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  #31  
Old 11th November 2009, 05:22 PM
John Caswell John Caswell is offline
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Default Re: Phono 3 with 5751 valves

Hi all,
Been watching this thread since the beginning and had a quick look at my Phono II/III with 5751 tried in output and input positions.

Test setup:
AWA very low distortion oscillator > Hagerman inverse RIAA filter > Phono II/III with Sowter 9575 20:1 stepup Tx > AWA distortion/noise analyser > HP180 50mHz scope.
Figures are compared to my standard PhonoII/III with Philips 12AX7WA input valves and Brimar 12AX7 output valve

Firstly - output position. As far as I could measure there was negligible to no change in either frequency response or gain when used in this position, which is not surprising as it is a cathode follower! There was no need to adjust the variable capacitor in the phono III unit to re-correct the square wave shape

Secondly - input position. Once again as far as I could measure negligible frequency response errors(Less than 0.1dB - measuring error?). Gain however was down -2dB to -3dB depending on valves so, as Richard M says, could be useful if you have an over sensitive pre/main amp.

Thirdly - Both positions with 5751s. Negligible frequency response changes but gain down -2dB to -3dB.

The only advantages I can see are lower gain possibly lower noise floor and possibly lower microphonics.

I will do some further tests using my phono II without the transformers as they will undoubtably have some effect on the frequency response.

There are also some other valves that may be suitable but I think are pretty rare/expensive e.g. 12AY7, 12AD7.

John - Dr John
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  #32  
Old 11th November 2009, 10:48 PM
Richard Richard is offline
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Default Re: Phono 3 with 5751 valves

Hi John,

Yep, that looks right. Checked carefully twice now with 2 amps and 4 sets of valves, comparisons in the same amp in the same sessions, today with GerryB. I used the 600ohm input and -40dB output of the Hagerman iriaa rather than include any step-up transformers and reffed at 2v p-p 1KHz on the scope at 0.5v/div showing a span of 20 graduations.

The drop in gain from ECC83 to 5751 is 14/20 on the scope = -3dB. That relates well with the difference in mu of the 2 valve types.

After a straight swap from ECC83 to 5751 there is a slight treble lift. After re-setting the trimmer caps there is a just-perceptible lift (still a net roll-off) which results in a -6dB point at 65KHz instead of 50KHz. I believe the slightly lower plate resistance of 5751 would account for this but we'd have to try a few more different makes to be more sure.

Usage-wise, we are back to that intangible quality - the sound - and to my first post where I did say these may not be better than anything else but are viable alternatives . I've taken so long to try them as I knew they were different electrically and wanted to know what effect they may have rather than just plonk them in.

I recd some 6072s today which would have been interesting to try, if the gain is enough, but they're not what they should have been so have gone back...

Rich

Last edited by Richard; 16th March 2010 at 08:19 AM. Reason: I believe the...
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  #33  
Old 17th November 2009, 07:52 PM
Richard Richard is offline
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Default Re: Phono 3 with 5751 valves

Just a quick note that Mike at Chelmer Valves now has Jim Fish (Wilson Valves) stock, on the same phone number, 01484 654650, and has GE Jan 5751 in at the mo, nos military boxed, £15 +vat.
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  #34  
Old 3rd December 2009, 11:38 AM
Richard Richard is offline
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Default Re: Phono 3 with 5751 valves

The 6072 replacements have now arrived so I looked at trying them in Phono.

They're considerably lower gain and lower plate resistance than ECC83 or 5751. They'll work but throw the third corner point a mile out without adjustment; the first 50Hz corner is better at 48.9Hz without a cathode bypass cap (51.8Hz with), the 500Hz corner is the same as before, but the 2122Hz corner is now at 2680Hz. This will put a step in the frequency balance, all lifted from 2122Hz upwards.

So, it will probably be a quite dynamic sounding change; the bass may be better and the treble will get a lift but we cannot just drop valves in willy nilly if we're concerned about fidelity. It can be corrected by changing the 220pF to 277pF but this is out of adjustment range for 3S trimmer so will wait for another day.

All told, I won't do it for a while at least as gain will be too low without a cathode bypass cap and the bass is then off. Perhaps it would be good to try in future with a full reworking to correct response and using bypass caps on both first and second stages to get the gain back.
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  #35  
Old 19th February 2010, 12:36 PM
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Shian7 Shian7 is offline
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Default Re: Phono 3 with 5751 valves

Quote:
Originally Posted by Gerry View Post
I was thinking of waiting for their promised KT88s but didn't in the end.
Great pity, but you never know what might emerge out of the ashes with thes "pre-Pack" deals when a firm goes insolvent.
Mike those valves might make some of the old NOS stuff look cheap in a few years time...don't burn them out!!!

Erm...

Something 'happened' but I'm not sure what!

I switched on one day to find all sorts of weird sounds and after a bit of valve swapping became convinced there was a problem with at least two of them. So I asked Rich if he'd take a look, and this is what he found...

Quote:
Hi Mike,

Sorry to be the bearer of bad news but 1 is good, 2 are knackered.

Test spec for the Avo is at 250V when they should draw 1.2mA current and show a slope of 1.6mA/V.
(A new lively one might have a higher slope for that current or meet that slope with less current. A worn one will draw less current and have less slope.)

Valve 1
anode 1 1mA / 1.7mA/V
andode 2 1 mA / 1.7mA/V

Valve 2
anode 1 0 / 0
anode 2 0.6 / 1

Valve 3
anode 1 0.7 / 1.4
anode 2 1 / 1.7

All test fine for heaters, gas, and shorts/insulation.

