Valve Wizard Medium Gain 6V6 in Stout Chassis

[dwinstonwood]

Hello all,
I've been working on this project for a while. It's basically copied from page 256 of Merlin Blencowe's Designing Valve Preamps for Guitar and Bass, 2nd ed.

I used the standard AB763 tonestack, but I scaled the values following Aiken's and Blencowe's info. I elevated the heater wiring, and added an extra filter stage so that the three triodes are not all on a single one (like I've usually done).

I built the amp using the chassis and transformers from my Musing 15. I had noticed some flashes or sparking in the JJ GZ34 in that amp. So, this time I went with a solid state rectifier. Because of that, I used a 50uF/50uF can.

I haven't fired it up yet. I'll use my dim bulb tester with no tubes in to check the voltages later today. I still need to go over everything with a fresh set of eyes. I'm sure there's at least one mistake hiding somewhere.

I enjoy making the drawings as much as soldering, so I spent a good amount of time on them. I also modeled the power supply in PSUD2. So, the B+ voltages on the schematic are estimates, but the percentages of the voltage dropping should be close, even if the actual voltage numbers are different. I'll update that and record other voltages when it's up and running with the tubes in.

As always, I'm glad to read any criticism, good and bad! 
Thanks!

[Ronquest]

Nice work there!   I've built that preamp and variations several times, it's a great circuit. 
Keep us posted.

[dwinstonwood]

Thanks Ronquest. Glad to know you like the circuit. It will be totally new to my ears. The gain stages are very well thought out, so I'm sure I'll like it.

[dwinstonwood]

My 25W dim bulb lit up and immediately dimmed way down on power up. So, I must not have designed in any shorts.

Without the dim bulb I measured 393.6VDC unloaded at the rectifier with no tubes. My PSUD2 voltages might be pretty close with all the tubes installed.

6.5VAC between pins 4/5 and 9 on the input tube socket. I thought that might be a bit higher.

[dogburn]

Very nice and neat work. I noticed you mounted the cap can inside the chassis on the side - I just did that with my latest project (which doesn't look near as neat inside).

[shooter]

I must not have designed in any shorts.

seldom are they designed, typically get installed post design; by hurry, impatience, and Murphy 

[dwinstonwood]

I must not have designed in any shorts.

seldom are they designed, typically get installed post design; by hurry, impatience, and Murphy 

I think Murphy might have been in a hurry. I almost fired this up: 

[kagliostro]

Ciao Dwinstonwood

VERY COOL

I planned the use of that preamp for the conversion of an old Geloso G1 1020 A

initially we tried to modify the TS to a 3 knob but ended to the use of the planned TS with 2 knobs

I'll be interested to know more about your use of the AB763 TS and how you modified it

Thanks

Franco

BTW: Also the Geloso Chassis has the same disposition of tube sockets and pot location like in the Stout

[dwinstonwood]

I played the amp for a short 1/2hr session (time for dinner). It sounds really nice. Very quiet, no hum. I just need to play it.

I recorded some voltages. The EH 6V6GT's are running full tilt at 14.3W/102%. That's with a Dale 1% 250 Ohm resistor. I might go up to a 270 Ohm, or just leave it as is.

[dwinstonwood]

Ciao Dwinstonwood
I'll be interested to know more about your use of the AB763 TS and how you modified it

Thanks

Thanks for that info kagliostro. For the tonestack, I multiplied the resistors by 2.2 and the capacitors by the inverse: 0.45. [fixed errors] The controls all seem to work just like the standard AB763 version. It's supposed to help if you're not using a cathode follower.

[kagliostro]

Many thanks for the shared schematic and info

So you also swapped the fixed resistor with a pot

I don't remember the reason for which we stopped to convert the TS to a 3 knob but I think because I didn't find any indication like those you are giving me here

Our Power Section is based on EL84 tubes and we planned to use a Mixed Bias, on the PS we use a Gyrator as choke

all the material arrived from Germany and USA but my friend had some work problem and must temporarily stop the build

Franco

[dwinstonwood]

So you also swapped the fixed resistor with a pot

Yes, well, I started with a Fender TMB. Here's what I built:

[dwinstonwood]

Very nice and neat work. I noticed you mounted the cap can inside the chassis on the side - I just did that with my latest project (which doesn't look near as neat inside).

