AC30 based ampdesign

[JRB]

Hello everybody,

Been reading a lot on this forum the last couple of months and I have to say the information here is excellent.
After having build a whole pedal board full of effects I have been wanting to build a tube amp for myself for sometime now. I'm well aware of the dangerous of working with electricity having read all the warnings on different tube amp websites together with learning and working with it for half my life. To educate myself in the world of tube amps I have gotten myself merlins book and so far its been a good read.

What I have been thinking of building is basicly a AC30 without the awful 30kg weight on it and the awful loudness, together with some switching capabilities. So the list I came up with is the following:

• Channel switching between the normal and top boost channel;
• Double volume control for the top boost channel;
• Two equalizers;
• Both channels going through the equalizers;
• 4-8-16Ohme output switch;
• 15W output.

So using the AC30 as a base point I came up with the schematic in the PDF, I still need to design the power supply but I think its going to be based on the Hammond 290PX.
Does anyone see any problems that may arise that I am not aware off?

JRB

[Dave H]

It’s too complicated and has too many switches

S1 won’t work. The grid of V1 is open as drawn and the cathode follower V3 has no bias voltage. It would normally be directly connected to V2 plate not through C8.

R17 should be about 47k not 1k5. R24, R25 should be replaced by a single 130R resistor. A 50R cathode resistor (R24//R25) is for 4 x EL34s.

I think I’d decouple the EL84 screen grids (and the preamp) with a 1k/47u and I probably also change the 100R screen resistors to 1k.

[JRB]

If you think this are many switches you should see the amounts I come across with my work :P

I had a look at your suggestions and I did some changes to the schematic,
Mirrored the S1.2 and S1.4 so it doesn't switches the signal I want to use to reference.
Fixed the copy error I made with R17, on the schematic I have it also said 47k.
Deleted R25 and changed R24 to 130R. Funny thing is that I was thinking of doing that since looking at the schematics for the AC15 I saw 120R everywhere.
Found a drawing error with the centertap coming from the output transformer.


For the cathode follower I am going to have to reread the chapter about them in merlins preamp book. I was expecting problems with that one to be fair.

Could you explain what you mean with decoupling the screen grids and the preamp and why you would change the screen resistors to 1k?

[jazbo8]

Some questions and comments:

Why does the second tonestack takes its input from the treble pot of the previous tonestack instead of the cathode follower (after the switch)? If the one of the tonestacks is always in the circuit, then C16 & C17 could be eliminated, as the coupling caps in the tonestacks already provide DC blocking.

The screen supply should always be decoupled from the rest of the PS (as much as possible), since it has great influence over the output power and power tube bias. The 1k screen resistors ensure the screens are well protected during the peaks - cheap insurance.

Slightly OT, what software did you use to get the layers on the PDF?

Jaz

[JRB]

Some questions and comments:

Why does the second tonestack takes its input from the treble pot of the previous tonestack instead of the cathode follower (after the switch)? If the one of the tonestacks is always in the circuit, then C16 & C17 could be eliminated, as the coupling caps in the tonestacks already provide DC blocking.

The screen supply should always be decoupled from the rest of the PS (as much as possible), since it has great influence over the output power and power tube bias. The 1k screen resistors ensure the screens are well protected during the peaks - cheap insurance.

Slightly OT, what software did you use to get the layers on the PDF?

Jaz

The second tone stack go's from the switch to the input so I think you must use a different drawing standard then that is used over here. In my country (the Netherlands) when drawing a schematic a T-junction means a connection, having the wires cross means its not a connection.

The screen supply sounds like a good suggestion everything that prevents stuff from getting damaged is a good thing in my opinion.

So the decoupling kinda ensures that the power amp keeps doing what it does even when the power supply decides to do something else for a while. I guess I would put one set of the resistor and capacitor between the power supply and R26 with R27 being connected to R26.
Do you happen to know any good sources that would explain power amps? I would have bought merlins book about it unfortunately he hasn't written or released it yet.