So,
Valve 1 is fine both sections well matched. It draws a little less current than spec and gives a slightly higher slope.
Valve 2 is all but dead on anode 1, there is a little movement showing it's not disconnected but it is useless. Anode 2 is low, will still make sound but you can see the way it's going.
Valve 3 is low on anode 1 but will still work. Anode 2 is fine.

Both bad ones have suffered in a similar way but anode 1 in partic. I'd first check V at the anode pins without valves fitted. If ok (a bit high due to no load) install other working ecc83 and check all voltages. Particularly the cathode voltages as either these valves simply have a short life or they've been run with high bias current on anode 1 in one or 2 valve positions in the amp. Is it possible you've a wrong cathode resistor value in one position for instance? Or could be just one bad position and they've been switched around only partially wearing out the other valve.

Rich
After poking and prodding around with a meter I can't find anything wrong, all the voltages are there or thereabout. So I don't know what exactly 'happened'!

It has to raise a big question mark over the reliability of these valves IMO... Has anyone else used them and had any problems?

Cheers,
Mike.
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  #36  
Old 8th March 2010, 08:09 PM
bearded fiddler bearded fiddler is offline
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Default Re: Phono 3 with 5751 valves

Hi Richard,

I need some assistance with the trim caps. I tried to adjust them, thinking there would be end stops, but now it turns out the trimmer can be turned indefinitely, so I don't know where the trimming range starts and ends. I'm looking for the middle of the range, to adjust for ECC83's. It will be clear by now I don't know anything about the blessed things. When using 5751's I understand the trimmer should be a little past the middle of the range. Your help will be much appreciated.

Jan
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  #37  
Old 8th March 2010, 08:43 PM
Richard Richard is offline
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Default Re: Phono 3 with 5751 valves

Quote:
Originally Posted by bearded fiddler View Post
Hi Richard,

I need some assistance with the trim caps. I tried to adjust them, thinking there would be end stops, but now it turns out the trimmer can be turned indefinitely, so I don't know where the trimming range starts and ends. I'm looking for the middle of the range, to adjust for ECC83's. It will be clear by now I don't know anything about the blessed things. When using 5751's I understand the trimmer should be a little past the middle of the range. Your help will be much appreciated.

Jan
Hi Jan,
Check the pic below carefully.
See the slightly protruding flat end of the white ceramic base.
See the brown mica disc with silver semi-circle that the screwhead is soldered to.
When the silver is turned to be fully at the flat end of the ceramic base that is max cap.
When the brown is at the flat end that is min cap.
When it is halfway with the screwslot aligned leg to leg it is half cap.
Thus, one complete turn of the screw will go from max through halfway to min then through halfway again and back to max. (So the full range is covered in half a turn of the screw, either way, and no end stop so no harm done.)
The full range measures 5-70pF.
The pic is set for 5751 at about 40pF (see the silver is more nearly at the flat end so is greater than half cap).
For Ecc83 you need about 20pF on this cap so wind it to a similar position but with the brown semicircle nearest the flat on the base to make less than half cap.
Hope this makes sense!

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  #38  
Old 8th March 2010, 08:44 PM
Richard Richard is offline
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Default Re: Phono 3 with 5751 valves

Quote:
Originally Posted by Shian7 View Post
Erm...

Something 'happened' but I'm not sure what!

I switched on one day to find all sorts of weird sounds and after a bit of valve swapping became convinced there was a problem with at least two of them. So I asked Rich if he'd take a look, and this is what he found...



After poking and prodding around with a meter I can't find anything wrong, all the voltages are there or thereabout. So I don't know what exactly 'happened'!

It has to raise a big question mark over the reliability of these valves IMO... Has anyone else used them and had any problems?

Cheers,
Mike.
Hi Mike,
How's it going? Is it working again with different valves? Did you find anything?
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  #39  
Old 8th March 2010, 09:44 PM
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Shian7 Shian7 is offline
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Default Re: Phono 3 with 5751 valves

Quote:
Originally Posted by Richard View Post
Hi Mike,
How's it going? Is it working again with different valves? Did you find anything?
Hi Rich,

I couln't find anything wrong so it's still a bit of a mystery!

I've only been using it with some ropey old ECC83's as I'm a bit weary of 'something happening' again and I'm not keen on wrecking any expensive valves.

It's out of action at the moment though as I'm soon going to re-plinth the TT and buy a 12" (with 'regular' bearings) arm. NickB has just bought my 9" JMW Unipivot, so I'm currently armless.

Cheers,
Mike.
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kudakutemo

ari mizu-no tsuki

Though it be be broken -
broken again - still it's there:
the moon on the water.

- Choshu.
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  #40  
Old 8th March 2010, 11:06 PM
bearded fiddler bearded fiddler is offline
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Default Re: Phono 3 with 5751 valves

Quote:
Originally Posted by Richard View Post
Hi Jan,
Check the pic below carefully.
See the slightly protruding flat end of the white ceramic base.
See the brown mica disc with silver semi-circle that the screwhead is soldered to.
When the silver is turned to be fully at the flat end of the ceramic base that is max cap.
When the brown is at the flat end that is min cap.
When it is halfway with the screwslot aligned leg to leg it is half cap.
Thus, one complete turn of the screw will go from max through halfway to min then through halfway again and back to max. (So the full range is covered in half a turn of the screw, either way, and no end stop so no harm done.)
The full range measures 5-70pF.
The pic is set for 5751 at about 40pF (see the silver is more nearly at the flat end so is greater than half cap).
For Ecc83 you need about 20pF on this cap so wind it to a similar position but with the brown semicircle nearest the flat on the base to make less than half cap.
Hope this makes sense!

Hi Richard,

Thank you for your very prompt answer which makes a lot of sense in my opinion. There is a slight problem though: the Phono 3S parts list mentions a "5-50pF variable capacitor" for C9A/C10A. So the question is, what range have I got?

Jan
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