12AT7 reverb tube for Deluxe AB763 circuit

[mikepukmel]

Good observation! When I replace roasted Rk's on reverb drive circuits, I generally use a 2200 ohm resistor, 1 to 3 watts depending what I have on hand. I've seen Fender use 2200 half watts down to 470 ohms. This tube does take a beating, and IMHO there's more than enough drive signal to spank the tank. A little reduction here isn't such a bad thing. I like your proposal for a branch off the main B+ supply too. There also the values can be altered to suit whatever you have on hand or can get cheap. For instance 10K, 10 uF, close enough for rock n roll. Especially if you're entertaining your neighbors with Jeff Beck licks. What must be going thru their minds, "are we being visited by flying saucers??? Martha, go have a look out the window!"

Hi Leo, re roasted Rk's, is that cathode resistor? Higher value = larger bias, which means smaller plate dissipation? This amp has 2K2 on the reverb driver, 800R on the reverb recovery. For lower dissipation, would you go up, in value from 2k2? How far?

Re Chuck's suggestion on branching a B+, I have a vague idea, but not quite sure what this means. Instead of taking the B+ for the plates for this tube from point B on the schematic (first filter cap), I would add a dropping resistor, and another filter cap (which I have a few extra), off that point (point B on the Deluxe AB763), then feed the reverb transformer from that? OK I will draw up what I think the suggestion is. and post.

Regarding my guitar playing, I love to play, but have no problem with admitting that most other humans, and 99% of dogs won't appreciate my versions of Jeff Beck. So far the police haven't showed up.

[mikepukmel]

Is this what you guys mean by branch off he main B+ supply? I have room in the chassis, but not sure where a good place to put this for noise, etc purposes.



IS the idea: don't muck up the voltages and currents going down the chain to the preamp and PI, but put a branch here to drop V to the reverb transformer?

R = 10k 2 watt. I don't know how to estimate the current here, so I don't know what the voltage drop would end up being, but the two existing dropping resistors in this amp are 10k as well, so I have a a bunch extra (always order 10 or 20x due to shipping).

C = 10uf? I have a few extra 16uf 450v, would those work?

Thanks!!

[mikepukmel]

Akk, sorry Chuck, I missed your post, please ignore above.

"Honestly, just because it's pretty easy I would just branch a 15k/5W resistor and add a 22uf/450V filter to the end of the branch. "

OK when you say 'end of branch" you mean, off the point like I made a crude drawing above?

[Enzo]

OK I get it, Fender didn't do it since they were a production shop, fewer components, cheaper, easier to manufacture, and ... the amps had good longevity.

In other words, they had no problem there to solve.

C = 10uf? I have a few extra 16uf 450v, would those work?

Remember? Just a guitar amp? Of course they would work.

This amp has 2K2 on the reverb driver, 800R on the reverb recovery. For lower dissipation, would you go up, in value from 2k2? How far?

How much dissipation is that 2200 ohm resistor allowing NOW? Measure the voltage across it and use Ohm's Law. And before you try changing anything, what is your TARGET dissipation? You need to have a goal before you start changing things.

yes, your drawing is an example of a branching B+ supply. A DC power supply isn't going to radiate a bunch of stuff, so placement is not much of an issue.

[Leo_Gnardo]

Hi Leo, re roasted Rk's, is that cathode resistor? Higher value = larger bias, which means smaller plate dissipation? This amp has 2K2 on the reverb driver, 800R on the reverb recovery. For lower dissipation, would you go up, in value from 2k2? How far?

Rk = cathode resistor, yes. Enzo did a calculation with 470R as the Rk value, 77 milliwatts dissipate in the resistor. You're not gonna wreck a half watt resistor in this position whether 470 or 2k2 or in between. Well, as long as the 12AT7 doesn't short . . . but I see a few cases each year where it has shorted, and there's the resistor sitting there with the stripes burnt off it, looking like a mouse size hot dog that was left on the grill an hour too long. (Play Frank Zappa's "Burnt Weeny Sandwich" here.) I figure there's a better chance for the resistor to survive using a larger wattage rating, but that starts to bring up the question, should we depend on cathode resistors to act as fuses? Hm, probably not especially carbon comps which have a disconcerting habit of reducing their resistance when roasted, thus making matters worse, instead of going open circuit.



You never know, the local gendarmes may eventually respond to your forays into Jeff Beck territory. I have a friend who's a police lieutenant in a nearby town, he's a big Jeff Beck fan. As long as it's not 3 in the morning...

[Enzo]

So you get Wired at 3AM and start cranking it out, and they call the cops, who then demand a Blow By Blow report.

[Leo_Gnardo]

So you get Wired at 3AM and start cranking it out, and they call the cops, who then demand a Blow By Blow report.

And if the cops give you a hassle, sneak on over to Frankie's House to continue the jam. It's so obscure the fuzz will never find you.

[bob p]

So you get Wired at 3AM and start cranking it out, and they call the cops, who then demand a Blow By Blow report.

... and don't forget to Stand On It while you're wearing your Led Boots. You do this Night After Night because it's just another Day In The House. (I could go on and on...)

[Chuck H]

Just like you drew it BUT!.. You have the wrong cathode resistor value on your recovery circuit. The reason the recovery cathode is 820R on all the Fender amps is that it's a shared circuit with another gain stage. Since you added this circuit I'm almost certain that your reverb recovery stage cathode circuit isn't shared with any other cathodes.?. If not it should be 1.5k. That may even help stabilize things since it will increase the headroom of the recovery stage.

