• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Help adding CRC to tube amp

Hello team. I need help/advice.

I have an old console stereo tube amp with an EZ81 rectifier. The amp runs fine but hums. It currently has a single 47uf cap as a filter on the DC side after the rectifier. I want to add an additional resistor and cap to try to limit the hum further. So, your thoughts on value of resistor and cap please? The DC voltage is about 220v from an initial AC voltage of 240.

Output tubes are ECL82

50uf is the max for the first cap post the EZ81 tube according to tube specs.


taking small exception to a R value.

I would replace the old capacitor at the same time. Use two 47µF 450VDC electrolytics,
with a 22 Ω 5W resistor between them.

Not bad advice, but the “corner” (–3 dB point) of the R and C2 is
F = 1,000,000 / (2 π RC) … Ω, µF
F = 1,000,000 / (6.28 × 22 × 47)
F = 153 Hz​
Better would be to use a somewhat larger resistor. If one shoots for 50, or 60 Hz divided by e (2.71828), it is pretty optimal. Luckily, it fits right back into the same formula:

F = 1,000,000 / (2 π R C)​
then by moving variables around
R = 1,000,000 / (2 π F C)​
F = 50 Hz ÷ 2.71828
F = 18.4 Hz
C = 47 µF​
so, now subbing in
R = 1,000,000 / (6.28 × 18.4 × 47)
R = 185 Ω​
This will have a substantially stronger hum-suppressing effect, but at the same time not likely to drop B+ very much. An “old amp” isn't going to draw more than what, 50 mA?
E = I R
ΔE = 0.050 A × 185 Ω
ΔE = 9.2 V​
So, 220 - 9 … is about 210 volts.

Plenty still, to “get 'er done”.

⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅

PS: alternatively, one could use a larger capacitor, since larger-value capacitors are brilliantly cheap these days!. If you still want the very modest ΔV of the 22 Ω resistor, then just use a proportionately larger capacitor. For example

R = 1,000,000 / (6.28 F C) … then with rearrangement
C = 1,000,000 / (6.28 F R) … and subbing in from above
C ≈ 160,000 ÷ 18 Hz × 22 Ω
C ≈ 400 µF​
And since capacitors-that-are-cheap-and-good are the ones that come in commercially competitive E–6 value sequences ( 10→15→22→33→47→68 …], then just go for the nearest larger value
C = 470 µF​
(edit…I said 'tada', but realistically, the 22 Ω resistor's backward facing current draw will exceed the vacuum-rectifier's nominal handling.

I take it back. Better to use an even larger resistor. 330 Ω to 470 Ω, again in E–6 [10→15→22→33→47→68] values, for easy access and low cost.

⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅

PPS: the output power will be impacted only by
dB = 20 log10((B+ -ΔV) / ( B+ … without CRC ))
dB = 20 log10(220 - (0.050 × 330 Ω)) ÷ 220)
dB = 20 log10( 0.925 )
dB = –0.6 dB​
Hardly enough to even slightly worry about. … GG
Last edited:


2020-01-01 2:22 am
Okay. Went with CRC of 47uf 220ohm 47uf as this is what I had available. As Goat Guy predicted there was hardly any change to DC voltage. The amp is single ended - one output tube per channel. The hum is half what is was and perfectly acceptable to my ears now. Thank you all for the advice. I had no inductors or chokes in stock.