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"A taste of tubes"

Posted by Wayne Parham (M) on March 23, 2002 at 20:54:44

Hi Doc and gang!

I hope you won't mind if I ramble on a bit on your forum. I still appreciate our E-mail talks last year, and I'd like your advice again.

As I'm sure you've noticed, I personally like to use high-quality, high-power amps with my speakers. I don't usually open them up much past "just barely," but I like having the option. When a speaker of 100+ dB sensitivity is used, then obviously a guy doesn't need much power. And I'm finding tons of guys are running Bottlehead amps and preamps with π Speakers.

So I'd like to buy a Foreplay and either a Paramour or a Parabee, and maybe it would be wise to have 'em both. Can't break the bank, it's almost tax time and Uncle Sam really needs my bucks. But certainly, the costs of your kits are more than reasonable.

The reason I'm writing is that I'd like some suggestions. I want to build the kits completely stock, without any "tweaks." I'll be careful about running sensitive conductors appropriately, and I'll do a good job when assembling them. And I have an oscilloscope, DVM and signal generator so I'm all set.

I'd like to build an example of what a person can expect if they purchase your kit and just follow the directions. The reason is that this system will likely double for demonstrations, if I were to ever decide to show my speakers with tube amplifiers. I want the demonstration to be of standard items. But I'd also liuke them to be your best options, and those that you would want to be used to represent your product offering.

If I were to go to a show like the gathering next weekend in Lima, I would like to take Professional Series four π Speakers and PiAligned JBL 2245 subwoofers to be used with Crown Microtech amps. I'd also like to take Theater Series four π Speakers with a pair of your best Paramour or Parabee monoblocks. I could then switch speakers and amps, and show each with the other. So that's my motive.

Besides all that - I just like having cool stuff, and I think your amps look really great.

When I was a kid, my grandparents gave me lots and lots of tube-based console stereos and things. They usually had a pair of 15" woofers with really tight suspensions, a pair of 2" cone tweeters and about 8 tubes. I would play with them for a while, and then pitch 'em. Sacrilidge! - No telling what I threw away! But there is one radio my parents gave me that I've kept, and I'm real proud of it.

Stewart Warner Model 13-5U five tube radio

This is what Stewart Warner was building back before World War II. I'm used to seeing them making tachometers and stuff, so this radio was always really cool to me.

The label says "Riley's 8-14-45 8 4508"

I'm not sure if the label was attached by a repair shop when fixing the radio or if it was put on when it was sold. But it is interesting to me that it dates the radio to at least 1945. I touched up the image file you see here so that you could read the label, but it is clearly legible to the naked eye.

Operating chassis. Notice the filaments glowing!

The five tubes are 35Z5GT rectifier, 12SA7 detector/oscillator, 12SK7 IF amp, 12SQ7 2nd detector and 50L6GT audio amp. The locations for each of these tubes is shown on a label on the bottom of the radio, so I went to Radio Shack and purchased the tubes. Remember when each store had a tube checker and they offered every tube with gold leads and a lifetime guarantee?

Even with new tubes, the radio still didn't work, so I had to dig in deeper. I could not find a schematic for this chassis, but I found schematics for a couple of other chassis that used the same tube set. Interestingly, neither had the same speaker or antenna configuration, but it was easy enough to modify circuit layout to suit the components used in the radio. Once I had come up with a suitable schematic, I made a couple of copies, and I keep one stored inside the radio cabinet.

Point-to-point wiring, of course

I had originally intended to keep all the original components, but found some coupling capacitors completely open. So it just wasn't possible because too many components were defective. I also found that the radio had been rewired and the filaments did not even make a complete circuit. Undoubtedly, a repair shop was missing one of the tubes that needed to be replaced, so the repairman made a substitution and rewired for its alternate pinouts.

So I decided to remove all wires and components, and start from scratch. That made it easy to clean the chassis completely. The components are good quality 1/2 watt and 1 watt resistors and polypropylene caps, and the wires are all the same ones that came out. Oddly, I had about two feet of wire remaining, in various lengths. Spare parts?!! Ahh, but it works well, and doesn't hum at all.

Field-coil speakers, legacy of the past

This was the part of the restoration that was a real drag. The field coil was poofed. I rewound that silly thing, because it just didn't seem right to substitute the speaker with a newer fixed magnet type. So I wound that thing for a week. The B+ supply passes through this field coil, but only about 70mA is required, so this 34 guage wire should be sufficient. I don't know if that's how the original radio ran the field coil, but that's how mine is done. It also has a humbucking coil, but there's 100uF across the B+ supply both before and after the field coil, so there is very little ripple and the humbucking coil really isn't needed.

Well, I hope I didn't bore you with my little radio and stuff. You guys over here probably all have a ton of these things. I just wanted to ask your opinions on the best stock version of Bottlehead amps to buy, and to show you my "tube rig."

Wayne

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