debugging adding reverb to deluxe

[mikepukmel]

Starting with a fairly new Deluxe AB763 build that did not have reverb before. Added 2 tubes, transformer and components for the reverb.

Got the reverb sockets, transformer hooked up. Double checked all of the connections. Put the tubes in, and fired the amp up.
Amp sounds as before with reverb at min. If I turn up the reverb a little (maybe to 2) I get some reverb sound. So far so good. When I turn the reverb up a little more I get a big drop in volume, and both output tubes glow brighter (a lot brighter), and my home made current limiter light bulb glows brighter. Its not white bright, but a lot more than expected.

I hit the strings a few times on my guitar before I shut it off, and there is a lot of reverb as expected (reverb sounds nice), but the overall volume is way lower, which is unexpected, and the output tubes glowing is scary.

After shutdown, nothing appears to have overheated, but can't tell what terrible things I did to the output tubes.

Will discharge the filter caps, and check all wiring again.

Any chance this is a bad tube? The two tubes 12at7 and 12ax7 for reverb are new out of the box. I don't have any extra tubes, was thinking to pull those out, and swap them into V1 and the PI and start it up with no reverb after checking all the wiring and components again.

It worked fine before I did all of these wiring changes!!

[Chuck H]

"output tubes glowing" is a very nebulous description. Is there red plating? Do you see a red spiral glowing? Blue coloration glowing?

Sometimes the dry signal volume seems to drop with reverb because of phase anomalies. But you describe an overall volume drop.

You also mention a light bulb limiter. Are you tone testing with the limiter in the AC line?

Since you seem to think it's all good with the reverb down, how about taking voltage readings without the limiter and with the reverb down. How do those look?

[mikepukmel]

"output tubes glowing" is a very nebulous description. Is there red plating? Do you see a red spiral glowing? Blue coloration glowing?

Sometimes the dry signal volume seems to drop with reverb because of phase anomalies. But you describe an overall volume drop.

You also mention a light bulb limiter. Are you tone testing with the limiter in the AC line?

Since you seem to think it's all good with the reverb down, how about taking voltage readings without the limiter and with the reverb down. How do those look?

Hi Chuck,

Thanks. Sorry my bad description. So, with reverb at minimum, the amp plays fine output tubes have some glow (don't know how to describe, but like they did before I added reverb, I think). Turn up the reverb to 1, amp is still ok, and I get a little bit of reverb, and the volume doesn't drop, brigthness in the 6V6's is about the same. When I turn up the reverb a little more, say 2 to 3, there is an instantaneous jump in the brightness inside both 6V6's, like I flicked a switch. I only know what red plating means by some photos on the web. I don't think they're redplating yet, but didn't leave it on more than 2 or 3 seconds after I saw the brightness change in the output tubes and the limiter bulb glow brighter.

Yes, the limiter bulb is on the AC line. Before I fiddled with the wiring to add reverb, I played the amp a couple of hours total, and everything seemed to run OK. If I turned the amp up quite a bit, (5 or 6 on vol) and had the room lights off (only my desk light on) the limiter bulb would glow a little but not very visible with the lights on. Now, when I turn up the reverb, the bulb glows bright enough to see it clearly and its a big jump with that small change in reverb setting.

I re-checked the connections, but maybe need someone else to look over my shoulder, might be missing something obvious.

Voltages: re-reading now. Which voltages should I read? plates on the 6v6 but also plates on the reverb tubes?

[mikepukmel]

"output tubes glowing" is a very nebulous description. Is there red plating? Do you see a red spiral glowing? Blue coloration glowing?


its red inside the tube, behind the (what I think is) the plate, the metal box looking thing inside the tube. The red color increases a lot. I don't see a spiral, but I didn't leave it on very long. Really surprised that the reverb control could cause this, hoping I screwed up a wiring path. Didn't find it yet though.

[Chuck H]

It sounds like it could be a filament issue. That could be caused by having the limiter in the AC line while testing. What's confusing is the limiter bulb glowing bright when you turn up the reverb. Does this happen simply turning it up, or only when you play the amp with the reverb up?

[Enzo]

If the thing is not blowing fuses, get rid of the bulb limiter. It can confuse readings, and alter operation. It is a helpful tool, but not transparent to the amplifier.

