6V6GT Single-Ended Project

[Willabe]

You've tried the lower screen dcv with the 8k2 B+ dropping R.

You can change that dropping R to 1K that'll bring up the screen dcv to close to the plate dcv.

See if you like it better.

[shooter]

I will follow your advice shooter and step down until I'm just at 100% - maybe a tad less.

the tonal "sweet spot" may be at 90% or 87%, so play long enough between changes to know "Oops, to far" 


also, play with NFB, then without, you may find you want that a switchable option.

[dwinstonwood]

Thanks Willabe. So, this is only the 7th tube guitar amp I've built (not counting a couple of rebuilds to optimize my earliest projects). All of those amps have the screen voltage at, or close to, the plate voltage. With this project I chose to do things a bit differently than my previous builds. That's what makes it fun; there were a lot of "unknowns" for me. I really like the way this amp sounds. It's full, not too bright, and definitely not muddy (through a Celestion Creamback). It must be a combination of the Octal tubes, the voltages, and the various preamp stages I decided on. I'm honestly a bit surprised that it sounds this good, since I'm not familiar with another amp using these same exact choices - though nothing with tube amps is actually new. I'm not going to change anything anytime soon. I have more than enough amps with conventional voltages, and it's cool to play through something different for a change. And, to be able to try and hear any audible differences.

Thanks shooter. I tacked a 2.2K across the 330 Ohm Cathode resistor which gives me 287 Ohms. Here're the results. Oh, I took out the sacrificial new EH6V6GT and plugged in an old GE 6V6GT. As you suggest, I'll stay here long enough to really get to know the amp.

Plate - 324
Plate to cathode - 312.8
Screen - 248.6
Rk - 287 Ohms
Across Rk - 11.9
Plate current - 38.4mA
Dissipation - 12.3W, 87.9%

And, here's the "final" schematic.

[Willabe]

All of those amps have the screen voltage at, or close to, the plate voltage. With this project I chose to do things a bit differently than my previous builds. ......I have more than enough amps with conventional voltages,

I know that. And it's not about conventional voltages.

Wow! I need to back way off on the voltage dividers I added. You all had me scared that this thing would be a howling beast, so I really chopped down the voltage.

With the Gain and the Volume knobs on 8 it is quiet but very clean, no distortion.

When I said try raising the screen dcv and see how it sounds, I didn't mean tone, I meant volume output and output tube distortion. 

It's not just the voltage dividers between the preamp stages. 

This is why I said to try raising the screen dcv. It's low and it's part of why your hearing what you said above. Lowering the screen dcv chokes the tubes output. The lower you go, the more it shuts off the tubes output. It's a grid, it's allows electrons to flow to the plate or it stops/impedes electrons from flowing to the plate. The grid is why the English call vacuum tubes values.   

Change the screen R and see it opens up the volume and allows some distortion.

[dwinstonwood]

Sorry Willabe, but if I crank up the Input Gain this amp has way more distortion than I ever really use ( but, it's nice to have it on tap).

With the Input Gain down around 4 or 5 and the post-tonestack Volume dimed it sounds great. Hair, but controllable by string attack, i.e., clean to breakup depending on how hard I hit the guitar.

It's very versatile. It does a lot, and does it at volumes that I can use at home at night. No, not everyone would like it. But, beyond my own expectations, it does what I hoped it would do. Plus, it's crazy with its 5693 Pentode, 6SF5GT input, 6SL7 Cathode Follower, and James Tone Stack. The most fun I've had building an amp. Maybe the best I've done yet.

Unless I feel like there's a need to mess with it, I'm not.

[Willabe]

With the Gain and the Volume knobs on 8 it is quiet but very clean, no distortion.

I was going by this, I must have missed something.

[dwinstonwood]

I've been twisting knobs all night. With the Input Gain on 5, the Post-Tonestack Volume on 10, the Treble and Bass both at 7, and my Tele in the middle position with the Volume up full and Tone about 2/3rd's up, it has more Chime that my AC15. Without the ringing. This amp is cool.

[dwinstonwood]

With the Gain and the Volume knobs on 8 it is quiet but very clean, no distortion.

I was going by this, I must have missed something.

That was before I went from 820K/220K to 220K/220K dividers.

[DummyLoad]

What brand/model# did you end up with for power trans, output trans, and choke?


--Pete

[shooter]

This amp is cool.

welcome to the dark side 


now for Christmas ask nice for a KT88, and Iron 

[dwinstonwood]

What brand/model# did you end up with for power trans, output trans, and choke?

