Fender Champ aa764 Secondary shorting reading 3 ohm instead of ~277 ohm

[abajor]

This amp has a little bit of a story it was my dads since the early 80s. Some time in the 2000's he burnt out the speaker transformer so I replaced it, also noticed his Jensen cone had a rip in it and replaced it with an Eminence 820H.  The power diode tube was a little worn so I replaced it with silicon main difference is it no longer added any load to the heater.  I noticed the 1kohm that looks like it goes to pin 4 (Screen Grid) on the other side of the 10kohm 2 W was burnt so I replaced that.

I made a goof where I tied the two output coil taps together on the output transformer resulting in very wimpy performance.  While trouble shooting that I also replaced the main triple filter capacitor with 3 military grade Kemet caps.  After I untied the blue and yellow taps of the output transformer secondary (yellow only) The amps sounded great, very clean and chimey.

When I was moving from my old place my house mate had a bunch of 6V6 tubes and I could use any I wanted. Since I have a tube tester I decided to go through them.  I found one with unusually high trans conductance and decided to use it.  It seemed to work fine.  Shortly after buttoning everything up, I needed to move. I pulled the amp out one day to practice and I heard a moderate hum and smoke started coming out of the amp. It appeared to have come from inside the original power transformer.

I ordered a replacement from Hammond the 290AX thinking that old transformer had just been over stressed and had given up the ghost, but the new Hammond one smoked as well, and now I'm reading 3 ohm on the secondary and the manual says it should be around 277 ohm DCR.

Anybody have any idea what can cause the secondary to short like this?  My guess was that the 6V6 from the Lowery organ with the unusually high trans-conductance (it pegged the scale for 6V6), may have been oscillating?  Really sure if there is something else I should check first, I don't want to just melt another power transformer.

Any pointers would be appreciated.

Thanks!!

[sluckey]

I would first look at the power supply. Check for a shorted filter cap and shorted rectifier diodes.

[abajor]

I would first look at the power supply. Check for a shorted filter cap and shorted rectifier diodes.

I should have mentioned that the power diodes junctions appear to be fine. No short or open and normal forward bias voltage drop.

The Capacitors all appear to be good positive and  negative in their respective positions it drifts down from a few meg ohms down to ~600k ohm after a long time.
I also added a 1M ohm drain resistor so the caps don't stay charged too long.

I don't think it's the rectifier diodes or the capacitors.

[Ronquest]

You need a lightbulb current limiter
 https://sluckeyamps.com/misc/Amp_Scrapbook.pdf

And a fuse or 3

[tubeswell]

What sluckey and Robquest said.  A short somewhere is sinking a massive amount of current through a part that’s causing it to burn. Make a current limiter and find the short systematically, or go on guessing and replacing random parts.

[abajor]

You need a lightbulb current limiter
 https://sluckeyamps.com/misc/Amp_Scrapbook.pdf

And a fuse or 3

Well I mean since the secondary is reading 3 ohms that's not going to be particularly helpful.  At this point I need to order a new Hammond Power Supply before trying anything else.
I do happen to have inherited a variable AC power supply with current meter.  I suppose the best I can do is put in the new transformer and at a non destructive voltage and look for an abnormally low voltage along the plate rails.  I'd be surprised if one of those Kemet caps failed https://www.digikey.com/en/products/detail/kemet/PEG124YL2470QL1/13176285.

The DC-R of the output transformer appears to be fine.

[sluckey]

You should be able to find the problem using an ohm meter even before installing another PT.

A lightbulb current limiter will be more useful than a variable AC supply at this time.

[abajor]

I made a very embarrassing discovery, I'm not sure what possessed me to do this, but for some reason I wired the center tap (ground) of the filament heaters to the cathode I'm assuming I intended to put it on the ground side of the bypass cap and bias resistor for the 6V6?  What's even more baffling is I tied one end of the filament supply to ground. Basically giving a low DCR path to ground for the Cathode voltage, so that's embarrassing.

My only excuse is Hammond didn't send a proper printout with my transformer, still have no idea how I made such an insane screwup.  It still doesn't explain why the original PSU burnt out, but it explains why the new one is now dead.

[abajor]

I made a very embarrassing discovery, I'm not sure what possessed me to do this, but for some reason I wired the center tap (ground) of the filament heaters to the cathode I'm assuming I intended to put it on the ground side of the bypass cap and bias resistor for the 6V6?  What's even more baffling is I tied one end of the filament supply to ground.

