Sunday, April 13, 2014

Picked up a couple Turner 500 microphones with native four pin Cannon/XLR plug and noticed an ALARMING LACK OF INFORMATION ON THE INTERNET in regards to refitting these with modern connectors. Tons of data regarding the HAM radio OEM stuff, but hardly a whisper in terms of studio quality stuff Turner churned out before their unfortunate demise.

The original design is actually pretty cool. You have a case ground (black) a high impedance (blue) and a transformer coupled low impedance pair (orange/yellow), so you could cable select impedance. Reminds me of the old SM-57 transformer bypass mod, just so happens to be BUILT IN and optional on these.

That said, sourcing the 4 pin connectors for it now is something of a nuisance, and I want to reach for the low-Z mode anyway, so what you're about to see is an easily reversible disconnect of the high impedance (pre transformer) circuit.

Form factor is identical, I happened to have a couple three pin Cannon male inserts lying around, which turned this into a full "bolt on" mod.

The dirt and crud even lend a period correct feel to these, so as not to bum out the vintage zombies. I jest, partially.

So a bit of heat shrink on the blue hi-Z feed, connect chassis to 1, and I decided to jam orange as "hot" on pin 2 because it's a "warmer" color than yellow. Completely scientific like. Whatever. The mic works, I like the sound, it will get used.

Apologies, I don't seem to have taken a picture of the completed unit. It looks like the connector end of a standard microphone, my guess is that if that isn't explanation enough, this mod might not be for you.

Friday, April 11, 2014

Drilled out the rivets securing the back panel of the IRP Voicematic to allow a bit of useful modification.

It's a little difficult to discern here, but there's an uninsulated wire in between the black and white twisted pair along the backplane buss. According to my scope, the bare wire is the audio feed line from the independent cards to the master summing section. Visible are my sloppily hand aligned holes.

The idea here is that since all the cards output direct to this buss wire, I can get away with installing interrupting jacks (Switchcraft 12A) so that the card feeds the tip directly, which is shunted to the combined buss when nothing is plugged into the jack. I've opted to only go so far as metal work on the 11/12 hole, as I don't have a card for that pair. So yeah, I'm leaving a HOLE in the case. Never know when it might come in handy.

Functional, though the recovery amplifier provides for considerable gain, so direct patching the two input card has a much lower level than what is available at transformer output. I guess that's what booster pedals are for.. HAHA.

Wednesday, April 09, 2014

Here are some more shots of the woodwork on the TIC 500 set (both of which are now recapped, and to varying degrees functional).

Assembly details, fairly apparent.

Precision.

Each unit is self powered, the large bodied grounded plug assembly just fits through this hole necessitating a keyway for other cord.

Another peek at woodwork, only one side is drilled like this..

..which makes for a nice view.

Though to be fair, the ample ventilation provides for largely unrestricted observation.

I found service data! TIC was evidently acquired by Acton Labs in the mid 1950s (or thereabouts), and in reading this thread it would seem TIC was H.H. Scott's company after departing GenRad? http://antiqueradios.com/forums/viewtopic.php?f=8&t=221316
Regardless, I'm happy to have a roadmap I can follow should I decide to tackle the motorboating issues. I'm somewhat on the fence about that, because of an interactive response to signal that nudges these into musical instrument territory.

Monday, February 10, 2014

Picked up a Technology Instruments Corp Dual Type 500 AR wideband voltage amplifier last year, because, well.. look at it.

Tired phrase, but: they don't make them like they used to.

Fairly no nonsense control set.

Yes, it glows in that logo.

Cracking the units open revealed a somewhat ugly bias battery assembly, comprised of two 625 mercury batteries bound together. This is only switched in at 1000 gain, and so I opted to remove it with the intent to replace in event of malfunction. My applications are confined to the audible spectrum of sound (as opposed to full 5Hz-2MHz claimed) so a little non-linearity may actually be welcome.

There's really only one way to find out.

Original circuit, gutshot topside.

Original circuit, gutshot from below. My intent is to replace the electrolytics and the selenium, and then give it a listen.

Modern parts make for a roomier experience.

A 100pf mica domino cap (visible on the original unit) has been installed on the wafer switch.

I've been going the budget route on electrolytic capacitors, building ugly little clusters costs less than a quarter of repop multisection can caps. These are not museum collection pieces, these are supposed to be used.

This was an easy fix that entailed shortening the power cable about a foot from the inside.

General purpose gutshot.

The earlier, original screw is quite evident compared to the newer screw, both of which exhibit a level of quality I don't see in a lot of current hardware.

So, after working one unit over I was slowly bringing it up on a variac and started to read voltage swings on output, one pulse every two seconds or so. Slow enough to confuse my multimeter on both AC & DC settings. The scope displayed a pulse that jumped toward the top of the scale, which indicated a frightful signal to be feeding gear downstream.