Glowing? Say what you mean. Heaters glow normally - they should glow to tell you the tube is heating. Some power tubes get a purple-ish glowing inside the glass - an overall aura so to speak. It will get brighter as tube current increases. If the metal structure starts glowing red hot in places, we call that "red plating". That means the plate is trying to conduct too much current. SO those are three common types of glowing. Maybe add a couple extra words to indicate which.

When you turn up some control and all of a sudden the sound diminishes and the power tubes look like they are screaming for all they are worth, it says to me the amp has gone into RF oscillation. If turning up the reverb causes it, that even more makes me think this.

[mikepukmel]

It sounds like it could be a filament issue. That could be caused by having the limiter in the AC line while testing. What's confusing is the limiter bulb glowing bright when you turn up the reverb. Does this happen simply turning it up, or only when you play the amp with the reverb up?

Happens as soon as I turn the reverb up past point X (say about 2). Its like a switch is turned on. Both the limiter bulb gets noticeably brighter, and both output tubes noticeably brighter red glow. (I don't think they are red plating). Took off the limiter, measured a bunch of voltages, will post below.

[mikepukmel]

If the thing is not blowing fuses, get rid of the bulb limiter. It can confuse readings, and alter operation. It is a helpful tool, but not transparent to the amplifier.

Glowing? Say what you mean. Heaters glow normally - they should glow to tell you the tube is heating. Some power tubes get a purple-ish glowing inside the glass - an overall aura so to speak. It will get brighter as tube current increases. If the metal structure starts glowing red hot in places, we call that "red plating". That means the plate is trying to conduct too much current. SO those are three common types of glowing. Maybe add a couple extra words to indicate which.

When you turn up some control and all of a sudden the sound diminishes and the power tubes look like they are screaming for all they are worth, it says to me the amp has gone into RF oscillation. If turning up the reverb causes it, that even more makes me think this.

Enzo, yep, not blowing fuses. Have the standard 1a slow blow, and amp was running for a while before I turned up the reverb and noticed the evil transpire.

its not a purple glow, its red/orange. The thing that bothered me was the noticeable change with the setting of the reverb dial. OK did as you all suggested: removed the limiter, moved the whole setup and took a bunch of voltage measurements with the limiter off.

Oscillation!!!! Ok, thanks, thanks, yeah, my lead dress here is not . . . stellar. I will post a photo as soon as I can get over my ego dent knowing that I did something wrong, ok stupid.

[mikepukmel]

Voltages!!

Measurements with all tubes in, reverb set at minimum.

For me, this is the scary stuff, HV, have to keep my wits about me and resist the urge to touch something to see how hot it is

The only other "mod" on this amp is 1000v solid state rectifiers on the inputs of the tube rectifier.
I did not check all of the heaters, just a few, look within range.


GZ34
-------
pin V
------------------------------
4 349/137 vac (before ss rectifier/after ss rectifier)
7 350/140 vac
2 417 vdc
8 417 vdc


6V6 (outer tube, closest to the rectifier tube)
pin V
------------------------------
1 (vdc) -62
3 410
4 382
5 -61
6 402

Voltage drop across 1 ohm resistor 0.064vdc (checked this twice, looks really small?)
Resistance across 1 ohm resistor (corrected for leads) 1.0ohms


6V6 (inner tube farthest from rectifier tube)

pin V
------------------------------
1 -68.6
3 414
4 388
5 -68.7
6 402

Voltage drop across 1 ohm resistor 0.046vdc (checked this 5 times, looks really small?) (this tube glows darker than other 6V6)
Resistance across 1 ohm resistor (corrected for leads) 1.0ohms


12AT7 phase inverter
pin V
------------------------------
1 202.5
2 49.4
3-8 60.6
4-5
6 177
7 51.2
9 3.28 vac (heater just checked a few tubes)


12AX7 Reverb Return

pin V
------------------------------
1 169
2 0
3-8 1.5
4-5 3.28vac (checked a few, not every one)
6 175
7 0
9



12AT7 Reverb Send
pin Vdc
------------------------------
1-6 397
2-7 0.004
3-8 8.27
4-5 (heater) 3.26 vac
9 H


12AX7 Channel 3 preamp tube

pin Vdc
------------------------------
1 167
2 0
3 1.46
4-5
6 173
7
8 1.5
9

[mikepukmel]

If its oscillation, Id really like to understand this better. The oscillation is controlled by the reverb pot. Dose this mean that the reverb output tube is what is oscillating, sending RF into the PI and then on to be amplified by the output tubes?

