AA864 Musings

[dwinstonwood]

Hey everyone, while I'm waiting on my 6G3 cabinet, I can't seem to stay away from imagining another project. I figured it was maybe time to think about 6L6's.

I've had an interest in the Blonde and BF Bassmans for a long time. I'm sure this has been done many times, and much better, but I got the idea to draw up an AA864 with one channel. It would use the three gain triodes of the Bass channel, but use the components/values of the Normal channel for guitar.

A major issue for me is the availability of a pre-punched chassis. I have a drill press and step bits, but making a custom chassis is a big task that I don't enjoy.

It turns out that the Mojotone 6G2 Princeton chassis is just right. I would only need to install an octal to 9-pin adapter plate for the PI tube. I would use the other 6V6 hole and the 5Y3 hole for the 6L6's, since the AA864 has SS rectification. The extra pot on the front could be used for a Presence control, etc. The ground switch on the back would become a standby switch.

Here's a layout I've been working on, even though I don't need another amp.

[shooter]

even though I don't need another amp.

I'm not a guitar player, but I always seem to have at least 2 amps ready, just in case 

[pdf64]

A 6L6 Express then 
From a technical perspective, standby and ground are equally redundant.
Whereas a switch at that end of the amp could be used to provide an alternative response characteristic, eg HT sag resistor, or, if you can source a PT that provides a proper HT that’s closer to 400 than 450Vdc, cathode bias.

[dwinstonwood]

I'm not a guitar player, but I always seem to have at least 2 amps ready, just in case 

Who knows, you could wake up one morning with the ability to instantly visualize every possible chord inversion on the entire neck.

A 6L6 Express then 
From a technical perspective, standby and ground are equally redundant.
Whereas a switch at that end of the amp could be used to provide an alternative response characteristic, eg HT sag resistor, or, if you can source a PT that provides a proper HT that’s closer to 400 than 450Vdc, cathode bias.

  I like 6L6 Express. I was thinking of a play on Bassman - maybe, "Guitarist." Pretty bad, I know.
That's good to know about standby switches. In my very limited understanding I assumed that with SS rectification standby was important to protect the tubes.

[shooter]

Who knows

I use the "dave" method;
yup, if I put my fingers here, sound pretty good - memorize
strum here with fingers there, yup, sounds good, memorize
as a climber you practice so much that muscle memory gets programmed
as a guitar player, seems to work about the same

[dwinstonwood]

So, I'm drawing up the schematic for my single channel AA864, and the bass channel - which I'm using with normal channel components - has this circuit (see image) right before the last triode before the PI.

I'm guessing that this is a Band Pass Filter, but I really don't know.

If it is, the HPF has a cutoff of 7.23Hz and the LPF has a cutoff of 723.43Hz. If I'm seeing this correctly, it seems like a narrow band, which I guess makes sense for a bass.

OTOH, if I've got it all wrong, what is this circuit doing? Should I just get rid of the second/lower 220K resistor and the .001uF capacitor?

Thanks, as always!

[PRR]

> 723.43Hz. If I'm seeing this correctly, it seems like a narrow band

Does it sound dull?

The high-cut may be closer to 1400Hz.

[HotBluePlates]

... draw up an AA864 with one channel. ...

It turns out that the Mojotone 6G2 Princeton chassis is just right. ...

Here's a layout I've been working on, even though I don't need another amp.

Does your layout draw each part to-scale?  If yes, what is the overall length of your board?  Does it fit in the space afforded by the 6G2 chassis?  While still leaving room all around the circuit board (to avoid being too cramped)?

I've found it's very easy to breeze past the basic requirements of "Will it fit?" when drawing up a new idea.

[dwinstonwood]

Thanks HotBluePlates,
Yes, I've been laying this thing out to fit in a 17" chassis. The 6G2 is 17 1/4" long x 7 18" wide. My board is 11 1/2" long. But, there are other options, too, like some of the Marshall chassis. With those I can use multicaps for filtering instead of a Fender doghouse setup. I thought about the Hoffman Stout chassis, but I'd need to redesign my board; it's only 12" wide.

I'll post a revised layout and a schematic tonight.

