12AT7 reverb tube for Deluxe AB763 circuit

[Enzo]

Over the decades, I saw most everything imaginable. But I have to say, if I were to make a list of problems I expect to see, a frotzed reverb drive cathode resistor would not make the list.

[mikepukmel]

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."

The chopstick test showed that BOTH brand new EH tubes are microphonic (12AX7 and 12AT7), so that is where some of the bad funkyness is coming from (turn up the reverb and its on the edge of feedback, and some other audible artifacts) So, out of the kit I just bought: 1 knob, the set screw is so loose it doesn't hold the knob on (bad machining job on the phenolic or whatever plastic its made of), 2 out of 2 tubes are no good. And my brother was over to check out the amp and noticed that one output tube is glowing a little more blue than the other one. I didn't check the bias on both tubes this time around, just one, the 'hot one' and its about 24ma into 410v to 414v or about 80%. Will one tube toast itself earlier than the colder one? Should I bias the tubes separately, i.e. put in another bias pot and hook up the two output tubes to separate bias sources?

[mikepukmel]

One of the bias methods uses DC resistance on either side of the output trans primary winding. I got 199ohms and 210ohms, so just about 5.5%. Is this a reasonable difference? Would this explain the difference in visible glow between the two output tubes?

[Chuck H]

Nearly all preamp tubes are microphonic such that they do ping or ding when you strike them with a chopstick. If they notably rattle or make a grump grump sound when thumped then they are bad for anything in a guitar amp. Otherwise, a ping or ring sound just means they may not be useful as the first gain stage in a high gain amp. Most preamp tubes ring or ding a little.

The blue glow is nothing to worry about. The difference in blue level is unusual though. I would definitely check plate, screen, bias voltage and current for individual tubes in that case. Probably no big deal, but I've only seen it with mismatched tubes. 199/210 ohms would be better than average for many OT's.

[mikepukmel]

Nearly all preamp tubes are microphonic such that they do ping or ding when you strike them with a chopstick. If they notably rattle or make a grump grump sound when thumped then they are bad for anything in a guitar amp. Otherwise, a ping or ring sound just means they may not be useful as the first gain stage in a high gain amp. Most preamp tubes ring or ding a little.

The blue glow is nothing to worry about. The difference in blue level is unusual though. I would definitely check plate, screen, bias voltage and current for individual tubes in that case. Probably no big deal, but I've only seen it with mismatched tubes. 199/210 ohms would be better than average for many OT's.

Thanks Chuck! Also, thanks for the info on preamp tubes. I went up the line, from V1, v2 etc. and none of the others made this much noise in the output, just the 2 reverb tubes. I don't have a good description, but like you tapped your finger on a microphone when it was on. Didn't ring, or make any sound that lasted, or sound like it was going to fall apart.

I'll let it warm up well and get all new V measurements.

[Chuck H]

FWIW when I stick a preamp tube into the first stage of a high gain amp, crank it up and tap the tube, I'm very happy if it only goes ping. That's a keeper. The best I've heard will just give a dull tap sound, as you described. That's rare. None are silent. Not in a high gainer anyway.

[mikepukmel]

All below is with the guru extra filter node mod.(for reverb. mentioned earlier in this thread) The 10k resistor didn't drop the voltage into the reverb primary transformer much. 397 to 392. Will try double the resistor. I had a big 2 watt 10k on hand.

Fiddled with the guitar pickup height, and all in all, the tone is excellent and its a quiet amp talking into consideration single coil pickups.

I re-measured all the tube voltages. Swapped the two output tubes, and the little purple/blue halo near the bottom of one of the 6V6 follows the tube (it was in the outer position, moved it to the inner). I turned all the lights off and don't see the plate turn red anywhere.

Let the amp warm up, played some torturous notes (lucky for the neighbors its winter and all the windows and doors are closed ,
then re-measured (values written down this time).

With the fluorescent lights off and with the guitar aimed just right, the amp is *very* quiet, even with reverb on. (chassis is out, and the reverb tank is sitting a foot away, adjacent to the chassis.).


6V6 (outer socket)
PIN VOLTAGE
---------------------------
1 -36.9
3 431
4 432
5 -36.9
6 434
8 0.020

Verified voltage across the 1 ohm resistor (pin 8) 0.021v


6V6 Inner Socket
PIN VOLTAGE
-------------------------
1 -37
3 433
4 433
5 -37.1
6 434
8 0.020

Verified voltage drop across the 1 ohm resistor on pin8 0.020v

Note that after I swapped the output tubes the voltages are almost the same.


12AT7 Phase Inverter
PIN VOLTAGE
---------------------
1 194.1
2 48.2
3-8 75.6
4-5
6 183
7 50.2
9 3.21vac



12AX7 Reverb Return
PIN VOLTAGE
--------------------------
1 186
2 0
3-8 1.6
6 184
7 0

12AT7 Reverb Driver
PIN VOLTAGE
--------------------------
1 392
2-7 0
3-8 8.1
4-5
6 391
7
9 3.21vac


12AX7 V2 channel 2 preamp
PIN VOLTAGE
----------------------
1 178
2 0
3 1.55
4-5
6 184
7
8 1.57
9

12AX7 V1 Channel 1 preamp
PIN VOLTAGE
---------------------------
1 178
2 0
3 1.55
4-5
6 188.9
7 0
8 1.57
9 3.20VAC

[mikepukmel]

Wish I could buy you all a beer or bowl of chili or something like that, for all the help, great advice, and absolutely wonderful sense of humor dealing with people like me who know nothing.

But seeing as how that would be tough to do, is there a pay pal account to donate a few bucks to the "Music Electronics Forum" business costs?

[g1]

Look up at the top of the page for the green button that says 'donate'.

[mort]

Look up at the top of the page for the green button that says 'donate'.

You know, I have never noticed that before nor had it ever dawned on me to consider donating some funds to the forum. The forum has treated me very well over the years. Things are tight at the moment but as business picks up in the spring I'll be sure to use it and will make a habit of it. Thanks for mentioning it.

[Justin Thomas]

Wish I could buy you all a beer or bowl of chili or something like that, for all the help, great advice, and absolutely wonderful sense of humor dealing with people like me who know nothing.

If you ask nice, and donate for the cost of ingredients, I hear Enzo makes a mean chili...

Justin

[Chuck H]

...for all the help, great advice, and absolutely wonderful sense of humor...

What the hell do you mean by that?

[mikepukmel]

Oh man all this time and I never noticed that dark blue "Donate" link. Thanks, will do when I get home tonight.
Maybe Enzo can do a chili by mail?

[Enzo]

I just made a large tub of guacamole for our big potluck here at the home yesterday. That count?

[Chuck H]

MMmmmm. Mail guacamole