Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: liagasg on January 11, 2012, 03:08:04 am
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Hello there,
I am thinking of building an amp based on JCM 800 2204. As a matter of fact I want to build a 2 channel amp.
One channel for the clean sound with it's own tone and gain control , and a second OD channel with its own tone and gain control. And a footswitch.
I don't know how to do it, since I am new in amp building.
What I DO NEED, is some info on channel switching. I 've been digging the forum for 3 days, and came up to the conclusion, that channel switching is done using relays. But haven't find any schematics so that i could understand how this is done.
So please enlighten me if you can.
Attached is the schematic of the 2204 amp. I will remove the 2nd input jack and keep only one.
Then I will use one channel employing V1B => Gain + Tone control (these do not exist on the schem). That would be the clean Channel.
I will use a OD preamp employing V1B + V1A=> Gain + Tone control (not the ones of the clean channel). That would be the OD channel.
Do I have to build 2 preamps? One for the Clean, one for the OD?
Or can I engage/disengage the V1a to create the distorted preamp and drive it to a separete tone stack? Is it possible?
Do I have to build 2 output stages, one for each channel?
Generally, a schem of a two channel tube amp, using relays for channel switching would be a good starting point, so if anyone can help, I would appreciate it.
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Here are a number of ideas for Marshall style topology with a switchable OD channel also. These ideas would probably require some amp building experience and/or patience and/or a willingness to persist in tweaking it to what you want.
http://www.el34world.com/Forum/index.php?topic=10776.0 (http://www.el34world.com/Forum/index.php?topic=10776.0)
And then there is Hoffman's hotswitch which is straightforward and simpler. You could simply add a volume and 5E3 tone control to that.
http://www.el34world.com/projects/hotswitch.htm (http://www.el34world.com/projects/hotswitch.htm)
And how to add a relay to make it footswitchable: http://www.el34world.com/projects/relay_switch.htm (http://www.el34world.com/projects/relay_switch.htm)
Or you could use a Weber VST relay switch: https://taweber.powweb.com/store/chansword.htm (https://taweber.powweb.com/store/chansword.htm)
And here is a Plexi 50 with a switchable OD that has a volume and tone control.
You have lots of options. with respect, Tubenit
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Hi buddy, and welcome to this forum. It seems to me that you are trying something a bit too challenging, according to your skills, experience in building amps, like Tubenit said.
If I were you, I'd start by building a Champ, with a master volume, to learn how things are related in a tube amp. Just building a quiet amp is a challenge and lots of variables are involved: lead dress, heaters, grounding, shielded cables, voltages, just to name a few. If your first build is too complicated, it may end up in a box , cause de-bugging it will be too complicated for your knowledge. BUT, I am not you and if you go ahead with this project anyway, we'll try to help you as much as we can :icon_biggrin:
Colas
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I would like to thank you for your responses guys :smiley:
I've been building guitar effects pedals for about a year, and I want to build an amp too. I do not have any experience, but I am good at following instructions in "step by step" mode. Of course it is "mega challenging for me", but I 've got a wild card : A very good friend of mine who's been building hi-fi tube amps. He will help me too, so I am not alone on that.
I do appreciate your feedback.
P.S. I am really happy I'm a member of this forum.
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Don't let us scare you away--but before you jump in you need to do some homework. Buy this book--it has a great section on switching and various preamp topagraphies: "Modifying and Custom building Tube Guitar Amps" If you can solder you can do this but mistakes are expensive. http://www.londonpower.com/catalog/product_info.php?cPath=3&products_id=3 (http://www.londonpower.com/catalog/product_info.php?cPath=3&products_id=3)
And here is my favorite right out of the book. You can build it with a switch first and tweak and then then add relay or solid state switching. Read the book--its expensive but it is worth every penny. Jim
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+1 for Jim's advice. I've build that preamp into a single-ended power amp with good results.
Here's a relay circuit I've used many times with success (I think it's from the Marshall Silver Jubilee, but I'd have to double-check to be sure)
http://i40.tinypic.com/29lzd6p.jpg (http://i40.tinypic.com/29lzd6p.jpg)
-John
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My main problem is that i have no idea how relays work. I have to dig more into it.
@JIM,
That's a design I was thinking about, but I think I'm missing something!
In the clean channel, the signal goes from A1 to A4, and with the use of the switch the signal goes from A1 to A2, A3 and then to A4.
There should be the phase split and after it the final stage (power amp).
Am I right (God I want to be!)
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Yup, you got it right. What is shown is only the preamp section. It would be followed by a phase inverter in a push-pull amp, or straight to the power tube in the case of my single ended version.
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Thats cool (the fact that I've understand it)!
On the attached file, where does the signal go?
One path is from V1a to V2, then to the 1M (master volume i guess) and then FX loop, to the phase split etc.
The other path goes from V1a to V2 then to V1b , then to Mater volume then FX loop, to the phase split etc.
I guess in the attached schem, the 2 channels share a common tone stack and master volume. Am I right?
