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 Post subject: Hisssssssssssssssss!!
PostPosted: 30 Nov 2015, 11:51 
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Joined: 26 Sep 2015, 13:59
Posts: 56
Please might I ask you guys for some help again? I'm sure I'll get this going in the end but at the moment it's getting very frustrating!

I have built my preamp according to the schematic attached.
    All voltages are correct as per the 6AK5 spec sheet: 180v plate, 120v g2, -2v k/g. Heaters a running 6-7v ac (which I plan to convert to regulated DC when I get it all working). All components are new high quality, audio spec; apart from the transformer which is a vintage one from eBay. It is a Phillips transformer from a tape recorder (or so I understand)
.
    I am outputting into a modified Quad 405 power amp which has a 100k pot on the input. The output is, in other words, driving across a 100k load.
    Input is from a highly modified Musical Fidelity VDac.

The problem is hisssssss! Hum seems low (but will need some work) but the hiss is really loud making the unit unusable. It plays music beautifully so everything is working - apart from the hisssss!

Now, I have been told that it could be a faulty component, usually a resistor. But all these are new (and expensive!) All the capacitors are well within tolerance (I think). So where do I look next? Could it be the transformer? Is that possible?

Please save me from throwing this thing through the window :mad:


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PostPosted: 30 Nov 2015, 14:22 
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Location: Arizona, USA
Hi, Anything is possible, but my guess is the power supply. I would get rid of the pass transistor and use the LR8 directly. It can handle the voltage and current. Second the adjust terminal of the LR8 should have a capacitor to ground. I use mostly 2.2uf polys there. The pass transistor can act as a capacitor multiplier as shown, but it may also have sufficient leakage to cause the hiss. If you do get rid of it...remember that the TO72 version of the LR8 can only dissipate 750mw, it is why I use the K version that can do 2.5 watts.

Just curious but why use a pentode? A more comon triode stage should provide enough gain unless there is something odd in your other circuitry.

Good listening
Bruce

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PostPosted: 30 Nov 2015, 15:03 
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Joined: 26 Sep 2015, 13:59
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Thanks for this Bruce,

I actually recently added the pass transistor (a TIP50) because I found the LR8 was shutting down at the slightest provocation and the output voltage was sagging to about 60v. I tried adding heatsinks (like little hats) to the LR8s but that was not enough. I'm using the LR8N which, as I'm sure you know, is a TO-92 package. It's the only one I can find in the UK that's not SM. I will try adding the cap you suggest and see if that helps. I can also, with some carefully placed resistors, try bypassing the regulator circuit completely to see if that helps. At least it woud narrow things down for me.

I'm using the pentodes simply because I have them and I also have a small, cheap Chinese preamp that uses them. This Chinese preamp is pretty amazing really. It runs from a 12v ac wall-wart. 12v ac to power the heaters in series and voltage doubling using capacitors to produce HT of c. 90v. It sounds pretty good too! I thought a properly designed and built unit using the recommended valve spec ought to give very good results. I thought it would be simple!

When I get rid of this hiss I will try the valves in triode strapped mode to see what they sound like.

Thanks for your help. I'll get back to you when I've had a play.

Mark


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PostPosted: 30 Nov 2015, 15:46 
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Whelp, 6AK5 is a pentode and pentodes are normally noisy, that's why people run them in triode for audio.

Another reason might be too low of current. Run the current up so you have at least 1/3 your maximum Pd. This takes advantage of tube transconductance and noise falls big time.

Cheers!

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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
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PostPosted: 30 Nov 2015, 16:12 
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Joined: 26 Sep 2015, 13:59
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Bruce,

You hit the nail on the head again! I've bypassed the regulator and the hiss has gone! Just need to get the hum down now, not that it's terrible but I'd like to get it as quiet as I can. I'll chase up one of the hum guides on this site. I would like to get the LR8 circuit working, but at least I know where to look now.

re. Pentode. I'm using it bacause I had it. I'm pretty new to this (resurrecting an old hobby) and this is a learning exercise for me. I'll try triode strapping and see if it makes a difference. My understanding is that to do this I need to put the control grid at the same potential as the anode? In the case of the 6AK5 90v seems to be the favoured potential.

Sorry, also being new to this I don't understand what
Quote:
Run the current up so you have at least 1/3 your maximum Pd
means, nor how to acheive it. Could you possibly expand a little?

Thanks again to all.

Mark


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PostPosted: 30 Nov 2015, 16:32 
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Location: Chilliwack, BC
RevM wrote:
You hit the nail on the head again! I've bypassed the regulator and the hiss has gone!


Just need to get the hum down now, not that it's terrible but I'd like to get it as quiet as I can. I'll chase up one of the hum guides on this site. I would like to get the LR8 circuit working, but at least I know where to look now.

Quote:
Sorry, also being new to this I don't understand what
Quote:
Run the current up so you have at least 1/3 your maximum Pd
means, nor how to acheive it. Could you possibly expand a little?


Then don't worry about it, especially since you found your problem ;)

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 30 Nov 2015, 17:04 
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Joined: 26 Sep 2015, 13:59
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Gregg,

Any things I might try to get the most out of this little valve I'd love to know about. As a learning exercise I have more options to play with a pentode than a triode.

Could you advise on a good triode valve for use in a preamp? I would like to build a good preamp in the next few months after I have learned all I can from this current exercise.

Thanks again,

Mark


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PostPosted: 30 Nov 2015, 17:54 
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Joined: 08 Aug 2009, 03:11
Posts: 2159
Location: Chilliwack, BC
12B4 is really good. You want lower gain for a line stage.

Another option is triode strapping an EF184/6EJ7. Voltage gain with resistor on the anode will be about 50, so you may want to use a step-down transformer on the output or a divider.

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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