Building a vacuum tube amp from scratch

[dlgoff]

After thinking about the feedback issue, I don't think it matters if the feedback is connected to the 8 ohm tap even if the amp is connected to a 16 ohm load.

Having just had a tooth pulled I'll only give this reference to see how the feedback connection could complicate things. Image from http://vinylsavor.blogspot.com/2011/08/speaker-impedance-and-amplifier-output.html

 

[Planobilly]

Hi Guys,

I got the amp up and running today. I still have a few things to clean up. There is no noise at idle...so no ground loops. It sounded pretty good at start up. I am sure I can tune it up to sound a good bit better.
As I think back over this project the chassis was not easy to make. I had a few issues at start up. Typical stuff like having the output transformer out of phase. I still have no idea how to tell which wire goes to what output tube without just trying it. Some stupid things like not having a 22K 3 watt resistor and having to use a 20K and 2K in series.
The project was harder than I thought it would be. There is a big difference in building a kit and building from scratch. It has been a good learning process. I deviated from the original schematic in several places. What I learned from that was not much is set in stone with component values and small changes in general don't affect the sound much.
Here is a photo of how it looks at the moment...

Cheers, and thanks for all the advice

Billy

 

[Tom.G]

Nice.

A project like that gives you a little more appreciation of the effort from the designers!

What did you end up using for the Heater supply?

Have you settled on a front panel implementation yet?

Audio Transformer Color Code (from pg. 524, "The Radio Amateur's Handbook", 1969 Ed., American Radio Relay League)

Blue - plate (finish) lead of primary
Red - "B"+ lead (this applies whether the primary is plain or center-tapped)
Brown - plate (start) lead on celter-tapped primaries. Blue may be used if polarity is not important.
Green - grid (finish) lead to secondary
Black - grid return (this applies whether the secondary is plain or center-tapped)
Yellow - grid (start) lead on center-tapped secondaries. Green may be used if polarity not important.
Note: These markings apply also to line-to-grid and tube-to-line transformers.

Another reference for transformers says there is an applicable EIA Standard (Electronic Industries Association), if you want to track it down.

 

[Planobilly]

Hi Tom,
I have learned a lot in this project about the issues a designer is faced with. First, it is a lot of work, and second designing a tube amp is not really very easy for anyone no matter what their skill level is. So my hat is off to the people who do this every day and the people who came up with the original designs.

I had some incorrect notions about electronics in general when I first started trying to learn about the subject. I assumed things would be very mathematical and clear cut. I assumed the end result of some implementation would be very closely defined by the theoretical calculations. I assumed I would always make my decisions based on pure science. I guess it never occurred to me how many other realities would come into play...such as "three o'clock in the morning and too much coffee"..lol

The implementation of the heater supply is a case in point. You had said to use a 47 ohm resistor in parallel to ground. Someone else had suggested using a 100 ohm resistor. For some unknown reason I used a 67 ohm resistor...to much coffee on my part I assume...lol The bottom line is 6.3 VDC appears on the heaters and I sort of assume that the 6.3 VDC would have been there with any value between 47 ohms and 100 ohms. The possible of creating ground loops with this design I assume exist but did not occur in this case. The amp produces more or less zero noise with a no signal condition and only displays noise when the second channel is turned up past 90%. There is no reason to turn the amp up to that level in normal use in any event.

The issue with the output transformer is that the primary high voltage has to be in phase with the output tubes...so do you connect the blue wire to the first output tube or the second output tube? It is not really a big issue because if it is not connected correctly the amp will oscillate and make a loud noise that will let you know your guess was incorrect...lol

I have not finished everything on the front panel yet so the lettering is not decided on. I also have not installed the master volume control circuit. I also have a issue with the second channel tone stack not working correctly. I think I will get this sorted out today. At that point I will start to make small modifications in an attempt to cause the amp to sound more to my liking.

This has been and continues to be a fun project. Not everything turned out a well as I would have liked. Actual experience in building something like this is a serious requirement to produce really good results. I will certainly be better equipped to do a better job on the next one.

Cheers,

Billy

 

[dlgoff]

Awesome Billy. It really looks nice. Any issues with the output impedance thing?

 

[Planobilly]

Hi Don,

Thanks for the kind words.
There are no issues with the output impedance implementation that I am aware of. I have only tried it on a 8 ohm speaker cabinet. There are issues with the NFB implementation. I need to install a switch and another resistor to provide three levels of NFB or perhaps a variable resistor would be even better. Currently the amount of negative feedback is too large. That is causing the output tubes to be less smooth as they enter into distortion. The upside is less distortion overall. What I am looking for is clean when I need it and smooth entry into distortion when I want it which will require a condition of no NFB to max NFB. The circuit is easy to build.

I solved the tone stack issue not working. I had placed the wrong value resistor in the circuit. Everything works now but I am not happy with the amount of Mid tone control that exist. I will save that issue for later.

The last control that I have not hooked up is the post phase inverter master volume control. I am working on that now but I have not come up with the circuit design yet. I am having to do some studying on that issue. I want the master volume control to color the sound as little as possible when in use and to not change the sound at all when not in use.

