26 pre

pre65 · Post by **pre65** » Mon Mar 25, 2013 9:02 pm

If I try an input cap, 0.15uf (150nf) was suggested.

I have not got one that size, what effect would using an 0.22uf (220nf) have ?

And also, would I need a grid stopper ?

andrew Ivimey · Post by **andrew Ivimey** » Mon Mar 25, 2013 9:54 pm

Look what do I know but .... you have 9volts D.C. bang in line with teh audio signal. What difference will a o.15 and a o.22 mfd cap make?

DUNNO

but it seems a tad pointless. A cap blocks D.C. if it makes any difference at all it will be to cut some of the high frequency. why not use a 4mfd cap. You will hear a difference.

won't you?

pre65 · Post by **pre65** » Mon Mar 25, 2013 10:32 pm

I'll go through my capacitor box and see what I can find.

This was plan B after I got the pre up and running.

pre65 · Post by **pre65** » Mon Mar 25, 2013 11:34 pm

Right, plan B has been actioned.

833a was taken off the operating table, and 26 pre received an input cap transplant. I only had two 220K resistors left, so I used those as grid leaks and a couple of World Designs SAX 0.22uf (220nf) capacitors.

It's too late in the evening for testing at realistic levels, but initial impressions are a better sound. I'll have a blast in the morning.

And, young Mr Bass, the squeak on Vals rocking chair is on the RH side.(my right, his left)

Mike H · Post by **Mike H** » Tue Mar 26, 2013 12:48 pm

So is this -

battery + to ground

battery - to 220k resistor, then to grid

Input capacitor from socket to grid.

pre65 · Post by **pre65** » Tue Mar 26, 2013 12:52 pm

Mike H wrote:So is this -

battery + to ground

battery - to 220k resistor, then to grid

Input capacitor from socket to grid.

Yep.

Mike H · Post by **Mike H** » Tue Mar 26, 2013 1:07 pm

Super.

What we want to know is how that sounds compared to battery in series as previously.

pre65 · Post by **pre65** » Tue Mar 26, 2013 8:21 pm

Mike H wrote:Super.

What we want to know is how that sounds compared to battery in series as previously.

Better.

It's not night and day, and as usual I have trouble describing the betterness.

Dave the bass · Post by **Dave the bass** » Tue Mar 26, 2013 9:08 pm

Can you hear the Nylon of Des O'Connor's shirt rustling against his tank top? If you can... that'll be an improvement in micro detailing

DTB

pre65 · Post by **pre65** » Tue Mar 26, 2013 9:18 pm

Dave the bass wrote:Can you hear the Nylon of Des O'Connor's shirt rustling against his tank top? If you can... that'll be an improvement in micro detailing

DTB

This is SO good it's got nano detailing, s'true.

Neal · Post by **Neal** » Wed Mar 27, 2013 8:31 am

Who would have thunk it? A cap better than a battery....tisk!

Dave the bass · Post by **Dave the bass** » Wed Mar 27, 2013 8:56 am

Neal wrote:Who would have thunk it? A cap better than a battery....tisk!

Heheheheheheheh, you are naughty Neal!

DTB

pre65 · Post by **pre65** » Wed Mar 27, 2013 10:16 am

Neal wrote:Who would have thunk it? A cap better than a battery....tisk!

To be honest, the battery was surprisingly good.

And I know that 'cos I have tried it.

IDM · Post by **IDM** » Wed Mar 27, 2013 1:30 pm

The only schematic I have seen with a DC blocker on the input a DHT linestage used an incredibly low value of 0.047uf. The quality of the cap was however, very high (teflon).

It might be worth having a play with whatever, you have any in your spares box (I guess you have quite a few types) to see if there is a real "sweet-spot" to the presentation.

I am very tempted to convert my 2A3 stage to grid bias, though I need to think how to derive the 30V needed.

Cheers
Ian

pre65 · Post by **pre65** » Sun Mar 31, 2013 11:25 am

Next step in the 26 pre could be a shunt regulated power supply.

Is it sense (or non-sense) to have a smoothing choke between the regulated supply and the anode choke ?

At the moment the supply is split and each channel has an extra 40H smoothing choke just before the anode choke.

audio-talk

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