[davehanc]

Hello,

The Schematic & Parts List show R4 as 220k 2w, but the Digikey part # given (P220KW-2BK-ND) returns no results, and I can't find a value above 1/2w at their site.
Is 220k correct, or should it be (perhaps) 2.2k?

Thanks
David

[PRR]

I'm not the Cymbal expert, but I've specified (and smoked) a lot of resistors....

> should it be (perhaps) 2.2k?

It surely is not 2.2K: the power supply would be very unhappy, and R4 or Q2 would fry.

220K is a reasonable value: it turns-off the red LED when plate voltage gets over 100V.

2 Watts seems, to me, to be a conservative value. A cheap designer might use 0.5W or even 0.25W (actual dissipation in normal condition is only 0.18W). But some 0.25W resistors are not reliable with 200V across their ends. And it has an important safety function, so over-specifying it is wise.

> I can't find a value above 1/2w at their site.

DigiKey has lots of power resistors, though hi-Ohm power resistors are getting rarer as high-voltage gets rarer in commercial/industrial gear.

Since it is "only" turning an LED on/off, and taps a very low impedance B+ filter, the "sound" of this resistor should be irrelevant.

Try PPC220KW-2CT-ND on catalog page http://dkc3.digikey.com/PDF/T053/1143.pdf

PR02 Type 5% Power Metal Film Fixed Resistors — 2 Watt 5083NW Series
A homogeneous film of metal alloy is deposited on a high grade ceramic body. After a helical groove has been cut in the resistive layer, tinned connecting wires of electrolytic copper are welded to the end-caps. The resistors are coated with a red, non-flammable lacquer, which provides electrical, mechanical and climatic protection. The encapsulation is resistant to all cleaning solvents in accordance with “MIL-STD-202E, Method 215” and “IEC 60068-2-45”.
CHARACTERISTICS: * Resistance Tolerance: ±5% * Temperature Coefficient: ±250 ppm/°C * Absolute Maximum Dissipation at TAMB = 70°C: 2.0W * Thermal Resistance, RTH: 75K/W * Maximum Continuous Operating Voltage: 500V (DC or RMS) * Operating Temperature Range: -55°C to 155°C

This is $3/5.

A cheaper, though ugly, alternative is P220KBBCT-ND 220K carbon-film 1/2W $1/10. In normal use, this is apparently within ratings. However at start-up, when the rectifier is almost-hot and the audio tubes are still heating, the B+ could soar momentarily to 300V which puts this resistor AT its ratings (0.4W is marginal, but the small body is rated 300V long-term). It would be safe with four P220KBBCT-ND in series-parallel to make a 220K <2W 600V resistor.

[hagtech]

David,

Try the "220KW-2".  Not sure what happened to the original panasonic version of this part.  DigiKey seems to run out of certain items once in awhile.  This time the part number even disappeared.

jh

[davehanc]

Thanks,
The digikey order has already shipped, but I can source a vishay locally which looks OK:

PF Part Number: 550322

Manufacturer: BC COMPONENTS VISHAY

Manufacturer Part Number: 2306 198 53224.
Description: RESISTOR, PRO2 5% 220K; RoHS Compliant:YES; Resistor element type:Metal Film; Resistance:220kR; Case style:Axial; Power rating:2W; Tolerance, +:5%; Tolerance, -:5%; Temperature, operating (a) max:155°C; Temperature, operating (a) min:-55°C; Voltage rating, DC:500V; Temperature coefficient, + (a):250ppm/°C; Temperature coefficient, - (a):250ppm/°C; Length / Height, external:12mm; Climatic category:55/155/56; Diameter, body:3.9mm; Diameter, lead:0.8mm; Length, lead:33mm; Temperature, power rating:70°C; Voltage rating, AC:500V

 David

[hagtech]

Looks fine.  PRR is correct, many times the resistor is big merely for the voltage rating.  I used to put two resistors in series, but that often cases confusion.

jh