Related question: this build, the main board is on standoffs, so it is far away from the chassis compared to original amps. Is it more important to leave the wiring going from the main board to the tubes longer, so that I can form the wiring down along the chassis to the socket pin, or shorter, which will get the wire to the pin with shorter leads, but not run along the chassis? Im guessing that this oscillation is caused by some wire inducing a signal into another wire that is running from the main board to the tube pins? Compared to you guys builds, repairs photos Ive seen, this amp the wire running from the board to the pins are too long and routed all over each other.

[mikepukmel]

I just went through the calculations on Rob Robinette's web site, and if I did the measurements right, the output tubes are now biased around 206% which according to the charts is a little high. Glad I didn't run the amp more than a few minutes. When I first got the amp running, my crappy meter measured something like 0.008v drop across the 1 ohm cathode resistor. Could be a junky meter, or the amp did change that much by the component changes that add reverb? Would over bias (way over bias) cause, or increase the chance of oscillation?

[Enzo]

It is all about circuits, not about parts. If the reverb control determines if it oscillates or not all it means is the reverb return stage is INVOLVED. It may be the the reverb tube is in fact oscillating on its own. But more likely it is forming a positive feedback loop with the power amp. That could come from power supply coupling - the B+ in one stage affecting that in another. That could come from sensitive grid wires coming too close to strong signals in other stages.

If your 1 ohm resistor was for one tube - as in a resistor for each - then the dissipation would be 19 watts, a bit steep for a 6V6. If the resistor is for the pair of tubes, then the 0.046v, which is 46 millivolts, is for two, meaning 23ma per tube. 410V x 0.23A = 9.4 watts.

[Chuck H]

Just getting home from work. All I can say is, THIS^^^^^^^^^^^^^^^^^^^^^^^^^

[mikepukmel]

Wow, was this an experience and a half. or more!

before adding reverb components: output tubes "glowed" a certain level, I don't know how to explain it, but with my office lights off, just visible at the bottom of the tube (close to the socket)

After adding reverb components, output tubes glowed a lot brighter with reverb at min. Turn up reverb to say 2 and the output tubes glowed a lot brigher, the limiter bulb glowed brither and the volume dropped like crazy (3/4?) like I turned the volume control way down.


Spent all after noon re-routing CAREFULLY, shortening all of the wiring. Tried very hard with my limited skill set to get the wiring to look like the layout diagram: wires to a given tube don't cross other wires to other tubes, and cross each other as little as possible.

Also, drastically shortened a big bundled of output trans wires that I had capped off (for 4 and 16 ohms) and also the wires to the output jacks.

Shortened and re-routed wires to the reverb send and recieve, and tried to not have them cross any other wiring.

Shortened and re-routed all wires to V1 and V2 preamp tubes, and also wiring to the phase inverter.


Turned amp back on, with reverb on 10: output tubes glowed very little.

Also, and this was extremely surprising, the current through the output tubes was back exactly at about 8 or 10 ma, almost exactly the way I had set the bias prior to installing all of the reverb components.

Blymey, this oscillation stuff is nasty. Im wondering how much I shortened the life of my poor EH 6V6's by this exercise.

Wondering if I should put grid stops on the reverb return and extra gain stage. its too damn late to crank the amp now (didn't get started due to some awful flu virus that had me in bed most of the day).

Thanks to everyone, your many posts and recommendations helped a huge amount. I just read a recent thread by a wizard tech who built that space age looking bassman type thing that also had oscillations.


Q: 1) Should I put shielded wiring on the reverb send and return lines?
2) Should I put grid stops on the 12AX7 reverb recv tube?


Wahoo!!!

[Chuck H]

If you practice good lead dress then you should only need to shield the most sensitive leads. The input lead and perhaps the second stage from pot to tube grid (if it's a cascade stage amp). For some reason very many cascade amps run straight from a pot on the front panel to a tube at the back of the amp across the preamp board.?. Also a good idea to shield PPIMV's. They seem to cause trouble for some, so just shield the leads and keep them away from the preamp. Everything anyone ever wanted to know about lead dress, transformer location/orientation, grounding schemes, etc. is here on this forum ready to be searched and viewed.

Our new friend with the "tubes in tubes" Bassman is having better than expected success in short order working out the issue. But he IS an engineer and was practicing some level of understanding when he bundled his leads.

You're issue seems to be working out too. Lesson learned and more to follow.