Pdf64 suggested "6L6 Express" as a name, and looking at the Trainwreck Express schematics there are similarities. It might be interesting to try some of those component values instead of the BPF I highlighted above. For example, the Trainwreck schematics suggest values like .001uF and 68K before the third triode. Or, .002uF and 150K, etc.

Then, again, I could just try building an Express!

https://el34world.com/charts/Schematics/files/Train_wreck/Trainwreck_express.pdf

[dwinstonwood]

Here's the schematic and an updated layout. I might still change some things before ordering parts.

[dwinstonwood]

Hi, I'm still mulling over this project (and raising the funds), and I have a question, or two.

In the red box in the attached image is what I think is a voltage divider that results in 50% attenuation. Could those two 220K resistors be replaced with a 500K linear pot to act as a gain control?

Also, - and this is where serious confusion sets in - I'm under the impression that at least three things are going on here:
1.) a voltage diver
2.) a high pass filter (with maybe some other frequency filtering, too)
3.) and, the setting of Zout and Zin between the two stages.

Since all of those things seem, as I understand it, to be interconnected and interdependent would using a pot have an adverse effect? This is an area of amp circuits that I'd like to get a handle on, and understand better.

Thanks!

[sluckey]

In the red box in the attached image is what I think is a voltage divider that results in 50% attenuation. Could those two 220K resistors be replaced with a 500K linear pot to act as a gain control?

It is a voltage divider and you can use a pot to dial in some more gain.

Also, - and this is where serious confusion sets in - I'm under the impression that at least three things are going on here:
1.) a voltage diver
2.) a high pass filter (with maybe some other frequency filtering, too)
3.) and, the setting of Zout and Zin between the two stages.

1. Yes, a voltage divider.
2. No, it's a low pass filter. Remember, it's a bass amp. I would remove that cap if this amp will be used for guitar.
3. Impedance is not really an issue here.

Since all of those things seem, as I understand it, to be interconnected and interdependent would using a pot have an adverse effect? This is an area of amp circuits that I'd like to get a handle on, and understand better.

No problem. I suspect the voltage divider was all about balancing/matching the bass channel output with the Normal channel two stage output.

[dwinstonwood]

Thank you sluckey! I'll add the pot into my schematic and ditch the cap (I assume you're referring to the second .001 cap). 

[PRR]

> a high pass filter

The 0.1uFd and the 440k of resistance is a 4Hz high-pass (low-cut) filter, yes. 4Hz is "nothing"; it's really just blocking the DC.

[tubeswell]

So, I'm drawing up the schematic for my single channel AA864, and the bass channel - which I'm using with normal channel components - has this circuit (see image) right before the last triode before the PI.

I'm guessing that this is a Band Pass Filter, but I really don't know.

If it is, the HPF has a cutoff of 7.23Hz and the LPF has a cutoff of 723.43Hz. If I'm seeing this correctly, it seems like a narrow band, which I guess makes sense for a bass.

Yep - its a band pass filter


1) Bass rolloff  = 1/[2Pi.100nF.440k] = 3.6Hz @ 20dB/decade (which is nuthin' much)
2) Overall attenuation from the voltage divider = 220k/440k = 0.5 (-6dB)
3) Treble rolloff = 1/[2Pi.1nF.(220k||220k)] = 1,446,281Hz* @ 20dB/decade (1.4MHz* Oops - not 1.4Khz - didn't put enough zeros in my calculator  )


It interacts with the (~40K) output impedance of the previous stage and the (~100pF) input capacitance of the following stage (so gain and treble rolloff point will shift slightly lower)

or in 'geetar amp speak' its a hard-wired treble control with the treble knob permanently turned down a bit with a 1nF to ground to slug resistor noise from the 200k 'grid stopper' - and that's my story and I'm stickin' to it
 

[dwinstonwood]

> a high pass filter

The 0.1uFd and the 440k of resistance is a 4Hz high-pass (low-cut) filter, yes. 4Hz is "nothing"; it's really just blocking the DC.

Thanks PRR. Yep, that's what I was looking at when I posted "high pass filter." I forgot about the second 220K resistor and the .001 as a second filter, i.e., a low pass, as sluckey pointed out. Although, oddly enough, in Post #5 (two months ago) I did comment on it!  Not surprising.

