AC15/AA964 Princeton/Tweed Hybrid

[dwinstonwood]

I've been slowly accumulating parts that would allow me to build yet another amp. It's hard to decide on one project, so I've been thinking about combining some stuff that interests me.

To begin, my apologies to sluckey for directly ripping off large parts of his AC15 schematic. I should have asked you first. I'll make my own rendition if I actually build this.

I don't know if this idea will work, or if it's been done already. What I have is the input and power sections of the AC15, and the tone stack, recovery gain stage, and cathodyne PI from the AA964 Princeton. But, because it's a cathodyne PI driving a cathode biased power section it sort of has an early Tweed, or Gibson element to it, too.

Anyway, thanks in advance to anyone who points out serious errors in my idea. Thanks!

[dwinstonwood]

I've ordered some parts; I've decided to built this thing. It's laid out to fit into a 6G2 chassis (with octal to 9-pin adapter plates, of course).

[sluckey]

That TMB tonestack is gonna be a heavy load on that pentode. Another guy just tried it. Read this...

     https://el34world.com/Forum/index.php?topic=26984.0

[dwinstonwood]

Thanks sluckey. I'm not set on the TMB.
I'm guessing that a 4 or 6 position coupling cap switch - like your AC15 - would be a better wat to go?
Or, just one tone control with maybe a bright switch?

[sluckey]

I'm guessing that a 4 or 6 position coupling cap switch - like your AC15 - would be a better wat to go?

Yes

[dwinstonwood]

OK, I ordered the shorting (make-before-break) P-H392 version:
https://www.tubesandmore.com/products/switch-rotary-2-poles-6-positions-shaft

if my research is correct, I don't need to use the 5M1 R's with the shorting version.

Now to sort out the best way to mount the caps... one end of the caps soldered to a terminal strip located close to the switch, the other ends connected directly onto the switch terminals... or, mount the caps on the board and run a bundle of wires up to the switch...

[sluckey]

I like to mount all the caps directly on the switch. Then you only have to run two wires to the rest of the circuit. Study this...

     http://sluckeyamps.com/VAC15/Vox_AC15_mods.pdf

[dwinstonwood]

Perfect! Thanks sluckey!

[dwinstonwood]

Revision 5... 

[dwinstonwood]

I've been reading up on this confusing world of impedance matching. I understand enough to see why a high output impedance tube like the EF86 is a bad match in front of a low input impedance TMB tonestack - that it results in too much signal attenuation.

So, I'm wondering what you all think of a pentode/triode tube like the 6U8A used to insert a cathode follower (the triode section) after the pentode section in front of a TMB? As in: Input > pentode > triode CF > TMB...

I don't know what amps have used this, or how it sounds. Does it retain any of the EF86 characteristics?

Thanks!

[dwinstonwood]

Waiting on parts delivery to get started. Ignore the 6U8A tangent.
I ordered a 6G2 chassis on the auction site and some octal to 9-pin adapters. Thanks bmccowan for the chassis seller tip in another thread. Great price.
Since I have to order my amp cabinets anyway, I’m thinking about a custom 2-10 with a WGS Green Beret mated to something else...

[DummyLoad]

Revision 5... 

ground pin 7 of V1 (ef86).


--pete

[dwinstonwood]

The B+ filtering for the AC15 has me confused.

Although it looks like a "capacitor-input" as far as the rectifier is concerned, it looks like a "choke-input" in terms of the current load - the OPT primaries and power tube plates are connected after the choke.

So, my question: doesn't this configuration increase the current requirements of the choke, i.e., wouldn't the choke need to be rated for 250ma-300ma or more (or, whatever the power tubes draw)?

I'm just trying to learn about all this. Thanks!

[sluckey]

It's not a choke input filter. See the cap between the recto and the choke? That makes it a cap input filter. That cap would not be present in a choke input filter.

Yes, the choke will need to carry all the current for the entire amp. But my full circuit AC-15 only draws 100mA. The choke you have listed is fine. Don't know why you think the power tubes draw so much current? My full circuit power tubes draw 92mA and my Dual Lite draws 67mA.

[chocopower]

Hi!


I did something similar.


https://el34world.com/Forum/index.php?topic=8349.msg94519#msg94519


Work like a charm!

[dwinstonwood]

Thanks sluckey. I'm still trying to teach myself how to add up the tube current from a schematic.

Do you simple add up all of the anode current numbers from the data sheets? Like, 1.6ma for a 12AX7, for example?

