Friday, December 30, 2011

Hammond organ tube amps! Crusty AO-29 (M3-M100) in foreground, cleaner AO-14 (M2) in background. I know which one I would feel more comfortable plugging in...'s not what you would think by looking at this picture.

7 pin miniature, B7G, heptal, small button. Too much cool nomenclature, if I had to pick one, it'd probably be "small button", just to pare down those who would know what I'm talking about..

AO-29 guts. Pretty clean considering the blanket of dust top side. Healthy amount of in/out going on at the bottom of the frame

AO-14 guts. Let's take a closer look.

Coupling capacitor failure. Interesting failure mode without the presence of burn marks or signs of violent expansion, looks more like to just extended its innards, like a push up popsicle.

However, the mica domino that was bypassing AC mains hash followed a different path to failure.

Wednesday, December 28, 2011

DeVry. I just spent a few minutes of study trying to craft a neat time-line fashion historical interconnect between this amplifier and the DeVry technical institute. I wound up digging up ties to Lee de Forest and Bell & Howell. They are all connected in some manner. I'll leave it at that. It's just too convoluted a maze to touch upon here, and I'd like to flow some solder sometime today. Lee de Forest is an interesting character steeped in early electronics design lawsuits, let me kick this off with a quote:
"So I repeat that while theoretically and technically television may be feasible, yet commercially and financially I consider it an impossibility; a development of which we need not waste little time in dreaming." – 1926

Had they listened, what would we be doing today?

To sum it up: Hot chassis film projector amplifier.

The enclosure really speaks to my growing fascination with modular synthesis. Observe that the selenium wafer stack is bypassed with a silicon rectifier. The silicon rectifier exhibits a much lower internal resistance of roughly 0.6 volt forward drop than the selenium wafer stack, which exhibits roughly one volt drop per wafer - almost ten times as much resistance in its conductive cycle. Therefore, during the conduction cycle the electricity will follow the path of least resistance and bypass the selenium stack. However, each selenium wafer only withstands roughly 20-25 PIV, which is why they are stacked to provide a working voltage to begin with. By not removing the selenium stack from the circuit, its 100-125 volt PIV rating still stands during the non-conductive phase of the AC cycle, which, may I remind you, is non-isolated 120 volt mains.

In short, bypassing in this manner is a terrible idea.

Engraved by hand on the chassis at the tube sockets are the following designations from left to right: 12AU6, 12AX7, 35C5 and between the two sockets at the right 35C5. The right-most socket is wired differently, and during a few moments in which I had blocked the memory of the solid state rectifier from my mind, I considered this an error until I did a little more hunting.

Discovering pictures of a similar DeVry projector at auction produced the layout legend above.

Another note. I won't tear this one up, sort of speaks for itself.

Gut shot! Finally some guts. One of these weeks or months I'll dispel with the commentary and just cut to the chase and post innards. I owe it to the internet.

Thordarson transformers. This company has been going since 1895, keep the dream alive. I personally feel that the creed from 1937 far outweighs their current motto, steeped as it is in the way of things...

For all the shit I've heaped on selenium, the rectifier stacks sure are pretty.

I'll post a proper retrofit eventually. I realize my content is flighty, best to just roll with it.

Monday, December 26, 2011

So as not to sow the idea that I'm of the opinion logo design met with universal deterioration as the years rolled by:

I like the embellishments that occurred in the 12 years since the 93-350 amp was built.

I present the Allied Knight KN3050HD. I love the look of this amp, functional. The black on the enclosure is spray paint which was applied in a less than perfect manner, original coloring was similar to the dark turquoise section of the lower control set.

Don't sit on your sheet metal amplifiers! Left to right: 5V3, two 6CA7 and a quartet of 12AX7; hiding behind the cardboard wrapped multisection can cap is a 6AV6.

I think I did this in the late 1990s with the intent of lodging a 1/4" jack in that hole. I hate looking at my half baked ideas from over a decade ago poorly (or in this case, not) executed, hopefully 15 years from now I'll look back on what I'm doing these days with a sense of pride. Otherwise all is lost (HAHAHAHAHAHA).

Top shot speaks of less than optimal living conditions, can't play the patina card on this one - we've shot into rust territory.

From the front this amp is quite approachable by the pedestrian operator, more complex configuration is tucked away in the set & forget realm of the rear.

Guts! Feature set similar to the Bogen M60A, but a little more open & inviting.

