Issues with voltages during testing

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Jac88
Posts: 4
Joined: Thu Apr 08, 2021 12:10 pm

Issues with voltages during testing

Post by Jac88 »

Hi Mark,

I hope you are well.

I have really enjoyed following your youtube tutorial as I build my Single Ended KT88 Amp.

I am currently in the testing phase and using a dummy load throughout the process. (I do not have an oscilloscope or signal generator).

The main issue I seem to be having is a much higher plate voltage (428V) on the 6AQ8 tube. I have a feeling that this is also the reason my cathode resistor voltage is also to high at 10.9V. The LED does not seem to light up like it did on the bench so I am guessing it has either blown or both are installed in the wrong direction (which I did check whilst wiring but could be wrong?).

I checked the 39K resistor whilst still soldered in the amp (power off). For some reason I was getting a reading of 32.5K Ohms on both resistors. I checked both of the spares I had on the bench and they measured 39K. Not sure if this means anything could this be the reason for the extra 200V I am getting on plates?

The other concern I have is regarding the KT88 cathode bias voltages which do not seem to match up on each side. I am not sure what this means. Maybe I need to change the voltage of the resistor? Also maybe the wattage as they do seem to get very warm.

I have attached a picture with my readings highlighted in blue to make it easier for your.

As always your assistance is much appreciated.
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blueglow
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Joined: Fri Feb 12, 2021 4:39 pm

Re: Issues with voltages during testing

Post by blueglow »

Make sure the LEDs aren't backwards or open. If they are, you aren't completing the circuit and causing a voltage drop across the input tube which might cause what you are seeing.
Use a DVM on diode mode to test them.
Jac88
Posts: 4
Joined: Thu Apr 08, 2021 12:10 pm

Re: Issues with voltages during testing

Post by Jac88 »

Thanks Mark.

It’s amazing how much the circuit was thrown out by one little component. Turns out I did have the resistors the wrong way around and now all the voltages measure great. Even tested the amp on some small old speakers today with a preamp I made. Worked without an issue thanks. Look forward to testing it properly on my Klipsch 600m’s on the weekend
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SLF99
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Joined: Tue Mar 02, 2021 4:15 pm

Re: Issues with voltages during testing

Post by SLF99 »

Is there a direction on resistors? Huh.
"The phlogistic theory is not without its difficulties. The chief of them is that we are not able to ascertain the weight of phlogiston, or indeed that of the oxygenous principle." - Joseph Priestley
Jac88
Posts: 4
Joined: Thu Apr 08, 2021 12:10 pm

Re: Issues with voltages during testing

Post by Jac88 »

Sorry guys, meant to say LED leads not resistor
Shoreman
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Joined: Sat Feb 27, 2021 3:00 pm

Re: Issues with voltages during testing

Post by Shoreman »

Yes there is a direction to wire leds .there a anode and a cathode pos. And neg they need to be wired right in a dc voltage.in ac it don't matter because it swings in pos and neg. When I wire in ac for 2 leds in curcuit I wire on in pos and one in neg so when one is off the other is on and vice versa hope this helps
KresimirKomar
Posts: 4
Joined: Mon Mar 08, 2021 9:55 pm

Re: Issues with voltages during testing

Post by KresimirKomar »

Hi everyone.
At the end of 2020 I finished building this amp and it sounds great.
I have done measuring voltages and there are some differences in numbers.
Please find attached my measurement after half hour amplifier was power up and I would appreciate for your comment and advise - should I leave everything as is or do some changes.
As you can notice some voltages I measure are quite different.

The voltage at the preamp tube’ cathode (E88CC in my case), where the LED is located, should be between 2.6-2.7V. I assume that this voltage is critical in driving the tube.
In my case it is 3.52V and 3.71V Is it critical that is higher then 2.6-2.7V?

I have used toroidal transformer for power supply :
240v Primary 380-0-380 secondary @ 200mA, 6.3V@5A, 5Vu@3A.

For output transformer I used :
HAMMOND 1628SEA
30 watts
Primary : 5000 Ohms
Screen tap 40%


Any comment and suggestion is appreciated.

Thanks,

Kreso
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SE-KT88 v6 Amp Only - Blueglow - Final Build - changes and measurements.png
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Tom_S
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Joined: Thu Feb 25, 2021 2:29 am

Re: Issues with voltages during testing

Post by Tom_S »

Hi Kreso,

I would start by trying to get your B+ higher. You are around 15% low compared to the original design. You can try adding a little more capacitance after the rectifier - maybe 10uf. Obviously other voltages will change in relation. I'm also wondering why you are getting different plate voltages on each side/channel of the 6DJ8.
KresimirKomar
Posts: 4
Joined: Mon Mar 08, 2021 9:55 pm

Re: Issues with voltages during testing

Post by KresimirKomar »

Hi Tom_S

Yes, this bothers me too, how the B+ is 15% lower than the original design. I have usted toroidal power supply instead of Edcore power supply  and Hammond choke with the same spec, but that is it. Also I notice one thing....after I switch off the amp the B+ drops immediately from 394V to 20V. I didn't put a bleeding resistor..could it be some problem with the capacitor in the power supply ?
As for the difference on the plate of the driver tube..the resistors I used have more tolerance then in original design..I think it is 5% instead of 1%
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Tom_S
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Re: Issues with voltages during testing

Post by Tom_S »

Ahhh...dopey me. The difference in plate voltages on the 6DJ8 is probably just a slight mismatch between tube sections.

Not exactly sure what might be causing the sudden B+ drop when powered down. I would guess it's the led on the cathode pulling more & more current as the B+ drops while the tube is still hot and conducting. But I see bleeding off all that voltage as a good thing for safety reasons at least.
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