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Old 12th November 2012, 07:30 AM   #11
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Join Date: Aug 2012
When hum and voltages change so dramatically with just movement of wires, my guess would be that there are some bad connections.
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Old 12th November 2012, 08:34 AM   #12
jazz is offline jazz  Netherlands
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Location: leiden, physically that is...

Looking at the picture there seems to be a cathode bypass cap missing on the most left socket.

oh, what a beautifull day
I wanna go out and play!
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Old 12th November 2012, 08:46 AM   #13
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Originally Posted by EEApprentice View Post
DeathRex: Yeah, I've been using Morgan Jones' Valve Amplifiers almost as a Bible haha.
Building Valve Amplifiers, by the same author, should also be in your library, very good reference for construction

Jacob, the Phase Inverter gets DC reference directly from the previous stage (so it is fixed biased), so no use for a grid leak. It does still need a grid stopper though...

Last edited by costis_n; 12th November 2012 at 08:53 AM.
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Old 12th November 2012, 05:08 PM   #14
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Location: Auckland, NZ
amongst all of hte comments I'd add that your grounding scheme (if it follows the schematic) is almost designed to induce hum. Check out the very good article on grounding schemes in hte articles section of the site.
"It may not be easy for some to not hear differences, even if they are not there." - Vacuphile,
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Old 12th November 2012, 06:30 PM   #15
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aardvarkash10: The actual grounding is not like the schematic, I have everything running to one main star ground near the back and center of the chassis, I thought that would eliminate ground loops.

jazz: Yeah, shortly after posting the picture I noticed my KT88 caps kept coming off.

DeathRex: You were right! The resistor values for my cathode bias are wrong, I used the equivalent resistance seeing by the caps (used to calculate the cap value) instead of the correct cathode resistance. I've already ordered those new resistors.

dgta: Because of the resistor thing, I'm going to wait until I get those DC values since some of the component values are wrong.

There is one channel that is actually reproducing music, however the bias for my KT88s on that channel is 55 [V] and 48 [V]. The music is not very loud and a bit distorted.

The other channel still has that LOUD, low frequency hum. I've tried disconnecting coupling capacitors going from the output towards the input. I've found that if I disconnect the input to the KT88s, the hum is gone. If I disconnect the input to my driver after the phase splitter, the hum is gone. If I disconnecting my input from the pot to the pre-amp before the phase splitter, the hum is there. So this hum seems to be originating between my pre-amp and phase splitter.

I rewired that entire section and the hum is still there....
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Old 12th November 2012, 07:25 PM   #16
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Location: london
Hi EE,
I have a pair of push pull kt 88 monoblocks. Your assertion above is correct. The thing to do now is try using the non humming channels front end to drive signal to the phase splitter on the humming side to assert that it is the pre at fault, and then try to figure out the difference between them. Use screened wire, seeing as how you will be linking over lots of circuit that can add hum. I am sure you will have a bad connection somewhere- it is very easy to do when you use the same colour cable everywhere- I have rectified a few diy'ers errors (and others who ought to know better) due to this.
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Old 12th November 2012, 08:16 PM   #17
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bernification: But the non-humming channel still has problems. The bias for those KT88 is way above what I calculated and they are mismatched.

Last edited by EEApprentice; 12th November 2012 at 08:38 PM.
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Old 14th November 2012, 12:35 PM   #18
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Location: london
Hi EE,

one problem at a time. Try to eliminate the source of the hum first.

Then check all your cathode bias resistors and caps, and the pinout of your kt88s.
It sounds like one amp has a good balanced output. Check to see if both amps cathode resistors correspond, and then try to swap valves and see if the values match.

Between both amps it sounds like you have at least one working amp. Should be easy to add them together and then cross reference to get both running.

Good luck.
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Old 14th November 2012, 04:18 PM   #19
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Join Date: Jun 2011
Before I have to run off I would like to say that construction techniques are important.

orientate tube sockets so the heater windings and leads are away from signal lines.

keep the parts mechanically stable by putting them between terminal strips then wire to the socket.

and also briefly looking at the schematics stay away from cathode bias unless you want to tweak the design to offset the uneven signal loading and amplitudes. use fixed bias method like a -40V supply adjustible type through 100k-220k resistors. like this:

Click the image to open in full size.
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Old 16th November 2012, 02:31 AM   #20
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So, I took the main advice and completely re-did the wiring, the only ones I didn't de-solder and re-soldered are in the power supply (middle section), the rectifier tubes and some heaters.

Also, I made DC measurements at several points for both left and right channel.

I have attached all this info on the post. (Original post has the component list and circuit diagram with the calculated values)

After re-doing the wiring and changing the resistor values for all cathode bias (turns out I had the wrong values). THE PROBLEM IS STILL THERE!

Left channel has CRAZY LOUD HUM.
Right channel dead quiet. Also before changing the wiring I unplugged the humming side and the right side did play music but it was a bit quiet and distorted.

Anyone has any advice? I thought for sure changing the connections would fix it, now I'm wondering if I have a busted tube or even OPT?

Thanks again to everyone who's taking time to help out.
Attached Images
File Type: jpg IMG_3207 - Copy.jpg (651.9 KB, 133 views)
File Type: png LEFT_DC.png (41.0 KB, 114 views)
File Type: png RIGHT_DC.png (41.4 KB, 108 views)
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