Single Ended 5881 with Musing preamp & sluckey Trem-O-Nator

[dwinstonwood]

Hi all. I'm working on the drawings to build an SE 5881 amp that I've been posting about lately, so I thought I'd start a new "build" thread.

One thing not shown is that I'll most likely ground the 6.3V at the 5881 cathode. Since this PT has a 6.3V center tap, can I just connect that right to the cathode?

Thanks.

Here's my schematic and layout progress so far. I'm sure there are errors:

[sluckey]

Since this PT has a 6.3V center tap, can I just connect that right to the cathode?

yes

[Esquirefreak]

I'll bet this build is gonna sound really cool 

[dwinstonwood]

Thanks sluckey!

Thanks Esquirefreak!

The reason I'm leaning towards a Vibro Champ chassis is because they are available, and MojoTone sells a VC cabinet with a 12" baffle. That opens things up to lots of speaker choices. 

https://www.mojotone.com/Blackface-Vibro-Champ-Style-Guitar-Amplifier-1X12-Combo-Speaker-Cabinet_20

[dwinstonwood]

This build is a good opportunity to try different tone stacks.

I'm sure this web site has been posted here before, but these folks have converted the Duncan Tone Stack calculator to JavaScript:

https://www.guitarscience.net/tsc/info.htm

I'm looking at the James which I've read was used by Ampeg, Magnatone, and the Blonde Twin, etc:

https://www.guitarscience.net/tsc/james.htm#RIN=38k&R1=100k&RB=1M&R3=10k&R4=180k&RL=1M&RT=470k&C1=470p&C2=4700p&C3=330p&C4=3300p&RB_pot=LogB&RT_pot=LogB

Lots of different tone stacks are listed along the menu at the top of the page. You can click the "Edit" button and change any values you want. The sliders are fun to play with.

[pdf64]

Were you intending this to be an overdrive preamp, ie capable of achieving a good depth of overdrive at low output levels?
If so, you may wish to consider moving the last gain control to after V2a, eg change to 470k grid leak for the output valve to a pot.
Adding a safety back up grid leak eg 2M2 would be a good idea, to protect the output valve if the pot wiper fails open circuit.

Although it’s technically correct, it’s unusual to label the final gain control as ‘gain’. ‘Master’, ‘master volume’, or ‘post gain’ are common terms used by manufacturers for it.

[dwinstonwood]

Thanks pdf64. Yeah, I've wondered about what I should call that second gain control. You're right, "Master Volume" is the industry standard in commercial amps.

I've just been calling it a gain control - for lack of another name - because I placed it after the 220K/220K voltage divider in the original AA864 circuit. So, I never consciously thought of it as an MV, but as a way to attenuate the gain going into that third triode. But, it's really doing the same thing that a volume control does.

With this build, my third version of this same basic circuit, I'm just replacing the lower half of that voltage divider with the 250K Intensity pot, as suggested by sluckey.

In the first version I built of this amp, I experimented with simply replacing the 220K/220K divider with the pot, but I didn't like it at all.

[dwinstonwood]

So... if I'm reading Merlin, and other's here, correctly, I should only make one ground connection to chassis?

In the image below, do I have it right? PT center tap to reservoir, and then daisy chain the power grounding onto the end of the preamp ground? So, the input jack is the only point of contact with the chassis for everything (except the IEC earth ground).

Merlin states that if no NFB is used, the speaker should ground to the power star, which I understand to be the reservoir's negative terminal.

Thanks!

[Esquirefreak]

I did a similar build recently, but with a 6v6 PA and one less triode. I used the "two-point" grounding principle and the amp is dead quiet.

/Max

[pdf64]

So... if I'm reading Merlin, and other's here, correctly, I should only make one ground connection to chassis?

Yes

In the image below, do I have it right?

It all looks good apart from the common bus at the controls.
The idea is for each grid leak circuit resistance to reference the same 0V common point as that stage’s cathode.

So the common for the volume and the mid resistor should connect to the common point of the 2nd stage.

Likewise the common terminal of the gain (master) pot should connect to the 0V common end of the V2 pin3 10k cathode resistor, and the speed common to the 0V common end of the V2 pin8 10k cathode resistor.
Etc.

Merlin states that if no NFB is used, the speaker should ground to the power star, which I understand to be the reservoir's negative terminal.

Yes.

[dwinstonwood]

Thanks pdf64. It's interesting that the Hoffman Stout chassis facilitates your recommendations much better, since the pots are on the same side of the board as the tube sockets.

[dwinstonwood]

Thanks Esquirefreak. I've done two point grounding on other amps and they're also very quiet.

I'm still noodling with this schematic. I decided to add NFB. On amps without LTP PI's - Vibro Champ and Princeton - Fender used a 2.7K and a 47Ω resistor (from an 8Ω speaker jack) connected to the preamp tube right after the tone stack. So, that's how I did it here.

sluckey, you said I might need to experiment with the tremolo connection point. So, I'll be prepared to also test it at pin 8 of V1-B (the Vibro Champ point).

[Esquirefreak]

I inserted the trem at the treble/tone pot wiper and it works really well. At first I tried it at the 6v6 grid leak R, but that made it act like a second volume pot. And adding resistance in series with the INT pot made the wobble effect disapear.

/Max

[dwinstonwood]

Thanks Esquirefreak. Now I'm thinking I might put a mini-alligator clip on the intensity wire and find the best place by trial and error. 

