Overhauling a National Dobro Mystery Amp-Mystery Solved.

UPDATE: The clip from the 1942 National Dobro catalog identifies this amp as a Concert Amplifier No. 400.





This amp arrived with a seriously cooked power transformer spewing goop, and mesne other complaints. A little research revealed.....nothing.

A lot more research revealed.....more nothing.

As yet I do not know what the model number of this amp is, officially, so I'm calling it a model 100-B. It seems to be a modified National Dobro model 100, for which the schematic can be found in the John Rider Public Address Manual.

The 100 schematic bears a date of 8-28-39, and I have this mental picture of some poor schlub at the National Dobro design office having finished his master work three days before the most destructive war in history broke out, but nevermind. 1939 must have been a helluva year.

There are significant component value differences, some of which are related to the use of 6SC7s for the second preamp and phase inverter functions. I have documented these from actual observation.

The original Stancor P6013 power transformer had breathed its last, expiring in a puddle of smelly wax. Looking around my laboratory I latched on to a Midwest Coil and Transformer unit from a Hammond organ, the part number being A-024110-1. The voltages were just right, but unlike the Stancor, the 5v winding was not center tapped. National Dobro chose to use the center tap for B+, no doubt in an effort to reduce hum in the Jensen G12RS field coil speaker. I pulled my B+ off one leg as is more customary and nobody's the wiser.

In addition, National Dobro chose to ground the high voltage center tap through a .05/600 capacitor, which I thought was surplusage and so it got left out.

The output transformer, as yet unidentified bears a part number of C-3569 but no other marks.

I also took the opportunity to map the voltages present in this amp which are used in my hand drawn schematic. I have not yet figured out how to use Express SCH, which is a freeware schematic drawing program.

How's it sound? The fact that the amp has two different preamp schemes is a gift. The instrument input is sweet and clean, whereas the grid biased mic input is louder and grittier. The speaker's in good condition, which has to help.

I'm going to hate to give this back to the owner. Maybe he'll trade for something....hmmmmmmmmmm.

Bustin' Caps With Victor

In my continual quest for better, low priced test equipment I've been using an IRT-2 Insulation tester as described in an earlier post for determining whether capacitors-particularly the wax paper and foil types-are fit for continued use.

It's an important thing particularly in the case of plate coupling caps in the final stage of an amp, where a smallish amount of leakage-measurable in microamps-can shift the bias point of the tube enough to cause it to be seriously overworked-red plating is also part of the mix. The IRT2 has done a workmanlike job but suffers from the problems of analog stuff in that unless you can get the right range, the precision reading will escape you.

I started searching around and found a number of insulation testers on fleaBay, which range in price from inexpensive on the low end to ridiculously priced lab equipment. I didn't feel the need to pay for lab grade precision with lab grade prices.

Enter the Victor.

It's a digital readout insulation tester from the Shenzen Victor Hi-Tech Company with three test voltages and the ability to measure AC voltage up to 750-a nice feature.

For our purposes we're using the 250v, 500v, or 1kv ranges, and with the two position range switch we can read from 0.1 to 20 megohms or 20 to 500 megohms at 250v, 0.1 to 50 megohms and 50 megohms to 1000 megohms at 500v and at 1kv, 0.1 megohms to 100 megohms and 100 megohms to 2000 megohms-a substantial increase in range from my trusty IRT2.

It came in a nice box with a set of batteries (a big plus, in my opinion), test leads and alligator clips, a carrying strap, and a cover for when the device is not in use, and an instruction manual written in Chinese. It took a little bit of head scratching and a trip to the Shenzen Victor website (http://www.china-victor.com) to suss out the connectors and how to use the beast. However, the fit and finish is very good, the display is large and bright, lighting up on test, and the ranges are good for what my purposes are.

It's evident that the Shenzen Victor people put a lot of thought into this tester and did a very nice job at a good price, shipped for around sixty bucks. Rather than the generic QC passed white sticker this one came with a card with the part number and serial number, the date of manufacture, and the inspector's stamp on it in ink.

Here's an old Sangamo capacitor on test, showing a resistance at 250v of about 79 megohms give or take. At higher voltages, of course, the leakage rate is likely to be higher. Some work with my usual poly caps and some Orange Drops will yield a range of useful figures.

It's well worth the price of admission because you can also use an insulation tester to check transformer windings or any sort of circuit for high resistance shorts.

