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Post subject: Oddball hum
Posted: Sat Nov 10, 2012 4:28 am
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Here's an odd one. Got a SR in for new caps and hum issue. Pulled all tubes except rectifier and two 6L6GC. Both have steady bias at 40mA with 475VDC on plates. Hum. 120Hz variety.

Now, pull one 6L6GC (one 6L6GC socket open) = no hum. Swapped open sockets = hum again (though a bit quieter, prolly do to less currnet draw).

Changed the 6L6GC and 5AR4 = same problem.

Quick check of sockets, tensioning, resoldering of all ground points, and points to socket = same problem.

Checked and resoldered all heater line points. Checked the 100-ohm heater-to-ground pseudo-CT resistors. All ok.

Will redo grounding points (pin 8 to ground) tomorrow. Re-route wiring. Maybe new socket. Will update.

I don't remember ever having this kinda problem. Where one output tube/socket is stone quiet. But, the other is noisy as heck. :?:


EDIT: Yes, the main filter caps were replaced. 40mfd/500VDC X2 first section. And three 20mfd/500VDC.


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Post subject: Re: Oddball hum
Posted: Sat Nov 10, 2012 7:10 am
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Beemer...you didn't check the Flux Capaciter :wink: I am no tech by any means...but that does sound like a pretty unique problem...is it possible for a socket to just be bad? Maybe just a really small crack in it somewhere?

T2

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Post subject: Re: Oddball hum
Posted: Sat Nov 10, 2012 9:40 am
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T2, probably a bad socket. This amp had some other interesting mods. The ground straps between the cathodes of each 6L6GC (pin 8) and ground was pulled. And a push-back wire was run between each of the cathode to a separate ground (by the speaker jack).

Then, where the ground straps were once soldered to the chassis a series of three diodes were soldered in series ffrom this chassis point to each anode of the 6L6GC. The diode are "probably" there to dump OPT flyback, when you turn off the amp.

Problem is, nearly all flyback networks I've seen have the diode tied from the OPT to ground. As I believe, the "flyback" voltage goes backwards through the OPT into ground and into the cathode. Like the diodes in this JCM800 Lead:

Image


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Post subject: Re: Oddball hum
Posted: Sun Nov 11, 2012 2:58 am
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Is the hum change with volume pot ? Or always same hum at any volume ?


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Post subject: Re: Oddball hum
Posted: Sun Nov 11, 2012 3:57 pm
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Stratele, always the same hum. No which pre-amp tubes in or not --or-- ALL pre-amp tubes out. I got rid of the hum by removing the add-on cascade diode network on each 6L6GC anode. And re-grounding their cathodes with short leads to the chassis.


The amp still has a big difference between the NORMAL channel (sounds nice and full) . And the VIBRATO channel, which soudn sounds a bit weedy and thin (even after replacing the incorrect coupling caps). I noticed that when rolling tubes the biggest difference in tone was swapping V2 (Vibrato input) and V4 (reverb recovery/stage prior to PI).


http://www.webphix.com/schematic%20heav ... _schem.pdf

All voltage values are within range. Except, second-half bias votlage of that V4. Think we may have a bad socket. One that maybe allowing voltage onto the tube. But, limiting current flow --- & thus idle bias cathode voltage, due to poor contact.

BTW... when did Fender go to 1-inch punched holes for their octals? I have bunch of 1-1/16 inch mil spec Amphenols and Beltrons. Had to use the old Dremel last night. What a drag.

Finish the thing after NASCAR race, in Pheonix.

Talking about AZ. Has anyone heard from Arjay, lately?


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Post subject: Re: Oddball hum
Posted: Sun Nov 11, 2012 6:05 pm
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Beemer, I use flyback diodes in the '67 B'Master. Three IN4007's in series off both 6L6 plates straight to the chassis(short & sweet). Cathode ground strap is unchanged from original set-up. I've experienced no adverse noise. Art

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Post subject: Re: Oddball hum
Posted: Sun Nov 11, 2012 6:54 pm
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Art, flyback occurs when the amp is turned off and the electromagnetic field around the trannies collapses. This energy is changed into REVERSE flowing electricity,

If this is an issue (AOHF -- An Old Ham Fart) told me to designed a series of diode set to avalanche point. And tie them between the primary side of the OPT and any ground point. Reversed polarity, so no flow, under normal conditions.

Because when the EM field collapses. The current flow is backward. Like Output tube anode to ground. Where ground has higher potential than B+. These avalanche networks should be inline with OPT and sometimes PT and ground. Once a specific voltage and current as voltage is reached ---the diodes pop, acting like fuses preventing large flyback current/voltage damaging the rest of the unit.

