Problem with push-pull KT88 - diyAudio
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Old 23rd April 2013, 07:49 AM   #1
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Default Problem with push-pull KT88

Hello,
I started to getting parts and building this amplifier some months ago, and yesterday I got one of the two channels assembled. After assembling a circuit, my usual procedure is to check everything against the schematic, and if everything's OK, then double check it by drawing the schematic from the actual assemble circuit. If comparing both the original schematic and mine results in no differences, then I go ahead and light things up. May seems to be a bit paranoid, but I haven been always taught to work on the safe side.

After cabling all heaters (important! only heaters, not any other voltages from PSU), pushed valves in sockets and all lighted, check for their voltages. Good. Then I took them out and connected the rest of the voltages, adjusted trimmers to bias KT88 at -65v, and ready to go! I put the valves in again, turn the thing on with multimeter probes between the resistor cathodes on both KT88 (it's a push-pull) to check for bias (1 ohm cathode resistor, so MM at 200mv scale), and display started to show strange changes, wildly varying, 20mv, -140mv, 60, you know, all at random.

I turn off the thing and visually check if there were any loose cable, any kind of short... nothing. Turned it on again, and voltage varies randomly.

While I am writing this post, I have just realize a very important fact. This is the website from where the design comes from. Along the months that I have been gathering parts, the schematic has changed dramatically! Please check attached schematics, version 1 is the "classic" one while version 2 is the new uploaded, one month ago. I have built the second version, which connect both KT88 cathodes to ground through a current sense reistor.
Should I try to connect it like in the previous schematic version?

Anyway I have made a call to Andrea who is the designer of that circuit to see if he can throw up some light on that.

Thank you!

PP2012 - KT88 Hi-End Push Pull Amplifier
Attached Images
File Type: gif PP2012sch-VERSION1.gif (92.8 KB, 298 views)
File Type: gif PP2012sch1-VERSION2.gif (96.6 KB, 288 views)
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Old 23rd April 2013, 08:00 AM   #2
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disconnect negative feedback at thr out put transformer.
if the problem magically disappers, reverse the secondary wire connections.
if not, then its associated by the power supply.

that is if the schematics are a proven design
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Old 23rd April 2013, 08:04 AM   #3
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Where are ther grid stoppers on the output valves? It may take off at weird frequencies!
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Old 23rd April 2013, 09:00 AM   #4
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Do you have a load connected to the amp? Don't turn it on without! A simple 8 ohm resistor will do.
As a side comment: a 10k output transformer is a bit high impedance for KT88 @ 500V. I would expect something around 5k.
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Old 23rd April 2013, 09:35 AM   #5
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Hi you all, I will try to do my bests to answer each one:


Quote:
Originally Posted by DavesNotHere View Post
disconnect negative feedback at thr out put transformer.
if the problem magically disappers, reverse the secondary wire connections.
if not, then its associated by the power supply.

that is if the schematics are a proven design
Before messing up with feedback I will try to see which schematic is the correct one regarding KT88's cathodes. May it seem like a bug to have them grounded?


Quote:
Originally Posted by Harleyjon View Post
Where are ther grid stoppers on the output valves? It may take off at weird frequencies!
I though that too... As far as my limited experience tell me, they are almost near to mandatory. What should do it, 1K ohm 1/2W? Anyway, I placed 1K serial resistor as grid stopper on the first valve (12AX7) that is not shown on the schematic, just because I used to with SS.


Quote:
Originally Posted by Parafeed813 View Post
Do you have a load connected to the amp? Don't turn it on without! A simple 8 ohm resistor will do.
As a side comment: a 10k output transformer is a bit high impedance for KT88 @ 500V. I would expect something around 5k.
Yes, I have an 8 ohm power resistor across the secondary of the ouput trafo. And I have shorted the input signal (RCA) to ground in order to avoid picking up aerials, RFI, etc.
As far as output trafo impedance, what would be the drawbacks/pros of using a higher impedance one? As I have read, a bit lower on the raw power side, but maybe a less of distontion? Just talking from my own ignorance...

Best regards!
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Old 23rd April 2013, 09:35 AM   #6
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Sorry for the mistake, the KT88 ground was missing in the first schematic.

The 2.2ohm resistors are necessary to set the correct bias with a voltmeter.

Last edited by audiodesign; 23rd April 2013 at 09:56 AM.
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Old 23rd April 2013, 09:45 AM   #7
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Negative voltage on a cathode resistor is 'impossible' so a classic sign of oscillation confusing the DMM.
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Old 23rd April 2013, 10:07 AM   #8
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Quote:
Originally Posted by audiodesign View Post
Sorry for the mistake, the KT88 ground was missing in the first schematic.

The 2.2ohm resistors are necessary to set the correct bias with a voltmeter.
So the correct one is actually the one of the website? with both cathodes grounded, right? I have used 1 ohm resistor instead of two 2.2ohm, is that fine, right?

I have attached some pics I had lying around from the prototype, components, just for the fun of it... sorry for the low Q, they were taken with my phone.
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Old 23rd April 2013, 10:07 AM   #9
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Grid stoppers should be as close as possible to the grids of the ouput KT88 valves, otherwise they will have no effect!
Mount them actually on the valve base as there is spare pin.
Use 10k ˝Watt carbon.
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Old 23rd April 2013, 10:07 AM   #10
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regiregi22,
is there other problems ?
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