New Project: 6U8/6V6 amp

[tunghaichuan***]

I got a start on my newest project: a single ended amp using a 6U8 triode/pentode for the preamp tube and a single ended 6V6 for the output.

Here are some pix.

The amp uses a 6U8 triode/pentode for the preamp. The pentode section is the input tube; the triode drives the 6V6. I have quite a few NOS 6U8 tubes as well as 6GH8 and 6BL8, which have the same pinout but slightly different ratings.

The output is based on the tweed Princeton/Champ. I decided to use JJ 6V6S tubes as I got a batch with the skinny pins. The ceramic tube sockets I got a while back grip the pins very tightly, which is good because the JJ 6V6S tubes are loose in the NOS sockets I have

One interesting thing about this circuit is that it has a built in compression control. More on that later.

I also plan on installing the VVR circuit using this layout. The VVR is a type of power scaling which uses a MosFet to control the anode voltages, and thereby reduce the ouput wattage. I've been wanting to try this for a while because it is supposed to give good OD sounds at whisper-quiet volumes. We'll see. The one bummer about using the VVR is that the MosFet costs about $12US. It really pains me to pay that much for a solid state device.

So far I've generated the schematic and the board layout. Next up is building the board, and then generating a chassis layout.

I've been wanting to do this project because eventually I want to build a push pull version of this amp using the 6U8, a 12AX7 phase inverter and a pair of 6V6 tubes. Maybe even a tube rectifier.

Stay tuned...

tung

[duhvoodooman***]

Cool! Waiting to see (and hear!!) how this one turns out! :

[tunghaichuan***]

Got the circuit board built this morning. As usual, I made a master drilling board so all I have to do is cut the blank to length and put the master on top and trace out the holes. I then drill the board using a layout guide and a fence on my drill press so all the holes line up evenly. Next, installed the eyelets.

Next step is to drill the chassis. Since this is a prototype, I'm not going to spring for face plates. There will only be four knobs on it anyway: compression, volume, tone on the front and a VVR level on the back.

Some pix:

Drilling pattern taped on master and blank board. The bag to the right is full of 1/8" eyelets, approximately 1,000.

Closeup of pattern taped on master board and blank board.

Drilled out blank board, master and extra pattern.

Finished board.

tung

[tunghaichuan***]

Got the chassis done yesterday.

Blank Bud AC-409 chassis It's even RoHS.

Chassis with blank circuit board

Chassis clamped to bench

Control panel holes drilled out

Finished chassis

Front of finished chassis

Board mounted in chassis

Parts mounted on chassis

Board stuffed with parts, not soldered yet

So far this is shaping up nicely. The next step is to solder the flying leads to the board and mount into the chassis. After that, I can begin to wire the circuit.

More later.

tung

[piebaldpython***]

I gotta tell ya Tung, I am totally in awe; : : : ; of what you and DVM do with your skills in making amps, pedals, etc.

OMG, I would be ready for the loony bin for sure.

[duhvoodooman***]

Comin' along nicely there, Tung! :

I gotta tell ya Tung, I am totally in awe; : : : ; of what you and DVM do with your skills in making amps, pedals, etc.

Don't be so impressed, Pie. If Tung and I can handle this stuff, how hard can it be??

[tunghaichuan***]

Comin' along nicely there, Tung! :

Don't be so impressed, Pie. If Tung and I can handle this stuff, how hard can it be??

Yeah, really. :

I gotta tell ya Tung, I am totally in awe; : : : ; of what you and DVM do with your skills in making amps, pedals, etc.

OMG, I would be ready for the loony bin for sure.

Thanks for the kind words, Python.

Update:

I spent the afternoon attaching the flying leads and installing the board. I spent quite a bit of time soldering down the leads from the transformer as they can be a real PITA to deal with.

Here are a couple more pix:

Flying leads attached to board

Board installed in chassis

I've got quite a bit more wiring done than shown in the above picture. More pix to follow.

tung

[duhvoodooman***]

Man, you are flyin' along on this one, Tung! :

[F_BSurfer]

looking good.......what kind of transformers you using on this one

[Ch0jin]

Nice work as usual Tung

I'm especially interested in what you think of the VVR in practice. I might have a lot of spare time in the near future, and I'm thinking of rebuilding my old frankenstein amp to sort out a few niggling issues. As it's probably going to remain around 50W (don't want to change trannies etc) being able to dial down the voltage and subsequent volume is appealing.