I just looked closer at my amp. I'm assuming this is how the CBS silver face Champ was wired, but the original champ didn't have a center tap on the filament supply.  The more interesting thing was that the heater filament pins "Pin 7 on the 6V6" are all tied to ground with a piece of BUS wire.  So one end of the heater tap is also tied to ground.  The ones that go to Pin 2 come from the un-grounded lead.

This is unfortunate since if I had tied the center tap to ground like I intended, I would only have 3.35V on my heaters, but instead I just made the choice that actually killed my secondary.

Interesting that the Hammond filament transformer comes with the center tap, I assume that could provide better hum rejection.

So I now can conclude that I probably killed my PSU due to accidentally picking the cathode instead of ground, and not understanding that I should have just not used the center tap in the first place.

I'm just going to have to assume the other one died due to old age and abuse for now.

Just a thought.  When I get the new Transformer, should I put a dummy load on the 5V tap instead of leave it floating since I don't have a vacuum diode to heat?

[sluckey]

I just looked closer at my amp. I'm assuming this is how the CBS silver face Champ was wired, but the original champ didn't have a center tap on the filament supply.  The more interesting thing was that the heater filament pins "Pin 7 on the 6V6" are all tied to ground with a piece of BUS wire.  So one end of the heater tap is also tied to ground.  The ones that go to Pin 2 come from the un-grounded lead.

This is unfortunate since if I had tied the center tap to ground like I intended, I would only have 3.35V on my heaters, but instead I just made the choice that actually killed my secondary.

You would have still killed the PT because there would have been a dead short across that half of the filament winding. That center tap lead should be taped off and connected to nothing.

Just a thought.  When I get the new Transformer, should I put a dummy load on the 5V tap instead of leave it floating since I don't have a vacuum diode to heat?

No. Just tape off the end of the 5V leads and tuck out of the way.

[abajor]

@Sluckey

Thanks that makes sense.  Probably not fry the High voltage secondary, but kill the heater taps.

[sluckey]

@Sluckey

Thanks that makes sense.  Probably not fry the High voltage secondary, but kill the heater taps.

Its hard to kill a filament windind just because of the big wire used to make the low voltage/high current winding. You said the high voltage secondary was reading 3Ω. Most likely the high heat from the big wire of the shorted filament winding melted the enamel insulation of the tiny wires of the HT winding. You could probably simulate this by probing around inside a PT with a 200 watt soldering iron.

[abajor]

Update:

Ok so I got the new transformer and installed it properly, then very carefully discovered what killed the original transformer.

I am lucky enough to have a SENCORE AC "POWERITE" so I can slowly ramp up the line voltage. I started testing again with no tubes connected.
As soon as I had about 60% line voltage I started hearing a tick/crack noise and seeing current draw spikes on the meter, so I backed it off and measure B+ at around 250V.

When I did my silicon Rectifier mod, I didn't have any 1N4007 equivalent diodes so I connected 2 diodes in series and didn't add a 100ohm current limiting resistor, so I decided to re-do it proper even though the junctions on all 4 diodes appeared to be fine.

Just as a sanity check I powered up the amp with the secondary removed from the circuit and everything was fine.

After replacing the rectifier Diodes I slowly powered back up and again started hearing the noise and seeing current spikes.

None of the filter caps measured a strange value, but I removed the first filter cap before the current limiting resistors for the other two caps.  It measured the 47uF that I expected from the Kemet capacitor.

Powering it up without that first filter cap showed the circuit to be stable, so somehow that extremely expensive military grade Kemet Capacitor had an internal failure.  I replaced it with a 450V 22uF which I was quickly able to pick up from the local electronics shop.

Smooth Sailing. I'm a little shocked the KEMET failed, I should see if they warranty their very expensive capacitors, especially after it destroyed my original transformer operating well within it's design limits.

[mresistor]

Hi abajor  - it would still be prudent to build the very simple light bulb current limiter device if you are going to be working on or building guitar amplifiers.  link was provided.

[acheld]

Listen up!

You need a light bulb current limiter.   You're making quite a few errors -- nothing wrong with that (is how we learn!) -- but you need to protect yourself and your equipment.  Variac with ammeter is fine, but you want something in circuit that actually limits how much current can flow when you make a mistake.

Vast majority of experienced folk on this forum use them when powering up a build for the first time until you know that all is well. 

Edit:  Don't mean to pile on.  I was writing this while mresistor was posting. 

[abajor]

Hi abajor  - it would still be prudent to build the very simple light bulb current limiter device if you are going to be working on or building guitar amplifiers.  link was provided.

Listen up!