The software I am using is Acad2012, it has a DWG to PDF converter build into it, I didn't even noticed it added the layers until you mentioned it. Its really a nice piece of software and I have been using it for over 8 years now. Once you learn the commands it really becomes a fast drawing tool. If you guys/galls want I could print the PDF's in colour next time, since I colour coded the wires maybe makes the drawing more readable.

JRB

[jazbo8]

Ok, that makes sense. As for the power amplifier design, you can check out the following:

Ampbooks
Turner Audio
Basic Audio see the other two volumes as well.

Jaz

[JRB]

Read the chapter about cathode followers again and I think I got the bias for the cathode follower sorted now. Only need to decide its bias point now but component wise it should be functional.
Fixed a little error in my switching system and filled in values for C8, C21 and C9. They should have their roll off point set to about 1-3Hz.I also increased the screen resistors to 1k. I am still looking into the poweramp part so I haven't done anything with the decoupling yet.

Thanks for all the help so far.
Its a same they don't teach tubes any more in university these days it seems they are a lot easier to understand then transistors or at least in my opinion.

JRB

[Dave H]

Could you explain what you mean with decoupling the screen grids and the preamp and why you would change the screen resistors to 1k?

It's the 1k resistor at the top right and the 47u to its left on the pdf below. The 1k screen resistors are cheap insurance as Jazbo8 said.

Poweramp.pdf

[JRB]

It's the 1k resistor at the top right and the 47u to its left on the pdf below.

[ATTACH]22654[/ATTACH]

I thought it was something like that. On the schematic I found of the AC30 its done by the choke and a 22uFcap in the power supply at least that is what I think its for. I did make a error in my first drawing by hooking up the output transformers centre tap to the same point as the powerampscreens.

[yunger]

why are there two identical volume controls (vr2,vr3) that do the same thing? are you trying to switch c5 in/out?

C9 seems unnecessary.

why are there two identical tonestacks that do the same thing when switched?

as shown, v2 is the end of the signal chain. I assume the switching schematic isn't correct.

[JRB]

why are there two identical volume controls (vr2,vr3) that do the same thing? are you trying to switch c5 in/out?

C9 seems unnecessary.

why are there two identical tonestacks that do the same thing when switched?

as shown, v2 is the end of the signal chain. I assume the switching schematic isn't correct.

The plan was to be able to go from full out distortion to just light crunch with the push of a button. I still have to find a way of keeping the volume level the same when you switch between them or else I might get ride of that option. In my understanding the AC30 gets most of its distortion from the poweramp so it would have to be something between the poweramp and the speaker I guess.

The two identical tone stacks is just a copy for now I am planning on experimenting with the values a bit when I get it running. Together with the fact that my bands playing style go's from blues to metal it is kinda useful to be able to switch eq's on the go. At the moment I am using a separate equalizer pedal but I am planning on decreasing my pedalboard size when I get this amp up and running.

C9 is there to block dc from getting on the switch.

The first two switches should indeed be mirrored to get the proper switching.
Just to make clear what the switches are suppose to be doing here is the list:
S1 is your clean/distorted channel switching with switches to mute the non active channel when its not active.
S2 would be the crunch/distortion switch.
S3 would switch the EQ in or out.
S4 is a selector switch for different output impedances.

[J M Fahey]

Please either place a dot at line junctions to show a connection is made *or* don't but jump "non connections" with a little arc.
The way it's drawn now is a shorted mess.
Not your intention, of course, so please follow either one or the other drawing convention.
So far nobody complained because we "know" what-goes-where, but it's poor Engineering practice and as soon as you start complicating things a little, will cause a lot of confusion.
Thanks.

[Dave H]

Please either place a dot at line junctions to show a connection is made *or* don't but jump "non connections" with a little arc.
The way it's drawn now is a shorted mess.
Not your intention, of course, so please follow either one or the other drawing convention.
So far nobody complained because we "know" what-goes-where, but it's poor Engineering practice and as soon as you start complicating things a little, will cause a lot of confusion.
Thanks.

I thought that schematic looked odd. I've never used Acad but all other CAD packages I've used automatically added a dot to a junction.

[J M Fahey]

I use humble very easy to learn and free ExpressPCB - Free PCB layout and schematic software which does it automatically.

[Dave H]

I used Express for the schematic I posted above.