Dr Z redplating

[nosaj]

I just had a conversation with the customer saying I will do the upgrades, and explaining that I don't think there is anything wrong with his amp except the design, and I can't give my usual warranty if it blows up next week or next month. He repsonded by saying he's used it 3 -4 nights a week for the last 2 years, so he will take that chance.

I gotta say, this has really tweaked my sense of what I thought was supposed to be acceptable amp design and tube operation. What were they thinking, "hey let's run those tubes at almost twice their design parameters"? I must admit, and I did tell him this, it is difficult sending an amp out of my shop fully expecting it to blow up again.

That's what Hot Rod cars are all about...Living on the edge and if it blows all to hell build another one.
Some people have more money than time and it's no big deal to plop down money for something.......me I have more time than money and I use it to learn more.
But you at least explained this to the customer and please do notate it on the service ticket and you won't feel bad when he comes back to let you repair it again.

nosaj

[Randall]

I replaced the cathode resisitors with 200 ohm 10 watt, and the screens with 500 5 watt. I am now measuring 23.2 v across the cathode resistor. 23.2/200=116 mA / 2 = 58mA for one tube. My Hoffman bias checker reads 59.5 mA. Not as bad as before, and the JJs are not redplating, but still very hot IMHO. Damn.

[Randall]

A follow up to this thread. The amp came in for it's yearly check up, and a fresh quad of JJ 6V6S. To my surprise, they were all idling at the same 57/58 mA as they were when installed a year ago, with B+ at 397v, which is 22/23 watts. He has used the amp a lot around town and on tour, and says it has been reliable. This still distorts my reality.

I installed a fresh set, and these are running hotter at 60/63 mA, B+ @ 395v. This puts them at around 24 watts. This is insanity. I'm tempted to tell him to keep the old ones in there, since they have proven themselves, but I'm not really sure about that. What an odd amp.

[Justin Thomas]

Well, at least it's a testament to the toughness of JJ 6V6s... I still wouldn't do it on purpose though.

Justin

[Chuck H]

I'm tempted to tell him to keep the old ones in there, since they have proven themselves, but I'm not really sure about that. What an odd amp.

Have him keep them on hand as spares I guess. But tell him to put them all in together as a set if it comes to that. As you said, they've proven themselves. Lucky for Dr. Z this tube is around. What would they do if we could only get NOS? I've pop corned a couple of golden age manufactured tubes in an SF VibroChamp that I use to service for a guy. Idle at 13 watts and 428Vp. The tubes were used RCA but tested "good". They both gave up the ghost while putting the amp to the dime test. I ended up with an Ei tube in there that survived. Probably not dissimilar from the JJ stuff, but that was many years ago and things change. The only 6V6 amp "I" own doesn't stress the tubes at all, I hardly use it and it has the same Ei's. They'll probably last forever.

[TomCarlos]

Dr. Z Remedy photos....

[Justin Thomas]

Thanks!

Jusrin

[HaroldBrooks]

I was going to say my guess is that it just eats tubes in it's stock form. Every cathode bias Dr. Z. redplates at idle, and biased over 100%. They stop redplating with signal, but still it seems crazy to design amp in mass this way. Anyone whose experimented a lot with cathode biasing will tell you the extra "tone" from biasing that hot is not worth all that money that could have otherwise been spent on pizza.

I never thought of it that way, but next time I have to shell out for Pizza for my band, I will think of this.

I have thought about every large household purchase in the currency exchange value I consider universal, that is : " How many vintage amps could I purchase if I avoided the new furniture and redecoration ? "

Not sure my wife thinks the same way...

[Randall]

Could part f the reason these tubes have lasted for this long be that the B+ is only 395v?

[Dave H]

I was going to say my guess is that it just eats tubes in it's stock form. Every cathode bias Dr. Z. redplates at idle, and biased over 100%. They stop redplating with signal, but still it seems crazy to design amp in mass this way.

You just have to make sure you never stop playing

I've simulated it. You can see that the plate dissipation drops as the output power increases so it's OK to bias at 90 to 95% but not over 100%

[Chuck H]

You just have to make sure you never stop playing

I've simulated it. You can see that the plate dissipation drops as the output power increases so it's OK to bias at 90 to 95% but not over 100%

[ATTACH=CONFIG]56546[/ATTACH]

Interesting. I was actually thinking this because I sort of live in el84 land. Where the power tubes are commonly biased at over 100% at idle in cathode bias even for amps that end up in an AB1 bias for most of their duty. But they just take it and don't red plate. 6V6's might be more tender about this. My personal amp (el84's) is biased close to 90% and I use the zener thing (you all know by now) to check the - bias voltage increase at a reasonable level that doesn't cause crossover distortion. Extensive testing has shown me that the hotter you bias with these small bottles, the more the bias V rises as current increases with signal conduction. So there's really no reason to bias at 100% or more for a push/pull amp. (<period)

EDIT: P.S. An AC30 is actually operating in AB1 when overdriven. Really. That "class A" thing for push/pull guitar amps gets pretty cloudy with sales promotion too. I've seen many review references that seem to straight up equate cathode bias with class A operation irrespective of any operating conditions. While it should be obvious that the bias method and the actual bias and operating conditions for the tubes is mutually exclusive, this seems to escape even some manufacturers in their advertised product descriptions.

But I digress.