You need a light bulb current limiter.   You're making quite a few errors -- nothing wrong with that (is how we learn!) -- but you need to protect yourself and your equipment.  Variac with ammeter is fine, but you want something in circuit that actually limits how much current can flow when you make a mistake.

Vast majority of experienced folk on this forum use them when powering up a build for the first time until you know that all is well. 

Edit:  Don't mean to pile on.  I was writing this while mresistor was posting. 

Ok.  I'm not sure how it would have helped with my initial problem after the amp worked fine for months and randomly smoked the original transformer the second time.
The current limiting bulb at the recommended wattage ratings sounds like an extremely poor fit for a fender champ as it will allow ~60-100 Watts worth of  current to flow through the primary, considering the champ is ~15 Watt amp I'm wondering how that would protect the internal circuit in the champ?  Would it be a sufficient to prevent the champ from burning up its secondary if it can allow 4 - 10 times the power the champ is rated for to flow? Shouldn't I be using a bulb rated to a lower power dissipation?

I'm more interested in understanding why the dielectric in my new capacitors were compromised.  The rectifier diode junctions were all intact.
From going back and reading more, I suspect the reason I killed the original power transformer was that not using a current limiting resistor caused me to have significantly more inrush current stressing the secondary as well as a longer charging period due to doubling the capacitance.

But as far as I know there was never a chance the capacitors had reverse voltage applied to them. Having them fail under normal charge is still surprising to me, but maybe there is something more experienced tube circuit engineers know of.

Ironically the Fender champ is the least ambitious circuit work I've done, and I've done almost all of the work with schematics and a DMM.
My first tube circuit modification was rewiring my Fender PA100 to essentially use the Mesa Boogie Mark 1 circuit.  It still works great. I just knew enough to keep well away from live B+ and make sure its discharged long before getting my fingers near it.

I am not really experienced working with tube diodes, and am not convinced there is any great reason to keep it in the champ. Inrush current killing the secondary wasn't something I was aware of.

The light bulb current limiter sounds very cool, but I am right in assuming I need a much lower wattage rated bulb for a low wattage amp, since current is on demand?

[sluckey]

I would use a 40 watt bulb for a champ. A 40W bulb will limit the current to .33A. If .33A flows then all the 120v will be dropped across the bulb, leaving zero volts for the champ.

[acheld]

Light bulb current limiters are useful whenever significant work has been done in an amp, especially when the power supply has been altered. My guess is that all of us here have made significant errors over the years (I certainly have done!), and light bulb limiters have saved many a transformer (eg, time and $). 

You are quite right that one must use the correct bulb with regard to wattage in order to achieve useful current limiting.   That is all part of the process.   

Inrush current is normally a problem for tube rectifiers, not so much for the filter caps (of course depending on the voltages delivered), nor for the secondary windings (since the inrush current is limited to a short period of time charging up the filter caps).   

With regard to your Kemet caps failing: Having more than one fail simultaneously (if that is what happened) suggests a wiring anomaly.  Other biggies in the electrolytic cap world are heat, over voltage, excess ripple (eg, heat), and age.   I have not used Kemet electrolytics before, but their data sheet looks great.   

It sounds like there have some mods made to this Champ.  Do you have a schematic that you're working toward? 

[abajor]

Light bulb current limiters are useful whenever significant work has been done in an amp, especially when the power supply has been altered. My guess is that all of us here have made significant errors over the years (I certainly have done!), and light bulb limiters have saved many a transformer (eg, time and $). 

You are quite right that one must use the correct bulb with regard to wattage in order to achieve useful current limiting.   That is all part of the process.   

Inrush current is normally a problem for tube rectifiers, not so much for the filter caps (of course depending on the voltages delivered), nor for the secondary windings (since the inrush current is limited to a short period of time charging up the filter caps).

With regard to your Kemet caps failing: Having more than one fail simultaneously (if that is what happened) suggests a wiring anomaly.  Other biggies in the electrolytic cap world are heat, over voltage, excess ripple (eg, heat), and age.   I have not used Kemet electrolytics before, but their data sheet looks great.   

It sounds like there have some mods made to this Champ.  Do you have a schematic that you're working toward?

Just getting it fixed really.
My dad fried the output transformer so I replaced that.  My first mistake was not just taping off the higher impedance tap and connecting the Hammond blue tap in parallel with the brown tap, assuming it would not effect the performance since the brown tap was least path of resistance. This gave me extremely weak output.

I was uncertain about the condition of the rectifier tubes condition and figured it wasn't worth the vintage tax to replace (I personally have no interest in rectifier sag).

The original Jensen speaker had a rip in it so I swapped it out for an Eminence 820H. That speaker might not be for everybody but I sort of love its very even mid-range and top end.

