Vox AC4 Question(s)

[dwinstonwood]

I'll add, that's with 270v on the B+

[sluckey]

I have just under 98v on the screen and about 96v on the plate. The cathode is about 2.2v. That got it way closer to sluckey's schematic.

Good enough. May just be the difference between a real world meter, tube, components, etc and a virtual spice world.

BTW, I had 266v for B+ when I made those measurements.

[dwinstonwood]

Cool. The EL84's measurements seemed closer, too. Plate and screen were 270ish and cathode was about 8.5 - 9v. Your Dual Lite is 317, 295 and 10v respectively.

The whole section of my schematic with the volume and tone pots, and the coupling caps needs work. I'm guess that is having an effect, too.

Thanks sluckey.

[dwinstonwood]

Good enough. May just be the difference between a real world meter, tube, components, etc and a virtual spice world.

Yeah, I played around with it some more. The only way I can get the screen a couple of volts negative of the plate is by adding a 68K resistor in series with the 1meg. At the same time that raised the plate voltage a bit. I think someone in another thread said, "everything effects everything" in tube amps.

Your design works in real life, so it's surely something quirky with my LTSpice model. But, I am learning stuff by watching how resistor values interact, etc. It's all new to me and really interesting.

[kagliostro]

May this be of your interest ?



Franco

[dwinstonwood]

Thanks Franco!

[dwinstonwood]

I really don't know what the real-world B+ will be. So, this is all just for fun. But, with the simulation B+ swimming along between 266v and 270v and a 56k resistor in series with the 1meg, I get 99v on the plate and 94v on the screen.

I'll probably do some research on affordable transformers next. It looks like I'll need a 150-0-150 PT and a 5K8ohm OPT. I guess I'll start by looking at Hammond? Let me know if that sounds wrong. Thanks.

[dwinstonwood]

Hey, I thought I'd share my latest partscaster, the reason I'm working on this project.

It's not for everyone, but it's everything I like. Vintage 50's Mexico 7 1/4" neck; Kluson Supreme 18:1 vintage tuners; 4 1/2 lbs. Warmoth Vintage White Hybrid Strat body; Alnico 2 Bootstrap PU's; reverse controls; Tusq XL nut. It's light and I love the Alnico 2's. I just need a decent home amp.

[dwinstonwood]

The Mercury Magnetics are too pricey for me with the current state of things.
The Hammond 269BX is 300V and 6.3V@2A for around $51.
The Hammond 125CSE 8Watt is $50.

It looks like these will work. And, $100 is about what I wanted to spend.

[sluckey]

I'd rather have the 269AX

[dwinstonwood]

I'd rather have the 269AX

Thanks. I see that it's 250V. That will lower my B+ so should I do away with the 470 resistor, or run the amp with lower voltages?

I got the EF86 wired up in DIY Layout Creator. I'm sure there's a cleaner and "electrically better" way to do it!

[sluckey]

Thanks. I see that it's 250V. That will lower my B+ so should I do away with the 470 resistor, or run the amp with lower voltages?

Just stick to the schematic. PT is 250-0-250, dropping resistor is 1K (not 470Ω), and use an EZ80 or EZ81 tube rectifier. Otherwise, your B+ will be around 400V rather than 270V and all that spice time just goes down the toilet.

[dwinstonwood]

Thanks sluckey. That sounds great to me. I appreciate all your help. It's time to focus on the layout.

[dwinstonwood]

The pot wiring needs work, as does the grounding layout. I'll research those things some more. I think everything else is ok.

[66Strat]

Unless I'm missing something, the 269AX voltage at 115 ma. is 125-0-125. The 250 volt rating is across the secondary winding (red to red).

http://www.hammondmfg.com/pdf/EDB269AX.pdf

[sluckey]

Unless I'm missing something, the 269AX voltage at 115 ma. is 125-0-125. The 250 volt rating is across the secondary winding (red to red).

Good catch. Huge mistake on my part. Sorry. 

Looks like the 270CAX or 270DAX would be good candidates.

[dwinstonwood]

Thanks 66Strat!

Here's an updated layout:

[sluckey]

There is no center tap on the 6.3 heater winding.

[dwinstonwood]

Thanks sluckey. I'll fix that.

I sort of used the 5F2-A as a reference to try and figure out how to wire the pots. This is an area that confuses me. Another change I made was to use a dedicated power switch instead of the Tone/On-Off pot shown on the original schematic.

I'm off to teleworking. Thanks again for all the help I'm getting here. I think it would be great if this turns out to be a successful amp that others might want to build.

[66Strat]

Not trying to be "that guy" but, the 270CAX wiring diagram shows a center-tap for the 6.3 volt heater winding. The 5 volt heater winding does not have a center tap. I like this power transformer. The 5 volt winding would allow the use of a 5Y3 in lieu of the EZ81 should you choose.
https://www.hammfg.com/files/parts/pdf/270CAX.pdf

Good looking guitar btw. I've emailed Bootstrap regarding some alnico ii pickups for my Tele.

