Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: gtrmac on August 15, 2026, 07:22:32 pm
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A new build of a Brownface Princeton clone and all is well except the tremolo footswitch operation. The schematic and layout show the footswitch shorting the oscillator grid to ground through a ,01mfd cap. This is how I assume it should work. The Tremolo works fine until I depress the footswitch and I get a loud motorboating type of noise from the speaker. here's the schematic:
https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_6g2_schem.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_6g2_schem.pdf)
Any thoughts from you guys? I'm stumped!
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Most likely a wiring error. Post some hi-rez pics. Maybe we can spot something that you are overlooking.
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Certainly a reasonable assumption but there is only one wire going to the footswitch jack and it's definately attached to the .01mf cap at the junction of the 1M resistor. As i said, the tremolo works fine without the footswitch attached, and even with it plugged in it only starts oscillating when the switch is shorting the connection.
Here's a pic:
(http://IMG_7157.jpeg)
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Disconnect the white wire from the footswitch jack. Now touch the dangling end of that white wire to the ground buss at the top side of the board. Does that kill the tremolo without any motorboating noise?
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... the footswitch ... with it plugged in it only starts oscillating when the switch is shorting the connection. ...
That seems topsy turvey, shorting across the socket should stop the oscillation.
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I clipped a jumper across the jack from the tip connection to the shield and it oscillates. Its like a buzzing sound and very loud.
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I'll try shorting the white wire to the ground buss and see what happens.
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shorting the white wire to the ground buss has the same result, a loud buzzing from the speaker.
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There are likely problems related to the Tremolo circuit in this amp. When to VOL is raised near maximum you can hear a hum from the output which oscillates relative to the settings of the SPEED and INTENSITY controls. With the VOL pot at minimum there is also a hum which decreases when the VOL pot is raised to around "3" on the scale. A lot of the hum issues were minimized with the addition of an aluminum shielding plate inside the top of the cabinet. The copper ground buss is connected to the input jack shield terminal and reads continuity to the chassis. The BIAS BOARD is grounded to an eyelet bolted to the chassis at a transformer bolt as you can see from the picture in the previous post. All intended ground connections show continuity to the chassis when read with a DMM. I found a cold solder joint at the connection of the output transformer to one power tube but repairing this had no effect on the footswitch problem or hum issues. Still poking around tryng to find a solution.
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This is weird. I don't see any clues in your pic, but I also don't see how the copper ground buss attaches to the chassis.
Where did you get that black board. I'm asking because not too long ago there was some black board circulating around and it was slightly conductive which caused a lot of weird illogical issues.
This is a stretch... Disconnect the NFB wire from the board. Any better?
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Sounds like the tremolo circuit is working properly - I built a 6G2, and when you turn the volume up, you can hear tremolo oscillation in the background. There's some hiss at that level in my amp and it basically varies with the tremolo, and the same happens if there is hum out of the amp.
You say you shorted the white wire, but did you actually disconnect it from the jack like Sluckey indicated? Because if there is a problem with the jack, you are still keeping it in the circuit unless you've disconnected. I would de-solder the wire from the jack, and test it by shorting it to ground, and then test by shorting from the eyelet (where the wire is attached) to ground. I also find it odd you are using a shorting jack for the tremolo.
I probably go overboard with too much solder, but it looks like your eyelets are rather light on solder - you might try adding some more at that point.
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shorting the white wire to the ground buss has the same result, a loud buzzing from the speaker.
I suggest to try a grid stopper to the V2 socket lug 2, something in the range 10k to 100k.
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Thanks guys for the tips and advice!
sluckey: the copper wire attaches to the chassis at the input jack shield terminal. I disconnected the white wire from the jack and connected it to the Ground Buss with a clip lead and it had the same buzzing noise. The black board is from Hoffman. I could try disconnecting the NFB.
dogburn: I did disconnect the white wire from the jack, no difference whan shorted to ground. The shorting jack is not shorting without the switched terminal connected to the shield lug. This particular jack doesn't short anyway, the switch tab is bent. btw did you have any problems with the Tremolo footswitch operation?
pdf64: there is a 1M grid stopper as in the schematic (edit: the 1M is the Grid Leak, no grid stopper here, tried 68K stopper but no effect on Tremolo FW problem)
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I did not have any problems with the tremolo footswitch - I used an RCA jack, so my problem was just having to acquire the right switch. It definitely cuts the tremolo entirely out of the circuit.
