Build thread all octal (6SJ7+6SK7) 5F2A with 6L6

Jerry garrcia

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So I guess I’ll try to tie it all together.
The planning started when I was looking for something to put in an old 12” Ampro speaker cabinet.
4CDF09CD-84FA-4FD6-B9DD-FA37E80EE8B9.jpeg 6F3E6FC4-6C8C-4D0E-8565-785F572DB1AF.jpeg It doesn’t have to much room in the cab and I wanted to have as much clean volume possible out, so I decide to go SE with a 6L6.
Only using tubes that I have at home and at least two of them.
Love the sound of the 6SJ7 but when pressed hard to squeeze out the maximum gain out of it it sometimes takes it own route in terms of mis sound. A bias switch so it can run between grid leak and cathode bias.

Also a bit afraid that with the added tone stack and the big power tube that a single preamp tube might not drive the 6L6 to it maximum potential.
So added a triode wired 6SK7 (remote cut of pentode with 20 mu when in triode connection) between the tone stack and the output tube. Not out for distorted heavy metal sound but that warm, clean jazzy sound.
Next problem was that the speaker I’m planning to use is an 15 ohm so the OT wouldn’t be optimal with regards to primary impedance. Went for the 15w 125ESE and will use the 5K winding. @2L man helped a lot with the plate load to maximise cleans and volume. Aiming for around 420 plate voltage on the 6L6 and was also suggested to try Local Cathode Feedback. Will wait with that for now but populate the turret board so it will be easy to add.
Initially planed to use one of my old radio PT’s but settled for a toroidal 345-0-345, 0.15A. An extra filter stage before the B+1.

So my questions are (before the last schematic is drawn) and the build will start is:
- any inputs on the above?
- the 4.7K cathode resistor for the 6SK7 seems a bit high but was read some were that it should be about 4-5 times higher than expected due to the tube’s characteristics. Inputs on that?
- should I go with higher values on the filter caps?
- the cab will be a closed back but I bill cut out part of the back to be able to access the amp and place a small grill opening above that for ventilation.

Any inputs appreciated on the above end below and also if someone has any experiences they will be willing to contribute with regarding their own 5F2A builds with 6L6’s.
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Jerry garrcia

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Thanks to @2L man this is the updated schematic. Initially I calculated with a larger voltage drop over the OT than was expected, a 1M master volume pot instead of the optimal 500K before the 6L6 and forgot the coupling cap before the power tube.
Hopefully the voltage drop resistors in the power stage is a bit more “theoretical accurate”. The build will probably start Monday next week.
Still open to inputs 😉. One good thing is that the tube tester DuoKit works fine according to my trials. Cheap and quite easy to assemble.
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dunner84

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Great project. Working on a few pentode projects myself right now.

I don’t want to derail your plan, but have you looked at the silvertone 1331? Single 6sj7 into a 6v6, and they scream. It’s interesting because the tone stack is on the input. Also grid leak biased for max gain.
 

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Jerry garrcia

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Great project. Working on a few pentode projects myself right now.

I don’t want to derail your plan, but have you looked at the silvertone 1331? Single 6sj7 into a 6v6, and they scream. It’s interesting because the tone stack is on the input. Also grid leak biased for max gain.
Thanks for your input. I was unaware of that circuit but have built a couple of similar once in the last year. Not with that tone stack though. Need to look in to that.
The reason why I planned this kind of design for this one is that I have this large cabinet with the 12” speaker and wanted an amp that can be gigged with in larger clubs and aimed for lots of headroom and volume. That’s the reason that I decrease the amount of voltage gain of the 6SJ7, went with the 6L6 and also made the bias switch to be able to make it more pedal friendly.
I’ll include the different 6SJ7 schematics from the last builds.

