K-502 Modifications (capacitors, etc.)?

[DerekVa]

OK. One more K-502M question:

I want to pull the volume pot from a K-502M so it'll run as a pure power amp. Should I just jumper the input/output connections to the pot on the circuit board? This is what I'd assume, but I've got a great track record of being mistaken...

Thanks!

-D

[EWBrown]

You have half of of it right, jumper the pot's PC board pads from the input to the output.

Since the pot is mounted "upside-down", the left side pad is the ground, the middle pad is the wiper / output and the right hand pad is the "input" (looking from the front of the PC board).

But, since the pot acted as the input tube's grid resistor, so you will have to add a grid resistor for each channel, and since the original pot was 100K, just use 100K, 1/4 watt resistors. The resistor can mount across the pot's ground (left) and input (right) pads, and then bend the "input" lead over and solder the end to the center "output" pad.

You could also connect the 100K grid resistor from the tube's input grid pin to the ground buss, either way is OK, it's just a matter of preference.

This way the input grids will have 100K resistor to ground.

/ed B

[TerrySmith]

Sorry to bring back an old thread but... I decided to play with my K-502 amp that I got a long time ago. Hum was getting really bad. All the electrolytics were replaced with Nichicon caps and the hum is gone! Looking at the old caps, they are the same brand that typically fail in newer TV's, etc.

[EWBrown]

The K502 & K12's original supplied electrolytic caps are rated at only 250VDC, which is exceeded when the amp is first powered on, before the tubes warm up,
and start conducting. Just call it bad basic designing on their part...

Replace them with same uF value caps, but with 350 or 400 VDC rating, then they should last a long time.

There was a question posted, a while ago, how to determine if an amplifier is Class A, just by looking at the schematic.

All single ended amplifiers must be Class A, they can be Class A1(no grid current) or Class A2, (draws grid current, needs to be driven through an interstage transformer). Class A means that the power tube draws current through the entire audio waveform.

Class AB1, AB2 are Push-pull. A PP amp can be Class A, and if the power tube cathodes are shared, then a cathode bypass capacitor isn't necessary, as the cathode current does not change. AB1 and AB2 require the bypass cap, else severe distortion, and reduced power output will result.

A seriously overdriven and underbiased single-ended amp will be driven into Class C, which is absolutely horrible for audio purposes, but is perfectly acceptable for
efficient RF power amplification, but then this is far off from the original topic

For a more technically accurate method, measure the power tubes' cathode-to-grid 1 voltage, and cathode / plate current, and then apply these to the the published tube curves and operating data to determine if it is truly Class A or not.

A Class A PP amp will deliver approximately twice the output power of a single ended amp, using the same kind of tubes. Class AB1 and AB2 can deliver up to 4X the output power of single-ended.

/ed B

[TerrySmith]

The K502 & K12's original supplied electrolytic caps are rated at only 250VDC, which is exceeded when the amp is first powered on, before the tubes warm up,
and start conducting. Just call it bad basic designing on their part...

Yep, I measured this and got 265v on a cold startup. The old caps feel really light, like they have dried out, maybe I'll bust one open and see what's inside.

[Shannon Parks]

Terry, what brand were they?

Shannon

[EWBrown]

I checked a couple of my K502 / K12, and K16LS (PPP 6AQ5s) the caps on one were all Samsung rated at 250V, the other has a mix of unknown brands (G-Luxon, Piberty (I think that is correct, it is in a weird stylized script font) and one Nichicon, but I think I put that one in form my junque box. They supplied the cheapest ones that they could find...

BTW, don't bother with the K16LS or it's weaker sibling K8LS (PP 6AQ5s), they are basically a POS

/ed B

[TerrySmith]

Both of mine have Samsung electrolytics. I've had good luck with Panasonic and Nichicon caps.