Thanks dogburn! The previous amp in this chassis had a tube rectifier, so that was about the only place for the can. Here, I went with a solid state rectifier, and since the clamp was already bolted inside, I just used it again, and covered the GZ34 tube socket hole with a sheet metal plug. 

[shooter]

14.3W/102%.

get "used" to that setup, know it sonically, then do the math to get it near 90%, drop G2 from 470 to 100, get used to it, or knot; "ya, that's not cool:)"

[roseblood11]

For the tonestack, I multiplied the resistors by 22 and the capacitors by the inverse: 0.045. The controls all seem to work just like the standard AB763 version. It's supposed to help if you're not using a cathode follower.

Wasn't it 2.2 and 0.45?

[dwinstonwood]

Wasn't it 2.2 and 0.45?

Typo on my part. Thanks for catching that roseblood11! Yep, 22 would lead you to use two 5.5M pots and a 2.2M slope resistor! 

In addition to reading Blencowe and Aiken - because I'm math challenged - I used this site to get a graphic idea of the scaling:
https://www.guitarscience.net/tsc/info.htm

The values I used come pretty close to the Fender TMB.

[dwinstonwood]

Whenever I'm starting up a new amp the first time, I get in a zone where I'm really focused on safety (I'm sure most folks do that).

Anyway, I overlooked/forgot to measure the plate to cathode voltage. I knew I'd forgotten something, but it didn't dawn on me until the next day. On the previous amp in this chassis that reading was about 6% lower that the plate to ground voltage.

I'll measure it after work, but assuming the voltage drop is sort of near 6%-ish, that would probably put me somewhere around 329V plate to cathode. That calculates to 13.5W/96.4%.  But, I'll get real numbers tonight.

Thanks everyone for the comments and input on this project! This one was a lot of fun from beginning to end. And, thank you Mr. Blencowe!

[dwinstonwood]

I'm going to consider this project finished and just play it... for now (I always go back in and fiddle with something).

Plate to cathode: 333.5VDC
Voltage across cathode resistor: 21.65VDC
Cathode resistor: 250 Ohms
Plate current per tube (less screen current): 40.9mA
Plate dissipation 13.6W
Dissipation: 97.1%

[pdf64]

Great project!
To reduce output valve dissipation a little, consider adding some series resistance to the choke, eg 470R or 1k.
Or reinstating the GZ34, it’s a pity to have everything in place but not use it.
Back up silicon diodes in series with its anodes will stop arcing and should great extend its life. A GZ34 should cope with a reservoir cap up to 60uF, assuming the transformer winding resistance isn’t super low.

[dwinstonwood]

Thanks pdf64!

I actually do intend to add back the 620 Ohm 1% resistor in series with the choke that was there in the previous version. I had used Merlin's formula for damping the choke's resonant peak, and 620 was the closest available value. And, it does lower the screen voltage a bit.

But, I'm staying with the solid state rectifier. I saw flashes or sparks inside the JJ GZ34 I was using before.

I went ahead and ordered some 1% 270 Ohm 10W resistors, and I found another 10W in a parts box that measures 266.9 Ohms. So, I have a range from 250 to 267 to 270 that I can experiment with.

[tdvt]

Great project!

To reduce output valve dissipation a little, consider adding some series resistance to the choke, eg 470R or 1k.

Agreed, it is a really nice project!


To the resistor in series with the choke; I am reading this as both components mounted in series, on the same link between nodes (between the reservoir cap & the node B/2 cap)

Is that correct?

Made a quick skim of my Merlin files, but didn't see that mentioned.

[pdf64]

Great project!

To reduce output valve dissipation a little, consider adding some series resistance to the choke, eg 470R or 1k.