Props to the good folks at Shenzen Victor. They're doing a great job of putting serviceable, modestly priced test equipment in the hands of people like me.

More Capacitor Testing Adventures

Having an ESR tester around the shop is a useful thing when it's time to start looking at the electrolytic capacitors in an amp. I had been using my ESR Micro 3.1, made by Radio Devices in beautiful downtown metropolitan Tver, Russia about which more later.
All was going swimmingly until I went to test a capacitor that had an open ground and was thus chock full of juice-the ESR Micro 3.1 popped and although this is supposed to be an easy fix the diodes and 100 ohm resistor that are supposed to protect the innards seem OK so I figure one of the ICs is gone....where was I?

Oh. I remember. ESR, or equivalent series resistance, is a good metric for checking older electrolytics. The idea is that as a capacitor deteriorates over time it will build up internal resistance that can be used as an indicator of condition.

Well. What to do?

I didn't have three weeks to get another from Mike Klykov although I think I may anyway, so I ordered this ESR tester off of Amazon that came for about $50 shipped.

Of course it's made in China, and of course it has the obligatory "QC passed" sticker and of course it didn't work right. It wouldn't zero but occasionally could be persuaded to go full span if the panel was pressed on. I guess that's what passes for QC in China.

So there I was, faced with the prospect of boxing it up, shipping it back, and screwing around for another couple of weeks.

I figured it was worth a try to fix it, so I whipped the back off, removed the circuit board, resoldered the tabs on the zero set trim pot, shot the pot full of contact cleaner, and put it back together.

Now, I can put my own QC Passed sticker on it because it works just fine.

The MAT Electronics MUL3333 ESR tester is an analog meter contained in a substantial case, large enough to hold the 6 AA batteries you're going to need. It comes with test leads which I shall modify with alligator clips. One thing I like about the ESR Micro is that needles are soldered to the alligator clips, making it easy to poke through the lacquer and get into a trace.

The instructions are pretty straightforward even if the translation into English is a little strained at times. It tells us that the internal frequency of this device is 66khz, and from what I read this is an AC ohmmeter with an internal power source.

Electrolytic capacitors ESR tends to increase over time as the electrolyte dries out or the integrity of the seal is compromised. If the ESR value rises, it is the first sign of a capacitor that's on its way to glory. It's a worthwhile test for any shop bench, and if you're like me and can't afford a Sencore LC3, a simple tester like this is the way to go.

We're going to give this tester a run for its money and we'll have a fuller report. And we'll still order another ESR Micro 3.1.

Further Adventures in Capacitor Testing: The IRT-2 Insulation Tester.

When you are in the amp repair business on any level you soon progress from slinging a handful of orange drops at whatever strolls in the door to trying to do some real component testing. With non-electrolytic capacitors that usually comes down to figuring out ways of determining whether the leakage level has become unacceptable.

Why should this matter? Leakage in a signal capacitor gets the wrong kind of voltage in the wrong places, and it can shift the bias point of a tube enough to, in some cases, cause a meltdown. I was first introduced to this when rehabilitating a Fisher 500c integrated receiver amplifier a few years ago. One of the four 7591s was getting a sullen red glow in the plates after some time operating, and it was leakage of the blocking capacitor between the plates of the phase inverter/driver and the power tube in question.

Another reason is that in vintage guitar amps we don't want to replace anything that does not need replacement, lest we be accused of not knowing our trade, shotgunning parts because we don't understand the problem that presents, or worse, mining out vintage parts from our customer's amps and reselling them on fleabay-which has happened to my knowledge.

Well, sometimes it's a chore to fire up the Sprague TelOhmike, the special purpose direct reading capacitor leakage tester I built here a while ago, or something spendy like a Sencore LC3 if you should be so lucky.

Enter the IRT-2 insulation tester. Like many of its ilk it is a high resistance tester used by the refrigeration trades that can suss out high resistance shorts in entombed electrical equipment. It does this by a small power supply that can pump out up to 1 kv of ac at very low currents. It's quite enough to test the windings of transformers on the bench for high resistance shorts, but it can also be used, as is shown here, to test capacitors for unacceptable levels of leakage. As a practical matter, anything less than 200 megohms at 1 kv gets rejected here.

Pictured is a red paper Astron showing about 10 megohms resistance-far too low for my comfort and thus it makes a detour into the waste basket.