(And, yes, I did see it as a capacitor-input circuit; I was only referring to the additional current load on the choke)

Thanks chocopower! Yes, your Gañan II looks very similar. Do you like the amp more with the bypass caps in-circuit, or switched out?

[chocopower]

     Do you like the amp more with the bypass caps in-circuit, or switched out?

You mean the coupling caps on the ef86 or the bypass switch on the 12ax7 cathode?

[sluckey]

I'm still trying to teach myself how to add up the tube current from a schematic.

It's easy if the schematic has voltages on it. Simply divide the cathode voltage for a tube by the cathode resistor value. This will give you the current flowing through that tube. Repeat for all the other tubes and sum all the individual currents. That's all.

For example, look at my AC-15 Lite.   http://sluckeyamps.com/hammond/hammond_ao39.pdf

V1 cathode voltage is 2.1V and the cathode resistance to ground is 2200Ω, so the cathode current will be .95mA. Repeat for V2 (notice that the cathode resistance to ground is 820 plus 47K). Do the same for V3 and V4 (doesn't matter that the tubes share a common cathode resistor). My calculator says the total is 68.86mA. What does yours say?

[dwinstonwood]

Yep, 68.85ma. I think back at some point in time I learned that (when I built my SET stereo amp last March), and since forgot it. Thanks! My project has the same number and types of preamp and power tubes as your AC-15 Lite, so it should sum up around the same, give or take.

You mean the coupling caps on the ef86 or the bypass switch on the 12ax7 cathode?

chocopower, both I guess!

On a different note, I talked with Dave Allen of Allen Amplification about his custom Heyboer transformers. I ended up going with his PT and OPT instead of the Hammonds.
TP22D PT: 270V-0-270V @ 120ma; 6.3 CT@3A;and 5V@2A (won't use with EZ81)
TO20 OPT: 20W; 6,600 ohms to 8 ohms

[chocopower]

The switch in 12ax7 cathode... i remove it.. 1k5/22uf work better for me.


The rotary switch for the coupling caps on EF86, you cant switched out. At less, you need ONE coupling cap.
I make it variable to control bass response.
For me, with 3 positions is enough,  and in amps recently build,  i use a 3 position toogle switch.

[chocopower]

If you check the topic about mine, you'll see i had problems with missing highs.


Those values you use with the EF86 (plate, screen and cathode), seem to me, are gonna show the same result. Too dark and too "gainy".


Maybe is whst you are looking for and you like it, but if its not the case, try the ones i use, adapting them to you supply voltage.


Valvewizar web have a nice example about how to get the more clean signal from a EF86




Saludos!!

[dwinstonwood]

Thanks David.

I see that you used 330K plate, 2.2M & .047uF screen, and 2.2K cathode.
I have 220K plate, 1M & .1uF screen, and 2.2K cathode.

So, you're saying that your values will make the amp brighter?

I'm using a six-position rotary pot per sluckey's documents. I have a .01uF before the pot, so one of my settings is with no cap, i.e., .01uF.

I've also been looking at Hoffman's AC30 schematic. I will build the amp according to sluckey's AC15 (the original V-1-5 schematic) and live with it for awhile. Then, I might try Hoffman's .047uF before the PI and his .1uF's before the power tubes. It will be a few paychecks before I buy a cabinet, so the chassis will be open to tinker with for a while. Maybe I will experiment with your EF86 values, too.

But, for now, the amp will be built (and de-bugged) as is.

[chocopower]

Thanks David.

I see that you used 330K plate, 2.2M & .047uF screen, and 2.2K cathode.
I have 220K plate, 1M & .1uF screen, and 2.2K cathode.

So, you're saying that your values will make the amp brighter?

I tried those values and didn't like it.
If you check the last schematic,  you'll see i finish using 100k for plate, 470k for screen and 470ohm for cathode.   
In some intermediate fase, i even installed a switch who change those 3 values... after play with the amp for a while, i remove it. Fixed values, for the most clean output.

[dwinstonwood]

Thanks for that info David.

The 6G2 Princeton chassis is going to work fine. I'll need to enlarge the PT opening and drill four #6 holes to mount the board, but that's about it.
I'm messing around with control panel ideas, too (again, I'll laser engrave the acrylic at work).
I'm leaning towards a head cabinet with Vox cloth. But, a 1-12 with a WGS Green Beret would be nice, too.

[tubeswell]

I have a .01uF before the pot, so one of my settings is with no cap, i.e., .01uF.

