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Old 18th April 2011, 06:18 PM   #1
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Default First DIY from scratch

I am going to build a simple amp that I hope will sound very nice but one of the main reasons for choosing the tubes is for the ability to roll tubes. The tube layout is simple 6J5G&GT/ 6V6GT/ and 5V4 for the recifier. I will be building mono blocks and want chokes and extra large oil caps of 6 uF and I want to use all high quality parts as I can afford them.

What I am looking for since the 6J5 is half a 6SN7 is the best voltage to use, best way to ground the cathode, what might be a better way to hook up the 6V6, basically any advice that has been found from trial an error.

Even though the 6V6 is going to be SE I am starting with 335 to 340 VAC because I have a pair of potted military power transformers with all the voltages I need for filaments, the rectifier and even another 100VAC if I need to do something completely different.

I have a pair of round potted 5 Henry chokes, will probably buy something to go under the chassis like the chokes used in Dyna power amp kits. I have some other chokes but they are potted chromed chokes and might look a bit odd unless I can get some more chrome on top of the chassis.

The chassis are thick aluminum 16"X7" and 2.25" tall. I will be sealing the ends with thick alum sheet and putting handles on. I haven't decided on polishing the chassis or painting yet.

I have good wire for everything, including some of the super magnet wire that was sold by Vampire for signal, old WE wire for power, thick copper for a grounding system.

I will be ordering resisters and caps for under the chassis, types and brands would be nice to know. I think I might get a bunch of the Russian PIO caps and have a good stock of WIMAs etc already.

Anything I should know. please tell me. I am an artist, can build things, work on cars that don't have computers in them, but math is beyond me. I will work on that but for the most part I just need some help on what is the best voltage to go where, what kind of part to use and I will follow along as best I can.

One other thing, the outputs were wound for Mullard 3.3 mono blocks. I won't be using the power supplies that were wound for them. They are pretty good transformers with 5K mains and I think I can get 4,8 and 16 ohms on the BPs if I can use the right kind of switch. They were made in England but I have no schematic showing how to wire up the speakers, but do know I have to combine 2 of the leads to get 16 ohms. That will be interesting.
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David Thatcher
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Old 18th April 2011, 06:34 PM   #2
tomchr is offline tomchr  United States
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If math and engineering are beyond you, then I suggest going with a proven circuit. I would probably look at the Tubelab Simple SE (Simple SE). In fact, I've built that amp circuit on a breadboard using a 6J5 as the input tube. You'll get about 6 dB less gain with the 6J5 as the it gives you about 20 V/V gain whereas the 5842 tube used in the SSE gives you a tad over 40 V/V. That's usually not a big deal. Just crank the volume up a bit higher.

I would suggest starting with the output stage. What voltage and OPT impedance do you need for a 6V6? Go from there to design the input and driver stage(s). Once you have that, design the power supply. You may end up needing different parts than what you have -- or modifying the parts you have to fit the project needs.

If you want to design from scratch by trial and error completely without math and/or engineering, then I wish you the best of luck. I won't be of much help in that situation. Take a look at the SSE. It's a good circuit and it would surprise me if George hasn't already tried 6V6 in it...

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Old 19th April 2011, 12:32 AM   #3
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Quote:
Originally Posted by tomchr View Post
I would probably look at the Tubelab Simple SE . In fact, I've built that amp circuit on a breadboard using a 6J5 as the input tube. You'll get about 6 dB less gain with the 6J5 as the it gives you about 20 V/V gain whereas the 5842 tube used in the SSE gives you a tad over 40 V/V. ~Tom
A minor correction; the Tubelab Simple SE uses a 12AT7 driver and the Tubelab SE uses a 5842. Also, the boozhound site has a SE 6V6 design. Boozhound Laboratories

Last edited by boywonder; 19th April 2011 at 12:36 AM.
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Old 19th April 2011, 12:59 AM   #4
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TE,

You say the O/P trafos in your possession are for Mullard 3-3 amps. In full pentode mode, a SE 6V6 will yield about 5 W., which is too much for the "iron". OTOH, triode wired 6V6s are good for about 2 W., which mates well with your trafos. Just tie g2 to the plate with a 100 Ω Carbon film resistor.
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Old 19th April 2011, 02:57 AM   #5
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Originally Posted by Thatch_Ear View Post
What I am looking for since the 6J5 is half a 6SN7 is the best voltage to use, best way to ground the cathode, what might be a better way to hook up the 6V6, basically any advice that has been found from trial an error.
This looks pretty good. I've used the 6J5 as a voltage preamp, and it works really well for that. Also, since this was designed as a TV vertical oscillator/amp, it's running well below spec here.