[dwinstonwood]

Thanks tubeswell! I knew there was more going on there than I could see.

[dwinstonwood]

It interacts with the (~40K) output impedance of the previous stage and the (~100pF) input capacitance of the following stage (so gain and treble rolloff point will shift slightly lower)

Now I remember an Uncle Doug video where he said you can think of a 12AX7 as a +/- 151pF capacitor. So, I can see how the 220K grid stopper and the tube would together act as a low-pass filter, in that respect.

[sluckey]

I think in more simplistic terms. That .001µF cap connected directly to the grid, is gonna dump a shitload of highs directly to ground. That's why I said its a low pass filter. I could have said it's a treble bleed cap. Same thing in that circuit. IMO no need to break out a calculator for that.

[tubeswell]

It interacts with the (~40K) output impedance of the previous stage and the (~100pF) input capacitance of the following stage (so gain and treble rolloff point will shift slightly lower)

Now I remember an Uncle Doug video where he said you can think of a 12AX7 as a +/- 151pF capacitor. So, I can see how the 220K grid stopper and the tube would together act as a low-pass filter, in that respect.

Cin = C (grid to cathode) + C (plate to grid) x A (where A = voltage gain of the stage. For a 12AX7 with a 100k plate resistor, and a fully bypassed cathode resistor Voltage gain is about 60)

So 1.6pF  + (1.6pF x 60) = 97.6pF

Uncle Doug would've assumed a voltage gain that was somewhat higher - goodness knows what he was thinking

[PRR]

............. = 97.6pF...

I pencil it as 100p.

160pFd sounds wrong.

"0.001uFd" is 1,000pFd. So another 100pFd on top is a small difference. Without the 0.001uFd it might be flat to 14kHz, flat for any guitar purpose. (Too flat, because the very high-order harmonics of a string are nasty, even by rock standards.)

[dwinstonwood]

I think in more simplistic terms.

I honestly try to! The clearer this stuff becomes, the easier that is.

In that respect, I'll use just two components: a .1uF blocking/coupling cap connected from the preceding plate to one side of a 500K linear pot; the other side of the pot connected to ground; and, the wiper connected to the next grid.

[dwinstonwood]

Here's the final version. I'm sure there are mistakes.
It will still fit in a 6G2 Princeton chassis. Same number of knobs - volume, treble, bass, gain instead of volume, tone, speed, intensity. I'll change one of the octal holes to 9-pin with an adapter plate. Might need to enlarge the PT opening. I'll maybe make this one as a head.

[tubeswell]

Will make a fine raucously loud geetar amp

[DummyLoad]

are the power transformer and output transformers suitable for a 50W amp going to fit that chassis without opening up / re-drill mounting holes? or did miss a mention about something other than 6L6 for the output stage? 

6G2 PT and OT are much smaller than AA864 parts.


6G2 PT dimensions - http://www.classictone.net/OOA-85282_Chassis_Cutout_C-100.pdf
AA864 PT dimensions -  http://www.classictone.net/OOA-85285_Chassis_Cutout_C-400.pdf


6G2 OT dimensions - http://www.classictone.net/40-18045.pdf
AA864 OT dimensions - http://www.classictone.net/40-18008.pdf

--pete

[dwinstonwood]

Will make a fine raucously loud geetar amp

  That's pretty much what I'm thinking. Thanks tubeswell!

are the power transformer and output transformers suitable for a 50W amp going to fit that chassis without opening up / re-drill mounting holes? or did miss a mention about something other than 6L6 for the output stage? 

--pete

Probably not! It just seems to have the right number of holes.
But, you are right. It should have a larger chassis that will not only handle the bigger tx's, but allow for the extra filter caps, too (like maybe room for a doghouse).
This definitely requires more serious planning before I buy anything. Thanks pete!

[mresistor]

Consider having a custom chassis made..   https://seasidechassisdesign.com/


and I had one made here  https://www.facebook.com/synapticamps/?ref=page_internal                     http://www.synapticamps.com/


and both do excellent work.

[dwinstonwood]

Thanks mresistor.

An AB763 chassis is closest to the original AA864 size, although the chassis I just used - the 6G3 - would also work and is a bit shorted.

