Debugging The Channel Switching On A Hot Rod Deville

(Note: This probably applies to any other amp in this series that uses a similar switching arrangement-Ed.)
(Note: The numbers refer to the Hot Rod Deville schematic, Fender dwg no. 050384 rev, B)

I've had a Hot Rod Deville here for a while as I was trying to sort out a number of problems. The customer complaint was that it had a sort of a crackle as certain notes trailed away. I concluded that it was one of the usual problems that these amps are prone to-bad solder joints on the tube board where the phase inverter resistors are located, a bad R97 resistor (these get hot), or a problem in the +/-16v supply for the solid state devices  that this amp has some of. Also on the list of possibilities were the usual bad solder joints, dodgy filter capacitors and so on.

After some initial testing I discovered that although the bias supply was appropriate for the pair of power tubes this amp had on board, the bias voltage could not be adjusted. That led to  having to remove the main board again, installing a new 25k bias potentiometer (R82) and ultimately replacing the tube board with another.

Great. That problem was cured and it was only then that I found that the channel switching had  disappeared down a rat hole. Since the owner does not use the dirty channel he'd probably never noticed it, all of which led to me dreaming up an amp checkin and evaluation form  so as to evaluate any and all systems on board before disassembly.

The problem manifested itself as the relays started chattering and a lot of noise was being injected into the signal. The system was unusable. Also the dual LED would not light red as it's supposed to when "more drive" is engaged.

After I'd replaced a bunch of small zeners and diodes with no luck I got suspicious of U3, which is a BA4560 dual op amp.  I installed a socket (good thing when removing ICs) and plugged in a 4558 op amp which is said to be equivalent. It quieted the relays but I still did not have the 'more drive' function (which is a couple of FETs).

So back to the drawing board and I ordered all the small parts I didn't have including some real BA4560s. They arrived today and plugging in a genuine BA4560 cured the problem. I think I'm home free on this one.

Points of information. I'd been informed by someone else that had this same problem that installing  a pair of new diodes D26 and D28 cured the problem for him, but the entire episode leads me to believe that if any of the small solid state components in the switching system fails you're likely to run into the same problem.

Best guess? They're cheap, so take the schematic and buy a few of everything you will need to 'cut and paste' your way through this problem.

And beware of "equivalent part numbers."

In the schematic I've marked out the different areas of the switching circuit. 1 is the +16/-16v power supply, 2 is the the switching circuitry including U3, 3 is the 820 ohm 2w resistor R97 and 4 is the more drive FETs.

Uncomplicating a jack swap on a Blues Junior

Every time someone asks me to get rid of one of those cheesy plastic input jacks on a Fender Blues Junior-or for that matter a Blues Deluxe, Hot Rod Deluxe, Devilles, Pro Juniors or any number of miscellaneous amps I always forget how it's done and have to go looking it up and wade through the dumpster that is the internet for that ".........OK. I remember now" moment.

The reason Fender uses them is simple-machine stuffing of circuit boards.

So here's a simplified drawing that will take you through it and remind me next time when I change the jack on my Pro Junior-which is a fun smallish amp.

We're looking at a 12A Switchcraft grounding jack, and two insulating washers from Mouser Electronics, a flat one and a stepped one. They insulate the jack from the chassis that way there aren't going to be any ground loops.

As always, check yourself and also me, make sure your amp is unplugged and the filter capacitors properly discharged. A test lead with an inline 100 ohm resistor will do fine for this chore. If you think it is beyond your range of normal competence, for crying out loud take it to a professional. There are three of us in the Des Moines area and all of us will serve you well.

For all that, 2016 was not a bad year overall, of course not mentioning politics. I dunno, the combination of social media, mass idiocy, and aggro rhetoric produced a perfect storm of sewage delivered right into my home that has left me cold. Some of the things I've seen people who claim to be adults give voice to were just that-liquid excrement. I've no interest in repeating the experience and I have terminated my association with them.

Racism and hatred has no place with me. Aside from being generally wrong, it takes up too much time that could be spent making music or helping others to make music.

I'm going to see what I can do to make my party more relevant, but in the meantime I shall stay close to the things I like to do and see and think about and let the other shit slide, along with the people who bought into it.

I think in 2017 a lot of people are going to find out they've been bamboozled and hornswoggled. Just my opinion, your mileage may vary.

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.

Disabling the Master Volume and Pull Boost On A Twin Reverb

There's a generation of hand wired Fender amps starting from about 1974 or 1975 that had a master volume and a pull boost added. These are generally despised as tone sucking turds, and although I wouldn't go that far, they can be easily done away with while you're reverting your Fender amp to an earlier and tastier configuration such as the AB763.

Enter the Quad Reverb. This was a 1975 issue Quad Reverb which is nothing more than the venerable Twin Reverb circuit in a 4x12" combo cabinet. In terms of wretched excess it is exceeded only by the Super Six and the early Peavey Vintage 6x10"-which I've only seen in pictures.

Well, nevermind. I acquired this Quad Reverb in a trade for some amp work I did for a guy, he got it from the widow of a friend in exchange for some house painting so we all made out good. The widow got her painting, my bass player pal got his Ampeg spruced up and I got the Quad Reverb.