Everything I am doing now is just modifications to the basic amp design. The amp is working as currently designed without any real issues.

Thanks,

Billy

 

[Tom.G]

master volume control.

This assumes you have about 20% extra gain already available in the preamp stages. To test this assumption, just tack a 1Meg resistor in place of the pot and see if you can live with it.

 

[Planobilly]

Hi Tom,

There are several ways to implement a master volume control I guess. I may give your idea a try. Here is another way I am considering. Take a look at this example.

With this dual pot idea and after the phase inverter I should have control over both channels. At zero ohms on the pots there should be no effect on the circuit. I have a 250K dual pot on hand but would have to order a 1m dual pot. One never has enough widgets...lol

Also I am finding the first channel not very useful to begin with. Here is a modification I may try to make too the first channel to make it more useful. See what you think.

Cheers,

Billy
EDIT...Here is what Rob has to say about the Channel mod.
"
Lead Channel Mod
Many AB763 players never use the Normal Channel since it has lower gain and no effects. If you don't use it you should consider voicing it as a "lead" channel by changing out only four components. Three of these component changes are from the 1987 Marshal "Plexi" lead channel preamp. They filter out excess bass frequencies that tend to boom or get muddy when severely overdriven. You will also gain some clean headroom and maximum volume because low frequencies use up a lot of the amps power so removing very low frequencies allows more amplification of the remaining audio frequencies.

With these four changes you can push the Lead Channel very hard with gain and boost pedals and get a nice, tight, modern overdrive tone. Reverb and delay effects and hot humbucker pickups will also sound better through this "lead" channel because the reduction in low frequencies will keep the amp from being overwhelmed. This mod will also make the channel more pedal friendly in general. I really love the Lead Channel Mod.

This mod will not affect the Vibrato Channel. Pairing this mod with the "Fritz Mod" above will send the Lead Channel through the reverb and tremolo effects and through the third preamp gain stage. You'll be able to run more reverb with the Lead Channel because the very low freqs aren't there to freak out the reverb circuit and springs.

The 2.7k cathode resistor is used in many high gain preamps and will bias the preamp cool and make creamy asymmetric distortion more likely. Thesmaller bypass cap will boost more mids and highs. The .0022uF coupling cap is standard in modern high gain amps and will trim unneeded low frequencies to tighten up the overdrive tone. This smaller cap will sweeten the overdrive tone by reducing bias drift recovery time of an overdriven second preamp stage. A .0047uF coupling cap is an option that will sound a little fuller than the smaller .0022uF but if you plan to hammer the piss out of the amp with boost and gain pedals then the smaller .0022uF cap would be better. The V1A 220k 1 watt plate load resistor will add gain to the channel's first preamp gain stage and make overdrive more likely in the amp's following gain stages.

 

[Tom.G]

For the dual pot master volume control mod, realize that it is within the negative feedback loop with unknown consequences. (Besides, why use two pots when one will do the job? )

The preamp mod sure looks reasonable, go for it and see if you prefer the sound. It's also simple enough to undo or modify as needed.

 

[Planobilly]

Hi Tom,

I was considering the dual pot idea because several people I know have said it was the better of several cost effective solutions. Most of the time when I would use a master volume I would most likely switch out the NFB circuit. The truth is that no master volume arrangement really does what most of us are looking for which is volume reduction without changing the sound quality which would be produced at high volume. That is in fact not possible for the simple reason the speaker cone movement is also reduced.

There exist another solution from a company in Canada. It is called "power scaling" and uses a MOSFET circuit. I have not ever seen it in use but I believe it works as advertized. The idea is to have control over the pre amp voltages and to regulate all the output tube voltages. Well...I have not seen a schematic so I am not 100% sure just how it works. It is touted as the best solution that there is. Of course, it still does not address the issue of reduced speaker cone movement. As far as I know there is no known way to reproduce the way a amp sounds and reacts to the guitar at high volume without the high volume. This is a classic issue all guitar players are faced with and becoming an even a bigger problem in today's world of smaller venues. The real answer is to use smaller amps on the stage that can be cranked up to near full volume and have the sound reinforcement system increase the volume to the audience. As you can imagine, the issues get even more complex in a recording studio environment.

Part of the reason for building this amp was to have a platform that lends itself to experimentation. I wanted to know for myself how some of these solutions actually sound.

Cheers,

Billy

EDIT: The other issue with using two pots is any slight variation in resistance would have to cause a certain amount of distortion. I really have no idea how close the resistance is when both wipers are rotated in tandem.

 

[Tom.G]

If you are counting on distortion in the inverter stage, then the dual pot approach may be advantageous. I was approaching it from an Engineering direction, not as a creative musician; definitely different thought processes!

The "power scaling" approach from Canada sounds interesting. It may be worth looking into.