[dwinstonwood]

Thanks 66Strat.

The Bootstraps are an amazing buy.

[sluckey]

Not trying to be "that guy" but, the 270CAX wiring diagram shows a center-tap for the 6.3 volt heater winding.

Haha. I'm gun shy now. Definitely gonna start looking at the pdfs. Hammond's quick listing did not show a CT.

[66Strat]

Not trying to be "that guy" but, the 270CAX wiring diagram shows a center-tap for the 6.3 volt heater winding.

Haha. I'm gun shy now. Definitely gonna start looking at the pdfs. Hammond's quick listing did not show a CT.

There is definitely a problem in Hammond's documentation. Either the listing is wrong, or the wiring diagram is wrong. I guess you have to buy the transformer to find out what it is. Gee, that has a familiar ring to it.

[dwinstonwood]

Would it be bad practice to build the vibrato circuit on its own small board? I watched some videos of 1960's AC4's and the vibrato is pretty nice. Maybe I could add it on later after the main amp is tested and working.

I'll probably try to draw a layout for it, too. It's good practice.

[66Strat]

If it were me, I would put it all on one board and use the AA764 Vibrochamp layout as a starting point.

https://el34world.com/charts/Schematics/files/Fender/Fender_vibro_champ_aa764.pdf

It wouldn't take much to adapt this layout to your project.

[dwinstonwood]

Thanks 66Strat. My main reason is that this will only be the second tube amp I've ever built, and I need to adhere to my original plan which was to keep it simple and make it work! My first project - that I finished last month - is a Tubelab SET EL34 stereo amp. It works and sounds great. Maybe I just got lucky, but it worked as designed from the first time I powered it up. But, I built the most basic version with no extras and used good parts.

I had nothing more interesting to do, so I went ahead and drew a layout for the vibrato circuit. Whether I use it or not, I got some more practice in Diy Layout Creator.

But, since I've already made a few changes based on sluckey's Dual Lite, it might not drop right in, anyway.

[dwinstonwood]

Well, I just ordered a custom turret board, components, and hardware from Hoffman. He had every component value I needed, and it was a lot easier selecting parts that it is at Mouser, etc.

I'll order the transformers, tubes and a chassis next.

[dwinstonwood]

Tubes and transformers are ordered. I haven't decided on a chassis yet.

[dwinstonwood]

I should be getting most of the parts in the next couple of days, or so.

I solved the chassis dilemma, though it's not pretty. I had an old solid state Fender Frontman Reverb with an 8" speaker that a neighbor gave me. The chassis space in the cabinet is 11.75" x 7" x 2" and Hammond has an 11" x 7" x 2" aluminum one. I figure I can have a local laser engraver make an 11.75" wide faceplate to cover the gaps on either side.

There's not as much to chose from in the 8" speaker realm, and I know very little about guitar speakers. Some people online seem to like the Weber Blue Pup. There's also the Celestion Eight 15. I'd welcome any suggestions or advice on speakers.

Thanks,
David

[66Strat]

If you haven't already ordered your chassis, you might consider having one custom made. I plan on using this guy for a 5Cish5 Pro project.

http://www.juicyamps.com/chassis.html

I haven't had any experience with him yet, but his products sound promising. I was thinking of polishing the aluminum and hand lettering the face plate with a paint pen. In a former life, I was an architectural draftsman and was pretty good at lettering.

[dwinstonwood]

Thanks JT, I'll take a look at that.

First package of parts arrived. Maybe start some soldering this evening...

[dwinstonwood]

I tried my best to follow these grounding guidelines: https://el34world.com/charts/grounds.htm

Preamp section, pots and input are grounded through input jack connection to chassis.

Power section, speaker jack (and OPT), 6.3 CT, and incoming AC earth ground will connect to PT mounting bolt.

The only thing I'm aware of that I changed is not soldering the bus wire to the backs of the pots.

I'd appreciate some expert eyes on this. I want to start soldering!

Thanks,
David

[kagliostro]

I didn't verified the layout, only one council, if I can say

often SE amp has at least a bit of humm problem

to try to minimize it, why don't use a third e-cap on the PS ?

say a ... 33uf or, better, 47uf connected as first cap and followed by a ... 120R till 330R resistor

Franco

[shooter]

don't need to solder to the back of pots, builder choice thing.

pin 8 of the el84 is not used, you have a jumper to pin 3 

As Franco stated, SE likes to hum if not heavily filtered.  My last build I wound up tacking in a couple extra PS caps, but not til I proved it was good working amp.

[dwinstonwood]

Thanks Franco and Shooter!

The two turrets on the far left of the board can easily accommodate an extra filter cap. The ground wire doesn't need to use them (as shown).

shooter, I also jumpered 8 and 3 on the EF86. Should I remove that one, too?

If I could also ask one other question: Does the wiring on my tone and volume pots look OK? I just can't get my head around how that circuit works. Here's the original schematic below.

And, I agree, let me get it running first, and then I'll work on improving it! Thanks, again.

