63 Bassman 6g6-b

[Billsack]

Hey there, I have a 6g6-b fender Bassman, which currently sounds amazing and is pretty quiet as far as hum/buzz. However, I have a small issue with it. I opened it up one day to find the red/blue transformer wire had overheated and seperated from the bias board, and all solder joints on the bias board burnt/blackened.
A few months ago, I bought all new tubes: vintage RCA grey and black plate 12ax7 in the preamps and two allegedly matched RCA black plate 6l6gc for power tubes. I remember smelling a burning smell only after putting the new tubes in which is what prompted me to open up the amp in the first place. I believe I may have ran the amp without a speaker load for up to a minute after popping all the new tubes in, but I'm not sure if I actually did that after getting the new tubes or if that was a different occasion. One of my preamp tubes also introduced very loud hum, which I swapped out for a different one since. Anyways, the amp seems fine now, it sounds great, no burning smell since putting the wire back, but:

-the power transformer gets kinda hot, and the wax or whatever on it gets soft after playing for a bit (half an hour to an hour and a half). Also there's like a drip on it, I assume it came like that from the factory and not evidence of it severely overheating?
-bias measures, with a bias probe, 20ma for the one beside the phase inverter, and like 35ma for the other tube. Does this mean my tubes aren't actually matched?
-Should I be replacing those diodes on the bias board? I probably shouldn't trust them now eh, with all the burning? What would I replace them with? Can i test them in circuit? Seems like they'd take out my transformer if they shorted
-Also, is there anything else I should check? This amp needs to last my entire life lol, I love it dearly. It looks original except filter caps, 3 prong cord and bias resistor/cap

Anyways, I attached some gut shots and related pics

[g1]

It looks like the solder joint where the red/blue meets the 1K just was a bad/cold solder joint. It needs to be re-soldered if it hasn't already. The bad joint caused the arcing/blackening. The other joints on that board look like the 'paint' you sometimes see on un-retouched solder joints. The transformer issues sound normal.
Try swapping the tubes and see if the higher value (35mA) stays with the tube or the socket.
Someone has added a 43K across the 27K to make the idle current a bit hotter.

[Billsack]

So I found two tubes that both measure 20ma... so how do I go about biasing this thing? I see the layout calls for a 27k resistor in parallel with the cap on the bias board, where there is currently both a 27k and 43k so would I just add the two and replace the resistors with a 80k trim pot? And would I leave the cap as it is? Its currently a 100uf cap there.. It looks like the schematic calls for 25-50

[Billsack]

It looks like the solder joint where the red/blue meets the 1K just was a bad/cold solder joint. It needs to be re-soldered if it hasn't already. The bad joint caused the arcing/blackening. The other joints on that board look like the 'paint' you sometimes see on un-retouched solder joints. The transformer issues sound normal.
Try swapping the tubes and see if the higher value (35mA) stays with the tube or the socket.
Someone has added a 330K across the 27K to make the idle current a bit hotter.

You're right actually, it does kinda look like paint, weird that it's only on the bias board though. Also I was mistaken, it was one of the red high voltage trans wires that go to the diodes in series. The one on the inside, not the outer one.

Swapping the tubes resulted in the current swapping too, yea but I found new tubes, two different matched new pairs the both measure 20ma, which is very cold, no? Should be 30-35ish eh?

[g1]

The solder joint where the 1K meets the red/blue wire still looks suspect so I would re-do it.
Fender biases their amp quite cold, and that is part of the sound. So I always suggest sticking with factory bias for them.
Have you checked the 470R screen resistors on the power tube sockets?
Schematic shows around 430V for B+ and you show around 415V. And around -54V bias where you are showing -46V.
Do you have good 120VAC line voltage?

[The Dude]

The less negative bias voltage is because of the added 43K in the bias supply circuit. I wouldn't worry about it unless bias current was excessive, which it isn't according to measurements. Do check the screen resistors as g1 suggested.

