AX84 November, Rev. 7. Anybody built this?

[Madhatter]

Hi Everybody,

I want to build an 18 Watts Marshall Plexi clone and found the "November" schematic on the old AX84.com page, see attached PDF. It is revision 7. There were some updates later but they did not have the 4 input jacks feature anymore, so I got stucked to this revision. Has anybody ever built this one? I just would like to have someone who is more experienced than I am to confirm that the schematic does not contain any "pitfalls".

Thanks,

madhatter

[HeyItsBen!]

https://sluckeyamps.com/november/november.htm

[Madhatter]

Hi,

thanks, that helps. Is that your site? There is no contact information and I have some questions.

Thanks,

Madhatter

[acheld]

Is that your site? There is no contact information and I have some questions.

No, that site belongs to Steve Luckey, a former member and much respected moderator for this forum.   Lots of good info there.  He is not active any more, but the forum is -- I suggest you ask your questions and we'll do our best to help out.

[Madhatter]

Hi,

got it, thanks. OK, I have several questions, let's start with these:

1.   What voltage is required for the capacities? I would go for 630V, is that OK? If not, which are different?
2.   Usually electrolytic capacitors are used at the cathode. C18 is not, is that OK?

Thanks,

Madhatter

[Merlin]

1.   What voltage is required for the capacities? I would go for 630V, is that OK? If not, which are different?

450V or more is fine.

2.   Usually electrolytic capacitors are used at the cathode. C18 is not, is that OK?

Yes that is OK. Non polar capacitors are always acceptable.

Do not include the standby switch S3, it can be damaging to the rectifier.

[Madhatter]

Hi,

thanks for replying. No Standby switch? OK ....
As for the rectifier, next question: Originally, an EZ81/6CA4 was used, later this was changed to a 5Y3, saying “OK to use EZ81”. Which option is better and why?

[Merlin]

As for the rectifier, next question: Originally, an EZ81/6CA4 was used, later this was changed to a 5Y3, saying “OK to use EZ81”. Which option is better and why?

5Y3 will drop more voltage. Use whichever is easier for you to buy.

[Madhatter]

Hi,

wow thats fast ... this forum is a dream 
Next questions about modifications.
1. The newer versions and also the one from Steve Luckey contain a feedback loop by just placing two jackets between preamp and power amp, no buffering or anything there. Is that enough? I plan to use some effects like tc Halloffame, echo, chorus flnger.
2. Steve Luckeys version also has a Master volume. There are several designs possible for this. But since he is considered a real expert, I think it is OK this way?

Thanks,

Madhatter

[Merlin]

1. The newer versions and also the one from Steve Luckey contain a FX loop by just placing two jacks between preamp and power amp, no buffering or anything there. Is that enough?

It's not exactly the 'best' way but it is certainly the simplest and cheapest.
However, I would recommend adding two back-to-back Zener diodes to protect the effects from excessive voltage.

2. Steve Luckeys version also has a Master volume. There are several designs possible for this. But since he is considered a real expert, I think it is OK this way?

Yes that is a standard MV, nothing wrong with it.

[Madhatter]

Hi all,

brilliant, thanks. We almost have it. Some questions on the switches

1. Does the option to switch between fixed bias and cathode bias make sense? Is the difference in sound significant?
2. Same for the switch between Pentode and Ultralinear, which was later removed. Is the difference in sound significant?

Thanks,

Madhatter

[Merlin]

1. Does the option to switch between fixed bias and cathode bias make sense? Is the difference in sound significant?

No not significant.

2. Same for the switch between Pentode and Ultralinear, which was later removed. Is the difference in sound significant?

Yes it is significant, but nobody likes the sound of UL

[Madhatter]

1. Does the option to switch between fixed bias and cathode bias make sense? Is the difference in sound significant?

No not significant.

2. Same for the switch between Pentode and Ultralinear, which was later removed. Is the difference in sound significant?

Yes it is significant, but nobody likes the sound of UL

So kick bias switch? If so, which option to build in, fixed or cathode?

Since few to no amp has the option of switch ultralinear, I would kick it.

[Merlin]

So kick bias switch? If so, which option to build in, fixed or cathode?

Personally I would keep cathode bias, it's simpler.

[Madhatter]

Good Morning,

we are almost through ...

Later versions do not have the plexi-typical four inputs, but only one. I would like to have them. Is there a reason not to do this?

Thanks,

Madhatter

[Merlin]

Later versions do not have the plexi-typical four inputs, but only one. I would like to have them. Is there a reason not to do this?

You can have them. I have no idea why you would need four inputs though!

[Madhatter]

Hmm ... why did the Plexi have them? More options, one can jumper two inputs etc.
Do I get that right that in the one-jack version the one jack is the same as jumper the two channels in the four-jack version?

