Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

Bell Air Mystery Deluxe

Discussion in 'Shock Brother's DIY Amps' started by magic smoke, Sep 13, 2017.

  1. magic smoke

    magic smoke TDPRI Member

    53
    May 28, 2017
    Quantum Entanglement
    In regards to the grounding scheme, this amp presents a dilemma. If it were a scratch build or new prototype, the answer would be to simply follow the modern convention; i.e. twisted pair heater wiring, center tapped heater winding or artificial CT, follow Merlin’s advice or Rob Rob’s grounding scheme, etc… However, things were done a little bit differently at the ole Bell Air factory in 1953. Inside the painted chassis, the grounding is run via an uncoated wire, which is soldered to the base of each of the sockets in a more or less linear order from rectifier to first pre. They have run the heater wires with one grounded at the base of the recto socket, and one heater wire that travels down the line of sockets terminating right next to the grid pin of v1! At this point, one would be tempted to just remove the old filament wiring and install twisted pairs and an artificial center tap. The only problem is that the point to point layout has utilized almost all of the extra socket pins as additional solder lugs. In order to “correct” the filament layout, one would have to redesign the rest of the p2p layout. Yikes! At this point it becomes obvious that we should just roll with the original design, get it fired up and listen to assess if there is any 60cycle hum before gutting. If it ain’t broke?
     
    boredguy6060 likes this.

  2. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Yes, I'd give the original scheme a listen before doing major surgery.
     

  3. magic smoke

    magic smoke TDPRI Member

    53
    May 28, 2017
    Quantum Entanglement
    I was immediately reminded that the super heavy wire I had attached when rebuilding the PT a few years ago is definitely overkill. It was way too heavy and a real bear to install. Note to aspiring DIY builders- It’s definitely worth it to buy the right wire.

    The filter section was pretty smooth, but this build’s token missing components from the triple checked parts order were the 10k dropping resistors. Luckily I found these buggers in the lab. Close enough for Jazz.
    IMG_0417(1).JPG
    The OT needed just a little extension to the leads. The primary side got the ole double heat shrink treatment and careful separation of the wires off of the tubes. When I apprenticed in a neon sign shop we’d use glass tube as an insulator over any splice to the HT leads.
    IMG_0418.JPG
     

  4. magic smoke

    magic smoke TDPRI Member

    53
    May 28, 2017
    Quantum Entanglement
    With the power side done, it’s time to work on the input section. I want to hear the most of it’s original voicing as possible. I’ll replace the cathode bypass electrolytics and wire up a fender style HI/LO 34/68 ohm jack set. For first fire up I left the .01 cap before the grid, but that’ll get removed when I change the volume topography. In the original circuit the volume pot is located between the inputs and grid of V1. Once the amp is functional and after an initial listen I’ll modify it to a tweed inspired vol/tone stack.
    IMG_0419.JPG
     

  5. magic smoke

    magic smoke TDPRI Member

    53
    May 28, 2017
    Quantum Entanglement
    Time for the first power up! Why not use the original tubes found with it? Inspired by my young son, an avid youtuber, I decided to document the first power-up for the immortal internet.

    I once killed a 5Y3, just to watch it die.
     
    robrob likes this.

  6. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Dude! That's a killer light show. I'd definitely double-check all the rectifier socket wiring.

    Was it just a coincidence that the next video offered up by Youtube was a B52 bomber mission? :D

    P.S. Your work looks really good.
     

  7. magic smoke

    magic smoke TDPRI Member

    53
    May 28, 2017
    Quantum Entanglement
    Thanks for the compliment Rob. You can rest assured that the first thing I did was pull all tubes and scrutinize the rectifier and B+ connections. The only weird thing I found was +5.5v on pin2 of the 5Y3, but no voltage at the point where the other leg of that winding connects to the first node of the B+ rail.??? It is entirely possible that I made an error when installing these leads at the coil of the PT.
     

  8. magic smoke

    magic smoke TDPRI Member

    53
    May 28, 2017
    Quantum Entanglement
    Thank you Mr. G. Weber for teaching me about lightbulb current limiters. This one saved my iron. The immediate hypothesis was that I’d botched the internal rewiring of the PT many moons ago. A thorough check of all the circuitry around the recitifier tube showed accurate wiring and no spots for potential arcing. The voltages off the PT were accurate. The only anomaly was the presence of +5v on the lead connected to the recitfier socket, and no voltage at the connection point of the 5v winding to B+ rail, (although I didn’t unsolder to check these voltages.)

