Dammit, Igor! I said I wanted to listen to The Pussycat Dolls, NOT your Mother singing WestBrook Falls!
Summer is waning fast, meaning that it is high time to get rolling on this year's Halloween amp! I've had the basics for this one running around in my head since February when I picked up a pair of Wurlitzer Branded 6L6's and 5U4GB rectifier. Pieces have been coming together in the basement since then, and include some spacecraft parts, contra-band Soviet goodies from the Cold War, and even an old Ward's soldering iron!
Dr. Frankenstein wouldn't build just any amp. The question I am trying to answer in this piece is: "Just what kind of amplifier would Dr. Frankenstein have built had he been into music instead of Re-animating dead flesh?" I have taken quite a few clues from the movies - including Mel Brooks' 'Young Frankenstein'. Here are a few bullets on the general design:
#1) Ugly. Dr. Frankenstein did not shop at IKEA or the Sharper Image. Most likely he had Igor source parts for him late at night from old Power Stations or Military Bases.
#2) Crude - As in completely handmade. No custom built PCB's or other parts. No custom machined parts. Everything is going to be hand drilled & filed to fit. I am not touching the CNC for any parts on this one. All wiring is going to be point to point and soldered. I'm sure some of the menial tasks will be passed off on Igor.
#3) Big and Overbuilt. Just like his monster, everything in the lab was big if not downright huge.
#4) Scary. You didn't walk into his lab without some fear of electrocution. Even though I can't in good conscience run line voltage out to an exposed knife switch, One thing this amp must have is a big exposed power switch. I plan on using 12VDC switched across an appropriate looking knife switch to power a relay that will switch the HV on the amp - not really scary, but still cool.
#5) Slightly off Kilter. To put it politely, Dr. Frankenstein, although Brilliant, was not quite right in the head. There have to be some idiosyncrasies in the amp that just can't be explained by logical thought.
With these guidelines, I plan on starting some layout work as soon as I get the workbench cleaned off and straightened up. Or, perhaps in the spirit of this design, I should just leave it as is, and try to either work around the clutter or incorporate it into part of the amp. Trying to work by torchlight may be taking it too far... I will post more updates as things start to roll.
I have finally started laying out and roughing in the amp. I was waffling for the longest time about the preamp section, but when I scored a set of absolutely grungy looking Mickey Mouse 7193s, I knew I just had to use them on this amp. For those of you not familiar with this tube, it is equivalent to the 6J5 (which is also equivalent to 1/2 of a 6SN7) with top mounted anode and screen taps. When I bought these tubes, I could already see in my head the big coiled feeds of heavy gauge wire leading to the caps. Right now I only have some cheesy old bakelite anode caps laying around, so I will have to wait until I get some nice ceramic caps to complete the vision. But below, please find a couple of pics; One of the schematic direct out of the old RCA Receiving tube manual, and the other of the tubes temporarily fit up to the preamp 'tower' assembly. You probably can't quite see in the photo, but one of the tubes has some pretty nasty cracking in the base. The tube tests as new, so I just stabilized the cracks with some epoxy, being careful to leave them open and visible. I have another tube from the same lot with a good base, but somehow the ugliness seems to lend some character and credibility to the design. On the front of the tower assembly I will be installing a pair of 1/4" mono jacks for the input as well as the volume potentiometer. The steel tower is definitely too new and shiny looking for this amp, but I haven't been able to decide whether I should paint it a drab color, or just leave it outside for a while where the lawn sprinklers are sure to hit it. Maybe I should do both.
Latest Update: 10/6/07
Here is a quick shot of most of the external components laid out on the table. Note the knife switch and monstrous Soviet Rectifier.
The B+ power transformer is a Vintage Paeco Unit built for HP. It has twin isolated 115V input windings with two separate outputs, one at 215V, the other at 265. I plan to centertap the pair for a 215-0-265 output; certainly not orthodox, and certainly not balanced, but just dandy for a good 500+ Volts.
The Big-Ass Svetlana 5C8S rectifier is something I just could not pass up for this amp. I was originally going to use the very nice Wurlitzer branded 5U4GB rectifier that matched the 6L6 Output tubes, but the sheer physical size, capability (well over 1kV at 420mA!), and bulky, odd shape of the 5C8S make it a no-brainer upgrade. I’ll save the Wurlitzer for something a little more tame.
The big Svetlana rectifier dumps into a small 180uF cap which is then dropped across a 100 ohm power resistor into the main storage bank – a pair of big 250V Sprague electrolytics run in series.
If you look closely, you will see the power resistor is an antique Ward’s Soldering Iron. I was originally going to use a CLC type power filter, but when I found the super-cool finned base soldering iron at a Goodwill Store, the choke I originally had in mind went straight back into the parts bin.
It would be easy enough to drop the 500+ volts feeding the B+ line to the 300 needed for the preamp through a resistor. Instead I am going to take the second set of 115V primary windings off the power transformer and set up a small full-wave voltage doubler circuit using a 1176Z6GT twin diode. I have built quite a few of these in the past, and they deliver a nice 280 volts at up to 60mA without any sag – Complete overkill for a preamp that won’t be drawing any more than 15mA.