Plugging this 75 ohm terminator from my video junk into the input happens to snub the oscillation. It took me a little while to connect this particular set of dots.. after discovery of the motorboating, I shelved this for a while to clear my head. Then I observed that it didn't oscillate at 10, various resistors strapped on the output, loading the input with a signal source and listening to output, generally just fooling around with it brought about this observation.

So, the plan is to terminate the input with the largest resistor I can to prohibit runaway, which, I might add, only affects this unit (before and after recap). I dug into this one first because it was operationally impaired, so I could have a functioning unit to A/B while troubleshooting, should the need arise.

All in all, I would not describe this as a high fidelity preamp, it does however go deeply enough into the realm of character to justify having around. Plus, it's pleasing to the eye.

Friday, December 13, 2013

Above my racks in the studio I have some old light bar lamps that have the sockets wired up in series. This is to achieve two aims: a nice mild illumination that allows me to see while not being bright and extended useful service life of the incandescent bulbs.

At least, extended (my intent was indefinite) life span was part of the idea. Turns out having a bunch of bulbs overhanging the rack provides temptation to hang cables. Mind you, light bulbs are not load bearing. This one remained illuminated for about 10 seconds after the glass fell away. I was about to grab the camera when it flared out into what you see here.

No more using conveniently placed lightbulbs as cable hangers for me! Happy Friday.

In other news, I'll be gigging the reverb unit tomorrow, so I expect I should have some "I'm finished!" content soon.

Monday, December 09, 2013

Our temperate region was hit with an abnormally cold storm, derailing my plan to drive across town and use appropriate power tools to fabricate an enclosure for the reverb amplifier. I was actually toying with the idea to clamp a temporary table to my radial arm saw project (future instalment) when I came across an old Bach trumpet case I had salvaged from a pile of enclosures that were slated for a stop in a dumpster.

According to the reverb pan, this case will work out just fine.

Gutting it was not nuisance free.

The glued in monkey fur (remaining after I peeled the fabric backing away, which was a chore in itself) gave me ample opportunity to oil up my language.

Ultimately, once the fire hazard levels of fluff were removed the case continued to show promise. The area to the right where the shim block is will be filled with the B+ transformer I'm adding to the party.

Before I get that far, however, I want to address the incorrect capacitor I noticed the last time I posted, because the additional transformer is too large to mount to the chassis and performing work with a tethered chunk of iron is going to push the nuisance aspect a bit. Here I've pulled the 0.01 Black Cat and have eased a 0.1 Aerovox Duranite into the slot.

Access to the leg of the potentiometer necessitated the temporary displacement of the Domino cap, here everything is in position and ready for soldering.

Once the potentiometer assembly was buttoned back up it was time to address connection to plate leg of the tube socket, however, the longer package in the 0.1 cap put the B+ a little too close to the grounded screw for my comfort.

Tasteful application of brute force corrected the interference fit.

Nice round bend taking up the slack.

With the chassis sorted it was time to address the previously discovered deficiency in the higher voltage secondary of my installed transformer. I had toyed with the idea of putting a step up transformer across the 50 volt CT winding, but tests indicated that it was overburdening the 50 volt winding, so that plan was scuttled and this transformer was brought in, which provides roughly 200 volts DC once rectified, which is plenty for me.

I got to fill in the empty eyelet with a ground reference due to no center tap on the transformer.

With my car still snowed in (we reached -10F/-23C and the old moisture bearing door seals quite literally have frozen this car into a solid shell, though I am certain the aircooled motor would fire right up - if only I could get inside to operate the car) I decide to hand carve the enclosure.

Naturally, the larger knife I would normally reach for in this instance is currently TRAPPED INSIDE MY CAR, so I had a go with an old box knife...

...which resulted in many shavings.

I think in the end it will all be worth it though.

Saturday, December 07, 2013

200th post! Let me take a moment to explain my extended leave. The notion that I was starting a bunch of threads then leaving them out to wither and fade away was weighing down on me a bit. See, as I would climb the archives for a kernel of data (these posts are probably more for my own benefit than anyone else's, though I do appreciate the support and positive feedback when it comes up) it became more and more apparent that the greater majority of my posts were either showcasing an object (not my intent) or lacked closure.

In short, it really seems a scatterbrained collection of halfway theres, and I made the command decision to NOT "pollute" these logs with more half finished business until something started was ushered to completion. That something is the outboard reverb build I have been dancing around for more years than I care to contemplate. So this blogging process was shelved while I did other things, make no mistake, I haven't wandered away from flowing solder - I just couldn't bring myself to post about anything other than the reverb, until it's done.

