RARE Heathkit SP2A Preamp Rebuild

A rare Heathkit SP2A comes into the workshop to get a full electronic rebuild.

[@jeffbrooke4892]

Thanks. I had one of these preamps several years ago and could never find any information or serious discussion on the unit. I got a chuckle over your remark on the aesthetic as I always thought the SP2 resembled something Heathkit should have called the "Samsonite Series." Great video and thanks again!

[@FrankC76]

I just purchased one of these from eBay. Amazingly it works very well, with only a slight hum at very high volume. I was shocked at how good it sounded. However I do plan on restoring it anyway and this video is going to be invaluable in my effort. Thank you!

[@pcbgraphix1903]

Great to hear!

[@stephenrowley880]

My Hi-Fi's were also built in 1958 by British Thompson-Huston Ltd (BT-H) who my dad worked for as a cinema engineer. The mono amps were built for the then new Todd-AO 6 channel sound used in cinemas then also sold as home Hi-Fi amps with a pre-amp, mono or stereo, then BT-H then made a stereo power amp. The mono amps had a pair of EL34 and the stereo amp only has a pair of EL84 in each channel. I have used my stereo Hi-Fi every day since I bought it in 2004 as new old stock. It was still in it's original unopened boxes date stamped 12th May 1958. Needless to say II have replaced all the capacitors and resisters in all 3 amps.

[@pcbgraphix1903]

Wow! That's amazing to find NOS equipment that old.

[@jdrissel]

In the late 80's Yamaha made car stereos with that kind of loudness control. They really did sound nice, and having a separate gain control helped with signals that had different max levels. This was an issue with cassettes, especially if you traded with others who recorded at a different level than you did.

[@pcbgraphix1903]

Some of those old car stereos were great. I especially liked the decks that would autoreverse and play the other side of the tape.

[@frankgeeraerts6243]

F&T still makes those capacitors !

[@austinld]

Thanks for the informative video. I have one of these that needs restoration and I appreciate the info about lifting traces and ground probems. I'm going to use it with a pair of W-5Ms that I completely restored: caps, resistors, ceramic sockets. I am interested in the caps (and their source) that you chose as replacements. Hayseed for the 2 in 1 cans? Looks like maybe you used orange drops? Thanks again, good luck with your channel!

[@brianbloom1799]

I use what the Older gentlemen gassinger, Video channel, He said CRC BrakKleen, Non Flammable Has Perk same as dry cleaner uses on our clothes. Works awsome.

[@bboehm]

VERY informative video! Really appreciate you putting this together. I'm going to be starting an overhaul on a new (to me) SP2 and the information you've shared here is very helpful. The one that I have is in need of the Aux AC power control and knob (missing both). If you or anyone else knows of a source for this (keeping an eye on ebay), I'd be very appreciative.

[@pcbgraphix1903]

I'm glad you enjoyed the video. Unfortunately, I don't have any parts and I don't know of any either. Sometimes parts will show up on ebay so you might have some luck there.

[@FrankC76]

Me again: I've been working on my own HealthKit SP2A as you can tell and something interesting I have run across: When you first started working on the power section and discovered that the resistors in the ladder did not match those called for in the schematics or the build instructions...well mine had the exact same values as your "wrong ones". I left them in when I did the diode upgrade and the resulting voltages were pretty close to spec. When I changed the resistors to match the schematics, the voltages jumped up considerably, especially the first two (300V give or take). I believe they are still within what the 12AX7 tube can handle, but still...it makes me wonder if there was some sort of addendum that came with the kit to install these other differing resistors? That's the kind of detail that would be lost to time. I'm torn on whether to leave the schematic version of these resistors or go back to the values I initally found.

[@pcbgraphix1903]

As long as the voltages are within the ratings on the tube datasheets, I would leave them at the higher voltage,

[@FrankC76]

A question: when you bypassed the selenium rectifiers with diodes, do you recall which diode you used and whether or not you used a resistor in series with the diode or just the diode itself?

[@pcbgraphix1903]

Just a plain old 1N4002 diode and no resistor.

[@user-ge5zb9gw9u]

LP-setting = Early Columbia LPs = NARTB equalization

[@brianbloom1799]

Hi sir ,I,m Just learning to try and work on old tube radio's and amps, I picked up a Pre-amplifier, Heathkit WA-p2, In the back theres what looks like a Potentiometer, That the arm rides over a spring, Mine is Burned out, would I be able to use a regular, Potentiometer as long as it is same ohms, Thanks for any help Brian

[@pcbgraphix1903]

Is it an input level pot or a hum balance control?