What’s your take on the tone stack compared to the Princeton one?
12ax7 6sj7 reverb.jpg

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Jerry garrcia

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So finally got some building time. The dog snatched the only 47uF filter cap that I had so had to use a larger one. Since SS it will probably be ok and might also be a bit better since the rest of the filter caps are rated 450V and the unloaded B+ would be around 485Vdc. Will use a variac when I run it without tubes. Hopefully I’ll get some drilling time tomorrow. The lead to the first filter cap is wrong
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Jerry garrcia

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Finally everything mounted. Hope to get som soldering time this weekend.
Now the fun begins.
Might have to move the PT a bit to free up some space for the power tube. Now a 6L6 have 5mm free space from the PT. A KT88 will hit it. Might as well do it now. A couple of cm will do it.
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Jerry garrcia

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So now almost all connected except for the main power.
- Have a switch that will let you choose between grid leak and cathode bias.
-56K NFB switch to the 16 ohm tap.
- isolated 8 and 16 ohm speaker outputs.
- two diode rectifier.
- single ground bus to input jack.
- to short main power ground wire (need to change that).
- triode connected 6SK7 as a driver.

Got help from @2L man @dan40 and @tompj with the toroidal PT and it’s multitaps.

Hopefully get some time tomorrow to fire it up.
Got a hold of a 12” Goodman 8 ohm, 10W speaker today to compare with the previous installed 16 ohm Goodman. The 10w might be a bit weak?
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Jerry garrcia

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Great project. Working on a few pentode projects myself right now.

I don’t want to derail your plan, but have you looked at the silvertone 1331? Single 6sj7 into a 6v6, and they scream. It’s interesting because the tone stack is on the input. Also grid leak biased for max gain.
Sorry for the late reply. Now you got me interested 🙂. No I have not. Looks really interesting. The pentode is really set up for maximum gain.
Have you built one or tried it? When will it start to break up? A lot of questions. I’ve just got a stack of +10, old but functional, PT’s for 5-10W amps. Unfortunately all the SE OT’s I’ve got as spare parts are primarily el84 OT’s. Maybe a Silvertone 1331 with an el84? But also quite a few unmatched 6V6’s🤔.
One thing I regret is that I’ve built a lot of combos. In retrospect heads would have been better to save the speaker costs.
 

screefer

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Regarding the 1331, here's a clip of one...



I like that old timey grid leak grit and am very curious to see/hear how your switching arrangement works out.
Great project!
 

Jerry garrcia

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Regarding the 1331, here's a clip of one...



I like that old timey grid leak grit and am very curious to see/hear how your switching arrangement works out.
Great project!

Also a big fan of grid leak but not very pedal friendly. That’s the only reason for the switch for this build. Sometimes a clean boost is a good thing except for in grid leak. In previous builds there’s just been a tiny difference in tone regarding the switching between the different bias modes.
 
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Jerry garrcia

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So some delay due to to high HT from PT. Supposed to be 345-0-345V but I’m reality 376V and filament voltage was 3.50-0-3.5V.
So increased the resistor between first filter cap and plate node from a 470R to a 680R. Realising that I forgot to add a dropping resistor on the filament windings. Need to fix that.
Will power up tomorrow. Don’t dare to do it now with the family at home due to the NFB and uncertain of the phasing.
If something strange please comment.
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2L man

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Have you already used 240VAC primary wire? It will drop all secondary voltages!
 

Jerry garrcia

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Have you already used 240VAC primary wire? It will drop all secondary voltages!
Yep. This is with the 240V winding. 393V and 3.65V with the 230V primary.
You have taught me a lot and somethings stuck😉

For the next build I will not solder the voltage dropping resistors in the power section from the beginning. Alligator clips would probably been a better idea to easier trim it.
 

Jerry garrcia

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So, an irritating afternoon. The good thing is that my layout was ok. Totally quiet amp. Think part of it is the input grid resistor. But also maybe the 80uF filter cap, toroidal PT, choke and beautiful lead dress😀.