[Merlin]

Do I get that right that in the one-jack version the one jack is the same as jumper the two channels in the four-jack version?

Yes. I would use two jacks, one for each input triode, but you can use four if you like the vintage Hi/Lo options.

[Madhatter]

OK, got it.

Once again for the bias: The parts to set the cathode bias are R27 (130R/5W) and C15 (250uF/25V). I can’t find these parts here in Germany, although we have some really good online suppliers for tube amp builders. Wouldn’t it make more sense to replace R27 with a trim pot to set the value more precisely?

[kagliostro]

Cathode bias has auto bias set, not strictly necessary to have a fine regulation

You can obtain the required values in resistance and capacitance via parallel and/or series of components, in series or in parallel resistors will see the power abilities summed (2 x 2W resistors in series or in parallel will result in 4W dissipation ability)

One other thing you can do is to have separated resistors and capacitors for each power tube

K

p.s.: I forgot to say, if you connect each tube to ground with a dedicated resistor value must be doubled (in resistance and may be halved in power dissipation)

[Madhatter]

Hi all,

OK, after all this clarifications I compared Steve Luckeys version from the link above with rev. 7, I saw many changes that I overlooked at first glance. He changed the way the bias is switched and the wiring in the tone control section. He also changed many values:
•   C4 from 470p to 270p
•   R22 and R34v have disappeared
•   R36 from 10K to 47K
•   C16 from 100n to 22n
•   R20 from 82K to 15K
•   R21 from 100K to 15K
•   R27 from 130R to 150R
•   C20 from 10uF to 25uF
•   C1a from 32uF to 47uF

No I don't want to ask you guys to go through/explain all these changes, but more generic: Is version 7 unsafe or bad in other ways? I ask because I already worked on the layout and don't want to start again if not necessary. The revision comments only talk about changing the rectifier to different type, etc. This does not explain (to me) the changes above.

Thanks,

Madhatter

[tubeswell]

I’m thinking those are just Steve’s tweaks - if in doubt, build it stock then tweak it to suit. There’s so many gain stages, the end result will be difficult to discern tone subtleties.

[AlNewman]

Neither design is unsafe.  Looks like Sluckey harvested the transformers so some of the changes reflect that.

2.5R output transformer tap would explain the NFB change R36.  There's a note at the bottom of his schematic about that.

He changed the gain and attenuation of the PI, the removed 330k resistors work together with the value changes of R20 and R21.  Probably makes for a little more headroom.  The PI has more plate voltage and won't break up as quickly.  I built a similar amp and used a 12AT7 for the PI to a fixed bias output.  I just prefer a 12AT7 in the PI position.  They all work.

The larger value electrolytic caps add more filtering, and were probably just what he had lying around that would work.

Since he used a donor PT, he would have adjusted the 150R resistor to get the correct bias.  It's possible you will have to adjust yours as well when you 1st test it.  It never hurts to have a few different value resistors on hand when you fire it up.

Edit:

I made a mistake.  R36 sets the DC elevation of the PI cathode.  Merlin has a good explanation on his site about the influence of elevation on an LTPI.

The NFB ratio is set with R37. 

[Madhatter]

Thanks for explanations. I compared rev 7 to oroginal 11 and the changes there are not so big.
Last question: Merlin said in a post above, that the standby switch can damage the rectifier. In rev 7 the standby is different than in rev 11, where it sits behind the first filter cap. Are both designs harmful for the rectifier? So, omit the standby switch completely or is there a way to have it safe/correct?

[kagliostro]

Discard Standby and, if you want, install a mute

Or, if you really, but really want one

https://valvewizard.co.uk/standby.html

Franco

[Merlin]

This whole project is really just a Marshall 50W preamp bolted onto a Marshall 18W output stage. Easy.

[Madhatter]

Hi all,

so I think I'm good to go now, everything clarified. Thanks again to all responders for their quick an professional help! 

Madhatter

[Beezerboy]

Hi Everybody,

I want to build an 18 Watts Marshall Plexi clone 

save the headaches and build Hoffman's 6V6 Plexi... its a great amp. I've built it twice... gonna build another

I just finished a similar version of the AX. I'm still tweaking it because its pretty harsh. I only built it because I was given the assembled board and had the trannys on the shelf.

[Madhatter]

Thanks for your note. The overall schematic looks not so different, 6V6 and no tube rectifier. Do you have sound files somewhere?
UPDATE Found them on the hoffman site. Does yours sound like this?

[Madhatter]

Hi all,

one more thing I found: I would like to have a rotary switch to select the impedance (4, 8, 16 Ohms), sinde I have some cabinets I want to run it with. But when I look at the wiring of the output transformer for the different impedances (see attached rev 11 schematic), it seems almost impossible to find a switch that is able to do that?
Other problem with this is where to put the feedback loop connection. OT is Hammond 1650F.
Anybody did this?

