Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: adamG on January 22, 2020, 07:38:38 am
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Hi Guys,
This time I prefer to ask before building.
Recently, I've finished a kind of JCM800, 2202 - 20W.
The amp is really ok. On coming weekend I'll try it on loud.
But, I'd plan to add separate clean channel with TMB, Vol. and Master.
Could it be an option of Bassman with 12AY7 and IRF820 as CF?
What solution would you suggest?
Thank you in advance.
Kind regards,
Adam
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Hi Guys,
I've added a kind of Bassman channel with mosfet CF.
How about now?
Kind regards,
Adam
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The basic circuit is sound. But you need to fix one issue... the grid of the unselected preamp triode is left floating. That's not good as it kills the proper biasing of that triode and also allows the unused triode to generate noise. It's common to ground the unused grid. This requires a DPDT relay at the input. But you are using one pole to switch the output.
There are a few ways to address this issue...
1. Use an uncommon three pole relay.
2. Use two relays, one for the input and the other for the output. This would allow you to also ground the unselected output.
3. Use two 220K mixing resistors to mix the two preamp outputs to an single line. This frees up the relay pole to be used at the input.
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Thank you Steve, very much.
How about now, is it ok?
If I connected the mixing resistors within the circuit ok, maybe each of pots before them need to be lovered?
What do you think?
BTW, today I've tested the amp. I'm just delighted. I'm the admirer of D-Style amps, but this 2202 is far beyond what I expected...
I can honestly say, it's fantastic!
I may descript the amp as "acoustic". Strange!. I can hear the wood of my LP replica plugged into the amp.
Fanny thing that I can use the pots of the guitar. So, a musician can fully cooperate with the amp, without unnecessary pedals/effects. Just fingers, guitar and the amp.
First, with the pots high, you can hear the real rocking machine. Then, with the 3-4 pots adjusted down and the amp becomes something completely different...Fanny/strange thing is that with a hollow body guitar you can play...jazz. Yes, that's correct.
Therefore, the amp is "asking" for the extension of separate clean channel.
Anyway, the glory belonges to mr Leo Fender for his...Bassman!
Regards,
Adam.
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I can use the pots of the guitar. So, a musician can fully cooperate with the amp
:laugh:
as a non-musician, I've been trying without much success to convince guitar payers that's ALL they need :icon_biggrin:
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...I've been trying without much success to convince guitar payers that's ALL they need :icon_biggrin:
In my country musicians follow the custom of even 3-4 channels amps with 20-30 knobs, together with pedal board, of course.
Is their music nice? I'm not sure :rolleyes:
Regards,
Adam
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Please can standard schematic representation for switches be used? A layout is not a schematic. I can make a guess what’s connected to what with these blocks with 6 terminals but why force a guess when a standard symbol makes it clear?
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> guess what’s connected to what with these blocks with 6 terminals
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Yes, that’s what I assumed but weren’t you also just guessing with that, by using the rest of the schematic to provide a context to educate that guess?
Hence a block with terminals isn’t a helpful means to represent a circuit element. As brainpower that would more usefully used analysing the circuit is wasted on interpreting.
I accept that as it’s a dpdt switch (rather than a more complex circuit element), my point may be perceived as somewhat pedantic.
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Yes, that’s what I assumed but weren’t you also just guessing with that, by using the rest of the schematic to provide a context to educate that guess?
Hence a block with terminals isn’t a helpful means to represent a circuit element. As brainpower that would more usefully used analysing the circuit is wasted on interpreting.
I accept that as it’s a dpdt switch (rather than a more complex circuit element), my point may be perceived as somewhat pedantic.
On this forum and not only here, We use a kind of brief forms on schematics. For instance, block of six contacts represents DPDT, so forth...
However, you seem to be interesting in this amp. So, you can freely build it. It's simply "small" JCM800 with 20W output power.
Regards,
Adam
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Hence a block with terminals isn’t a helpful means to represent a circuit element. As brainpower that would more usefully used analysing the circuit is wasted on interpreting
:dontknow: Seems remarkably straight forward to me.
With respect, Tubenit
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Drawn like this, doesn't this switch have DC-voltage on it?
And in other position it lets that DC-voltage to pots too?
I think spdt switch connected after the treble caps would be better.
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One could do something similar with caps in series
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> brainpower that would more usefully used analysing the circuit is wasted on interpreting.
Agree. Especially since as VMS says, it looks like that switch near the Treb pot may be big trouble.
We also note that many of these switch-points could logically be SPST. I get it: SPST is not cheaper, DPDT has lugs to hang parts on.
OTOH: nobody draws DPDT toggle switch contacts like that. Everybody does relay-style with the throws together and the common off to the side. (That symbol I found was called "slide".) Except Reversing Switch is drawn like the DPDT lugs, but comes with a big X in the middle.
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...
:dontknow: Seems remarkably straight forward to me...
(https://i.postimg.cc/3WWX9Hb3/2020-01-27-3.png) (https://postimg.cc/3WWX9Hb3)
A block with 6 terminals, which yes looks like the base of a typical DPDT switch, could be pretty much anything with, err, 6 terminals. Including, as above, a ST3P switch.
And switch pole terminals aren't universally arranged between the 'throws', eg (https://i.postimg.cc/94nMQsPB/2020-01-27-4.png) (https://postimg.cc/94nMQsPB)
OK it's a relay not a manual switch.
A switch is the most basic circuit element with a pretty universal schematic representation.
Except here.
For goodness sake why the need to reinvent the wheel? Especially when the result seems, to me at least, kinda wonky. What's the advantage in doing it differently to / the big issue with doing it as per, the rest of the electronics world :w2:
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Why keep bitchin' about it? You always knew exactly what it was. Probably everone that looked at it also knew.
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I understand the point, and agree. I also think cats should be easier to handle when herded.
(https://i.kym-cdn.com/photos/images/original/001/346/869/db7.jpg)
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also think cats should be easier to handle when herded.
:l2:
Cats could care less what humans THINK :laugh:
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One could do something similar with caps in series
Jeff,
As usually, big thanks to you!
This is not an issue now. Later on, I'll go back to it. Of course, I'll try your suggestion, as well.
Best regards,
Adam
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Why keep bitchin' about it? You always knew exactly what it was. Probably everone that looked at it also knew.
I must agree with Steve.
My point, to persons interested in building of this amp, is just to do it instead of focusing on "details"...
The amp made me happy that I needed to share with everyone.
That is why I've got started an idea of the amps' extension with the clean channel...
I seem to be quite smart :smiley:
Regards,
Adam
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Hi Guys,
Here's few photos of the present, "standard" amp...
The only external servise are the silver faceplates, made by an engraver. The rest is my made.
Also, the string is of silver colour.
Main board is as pcb. Caps are Mallory 150. Electrolytic Nippon. Pots Alpha 24.
As said, the amp's fantastic, even after many weeks.
I think, the whole amps looks quite nice.
Have a nice viewing.
Kind regards,
Adam
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...Few more photos...