My first foray into the KT88 world

Looking at the schematic, I realized that the first stage cathode capacitor I bypassed was not tied to ground directly, but was part of a feedback circuit, I will remove them.
 
I removed them and I still have noise.

Before adding a second 4.7uf 630V CBB capacitor and a 0.1uf 1000V Ceramic disc, I don't remember hearing noises.

I think this is a grounding problem because filtering caps are suppose to filter, not adding noises.

Edit:
I removed thermistors on rectifier inputs to be sure I do not have a noisy thermistor and still have funny ticking, buzzing noises in both channels.

Removed the 0.1uf 1000V ceramic disc bypass cap for the B+ cap, did not help.

The next thing to try is to remove the 4.7uf 630V CBB bypass capacitors for the B+

Not funny because it started to sound very good before the problem happens.
 
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And note, this is not the HV center tap that goes to the positive of the small capacitor at the right, it's the center of the 3.15V 6SN7 heaters, one of the resistors leads to the positive B+ and the other to the negative.

I don't really understand this sheme.

If I had the heaters center tap to the ground, eliminating the small cap, the HV center tap to the ground (the negative of the 1st caps to the ground), the negative of the B+ cap to the ground, and everything that is marked as "to the ground" on schematic , to the ground, can I have less issues ?

Edit:
I read and understand that the heater center is elevated, I don't know at which voltage.

What happens is weird to me, suspected the preamp tube first, and seemed to first be corrected by replacing tubes, but the problem came back, maybe it was intermitent but more frequent now, I have the same noise in both channels...
 
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And note, this is not the HV center tap that goes to the positive of the small capacitor at the right, it's the center of the 3.15V 6SN7 heaters, one of the resistors leads to the positive B+ and the other to the negative.

I don't really understand this sheme.

If I had the heaters center tap to the ground, eliminating the small cap, the HV center tap to the ground (the negative of the 1st caps to the ground), the negative of the B+ cap to the ground, and everything that is marked as "to the ground" on schematic , to the ground, can I have less issues ?

..
Can you draw this circuit out as your description is hard to follow.
 
I posted the schematic here:

http://audiokarma.org/forums/index....to-the-kt88-world.829357/page-2#post-11775811

-> http://audiokarma.org/forums/index.php?attachments/schematic-jpg.1231584/

But the heater center tap elevation is not on the schematic

I think this looks like what you find here beside of The Heater Elevation paragraph

http://www.valvewizard.co.uk/heater.html

But instead of going to the ground, this is the B+ cap negative.

Please look at this picture: https://www.cjoint.com/doc/18_07/HGkriBpp0qs_IMG-2155.JPG

This is the small cap at the right.

PS: The chassis use RCA input jacks for ground point, besides of that, nothing else is really grounded in this amp.
 
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The amp is back to original condition, removed all decoupling caps and thermistors I previously added, even re-checked the B+ cap solderings, and that ticking, low level popping noises continue, only as the KT88 cathodes gets very hot, after the time they can be able to begin to make sound at startup I think.

Ever seen Shuguang KT88-98 making noises in a single ended amp, and in both channels at the same time ?

I have to purchase a new pair of KT88 to see, I hope the tubes are the main problem.
 
My problem was one of the tubes, I installed other tubes and it's fine now.

I remember to have bumped one of the KT88 base on the power transformer when removed it, maybe the soldered connexion inside the tube pins broke , I will try to save this tube by solder inside the pins.
 
Works fine since I have the Genalex Gold Lion KT88

I decided to not install thermistors to the rectifier input, because of possible difference in resistance between the positive and negative phases, I installed two 120 ohms in series to the AC input instead.

I replaced the two preamp tubes second stage 100uf 100V cathode caps for two 100uf 200V

I installed two 4.7uf 630V CBB capacitors to the B+ capacitor and six 4.7uf 450V CBB capacitors to the preamp tubes plates capacitors and second stage cathodes capacitors.

Picture of what it looks like now.

Higher resolution pic: https://www.cjoint.com/doc/18_09/HIbmLRdbNys_IMG-4.jpg
 

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I am tempted to remove the 6N8P cathode feedback that go to the 8 ohms speaker output , tie the 2.7K resistor and electrolytic to the ground and add a 4.7uf bipolar bypass cap in parallel.

I think that I must not add a bipolar bypass cap to the cathode capacitor if I use in the feedback, so if I want to improve it, removing the feedback loop is is the only way.

If I do the move, I think there is no reason anymore to have the center tap ot the 6N8P heaters "elevated" and will be better "grounded", no ?

A 120 ohms resistor between the center tap and ground is a good value to use ?
 
Grounding of the heater circuit is needed for lowest hum. Even if there is a voltage bias on the heater I believe you still need to ground it.
 
The center tap is connected to a cap that have positive connected to a 120K resistor to the positive of the B+ cap , and with a 46K resistor to the negative as bleeder for the two caps, the negative to negative of the B+ cap.

I read that someone recommend to ground the center tap with a resistor like 100 ohms or higher, so I am not sure.
 
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After removing the cathode feedback, avd grounding the chassis with the 3rd prong of AC power cord, I feel like the volume control is very lower for the same output power.

I also grounded by passing through a 120 ohm 2W resistor the center tap of the 3.15V heaters.

I hear a very slight hum when I put my ear close to the woofers, but not excessive.

I added a resistor for the center tap grounding because I read that if the heaters circuit becomes shorted, the resistor can act like a fuse.

I am not sure if 120 ohms 2W is or is close to the best value I can use, maybe not perfect but fine.


Edit: I read again, and it's writen (anything up to 100 ohms) here under the Transformer Centre Tap section : http://www.valvewizard.co.uk/heater.html

So I ordered 75 ohms 2W Metal Oxide resistor, maybe the hum will be reduced with a 75 ohms resistor instead of a 120 ohms one.
 
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As there is very little current that is supposed to go from the PT center tap to ground, the resistor is supposed to act like a fuse, and hum will be lower with a lower ohmic value resistor, would I be safe to try a 0.12 ohm 1W resistor ?
 
I removed the cathode feedback and replace the 2.6K + 100 ohms by a single 2.7K , and add a 4.7uf CBB.

That caused slight hum, maybe because of too high gain.

I ordered 5.6K resistors for the first stage cathode to ground instead of 2.7K , maybe diminishing the first stage bias will slightly diminish the gain and hum noise, I hope.
 
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I received the resistors and replaced the first stage cathode resistors by 5.6K and the hun is lower.

The gain is slightly lower, (but still very higher than before removing the cathode feedback), but confortable.

Sound great.

index.php
 
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The hum level still annoys me when there is silent passage in music or no music, I ordered 10K and 16K resistors to try, as well as new capacitors.

Maybe I will have to work on the grounding scheme.
 
I think I know what the problem is, the KT88 6.3V heaters are not center tapped.

There is two independent windings, one for each tube , I have some 180 ohms 3W resistors, I will try to make a virtual Center taps with that
 
Done, and now I only hear slight hum at 8 inches from speakers.

I Added a 0.022uf 2KV cap between the rectifier anodes

I also added 0.1uf 1KV ceramic discs to almost of the cacacitors, except for the KT88 cathode caps.

I plugged back the 3rd prong of the AC socket to chassis.

What it looks like now: https://www.cjoint.com/c/IAwrkuJXJXs

And he sounds very good, very clean and detailled.
 
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I have more hum in the left than the right channel, maybe because it is closer to the PT , low hum compared to before but...

I will try 100 ohms 1W resistors.
 
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