--Pete

PT = Hammond 270EX
Choke - Hammond 194B
OPT - Musical Power Supplies OT10SE 12W

[DummyLoad]

If the 5693 is not disposable (or too expensive) I can suggest to give a try to the russian 6Ж8, it was planned for military use and it has a sturdy structure

Here’s a detailed comparison between the **6SJ7** and the **6Ж8**:

### 1. Origin and Purpose
- **6SJ7**: An American-made tube widely used in audio amplifiers, radios, and low-frequency circuits. It was designed to provide stable gain with low noise for commercial and home applications.
- **6Ж8**: A Russian tube with similar specifications, often produced for military and industrial purposes. It's designed with greater resilience to mechanical and environmental stress.

### 2. Structure and Build
- **6SJ7**: Typically comes in a glass tube or metal case (depending on the version), offering decent shielding. The glass versions tend to be more prone to microphonics than the metal-bodied ones.
- **6Ж8**: Generally more robust and built to withstand shocks and vibrations better, making it less susceptible to microphonics. Its construction quality tends to be more durable due to military-grade standards.

### 3. Performance
- **Gain**: Both tubes offer similar gain levels, though the 6Ж8 can sometimes provide slightly more stable performance under high-gain conditions, depending on specific production batches.
- **Noise**: The 6Ж8 typically generates less noise than the 6SJ7, especially in noisy environments or with potential interference. This makes it better suited for applications requiring signal purity.
- **Microphonics**: The 6Ж8 generally has lower microphonics due to its sturdier construction, while the 6SJ7 may exhibit microphonics at high gain levels or in vibrating environments.

### 4. Availability and Compatibility
- **6SJ7**: Still relatively easy to find due to its historical popularity and large-scale production, especially in vintage American versions.
- **6Ж8**: As a Russian-origin tube, it may be less common in some Western markets, though it's readily available in Europe and the US through specialized suppliers. It may also have slight pin configuration differences, so compatibility should be checked before use in circuits.

### 5. Ideal Applications
- **6SJ7**: Recommended for vintage audio amplifiers and projects where microphonics are not a major concern. It’s also appreciated in DIY audio projects.
- **6Ж8**: Better suited for applications with concerns over vibration or electromagnetic interference, such as precision instrumentation and circuits intended for military or industrial use.

### Conclusion
Both tubes are great choices but with slightly different optimal use cases. The **6SJ7** is well-suited for general audio applications and hobbyist projects, while the **6Ж8** provides enhanced durability and lower microphonics, making it preferable for demanding environments or where high reliability is needed

.

Of course the 5693 is the better of the three

Franco

Franco, the 6zh7 is a copy of the 6J7 - It is dissimilar electrically to the 6SJ7. The 6J7 has less transconductance, so lower overall gain in like bias states. Typically the 6J7 were used in PA amps that were built in the 40's into the early 50's as a first stage amp. I shy away from them because of the grid cap, however, they do sound really nice when pushed hard, and IMO, they are better sounding than the 5879 in a guitar amp.

Hope all is well in Italia mio amico.

Con rispetto,


--Pete

[kagliostro]

Ciao Pete

Health is going up and down but this last days the tendency is to be better than the last year

The tube I refer is the 6Ж8 (in Cyrillic) (6J8 in our alphabet) (it is a no topcap tube) and also there is a bit of confusion because there is one other tube that calls 6J8 (in Cyrillic) but isn't the same tube (this last is a pentagrid + triode)

Anyway I'm not referring to a 6J7 tube but to a 6J8 (in our alphabet)


The spec I've find ate this
6J8 / 6Zh8 / 6SJ7 tube

6J8 is a Russian HF pentode tube. The tube is covered with black metal screen. Equivalents to the 6SJ7.


Name: 6J8;
Type: HF pentode;
Application: HF voltage amplification;
Cathode type: oxide, indirect heating;
Envelope: metal;
Mass, g: 42;
Filament voltage, V: 6,3;
Filament current, A: 0,275-0,325;
Anode voltage, V: 250;
Anode current, A: 0,002-0,004;
Anode power, W: 2,8;
Grid1 voltage, V: minus3;
Grid2 voltage, V: 100;
Grid3 voltage, V: 0;
Steepness, mA/V: 1,3-2,0;
Reverse grid current, uA: 0,5;
Microphnic noise, mV: 100;
Socket type: rsh5-1


And in many comparison sheet they say (may be wrongly) that they are a 6SJ7 correspondent tube

Franco


p.s.: For Cyrillic to our alphabet conversion I use this correspondence table find at duncansmps

[kagliostro]

Ciao Pete

I reread your last post in a different way ....

