1965 Gretsch/Valco 6165 noise

[wsscott]

I've had this amp since 1965 and its not had a lot of use over the years.  Anyway, it has some noise now that's noticeable on Channel 1. I tested by turning the Volume on Channel 2 all the way Off.  Then when I adjust the Volume pot on Channel 1, the noise intensity changes, but there's no impact with adjustment to Treble or Bass pots on that channel.  If I turn the volume on Channel 1 all the way off, then there's some background noise remaining which I would expect.  So with Volume on Ch. 1 off, if I then increase the Volume on Channel 2 there's very little change.  Pretty quiet.  So it seems to be associated just with Channel 1.  I'm attaching the schematics for the Pre-amp section and the PS section. 

My question is what should I check out to see if I can locate the source?  I assume its in the pre-amp section prior to the volume control?  Its never been worked on except to replace 1 filter cap several years ago.  Everything else is original except for a 3 prong cord I have installed. 

Also, does this circuit have the "death capacitor" that I should remove?  It doesn't look like its an issue here since the 3 amp fuse is before the caps and the rest of the circuit. Am I correct? 

Thanks as always.

[shooter]

Identify the kinda noise
sssssssssssssssshhhhk
fizzzzzzzzzzk
sizzchk
huuuuummmmmmm


use your tools, clean sin wave, prob the signal path
pull preamp tubes one at a time starting at V1 (1st tube), stop at PI (phase invertor)

[wsscott]

Shooter-Attached is a recording of the sound--(Well the forum wouldn't let me upload a m4a file that I recorded in Windows 10).  Anyway its like a ssssssshhhhhh sound along with a hum.

[shooter]

the ssshhhh noise it typically carbon plate resistors
the hum, tube, 120hz ac on DC, old leaky caps.


the ssssshhhhh sound should show itself as spiky, noisy stuff riding on a clean sinewave
the hum should show itself as a slow rolling sinewave superimposed on a clean sinewave

[Williamblake]

I have a tube that generates oretty stiff schsch.

[wsscott]

Shooter, thanks for your leads.  What are your thoughts about the "death capacitor" question?  Thanks.

[shooter]

typically they are on the AC primary side, I didn't see one there.  since you've owned this amp long time now, and we're still communicating....

[sluckey]

Look on the preamp schematic. There are two caps, one across the line, the other from line to ground. My fix would be a three wire power cord and replace those two caps. Use one class X and one class Y.

Otherwise, just snip both the caps and use a three wire power cord.

[wsscott]

Thanks.  I changed it over to a 3 prong plug several years ago, but didn't do anything with the caps.  So I may just leave it as is for now, or put in the X and Y safety caps at some point.  Are those two .02 caps there for noise filtering purposes or something else?  Are they tying into a neon power light bulb?

[sluckey]

Caps act as a line filter. They have nothing to do with the neon indicator lamp.

[wsscott]

I was looking at the schematics on this amp that I uploaded, and the B+ to Channels 1 & 2 comes off the end of the filter caps, and then splits and goes to the 2 channels.  Am I correct about that?  Since the noise is very low on Channel 2 even when it's volume is turned all the way up and Channel 1's volume is off, but on the other hand is very noticeable on Channel 1 and intensifies as its volume is increased with the volume of Channel 2 off, this leads me to believe that its not in the Filter caps or the 6L6's.  It also is not affected by adjustments to the Reverb or Tremolo--no effect in noise with intensity, speed, or on/off.  So it seems I can eliminate those 2 circuits also.  Since both Channels also tie into the PI section of the circuit, and only Channel 1 exhibits the noise, that would eliminate that part of the circuit.  This leads me to focus on the Channel 1 pre-amp section.  So does this make sense?  If so, then that's where I'll start exploring issues of resistors, tubes, etc.   Thoughts?

[shooter]

makes sense, an easy start though, identify the hum frequency, 60hz = filaments, nearby power noise.  120hz = unfiltered/poorly filtered DC power, so PS filter caps, possibly bias caps, cathode caps.

[wsscott]

Okay, so I checked the pre-amp tubes today.  I took each one out to see if the noise stopped or changed.  When I removed the 12AX7 for V-4 the noise cleared up.  When I put it back in, the noise returned.  I also pulled V-5, and when I did that no sound at all came through.  Am I correct that V-5 is the Phase Inverter part of the circuit, and it would make sense that there would be no sound if it is removed?

All the other tubes were fine.  One of the 6L6GC tubes has some microphonics.

So what does the V-4 tube do, and any thoughts on checking that part of the circuit?  Thanks.

[wsscott]

I forgot to mention that I replaced the 12AX7 in V-4 with another tube to see if it was the tube, but no luck.  Makes the noise even when a different tube is put in V-4.

