Can I remove the input caps?

[Nightpuma]

Hi.

I don't have much experience fiddling with electronics, so I need a little advice her.

I have a Graaf GM 13.B preamp which I have decided to try to sing more beautiful. Taking one thing at a time, I have decided to change the output caps. The amp also have 0.18 uF input caps to block DC I suppose. Do you think it would be wise to remove these?
It's a balanced design.

I've made a drawing of the beginning of the circut.

 

[SY]

The drawing makes very little sense. You need to trace this out much more thoroughly.

If you don't have a schematic and don't have a clear idea of what you're doing, the question is not whether or not you'll get smoke, but how much and how badly.

 

[Nightpuma]

Hi.

I understand. I'll draw a complete circut diagram and return.

Theo

 

[Svein_B]

I think the input of this amp is a diferential/LTP stage.
Measure the voltage at the grid, and I think you will find that it is not at ground level.

If my understading is right, the caps must stay.

Svein.

 

[rdf]

As drawn I think C14, by removing any DC grid reference to ground, will temporarily turn the tube into a nightlight. But only temporarily.

 

[Nightpuma]

Hi.

Thanx alot. The cap stays.

But what about changing the value down to .15 uF or up to .22 uF? Easier to find good cap.


Theo

 

[rdf]

Sorry, I should have been more specific. I don't understand what keeps that tube from burning up with C14 left in the circuit. Unless it's something completely new to me (not that big a challenge) C14 prevents will cause the grid to float positive and make bad things happen. There is no external DC path between grid and cathode as drawn. Are you sure about that schematic?

 

[Svein_B]

I think the confusion regarding C14 is because some important parts are not shown.

If we imagine a voltage divider between plate or b+ and ground to feed the top of c14, we establish a grid reference voltage, presumably somewhere in the range 50-100V, for the tube.
If we further assume that the tube shown is 1/2 of a differential LTP, we have established some "space" for a common cathode resistor.

This is just my assumptions from looking at a similar amp a few days ago.

Svein.

 

[Nightpuma]

Hi.

Thanx for helping me out. Found out that I needed some more teoretical understanding, so I've started reading

But, can any of you identify the caps?

http://www.hifisentralen.no/album/Nightpuma/DSCN0533_rs.jpg

Brand and type?

Theo

 

[Nightpuma]

Hi

I got it figured out. Cornell Dublier PIO caps.

Theo

 

[Rocky]

Are you sure you want to take those caps out for replacement? Those are fairly decent caps and the circuit look well built (from the very small part I can see in the picture), if you're out to make "just some mod" for better sound, perhaps the electrolytics in the power supply wants an upgrade? To Black gates or Cerafines?

If you decide to take dubilier caps out, beware that since you have no clue about the grid voltage and the caps already inplace are rated 400V, it might be a good idea to use capacitors rated for some DC. Or measure and see what you've got on the grid.

EDIT: From time to time, you can get silver mica capacitors in that range on eBay. I have some 0.2uF/350V in an amp, and something like that would surely be an upgrade to what you have with the dubiliers.

 

[rdf]

EDIT: From time to time, you can get silver mica capacitors in that range on eBay. I have some 0.2uF/350V in an amp, and something like that would surely be an upgrade to what you have with the dubiliers.

Hi Rocky. That caught my eye. I'm guessing you mean the rectangular brown jobs. I'm also (standard disclaimer: 'currently') a big fan of the Russkie silver micas. Same as you I think they handily better the PIOs, particularly up top where they're the smoothest, most extended I've heard. Since you're the only other person I know using them, spare an opinion?

BTW, you may already be aware but there's a very simple mod for them. Heat the pins and remove the solder tabes. Remove the excess solder from remaining hollow steel tube over which it slipped. From the end of that tube is a tiny wire, it's the direct lead to the mica. A 26-28 guage wire will fit onto the extra space in the hollow pin, bypassing the steel sleeve. If you solder a wire into the tube be sure to mechanically support it with teflon tubing or equiv, if it breaks at the pin entry it's never coming out. The entry tube is also embedded in a glass seal, I scrape all the paint overspray from the glass. FYI.

 

[Nightpuma]

Are you sure you want to take those caps out for replacement? Those are fairly decent caps and the circuit look well built (from the very small part I can see in the picture), if you're out to make "just some mod" for better sound, perhaps the electrolytics in the power supply wants an upgrade? To Black gates or Cerafines?

If you decide to take dubilier caps out, beware that since you have no clue about the grid voltage and the caps already inplace are rated 400V, it might be a good idea to use capacitors rated for some DC. Or measure and see what you've got on the grid.

EDIT: From time to time, you can get silver mica capacitors in that range on eBay. I have some 0.2uF/350V in an amp, and something like that would surely be an upgrade to what you have with the dubiliers.

Hi Rocky.

Thanx a lot for the advise. Since I now found out that the caps are CD I let them stay for now and concentrate on other parts of the amp. I'll remember the silver micas on eEbay. Also considered thos expensive teflon V-caps. Tried them?

About the electrolytics in the power supply, I recently replaced them since they were about 10 years old. Its' Kendeil brand.
You can see the pic here:

http://www.hifisentralen.no/cgi/yabb/YaBB.cgi/YaBB.cgi?board=diy2;action=display;num=1144783028
First picture.
So you think changig those would be a good route?

Theo

 

[Rocky]

rdf,
Nice to meet other fans yes, I really like them, they have almost no sound at all.. Mica caps are really the closest one can come to a ideal capacitor, and the only really special thing about the ruskies is the relatively big capacitance that makes them come in handy in a wider range of applications.. As signal capacitors, I would rate them higher than all others I've tried, including K76P2 teflon, Sprague VitQ, Mundorf silver, and alot of others.

I did hook up the bolt to try your mod, I removed the solder lugs, but the tubes they were fitted on didn't seem hollow? I tried a fairly thin copper wire, and couldn't get it to slip in.. ah well, I guess I need to find my solder sucker tool to get a clear view.. Thanks for the tips by the way



Theo,
I've tried K76P2 teflon, and they are very good coupling capacitors. Excellent value, but personally I like the micas better.

I think your power supply would be the obvious place to put the bucks. See if you can get Elna Cerafine (or Black Gate if you have the $$) electrolytic replacements. That will probably do more good than replacing the coupling caps..

 

[Nightpuma]

Rocky.

There are 3 Kendeils now, each 470uF/400V.

Seems hard to find that value in Cerfines. Do you think lowering the value a bit would matter, and if so, what do you think would be OK? Or should I use 6 220 uF?

Theo.

 

[rdf]

Nice to meet other fans yes, I really like them, they have almost no sound at all.

Also nice to get independent confirmation my ears aren't playing with me. On a spec basis mica makes almost no sense, but I agree with your description. Thanks for the sanity check!

I did hook up the bolt to try your mod, I removed the solder lugs, but the tubes they were fitted on didn't seem hollow?

If like mine the pins are just full of solder. he solder sucker or braid will usually remove enough to get a wire started.

 

[Rocky]

Theo,
Try machmat.com, look up the electrolytics section, and see if Matjis has any Phillips LL that fits your purpose. They are good and comes in a variety of ratings probably better suited for your purpose than Elna or BG. They come reasonably cheap too.



rdf;
Thanks for the tip - i still haven't dug it up from my boxes in the basement (I'm on vacation at my parents house, but I know there's one down there somewhere)

 

[Nightpuma]

Hi Rocky.

Think I'll stick with the Kemdeils for now. They are brand new and from what I read, quite good. Thanx for the link though, he shure had a lot of snacks.

Theo