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Old 18th December 2010, 11:22 AM   #1
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Default Anyone care for some troubleshooting? Hum issue

Just finished my latest amp project, PP KT88 integrated. Itīs Triode Dicks BILL Click the image to open in full size.

with a converted PS that uses 2 GZ34 rectifiers

Click the image to open in full size..

Powered it up for the first time and I have hum. Strangely enough the hum dosnīt come instantly, it rather builds up to come to peak after about 30-40 secīs. I have tried pulling the rectifiers out with the result of NO hum. So I tried pulling the KT88īs out and NO hum. I also tried playing some music with the hum humming away and the music was slurred and lacked power. I should have known better but I also tried pulling the 6SN7 out and this resulted in sparks of one of the ECC99īs, I believe I fried it so I have to order new ones now, sucks!

there are pictures of the amp in the "pictures section"

Anyway, if anyone would be so kind to help me with this I would be very grateful.

Cheers
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Old 18th December 2010, 12:06 PM   #2
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How is the heater supply wired up? To gnd or a positive bias? The LTP resistor (10kohms) could be two 5kohms in series, giving a perfect 65volt bias for the heaters.

Pulling the 6SN7 effectively gives b+ on the ECC99s grids, so they may have been blown by that. Bad luck on that part, but such things happen
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Old 18th December 2010, 12:08 PM   #3
DF96 is offline DF96  England
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What is your grounding scheme? Having two HT supplies sharing a secondary winding creates opportunities for problems, although it can be OK if done properly. The two pairs of silicon diodes will be in parallel, so it is likely that only one of each will conduct. This could inject hum into the ground unless the two HT supples share a ground connection. What would happen if one of these sets of diodes was snipped out (as may actually happen if they don't conduct).
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Old 18th December 2010, 12:47 PM   #4
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Quote:
Originally Posted by SemperFi View Post
How is the heater supply wired up? To gnd or a positive bias? The LTP resistor (10kohms) could be two 5kohms in series, giving a perfect 65volt bias for the heaters.

Pulling the 6SN7 effectively gives b+ on the ECC99s grids, so they may have been blown by that. Bad luck on that part, but such things happen
Hello SemperFi

Not quite sure I follow you here. The heater supply is wired exactly as in the schematic. The KT88īs are biased by the bias supply -58V to cathode.... Please explain what the LTP resistors you are refering to are.

Cheers
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Last edited by h00hbt; 18th December 2010 at 01:02 PM.
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Old 18th December 2010, 12:58 PM   #5
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Quote:
Originally Posted by DF96 View Post
What is your grounding scheme? Having two HT supplies sharing a secondary winding creates opportunities for problems, although it can be OK if done properly. The two pairs of silicon diodes will be in parallel, so it is likely that only one of each will conduct. This could inject hum into the ground unless the two HT supples share a ground connection. What would happen if one of these sets of diodes was snipped out (as may actually happen if they don't conduct).
Hello DF96

The grounding is wired exactly as in the schematic. Both HT supplies have separate ground bus that goes to star ground.

What would be the option of having pairs of diodes? If you are right what would you suggest instead?

Here are better pictures of schematics:


PSU


AMP

Cheers
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Old 18th December 2010, 02:33 PM   #6
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I dont think the problem is in the schematic; lots of "bills" have been build without hum.
Could you make a picture of the inside of your amp?

You should probably never pull a tube from a working amp; you could fry al kinds of things, (including your fingers)

By adjusting the bias does the hum change?

Do you have a scope or multimeter?

Try measuring your b+, but in the AC setting of the multimeter, and give us the value.

Succes, Paul
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Old 18th December 2010, 03:08 PM   #7
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Quote:
Originally Posted by pauldune View Post
I dont think the problem is in the schematic; lots of "bills" have been build without hum.
Could you make a picture of the inside of your amp?

You should probably never pull a tube from a working amp; you could fry al kinds of things, (including your fingers)

By adjusting the bias does the hum change?

Do you have a scope or multimeter?

Try measuring your b+, but in the AC setting of the multimeter, and give us the value.

Succes, Paul
Hello Pauldune

Here is a pictura of the internal wireing

Click the image to open in full size.

Click the image to open in full size.

I can ensure you I never pull tubes from a powered up amp, I turned it of before I pulled the tubes and then powered it up.

I did try adjusting the bias but the hum didnīt change.

I do have a multimeter

I have measured all the voltagepoints and they are within 5% of the voltages in the schematic. I havnīt measured B+ vith AC setting on the multimeter. What would I be looking for?

Im not able to do any thing at the moment due to beeing out of ECC99 tubes that where frried, stupid of me.

Anyway, if you have any ideas of what I could check when I get my new ECC99 tubes please let me know and I will try them.

Kind regards......
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Last edited by h00hbt; 18th December 2010 at 03:16 PM.
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Old 18th December 2010, 03:29 PM   #8
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Default It's just a convenient place to bias up your heaters............

Quote:
Originally Posted by h00hbt View Post
Hello SemperFi

Not quite sure I follow you here. The heater supply is wired exactly as in the schematic. The KT88īs are biased by the bias supply -58V to cathode.... Please explain what the LTP resistors you are refering to are.

Cheers


If you replace the 10K R with 2 5K's in series, and attach your heater winding center tap between them, it will bias up the heaters to around 65V, which can help reduce hum by reverse biasing the cathode-heater filament in the tube. Sometimes hum can be caused by the current leakage of the pseudo-diode formed between the filament and the cathode.

If you have significant hum, I would check other stuff, like how the volume pot is grounded, how the tube sockets are wired for the heaters, etc.
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Old 18th December 2010, 04:19 PM   #9
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Quote:
Originally Posted by boywonder View Post
If you replace the 10K R with 2 5K's in series, and attach your heater winding center tap between them, it will bias up the heaters to around 65V, which can help reduce hum by reverse biasing the cathode-heater filament in the tube. Sometimes hum can be caused by the current leakage of the pseudo-diode formed between the filament and the cathode.

If you have significant hum, I would check other stuff, like how the volume pot is grounded, how the tube sockets are wired for the heaters, etc.
Hi Boywonder

Ok, you mean the CT winding for the ECC99 on the TX connected to CT in this schematic?

Click the image to open in full size.

I do have significant hum, the volume pot looks ok but what do you mean when you say "how the tube sockets are wired for the heaters"?

Thanks for suggestions

Cheers
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Last edited by h00hbt; 18th December 2010 at 04:30 PM.
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Old 18th December 2010, 04:46 PM   #10
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Take a 330K and a 68K 1/2W resistors.

Join 'em together.

Place the end of the 68K to ground and the end of the 330K to the B+++ line.

Disconnect the Centre-Tap off the 6SN7 Heater supply At The Mains Transformer--Currently its attached to Ground.

Attach the centre-point of the resistors to the point on the Mains-Trans. that you disconnected the Ground from....

Job done.....
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