Tube amp RC-Filter and power supply bypass capacitor voltage rating

Thread Starter

Ryuji

Joined Apr 9, 2022
12
Hi everyone

I have a single-ended headphone tube amplifier and I'd like to upgrade some of its components. I understand that when talking about audio equipment everything ends up in a debate. To be perfectly honest, I'd like to avoid that. I really would like to see this project through and decide for myself if it was a waste of time and money or not. But some help in choosing the correct values for capacitors would be amazing.

The components in question are the electrolytic capacitors of a low-pass filter. They are rated at 22uF and 500V. I'd like to replace them with foil capacitors. The capacitors I had in mind, which I believe are built well and don't cost too much, only have 450V. Would that work as well?

I am also planning to add some foil capacitors in parallel to the power supply capacitors (220uF 450V). Would the parallel capacitors also need to be rated at 450V or could I go a bit lower?

Someone, who I thought had a lot of knowledge about this, but now I have my doubts, recommended me to add some parallel capacitors to the cathode bypass capacitors aswell (220uf 16V). He recommended me to use 10uF in this specific position because it provides, from 500 to 20000 Hz, the same electrical path as a 220u cathode bypass cap.

Is 10uF a good value for this?

I'll attach some pictures of what the amplifier looks like inside and the circuit design (not sure if the PSu schematic is correct).

Thank you!
 

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MaxHeadRoom

Joined Jul 18, 2013
25,211
Normally you would use a lower value below 1uf for a electrolytic HF decoupling by-pass, if this is what it is intended to do.
Ceramic version, usually.
 

MrChips

Joined Oct 2, 2009
26,099
What is the +B voltage?
You want the capacitor rating to be higher than +B. For example, if +B is 350V, you want the smoothing capacitors to be about 450V.

As for the C2 cathode bypass, this affects the gain of the amplifier.
Look at the cathode resistor and calculate or look up the cutoff frequency of an RC low pass filter.
For example 1kΩ//10μF rolls off at 16Hz. There is nothing to gain by going higher than 10μF.
The voltage rating of C2 should be about 20-50% higher than the voltage on the cathode. A rating of 50V should do.
 

Thread Starter

Ryuji

Joined Apr 9, 2022
12
What is the +B voltage?
You want the capacitor rating to be higher than +B. For example, if +B is 350V, you want the smoothing capacitors to be about 450V.
At the risk of making a fool of myself.

I don't know, can I measure it?

Where exactly in the amplifier is +B? Where do I connect the voltmeter?
 

MrChips

Joined Oct 2, 2009
26,099
But do the rc-filter caps smooth the voltage to the 6922 input stage tube or the EL84 tubes?
Both.
Measure the voltage at the plate of the EL84.
Also measure the voltage at plate and cathode of 6922 tube, all measurements with respect to GND.
 

MaxHeadRoom

Joined Jul 18, 2013
25,211
If I understood correctly B+ is the plate voltage of the power tube.
My electronic experience goes back to the tube/valve age, the power supply was always referred to as B+.
Per Wiki:
  • B+, the plus voltage of a battery or plus voltage of an electronic circuit.
The power supply caps smooth the ripple of a rectified supply and decouple the power supply conductors in both AC & DC fed types..
In larger AC powered tube circuits, this often consisted of a Pi filter , electrolytic-inductor-electrolytic cap.
.
 
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Thread Starter

Ryuji

Joined Apr 9, 2022
12
Look at the cathode resistor and calculate or look up the cutoff frequency of an RC low pass filter.
Could you do me a solid and mark which one/s is/are the cathode resistors? What is the ideal frequency cutoff?

Also while we are at it, is the schematic even accurate?
Thanks and cheers
 

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Thread Starter

Ryuji

Joined Apr 9, 2022
12
We don't know what power supply is driving this circuit. It can be any voltage.
Seems like 180v is very plausible for a single ended headphone amp.

So now that we know that B+ is 180V, the smoothing capacitor could even be 400V?
 

AlbertHall

Joined Jun 4, 2014
12,023
You don't have any voltage higher than 180V. Hence capacitors rated for 250V are fine.
If the power supply uses solid state rectifiers then the initial voltage will be higher until the tubes warm up and start drawing current. The capacitors will see this higher voltage and must be rated to handle it. Measure the voltage immediately after switching on from cold.
 
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