Did you try re-soldering that eyelet?
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Post a big (not 640x480) hi-rez pic of the entire guts. I want to also see filter caps, bias board, and PT.
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Here's a jpeg of the whole chassis. The filter caps are a multi section can, 3X40mf, I couldn't find a 3X30mf
dogburn: I resoldered the eyelet but no change. I wan't terribly concerned about the TREMOLO bleed at max volume but does the footswitch stop this?
pdf64: I tried a 68K grid stopper at V2 pin 2 but no joy there, at least as far as the footswitch issue goes.
Some other weirdness: the VOL pot at "0" has some humming and wiping it between "0" and around "2" has some scratchiness as if there was DC on the pot. DMM doesn't read any DC on any of the VOL pot pins though. Maybe it's a scratchy pot. It's a new CTS which I thought would be reliable though.
When I first powered this amp up I had to reverse the O/P Transformer Primaries because the Neg FB was not Neg and the amp was squealing like a stuck pig. Reversing the wires fixed this.
I must say, this is the fourth tube amp I've built and this is the first one that had such a difficult birth.
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Yeah, the footswitch should stop any tremolo bleed into the signal.
One thing I notice is that your heater wires are run close to the jack for the tremolo switch, but that shouldn't matter since you said you disconnected the wire to the jack and shorted it to the ground bus.
I certainly can sympathize with trying to chase down frustrating issues in an amp - hopefully you will get to the bottom of this soon.
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Those tiny 640x480 pics are way too small to show the detail we need to see in order to help troubleshoot. Surely that iPhone 12 mini can take a much bigger high resolution pic.
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Can you verify where that yellow wire from V2a connects? Looks like it goes directly to the speed pot but it should go to an eyelet on the board. And yes, higher res photos would really help.
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dogburn-
Yeah, I was concerned about the proximity of the heater wires to the footswitch jack myself. The wires are tucked into the corner of the chassis and probably not as close as they might appear in the picture. Laying them in the corner of the chassis seems to be a popular method although I've seen photos of vintage Fenders with them looping up over the tops of the sockets and that certainly worked for the original amps. The amp is pretty quiet otherwise.
In other news, plate voltages are a little high on V1, reading 200V on V1A and 190V on V1B. I'm wondering if I should be concerned. Fender usually notes that voltage readings could be +/- 20% and we're not seeing 117V coming from the wall anymore. I would by Mercury Magnetics transformers if I wasn't poor since they are corrected for modern line voltage.
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The yellow wire from V2a connects to the 1M -> GND and .01mfd -> Trem Pedal Tip
I soldered it into the eyelet from the underside.
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I would like to make a brass grounding plate for this amp and scrap the copper bus wire. I also wonder if I should have soldered the components at the pots directly to the back of the pots as I have done in the past.
dogburn sir, do you have any photos of your 6G2? I'm especially interested in the grounding strategy.
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I would like to make a brass grounding plate for this amp and scrap the copper bus wire. I also wonder if I should have soldered the components at the pots directly to the back of the pots as I have done in the past.
The brass plate is not necessary. Your copper wire buss works very well. I count 10 different chassis ground points. There may be others that I overlooked. We do need to discuss your grounding scheme.
But first, let me ask about your chassis. Is it aluminum? You cannot solder to aluminum. This is important to know.
There seems to be some kind of coating on the chassis. I can see bare metal in the PT area. That coating will prevent a good electrical connection to the chassis. You should remove every pot and jack and remove the chassis coating down to bare metal so the bushings will make good contact to the bare metal. Repeat this for all those terminal lugs installed with screws.
The cap can needs special attention. If the chassis is aluminum, forget about trying to solder to it. If it's steel, you need a BIG iron or gun and some flux to be able to solder to it. In either case, you must remove that coating from the chassis around the cap can. Bend three of the can tabs over to secure the can to the chassis. Leave one tab standing upright to use as a solder lug. Now connect the PT red/yellow wire directly to the upright lug.
There's more to talk about but doing as I suggest will get you a more reliable ground system to work with. May even fix that weird problem.