What I would like, is to have separate tone stacks and master volumes.
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I use the Weber DPDT relay board. They're only $10 if you solder the relay to the board (easy); $14 if they do it. You have to use a plastic Marshall style jack for the footswitch because the relay runs off the heater circuit and must be isolated from the chassis. They take about the same real estate as a preamp tube (plus the area left around the tube). I've used 15 or so and never had one fail. I thought one did once but it turned out to be something else. It's here:
https://taweber.powweb.com/store/chansword.htm
I just mount it in line with the tubes at the point where the relay enters the circuit. Easy and cheap.
Dave
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I guess in the attached schem, the 2 channels share a common tone stack and master volume. Am I right?
What I would like, is to have separate tone stacks and master volumes.
Look carefully, please.
The clean channel has it's own tone stack and master volume.
When you load the clean channel into the Overdrive channel, the clean master volume is OUT of the circuit and not used by the OD channel. So "no" the two channels have separate master volumes and do not share a master volume.
The clean channel tone stack is still prior to the OD tone stack, but the OD has a separate "quasi-5E3" tone stack also. This approach works quite well and I've used it a number of times with success. Functionally, the clean tone stack controls only the clean ........... AND the OD tone stack (which is separate) controls the tone coming from the OD when it is engaged behind the clean channel. Functionally, they are separate tone stacks.
I'd like to point out that you specified an amp based on JCM 800 2204
Show me the Marshall topology and cathode follower in the London Power design?
With the attached schematic, look at the triodes in the preamp & OD which are re-labeled so hopefully you can follow them more easily.
With respect, Tubenit
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My main problem is that i have no idea how relays work. I have to dig more into it.
.
Relays are quite simple. Apply power to the coil and it energizes closing and/or opening a set of contacts that current can pass through.
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The clean channel has it's own tone stack and master volume.
Yes, that's obvious. After the V1a, there is a 1M pot (guess it's the gain control) that goes to V1b and V2a. Then there is a tone stack and a master volume.
When you load the clean channel into the Overdrive channel, the clean master volume is OUT of the circuit and not used by the OD channel. So "no" the two channels have separate master volumes and do not share a master volume.
OD's master volume is the 500K pot before the relay connection? And the 1M pot (labeled tone) controls the Bass?
The "quasi-5E3" is a term I don't understand :(.
I was expecting to see a bunch of pots on V2b just like after V2a. I also don't know what the IRF820 is.
I also didn't get how the gain controls work. Ok, the 1Mpot after V1a is the clean channels gain.
But when V2b is in the circuit, the 250Kl pot controls the OD gain. Does it throw the 1M pot out? I can't see how this is done.
What I want to build in a 2 channel amp, is to have 5 pots per channel. Gain-Treble-Bass-Mid-master volume.
If that's depicted on the shcem (but i can't figure it out), then that's fine! By studying it, I will figure it out!
But if not, then I must do the modifications.
I am sure I've been asking a "3 years old kid's questions", but I am trying to understand. Creating a layout from a schem is easy, understanding the circuit is the hard thing.
Thank you ALL for your feedback, but I still have questions..Hope I won't give up trying.
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This is a way for homemade relay board
you can build it with all eyelets
or eyelets for connecting the relay and turret for connecting wires and other components
Dumble builders rule
One thing you don't say is if you want
two inputs with two separated preamp to be switched into a single Power Amp
or
one input switched between two separated preamp that than are switched into a single Power Amp
Kagliostro
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Because you originally specified a Marshall type amp, I went with a design that has three 12AX7 type tubes. The IRF820 is an inexpensive mosfet that works wonderfully as a cathode follower. NO tone loss from it. I think it sounds as good as a 12AX7 triode in the cathode follower position. This allows you to mount it on a layout board, it's inexpensive, it saves space, & it sounds great.
It is also because you specified a Marshall type amp, that I did not draw up an OD with two gain stages which would involve adding a 4th 12AX7. Plus, I have found that having a cathode follower in front of a passive effects loop seems to help the tone of delay pedals, IMO.
Quasi-5E3 tone stack simply means it has some similarities to a 5E3 Deluxe tone stack.
The reason I did NOT originally draw the OD channel with the 3 knob tone stack is that I have built one and did NOT like it at all! I could NOT get a good tone out of it. I think Geezer did something similar and did not like the 3 knob tone stack on the OD either. So I didn't send you that direction because I viewed that tone stack to a failure in getting the tone that I wanted. On Dumblish style amps this 3 knob tone stack is called an HRM. Many of the Dumblish clone builders have complained about the HRM tone stack being difficult to dial in good tones also. Some people like it. I don't.
In contrast, the Quasi-5E3 tone stack sounds great to me. I think it allows for a very transparent OD tone that is easily shaped.
Since understanding the relay switching on the clean and OD is a struggle for you, I have drawn it out & hope this is understandable? I don't know how to make it clearer?
