
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#6 Re: Another insane valve for phonos

Post by JamesD »

I found this thread had some 6F12p pentode models - post 1191 onwards.. There are a few different models and I have yet to try them myself - they are just sitting on the shelf for future use - much like most of my other bits and pieces :D

https://www.diyaudio.com/forums/tubes-v ... s-120.html

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#7 Re: Another insane valve for phonos

Post by JamesD »

I had a thought on the shunt feedback problem with the parallel capacitance shorting out the anode load as the frequency increases reducing the pentode gain and headroom. Two choices that might work - 1) a series inductor that starts to be effective just above the audio band or 2) implement the 2122Hz pole as an LR pole rather than CR pole.

The second option is the probably the better one but more complex to calculate values for... but LTSpice helps get it right - I haven't tried this yet but its now on the list :D
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#8 Re: Another insane valve for phonos

Post by ruffrecords »

Two things annoy me about articles like this. First they assume that a hghi gm automatically means lower noise - it doesn't, and they rarely if ever include actual noise measurements of the final design.

Cheers

Ian
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#9 Re: Another insane valve for phonos

Post by JamesD »

Thats a damn good point - particularly with the way that 1/f noise in a valve has a corner frequency ~ 1KHz rather than the 50Hz or lower that good low noise bipolars have...
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#10 Re: Another insane valve for phonos

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ruffrecords wrote: Tue May 25, 2021 11:12 am Two things annoy me about articles like this. First they assume that a hghi gm automatically means lower noise - it doesn't, and they rarely if ever include actual noise measurements of the final design.

Ian
Actually, I don't think they assumed that at all, the actual text is:
the tube has an amplification factor of 135 (higher than 12AX7),
transconductance of 14 milliamperes / volt (8 times more than 12AX7);
that means higher amplification factor with lower noises and output resistance.
So they are talking about the combination of high gain, low nose and low output resistance, which in general is true of most high gm valves I can think of. The high gain and low output resistance is by definition a high gm device. In this application the gain and Rout is more important than noise.

I agree, that some measurements would be nice, or even evidence that it was actually built.
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#11 Re: Another insane valve for phonos

Post by ruffrecords »

Odd that you should think high gain and low output impedance is more important than noise in the first stage of a phono amp. Care to justify that?

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Ian
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#12 Re: Another insane valve for phonos

Post by izzy wizzy »

ruffrecords wrote: Sun May 30, 2021 10:34 pm Odd that you should think high gain and low output impedance is more important than noise in the first stage of a phono amp. Care to justify that?

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Ian
My understandingis hi gm devices are usually low noise esp in this application as the ones usually used are also high gain. This leads to low Z inthe EQ which also benefits noise
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#13 Re: Another insane valve for phonos

Post by Nick »

My understandingis hi gm devices are usually low noise esp in this application as the ones usually used are also high gain. This leads to low Z inthe EQ which also benefits noise
Yes, that, but in this case the article in question was describing a phono stage using a LCR EQ. I am happy making phono stages with ecc83's and 88's on the front, but if you want to drive a 1k5 or 600R network, then the requirements change.
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