As for how to ground the cathode, you have a couple of options. You could go with LED cathode bias if you're not using gNFB. If you are, just let the cathode bias resistor do double duty: grid bias and the feedback divider resistor. Also, don't forget to include a grid stopper: 4K7, 0.5W, C-comp works well for that.

It's not so much the cathode you need to be concerned with, it's grounding the metal envelope. The envelope must not be allowed to become positive with respect to the cathode. This usually isn't a problem unless you're connecting the cathode to a negative rail. You could always connect the metal shell directly to the cathode at the socket so that they are always at the same potential. (May present a shock hazard if using a negative rail in the cathode, since all that's standing between you and that voltage is a thin layer of paint.)
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Old 19th April 2011, 07:14 AM   #6
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Originally Posted by Eli Duttman View Post
TE,

You say the O/P trafos in your possession are for Mullard 3-3 amps. In full pentode mode, a SE 6V6 will yield about 5 W., which is too much for the "iron". OTOH, triode wired 6V6s are good for about 2 W., which mates well with your trafos. Just tie g2 to the plate with a 100 Ω Carbon film resistor.
A 6V6Gt and a 6BQ5 should both put out the same power. I am using a 5V4 instead of a 6CA4, one because I like the way they sound so much better and there is a 5V tap on the PS. I can'tsdee that using a 6V6GT instead of a 6BQ5 should make any difference at all with the opt 6V6GT class is 5.5 watts. 6BQ6 class A is 6 watts. Max plate volts on the 6BQ5 is 300 and the 6V6GT is 315, not much difference there. In fact 3 watts is probably about where it will be, but unless the 6BQ5 was triode strapped in the Mullard circuit then the 6V6 shouldn't be either. I will take a look and if the 6BQ5 is triode strapped then of couse I will need to do the same with the 6V6. The 6BQ5 was made to replace the 6V6 because it takes up less room, but the power requirements and what they do are extremely close together. Maybe it was the 6AQ8, but that is close too. Looks like the 5K load will be fine too. The 6V6Gt likes it a bit higher but the GTA is 5.5, 6BQ5 is 5.2. Ought to be great.
Found the Mullard schematic and the pentode is not strapped for triode. I need to find one that has the voltages listed at the tube pins.

Thatch
I have 335V Ct of .125A. There is a 25 volt drop with a 5V4. Couple of 5 Henry chokes make 150 ohms, a few big chassis mount oil caps say 6to 4 uF per stage the AC ripple ought to be gone, so Do I go with what the GE book says on the 6J5 or have anybody played around with them or 6SN7s?
I don't see this as difficult and I am going to do point to point wiring, I have probably everything I need but a bit of advice.
I just bought a new computer and I heard tell that Tube cad and SE cad don't get along well with windows 7. My new computer has Windows 7. I don't know if it is true or not, but even if it was I want to hear from people who have used 6J5s or 6SN7s as the amplifiers in a SE amp. Tweakers always find out things that models can't tell you.
The do everything except use a good tube in the amplifier section looks like an interesting board. It doesn't seem to use 6V6s though and that is what I want.
I have built amps before. I can read schematics, do layouts and solder. I have military grade parts. I can't see that not being able to use Ohm's Law should prohibit me from making these mono blocks.

RCA plug to a good audio taper pot to the grid of the 6J5, get the right DCV into the plate, ground the cathode with a good cap and a resister, run the signal over to the grid of the 6V6 who's plate is run off the B+ after it fires up the OPT. Connect the filaments, filter that with a cap if that is a good idea. Connect the proper speaker to the correct tap, leave the power tube out while it is warmed up on the variac to 90V and leave for 2 days, come back, turn it off, give it a source, put in the 6V6 and give it 120V and find out how it sounds. I can get all the PIO or teflon caps I want, I have Polies and huge oils, probably some PCBs but nobody knows for sure. I have grounding wire, wire for the PS and Vampire 26 guage OFC magnet wire for the signal wire. I have meters, all kinds of things to solder componants into and bolt onto the chassis. I have adjustable wire wound resisters if I need something for bias. I started drawing the schematic today. I have a soldering station, a solder pot, multi meters and a capacitor tester on the way.
I don't like 12AT7s, I want to use all octals.
I don't plan on killing myself, but if I do it shouldn't bother anybody.
I know it is a bit harder than that, but that is why I am asking here. I don't want to stuff a board someone else designed. This is my learning amp(s)
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Old 19th April 2011, 07:16 AM   #7
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Also, the boozhound site has a SE 6V6 design. Boozhound Laboratories[/QUOTE]

Boozehound schematic will help, Thanks

Thatch
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Old 19th April 2011, 07:43 AM   #8
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It's not so much the cathode you need to be concerned with, it's grounding the metal envelope. The envelope must not be allowed to become positive with respect to the cathode. This usually isn't a problem unless you're connecting the cathode to a negative rail. You could always connect the metal shell directly to the cathode at the socket so that they are always at the same potential. (May present a shock hazard if using a negative rail in the cathode, since all that's standing between you and that voltage is a thin layer of paint.)[/QUOTE]

I'll have to check this out. I was planning on going with a star ground running a thick copper wire that is grounded to chassis and then grounding to it.