Yes, it really has taken me months to move on beyond chassis layout and punching holes (arguably more involved) to actual circuit wiring (you know, the FUN stuff). Ask me about painting my car next.

No, don't.

At any rate, progress has been made so here I am. For those that don't already know, the Fender 6G15 reverberation amplifier is loosely based on the Fender Champ - driving a spring reverb tank in lieu of a speaker, with an additional gain recovery/dry mix stage on the output. For reasons I have quite simply forgotten at this point, I decided I wanted to oust the 6K6/6V6 Champ inspired power section with a 6BQ5/EL84 single ended section. So, since I have one underfoot that I can observe directly, I turned to the Kalamazoo Model 1.

However, while the Champ and Model 1 are of similar class, they are of different pedigree. There were strengths I wanted to combine, and so we launched into hybrid territory.

Make no mistake though, when it came down to actual assembly I did not consult the gods of audio. I grabbed whatever I had on hand, you know, like the Old Masters did. Pictured above is the initial 12AX7 tube, first stage at high numbers (socket pinout) feeding a preamp gain (dwell) ala 6G15. This returns to the low numbers triode which is wired up more like the Model 1 sans tone control feeding the power section.

Here it is evident that I am using previously built components, and did not exert a lot of care in cleaning the vestiges of whatever this used to be. It's a 6BQ5 single ended power section now, please ignore any other disarray. AT LEAST I DIDN'T LEAVE REMNANTS OF WIRING!

I'd like to note that while the power amplifier stage is pretty much lifted directly from the Kalamazoo, the B+ power supply feeding it is a choked configuration more in line with the 6G15. The single ended amplifier stage feeds an output transformer coupled to a spring reverb tank.

The return from the reverb tank is seen here, direct coupled to the low numbers side of the 12AX7 gain recovery tube. This feeds a tone control connected to the high side of a mix control to output. High number side is a cathode follower dry feed taken from input jack, which drives the low side of the mix control.

Depicted above is the aforementioned tone and mix controls. Allow me to name off the imbroglio of capacitors, as there is no single photographic vantage point that could make sense of this mess.

The burnt sienna .03@600 CDE to the left is the DC blocking input cap for dry stage. Grid is elevated ~ 100 volts or so, and this fine capacitor is what keeps that off my pickups (or transistors, opamps, transformers, whatever.. outboard spring reverb will see more duty than just guitar).
The cyan/turquoise .1@400 is the DC blocker at dry cathode follower to mix pot (at right).
The .01@400 Sprague Black Cat is my reverb recovery section coupling cap to tone control. I think this should actually be a .1, that's a mistake (see if you can spot two more amidst these pics!) but I guess I'll leave it be unless reverb return sounds too thin.
The black .01 disc cap is the tone cap from control to ground, doing what it can to shunt ALL my signal away since I botched the coupling size.
The domino mica cap is a 500pf coupler from tone wiper to mix. I figured I would double the 6G15 design of 250pf here, because, well, you know, I like a little bass. Hahaha, apparently this was a wasted effort. I'll revisit this later. We're not done yet.

Before anyone pipes up about it, yes, I am aware of capacitive coupling, lead dress and ground loops. Thankfully I'm okay at (read as: used to) working in tight configurations, what with the rework I have in store.

The unit is going to be a controls on face, at bottom type of configuration (ala Marshall heads); so this is the natural finished stance of the chassis. Rear support is not going to be via standoff, it will be supported all sides by wooden runners.

So, with everything more or less squared away I set about the initial slow warm up on a variac. Something, however, was amiss. My voltages were way off, B+ being abnormally low. Concern with a short circuit proved unwarranted. Everything tested out okay, it was just that my B+ leg was reading roughly 10% of expected.

I had tested this transformer prior, and it was concluded to be a ~500 volt center tapped secondary. You know, because I measured it. Having the luxury of an uninstalled identical transformer I conducted the measurement test again on a part out of circuit.

Imagine my surprise when it turned out the high voltage secondary of my chosen power transformer was in fact delivering only ~50 volts across the windings! DECIMAL POINTS MATTER.

Above is an example of finished fitment, tight gap between the tube and chassis. Too tight? I'd probably bevel that on a production unit, but this is not, so..

Anyway, I'm wary about trying to fit enough discrete voltage multiplier stages to make use of the 50 volt center tapped PT work, so I'm thinking about piggybacking another transformer in there, either something with a proper B+ right out of the gate, or a step up I can drive with the 50 volts (I think a 120:24 volt transformer may work nicely here). Sadly, I couldn't find a correct voltage range with identical mounting in my piles, so the fix may wind up being less than visually perfect.

Of course, it'll all be encased in a cabinet anyway. Stay humble!