Agreed, it is a really nice project!


To the resistor in series with the choke; I am reading this as both components mounted in series, on the same link between nodes (between the reservoir cap & the node B/2 cap)

Is that correct?

Yes, reducing the screen grid voltage reduces anode current.
Think of anode current in pentodes as being determined by the relationship of the control grid and screen grid voltages to the cathode.
As either / both move in a negative direction, anode current increases.
Whereas if either / both move in a positive direction, anode current decreases.

Of course HT sag (at anode and / screen grid supply nodes) and cathode biasing can complicate matters 

[/quote] Made a quick skim of my Merlin files, but didn't see that mentioned.
[/quote]
Maybe a Duncan PSU2 thing 

[dwinstonwood]

Thanks again pdf64 for that info.

tdvt, the formula for calculating the correct total choke resistance to dampen the resonance peak is in Blencowe's book on power supplies: http://www.valvewizard.co.uk/Book4.html (if I'm not mistaken) Below is a detail showing the way I installed the resistor on my previous build. It's too late to add two new turrets to this amp's board, so I'll just use a terminal strip.

And, I'm glad some folks find this project interesting. Big thanks to Hoffman's forum for being such a great place to borrow and share info and ideas. I've borrowed a lot more that I've shared. 

[dwinstonwood]

OK, one more numbers post!  I added the 620 Ohm resistor in series with the choke.

Average plate to cathode: 328.35VDC
Voltage across cathode resistor: 21.19
Cathode resistor: 250 Ohm 1% 10W
Plate current per tube: 38.2mA
Plate dissipation per tube: 12.5W
Plate dissipation percentage: 89.3%

I'm happy with where things landed.

And, the "final" schematic with more voltage readings...

[danhei]

Will you be recording any clips? I’ve been curious about the sound of that preamp.

[dwinstonwood]

Hey danhei, currently, I'm not set up for recording. But, the next time I'm over at my old bandmate's house I'll mike it up. I can't promise when that will be.

But, it's a pretty standard amp, nothing unusual about the circuit. The one thing that is new to me, that I really like is having the tonestack at the end of the gain chain, right before the PI. So, the Gain pot is right after the input triode, and it controls the signal level into the second two gain stages. That signal level enters the tonestack, and here, the scaling of the tonestack values comes in to play in terms of loading. The volume pot then controls that signal level as it feeds the PI. It seems to be a lot more logical and versatile than my Musing circuit which is basically an AA864 (or, Trainwreck Express) signal path.

Does that help any?

[danhei]

Thanks. Yours looks like a good and sensible design end-to-end. I’ll have to prototype something with the medium gain preamp.

[sluckey]

Nice project. Excellent drawings!

[dwinstonwood]

Nice project. Excellent drawings!

Thanks sluckey! I think you'd agree that the graphic work is half the fun. It's where I do all of my "thinking" on a project.

Thanks. Yours looks like a good and sensible design end-to-end. I’ll have to prototype something with the medium gain preamp.

Thanks danhei! I've played it a few more hours. It sounds great through the Celestion Creamback in my AC15 clone cabinet (I need to invest in a speaker cabinet, but I'm running out of space). I messed around in PSUD2 a bit, and it would work well using a Hammond 270DAX and an EZ81 which would get the power tube voltages down to around 300VDC for EL84's, for example.

[sluckey]

Thanks sluckey! I think you'd agree that the graphic work is half the fun. It's where I do all of my "thinking" on a project.

Yep. I have an appreciation for nice drawings.

[dwinstonwood]

I bought some new tubes for this thing. I had played it with EH 6V6's and JJ's. The JJ's sounded better to me, but something wasn't there.

So, I took a chance on a matched pair of ANOS USA 6V6's from TubeDepot. One is a GE 6V6GTA and the other is a JAN Sylvania 6V6GTY. I believe TubeDepot matches them by current, I guess. The plate to cathode voltages only differ by 0.4V, and the screens by 0.2V.
I also changed the 250 Ohm cathode resistor to a 270 1%.
This biased the amp at 90.7% and 12.7 watts, with 37.5mA per tube.