    At the time I also considered the possibility that this was just a bad tube, and because I have a few lying around I decided to try another.
    My theory was that if the next tube did the same thing, I'd proceed with a lengthy re-work of the circuit, but if it worked I could maybe avoid it.

    With great trepidation, I installed another 5Y3 and flipped the switch. This time there was no star in a jar. Less exciting, but a huge relief. Installed the rest of the tubes with no issues and then the guitar was plugged in!!!!!!!!
     
    Karl Pilkington likes this.

  9. magic smoke

    magic smoke TDPRI Member

    53
    May 28, 2017
    Quantum Entanglement
    I finished the amp last Wednesday night. Despite my best efforts, I still managed to make a mirror image orientation of the input jacks. I guess those are the types of mistakes to be expected when working at the end of a 10hr build session. I was also in a bit of a hurry and wired the pots with a bit of extra lead length on the caps and wires. I do plan to tidy these up, but the amp sounds so good right now I’ll probably wait until the head cab is finished.

    Wow! Talk about tone, this is THE BEST sounding amp I’ve ever played. I can’t help but smile every time I plug into this thing. Rich harmonically thick drive with chimey highs and tons of string to string clarity. Please forgive the guitar playing, but I have to post a demo video.
     
    kleydejong likes this.

  10. magic smoke

    magic smoke TDPRI Member

    53
    May 28, 2017
    Quantum Entanglement
    I’ve run the amp for around 5 hours now and haven’t had any problems yet. The old school heater layout is dead quiet! The metal jacket 6j5 is a bit microphonic, but I do have a glass version I will swap in if it becomes a problem. The power transformer does tend to get warm at the bells, but its not hot. At it’s warmest I can still wrap a hand around it and hold on comfortably. I’m still a bit confused about the missing 5Y3 heater voltage, but the amp is working perfectly so I’m hoping it was just something I overlooked when checking voltages the first time. I’m headed to my shop today to take some more measurements and find out what the actual parameters are.

    *Note: Although my schematic shows a cathode bypass cap on the 6V6s, it was not in the original circuit and I did not install one in the current version of this amp. It just sounds way too good as-is.
     

  11. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Nice! You and your amp sound great. Congrats on bringing an instrument of joy into the world. Due to your efforts the world is indeed a better place today.
     
    boredguy6060 likes this.

  12. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Alligator clip a bypass cap on the cathode resistor and give her a listen. You could always put the cap on a switch.
     

  13. magic smoke

    magic smoke TDPRI Member

    53
    May 28, 2017
    Quantum Entanglement
    Thanks Rob.
     
    Last edited: Sep 19, 2017

  14. magic smoke

    magic smoke TDPRI Member

    53
    May 28, 2017
    Quantum Entanglement
    Whoa, checking voltages right now and the 6v6 are waaaaaay high, like 622 on the plates and 516 on screen. I guess I know what that extra wirewound 200ohm resistor was for....

    Of course, it helps if the multimeter is set to measure DC voltage instead of AC. Insert sheepish grimace.
     
    Last edited: Sep 19, 2017

  15. magic smoke

    magic smoke TDPRI Member

    53
    May 28, 2017
    Quantum Entanglement
    Disregard
     
    Last edited: Sep 19, 2017

  16. Platefire

    Platefire Friend of Leo's

    Age:
    69
    Nov 3, 2003
    North Louisiana around Many
    Just following up to see where you went with it. Glad you like it! Most of my projects evolve in time with mods. There are always tweaks that will present themselves to you that you'll have to try:>} Happy Amping! Platefire
     

  17. magic smoke

    magic smoke TDPRI Member

    53
    May 28, 2017
    Quantum Entanglement
    Hey platefire, sorry I missed this comment. I got into a rebuild of an el84 magnavox stereo right after buttoning this amp up. I've been playing the Bell Air as is for the time being, however the switchable 6v6 bypass cap is definitely on the list for future mods. Currently messing around inside a Hammond AO-35 chassis and discovered your Hammond/Ghia thread on another forum Rock on!
     
    robrob likes this.

IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.