Asteroid unpleasing distortion

[romberg]

Take a look at this post found on another forum. In particular the copy of page 39 from the 1958 Radio and Electronics.

http://www.tdpri.com/threads/fender-cat ... st-7931643

Does that scope trace look familiar? It does to me. The cure seems to be to use BIG grid stoppers. Not just a little bigger (5k instead of 1.5k) but an order of magnitude bigger 50-100k. I don't think really big ones have been tried on your amp.

There is a maximum size for this (Merlin covers this in his book but I don't have it with me). But you can go way higher than a few paltry k. Try some stoppers on the power tubes of around 50k and see if that has any positive effect.

Mike

[arjepsen]

I actually tried 100k gridstoppers on the powertubes a while back.
But thanks for the suggestion

Hmm... maybe i could try a larger stopper on the pi. I only tried 10k there I think.

[Mant]

Any updates?

[arjepsen]

Not at the moment.
When I used the headphone amp to track the audio through the amp, the harsh sounds seemed to originate in the PI.
Unfortunately, I haven't been able to really make it go away from there.

So for now, I've been been working on a different amp.

[arjepsen]

Ok, it's been quite a while, but this weekend I got back to messing with the amp.
And I think I have solved the issue.

I was a bit concerned that there was going too much voltage across the pi, so I though about ways to lower the voltage a bit.
In the end, I remembered reading about putting some current limiting resistors on the rectifier, so I ended up trying two 100R/5W on each plate of the GZ34.
It's been a while since I messed with the amp, but now I couldn't hear any of the bad distortion I mentioned earlier. NICE!
I did some measuring around, and was surprised that there seemed to be a much lower voltage drop across one of the limiters, than the other. After turning off the amp, I could feel that one resistors was too hot to touch, while the other was just a bit warm..... Hmm...
It seems my rectifier had gone bad. I tried another one, and now the amp is finaly starting to sing!

I'm now trying to figure out how much wattage these resistors should be able to handle (they do still get a bit too toasty...).
However in merlins calculations on his website, he posts a formula for figuring this out: P = (1.1 × Idc)^2 × R
I'm slightly confused by this - I assume "Idc" is dc current through the resistor, but since there's ac going through them, how do I find this value?

[sluckey]

Measure the AC voltage dropped ACROSS the resistor. Square that voltage number then divide by the resistor value. This is the actual power dissipated by the resistor. Double the actual power rating for safety. That's the resistor wattage you need. May have to go to the next higher available wattage, ie, if you calculate 3.75 watts, go up to a 5 watter.

[arjepsen]

thx