Bad thing is that my voltages was a bit of but that’s one of the joys with building an amp. A present that keeps coming.
So due to the measured PT secondaries I did increase the resistor between the first filter cap and plate node to a 680R, 5W.
Values without load:
B+ 509, B+1 (plate node) 510, B+2 510, B+3 500V. Filament 6.6V.

Inserted tubes
B+ 466, B+1 (plate node) 405, B+2 404, B+3 386V.
6L6
Plate 393V
Screen 398V
Cathode 27.53V
6SK7 (triode)
Plate 131V
Cathode 12.2V
6SJ7
Plate 92V
Screen 34.6V
Cathode (cathode bias mode) 1.3V
Grid (grid leak bias) -0.93V

Aimed for plate voltage of 420V of the 6L6. And 103% of plate dissipation.
So decreased resistor between first filter cap and plate node (B+ - B+1) to a 470R, 5W. Didn’t have a larger 10W cathode resistor than the 330R.

Under load:
B+ 476, B+1 (plate node) 431, B+2 430, B+3 410V.
6L6
Plate 421V
Screen 428V
Cathode 29.8V
6SK7 (triode)
Plate 140.2V
Cathode 13.8V
6SJ7
Plate 103V
Screen 45.3V
Cathode (cathode bias mode) 1.4V


So plate voltage good now but 120% plate dissipation. Old west German 6L6.
So need to solve and lower the plate dissipation.
I do have (in 10W) 100R, 270R, 330R and >1K. So thought that I’ll try to add another resistor in series. Will start with a 100R so I’ll get 430R as a cathode resistor. Hopefully it will solve it.

@printer2 your grid leak switch works fine as always.

Played for a while and the amp is super! Clean tone up to 11/12 and the closed back cabinet together with the Goodman speaker is just right. Whas louder than a regular champ but I’d hoped for even more. The speaker might not be the most efficient but nice sounding.
1) Any inputs on how to handle the cathode resistor and what’s the best value for plate dissipation to aim for and if 6.6V is an ok value (hope for that since the lowest value resistor I’ll have is two 1 Ohm, 2W)? Since it’s a CT 1 Ohm on each leg might be a bit to much.
2) To the octal pentode guru @printer2 , anything to do with the gain circuit?
3) have a 10W alnico 12” Goodman speaker. It’s too low wattage for this amp? For a 5W amp?
4) Any experience with a Isophone P30/37/12 speaker for guitar? A west german speaker maker.

This amp is a true keeper. Need to find a way to make an easy built head for it.

Next project(after this one is done) I’m gonna order parts for a single channel Ampeg B15 and this reverb tank that I goes a recommendation about https://valveheaven.com/2016/10/the-lamington-reverb-an-easy-to-build-valve-reverb-unit/ with a 6BX6 and 6aq5 tube
Have some tanks and I haven’t really got the LNDVerb to work perfectly. When incorporated in the circuit hum problem.
 
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2L man

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You could drop bias and let voltage get higher. Now bias is bit left. Onthis attached loadline that green is about 30Vpp grid drive signal between -15V...-45v and when anode current "hit the wall" on left there is 36mA current to release on right and it would increase anode voltage sweep perhaps 50V...100V
 

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Jerry garrcia

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You could drop bias and let voltage get higher. Now bias is bit left. Onthis attached loadline that green is about 30Vpp grid drive signal between -15V...-45v and when anode current "hit the wall" on left there is 36mA current to release on right and it would increase anode voltage sweep perhaps 50V...100V
Ok. So increase cathode resistor with 100R and increase plate voltages by 5-50?
What’s the optimal bias value, if I mA ask?
It sounds really good. Did also think of maybe decreasing the cathode resistor of th 6SK7 to biasis hotter and maybe get som more volume out?
Now the plate voltage is 420v. You mean around 480-500? Then the cathode resistor migh need to be 430R or 570 or 660R with the resistors I have.
Currently 330R.
 
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