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Curious Tube 'issues' (Read 1795 times)
Ryan H
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Posts: 9
Curious Tube 'issues'
11/06/09 at 03:33:15
 
Apologies up front for a long post.  If you're patient, I'd appreciate any ideas the forum may have...

I have some curious behavior and I'm wondering if someone can shine some light (glowy tube light of course) on what might be happening.  Here's the chronological summary of things...

My original kit build from Jan. 09:
I had NOS 6P15PEV Reflektor tubes (at least as far as I can trust the e-bay seller).  I kept getting no voltage drop across the cathode resistor so Steve suggested I jumper pins 2 and 8 on the tube socket.  (I know, pin 8 is 'no connection', or 'internal connection', but bear with me here)  The cathode began conducting.  All the voltages checked and that brought the system to life.  Things worked great.

In Sept/Oct of this year, I had an output tube short (Thanks Steve for the diagnosis).  Since I was 'fixing things', I did the CCE mod (cap between pins 6 and 3) and DISCONNECTED the pin 2-8 jumper.  That returned me to the non-functioning amp status.  So I reconnected pins 2 and 8. That (with the CCE mod in place) returned things to normal operation.  I replaced both output tubes with new JJ el84/6BQ5 and all was well.

Early November (today) I thought I would try some NEW Svetlana SV-83 that I purchased.  Yes they have all the SV markings.  Underneath one can see the same glass graphics that appear on my NOS 6P15PEV.  When I put those in today, nothing.  One of them glowed purple-blue, which is normal operation from what I have read.  The other did not glow.  There was no sound in either channel.

So I pulled them and replaced them with another pair of NOS 6P15PEV.  The amp returned to life, save for some crackling in one of the channels (I haven't had a chance to swap to tubes to hear if the crackling moves to the other channel).  Again, minus the crackling, things sound lovely.  

But I'm very confused by the "pin to nowhere" connection (2-8) and why it makes my amp work.  And I'm puzzled as to why my shiny new set of SV-83 (with 6P15P markings underneath the Svetlana markings) did not work when the 6P15s and jjel84s worked just fine.  

I know this is an experimenters amp. When I have some time to spare, I will disconnect the 2-8 connection and try my SV-83 (if they are not trashed from my previous attempt.  I enjoy the fiddling... but I'd also like to know why things behave the way they do.

Thanks in advance for any advice.

Ryan
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dank
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pair of dual 18
Imperials

Posts: 275
Re: Curious Tube 'issues'
Reply #1 - 11/06/09 at 13:50:05
 
Ryan

Sounds like you have the same Zkit1 circuit board that I have.  It has a couple of layout errors on it:

1)  The output tubes were laid out as if the control grid was on pin 8.  In reality, pin 8 is a no connect and the control grid is pin 2.  Your jumpers put the control grid signal on the control grid pin (and leave it on n/c pin 8 also).  Leave those two wires alone, the amp won't work without them.

2)  Pin 6 of the output tubes (the screen grid) was left n/c.  Some output tubes have an internal connection (pin 6 to pin 3, screen to cathode) but some do not.  The CCE mod requires you to remove this (in your case non-existent) connection and connect pin 6 to pin 3 through a 0.1 uf 400v cap.  If you leave pin 6 open, use output tubes without the internal pin 6 to pin 3 connection (6p15p), and don't have the CCE mod installed you wind up with an amp that sounds ok at low power but begins to distort at about 1/2 watt.  As long as you have the .1uf CCE capacitor in place you should be ok, the output tubes that have the internal pin 6 to pin 3 connection work, they just don't benefit from the CCE mod.

So, with your circuit board, if you have output tubes jumpered pin 8 to pin 2 and the .1 uf caps installed pin 6 to pin 3 you ought to be all set.  As for your other problems:  if one of your new SV83 tubes isn't glowing and the amp works ok with 6p15p tubes I would guess that you have an open heater in your new SV83.  Pull it out and check with an ohm meter, pin 4 to pin 5.  If its open your tube is shot.  As for the crackling, make sure the tube pin connections are clean and tight, make sure your rca jacks aren't dirty (just had hum issues there on mine), then try to isolate to a tube or to a channel and post where you are.

Dan
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Ryan H
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Posts: 9
Re: Curious Tube 'issues'
Reply #2 - 11/06/09 at 20:15:31
 
Dan,
Super explanation!  Thank you.  I've isolated the 'crackle' to my pre-amp.  Though it's sporting brand new 12AX7s, it appears one of those has an issue.

Anyway, back to the ZKIT-1.   As you suggested, I checked the SV-83s for continuity of the heater elements and they 'beep' through just fine.  I measured the resistance between pins 4 and 5.  One measures 4 ohms.  The other measures 3.9.  The former is the one that didn't achieve blue-glow status. I rather doubt 0.1 ohm difference matters in that regard.  Out of curiosity, I measured the resistance between pins 4-5 on my JJ el84s:  1.4 and 1.3 ohms respectively.  Similarly, I pulled out a pair of NOS 6P15Ps:  3.9 and 4.0 ohms, consistent with the SV-83s.  It's clear as mud.  Apparently the dc resistance of the heater filament, while different between el84 and 6p15P (sv-83) is not the factor that is causing the New SV-83s not to work.

So do you have any other ideas as to why the brand new SV-83s don't appear to work?  Firstly, there was no sound whatsoever with the SV-83s in place.  And, in particular, why one would appear to work (if the blue glow is any indication) and the other would not?  

One other clue, I was inspecting the SV-83s while in the amp (and it wasn't working) and I did spot some intermittent arcing between tube elements.  It was at this point that I shut 'er down.

Again, I appreciate your previous response and help.  The amp is working fine at this very moment with NOS 6P15P-EV, CCE mod, and 8-2 pins jumpered.

Cheers,
Ryan
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Ryan H
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Posts: 9
Re: Curious Tube 'issues'
Reply #3 - 11/11/09 at 23:40:25
 
The strangeness (or my naivete) continues...
I'm simply documenting my experience here for those who might follow or have similar experiences.  If you can shed light on the curious results, be my guest.

Emboldened by the simple measurements of the filament resistances and the consistency between tubes noted in my previous post ...  I decided (just now) to retry the SV-83s.  I marked the boxes on my original attempt and noted which channel each had been placed.   On this try, I simply switched them.

Now the new SV-83 tubes work!?  No blue glow, no arcing.  It just works.

I play the amp almost daily for a few hours or so.  It sounds very good with all tube variations I've tried.  I haven't had time to focus and to study if there's a particular set up I like better than another.  It is indeed all good, very good.

Ryan
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