[wsscott]

Sluckey/Shooter-I replaced the 2 prong plug with a 3 prong several years ago, and tied its green ground wire to the chassis via one of the screws on the transformer for the Reverb.  I was wondering if this circuit needs two (2) 100 ohm resistors installed somewhere to cut the hiss/noise?  Thoughts?

[sluckey]

I was wondering if this circuit needs two (2) 100 ohm resistors installed somewhere to cut the hiss/noise?  Thoughts?

No. Your PT heater winding already has a center tap.

[wsscott]

Yes, I should have looked at the schematic, I just didn't know if changing the plug so it had a separate ground would have an impact.

[shooter]

cut the hiss/noise? [/quote]
I would replace V4s plate R's with 1W metal film
clean n tension the pins

[wsscott]

Shooter--This is an unrelated question for a friend's 1977 Princeton Reverb.  He's had issues with it being slow to heat up and he checked the filament voltages on the 12ax7 tubes at pins 4 & 5 & 9, and each pin read 3.3 VAC.  I thought they had to read 6+VAC each.  Am I wrong?

[wsscott]

Sorry, I realized I was reading to GND and should have been reading between 4 & 9 and 5 & 9.  In that configuration they're reading 6.6.  So okay.

[wsscott]

I know this is going to sound pretty dumb, but I'll ask anyway.  I was going to check the bias on my Gretsch amp and wanted to know if I have to have the 6L6's installed when measuring the plate to cathode voltage?  The power supply of this amp is on a separate chassis that is bolted to the bottom of the cabinet, and I would have to take it off if I have to measure the voltage from the pin side of the tube sockets.  If I don't have to have the tubes installed, then I could just put the probes into the 3 and 8 sockets from the top.

[shooter]

but I'll ask anyway. 

won't work,....... sorta
current need's to flow, current is "set" by the tube + cathode R + OT
voltage is based on that current, using a meter as a tube "substitute" (by setting it to read amps) really isn't valid at this "level"


you can just measure plate VDC at the PS tap and call it good. If you do pull it to measure, grab the screen and cathode VDC also

[wsscott]

Shooter, thanks-I went ahead and pulled the PS chassis and did the readings on both 6L6GC's, and then used Rob Robinette's Tube Bias calculator program.  Attached are the results.  How do they look to you?

The cathode resistor is shared by both tubes and it's actual reading is 198.1 ohms vs. 200.
The Plate Currents are 69.6 ma and 67.1 ma.
The Screen voltages are 396 and 395 VDC.
The Cathode Voltage Drop is 28.47 VDC.
The Plate to Cathode Voltages are 372 and 371 VDC.

Thoughts?

[shooter]

you didn't complete the problem 


372vdc X .0696A = 26W/tube




i believe that's within reason for a 6L6, although it might be close.   

[wsscott]

I thought the 6L6GC was a 30 watt tube. 

[wsscott]

Sorry, I guess the calculator would want it to be around 80-81 and I guess its around 83?  I could put in a new cathode resistor.  Maybe 220?  Or just leave it be?

[sluckey]

The majority of cathode biased push/pull 6L6 amps ever made use a shared 250Ω cathode resistor.

[wsscott]

I guess given the age of this amp, ie. 1965, the 200 ohm 7W resistor would have done it. So I guess with current line voltage around 120 that's the difference. 

[shooter]

my brain is competing with pain so;

The Plate Currents are

Assuming the calculator took care of "common" cathode R


if so;
i'd let it ride, tweaking Rk does work, but it comes with "tonal consequences"

[wsscott]

I put in a NOS 6L6 about 2 weeks ago to replace the original one that had gone microphonic and was noisy.  The new tube seems fine and has much better plate current reading than the old one that was replaced.  In any event, the amp seems to pop and snap now a couple of times during an hour for a second or two, and then everything is fine after it stops popping, until it does it again. I'm not saying it only happens during the first hour of playing, but that's about how long I'm running the amp when I turn it off.  It occasionally did that before I replaced the tube, but now it does it each time I run it up.  It doesn't do it until its been running 20-30 minutes.  The old tube that remained does seem to glow brighter than the replacement.  Its not that big a deal for me, but a friend uses it sometimes to record and obviously that's a big issue if in the middle of a recording you hear this loud pop/snap.  Suggestions?  Thanks.

[shooter]

loud pop/snap.

typical is something went momentary open, or something arc'd
so bad solder joint, dirty/sloppy socket.


quality time powered down on the bench with bright lights hunting, or powered on, signal playing and chop-sticking, taping, wiggling.

[wsscott]

Thanks Shooter for the lead.  I'll see what I can find given the direction you've suggested.