On Another Super Reverb question:

Ever notice the big difference of the SR AA763 and AB763 Vibrato driver setup?

AA has the two grids tied. AB seems to use the two-halves as separate gain sections.

Anyone work on an AA763 SR? Just wondering about tonal and control differences.

Thanks!

http://www.webphix.com/schematic%20heav ... _schem.pdf

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: Oddball hum
Posted: Sun Nov 11, 2012 10:12 pm
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Concur, my friend!!! But the electromagnetic collapse from the speakers can occur sporadicaly, during normal operation as far as I know. Bill, probably can clear this up! My info is from an old PA designer, many years ago. So my understanding may be way off the reality of this!!! Art

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Post subject: Re: Oddball hum
Posted: Mon Nov 12, 2012 10:30 pm
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The flyback diodes don't pop, they just conduct the collapsing field from the OT primary to ground (reverse voltage) instead of conducting through the tubes. Any reverse voltage above 0.7 volt will conduct through the diodes. The diodes should be rated for high enough voltage and current to conduct the high reverse voltage, which can actually be much higher than the normal forward voltage. There is no noise or affect on amp sound with the diodes in place.

The vibrato tube uses one half as an oscillator, and the other half drives the output of the circuit to "wiggle" the output tubes' grid bias voltage, neither is a gain stage, and has no affect on the tone of the amp.

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Last edited by shimmilou on Mon Nov 12, 2012 10:52 pm, edited 1 time in total.

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Post subject: Re: Oddball hum
Posted: Mon Nov 12, 2012 10:50 pm
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Shimmy, you are right, again. From the schematic of that Lead Marshall, the diodes will not conduct electricity from OPT to ground, until a threshold level is reached.

They are not acting as fuses.

Thanks, for clarification!


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Post subject: Re: Oddball hum
Posted: Mon Nov 12, 2012 11:06 pm
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The "threshold" is really any reverse voltage. Since the diodes are installed "backwards", only conducting during reverse voltage, the forward voltage of the OT would have to exceed the PIV (peak inverse voltage) rating of the diode for it to pop during normal amp use. Usually this PIV rating is in excess of 1000 volts, which you'll never see in a typical Fender amp. The diodes don't conduct during normal use of the amp, only conducting when the OT primary field collapses fully, creating the reverse voltage, such as when amp is switched off or put in standby. Notice that the flyback diodes are connected directly across the output tubes, Plate to ground, which is the same electrical connection as OT primary to ground.

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Post subject: Re: Oddball hum
Posted: Tue Nov 13, 2012 7:41 am
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Shimmy, thanks! So two or three 1N4007 wired in series to the plate should be fine?


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Post subject: Re: Oddball hum
Posted: Tue Nov 13, 2012 8:18 am
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I don't see a problem with that. It's probably done that way to get a higher PIV rating without using more expensive diodes, since each diode would be able to handle 1000 volts PIV. So, 2 diodes in series would give a 2000 volts PIV, and 3 diodes in series would give 3000 volts PIV. The reverse voltage would still cause conduction through the diodes, but now the "threshold" for the reverse voltage would be 1.4 volts for 2 diodes, and 2.1 volts for 3 diodes, which is not really much difference in that respect, as any reverse voltage will quickly exceed those voltages.

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Post subject: Re: Oddball hum
Posted: Tue Nov 13, 2012 5:18 pm
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Yep, that's the deal. When the voice coil(an inductor) is charged over time and is reduced or shut off suddenly, voltage is sent back to the charging source. The OT windings multiply that voltage, depending on their ratio. The diodes just steer the high volts away from the power tubes and sockets....JMO. On another note, Shimmy, What do think about using FRED's in the rectifier section ? Art

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Post subject: Re: Oddball hum
Posted: Tue Nov 13, 2012 7:14 pm
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Just about right, but the collapsing field voltage from the voice coil of the speaker is really insignificant, and the flyback diodes don't see it (see below). For a 40 watt amp with a 2 ohm speaker load, there is approx 6.3 volts at the speaker output at full power [ E = √(40 watts x 2 ohms) ]. I say approx because there are losses, and it depends on the frequency. The amp watt rating is measured with a purely resistive load, not an inductor such as a speaker. The speaker field has already collapsed by the time the amp is shut off or put into standby, because when you stop playing, there is no longer a voltage generated to the speaker (AC), so the speaker is pretty much out of the equation at that point. But, the OT primary still has a huge field that will collapse because of the idle current through it that gets switched off.

I don't see the fast recovery diodes as necessary, but they certainly couldn't hurt, and they might be better than the diodes used. You're only dealing with 60 Hz, how fast do they need to be? :lol:

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