Are you suggesting to recover and also use the 6J7 tube (and Russian versions) as a good one for our Old Wild Amp constructions ?

Thanks

Franco

[dwinstonwood]

Tonight I dimed everything. Gain, Treble, Bass, Volume, and Telecaster Volume and Tone.

Through a Celestion Creamback it sounds insane! Very edgy, but not harsh. At full on overdrive it has great distortion. Maybe it's the James tone stack scooping the mids when, or the tubes I used.

At any rate, it really does its thing when cranked.

[kagliostro]

Happy to hear you are happy 


Franco

[dwinstonwood]

I just played it dimed through my Warehouse G12C/S. This speaker is brighter and more efficient that the Cream Back, so it's somewhat louder. There's no oscillation, or weird sounds that I can detect. It seems to be stable.

[kagliostro]

Very Well 

Can you post the last version of Schematic and Layout ?

Franco

[DummyLoad]

Ciao Pete

I reread your last post in a different way ....

Are you suggesting to recover and also use the 6J7 tube (and Russian versions) as a good one for our Old Wild Amp constructions ?

Thanks

Franco

Yes, I do recommend using the 6J7, but the grid cap can can cause problems with hum if you don't use a shielded cap. I haven't built anything permanent with them, just on a breadboard. If I repurpose an old PA amp that used 6J7, I rewire the socket to use 6SJ7. 

--Pete

[kagliostro]

And TopCap Shields aren't easy to be find, I searched for it some time ago

I must ask to my ungarian friend, he is my russian tube supplier and he has acces to things that are difficult to be find here

Thanks Pete

Franco

[shooter]

dimed through my

 


I rolled about 8 different speakers, sticky notes on each, I just couldn't find "That perfect speaker".  someone suggested 2 at once, bought a 2 X12 cab, rolled them again, more sticky notes, settled on a Cream-back and Eminence and quit rolling, that cab became "the standard"
never rolled speakers again 

[dwinstonwood]

Very Well 

Can you post the last version of Schematic and Layout ?

Franco

I've been sidetracked with other stuff. And, I wanted to add my measured voltages after a few hours of banging on it.

There's a lot of room in this design to change things up (voltages, tubes, tonestack, etc.). But, as I built it, it was relatively inexpensive and uses all "cheap" NOS tubes. Plus, wiring up those nice Belton Octal sockets - especially the heaters - has made me never want to use a 9-pin tube again.

[dwinstonwood]

I installed a NOS RCA JAN CRC 6V6. I rewired the socket with Pin 1 (metal shield) to ground, and the 1.5K Grid Stopper connected directly to Pin 5.

It's a really nice sounding tube. The box is dated July, 1953, so right at the end of the Korean War. But, when that black paint started heating up for the first time it got a bit stinky.

[bmccowan]

It's a really nice sounding tube. The box is dated July, 1953, so right at the end of the Korean War. But, when that black paint started heating up for the first time it got a bit stinky.

That's how you know it really is NOS.
I've got some metal 6L6s like those - they are about 8" tall and sound great.

[DummyLoad]

My spin on this - See attached. I biased the 6SJ7 for Va to be at ≈1/2 B+, CF trim adds some dark tone, reduces output swing to driver triode, so better for cleans. I like the 6L6 tone better when B+ is pushed with a class A output. EL34s in parallel sound ever better. I had planned originally to build || EL34s but I don't have an OT with the proper reflected load. In this iteration, I used a One Electron UBT-2, which is no longer made, too bad - Use a Hammond 125GSE w/5K reflected. Built on the breadboard & sounds killer.

On the breadboard I didn't build in the vari-NFB shown in schema, nor the switched selectable bypass for the driver, however, I did wire a switchable .68uF. Bypassing the driver really kicks the output stage in the pants. I plan to wire in the vari-NFB in the next day or two. Lastly, I'll most likely ditch the CF trim pot, i tend to favor it dimed (97K setting) anyway.   



--Pete

[shooter]

EL34s in parallel sound ever better.

 
he's almost there, i know he's smiling after a good session, but......

[dwinstonwood]

Here's a good candidate for an All-NOS-Octal build using affordable metal power tubes.

[bmccowan]

Here's a good candidate for an All-NOS-Octal build using affordable metal power tubes.

6SFGT? - Is 6SF5 what you intended?
That preamp with the 5693 would be very similar to many early Fender tweeds and early Gibson and Valco amps, but with fixed instead of cathode bias.
I built a Rickenbacker M11 with that preamp - sounds great. The preamp is very similar to that used in the earliest Fender Pro/Super which is on my build wish list.