I was also concerned about tracking of dual pots but didn't want to introduce yet another subject. To check tracking, connect them in parallel across a voltage source (maybe 10-20VAC, or even 6.3V filament voltage) then connect a voltmeter between the wipers and see how the difference voltage changes when rotating the pots.
If you try this dual pot test, let us know the results.

 

[Planobilly]

Hi Tom,

lol...turning the gain all the way up on the pre amp stages and turning down the gain on the output stage is most likely not the first thing that comes to mind for an audio engineer...lol When my lead guitar player ask me to get a little "fatter" tone from the amp I have to remind myself that does not mean eat more donuts!...lol I guess art and science are always intertwined to some extent. The approach is usually different...left brain right brain sort of thing.

I have been busy with some other work and not had much time to experiment. I did connect a single 1M pot to both pin 5's, the grids, of the output tube. This mixes the two signals from the phase inverter and causes them to cancel out. That result was not bad and does not affect the circuit when not in use. All these various circuits sound a little bit different from one another and like everything in electronics there will almost always be some sort of compromise I guess.

I placed an order for several dual gang pots of different resistance. I will test them when they arrive as I would like to know for future reference how close the match up.

The good news from south Florida is, it looks like the tropical storm is not forming into anything to be concerned about.

Cheers,

Billy

 

[Planobilly]

Update

I think I am close to finishing this project. I made several modifications to the basic circuit. I connected the first channel through the reverb and tremolo circuit. That also means I can externally jumper the two channels inputs together as they are running in phase now. I increased the first channel gain and added a master volume control. I jumped a .0022 cap on the reverb return to the reverb tank to ground. That cleaned up the reverb circuit a bit. I may experiment a bit with the tone stack. I am pretty happy with the sound at this point. Here is a link to a free cool software for tone stack modeling. http://www.duncanamps.com/tsc/

I had in mind to experiment with a set of 5881 tubes instead of the 6v6s tubes I am using. That may push the power transformer a bit to hard and I only assume I can bias the tubes with the 50V negative supply. Actually there exist both a power transformer and a output transformer that will run 6v6, 5881, and 6L6GT. The two transformers are pretty pricey.

I have in mind to build a single 12 speaker cabinet and try a WGS ET65 at $70 or a Celestion G12 Greenback at $120. Big difference in price for little difference in sound.

Everything else left to do is cosmetic such as the instrument plate on the front.

I learned a good bit doing this project. Only issue is, the more I learn and understand the more questions I have...lol Then there is the EL84 based amp I want to build...I my need to go to amp addiction rehab...lol

Cheers,

Billy

 

[dlgoff]

I think I am close to finishing this project. I made several modifications to the basic circuit.

That's a fairly big project to undertake. Love the persistence; makes for a good engineer. I wouldn't have taken on a tube circuit that big; solid state maybe. I've built several solid state audio circuits by just using a VOM and my ear. Learned, as you've said, there's a lot of variability.

 

[Planobilly]

Hi Don,

There were a few moments that this seemed like a big project. I would be lost if I had to build a one transistor amp, no heaters..lol I guess anytime anyone is learning new skills everything seems like a big deal. I sometimes find myself wondering if I can learn electronics or at least beyond just the very basic stuff. Tube amps are less complex in some ways, well at least most guitar amps. I can say for sure trying to learn electronics has been less stressful than learning to fly airplanes!

I am not sure why I think solid state electronics is so scary to work on. It is not so visual and many components are very small and a bit hard for me to read. I am also not sure why I feel pretty comfortable poking around in some 700 volt tube circuit. That does not seem very logical...lol

Actually, I did not find too much use for anything other than a VOM and my eyes and ears in this project. Well, I did measure the ripple and had a look at the crossover distortion on my scope. I have to at least pretend I can use a scope...lol If not Jim Hardy will revoke my license...lol

With electronics more than some things, there are many kind people willing to help people like me along the road. I am very appreciative of all the education I get here on this forum. I wish I could be of service to more people but I feel like I don't know very much so I read other post and keep quite for the most part.

Thanks for all the support,

Billy

 

[dlgoff]

I am not sure why I think solid state electronics is so scary to work on. It is not so visual and many components are very small and a bit hard for me to read. I am also not sure why I feel pretty comfortable poking around in some 700 volt tube circuit.

Small, yes. But you can, for the most part, rest your fingers on some components while probing without seeing fire. One gets sloppy troubleshooting solid state then gets bit when checking out a 700 volt circuits.

I have to at least pretend I can use a scope...lol If not Jim Hardy will revoke my license...lol

Knowing Jim, he'll find the problem without any test equipment. Just sayin'

I wish I could be of service to more people but I feel like I don't know very much so I read other post and keep quite for the most part.

Thing is, others are reading your post, learning, and keeping quite. But I understand as I feel the same when it comes to the mathematical formalism of quantum mechanics. You don't see me posting there but I read a lot there.

 

[Planobilly]

Knowing Jim, he'll find the problem without any test equipment. Just sayin'

Perhaps working that close to all that nuclear stuff for all those years is the reason...lol

It is past Jim's bed time so we can talk about him..lol

Billy

EDIT: and past my bed time...good night