My IRT2 was obtained for five bucks at an auction of the estate of an old refrigeration and electronics man. A set of batteries and I was in business.

I'd advise against using a crank operated megger for this purpose-too many hands going in too many directions at once even though they do look cool. Good serviceable digital readout insulation testers are available for about fifty bucks shipped from the PRC, brand new with batteries at the usual place and I may get one if I can find a reason.

The Last Mystery: Unraveling The Standel 25L15

One of the last great mysteries of the musical world can be posed as a question: "What's inside the Standel 25L15?".

It's also a question that may soon be answered, although if Bob Crooks, the creator of the 25L15 was anything like Leo Fender, there were a lot of changes made along the road without a whole lot of documentation.

Here's what we know about it:

The 25L15 guitar amplifier was developed and built by Bob Crooks in the early 1950s. It's unusual in that it used 807 output tubes instead of the more commonly available 6L6 variants. As was common in those days, it was probably seen as good practice to put the power supply in the basement and the preamplifier stages upstairs. This arrangement could certainly cut down on crosstalk between the power supply and the preamplifier.

Crooks also used a JBL D-130 speaker, and it was probably the best choice for high volume, distortion free reproduction that could be had in those days.

Probably the mojo's in the preamp and tone circuitry and that's been the subject of much speculation. I've seen at least two schematics that are alleged to be 25L15 clones, but one of them is, according to Ted Weber the Hoosier Electronics Wizard, "not even close." Ted is contemplating releasing what he calls the Pennebaker 25, and if he does, then the cat will be well and truly out of the bag because he is a fellow whose veracity is unquestioned.

A fellow named Bill McKenna has also produced a schematic on DIY audio that you can obtain and read with jschem, a freeware utility.

Much of the speculation centers around the circuitry and its relationship to the Williamson circuit that was popular in the old days of high fidelity. That may well be so, although I cannot vouch for it and nobody who hasn't seen an original 25L15 circuit laid bare on the autopsy table or has an original schematic can.

We also know that production of the 25L15 by Crooks was very limited. There were maybe 100 or so built and they command astronomical prices on the rare occasions when an authentic one appears on the market. According to the list maintained by the good folks who are reissuing the 25L15, most of the originals are unaccounted for.

There are a number of people around who say they've reverse engineered the original 25L15 (as the people in California allege) or that they've reproduced it in some way, but they're bound to silence by a dark and bloody oath.

One thing's for sure. There're some rumblings that may well let this cat out of the bag. Then, and only then will we see where the mojo is hidden.

Further Adventures From The Tonal Fringe: The Baldwin Professional "Willie Nelson" Model

Here's a chunk of vintage solid state gear that followed me home the other day for a rather modest fifty bucks.

It was crusted with the remains of a million Marlboros and Camels, and it stank up the cab of the pickup all the way back home. A couple hours scrubbing, a bottle of 409, and a roll of paper towels later this is what was revealed.

It's a Baldwin D-1 Professional guitar amplifier with-a drum roll, Maestro-Supersound, which consists of five different voicing selections that can be used singly or combined with the standard voicing.

It's an interesting piece of gear, and it dates from the early days of solid state amplifiers. It was hand wired on perforated phenolic boards, which seems to suggest that the production was pretty low. I've never seen another like it.

It was made by the folks who make Baldwin organs when they took a brief flyer in the amplifier field in the heyday of sixties guitar mania.

Tonally, it's smooth and mellow, and getting clear tone requires something with single coil pickups like a Telecaster. No humbuckers need apply. The nonoriginal speaker's nothing to write home to mother about but a Weber Chicago has been obtained from Gregg Levy and will arrive soon. That alone ought to provide a quantum improvement in tone.

The vibrato is lush, but the reverb is a bit of a disappointment. It's adequate, but that's all. Overdriving the preamp with a Tubescreamer is better forgotten about and yields little improvement of the kind that can be gotten with similarly smooth sounding tube guitar amps.

It is said that Willie is enamored of a Baldwin that he is known to use, and I can see why. It's easy to love and makes no great psychic demands.

Output? Allegedly 75w peak, which really means 25 or 30 on a good day when all the planets are in alignment.

Rollin' With Bone

I figured out how to get the CD player working in the Ford Ranger this morning and loaded in T-Bone Walker: The Complete Imperial Recordings from 1954.

I do not think there is much to top tooling down the highway listening to the blues.

What do I mean?

Give ear, o ye multitude.