Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: johnfromcyrene on May 11, 2026, 01:44:01 pm
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Hello,
Was looking to do a single channel zodiac twin 30, the amp from house of the rising sun. Was looking to make it a single channel with rotary control only instead of the click buttons and to combine the preamp and power into a single chassis.
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Started here. Will clean it up
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I've stitched together a single page schematic. Hope this is what you wanted...
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yes thank you.
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This is an interesting looking schematic. If I had run across this schematic 10 years ago I would have built it.
Not convinced this amp was a major contributor to the Animals' sound. I think it was probably just the amp they had in hand. The AC-30 or AC-50 would probably have performed just as well.
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In the click tone buttons I notice what appears to be a choke in there. Do you know why that is there?
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Don't call it a choke. Just call it an inductor. Inductors can affect tone in a very similar way that capacitors do.
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Ok I see.
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I'll probably do a layout for this amp when I get all my house chores done. Could be a while.
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Great, You will have a better layout then mine I think. Will wait for that.
One thing I cant figure out is this part here, does this go into the tone pot when rotary control is engaged? Or is that part of the high treble? Im not sure what this does.
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I left that cap/resistor out on purpose. This is what the schematic should look like.
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what does it do? give you more treble?
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Probably. I left it out because Channel 1 did not have it.
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What does the filter after the first 12AX7 do? It almost looks like a fixed TMB tone stack.
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I don't know. I just call it a tone network and try not to think about it too hard. :icon_biggrin:
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I think its a treble bleed circuit.
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May be someone will be interested on this (part one)
Franco
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May be someone will be interested on this (part two)
Franco
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May be someone will be interested on this (part three)
Franco
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Thanks for doing that Franco. That 2 pole 6 position switch makes it easier to see what's happening. A Hammond 154G should work for the 9.5H inductor.
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If a Hammond 154G is used I'll try to add, in series to it, a 270R (5% or better 1%) resistor + a 500R multiturn trimmer (with the purpose to decrease Q), then adjust the trimmer at your taste
Franco
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is the inductor a big part of the sound? Can it be replaced with a cap?
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Yes and no.
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I think you need a footswitch to activate the tremolo. Is that right? is it hard to make the footswitch deactivate the tremolo so its always on or does that not make sense?
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According to the schematic you do need a footswitch plugged in to enable the tremolo. But, all you have to do to be able to use the tremolo without the footswitch is replace the footswitch jack with a switching jack such as Switchcraft 12A and connect the switch lug to the ground lug.
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I see thank you.
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BTW
The Rotary Selector Tone
https://el34world.com/charts/Schematics/files/Selmer/Selmer_rotary_selectortone_15w.pdf (https://el34world.com/charts/Schematics/files/Selmer/Selmer_rotary_selectortone_15w.pdf)
Franco
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In the schematic there is a .01 and then .047 in ()
does that mean either one??
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There is also a resistor that's marked 10K (22K). That crappy looking schematic shows .01 and 10K. Don't use the values in parentheses.
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I put together a layout for the board. its not as good as the ones you do. Any plans for a layout?
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I put together a layout for the board. its not as good as the ones you do. Any plans for a layout?
I'm working on one. About 70% done. I've included a 6 position rotary switch to handle the function of the push button selector and I dropped the magic eye to use a red LED as a visual indicator.
Show me what you have. Here's the schematic I'm working with...
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I put the schematic in kicad and then arranged the components on the pcb board with a ground plane on the back. Will probably just wait for yours because you know more then me.
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Here are my drawings. I've included drawings for your original request (no tone selector) plus drawings for a cheap rotary switch instead of the push button selector. I included my choices for iron. I've proofread these drawings several times but sometimes errors just hide in plain sight. LOOK FOR ERRORS!
I'm never gonna build this so if you run with it just know that you will be the first to finish. :icon_biggrin:
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Ok thank you. For the EM84 magic eye. it says not used but I still put that tube in there?
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You can if you want to. I also included a red LED that can be mounted on the front panel to be used for a flashing tremolo indicator. The magic eye looks a bit nostalgic. Some people like them. If you do use it, be sure to rotate the socket for best viewing angle BEFORE drilling mounting screw holes for the socket.
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But if I dont use it then I delete that tube along with the two resistors the cap and the connection and the connection to the 330k resistor?
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But if I dont use it then I delete that tube along with the two resistors the cap and the connection and the connection to the 330k resistor?
Correct.
Or you could install the circuit but just don't put a tube in the socket. The magic eye is monitoring the LFO tremolo signal. It is not connected to the signal path and will cause no effects.
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Ok what happens if you put the tube in and have the led?
it wont cause any problems correct? they just both will flash?
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The LED and magic eye will coexist happily together.
The LED simply flashes at the tremolo speed. So, it's an indicator that tells you the tremolo is on and you can see how fast it is. But the LED cannot tell you how deep the tremolo will be.
The magic eye has two green "lights" that pulse with the tremolo speed. But it is also sensitive to the depth/strength of the tremolo. Stronger tremolo will cause the two lights to actually pulse harder until they may even merge. This gives you an indication of how strong the tremolo will be. The depth pot will control the magic eye.
This video will demonstrate much better than words. The one on the right is the EM84
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And here's the amp. He turns the tremolo on at about 1:40...
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Is the purple wire in the following screenshot connected to the .068 cap here? Just want to make sure im reading it correctly. Also i assume its ok to move the other wire from the 220k to the .0047 cap as indicated with the arrow. It works better for my layout. Is that ok? its the same yes?
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Is the purple wire in the following screenshot connected to the .068 cap here?
Yes. Actually, there are two purple wires, one from the depth pot, the other to V3. This is labeled tremolo signal on the schematic.
Also i assume its ok to move the other wire from the 220k to the .0047 cap as indicated with the arrow. It works better for my layout. Is that ok? its the same yes?
That's fine. Both of those turrets are connected together with a jumper that runs beneath the node B cap.
It's good that you are studying and questioning the layout. Compare it to the schematic and you will have a better understanding how this amp circuit works. Let me know if you find any inconsistencies between the schematic and layout.
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Im putting mine on a pcb so thats how Im doing it. I have a ground plane on the back. I was thinking of puting a plane on the front for the nfb. That way I dont have to run a wire from the top if need be. Would you get any weird effects do you think from doing this? weird hum or anything.
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I have no interests or opinions about pcb design.