[acheld]

I've tried a number of differrent solutions to this problem, but in the end, a dedicated impedance rotary switch is the most reliable.

I like this one:  https://www.tubesandmore.com/products/switch-rotary-impedance-selector-3-position,  and it's available in Europe also:  https://www.tubetown.net/ttstore/en/components-10662/switch/rotary-switch/.

[mresistor]

I use these in many builds   they are very good and reliable as well and small - the 6mm knob thing isn't that much of a hassle. https://www.tubesandmore.com/products/switch-rotary-1-pole-3-position-0

[Willabe]

I use these in many builds   they are very good and reliable as well and small - the 6mm knob thing isn't that much of a hassle. https://www.tubesandmore.com/products/switch-rotary-1-pole-3-position-0

I've tried a number of differrent solutions to this problem, but in the end, a dedicated impedance rotary switch is the most reliable.

I like this one:  https://www.tubesandmore.com/products/switch-rotary-impedance-selector-3-position,  and it's available in Europe also:  https://www.tubetown.net/ttstore/en/components-10662/switch/rotary-switch/.

No those won't work if he's using the Hammond 1650F OT in the schematic. Look at the schematic in reply #29. The 1600 series (NOT the 1600A series) has to be re-wired for the 3 different ohms out.

From tubes and more;

"3 pole, 4 position rotary switch (3P4T), ¼" output jack and black chicken head knob. Hammond 1600 series transformers all provide secondary impedances of 4, 8, or 16 ohms but due to the unique wiring of the secondary, the impedance cannot be changed without rewiring... Until now. We now offer a selector switch, pre-wired and complete with ¼" output jack."
Some of the Hammond's have an odd OT secondary winding.

This 1;

https://www.tubesandmore.com/products/switch-rotary-impedance-selector-hammond-1600-series

[SEL49]

I would like to have a rotary switch to select the impedance (4, 8, 16 Ohms)... OT is Hammond 1650F.

You cannot use a simple one pole three position switch with that 1650 OT. Best you can do is strap the secondaries for 4Ω operation. This will allow use of the 4Ω and 8Ω. (16Ω operation requires changing the secondaries strapping.) There is a cheap multipole switch that can handle the selections but it's very flimsy. I would not trust it with the high current it will see.

But, Hammond sells an "easy wire series" that will connect to a single pole three position switch, allowing selection of 4, 8, or 16.

1650FA (easy wire series) has the same specs as the 1650F.

I have used both. They're a perfect match for most any EL84 P/P amp.

Hammond 1650F
Hammond 1650FA

[Beezerboy]

. Does yours sound like this?

... it can. it can also be made more clean. or more dirty. it won't do Fender clean but thats not the point.

the 2 preamp inputs give you a good way to balance the initial EQ. then I use the tone stack and the presence control for the final tone shaping depending on what guitar I'm using. the "dirt" level is a play off between the V1/V2 input and the Master. there is a lot of control available to shape the sound.

I've built it a couple times and was "forced" to sell one to a friend that still gigs regularly. I modified that one to include Tubnit's  "one tube reverb" I got off this site. there is a long thread on that

[Madhatter]

I would like to have a rotary switch to select the impedance (4, 8, 16 Ohms)... OT is Hammond 1650F.

You cannot use a simple one pole three position switch with that 1650 OT. Best you can do is strap the secondaries for 4Ω operation. This will allow use of the 4Ω and 8Ω. (16Ω operation requires changing the secondaries strapping.) There is a cheap multipole switch that can handle the selections but it's very flimsy. I would not trust it with the high current it will see.

But, Hammond sells an "easy wire series" that will connect to a single pole three position switch, allowing selection of 4, 8, or 16.

1650FA (easy wire series) has the same specs as the 1650F.

I have used both. They're a perfect match for most any EL84 P/P amp.

Hammond 1650F
Hammond 1650FA

Perfect, That's what I have searched for. Second part of my question above: where to place the feedback loop then? Is it ok if it is e.g. connected to the 16 Ohms wire but my switch selects e.g the 8 Ohm wire? It should, since the voltage is still on all wires, just only one is connected to the output jack. Do I get that right?

[SEL49]

Second part of my question above: where to place the feedback loop then? Is it ok if it is e.g. connected to the 16 Ohms wire but my switch selects e.g the 8 Ohm wire? It should, since the voltage is still on all wires, just only one is connected to the output jack. Do I get that right?

You don't want to be changing the NFB when you change selector switch. So, you're right, connect the NFB directly to whichever OT tap you prefer.

[Madhatter]

Thanks, great. I prefer ... nothing Is there a technical or sound related preference to connect it to 4, 8 or 16?