I do plan on floating the earth ground connection with a 100 ohm few watts and a .1 with 400V or so to make sure the chassis ground and earth ground have a good filter between them. I was taught that that is floating the earth above the chassis. I am sure it is called a few things.

Main thing is to keep it as simple as possible. I don't plan on using feedback if I don't have to. If I do I might use a pot to make it variable, or a stepped attenuator. If I can get away without using the negative feedback then I won't. I do like a bit on female vocals, but acoustic instruments seem to sound better with none.
To keep the wattage down to 4 or so I might need to drop the voltage on the power tube, but with the medium mu on the 6J5 I ought to run it close to full out. The original circuit uses an EF-86 and if I was going to strap one I think I would strap that little pentode. The gain is extremely high and prone to microphonics, It is too bad there wasn't a single triode 12AY7 made in octal. Maybe I should get some WWII 6SN7s (VT-231s?) and just use one side. A friend was telling me about some tube you can drop into a 12AT7 socket without changes though t is quieter. I'll have to get that number and maybe some iron wound for Chimera labs that would take 6L6s EL-34s maybe 6550s and any kind of 12AX7 type almost 15 years ago. I think they have to be lammed, but that is real DIY.

Time for sleep. The circuit board looks interesting in that it can use almost any kind of pentode developed out of the 6L6 family but odd in that the only dual triode is 12AT7, but with all the power choices dropping a 12AY7 should work, I have used them instead of the 12AX7s in Fisher gear and the ones in that large family of kissing cousins. 5751s etc.. Fun stuff.

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Old 19th April 2011, 07:19 PM   #9
tomchr is offline tomchr  United States
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Originally Posted by boywonder View Post
A minor correction; the Tubelab Simple SE uses a 12AT7 driver and the Tubelab SE uses a 5842.
My bad. It's the SE not the SSE I've messed with then. I get names confused.

The 5842 is a nice little tube. Good driver. I liked the sound better with the 6J5, though. 6J5 as an anode follower or inverting amp with local feedback can be interesting as well. I had that running for a while driving a 300B but ended up moving to a high-mu triode DC coupled to the 300B.

The Boozhound circuit looks pretty solid. Note, however, that it uses a 6SL7 not 6SN7. 6SL7 is a dual, high-mu triode. I don't see any reason you couldn't use a 6J5 instead, you just won't get as much gain. Also, I'd use a CCS on the anode and an LED on the cathode for bias. Or you could use a 6J5 for the input, 6BX7 for the driver and 6V6 final. Options, options...

~Tom

Last edited by tomchr; 19th April 2011 at 07:23 PM.
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Old 21st April 2011, 01:37 AM   #10
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I looked around for alternatives and there is a nice Raytheon BP drop in that runs about $20 and it does look like fun. I still want to build this circuit with the 6J5. If is isn't enough gain I can triode strap a 6Y6G since I have a bunch of those. Running straight of a CDP usually gives enough voltage that if you are running 97 db speakers that there are no problems. The 6J5 is half a 6SN7 which is like a 12AU7 and I am hoping nice and quiet.
The whole reason behind this amp is to have fun rolling tubes, not because it will be super high fidelity. I have a 6SL7 SRPP driving 300Bs using those PP EL-34 Sansui OPT that have been air gapped. Not by me, but I have lammed audio chokes that were air gapped so I get the idea. The OPTs I have for this are air gapped 5Ks and I probably could do lots of things with them.
The boards you stuff can be changed to all kinds of stuff without having to build a 3D jigsaw.
I was hoping there was a 6SN7 nut around here who used them one side at a time and would be helpful in all the ways for hooking up the 6J5s. Same thing for the 6V6s in SE that is not a Fender Champ, ya know.
Maybe I should change the title of this thread, or start another. I don't think these tubes are anywhere near strange enough that they haven't been played with a lot. Price point, being common, everything made me think there would be guys with lots of info.
I guess I better hit the search here or Google because this hasn't gotten me much to work with.
Big thanks to those that have helped, by the way. I am grateful, but not Jerry Garcia.
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