Be aware that each of the caps on your 6-pole rotary will interact with the 0.01uF blocking cap to further decrease the blocking capacitance, e.g.:

220pF||0.01uF = 215pF*
0.0047uF||0.01uF = 0.0032uF*
etc

*By that I mean the caps are in series, but its my shorthand math for the sum of the inverses, inversed https://en.wikipedia.org/wiki/Parallel_(operator)

THis won't affect the small caps much, but you'll get a higher (i.e. even more subtle difference in) rolloff point than you might expect on the bigger caps.

To minimise the reduction, you could change the 0.01 to 0.1uF (or 0.047uF).

[dwinstonwood]

...To minimise the reduction, you could change the 0.01 to 0.1uF (or 0.047uF).

Thanks tubeswell! That's really good to know. I have some .1uF's and an .047uF, so I can experiment.

It's hard for me to keep up with the series vs. parallel rules for caps and resistors.

[sluckey]

I like the range of tones I get with my AC-15. Well, most of them. I don't care for the two pF cap positions. My favorite position is no cap, ie, only the .01 coupler. Ed Chambley used a 12 position switch for even more subtle steps.

But consider that the original AC-15 used only one cap with a shorting switch. You go from full Vox tone to ice pick! The 6 position switch is a definite improvement.   

[dwinstonwood]

Yeah Steve, this is the one amp project I can't wait to hear. I love the Vox sound. Thanks for letting me pilfer your schematic! Now, I think I want to go with a combo cab so I can play it through a British-type speaker. Guitar Cabinets Direct sells a 12" 6G2 cabinet for $260 - a great price. One Celestion alnico blue is actually more at $279!

[shooter]

It's hard for me to keep up with the series vs. parallel rules

my experience in "public education", having the info stored in your brain was best practice
my Navy education best practice was know where the data is when you need it.  So a test included, notes, books.....just NOT the human next to you

[dwinstonwood]

I read somewhere that Einstein never memorized phone number, addresses, etc. Apparently, he said something similar to you, like, "I don't clutter my brain remembering stuff that I can just look up." And, then there are all of the online calculators... I like Digi-Key's the best:
https://www.digikey.com/en/resources/online-conversion-calculators

[HotBluePlates]

I've been reading up on this confusing world of impedance matching. I understand enough to see why a high output impedance tube like the EF86 is a bad match in front of a low input impedance TMB tonestack - that it results in too much signal attenuation. ...

"Matching" is a term fraught with problems.

We "match impedance" when we seek to achieve maximum power transfer.  Or when we're connecting a speaker load to a tube, and the transformer winding-ratio is chosen to give the tube a desired loading when a specific speaker-impedance is attached.  Regardless, here we're looking for Source Impedance = Load Impedance.

Preamp stages are mostly Voltage Amplifiers.  We don't want maximum power transfer, we want maximum voltage transfer from stage to stage.  To get this, we want Source Impedance << Load Impedance, and ideally a load 10x (or more) the source impedance.

Pentodes have a high plate resistance; that's why their curves tip-over horizontal.  To get good gain, they have high-Ω plate resistors (though still less than their internal plate resistance).  Attach a lossy tone circuit that is few-kΩ to ground, and the pentode loses the ability to throw an output voltage.

[dwinstonwood]

...we want maximum voltage transfer from stage to stage.  To get this, we want Source Impedance << Load Impedance, and ideally a load 10x (or more) the source impedance.

Pentodes have a high plate resistance; that's why their curve tip-over horizontal.  To get good gain, they have high-Ω plate resistors (though still less than their internal plate resistance).  Attach a lossy tone circuit that is few-kΩ to ground, and the pentode loses the ability to throw an output voltage.

Thanks HotBluePlates. As you say, thinking about voltage is the key to understanding this stuff. Thanks for clearing up my misuse of "matching" when thinking about preamps as opposed to speakers and power.

I found a website that simplifies it to the point that I can start to grasp it. Here's a quote that makes sense to me:

"The output voltage from the source is developed across the input impedance of the destination (often called the load impedance, or simply the load), and therefore the signal voltage is passed from source to destination."
https://www.soundonsound.com/techniques/understanding-impedance

[dwinstonwood]

I'm still waiting on the transformers; all the other parts on on hand.
Since this "experimental" project is using the cathodyne phase inverter from the Princeton, I came up with a name for it...

[dwinstonwood]

The Allen/Heyboer iron arrived. It turns out the PT is a perfect fit in the chassis, no drilling or filing needed.
I will need to juggle the choke and OPT placement around a bit, and drill some mounting holes, but that shouldn't be too hard.