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... There seems to be some kind of coating on the chassis. I can see bare metal in the PT area. ...
My guess is that the chassis is plated steel, and that there's a stainless steel adapter plate between the PT and chassis, to allow a smaller than intended PT to be used.
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I don't currently have any decent pics of the inside of my 6G2 - I do need to open it up to address an issue, and will do that pretty soon, so I can take some good pics then. I used a copper wire ground bus as well for the board components, etc., which works just fine. One thing I did was add a separate filter cap for the preamp stage, mounted it alongside the board, and grounded it to the copper bus wire.
Another thing I notice regarding the heater wires is that for V1a and V2a, the heater wire goes right under the wires for the 1st preamp and the tremolo sections. It looks way too close and could interfere with the tremolo functioning. I would re-route those so they come down from above to attach to pin 9. Just re-route the length of the wire that comes over from the side (you don't have to re-do the entire heater wiring) so it arcs over the top and gets away from those preamp wires.
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My guess is that the chassis is plated steel, and that there's a stainless steel adapter plate between the PT and chassis, to allow a smaller than intended PT to be used.
Looking closer, I agree. There seems to be only two screws holding the adapter plate.
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The chassis is steel. I don't think it's plated it seems to be the same kind of material that the original Fender chassis were made of. It is not plated like the 1950's models, but more like the 1960's era. Solder does adhere to it and i have a 35 watt Weller that seems to have enough heat to flow solder onto the metal.
The grounding scheme is another story.
I relocated the PT Center Taps for B+, Filaments and the BIAS Board to a tab on the cap can. I am reading ZERO Ohms between the ground points and various locations on the chassis. If the chassis does have a coating it is conductive, maybe zinc?
Thanks for all the suggestions guys. I'm still stumped.
I could try re-routing the heaters I suppose. The signal connections to V2a for Tremolo and V2b for the Phase Inverter are arched up, away from the heater wires at the moment. This method has worked in the past. As I mentioned I have seen the way Fender did it which was to arch the heater wires and lay the signal connections down and I might use that in future builds. the way things are going the next build might be re-building this amp!
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Solder does adhere to it and i have a 35 watt Weller that seems to have enough heat to flow solder onto the metal.
35W is not enough. You need at least 100W to 250W to get the chassis hot enough to get a puddle of solder that is actually bonded to the steel. Too little heat and the solder is just kinda stuck to the chassis and is not considered a good solder joint. That's what I see in your pic.
Look at the pic in this post to see a properly soldered cap can to chassis.
https://el34world.com/Forum/index.php?topic=27522.msg302310#msg302310
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Regarding the heater wires, while you arched the preamp wires up and over, the heater wires to pin 9 still go very close to the pins where the wires connect. No need to re-wire the heaters, just unsolder the ones going to pin 9 for both sockets and arch it over - it's worth testing to see if that's the root of your problem with the tremolo.
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Sluckey- yeah I see the solder blobs. Guess I need one of those nuclear soldering irons. Nice to see an original 6G2 also.
Dogburn- I could try rearranging the heater wires, nothing to lose I suppose. Shorting the .01mfd/1MEG eyelet directly to the copper ground bus still produces the fault btw.
Another annoying fault is that with the VOL pot at zero I get a hum pulsating in time with the tremolo but when I turn up the VOL above 2 it goes away.
If not for these faults this amp sounds really good. Very bright with a Telecaster or a Stratocaster but not in a bad way.
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This is not a heater wiring problem. This is a very weird problem. Weird problems can be caused by poor grounding, conductive turret board, or some other weird issue that may have a very illogical solution.
Something to try... As is, the trem tube gets its B+ from the plate node of the power supply. Change that to get B+ from the screen node. Look at your pic. On the cap can the top lug is the plate node. It has one yellow and two red wires. The red wire that goes under the board feeds the trem tube. Move that red wire to the cap can terminal that has only one white wire. Does this help? If not, put the red wire back on the plate node.
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The chassis is steel. I don't think it's plated it seems to be the same kind of material that the original Fender chassis were made of. It is not plated like the 1950's models, but more like the 1960's era.
All Fender chassis were plated. Early 60s chassis got a nickel plating, and mid-60s and after got a zinc plating.