And that's where things stood for about five years until last weekend when I asked myself what in heck I was doing letting this amp sit.

I'd recapped it way back when, so that was OK. The capacitor board was dangling by the wires and the cake pan was long gone. I decided it needed to be reverted to the original AB763 configuration and that involved changing it from bias balance to bias level, removing the master volume and boost switch, working my way down the circuit board and changing out everything that was non conforming and finally replacing most of the blue Paktron capacitors with Mallory 150s and Mojo Dijons. I finished it off with an earlier Twin Reverb faceplate to cover the hole and polished it up.

There are plenty of resources on the internet to guide you through this process. Suffice it to say, here are the steps and the links to the steps.

1) convert bias balance to bias level. http://www.premierguitar.com/articles/20025-ask-amp-man-better-biasing-for-silverface-twin-reverbs

You may need to tailor the 15k or 27k resistor a bit to give you the range you need to bias your power tubes correctly. In my case an 8.2k did the trick.

2) remove and discard master volume and pull boost. Props to Mike Pascale for this.

3) Retrogress to AB763 configuration. You can either use this link http://www.premierguitar.com/articles/19916-ask-amp-man-twinterventionrescuing-rotten-twin-reverbs

or do as I did and take the board layout and work your way from right to left and conforming your circuit board to what's on the original schematic. There will be a couple of orphan parts left over from removing the pull boost which takes its drive off the reverb circuit but if you leave them I won't tell.

Removing riveted in tube sockets on the Thunderclunker

Quite a number of amps have tube sockets that are riveted in. I can think of a lot of Ampegs for one, and a lot of Valco made amps, the Thunderclunker included.

It's quite simple for an old aircraft mechanic like me, but for those of you who are not used to this, do the following.

Use a spring loaded punch to put a small divot in the bucked head of the rivet-that's the round side.

Drill with a smallish drill so as to get a good pilot hole started. Then finish it off with a larger drill. I can't remember the sizes offhand but you can either punch out the rivet stem or use the butt end of a drill to snap the head off.

For tube sockets it's easier to make a little map of what goes where and then go ahead and cut the pins off with a pair of surface cutters. Then, once the new socket is installed clip the wires, strip them a little and reinstall.

Note the burned plastic on the old sockets.

them one by one.

Reviving the Thunderclunker a/k/a Supro Thunderbolt Harmony 420

I've spent a couple of months on and off negotiating with a guy down south in Clarke County for a defunct Harmony 420, a/k/a the Thunderclunker. My reasons for doing so were because with the exception of a treble and a bass control it's the same as a Supro Thunderbolt of the later, no rectifier tube variety. There are both kinds, y'know.

A little research in the schematic library revealed that the Harmony 420, a couple of Airline amps, and the Supro are all pretty much identical to wit, two 12AX7s and a pair of 6L6GCs, cathode biased, nominally 35w (which I don't believe for a minute) and a Jensen C15P ceramic fifteen inch speaker.

UPDATE: I put this amp on the dummy load and it produced 37w maxed out. It was clean on the scope right up to 25w, which suggests that a nice JBL speaker would not be a bad thing. I have a couple in need of repair sooooo........

The amp was completely dead with no glass and no sign of life, so a complete-and I mean complete-rebuild was in order.

First to go were the tube sockets (burned and cracked), the Mallory can capacitor, and the taped in output transformer, the two way switch, neon lamp, fuse holder, dropping resistors and power supply diodes which got replaced with 1N4007s. The old ones tested OK but for about ten cents apiece I wasn't going to scrimp.

 This is more or less what I found inside. It became a complete gut job of the power section.

Here's the pile of crap I removed.

And here's the finished product. The output transformer is a replacement for a Fender Vibrolux which I had. and the power transformer is original. About the only thing saved in the power section were the terminal strips. The fuse holder was broken so that went. The three way switch wasn't needed with the three conductor grounded cord, and I dumped the neon pilot light for a 6.3 volt Fender type lamp with a cool old vintage red jewel.

I measured up the finished voltages with a full complement of tubes and they are AC: 178 & 158 for a total of 336v, B+: 405vdc, Vscr 395, Vp 399 vdc, first preamp tube pin 1 244vdc, second preamp tube pin 1 219 vdc, pin 6 181 vdc, all of which are at reasonably healthy levels.

At the first power up all was well for a minute or two and then a heavy hum originating in the power section commenced to build to intolerable levels. After a minute's thought I remembered rassling with a Deluxe I used to have that did the same thing. It was as easy as isolating the transformer filament center tap and clipping a couple of 100 ohm 1/2 w resistors to the ends of the filament strings to beat some sense into this thing.

After some decent glassware was installed, the results were pretty serviceable with  the original speaker doing its chores.

Remember that this amp is of the later type with a full wave bridge so there's no transformer high voltage center tap, despite what you may see on the schematics that are floating around. I'm thinking that obtaining a a replacement power transformer for the solid state rectified amps might be a chore, if one was to need one.

Here's a layout of the power section I did to keep track of things while taking frequent coffee breaks and doing domestic chores.