[sluckey]

I also jumpered 8 and 3 on the EF86. Should I remove that one, too?

You must have a jumper between pins 3 and 8. You also need a jumper between pins 2 and 7 that connects to ground.

[sluckey]

Your tone control is not wired correctly.

[dwinstonwood]

Thanks sluckey.

The EF86 wiring diagrams I could find online, and the original schematic, indicate - to my eyes - that 2 and 7 can connect to ground via the cathode bias cap (please see my new layout). I did it that way.

I've studied the schematic some more, and it shows the tone pot Wiper connected to ground. So, I've rewired it that way.

[kagliostro]

A small thing

the yellow wire that joints the 6.8K resistor to pin #2 of the Power Tube isn't placed correctly

Franco

[dwinstonwood]

The Hammond chassis is an almost perfect fit. There's just a small gap on either side that a faceplate will cover up.

I also made a few changes to my layout:

1.) I put the power switch before the fuse so that no one can get shocked by changing a good fuse if the amp is off, but still plugged in (of course, someone could still try to change a fuse with the power switch on, but that's a symptom of a bigger problem).

2.) I moved the grid stoppers to the tube pins as I learned that the closer they are to the grids the better they work.

I'm just waiting on a few more parts to arrive. I bought the wrong socket for the EL84; I bought the kind with the shield and the tube is too tight. (Hoffman explains all that in his parts store, but I failed to read it!)

[dwinstonwood]

A small thing

the yellow wire that joints the 6.8K resistor to pin #2 of the Power Tube isn't placed correctly

Franco

Thank you Franco. Please tell me how it should be connected.
David

[kagliostro]

In the image I see it connected near the pin #3 (also if not exactly to pin #3)

Sorry, you only miss pin #2 on the socket layout

Franco

[dwinstonwood]

Thanks! Just sloppy drafting on my part. Here's an updated layout:

[66Strat]

Would there be any potential noise reduction benefit in connecting the EF86 shield directly to ground, rather than elevated at cathode potential?

[dwinstonwood]

66Strat, I found this thread:
https://el34world.com/Forum/index.php?topic=432.msg454#msg454

After your post I was thinking the socket's mounting bolt would be good. I don't know what's best.

[ChopSauce]

I have neither a great knowledge of electronics, nor a valuable experience in the guitar world; but I've been messing around the schematic of the AC4 for two builds now, so I felt like I had to share some 5/6 thoughts - below - for what they're worth... 

Firstly, I haven't checked the schematic in great detail but the 5.0V mention close to the HT leads to the EZ81 is pretty confusing, especially when dealing with people used to work with US rectifiers - as a starter.

Secondly, your schematic appears to lack filtering. In my archives, the original schematic shows two 33uF then a 8(10) uF capacitor and the EF86 is NOT connected to the B+, but to some C+ sharing the HT node of the third cap.

Thirdly, there are a few boards layout for the amp that have been posted on a few forums so you should be able to see them and compare with yours.

Fourthly, if you want the "vintage tone" you could consider a smaller OT. The originals used to show a ridiculously small one and - if you're in the US, it seems that Mercury Magnetics have a replacement part, with ref. #VXP-4. I won't be surprised that they also have a PT too. It just depends on what you're after: a replica or some modded version.

Fifthly (this word sounds pretty unusual, doesn't he?), I feel like the PT with its 5V windings is overkill. See the fourth point above. NB: I might be wrong!

Finally, I decided to build one using the point to point layout of the late sixties - the one with a ground bus - mostly, AND with the tremolo, because I felt the effect was worth the little extra effort. I started wiring the heaters then the tremolo. That way I had no temptation to forget it in the end... 

Might https://ampgarage.com/forum/viewtopic.php?f=10&t=33245 be of some interest to you?

[dwinstonwood]

Firstly, I haven't checked the schematic in great detail but the 5.0V mention close to the HT leads to the EZ81 is pretty confusing, especially when dealing with people used to work with US rectifiers - as a starter.

Oops! Thanks ChopSauce. Since I had never used or heard of Diy Layout Creator before this project I stole an existing file from Rob Robinette's site and used it as a template. The "5.0V" text was left over from his layout, and I missed it. Thanks for spotting that; I deleted that text.

[dwinstonwood]

Nice! Did you build that cabinet? Looks great.

From your build: "Mais qu'est-ce que vous allez faire avec le chocolat?" - I was also wondering how you point-to-point wire a candy bar.

[dwinstonwood]

Concerning filtering: Is there a maximum size capacitor that the EZ81 can safely see in front of it? The datasheet below indicates a limit of 50uf - see where I outlined it in red. Am I reading that correctly? Does that only apply to the first capacitor in the filtering chain? So, if I install an additional 47uf in front of the two 22uf capacitors is that within the tube's specs?

Thanks,
David

[shooter]

that's how I read it.  You have a 47uF + 22uf for a total of ~70uF, you might
get away with that.  keep an eye on the rectifier when powering up to see if it gets sparky inside