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Old 16th November 2012, 05:28 AM   #21
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Have you tried to run the amp with the NFB loop disconnected? Are you sure the phasing of the feedback is correct,and that it's Negative feedback,and not Positive feedback?

Also,I would put grid-stopper resistors on the grids of all the tubes. Maybe something ~1K ohm.
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Old 16th November 2012, 10:58 AM   #22
DF96 is offline DF96  England
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The left channel seems to be taking much more current. I'm not sure which is correct.

The output stage screen resistors should be mounted right on the g2 pin, so they act as screen stoppers. Not inches away on a tag strip. Generally you seem to have long wires. OK for a guitar amp with big grid stoppers, but not so good for hi-fi as it gives valves the chance to oscillate at RF.

Making the input stages mirror images of each other is a good way to increase the chance of wiring errors, and make subsequent fault-finding harder. There are good reasons why stereo loudspeakers are sometimes done as mirror images, but these reasons don't apply to circuitry.

There are lots of DC voltages missing from the circuits. On the right channel, is the lower driver cathode really at 385V?
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Old 16th November 2012, 01:48 PM   #23
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An oscope is invaluable here. What you want to do (I hate to give an engineer advice because I'm only a ET) is disconnect any feedback and troubleshoot each side seperately starting in the middle (I forgot what they called it in the Navy), preferably on the left side. See how much hum you have on the driver tube's grid, even 10mv, go backwards until it disappears. If there is none, then move forwards until it shows up.

If you have 385 volts on the cathode of that driver, the resistor has to be open or not connected correctly. One thing that would help us, is putting all the component values on the schematic, along with all voltages of the tubes.

I do like the new component placements. Looks alot neater, and no flying connections. Grid stopper resistors and screen stopper resistors need to be directly connected to the tube sockets. You need to add grid stoppers to the KT88s control grids. Usually 1-3K, preferably carbon composition, carbon film is ok too.

Can we see the top of the chassis?

Last edited by DeathRex; 16th November 2012 at 01:50 PM.
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Old 20th November 2012, 12:52 AM   #24
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Update!

The 385 [V] at the cathode was because of a wiring mistake. Unfortunately, I also mixed the grids for one KT88 and now the tube is toasted, however I had a replacement from a previous amp.

I disconnected the negative feedback... And holy smoke this thing works! And it sounds good!

The voltages from the PS are a bit high but I reworked the loadlines and they are actually pretty good. All the biases are around <1[V] from each other and from each channel, however now I'm dissipating around 6[W] from the cathode of the KT88s so I ordered some 10[W] wirewound resistors. I can't listen to music for too long because the solder gets hot and that bypass resistor comes off, this should be fixed with the WW resistors...

I guess this is good news! But I'm still confused why the NFB didn't work, and why it created hum on only one side... Jones Morgan's Valve book doesn't do a very good job explaining NFB, so I guess I'll have to research that.

If anyone has any input on the NFB, I would appreciate it. Also I'll be posting a picture of the top of the chassis soon.

And again, thanks to everyone who's thrown in their 2 cents... It has been really helpful!
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Old 20th November 2012, 01:00 AM   #25
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Ok, here is my 2 euro cents.

You dont have to pay for WW resistors. Just Use 5-6 2W cheap resistors in parallel to make the total resistance, usually the eylets in octal valves can accomodate that. Just keep the all the same value, or else u gonna fry the biggest R.
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Old 20th November 2012, 01:34 AM   #26
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With the cathode resistor doing around 6 watts, the KT88s should be going close to 40 watts each. Are you in class A doing push pull? KT88s can do 45 watts, but at 40 watts they will have a shortened life. You can increase that life by going into AB1 and about 30 watts each. You can also put a small negative voltage on the grid resistors of the KT88s of about 10-15 volts to help the tubes out and the cathode resistors. Of course with self bias you'll want a push pull output transformer at about 5K to 6K primary. Fixed bias is about 4.2K primary.

Maybe you have too much feedback. Put in a test signal and get 10 volts out of the amp. Replace the feedback resistor with a 22K and see what voltage you get out. Keep replacing the resistor with lower values until you get about 5 volts out of the amp. See what it sounds like with the resistor in and out.
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Old 30th November 2012, 08:33 PM   #27
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Ok, so the amp is working without feedback and it sounds pretty good, but I was expecting it to be much better. There is one last issue that I can't figure out:

The right channel sometimes you a hear some sort of a... sputter? spitting? It happens at loud volumes but I can't find a pattern, it seems to happen with high volume and at sudden quick surges of volume within the music. It even makes the cones on the speakers "jump". What can I look for to find what might be causing this? I'm a bit dumbfounded since it doesn't happen at low volumes and I can't seem to replicate it when I want it, also I don't want to damage my speakers hehe.

Any ideas??

Oh! And here's a pic of the amp:
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File Type: jpg 20121130_124255.jpg (596.5 KB, 65 views)
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Old 30th November 2012, 08:51 PM   #28
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Wanna add quickly that I've tried different speakers and the problem is still there, so its not the speaker.
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Old 30th November 2012, 08:53 PM   #29
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What are your rectifiers? Hook the feedback up, it will sound much better.
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Old 30th November 2012, 08:54 PM   #30
DF96 is offline DF96  England
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Could be RF oscillation. This can be sensitive to signal levels, because to the accidental RF oscillator you may have built the audio signal is a varying bias. Add grid stoppers to the output valves.

The problems when you close the feedback loop may be loop instability. Have you done a Bode plot? Four stages plus an OPT can generate a lot of phase shift at frequency extremes.
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