You gave me some original guidelines: Somewhat novice to amp building. Want a Marshall type amp. Want an additional OD channel. Want
seperate clean and OD gain & tone controls.
I took all that into consideration when I drew up an amp for you. I think what I have suggested to consider meets those guidelines. There are other ways of approaching it also, but this one made the most since based on my experience.
With respect, Tubenit
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I have absolutely NO agenda for you other than that you have an amp that you enjoy ALOT that you are able to successfully build. That's it.
:thumbsup:
This may or may not be a good amp for you? I do think if built and tweaked that it will be an excellent sounding amp.
IF you decide to try this idea, I will also draw up a layout board for this amp design that you can compare with your own layout board.
With respect, Tubenit
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Tubenit, in the last drwing, V1a and V1b have two different b+ supply ?
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Hi Tubenit
The IRF820 is an inexpensive mosfet that works wonderfully as a cathode follower. NO tone loss from it. I think it sounds as good as a 12AX7 triode in the cathode follower position. This allows you to mount it on a layout board, it's inexpensive, it saves space, & it sounds great.
That was a great idea, since when the mosfet in that position appeared for the first time in your schematic I'm thinking to thank you for sharing your experience
Many thanks Tubenit
Kagliostro
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Off topic question about relay board. What eyelets are you using and is that 1/8" board material ? Thanks, Jeff
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The board is 2mm and eyelets comes from shoe repairer industrial shop :icon_biggrin:
(in my case)
Kagliostro
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Look at pages 1 through 5 from Dougs library on building a turret board for the relays he sells with the turrets he sells.
http://www.el34world.com/projects/relay_switch2.htm (http://www.el34world.com/projects/relay_switch2.htm)
Also Mouser sells a good number of different sizes of eyelets and turrets.
Brad :icon_biggrin:
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V1a and V1b have two different b+ supply ?
They should be the same. I grab old schematics and quickly edit them to do these illustrations. I think most of the time, experienced builders catch an error like this (as you did) and simply edit it to make it correct.
I'd use node E for V1 and node D for V2
With respect, Tubenit
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Tubenit, many thanks for your patience. :worthy1:
Thanks for your drawings too. That's very kind of you. I own you a favor for that.
I had already figured out the way the relay drives the signal. Your notes just acknowledged it.
I figured out a design and I would like your opinion about it. It's not a technical one, but it's most of what I am thinking of.
I thought not to use the MOFSET, but an extra tube.
What do you think? Could that be applicable?
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You show plate-drive tone stacks whereas the JCM is driven from the cathode. Nothing says you can't make it plate driven, but taming four cascaded gain stages might be ambitious for a first build.
The (traditional) "Presence" control is a power amp feature, so you wouldn't have one for each channel as you have shown.
For a clean channel with this topology you might look at the Trainwreck Rocket design, which is based on the Vox AC30 top boost. It would make a nice clean channel. It has a cathode driven tone stack as well. The traditional design only has Bass and Treble, but adding a Mid pot would be trivial.
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@Boots Deville
Thanks for the info concerning the presence control.
I don't even like it on old Marshalls!
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IF you add a 4th 12AX7 .............
Sure that will work. I personally would have the clean and OD share the same initial gain pot and switch after the gain pot. .
I would advise a drive pot in the OD or at least a trim pot that you can adjust instead of using a voltage divider. But that is a personal preference thing for me. That would increase your OD options, gain wise
However, what you have done should work OK. Be sure to have a coupling cap between the V1a plate and the relay switching.
One presence pot for both channels as was mentioned.
The second DPDT relay in the example is simple an illustration to show how the switching works. There isn't really a 2nd DPDT.
With respect, Tubenit
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Hi guys, I put this together based on Sluckey's Plexi/JCM800 amp and thought others might have some ideas that could be added. The plexi has Doug's hot switch hard wired to give it more the overdrive effect for the OD channel. I even kinda like this one myself.
Welcome liagasg and enjoy the ride. :icon_biggrin:
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Using your approach, here is a schematic to consider. Not sure of some of the values or whether you need an additional coupling cap after the DPDT relay before the next gain stages.
There is an editable schematic attached.
With respect, Tubenit
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Tubenit, on the last schematic, what is the 500K pot after the V1B for?
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what is the 500K pot after the V1B for
To adjust the amount of gain to the next gain stage.
It allows you more options to dial in the tone that you want. For example, you might find that you like the first volume pot on "10" & the 500k pot on "3" ............. or you might find you like the volume pot on "4" and the 500k on "8".
With respect, Tubenit
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Many thanks, Tubenit!
Is the 10K pot on V4b the "presence" pot? On Marshall schems, it's usually a 22K. What's the pros/cons of using the 10K? Better sounding? That halves the cut off frequencies, right?
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On Marshall schematics, there are 50+50/500V filter caps. On the schematics attached so far there are only 50uf/500V. What's the difference?
Also, how do I calculate the power supply that will go from the PT to the tube's plate?
Suppose that I will have a 300-0-300 TX.