Tube matching to the Max: The Maxi Matcher cometh.

I've been hankering after a Maxi Matcher for a while. It's a stand alone tube tester and matching device for certain power tubes and it's fast becoming the standard for the industry, so I thought I'd get one. Keeping up with the competition and all that.

I figured it was also a good bet for matching anything but KT88s and 6550s, as those I will still use Iron Gertrude for. That's my home brew tube matching device, it's completely analog and what's important is it'll put out 650v when called upon without even breathing hard, which is mandatory for high voltage Ampeg productions.

In any event the $695 was coughed up and the tester arrived here today.

Right away I knew something was wrong, and it is either me or the tester.

Doing the setups per the instruction sheet in the lid of the nifty hard plastic case I pulled a set of new Shuguang EL34s off the shelf, set the plate voltage to 400v, set the bias to -36v and proceeded to read the outputs.

The plate current should have been between 21 and 50 ma, and the transconductance should have been  greater than 4.0. What I got was around 6 ma per tube for plate current and the transconductance was between 1.76 and 2.31.

This was not good so I pulled another set of EL34s out of their boxes, this time a new set of JJs. The results were quite similar.

That got me worried. I measured the filament voltage and it was about 3.4vac, not the 6.3v one might expect. The 350v and 400v plate settings yielded  177v and 212v respectively.

I'm not convinced that the transformer output voltage is correct and I suspect that it has a 240v power transformer installed.

I'll keep you posted but until I hear back from the makers it's an expensive piece of eye candy, and I'll be using Iron Gertrude for my matching chores, as fussy as she is.

Stay tuned.

UPDATE:  I got a call from the folks at Maxi Matcher and what they told me was that they use a dual winding power transformer that is suitable for 120 or 240v depending on how it is wired. I got an email detailing how to switch the necessary jumpers.

In the lower left hand corner of the circuit board are  three places for jumpers. Bridge the inner one and you have 230v. Remove that and bridge the outer pair and you have a 120v transformer.

Smooth, Very smooth.

Rather than dismounting the circuit board I sweated out the existing jumper and installed two other jumpers-they look like staples-made from the wire shaft of a resistor.

It was easy enough to do and now it works just as it should, and the socket voltages bear witness to its proper function.

It still can't deliver the plate voltage that Iron Gertrude does, but it is quite a bit more stable and the digital readout is good for us old folks who tend to squint a lot.

I tend to think I'm going to be using this a lot.

UPDATE 2: I recently had a Blackstar HT60 in for a high volume crackling that it exhibited. The first task was to put the TAD EL34s -a/k/a gussied up Shuguang-on the Maxi Matcher and after about a minute of cooking the overcurrent LED illuminated and things shut themselves off as they are supposed to. This was a fault I could not have detected on my Hickok tube tester. A reboot confirmed this. A new set of EL34s from JJ and everything was peachy keen, no more unwanted crackling and distorting.
So score one tough fix for the Maxi Matcher that was worth it alone for what I paid for it.Colleagues have reported similar occurrences so I'm on solid ground here.

This is the second set of TAD EL34s in Blackstar amps that have failed or otherwise exhibited bad behavior recently. As of this moment they are all suspect and I'm not buying any.

Mystery Amp: The Epiphone Kent-or Maybe Asta.

Here's an interesting small amp I acquired recently and it was no stranger to me. I'd been at an auction a number of years ago and bid on it but failed to score it. So I consoled myself with a dolphin head Danelectro guitar. The amp itself showed up on craigslist on Saturday and I made my deal over the phone and picked it up yesterday. It was considerably less than it sold for at auction, plus it had been updated with a grounded power cord.

So then, the question was, what is it?

The unofficial New York Epiphone Registry thinks it is an Epiphone Asta, of which no catalog pictures exist. The amp they show on their site has four tubes-two 6V6s in series no doubt, a 6SL7 preamp, and a 5Y3 handling the rectifier chores.

I have my doubts. It looks awfully similar to the Epiphone Kent shown above from the 1950 Electar catalog, pictured in Epiphone: The House of Stathopoulo  by Fisch and Fred from 1996. This is described as a three tube amp developing 6-1/2 w and a single volume control.

The serial number on this amp is 2136. It's covered in tweed and has a 10 inch Rola PM speaker with a production date of 1949. It also carries a Freed power transformer which suggests that it was made in the New York area.

The extra large sized mud dauber nest is an extra cost option.

Nat Daniel made amps post war for Epiphone but according to him he and Epiphone had parted ways by 1946.

I would respectfully suggest that that is inaccurate, because the redesigned Epiphone amps in the 1955 catalog are quite similar in the panel control placement  and graphics to contemporary Danelectro amps including the placement of the pilot light . Nat Daniel was also known for using Freed transformers.

There is a legend on the later amps that says "distributed by Continental Music Champaign, IL-San Francisco CA-Atlanta GA". In further research, Billboard's list of exhibitors at the NAMM convention for 1951 in Chicago shows Continental Music, a division of C.G. Conn four doors down from Danelectro.

Fisch and Fred go into some detail concerning the relationship between Continental Music and Epiphone. which began in 1952 and continued to the end of the line.

I've yet to examine one of the middle fifties Epiphone amps closely, but I suspect not too many were sold because The House of Stathopoulo indicates that by 1957 Epiphone had become moribund and was acquired by Gibson.

If you have a schematic for an Epiphone Kent you can send it right along.

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.