[johngovan]

Thanks Esquirefreak. Now I'm thinking I might put a mini-alligator clip on the intensity wire and find the best place by trial and error. 

Do you have updates in your project? I really love to hear the sound of it.

[dwinstonwood]

Thanks johngovan!

No real updates. Other stuff keeps intervening in my free time. 

But, I'm still making changes to the drawings. At this point I have the 6G8 Blonde Twin/James tonestack. Endless noodling.

[dwinstonwood]

I ordered the iron. I switched to the Hammond 270DAZ:
https://www.hammfg.com/files/parts/pdf/270DAZ.pdf
It's a smaller, horizontal Z-type that will be a better fit in a Fender style chassis cutout. It's rated at 122.4mA max, which I think is fine for one 5881 at around ~350V.

I changed the tone stack to the 6G8-A version.

[dwinstonwood]

I thought it was interesting that "A" (440Hz) is -26dB at the lowest point of the mid scoop.

I guess it's time to stop "designing" and start building. 

[shooter]

 
yep
you'll have plenty of time after it's up n running to be completely baffed n amazed that it "doesn't seem to do what the designing said it would"

[tubeswell]

I thought it was interesting that "A" (440Hz) is -26dB at the lowest point of the mid scoop.

I guess it's time to stop "designing" and start building. 

What program are you using for the plot?

[Esquirefreak]

I was just about to say that I used the "revision -A" brownface tonestack in one build, and was gonna recommend using a 22nF mid cap rather than 47nF. It cut through better to my ears.

But now I see that you have a 4n7 mid cap and a 47nF cap prior to the 250pF/100k junction. Any benefit with that design?

/Max

[dwinstonwood]

What program are you using for the plot?

tubeswell, it's this site: https://www.guitarscience.net/tsc/info.htm
The various tone stacks are along the very top of the page. Click "Edit" to change values, click the down arrow next to the sliders to change pot taper.

I was just about to say that I used the "revision -A" brownface tonestack in one build, and was gonna recommend using a 22nF mid cap rather than 47nF. It cut through better to my ears.

But now I see that you have a 4n7 mid cap and a 47nF cap prior to the 250pF/100k junction. Any benefit with that design?

/Max

Esquirefreak, I still haven't settled on the values yet. The values in my schematic in Reply #16 are straight from the 6G8-A schematic.

But, that .047uF cap right before the tone stack is left over from the 6G8 schematic I originally drew. So, it's an error. I need to delete it. Thanks for pointing that out! To make things more confusing, the values in the drawings below are from the TSC website's example.

Here are updated drawings.

[kagliostro]

Ciao Dwinstonwood

I like very much your realization

Cool amp

But there is something odd on your layout

Things are two, or I've the wrong pair of glasses or your layout need a bit more of definition, I'm not able to read component values

Franco

[dwinstonwood]

Ciao Dwinstonwood

I like very much your realization

Cool amp

But there is something odd on your layout

Things are two, or I've the wrong pair of glasses or your layout need a bit more of definition, I'm not able to read component values

Franco

Thanks kagliostro. Here's the PDF; it's easier to read:

[kagliostro]

Many Thanks

Now I can read all the Layout values



Franco

[dwinstonwood]

kagliostro, that image is huge if you click on it! 

[kagliostro]

Yes, but the weight is supported at Imgur, not the forum server 😉


Franco

[dwinstonwood]

Yes, but the weight is supported at Imgur, not the forum server 😉


Franco

Thanks, I see... https://i.imgur.com/bdqcXCO.jpg
Photoshop says: 11550 x 6825 pixels.

[dwinstonwood]

I've edited this post to delete the 6SL7 & 7591A idea from the other day.

[dwinstonwood]

Since I don't have an EL34 amp, I'm going with this plan that's really pretty close to an SE Express with sluckey's Trem-O-Nator.

Here's my schematic and PSUD2 model. I think with the Hammond 270EX I can get very close to my 370VDC target. https://www.hammfg.com/files/parts/pdf/270EX.pdf

[AmberB]

I have 2 amps that I built with a single ended 6L6GC.  I used iron from donor amps, one a Stromberg-Carlson 10 watt PA amp, the other from some old reel to reel deck.  One has a 2 band Fender type tone circuit, the other has a 3 band Fender type tone circuit.  Both work pretty well for what they are.  Neither has a tremolo circuit, just one 12AX7, a 6L6, and a 5Y4.  Kind of like a super Champ.  A SE 5881 would be pretty similar.  The EL-34 will require more heater current, but would otherwise be about the same.
If I could afford to buy the output iron, I'd want to make a SE amp with a KT-88 for about 18-20 watts, but that output xfmr is expensive.

[kagliostro]

An El34 will require more heather current but much less drive

EL34 sensibility is 11mA/V

5881 & 6L6 is around 6mA/V

So to drive a 5881 you require a higher signal

Franco

[dwinstonwood]

Thanks AmberB and kagliostro.

I also thought about the KT88. But, as you say, SE OPT's with an adequate power rating get pricey.

The Hammond 270EX is rated for 4A@6.3V. The (JJ) EL34 datasheet states 1.5A@6.3V. So, I'll be fine with two 12AX7's, and one EL34.

I've never taken power tube sensitivity into account before, since I always copy other successful designs. But, that's something to think about. I'll need to read up on it. Thanks.