[SEL49]

Let your ears tell you which you prefer. If you just want to maintain the same amount of feedback as in the November Rev. 11, you would change the value of the feedback resistor. Look at note 4 on this schematic...

     https://sluckeyamps.com/november/november.pdf

[Madhatter]

Hi all,

OK, I am good to go now. Once again, thanks to everybody for advice. This forum is fantastic!

Madhatter

[mresistor]

Willabe thanks for pointing that out..   


SEL49 thanks for the info on the new improved Hammond easy-wire OT.

[acheld]

Hmmm, did not realize that the 1650F exists . . .  been using the 1650FA for a long time   

The rotary switches of course do work with the FA model.

[Beezerboy]

if you add more wafers to the rotary you can make it do anything you want, even with the 1650 F. add another and you can also change NFB

[Madhatter]

Hello all,

have been off for a while, but did layouting since.
An idea came to my mind: The original circuit has four inputs. Now, you can get a cool sound out of them by jumpering the two channels, connect a cable from channel 1 bottom input to channel 2 top input and plug the instrument into channel 1 top input. That should give a fat lead sound. If I do not do this and just plug the instrument to one lower input, I should get a good clean sound.
Question (roast me if stupid): How to change the schematic if I want ONE input but a switching option between these two sounds? Ignore the question of having two master volumes and / or two tone control sections. I am only interessted in the start of the circuit around the input jacks.
Thanks

Madhatter

[SEL49]

Look at this schematic to see how to connect a single input jack and switch in order to select either channel or both (same as jumped).

     https://sluckeyamps.com/phoenix/phoenix.pdf

[jbrrrrr]

This raises an issue I've been chewing on myself - in the case of paring back a 4-input amp design, the inputs can be internally jumpered with a switch.  You can even wire in a "lo/hi impedance" switch for a one-input version to get one or the other channel, or both jumped together, either with a low impedance or a high impedance.

Two questions - I think I remember seeing somewhere on Sluckey's page about consolidating inputs that even if you add in a switch to change the resistor to ground on the input jack, there's something about the paralleled combination of resistors/shorting that would still be slightly different w/r/t high end rolloff compared to using two jacks wired together, with one shorting out the other.  Does anyone have any info on whether that's accurate or if there's a fix for that beyond just switching between 68k or 1Meg?  Sometimes the high end rolloff from the low input can be useful in  shaping the overall tone early that I've always thought about trying to keep somehow.

Second - is there any missing benefit to being able to jump low into high or high into low, both high inputs or both low inputs - that you wouldn't get by scaling down to 1 or 2 inputs from 4?  I guess it'd be really splitting hairs in the overall sound, but it's still worth trying to get an answer for if anyone has a quick answer for it.

I guess the solution that retains most of the original implementation would be two jacks: one low and one high, with a DPDT on-on-on switch toggling either/both being sent down through the signal path.  You'd still lose the low-into-high or vice versa, but at least the input impedances would be the same as the four hole layout, right?

[Merlin]

Two questions - I think I remember seeing somewhere on Sluckey's page about consolidating inputs that even if you add in a switch to change the resistor to ground on the input jack, there's something about the paralleled combination of resistors/shorting that would still be slightly different w/r/t high end rolloff

Assuming you're talking about a Bassman-style 4-input arrangement, there is no difference in roll-off between hi and lo at the input tube, its the heavier loading on your pickup that dulls the tone in 'lo' mode.

Second - is there any missing benefit to being able to jump low into high or high into low, both high inputs or both low inputs

Jumpering them how? With a Y-splitter cable or with a jumper cable? How many combinations do you want?? Yes you are splitting hairs.

[jbrrrrr]

Thanks, Merlin - I was conflating the Fender-style hi/lo jacks with the Marshall inputs either way, since there's actual gain stages involved with each jack.  As far as this amp design is concerned, it wouldn't be of any issue.

https://sluckeyamps.com/misc/Amp_Scrapbook.pdf - I was thinking of the 68k || 68k interaction under the Vox inputs on p.13 that isn't totally replicated by simply switching the grid leak at the input - I had just remembered mentally earmarking this as a potential area of nitpick-ery if I was building something spartan for someone else who really had a strong opinion about that high/low jack interaction.

[tubeswell]

How to change the schematic if I want ONE input but a switching option between these two sounds? Ignore the question of having two master volumes and / or two tone control sections. I am only interessted in the start of the circuit around the input jacks.

Attached is a simple circuit with 2 input jacks where:


1 plug inserted in the parallel stages jack gives a 'fatter sound', while 1 plug inserted into the single stage jack gives a more normal sound, or plugs inserted into both jacks at the same time can be used to run a stereo input from a 'stereo out' pedal board (where you can have even more control over the types of signals going into each amp input.