[DummyLoad]

https://www.ebay.com/itm/Modified-JTM45-16Ga-Gal-steel-20X8X2-1-2-inches/223193599066?hash=item33f7600c5a:g:HasAAOSwVW9bx4nT


that linked above would work - build a marshall style combo, without the typical marshall circuitry.   


--pete

[dwinstonwood]

Thanks DummyLoad. I like the idea of an amp with the tubes right-side-up.

On a different note, I found this online PT calculator. I'm sure it's not exact:
https://thesubjectmatter.com/calcptcurrent.html

[sluckey]

You could put that amp in Hoffman's Stout chassis.

[dwinstonwood]

You could put that amp in Hoffman's Stout chassis.

And, the price would be hard to beat! Thanks.

One question: will the multi-cap hole on the end accept an 8-pin socket (in case I wanted to go GZ34)?

[sluckey]

yes

[sluckey]

Your amp might look like this if it were built in Hoffman's Stout chassis...

[dwinstonwood]

Wow, I feel honored that you put that together sluckey.

I'm studying it now. Your organization and grouping of components is super-logical and compact. I would not have been able to come up with anything near that sensible. Thank you!

Well, I no longer have an excuse for not starting this project. So, it looks like I'm going to have another guitar amp. And, they keep getting louder! AC4 > 6G3 > AA864.

Much appreciated!

[sluckey]

I put this together just as a mind exercise. You may not like having that much amp in a tiny box. I kinda like it though. I've built several amps in that size chassis.

If you want to pursue this I'll put together a document file just like all my other documents. It will include your schematic, this layout, and a full size drill template to make drilling and staking the board easy. LMK.

[dwinstonwood]

That's very generous sluckey! I think that sounds like a good idea.

I guess the big question is, do I get a link on this page? 

https://el34world.com/charts/TubeAmpSchematics.htm

[sluckey]

Sure... as soon as you build it.   

[sluckey]

Here's the document file. I've compared the layout and schematic several times. I don't see any errors. Doesn't mean there aren't any. I'm confident enough with the layout to build this if I wanted one. I do suggest you compare the schematic and layout looking for errors but mainly to get familiar with the layout. If you spot any errors or just have questions, speak out. Revisions are easy.

EDIT... removed attachment

[dwinstonwood]

Many thanks sluckey! Your layout takes care of the of the hardest part (for me) of the design work.
Also, thanks for improving the rectification and B+ filtering on the schematic.

I like the push/pull volume pot for a bright switch. The only thing I can think of at the moment is to buy an OPT with multiple speaker ohm leads, and add a switch, or extra jacks (not sure which is better). My plan is to build it as a head, so that would be a good feature to have.

I'll spend some time tonight going over things and write up a BoM!

[sluckey]

Take a look at this project...

     http://sluckeyamps.com/phoenix/phoenix.htm

It's the same power amp you have. The Hammond 1760J is perfect for your project too.

[dwinstonwood]

Very cool. Your writeup and photos are worth a great deal.
The Hammond 1760J it is, then.

Hammond lists the 290EX as their replacement for the 125P7D:
https://www.hammfg.com/files/parts/pdf/290EX.pdf
Or, the slightly more expensive Classic Tone 40-18005:
http://www.classictone.net/40-18005.pdf
But, I've had good luck with Hammond, so far, I'll stick with theirs.

Hoffman sells this 6A/125V rotary:
https://el34world.com/charts/images/rotarydiagram2.jpg
Maybe, similar, or the same, that you used?

And, where did you get those cool, black tube shields? 

[sluckey]

Very cool. Your writeup and photos are worth a great deal.
The Hammond 1760J it is, then.

Hammond lists the 290EX as their replacement for the 125P7D:
https://www.hammfg.com/files/parts/pdf/290EX.pdf
Or, the slightly more expensive Classic Tone 40-18005:
http://www.classictone.net/40-18005.pdf
But, I've had good luck with Hammond, so far, I'll stick with theirs.

Either looks like a good choice. Keep in mind that you will likely have to modify the Stout chassis cutout a bit.

Hoffman sells this 6A/125V rotary:
https://el34world.com/charts/images/rotarydiagram2.jpg
Maybe, similar, or the same, that you used?