Solder does adhere to it and i have a 35 watt Weller that seems to have enough heat to flow solder onto the metal.
35W is not enough. You need at least 100W to 250W to get the chassis hot enough to get a puddle of solder that is actually bonded to the steel.
I agree with Sluckey here: I would want 150w minimum, and a physically-massive iron/tip. That high power & large size are needed to overcome "thermal inertia" in the same way its easier to push a pickup truck with something big & heavy, while something light & tiny can push a toy truck just fine (because of the difference in their inertia).
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You can bolt the ground to the chassis if you take care in doing it correctly.
I don't know why this shows 'other grounds' stacked on top of the safety ground. I would keep the safety ground by itself and add a 2nd bolt away from the safety ground bolt.
From Merlin's web site;
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Willabe- I have since isolated the Safety Ground. I moved the Bias Board Ground to a lug on the cap can. The secondary center taps of the power transformer are also now located here. I get a "0" ohm reading between all the ground points and various points on the chassis although I haven't yet acquired a heavy duty soldering iron to re-solder the cap can tabs.
HotBluePlates- I figured it was zinc, basically galvanized. Zinc is conductive as far as I know.
Question to all: the first power up of this amp produced a runaway oscillation that was remedied by reversing the O/P Transformer Primaries. Could this have any bearing on this? I don't see how myself but I'm grabbing at straws here.
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.... although I haven't yet acquired a heavy duty soldering iron to re-solder the cap can tabs.
You don't need to, just bolt it. Many, most guys here bolt the main ground point to the chassis.
Just sand the chassis, on both sides, with a little fine sand paper or emery cloth in the spot where you put the bolt. A little anti-corrosion paste can be applied to the chassis and bolt/star washers too. They sell little tubes of it at the big box stores. Electricians use it. And I use a star washer on both sides of the chassis.
Lock the 2nd nut, 'lock nut', down real good, so it wont come loose. You can use a little lock tight on the nuts too. And that 2nd nut you could use n actual lock nut for that 1. But I'd still use 2 nuts and lock them up tight with 2 wrenches, or a wrench and a nut driver, against each other with a couple drops of lock tight.
It works fine.
Question to all: the first power up of this amp produced a runaway oscillation that was remedied by reversing the O/P Transformer Primaries. Could this have any bearing on this?
No.
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Wallabe- I have several grounds bolted to the chassis already. The power tube pins #1 are soldered to eyelet tabs that are attached to the screws holding the tube sockets. The Safety ground of the power cord is also attached to an eyelet tab. As i said i isolated this ground from the other ground points.
I didn't think that the reversed O/P transformer primary wires was causing the Trem switch but thanks for confirming. This still bugs me anyway.
I'm going to go through the Tremolo circuit again and make sure this is not a wiring problem. I'll look at all the voltages too.
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I have several grounds bolted to the chassis already. The power tube pins #1 are soldered to eyelet tabs that are attached to the screws holding the tube sockets. The Safety ground of the power cord is also attached to an eyelet tab. As i said i isolated this ground from the other ground points.
You want as few chassis ground connections as reasonably possible. The best being only the safety ground and the circuit ground. Although more than 2 chassis grounds can work too.
Have you read this from forum member Merlin's web site on grounding?
https://www.valvewizard.co.uk/Grounding.pdf
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Good article. I'm familiar with the concepts having read about this subject before. Thanks!
Another weird symptom with this amp is that when the VOL pot is at zero you can hear a hum which is modulating as if it is being controlled by the Tremolo circuit. It goes away when you turn the VOL pot past "2".
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Have you tried moving the tremolo B+ wire to the screen node as suggested in reply #30?
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I missed that one. Well, it didn't have any effect on the Tremolo F/S problem unfortunately.
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Still no solution to problems here. I ordered a 100 watt Weller soldering iron to see if I can get some solder to stick to the chassis.
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Let's try a logical approach and see if there are any clues...
Voltage measurements for all filter cap B+ nodes. And voltage measurements for all tubes (plates, grids, and cathodes). Some grids will be at zero volts. That's important so include zero volt readings as well. For chassis ground, clip your negative meter lead to the point where the PT red/yel wire is connected.
With power off, measure resistance of every tube grid and cathode in reference to chassis ground.
Post all readings. Maybe we can find a clue.
