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  #1  
Old 13th August 2009, 09:33 AM
Richard Richard is offline
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Default Phono 3 cathode bypass caps Jekyll and Hyde

Phono 2, 3 and 3S are similar and have been around quite a while but many recent owners may not have tried this with them.

The original Phono 2 circuit shows 220uf 10v cathode bypass caps on the first 2 valves. When the kit arrived the circuit no longer had these.

It was mentioned a time or two back then, but no real reason was given as to why they were left out, so I tried fitting them to the first valve only for a listen.

The result is startling. Quite a lot more gain (I use Denon DL160 high op MC which is about half the op of a typical MM and gives volume a little less than the CD into Pre 3) but so much more lively and transparent as well. I have CDs and vinyls of the same recording that I'm familiar with comparing switching between the two synced up in real time. I spent a good afternoon repeating this after the mod and it is a real effect all right.

Volume is now greater than CDp so perhaps the reason for leaving them off in the first place was simply a way of adjusting gain to a suitable level for the finished kit. What that did though was to cause local feedback and increase the output resitance of the valve. With the caps fitted, the increase in level is obvious but the improvement in drive has to be heard to be believed.

In short, an easy mod to try; put 220uF 10v, 16v or 25v caps (whatever you have to hand) across R7/R8 in Phono 3S or across R17/R18 in Phono 3 and Phono 2. These caps were C10/C11 in the original Phono 2 article. There's no dedicated space for them so I soldered them directly to the cathode resistor legs making sure the neg lead goes to ground end of the resistor.

I did it quickly first, didn't bench check, and just had a listen. I was so surprised by the difference I thought it would be due to a change in frequency balance. Putting it on the bench though and testing with a reverse riaa filter on the sig gen and into a scope shows this isn't the case at all. The response is exactly the same as when last set up a few months back and no adjustment at all was needed (mine's Phono 3S with adjustable hf caps).

Rich
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Old 13th August 2009, 10:22 AM
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soulminer soulminer is offline
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Default Re: Phono 3 Jekyll and Hyde

Nice one Rich. I will have to try this out myself.

Cheers,
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  #3  
Old 13th August 2009, 06:33 PM
colin.hepburn colin.hepburn is offline
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Default Re: Phono 3 Jekyll and Hyde

HI Richard/Soul
Yep this is something I have yet to try as well but with my 47uf 25volt OS-CON the lower value of 47uf as I don’t need more gain for the MM 1042 6.5mV seems OS-CONs are very good for Cathode bypass caps as there the closest thing to polypropylene I also want to try the battery bias removing R17/18 and inserting a 1.2 volt Ni Cad batt in there place and iv still to add the Phono II 3.18uS correction
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  #4  
Old 14th August 2009, 06:16 PM
Dik Dolan Dik Dolan is offline
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Default Re: Phono 3 Jekyll and Hyde

Sounds like it's worth a try, thanks Richard. I think I have a couple of used cheapo no-name 220uF 16v electrolytics kicking about-does quality matter here?
I'll try bunging 'em in, in the next day or so. I have to take the lid off, I think one of my TJ valves is getting noisy/microphonic which is disappointing, they were bloody expensive.
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Old 14th August 2009, 09:59 PM
Richard Richard is offline
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Default Re: Phono 3 Jekyll and Hyde

From swapping bypass caps in the past Dik I'd confidently say you will hear a difference between a cap and no cap and maybe hear a difference between a nono-brand and a BG Any old cap will give you at least 95% of this result I'm sure. I use the EI smooth longplate ecc83 valves from Langrex, six quid each and way best of several types I tried....

Colin, both battery and your 47uF cap will give (restore) full gain to the stage. With a lower value cap tho it will not restore full gain at the very low frequencies, it will roll off the bass just a little earlier than with 220uf, a useful way of implementing a roll off filter if you want one but 47uF should still give a decent sound. Yes loads of gain already for a MM with Phono isn't there. Useful to have a bit more with a low output cart but it's the change in quality that surprised me. See what you think.

Rich
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Old 15th August 2009, 10:10 AM
Guy Pettigrew Guy Pettigrew is offline
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Default Re: Phono 3 Jekyll and Hyde

Quote:
Originally Posted by Dik Dolan View Post
Sounds like it's worth a try, thanks Richard. I think I have a couple of used cheapo no-name 220uF 16v electrolytics kicking about-does quality matter here?
I'll try bunging 'em in, in the next day or so. I have to take the lid off, I think one of my TJ valves is getting noisy/microphonic which is disappointing, they were bloody expensive.
Is the valve covered by any sort of guarantee? You could always contact the place you bought it from and see if they'll exchange it.

Guy

Edit---Aren't "noisy" and "microphonic" two different things?
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Old 15th August 2009, 10:51 AM
Richard Richard is offline
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Default Re: Phono 3 Jekyll and Hyde

Re noise/microphonics, a couple of times I've had success by dropping the valve (gently) from a few inches onto a wooden suface. The knocks may allow the elements to realign and the valve was then quiet. Worth a try before binning it.

Are you a Phono 3 user Guy and have you tried bypass caps or battery?

Rich
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Old 15th August 2009, 11:44 AM
Guy Pettigrew Guy Pettigrew is offline
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Default Re: Phono 3 Jekyll and Hyde

Nice trick, Rich. Would have to be done with care, though. A drop from too high, onto a hard surface, would probably render the valve totally silent!

Yes, I have a phono 3.

Perhaps it would be worth trying the bypass caps. My knowledge of electronics is almost non existent, although I'm good at following instructions!

So, do these capacitors go across the resistors for all 3 valves? This would put them across R7/8, R17/18 and R23/24.

The other thing is, I may not have 6 spare 220uF capacitors, so couldn't do it anyway.

What's the battery thing about, please?

My phono 3 and pre 3 are powered from the WD power supplies, one for each amp. Would a battery be better?

Guy
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  #9  
Old 15th August 2009, 01:12 PM
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soulminer soulminer is offline
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Default Re: Phono 3 Jekyll and Hyde

Just tried this mod. I didn't have any caps of the right value so I just stuck a couple of BG NP 470uF in. It certainly has increased the gain, which by-passing a cathode resistor always will. Not that I needed any extra gain though. I can't say it has done much else at the moment either. I will leave it in for a couple of weeks and see what else I can perceive, if anything, before removing them for a quick a-b-a. I may even wire in a switch to try it "on-the-fly". Certainly no worse anyway. This is similar in outcome to changing the windings on the SUTs for a higher gain ratio. I will leave it in for now anyway.

BTW, the Phono II has provision on the PCB for these by-pass caps for the first two stages if you wanted to permanently fit them. I couldn't be bothered taking the PCB out, so just tacked them across the resistors.

I agree Rich that they were probably removed by WAD to a) reduce the (unnecessary??) gain and b) save them some money.

Cheers,
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Last edited by soulminer; 15th August 2009 at 01:22 PM.
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Old 15th August 2009, 01:19 PM
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Shian7 Shian7 is offline
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Default Re: Phono 3 Jekyll and Hyde

This sounds intriguing... gotta be worth a try!
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