Same

And, where did you get those cool, black tube shields? 

My shields came from a decommissioned FAA radar system. You can still find them on eBay...

https://www.ebay.com/itm/4-IERC-TR6-6020-Vacuum-Tube-Shields-for-9-Pin-Miniature-12AT7-12AX7-Similar/353164660945?_trkparms=aid%3D1110009%26algo%3DSPLICE.COMPLISTINGS%26ao%3D1%26asc%3D20200818142652%26meid%3D7cfaadf7973a4f4e9f42f4969139aaef%26pid%3D101196%26rk%3D4%26rkt%3D12%26mehot%3Dnone%26sd%3D192625646289%26itm%3D353164660945%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3Ddefault%26brand%3DIerc&_trksid=p2047675.c101196.m2219

[sluckey]

Here's the final layout. The board is 3/8" shorter. Keep in mind that the board is optimized for 1/2W metal film resistors and Xicon "chicklet" caps. Mallory 150s will also fit but don't even try to use orange drops or some of those big booteek caps. If you have any questions just ask.

EDIT... removed attachment

[dwinstonwood]

Thanks for all the work sluckey.

I'm ordering parts today.

I'll use 1/2W resistors everywhere except where your layout states 3W parts.

Glad I read your post about using the smaller Xicon's before ordering!

I'm taking a 4-day vacation a week from today; I should be able to get everything by then if I order now. So, I'll have plenty of time to work on this thing.

[dwinstonwood]

Everything's ordered. Most stuff, including the chassis, from Hoffman.
Going with the Classic Tone PT, OPT, and choke from TubeDepot worked out the cheapest after including shipping. Plus, I got the tubes, too.

As for a cabinet, I saw this thread, but it's three years old, so probably none left:
https://el34world.com/Forum/index.php?topic=21891.0
SILVERGUN, I might send you a message about the plans.

I was warned about this hobby. It's all out of control, now.

[sluckey]

Couldn't leave it alone! I've added one turret to the board to make it easy to add a voltage divider resistor in series with the GAIN pot just in case the amp has too much gain. I also added a note to your schematic. This is the last final change unless you spot an error.   

I highly recommend bookmarking the page on Hoffman's plexi6V6 build in this chassis. Lot of good building tips and pics that show a good way to do the wiring. Worth the read. Keep in mind that this is the prototype chassis and is very slightly different from his version 2 chassis. You will likely have to do a bit of chassis "tuning" to make your PT fit the precut hole.

     https://el34world.com/projects/Plexi6V6.htm

I'm gonna remove the previous docs for this project so use this attachment and delete older files you may have.

[dwinstonwood]

Thanks again sluckey. I've bookmarked that page. Nothing beats having clear hi-res pics.

Since I bought a blank board and a bunch of turrets I'm going to make one of these turret press tools for my drill press. Both Hoffman and TubeDepot have all the info needed.

I need to read up on gain in guitar amps. From what I understand, there are definite audible differences between preamp tube vs power tube gain. I don't know enough to really understand how overdriving the third triode before the PI will sound. So, thanks for the additional resistor option. I suspect the amp will want some tweaking here and there. But, it's a basic enough circuit (no trem or reverb) that changing caps and resistors - one at a time - should be relatively straightforward.

[sluckey]

I wanted to point out something about your anvil.jpg. It implies that the part of the tool with the hole in it will be mounted to the drill press table. That's wrong and if you try it you will quickly see why. Mount it as shown in this pic and load several turrets. Now you have to turn the board upside down to press them. I promise that some of the turrets will fall out.   

You want to mount the pointy part to the table and put the part with the hole into the chuck. Doing it this way makes it possible to put a lot of turrets on the board and press them all at the same time.

[dwinstonwood]

I wanted to point out something about your anvil.jpg. It implies that the part of the tool with the hole in it will be mounted to the drill press table. That's wrong and if you try it you will quickly see why. Mount it as shown in this pic and load several turrets. Now you have to turn the board upside down to press them. I promise that some of the turrets will fall out.   

You want to mount the pointy part to the table and put the part with the hole into the chuck. Doing it this way makes it possible to put a lot of turrets on the board and press them all at the same time.

Done.