Reviving A Sick Twin Reverb

I got a call from a new customer about a sickish Twin Reverb that he had. It had died on the job and another local tech had retubed it but for whatever reasons decided he was done with it. I talked to him over the phone and it appeared as if the amp had been inartfully modified by person or persons unknown and the preamp tubes flashed on powering up. There was no output to speak of.

So I hauled it home and performed an autopsy. Sure enough as you can see the center power supply capacitor was missing, and the bias setup was still the balance control with insufficient voltage at all the relevant grids. This requires using the layout (easiest way) and duplicating the original component values.



The first task was to do enough revival to assess the rest of the amp's condition. With no tubes in it the B+ voltage was well over 525v which is very high even for no tubes. A rebuild of the capacitor board was in order and a careful tracing to determine that there was no kooky stuff going on with the wiring.

After that, a look at the bias board convinced me to make another one with new diodes and such and convert the bias balance to the preferable level set system of the previous Twin Reverbs. Here's what it looks like.

With all that the voltage was still way too high, and then I noticed that the center tap (red-yellow) from the power transformer had broken off at the ground lug. Once that was taken care of, the voltage came right down to normal, and reinstalling the power tubes and setting the bias around 36 ma was a cinch.

And that flash from the preamp tubes? They were Groove Tubes, but  they must have been old stock EIs from Serbia. They're well known for doing this on startup, along with Bugle Boys, Telefunkens and some others.

A few tweaks got the vibrato (broken 100k resistor on the control) and reverb working, and I'll finish the cathode capacitors in the morning and send it back home.

Improving A 68 Custom Vibrolux Reverb-Part 1.

I recently acquired one of these amps in a trade for a well used Pro Reverb and an overhaul of a drip edge Bassman. The owner didn't love this amp, it was only two months old, but I figured that there was good value to be had in a nearly new amp if a few well thought out improvements and modifications were incorporated.

I gave it a good workout when it came in, and it's true-the tone was sort of flat and without the old Fender sparkle that is what I admire and look for. It does have slightly differently voiced channels and the reverb and vibrato run on both, so that's a plus. The normal channel definitely has more girth so that's where I'm going for now. (Note: There is no such verbiage as 'defiantly'. Stop using it for 'definitely'. Autocorrect is not your friend-Ed.)

The first thing I noticed is that the power tubes were developing a low moan and rumble when tapped so they had to go. I hunted around my collection and found a nice pair of dual getter STR387s that married up to within 2 ma. If you have to ask what STR387s are, it's probably time to go back to tubes for beginners. I was originally going to use a pair of black plate RCAs but one was showing signs of high hours so out it went. I still may go that route one of these days, or maybe some 5881s which I have a pair of.

So I removed the idiotic hamster cage over the power tubes and got to work with the chassis on the bench. The first thing to go were the Illinois power supply capacitors. They're bottom of the barrel, lowest cost doggy doo, and not very robust in the bargain. So out they went to be replaced with some nice German made F&Ts. The easiest way to remove the power supply capacitors is to clip the leads and slice the plastic sleeve with a razor blade, pop the capacitor out, peel off the plastic and  clean off the silicone rubber on the board. You can use a little hot melt glue in place of more RTV to keep them from bouncing around.

The stub ends of the leads can be heated up and shoved out from the outside to the inside. Be careful. The pads on Fender circuit boards are fragile and easily damaged if one is too rough with them.

I should note here that this amp is built to ROHS requirements which means no lead solder. It's difficult to work with, so make sure you have plenty of desoldering braid and liquid flux handy.

The next job was to put in the STR387s and bias them properly. The Weber  bias calculator tells me that about 49 ma per tube is in the 70 per cent range so I stopped a couple of ma short of that. This amp comes with a fully functional bias adjust pot so it's easy cheesy to get it where you want.

I installed a new old stock GE 12AX7 in the first position and a British made Mullard in the second position, reassembled, and went for a test drive. It's slightly improved, but it still needs livening up.

Sometimes reinstalling the chassis is a chore because it peels up the tolex covering in places. You can either put in a few staples or use a butter knife to slip the chassis past the leading edge of the covering material.