I ended up replacing the caps (this model has that annoying triple cap canister) because I get a bit of poopy bottom end fuzz when using my neck pickups or playing some tritones.
I may be just expecting too much out of this amp, but it was a shot in the dark. Unfortunately that triple cap canister isn't easy to swap out so bent the leads down and consider it retired.

That would be my only other mod/fix I'd like is to see if I can improve the bottom end?  The top end and mid-range is stellar now but the bottom end distortion almost sounds like a mechanical vibration/rustling.  My best guess is my 6V6 bias might not be quite centered? Its a very inconsistent breakup.

I would use a 40 watt bulb for a champ. A 40W bulb will limit the current to .33A. If .33A flows then all the 120v will be dropped across the bulb, leaving zero volts for the champ.

That makes sense.  I figured it would become the dominant voltage divider at some point, especially if the secondary winds are shorted.  I learned a tiny bit about how the primary impedance is effected by the load on the secondaries from some EE101 labs in the past.

[abajor]

Another Fun update. So after I originally replaced the output transformer on this amp, I discovered the Jensen speaker had a ripped cone, so I replaced it.

One of the things that motivated me to replace the caps, and change the rectifiers was getting a bunch of weird distortion on the low notes after installing the speaker.

Turns out the coil form on the brand new Eminence 820H was chattering.  I connected an EVM12L to the amp and it had zero problems with any low notes. Just deflecting the cone or spider a tiny bit on the 820H makes a crackling noise, not connected to anything.

So that's fun what a comedy of errors. With the lack of use (less than an hour) and lack of power available to drive the speaker to it's rated limits, I'm keeping my fingers crossed that Eminence finds it to be a manufacturing error.

[HotBluePlates]

... the coil form on the brand new Eminence 820H was chattering.  I connected an EVM12L to the amp and it had zero problems with any low notes. ...

Make sure you're not cranking down the mounting bolts.  Go finger-tight, then maybe another half-turn or a bit more.

Over-tightening can warp a lightly-built basket/frame.  However, speakers with heavier frames may not be distorted as badly (or at all).

[abajor]

Well I'm not sure if Cranking down the speaker bolts could cause this, but the guy at Eminence said my coil was scraping against the center pole.  I'm assuming he meant coil form unless the coil is wound on the inside of the form?

Anyhow they're sending out a Warrantee replacement speaker so hooray.

I did have a question though, does rewiring the heater using this aftermarket Hammond PSU to have the +/- heater coil on either side of the Tube heater (while twisting the insulated pair together) and connecting the center tap to ground have a significant effect on hum induced into the preamp tube? As opposed to the stock config where there is no center tap and one end of the heater transformer is tied directly to chassis  ground?

I do notice a little, it would be nice to have less, but its not a deal breaker by any means. 

[Esquirefreak]

My general experience is that using two 100r resistors as a virtual heater CT, tied to the power tube cathode is the way to go with SE amps. In some amps it doesn't make a difference, but easy enough to try.

/Max

[acheld]

If I've understand the question correctly:     

The heater circuit must be referenced to ground.  IF your PT has a heater circuit with a center tap, then use it.  Not sure which PT you are using, but the Hammond 290AX does have such a center tap.

If your PT does NOT have a heater center tap, then the simplest arrangement is to ground each leg of the heater circuit through a 100ohm resistor.   Do NOT use both methods.

See:  https://www.valvewizard.co.uk/heater.html for a better explanation, and some refinements.

[abajor]

If I've understand the question correctly:     

The heater circuit must be referenced to ground.  IF your PT has a heater circuit with a center tap, then use it.  Not sure which PT you are using, but the Hammond 290AX does have such a center tap.

If your PT does NOT have a heater center tap, then the simplest arrangement is to ground each leg of the heater circuit through a 100ohm resistor.   Do NOT use both methods.

See:  https://www.valvewizard.co.uk/heater.html for a better explanation, and some refinements.

No I don't think you have.

Yes the aftermarket transformer is a Hammond 290AX, which does have a center tap; however, it is currently stock wiring, this is why I'm asking if it helps reject more hum than stock to wire the +/- legs directly to the heater elements with the center tap referenced  to ground.

The diagrams I have seen show the +/- wires twisted together and center tap at ground, I'm assuming this provides common mode rejection. I just don't know if it's going to reduce 60 cycle heater hum getting into in the gain stage.
I already hear some hum without an instrument cable. With the 12AX7 removed, I don't hear any hum with only the output tube.  So clearly the 12AX7 is amplifying a little 60 cycle hum.

