Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: wsscott on October 06, 2024, 07:49:19 am
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I got this amp new back around 1965, and it hasn't been played much over the years. The only work done to it was to replace one of the filter caps and one of the 6L6 tubes. So it's in great shape, I think.
The issue is that I've noticed for several years is that after I turn it on, and even if its in Standby Mode (which I think is just a muted speaker connection and not a Standby like Fender--am I correct about this), after several minutes of warming up it will occasionally make a loud Pop sound. Scares me every time it happens. It doesn't do it every time it's turned on and so it's not predictable.
It might do this once or twice over a couple hours of being turned on.
Any ideas on what it could be? I'm attaching the schematics for the Preamp and Power amp sections. Thanks.
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it will occasionally make a loud Pop sound.
from within the amp, or at the speaker?
a "popping" sound is typically a sudden discharge, think of a cap holding a "charge" then momentarily "shorts"
a switch with "stored up" potential on one side closes and an "inrush" happens.
a sketchy tube socket, plug/jack, solder connection, builds up a potential then "arcs over", builds up, arcs over.....
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It comes through the speaker.
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I'd start with a spit-n-polish on the 2 connectors from the pre-section, and the tube pins n sockets
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Cleaning just the Output tube pins and sockets, or all of them?
Am I correct that when this amp is in Standby, nothing from the Preamp section would be getting to the Speakers to make the Pop sound, so the noise must be coming from the Power Amp section?
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the jury is still deliberating on that one :icon_biggrin:
It appears the standby switch grounds ONLY one power tubes grid, so my sense is there is a sneaky Pete way for a PoP to get through.
my logic thinks if that "grounded" connection is gritty, tarnished n varnished, in standby a "potential" could build up on the grounded grid then "arc over", thus Popping.
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Hay wsscott
I got a 6164(Gretsch Variety) new in 1965. Was my first good quality amp. Wish I had kept mine! :sad2: If you got a picture of it, post it if you don't mind
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Valco's of that era are known for cheap tube sockets which cause weird problems. Definitely clean, and retension all of those too although my first thought would be a cap going bad in the power supply.
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Platefire-Mine is the Variety Plus--35 Watts--2 12" Jensen speakers.
The amp is with a buddy being used in a recording session, but I'll take some pics when I get it back and post them.
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Shooter-I see what you mean about the Standby switch only grounding one of the grids.
But why would they do that?
Could it be that the GND connecting the Pin 7 grid between the 12K and 470K resistors may be the culprit, and that's how its getting through?
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Could it be
...........
there were 2 types of Field Engineer during my 32 years, those educated in Military Electronics and those from 4yr Colleges
while we from military ed were busy checking the "known to cause problems" areas, the College kids were drinking coffee asking
"could it be......." :icon_biggrin:
when you have a unknown, intermittent problem, you do as I mentioned above, if the problem should still poke it's head up in the future, you'll have a "list" of things already "taken care of"
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Shooter-I see what you mean about the Standby switch only grounding one of the grids.
But why would they do that?
This is a paraphase inverter. That simple SPST standby switch directly kills the signal sent to one of the power tubes. It also kills the signal sent to the grid of the lower half of the phase inverter and this removes the signal sent to the other power tube. Think "kills two birds with one stone."
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... after I turn it on ... after several minutes of warming up it will occasionally make a loud Pop sound. ...
I'd start with a spit-n-polish on the 2 connectors from the pre-section, and the tube pins n sockets
See the Servicing Fender Amplifiers (https://el34world.com/schematics.htm#Servicing%20fender%20amps) page that Doug put in the Library of Information. Specifically the "Jacks, Pots and Tube Sockets (https://el34world.com/charts/fenderservice2.htm)" section, as well as the "Oddball Amp Noises (https://el34world.com/charts/fenderservice6.htm)" section. The point of these is that there are some routine maintenance items one can do that resolve most noise issues in old amps.
For a first-pass, don't even try to localize & isolate the noise-source, just clean & re-tension everything. Most of the time, you will kill the problem (and there could be multiple individual "problems") just because everything got touched.
If you still have the popping noise afterward, report back & we can attempt to isolate the noise-source.
Separately, your amp's schematic (https://el34world.com/charts/Schematics/files/Gretsch/Gretsch_g6165.pdf) is in the Gretsch section (https://el34world.com/charts/Schematics/files/Gretsch/Gretsch_Schematics.htm) of the Tube Amps Schematic library (https://el34world.com/charts/TubeAmpSchematics.htm).
Whenever possible, try to create a link (https://el34world.com/Forum/index.php?topic=501.0) to the schematic, rather than adding it as an attachment. That saves server space, and makes it easy for folks to view the document.
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Thanks for the leads, and also info about posting.
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wsscott
Here is an old ad that outlines the difference between the Variety and Variety Plus. For some reason there is not many regular Varieties around, most of Variety Pluses.
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Yes, I've actually got the full sales brochure that shows the various models.
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Would love to here the "Rest Of the Story" on outcome of the "Pop" of from where it came from and where its gone?
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Will do. I just don’t have my amp now. A buddy has it for some recording sessions. So it will probably be a few weeks before I can get back to it. He says it’s been fine and no pops, so it will be a challenge, most likely.
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I had my buddy take some pictures.
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Here's the last photo.
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Thanks! Looks like She's in very good shape. You were wise to hold on to it. Attached is the only picture I have of mine taken with my Kalamazoo Electric Guitar (Gibson's Fender Mustang copy). If I remember correctly, think I traded mine in on a Silverface Deluxe reverb
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A question about discharging the reservoir and filter caps in this amp and other tube amps with rectifier tubes.
Can you properly discharge the caps by unplugging the amp, removing the rectifier tube, and then grounding the cathode Pins 2 or 8 using a normal capacitor discharge tool connected to ground?
In a Fender you'd leave the Standby switch in the "play" position, but in this amp it wouldn't matter since the filter caps aren't isolated.
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I would go pin 2 to chassis
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You can use a gator clip cable on a preamp tube plate R.
Hook 1 clip to ground and the other to the tube socket side of the plate R.
Always do a quick dcv check after you think you've discharged the filter caps. Just to confirm you did drain them fully.
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Can you properly discharge the caps by unplugging the amp, removing the rectifier tube, and then grounding the cathode Pins 2 or 8 using a normal capacitor discharge tool connected to ground?
Does this discharge tool have a resistor in series, so the caps don't discharge instantly?
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Yes, it’s a snuffer stick. I also use a 22k, 3 W, resistor in a banana plug adapter that I plug into my DMM and then plug the probes into that, so I can discharge and read the voltage at the same time-a D-LAB trick.