[mikepukmel]

Just like you drew it BUT!.. You have the wrong cathode resistor value on your recovery circuit. The reason the recovery cathode is 820R on all the Fender amps is that it's a shared circuit with another gain stage. Since you added this circuit I'm almost certain that your reverb recovery stage cathode circuit isn't shared with any other cathodes.?. If not it should be 1.5k. That may even help stabilize things since it will increase the headroom of the recovery stage.

Thanks Chuck, went back and studied the schematic and also studied what I hacked in, and I *think* I got it right. 99% sure. Maybe 98, or 95. Definitely over 80% ...

The reverb recovery, and extra gain stage right after the wet/dry mixing resistor/cap are in the same 12AX7 tube, so that 820R is feeding both sides of that tube, just like in the schematic. In that tube, pins 3 and 8 are tied together and go to that cathode resistor/cap.

[mikepukmel]

And if the cops give you a hassle, sneak on over to Frankie's House to continue the jam. It's so obscure the fuzz will never find you.

I'll use the secret passage (well ok not secret now that Im posting it on the internet) under the back porch, across our back yard, through the swamp to the next road over in the development, then off I go! Since the amp is so light weight, I'll have no problem carrying it and my guitar! They'll never catch me!

[mikepukmel]

Rk = cathode resistor, yes. Enzo did a calculation with 470R as the Rk value, 77 milliwatts dissipate in the resistor. You're not gonna wreck a half watt resistor in this position whether 470 or 2k2 or in between. Well, as long as the 12AT7 doesn't short . . . but I see a few cases each year where it has shorted, and there's the resistor sitting there with the stripes burnt off it, looking like a mouse size hot dog that was left on the grill an hour too long. (Play Frank Zappa's "Burnt Weeny Sandwich" here.) I figure there's a better chance for the resistor to survive using a larger wattage rating, but that starts to bring up the question, should we depend on cathode resistors to act as fuses? Hm, probably not especially carbon comps which have a disconcerting habit of reducing their resistance when roasted, thus making matters worse, instead of going open circuit.



You never know, the local gendarmes may eventually respond to your forays into Jeff Beck territory. I have a friend who's a police lieutenant in a nearby town, he's a big Jeff Beck fan. As long as it's not 3 in the morning...

I've only ever met one member of our police department. Not long after we moved here in 1996, a pissy neighbor (who never spoke to me or asked me to move my car) called the cops that I parked my car overnight in *front* of my house, after November 15th. There are no street signs, but it is supposed to be "well known" that no overnight parking due to snow regulations. Instead of giving me a warning, he wrote the ticket out and snarled "Its WELL KNOWN that you can't park overnight ..". These are the same neighbors who had their kids friends park in front of my driveway, overnight, all year long. So, not looking forward to meeting that officer again. Don't know if he's still on the force, but I bet he'd love to write out a handful of disturbing the peace tickets.

[mikepukmel]

Oh boy, burned cathode resistors due to shorted 12AT7 tubes. Another sleepless night. Smaller wattage resistors that fail open?

[Chuck H]

I've never had a shorted preamp tube !?! That being my experience I would think it's rare, but I don't know. I did recently toss a 12ax7 that was drawing excess idle current in a PI circuit. I noticed a 30VDC differential between the triodes and checked it out. Tube was bad, but not shorted. Unfortunately it was a gray plate RCA with only a few tens of hours on it. But it happens. I get new preamp tubes that are noisy sometimes. I've had preamp tubes do something weird where they pass DC through the grid onto the input of the amp. You couldn't turn the amp all the way down. Even at zero there was still a small signal and the pot was scratchy. Interesting. And of course I've had mechanically microphonic tubes. Grossly. Tap them and it sounds like someone hitting a bag of glass with a hammer. Anyway, I've never seen a shorted 12a*7 tube. But you did check current in the circuit and that seemed fine. Huh.?.

[Leo_Gnardo]

I've never had a shorted preamp tube !?! That being my experience I would think it's rare, but I don't know.

I work on at least a hundred old Fenders each year besides all the other amps that find their way here. So I get to see "all" the problems, and sometimes get surprised with a new one. In the case of a defective reverb drive tube, it's sometimes a matter of deductive reasoning. HOW did that cathode R get roasted? Sometimes the tube seems to be acting OK, in other cases it's obvious, I can see tiny lightning bolts coursing thru the 12AT7's interior accompanied by hearing the reverb sound like someones whacking an anvil with a hammer at the far end of a subway tunnel. Less of a hazard to the Rk, sometimes I hear a less present clack clack clack at random intervals, again at the far end of the tunnel, and it goes away when I replace the reverb drive transformer. Sometimes it's both - current spikes thru a dying drive tube may cause arcs in the primary of the transformer. In amps where there's a cathode bypass cap on the reverb drive tube, and the resistor is roasted or sometimes even broken in two, that cap is often driven to failure, sometimes a dead short, by having been subjected to voltage way beyond its rating. Some may scoff but like Ripley "believe it or not," I've seen it plenty of times.

Also tubes such as you describe Chuck, with DC leak to grid, excessive crosstalk, sound like a box of busted glass, yup I've run across them too. Plus pre tubes that make pop noises, ticks, whoosh, whistle, howl, moan, rattle like chains of Marley's ghost, and the best of the bad ones that spew noises I can only describe as "Gas Music From Jupiter."