How does "Power Scaling" for tube amps work?

[Bigun]

Hi Bigun. What model of Vox amp do you have? My only experience working on one has been an early AC30 that I did some wiring on years ago, burned from overheating, and maintaining and modifying my son's Ac50cp2. I Have worked on other amps of similar design that used EL84 output tubes.

I see that you are from Canada where the London Power company is, but it sounds that you are doing the scaling yourself. I have to admit that I am not up to snuff on Mosfets as you must be. Can you tell me more about the D.C. current getting to a potentiometer?

As I stated, I really did my experiment before even doing much research on power scaling, not expecting the good results that I got. Until this installation I had found my best results with a Weber *** attenuator. I do not like the Hot Plate types and have not found an amp that comes with attenuation "built in" that I liked either.

I don't look for a large amount of attenuation for the tone I have always striven for, but it is hard to even talk about something in an intelligent way about something so subjective in the first place. This is especially true when I love both the "sweet" overdriven sound tone that Otis Rush got on So Many Roads and Eric Clapton's Bluesbreaker days and with Cream. The difference in equipment used there is significant and perhaps impossible to replicate with one "rig" to say the least.

However, I have nothing that I enjoy better than trying. I would like to hear about the Mosfet prevention of D.C. thing, as well as any opinion you might have about Mr. O'Connor/London Power circuits if and when you find the time. Mr. O'Connor is quite opinionated and he has evidently has some "issues" with others use of public domain work and trying to patent it?! I am just trying to get a handle on the whole thing. Thanks.

My project is based on building an AC4, for use as a practice amp. The AC4 should be plenty loud enough for home use, probably too loud. Interestingly, I read that louder amps became necessary when the Beatles couldn’t make themselves heard over the din of screaming girls. I don’t expect to have that problem! The earlier amps were single ended too, just an EF86 up front driving an EL84, which simplifies things a bit.

I’ve attached my original ideas on the power scaling for an AC4. You’ll see this dates back to 2018 and it’s still not built, I start way too many projects! I have made two 12” cabinets, so I have two amps that need to be built. One will be based on the AC4, not sure about the other.

The thing is, regular pots are small current devices with delicate sliding contacts on a conductive track. Over time the contact and track can get a bit grubby which can result in less than perfect current flow and this can lead to scratchy sounds when you turn the pot. The screen grid needs dc current at about 15% of the plate current and I don’t want that flowing through a pot used to adjust the screen grid potential for variable power scaling. The FET solves that problem.

Of course, you don’t need to have continuously adjustable power scaling, you could just put in a switch with one or more fixed resistors.Then no need for FET and pot.

Later on, with higher power amps like the AC15 and then the AC30 it became a problem having so much power that the EF86 was starting to misbehave, all that gain and slightly microphonic so the Pentode was replaced with triode stages. I think the current production AC4 equivalent uses triodes up front now too.

 

[dimwatt]

Bigun. I know just what you say about starting too many projects. I had to build another bench for mine recently. The AC-4 is a nice practice amp and you have a nice cabinet for yours too. I build my own custom cabs as woodworking is a second hobby for me. I use exotic wood to build some and have some Bocote and Canarywood for use on one now. Too bad wood in general is getting so outrageously expensive now. I used to buy Brazilian Rosewood which can't be imported anymore for what Walnut costs now. I will be looking for the results you get with the AC-4. Those sliding pots are prone to "grubbiness" I have to agree. I have had good results with De-Oxit on those I could get it into, but not the spray. I use the stuff that comes in a tube on them as I think it is a different formula. It has to be if you see the results in comparison not to mention how it looks. I get it on Amazon and like the spray, it isn't actually cheap, but it does a much better job in the long haul. I think it "sticks around" longer as an enhancer.

Your knowledge about power scaling is in general, superior to mine. It will be interesting to see what how it pans out. I checked out the video posted using a Variac with a Marshall 100 Watt head and a Park, but it is not what I am looking for tonally. I need to reread what O'Connor said about it in one of his books. He had some reasons that it was not a good thing to do, but then he sells a power scaling circuit. It was interesting but like so much, at my age things just don't soak in like they used too anymore and I am so busy with my tinkering and experimenting that my studying sort of takes a back seat right now. Perhaps I have that backwards but I Do so love messing around my shop. I have always been more of a hands on guy even when my hands foul up something so bad that it ends up in the dumpster!

Although I have heard the AC-4 I have not worked on one. I did not realize that it used an Ef86 as a preamp for the El84 and I have no experience with that tube. I wonder what the quirks were with that tube like you mention? You are right about not having to worry about the screaming girls I suppose and perhaps most importantly you won't have another problem that the SuperBeatle amp had...Flameouts! I have worked on an AcC-30 that got Real hot and burned some wiring at 30 Watts. I can imagine what the increase of output would have done before they had the muffin fans there are now. I put a 60mm X 60mm one on every amp I modify just to be safe and some say that it even improves performance of the tubes. I haven't found that it hurts any.

I haven't checked out your attachments yet except to check out your cab work, which looks very nice. i will definitely do that tomorrow after "work". I am finding that retirement is harder on mind and body than when I was working. At 66, my "career as a web fed pressman for a check printing company took a toll but now that I am doing something that I love I am slowly getting energized again. You younger guys( I am assuming you are) have
me at a disadvantage when It comes to discussing many things here but I am trying to catch up, and it is quite helpful with the kinds of things I am reading here, plus it is a challenge.

Keep up the good work and be sure to post further here so I can try to keep up with what you are up to and learn from your achievements, and any mistakes, God forbid you might make! Now, if you are like me you don't make any! Ha! have a good one Bigun.

 

[Bigun]

I can't claim to be a Young'un any longer as this year I have a milestone birthday. Anyhow, sleep well, eat sensibly, exercise and keep social - all ways to stay feeling young for as long as possible.

FYI - the EF86 was in the original AC4, no longer though, new one's don't use 'em as far as I know.

 

[dimwatt]

Congrats on the milestone birthday in advance Bigun. I hope you stay healthy for a loooong time! I've never even seen an EF86 tube, let alone in circuit, but I AM studying a good vintage article from the robrobinette.com site on the triode's nonlinearity as part of the "tube guitar sound" at present. It's pretty technical, so a lot of it is beyond my understanding, but I keep at these things so as to learn as much as possible in the time I have left in this world. I have some health concerns that I am trying to get a handle on right now that has kept me out of action much more than I would like. Please, keep me in your thoughts/prayers and Anytime you have something to share on your work in progress too.