1964 Tremolux Redux

I got this 1964 Fender Tremolux head and cab specifically as a non-working project.  I knew the following when I got the amp:

-it didn’t make any sound
-the red power light came on and tubes lit up

So that was it –  my great adventure would begin with only that knowledge.  I know it’ll need *something* to come alive, but we’ll found out just how much and what butchery has been done over the last 53 years.  This blog post will cover logs of interesting things, like rectified voltage and Russian cyrillic characters.  Yes, you read that correctly.

Checking out the Chassis

Every amp of this age needs a general service no matter what – old dried out capacitors, cleaning contacts, spraying out pots, the usual things that a vintage piece of electronics needs.  But with this guy, even a quick glimpse of the chassis shows something is up.

Tremolux Power Transformer

Tremolux Power Transformer

Here we see the power transformer (and 50 years of smoke and oxidation on the chassis) but it’s obvious this isn’t what Fender installed in 1964.  A bunch of extra wires taped off to one side is suspicious, but if it works properly I’m fine with it being a replacement.  Here’s the back side of the transformer as well, with codes that also don’t point to a 60’s Fender part:

PT Bottom

PT Bottom

After some poking around I found this was indeed a Fender part, but it’s the power transformer from a Fender 30 amp manufactured in the 80s.  We’ll see why this is a problem in a bit.

So moving on, I’m performing the usual work, replacing the electrolytic capacitors.  When I flip the amp over to do the filter caps underneath in the dog house, I see they are all exploded (one had already been replaced with a wrong-value part, so I only have 4 of the originals to show you):

old capacitors

old capacitors

It’s pretty common to see one or two leaking, but that third one actually has a hole in the side.  Certainly not ideal, but it’s a 50-year old part that’s getting replaced anyway.

I finished up the service and I wanted to be cautious with the startup since I had an unknown power transformer.  I didn’t know why it was replaced and I didn’t know if it was going to behave properly in this circuit.  I had my multimeter setup to check the voltage in the power supply as soon as I turned the amp on and I found about 520VDC in my power supply – way more than what the amp ever called for.  Not only is this value higher than I expect, but it exceeds the rating for the parts in the amp, both the ones that Fender used originally as well as my replacements, as those are only rated for 475V.  I shut off the amp and did some digging.

It turns out the power transformer from the Fender 30 uses a much higher voltage than what the deluxe wants to see.  I’m guessing this transformer was installed some time ago, as the wires and solder joints all had a similar patina as the rest of the amp.  When the amp was turned on after this work the filter capacitors exploded after a short time, and no further troubleshooting was done.  It likely sat in a non-working state for some time.

Fender 30 PT test

Fender 30 PT test

Here’s that Fender 30 transformer out of the amp and on my workbench.  We can see that one side of the high voltage winding shows 380V.  I would order a replacement transformer that has 325 volts of AC, which I would expect to come out to be about 460V DC after it’s rectified.

New transformer

New transformer

Here’s the new transformer showing 332V AC.  A much better number for a Tremolux.  I’ll get this installed and see how accurate my math was.  Here’s some pictures showing the installation process.

new PT in place

new PT in place

It looks way less messy once the wires are twisted and installed, shown here:

New PT Installed

New PT Installed

So the new transformer is installed, and I had the voltages I would expect.  With the slight drop caused by the rectifier tube itself the voltages were safe even with no tubes installed or any current being drawn.  Nothing is going to explode this time around.

I was preparing to fire up the amp for the first time so I checked the tubes that were installed.  The preamp tubes looked fine but these were the power tubes installed:

What tube is this?

What tube is this?

If you click to enlarge that image you’ll see this tube is labeled 6N3C.  All the big Fender amps use 6L6 tubes, so I had to look this guy up.  This proved to be a challenge as I couldn’t find *anything* on this tube.  After a bit of research it turns out this is a Russian tube and those are cyrillic characters.  The N is actually the “pie” symbol you see in math equations and the C is really an S, making the English designation for the tube 6P3S.  Looking this up this is a suitable replacement for a 6L6, rated for 20 watts, so we’ll leave them in for now to see how they sound.

I fire up the amp since it should be ready to go at this point, and there’s no sound at all.  No signal is getting to the power tubes.  I traced the problem to the area of the phase inverter and found this:

bad phase inverter

bad phase inverter

While those resistors look fine, each one is actually cracked in a different spot.  Some were cracked where they were soldered to the board, and some were cracked where the wire came into the resistor body.  I replaced all three with shiny new ones…

yay new phase inverter!

yay new phase inverter!

…and the amp came to life.  Alive, but sounded terrible, with lots of hissing and sputtering.  A bit of troubleshooting later and I found two resistors in the preamp needed to get removed since they were noisy:

new plate resistors

new plate resistors

And the first preamp tube also needed to get replaced.  Sorry old Mullard, you had a good run.

old dead Mullard

old dead Mullard

Don’t worry, I replaced it with an old GE I had that sounds great.

After that the amp sounded great and idled quietly.  I hadn’t opened the speaker cabinet to see if there was anything changed there, but it was only good news from here on out.  You know the feeling when you turn screws that have never been removed?  This speaker cabinet had never been opened since it left the factory.  Here’s the nicely arranged insulation:

speaker cabinet insulation

speaker cabinet insulation

and the original Oxford 10L-5 speakers, with matching date codes from the 26th week of 1964:

original speakers

original speakers

I made sure they worked, sounded right, and were in phase, and then put the screws back in the cabinet.  Nothing much else to see or do here.

The only other minor tweak that was necessary was to one of the tilt-back legs.  The stop button was pressed deep into the wood and the leg wouldn’t rest properly.  So I added a washer behind the button and now things line up great.  No one is every going to notice, but it makes the legs work how they should.

washer shim on the tilt-back stopper

washer shim on the tilt-back stopper

The only thing that couldn’t be saved, besides the worn out electronic components, was the piggy back mounting hardware.  Both pieces were bent, likely from a drop or hit the amp took while in use.  These will get replaced with shiny new ones.

bent thumbscrews

bent thumbscrews

So those Russian tubes?  They sounded good, and biased at around 60% they were putting out about 12W of power for each tube.  I put in some new Tung Sol 6L6 tubes, biased those to 60%, and the amp is putting out a much more healthy 36ishW of power.

So, a little wipe down with some Windex, putting everything back together, and it’s good for another 50 years or so.  Here’s the full stack ready to rock!

Fender Tremolux head and cab

Fender Tremolux head and cab

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