I'm also waiting eagerly to hear about the compression control

[tunghaichuan***]

looking good.......what kind of transformers you using on this one

Thanks!

The power transformer is a vintage Triad R6A. I've used it for several tweed Princeton builds, and it works great. The output transformer is a Fender Champ 12 unit. 5K:8 ohms, rated at 10 watts. It should match well with the JJ 6V6S.

tung

[tunghaichuan***]

Nice work as usual Tung

I'm especially interested in what you think of the VVR in practice. I might have a lot of spare time in the near future, and I'm thinking of rebuilding my old frankenstein amp to sort out a few niggling issues. As it's probably going to remain around 50W (don't want to change trannies etc) being able to dial down the voltage and subsequent volume is appealing.

I'm also waiting eagerly to hear about the compression control

The VVR is the last circuit I'm going to put in. I still need to order some of those NTE mosfets, which are pricey. The VVR shown previously only works with cathode bias amps. There is another VVR circuit for push pull amps which automatically tracks the bias voltage.

The compression control is merely a pot to regulate the screen. Randall Smith has a patent on it, so I would never be able to use it in a production amp

I'm attaching the preamp schematic I'm working from so that you can check it out yourself.

tung

[Jipes***]

That's fascinating Tung I'm really curious to her and see the final result I have a friend who loves to play around with Tube amp but you seems to be kind of a "Tube Guru" isn't it

I have unfortunately two left hands (I'm right handed) so I will never ever give it a try on this type of construction but I'm really curious and admirative for your work :

[tunghaichuan***]

That's fascinating Tung I'm really curious to her and see the final result I have a friend who loves to play around with Tube amp but you seems to be kind of a "Tube Guru" isn't it

Thanks, but I'm really just a hack. I like messing around with tube circuits and trying new things, but I really could stand to learn some theory.

I have unfortunately two left hands (I'm right handed) so I will never ever give it a try on this type of construction but I'm really curious and admirative for your work :

Tube electronics isn't something you should mess around with unless you're really committed to learning safety procedures. It is a lot of fun for me. And I'm looking forward to finishing this project to see how this amp sounds. I have high hopes. OTOH, it could sound like crap.

tung

[piebaldpython***]

C'mon Tung.....you can do it! SHINER set the bar (the limbo bar, not the drinkin' bar ) kinda high with his FIREFLY amp. :

Seriously, you (along with DVM) are much too modest about your skills and knowledge about amps, pedals, etc. : : You are NO hack, that's for sure. I look forward to reading/hearing about your amp.

A long time ago, I installed a couple of ceiling fans in my bedrooms. I was scared half to death when I turned the electric back on. I half expected to see SMOKE and SPARKS coming from the outlet.

So, when I read about what you guys are able to do, it just earns all of my admiration for the time and talent it takes to do all that stuff. KUDOS.

[tunghaichuan***]

Again thanks for the kind words.

A long time ago, I installed a couple of ceiling fans in my bedrooms. I was scared half to death when I turned the electric back on. I half expected to see SMOKE and SPARKS coming from the outlet.

About 3-4 years ago I replaced all the switches and AC receptacles on the first floor of my house. I did check out a book on electrical wiring to make sure my job was up to code. Haven't burned down the house, yet :

tung

[tot_Ou_tard]

Really clean work Tung!

I'm all eyes. My ears are getting jealous.

[tunghaichuan***]

Really clean work Tung!

I'm all eyes. My ears are getting jealous.

Wednesday and Thursday were my days off, so I had a lot of free time to work on the amp. I probably won't get a chance to work on it again until tomorrow or maybe Tuesday.

One of the reasons I'm working on this circuit is that it would be a direct replacement for the Champion 600 amp. With a few mods it may even work for the Little Giant amp.

I became interested in pentode preamps after building the EF86 amp which is a hybrid Marshall 18W and Vox AC15. The 6U8/6GH8/6BL8 and a few others in that family were used in TV sets and made by the millions. They never were used in guitar or hifi amps with one or two exceptions so there is lots of NOS stock available inexpensively.

The other reason I'm building this prototype is to see if the 6U8 pentode/triode sounds good in a guitar amp. I have a push pull version on the drawing board that I plan on building for a friend. This is the friend who sent me the MI Audio Tube Zone pedal, so I wanted to return the favor with a well made boutique style amp.

Stay tuned, more on the prototype as I have a chance to work on it.

tung

[tot_Ou_tard]

Has the 6U8 been used in guitar amps before?