I am assuming it is coming from the heater voltage.

Another option would be to supply DC to the heater but rectifying it would only give half the voltage - the diode drop.

[dwinstonwood]

abajor, try this first. It's easy to simply move the 6.3V center tap from its current ground point to the cathode of the 6V6. Look at the "F" connection in the schematic attached below. (It's a different amp, but the principle is the same.)

Next, if you still hear hum, I suggest buying one of these $12 chokes. Remove the 1K/1W resistor between the first two 20uF filter caps (shown on the AA764 schematic: https://el34world.com/charts/Schematics/files/Fender/Fender_champ_aa764_schem.pdf ) and wire in the little choke instead. It's small enough to mount inside most chassis; that's what I did in my AC4. These two things almost completely eliminated the hum in my amp.
https://www.tubesandmore.com/products/choke-4h-50ma-fender-6v6-amps-125c3a-replacement

If those two things don't eliminate the hum, it might be due to lead dress, transformer orientation, or something else.

[abajor]

abajor, try this first. It's easy to simply move the 6.3V center tap from its current ground point to the cathode of the 6V6. Look at the "F" connection in the schematic attached below. (It's a different amp, but the principle is the same.)

So is this a confirmation that simply tying one leg of the heater transformer to ground can be a source of hum?

Interesting does tying the center tap to the Cathode of the 6V6 give a little out phase feedback to the 60 cycle hum between the preamps and output stage?

I should just measure it all on an oscilloscope and see what happens between stock wiring and centertap reference choices.

[Esquirefreak]

Connecting the heater CT to the power tube cathode is a form of DC elevation.

/Max

[abajor]

Connecting the heater CT Is a form of DC elevation.

/Max

I'm not sure what that means.

If the +/- ends of the heater coil only connect between the heater terminals and you attach the center tap to the cathode the cathode is ground with respect to the heaters, there is no part of the heater circuit that is connected to chassis ground so it should not contain any DC Bias. I do see the potential to pass audio signal into the heater coils.

[Esquirefreak]

I forgot half of my sentence. I was interupted by something. Haha. Edited my answer. Sorry!

[tdvt]

Next, if you still hear hum, I suggest buying one of these $12 chokes. Remove the 1K/1W resistor between the first two 20uF filter caps (shown on the AA764 schematic: https://el34world.com/charts/Schematics/files/Fender/Fender_champ_aa764_schem.pdf ) and wire in the little choke instead. It's small enough to mount inside most chassis; that's what I did in my AC4. These two things almost completely eliminated the hum in my amp.
https://www.tubesandmore.com/products/choke-4h-50ma-fender-6v6-amps-125c3a-replacement

I will confirm that adding a choke to an SE amp makes a BIG difference in hum levels. One iteration of early Champs had them, presumably dropped due to cost.

So is this a confirmation that simply tying one leg of the heater transformer to ground can be a source of hum?

I have never had an amp with the chassis conducting one side of the heater circuit but that seems like a good way to have a source of AC hum running rampant throughout the amp.

[mresistor]

And yet thousands of original Champs and Vibro Champs were made with one side of the heater circuit running through the chassis and they exhibit very little hum, even when played through a larger speaker.

[sluckey]

Most of the 5XX tweed amps connected one side of the filament winding to chassis.

[abajor]

So there is no consensus on what I'll call Floating the heater coils and tying the center tap to some reference point (be it Chassis or output tube Cathode) and using no center tap and tying one end of the heaters to ground?

If I get the time I'll make measurements with the different methods. The hum I have isn't horrible I just notice it when I'm close to the speaker, it clearly isn't present when the 12AX7 is absent from the circuit.

[sluckey]

So there is no consensus

Sure there's a consensus. Using a center tap (whether real or artificial) connected to ground, gives better hum reduction than simply connecting one side of the filament winding to ground.

Your hum may not even be heater hum. Your champ would have even less hum if there was a filter cap and choke connected between the rectifier and first B+ node.

[AlNewman]

I don't understand.  If you are running heaters with a chassis return, and you ground the center tap to the chassis, wouldn't that just short out one side of the heater winding?

[abajor]

I don't understand.  If you are running heaters with a chassis return, and you ground the center tap to the chassis, wouldn't that just short out one side of the heater winding?

I accidentally fried my secondary that way because I didn't realize my original transformer didn't have the center tap.  Currently the Center Tap is taped off.

[acheld]

Photos of your heater circuit would help us help you.

A careful reading of Merlin's heater circuit page (cited above) will give you some ideas about how to approach this problem.