Here's where the spray paint causes problems, masking a sheet of paper isn't difficult. Thankfully the schematic is still legible, though marginally presentable. Since I can read it I'm not going to mess with cleaning it up.

Somewhat blurry, but this too is readable. High tech date code.

All in all, the interior looks good. Hum balance pot above shows the trace has separated from its backing, but it looks serviceable none the less.

Saturday, December 24, 2011

Here's a Lear Siegler era Bogen M60A tubed PA head.

Compared with the Bogen logo from earlier days, I'm going to set my expectations a touch lower on this one. Granted, it's probably twice as powerful, but power isn't everything.

Classy design & good looks still count for something. Back to this amp, survey says: 1967.

A note! This isn't going to be the last note published here, nor is it going to be the most interesting, but - this one is highly informative. Let me run down my analysis of this note (spanning the next couple pictures as well).

1st: The unit was shelved due to low power output, not because of some more serious problem.

I'm going to go ahead and quote the note here, because I'm going to wind up coming back to it in bits and pieces and it'll simply be easier if I'm dealing in text.

With 12AX7 in input 2
it works but is nowhere
near 60w @ 8 Ω.
Check out if worth fixing
(maybe tube refit: eg:
6L6's & 12AX7's)

Here's the top side of the chassis. By the late 1960s sound reinforcement was fast turning into a more sophisticated endeavor, as evidenced by the input/output block at the rear of this unit. At first I thought that the low output power may have been the result of improper configuration of input selector, but wait, let's re-read part of that note:

With 12AX7 in input 2

"input 2"

Well, I do see a 9 pin Noval socket down there labeled "INPUT 2", and seeing how 12AX7 are also Noval base the parts will physically fit, but, 9 pin miniature does not automatically equate to a tube, much less a 12AX7. Note, there is no tube designation call out in the proximity of those sockets. That is because those sockets are wired for input transformers.

Like this. TM-200 input transformer can, on a Noval 9 pin base. Clearly an unpowered tube is going to pose a hindrance to signal. But I digress.

(maybe tube refit: eg:
6L6's & 12AX7's)

I'd also like to point out that there isn't a 12AX7 or a 6L6 in this entire amp. Thus concludes my note decomposition, there's hope for this amp yet.

Guts! The thicker wires are shielded, build quality is pretty robust. Busy, but not overly convoluted. This also pretty much looks exactly like the other LSI Bogen guts I've seen.

I found the schematic on this page on, ton of other Bogen stuff there, in case you wound up here looking for a schematic I didn't post.

This grill hides a spot for a ~3 inch speaker. I can laugh at the notion of a 60 watt powered mixer pushing a front panel mounted 3" speaker, can't I?

Yes, yes I can.

Thursday, December 22, 2011

Once I get past the fact that this is painted in one of the most hideous colors imaginable, this amp is pretty adorable. Behold, the Sears Silvertone 1451 combo amp.

Quick investigation reaped this hit: Silvertone World page on the 1451, which indicates this was a one year only (1959) "wish book" item that carried an asking price of $22.95, which apparently equates to $174.05 in today's money. It would also appear that the finish is not original. I get the impression that this is the combo version of the classic "amp in case" Silvertone.

It's also another hot chassis death amp, I'm guessing someone cut the cord after one too many jolts. The tiny bits of darker brown speckled finish that is all manner of awesome would be what was there until the mannequin fetishist slopped some "honky flesh tone" paint over it.

Looks like this amp has enjoyed some use in the past though, judging from the heat tan at power and rectifier sockets.

Free-form point to point type construction, neater than a rat's nest but only by a small margin. I don't know, having spent a little time with these hot chassis amps, I'm tempted to do a few isolation transformer modifications & update the mains safety aspect. See what these things can do. Yup, even the Decca.

Tuesday, December 20, 2011

Mystery amplifier!

Beyond this being another hot chassis amp with series string AC mains filament, I'm at a loss in terms of identification. That is not to say that it does not have any identifiable or rare qualities, rather, there's no visible mark manufacture, and plugging search terms based on what I do know and conclusions I've drawn based on the object itself, I keep getting flooded with results that do not match.

You can call me captain Run-On-Sentence, thanks.

From left to right, three 6AV6, a 50C5, and a 35W4. Supporting passive components litter the topside of a printed circuit board. I'm forwarding the guess that the caps are not original. The presence of a fuse is a nice touch.