As for the 6.3V filaments, I have an ancient Waltham Model Railroad power supply wired for two 6 volt circuits, one AC, the other DC. The solid state diode rectifier in this unit has to be one of the very first solid state ones ever commercially produced, as this unit dates from the late 40’s. I haven’t decided to mount it on the amplifier in the existing separate chassis or transplant the components into the amplifier main deck. I am still on the hunt for a 5V transformer capable of delivering the 5 amps required by the 5C8S rectifier.
Probably the most important part of the entire amp has got to be the power switch. It simply cannot be Frankenstein’s amp if it doesn’t have a big knife switch for the power. I doubt that most people will want to don a pair of rubber lineman’s gloves every time they turn the amp on or off, so wiring the line power across the switch directly is out of the question. Instead, I plan on wiring a small 12V transformer that is always ‘hot’ across the switch to operate an octal mount relay that will bring the HV and filaments online.
About the only thing left to procure are the output transformers. I don’t have anything in the junk bin worthy of this amp, so I am actively looking. Right now I am leaning pretty hard towards just buying a new pair from EDCOR, but something else old and ugly might pop-up. For those of you not familiar with the EDCOR brand of transformers, do yourself a favor and at least check their site before you embark on your next project. Their sound is fantastic, the prices are dirt cheap, and best of all, they are made here in the USA!
With everything laid out, I just have to get going on the actual build. There are a couple of suprises that I am saving for later, but most further updates will just be on the execution of the plan above. Please feel free to email me with any comments or questions you may have. Also, I have had a lot of questions about the release date for the amp. I’m shooting a late September initial power up, with the Ebay auction start date of October 5th, ending October 15th, leaving plenty of time for shipping to ensure an arrival before Halloween.
The preamp is done, and the look came out just the way I wanted it! As shown in the photos, there is a pair of ¼” mono inputs in the front, each feeding one half of an Alps stereo potentiometer volume control on the left side. From there, the signal is routed up and out of the base assembly to the top mounted screen tap. I mounted the 1K gridstopper resistors directly into the caps. I waffled around about whether I needed shielded cabling for the signal lines, but in the end I chose some nice solid core cloth wrapped unshielded wire since the only AC inside the preamp is for the filament heaters, and those are well shielded. I will probably also epoxy the plate caps in place once I have run a final checkout on the amp. I really have become attached to the ugly broken base tubes, and am going to leave them as is. They are still completely solid and have no signs of wiggle, although I did fill the crack tips with black epoxy to prevent any further propagation. The big capacitor hanging of the right side of the chassis is the final filter stage from the standalone preamp power supply. There will be a 117Z6GT set up as a voltage doubler mounted somewhere else on the amp to feed it. Per the previous schematic, the tubes are cathode biased. I used Elna Silmics as the bypass capacitors. I have found that although they take a while to burn in, they give a wonderfully warm and clean sound. It was a little strange working on this piece. Normally I have a nicely painted chassis that I have to be careful working with to avoid scratches, nicks, or resin splatter. This time around I had a solid chunk of ugly metal that could care less about scratches or other abuse. It still came out quite nice with no external gouges in the metal. I originally thought the rather bright galvanized chassis would make the preamp look too ‘new’, but the ugly tubes, brown capacitor and wiring age the piece just right. More updates to follow as more of this beast comes together…
The Tube Driver Towers are done as shown below. I can't decide whether I like the look of Wurlitzer 6L6GC's or the ST glass Sylvanias better. Email me if any of you have a preference, because I can't decide. The Wurlitzer's are maybe a little stumpy looking, but you could also say the Sylvanias are tall and Gangly. I can make up my mind on the output transformers. I loved the sound of the Edcor transformers in the Benningtone amp so much, I decided that I would have to splurge for them on the Frankenstein amp as well. The 6L6's are now set up for ultra-linear operation with some very nice Edcor Iron.
Amp Complete 10/6/07:
It's Done! A few changes along the way, but it is finished and runnning! Updates from last time: I finally got around to slicing down some aluminum sections and building the chassis base. I gutted an old HeathKit receiver (God Bless her soul!) for the aluminum baseplate. I could never get happy with the absurdly long shape the amp took with the Paeco power supply and extra filament transformers, so I ditched them all and stuck in a power transformer from an old Westinghouse TV set. Final assembly was as simple as bolting all the components onto the chassis, and powering it up. The power relay gives a nice 'clunk' when you close the power switch, and the tubes all give off a nice warm glow. (You can see the traces of blue given off by the 6L6's). The sound is great, actually better than I expected. I credit the Edcor output transformers and ultra-linear operation for most of it. I will let the pictures do the talking for the rest of it - I'm headed down into the basement to do some more listening. Better pics to follow soon - it's late and my hands are jittery from too much coffee.