I also bought a NOS JAN Philips 12AT7WC for the PI.

Anyway, it sounds great. Maybe the best sounding amp I've built. I live in an old townhouse with close neighbors, but I'll be taking it out to a friends house in the "country" this weekend to turn it up.

I'll have to see how these ANOS 6V6's hold up, but they definitely bring something good to this amp. Just thought I'd give an update. 

[Glenn]

Thanks for the update. I have some 18 watt heyboer iron I’ve had for a bit, and I plan on building this in the next month. Thanks for all the work you put in
Glenn

[shooter]

always good to know someone livin in the sticks 


I hired my son's band for a Bday party I threw, he's a city dweller, got to play his twin on 10 for the 1st time, I got all the dust shaken outta the barn, the neighbors 1/2 mile away commented "sounded like a great party"  had set up a shooting range, everything from a handmade long bow to a Thomson Sub-machine gun, country livin has it's upside 

[dwinstonwood]

shooter, I come from rural stock. My dad and his parents were dairy farmers. I almost was one myself.
I put country in "" marks because forty years ago that area where my friend lives was farmland. Now, it's on the outer swath of commercial development.

One day I hope I get to dime a Twin. Actually, if I can fix the solid state Fender Stage 160 I'll soon have...

[kagliostro]

I think there is nothing better than Farms

Both my grandparents were farmers, albeit with only a small portion of the countryside

My great-grandparents were too, only that one of them had so little land that it was little more than a vegetable garden,

so to support the family he accompanied the rich by boat to hunt for ducks in the Venice lagoon and his pay consisted of cartridges (and sometimes even a little wine)

with wich he go hunting to support his family, so he hunted a little, fished a little and grew something on the small piece of land he had

Franco

[bmccowan]

I just came in from brush hogging the back 40 (well  back 14 in my case) I spent 1 year of 70 living in a place where I could not run tractors and chainsaws. I suffered severe withdrawal. Legend has it that my Grandad had the biggest Victory Garden in Rochester NY, so I guess it's in the genes.

[shooter]

I spent 1 year of 70

Nice!
being in the Navy we lived in apartments for 6 years, getting deployed was a blessing!
then was stuck in suburbia, where they had their OWN LAWS!!
at 29 I made it back to farm country and never looked back.

[dwinstonwood]

Mom could drive a tractor before she ever drove a car. My older sister was born at that farm.

[shooter]

working at the beerBaitnGas you get to watch the kids grow up, make fun of the parents and hangout with the grandparents.  Always a treat seeing a 12yr old with an ear to ear grin "Dad let me drive the tractor ALL THE WAY HERE"  I would tell the kid pick anything in the store under $10, they'd run off n Dad's like "you didn't have to do that"  I'd just smile at Dad, "I'm not doing anything but adding $10 to your tab" 

[PRR]

> Calais.png (796.55 kB, 1113x471 - viewed 5 times.)

That's a big FarmAll. Is that Calais Maine? Probably should get a copy to a historical society.

[dwinstonwood]

> Calais.png (796.55 kB, 1113x471 - viewed 5 times.)

That's a big FarmAll. Is that Calais Maine? Probably should get a copy to a historical society.

The farm is still there, on the south bank of the James in Virginia. My grandparents owned another dairy farm on the north bank of the river named Dover. Hence Dover and Calais. My dad was born at Dover. It's still operating as a dairy farm under different owners.

Mom plowed the cars out of the snow, too. I guess dad stood around with a camera.

[dwinstonwood]

Sorry for getting way off track.

At any rate, I found a person on eBay that will make custom pine cabinets at good prices. I think I finally need to put this Stout chassis in a safe enclosure. At least something with a handle on top. I made the Illustrator files for the faceplate and backplate. So, I'll get those laser engraved next.

[bmccowan]

Sorry for getting way off track.

A very enjoyable side track