Early printed circuit board.

Three inputs..

..chained together. Some sort of load resistor here may have improved multiple instrument performance. May have... I do not approve of the mechanical ground reference either, but if I'm going to spend time critiquing every last aspect of what was originally a cheap amp we'll be here all day.

A trio of 6AV6. I've seen these in a variety of amplifiers, as the triode section carries similar characteristics to the 12AX7. However these are single triode / dual diode valves, and anyone familiar with building fuzz circuits should already appreciate diodes. I've yet to see a 6AV6 deployed in a stock vintage amplifier that makes use of the diode (for distortion purposes, AGC and detection do not count). I have run signal through these diode stages, and while it takes a healthy level of voltage to make it work, I found it wholly worthwhile.

Of course, I like noise. It is safe to say that this isn't a holy grail of classic popular guitar tone, but it's also safe to say that there are those among you that are not purists. We'll touch upon this tube later on.

Those grey dog bone resistors make me think Japan is the likely candidate of country of origin, but again, it's a mystery.

Another shot of a 35W4 silkscreen. I really have no good reason for the these last two shots, other than it being easier to discern what the tubes were by looking at the pictures than by looking at the tubes themselves.

50 volt filaments, I'll find a spot for you yet.

Sunday, December 18, 2011

For some reason I started associating Decca with the early electrified blues, which is goofy, since Decca as a music label seems pretty main stream. Here's a Decca DMI-60 combo amplifier, perhaps we can get to the root of my misconceptions.

Nothing out of the ordinary, it's a small combo practice amp.

Sarcasm is evident here, but honestly, there's little else offered in terms of feature set. Cost of admission is a touch steep in my opinion, but clearly the thing didn't sell & wound up catching dust in the attic.

I'm really having a hard time not belaboring the obvious and running with that kernel of sarcasm planted above. I want to dissuade any one from concluding the sixty numeric in the model number has anything to do with output power; I'm guessing it has more to do with ever apparent line frequency hum. While I'm openly mocking this vintage tube amplifier, let's take a closer look at that strategically placed paint spot to the right.

Was this an attempt to conceal pertinent information, or a good old fashioned mishap? Like it or not, having seen the guts first hand, this amp does have an "American" feel to its construction.


Ever get the feeling you've been cheated?

Which brings me back to my association to early electrified blues. Early blues evolved in a poor, downtrodden environment. We're talking melanin rich people living in apartheid era United States, who by and large had to exert Herculean effort to afford even the cheapest shit, like this amplifier on hand. The Lo-Fi counter movement in the digital era holds nothing on the original people who made trash sing. Anyway, with the press-board baffle, laughably small speaker and absolute hazard this amplifier embodies pretty much every facet of instrumentation I associate with the birth of electric blues music. I'm left with the impression they were shafted by the record companies too.

Something is missing here. Something important..

I'd like to take this opportunity to point out that "domestically produced vintage audio gear" does not automatically equate to quality, take that "vintage" label!

Also, that rubber band is structural. I added it in hopes of inducing a little reverb, should I actually go against my better judgement and revive this amplifier.

So, what we have going on here is a power transformer-less circuit, ala the "All American Five", though clearly only running three tubes. I'd also like to point out the non polarized flippable mains plug. This amp would not be my first choice for bath time guitar strumming.

Who needs a fuse? Fuses cost money. I guess that 100K resistor between floating ground and chassis ground (aka, your guitar strings or microphone shell) sort of counts as a safety measure. If I had the power of omnipotence, assessing an exact count of lives saved by 100K carbon comps would be the sort of thing I'd contemplate over my morning coffee, bolstered by the fact that I am far from omnipotent I must say that's something of an insult. 100,000 ohms, meditate on that.

So, I've taken to reacting to the presence of 35W4 or 50C5 in much the same way I react to someone else's fart. I realize they exist, and I have no control over that. However, I'm fully okay with cringing in their midst and wouldn't object to never seeing them again... unless the tubes are on a proper secondary of a transformer and the farts are dutifully expelling themselves in an internal combustion engine.

The 12AU6 however, has uses elsewhere. I recall plucking one to populate a Tektronix type CA plug in for a 545 scope. My immediate sense of values leans on parts extraction, there is that tiny idea however, that having a (safety modified) horrible POS amp on hand for recording weirdos has value too.