[g1]

Re-reading, I see you first said it sounds amazing. All the more reason not to try to bias it hotter.

[Jon Snell]

Not a good idea to place a 43k across the 27k bias load as it will load the bias winding further. The mains transformer is suffering by the look of your photo with wax dripping off it, possibly a shorted turn will make it run hot. Remove the 5881s/6L6s and run it in standby for an hour, check for a hot transformer. If it gets hot, there is/are shorted turn/s. Time for a new transformer.
20mA on one and 35mA on the other valve is a misbias. Get a matched pair or bias them separately. The 'vintage' 6L6s, are probably second hand.
The stock diodes are fine, leave them where they are.

[Helmholtz]

Don't jump to conclusions.
I've never seen a wax potted Fender PT.
Rather they were potted with someting like polyurethane or epoxy.
The shiny outer trace seen in the picture is normal and not a spill-out but a leftover from the potting process.
It might feel sticky when hot, but still can't be removed with a fingernail (like wax would).
PT potting efficiently hauls out the inner (wire) heat while avoiding high temperature hotspots.
Those black Fender PTs do get hot in normal operation.
Make sure to use the correct mains fuse.

[g1]

Those black Fender PTs do get hot in normal operation.

This has been my experience as well.

[Tom Phillips]

As others have pointed out, many of the things you mentioned are normal for this amp .
Drips of transformer dipping varnish, black dope coated solder joints on diode board, hot running power transformer. All normal and all common for this model.
It's a mystery to me why Fender coated the solder joints on the rectifier/bias supply board. They stopped doing that on later production amps.

Edit - To be clear, those drips of varnish you see on the power transformer have been there since the transformer was made. They did not drip out of the transformer later after the transformer was installed in the amp.

[eschertron]

...and from my limited experience with vintage amps, the overall condition (judging from the gut shots) looks really good! As long as the PT is okay - test as per recommendation in post #8 - you have a solid unit there. Good find!

[Justin Thomas]

I'm wondering if that PT was already replaced? It's either a 58 or a 68, maybe a 78? I'll look up the part number meanwhile. The others seem to be original for the model. I know there can be some spread with Fender parts but I wouldn't expect 4 years for transformers. Wouldn't bother me at all since I'm a player & not a collector.

That's a beautiful amp; I've never seen the inside of one & I'd think it'd be a little more "busy" on the inside. How's that bass channel sound? I seem to prefer the more "atypical" Fender circuits...

I tend to bias my amps as cool as possible while still sounding good at gigging volume. Best for the tubes, transformers, my wallet, & everything else in general. And your PT looks perfectly healthy. The last time I had a melted transformer in one of my amps (courtesy of a friend not following my orders to not dime the amp for hours with a mismatched load), that smell lingered for weeks.

Jusrin

[pdf64]

I'm wondering if that PT was already replaced? It's either a 58 or a 68, maybe a 78? ...

Good point!
I suppose that a 1958 transformer wouldn't have that 6 number code though, rather a 125Pxx.

Just to note that the mains transformers for vintage valve amps can be expected to run fairly hot; their materials and design make it inevitable, and the resulting slight degree of HT sag helps towards the dynamic response that makes them attractive to musicians in the first place.

Wouldn't a shorted turn collapse the magnetic field and blow the mains fuse?

I wouldn't describe 20mA here as 'very cold'; 'cool' seems more appropriate.

The use of a 'through stack' mains transformer fastener to retain the chassis safety earth connection seems a bad arrangement to me, worst possible option really.
The mains transformer fasteners already seem to be the most mechanically stressed on the amp chassis, and 'through stack' ones will be subjected to magnestriction whenever it's powered.

When servicing vintage amps, it's typical to find those mains transformer fasteners loose.

[Tom Phillips]

Regarding the comments that the original power transformer may have been replaced. I agree and another bit of evidence is that the existing PT does not have a center tapped heater winding. That's why the two 100 Ohm balancing resistors have been added near the pilot light.