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Thanks sluckey for sticking with me on this. I'll post a table with readings ASAP. I will first say that plate readings at V1 and V2 looked above spec.
The multi can cap I used is a CE 40/40/40uf @525V. Schematic called for 30/30/30uf. Is this a possible problem?
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The multi can cap I used is a CE 40/40/40uf @525V. Schematic called for 30/30/30uf. Is this a possible problem?
The value difference isn't an issue. However, one or more of the caps in that can may not be performing very well. Temporarily connect a 20µF to 40µF cap across each of the cap can terminals to see if that eliminates the issue.
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I could look at the B+ nodes with my scope. I wasn't expecting a new cap can to be faulty but anything is possible. Here is a table of the voltages and resistance at the tube pins:
(http://Princeton Measurements)
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Min - Max values are shown where values are fluxuating. These rate of flux is affected by the rotation of the trem SPEED knob.
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A thing I noticed in the layout in your photo is you have that funky extension of your copper ground bus bar coming quite close to a B+ wire attached to your cap can. Could that be causing the interference generating the oscillation? Maybe just a wild guess, but you could easily bend them away from each other to check.
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I'm going to re-think the grounding once I get a 100 watt heavy duty soldering iron so that I can solder directly to the chassis. I'm going to make sure the cap can is grounded better for one thing.
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Do the 6v6 pin5 dc voltages stay the same without the oscillation and then when the white wire is grounded.
Use alligator clips to ground the speed pot resistor and the intensity pot cap to the chassis ground near the transformer and then ground the white wire to chassis or alligator clip it to the same spot near transformer.
What is the 6v6 plate current.
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When the footswitch connection is shorted to ground the -35V at the PWR tube grids stops modulating as it's supposed to.
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I'm going to re-think the grounding once I get a 100 watt heavy duty soldering iron so that I can solder directly to the chassis. I'm going to make sure the cap can is grounded better for one thing.
You don't have to do much to test whether the B+ wire is interfering with the copper bus bar - just push that yellow B+ wire from the cap can away from the copper wire.
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I was not happy with the grounding so I re-worked that part of the wiring. I bought a 100 watt Weller iron and soldered ground nodes directly to the chassis. It was necessary to rough up the locations with some sandpaper but now the ground points are solid. I was also able to solder the tabs of the cap can to the chassis this way.
The Tremolo Foot Switch still doesn't work properly. When the wire going to to switch is connected to chassis ground there is a buzzing in the output. :BangHead:
I think I'll go have dinner and drink a couple beers.
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I feel your pain. I have one last suggestion. Temporarily ground the speed pot lug that has a yellow wire attached. Does that kill the tremolo without any buzz? If so, then connect your F/S jack to the speed pot.
My next level is to replace that black board with one of Hoffman's green fiberglass boards. Not necessarily saying the black board is the issue, but the process of replacing it will likely fix whatever is causing the issue.
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I suggest to try a grid stopper at V2 pin2, in the range 10k to 100k, properly mounted at the socket lug.
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I bought a 100 watt Weller iron and soldered ground nodes directly to the chassis.
Works even better if you use solder flux paste.
And a powerful flat tip iron. Fender's factory ladies had big old clunky flat tip irons that were on all day
And you really need to get the chassis to immense heat for this to work - if you don't have lots of heat and solder flux, you won't get good connections - and in doing this you risk melting film caps etc.
A better method is to use a common ground buss wire that is attached to the chassis at the input jack. Much easier to solder ground return wires to this.
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Look for a vintage American Beauty soldering iron. You can usually find one for $40-50. I bought one that's 200W, if I recall. I only use 'Big Bertha' for soldering to the chassis in vintage amps. The chassis acts like a heat sink. That big hunk of iron stores thermal energy so the chassis can't wick all the heat away. I use a copper ground buss wire in new amps.
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A steel chassis is not a very good conductor (so adds resistance) and it's ferromagnetic (so adds inductance).
It should not be used for any signal or power supply currents.
Multiple chassis grounds invite ground hum and can cause unwanted coupling effects.
Single chassis ground and a copper ground bus is a way better solution.
I like a 1.5mm dia silver plated copper wire.
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Did you try the suggestion in reply #52 for the trem f/s issue?