6HC8 SE Build Part 1: Overview // Gray Bench Electronics

Hello and welcome back to the bench.
Today I'm going over the beginnings of a new tube amp build using some rare tubes.
Enjoy!
#tubeamps #diytubes #guitaramps #tubes #electronics

[@supportsquid]

Looking forward to following this build. Good luck, man!

[@JonnySoundSounds]

ive used a push pull output on singled ended you use the center tap and one side and float the other i did it in a single ended princeton build

[@thomaswilhelm3384]

I would try the 10K tap first and maybe have a switch to select between 10K and 5K to see how the tube performs.

[@oldasrocks9121]

Check out the 6AD10/6AD10-A. Compactron, usually about $5. Not as spicy as your choice but you might need a flea watt in the future!

[@JonnySoundSounds]

its crazy you could make an amp with that one tube if its a 1 gain stage to the power stage. id go solid state rec also just so its a 1 tube amp

[@dvhx]

Spice can simulate vacuum tubes, I would start there.

[@oldasrocks9121]

Fun fun! You can halve the 5k primary output trafo winding if needed and have 8/4/2 secondaries. Also the 6GW8/ECL86 is in on this game, most know it from the Hammond AO-44 reverb amp. It has a shield between the sections, and it is as well pretty spicy output wise. The AO-44 is good reference for the power supply. The 6BM8 has troubles if you use the triode as the s.e. driver stage, it gets too excited. Which I believe is an actual tube amp designer term of art! I can tell you when the ECL86 fails it really does so. Like your amp had that accident with a car trunk full of fireworks. Its the pentode that bites it. The super rare Gibson GA1-RVT (not the GA 1 RT) is a flea watt version of this with verb and trem, uses a 7199 for pentode input. 6AN8 is close and way way cheaper. You could run parallel triode/pentode input stages and use the triode in the 7199 type tube as the driver. 2 sets of hi/lo inputs, 2 volumes, 5E3ish, that would be super cool.

[@goodun2974]

Uncle Doug is currently posting videos of a GA1RVT clone build on his channel.

[@oldasrocks9121]

@@goodun2974 Yessir, nearly have all my parts! Building mine in a VHT Special 6 combo: reusing that PT (reclaiming the rest for a KT66 Super 5F2A head,) new turret board, Hammond 125BSE, 10" '53 Rola alnico. Gathered all rebranded Mullard glass but the rectifier. Chuffed!

[@goodun2974]

@@oldasrocks9121 , I'm glad you're building your clone on a turret board. I don't know what Doug was thinking trying to do this with terminal strips especially when he's mounting them on either side of the tube sockets on the bottom lip of the chassis. When you come up with a layout for the GA1RVT on a turret board you should share it with us. I was thinking about drawing one up Fender style with similar layout but it sounds like you're already just about there.

[@goodun2974]

@@oldasrocks9121 , Rola! Everybody else is stuck on Jensen and Celestion, but Rola made some great speakers. I've got a few vintage ones here pulled from old organs and organ tone cabinets but I also once had a Unicord 4x10 or 4x 12 cabinet that had 4 Rola speakers in it. . At some point years ago , Marshall was distributed in the US by Unicord in New York; It was a great sounding cabinet but I sold it to somebody who was more likely to use it than me just playing at home.

[@oldasrocks9121]

@@goodun2974 It was a bargain, also from a Hammond tone cabinet, it's the speaker with the small cylindrical magnet. The pair of 12 inch 60s Heppners from the Hammond L100 spinet are amazing too tho the ribbed cone alone will do a fella right. That original GA1 pictured sold by Flipside shows the best clue to the physical aspect of it's sound: the huge baffle with the 8" right in the middle. My baffle is about 14x15, the Maestro has to be at least 20x16, very little baffle step notch and if it's on its feet on the floor the LF will couple pretty well. I'd be curious to know how a similarly sized baffle with 2 8 inchers in it would sound. A guy could definitely get 2 8 inch speakers in a new baffle in the 5E3 Mojotone cabinet. Chopping my way thru circuit layout dyslexia presently, lol. Got a bargain on the VHT, wouldn't power on and the music shop tech took a pass on repairing it, complete $165 shipped. Can't buy a good new pedal for that. Hey, deflation!

[@wiggyjones]

Looks like an interesting project.

[@kevintaylor6375]

ECL86 is another triode/pentode in a single package tube. though they are a bit $$.

[@TubelabCom]

I realize that you have likely already started building on this amp, but I question the use of tubes you know are rare. Googling didn't find any NOS tubes. I have been tinkering with tubes for 60+ years. I have kept a list of every tube that I have ever encountered and collected all of the data sheets once the PDF was invented in the early 90's. I have never seen the 6HC8, so I added the PDF to my collection. For a one off that you will build one of and nobody will copy, it's a valid choice. The only common tube that shares the same pinout is the 6BM8 which has been seen in guitar amps, though it has a lower plate dissipation rating. I would tend toward a tube with multiple pin compatible substitutes for a wide range of different tonal possibilities. I chose the 6HZ8 when I was experimenting with a single tube amp during the Hundred Buck Amp Challenge on diyAudio many years ago. It shares a common footprint with 22 other different tubes! I would choose a tube from the 9AE / 9DC (6U8, 6BL8) basing for the first tube instead of the 6AN8 for the same reason, there are 19 pin compatible tubes there. As another commentor suggested, run all 4 OPT taps to a switch just for the sonic choices offered. At the 5 watt or so power level seen here a mismatch will not blow anything up and can offer up some unusual tones when overdriven. I did this with the 125CSE OPT in all of the "Turbo Champ" amps (6L6GC or 6550 tube on 400 to 450 volts) that I built in the late 1990s.

[@grundunterrecording4197]

This is going to be really interesting. Looking forward to the series!

[@charlesdeal771]

the heater current on that tube is 1.2a, that's a heavy load.

[@graybenchelec]

Yep, gotta make sure your power transformer is up to the task. Luckily the 270CAX has a 2.5A 6.3V winding!

[@charlesdeal771]

@@graybenchelec Have you considered the 6AF11? it's a 12 pin, with a high mu triode, medium mu triode and a pentode. It draws a tiny bit over 1a.

[@graybenchelec]

@@charlesdeal771 That's an interesting tube too. I know there was at least one guitar amp manufacture using compactrons. Might be interesting to do a push pull amp with two 6AF11. I'll look into it. Thanks!

[@charlesdeal771]

@@graybenchelec TWO ?! That's a wild, cool idea!