[dwinstonwood]

Before I start drilling holes, is there anything terribly wrong with this setup?

My reasons are: this OPT sits lower than the choke and is less likely to interfere with a speaker basket; the OPT leads line up well with the holes intended for them; and, this is where the original 6G2 OPT mounting holes are located.

My concern: is the choke too far from the cap can? I will route the choke leads under the circuit board away from the pots and front of the amp. They won't cross or be near any other wires.

Thanks!

[Glenn]

you should rotate the OT  90 degrees. with lay down PT's is hard to determine the bell ends - in this case they face East/West
so your OT Bell ends should face North/South. Hope I make sense. Look at pictures of Fender amps to see how the iron is oriented

[dwinstonwood]

Thanks Glenn! Well, I actually was looking at photos of original Fender Princeton amps. Of course, mine isn't a Princeton, but it's a Princeton chassis, and I'd like to use the original OPT mounting configuration.

- edited to add both AA964 and 6G2 pics -

[Glenn]

Lol... well the proof is in the pictures. I stand corrected.... good luck with the build

[dwinstonwood]

Thanks Glenn. I've read up on a technique where folks connect wall AC to the PT primary and connect headphones to the OPT secondary. Then, they slide the iron around on the chassis to find the least amount of hum.

I don't think I'll go that far. But, if there is a lot of hum (moderate hum doesn't bother me) I guess I could move the choke around after the amp is built to see if it helps.

David

[dwinstonwood]

I'm making progress - at my usual snail's pace.

Some of the outside turrets along the long edges have only had parts "tack" soldered since they will have tube and pot wires going into them. They look like poor solder joints right now, but this is easier for me. I'll go over every connection one final time.

I left enough free space around the .01uF caps (smallest yellow ones) so that I can go up to .1uF in size later, if I want to.

[brewdude]

Looks to me like the lay down PT end bells are oriented “up/down”, neither east/west or north/south.

[dwinstonwood]

Looks to me like the lay down PT end bells are oriented “up/down”, neither east/west or north/south.

Yes, that's how I see them, too. Thanks.

Next thing to do is wire up the filament connections. I'm going to put them flat down onto the chassis, and pushed up against the back edge.

[dwinstonwood]

Ready for first power up with dim bulb, no tubes.

[dwinstonwood]

With only the EZ81 rectifier in:

Node A: 419.7VDC

Inner EL84 pin 7 plate: 419.5VDC
Inner EL84 pin 9 screen: 419.0VDC

Outer EL84 pin 7 plate: 419.4VDC
Outer EL84 pin 9 screen: 419.1VDC

EF86 pin 6 plate: 403.7VDC

12AX7 pin 1 plate: 409.2VDC
12AX7 pin 6 plate: 414.1VDC

Those voltages seem high, even though I know they'll drop with the other tubes installed.

Should I proceed by putting in the other tubes and hooking up a speaker? I don't want to burn up my EL84s.

Thanks!

[sluckey]

Should I proceed by putting in the other tubes and hooking up a speaker?

Yes. Use the dim bulb for a few minutes. If no problems then plug straight into the wall. Now you can test for sound and measure voltages.

[dwinstonwood]

Thanks Steve! I'm down to one 200w incandescent bulb. Will that be ok, or should I run to the store and get a 40w?

Thanks.

[sluckey]

Why would you use a 200W bulb to check a 15W amp? You been watching Uncle Doug again? Get a 40W appliance bulb. You may even want a 25W appliance bulb while they are still being sold.

[dwinstonwood]

...You been watching Uncle Doug again?...

  yep

Cool, I need to go to Kroger anyway.

[dwinstonwood]

No pyrotechnics with the 40w appliance bulb. But, the tubes barely glowed.

Plugged straight into the wall the EL84 plates settled down to 316.6VDC and 316.8VDC after about five minutes.

I'll measure the little tubes in a while, after I relax. I still get nervous with these first power ups - I'm working in a townhouse with attached units. I don't want to start a fire!

Anyway, those two readings look good to me. We'll find out if I might need to adjust the two B+ dropping resistors for the first two tubes...

Thanks again Steve!

[sluckey]

No pyrotechnics with the 40w appliance bulb. But, the tubes barely glowed.

And that's what I want to see. I bet the bulb was glowing dimly too. At this point of testing I'm not interested in voltage readings or even if the amp can make a noise. Once I see this, I'm ready to plug into the wall for some excitement!