Jcm 800 2204 build voltage question

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franksl
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Jcm 800 2204 build voltage question

Post by franksl » Mon Jul 13, 2009 12:29 pm

Hi,
First message for me...

I'm building a clone of a jcm 800 2204 50w head, at least for now, as this is my first tube project I wanted to start from this and then apply some mods.
I'm using power and output transformers from http://www.inmadout.com" onclick="window.open(this.href);return false; as this is in my country. In particular the power amp is model TA34M52.
I tried wiring the B+ secondary as the 2204 schem but ended up with too little voltage, so I wired like the 2203 schem (diode bridge) to double the voltage and have what seemed to me correct for output tubes.
The problem now lies in preamp tubes, as looking on the forum I noticed that usually people have smaller voltages than what I read for preamp tubes.

Here's my voltage chart:
Pins: 1,2,3,4,5,6,7,8,9
V1: 270, 0, 2.4, -, -, 315, 0, 3.46, - (B+ is 376V)
V2: 204, 0, 1.49, -, -, 392, 204, 214, - (B+ is 392V)
V3: 259, 13, 42, -, -, 248, 14.1, 42, - (B+ is 391V)

Pins: 1,2,3,4,5,6
V4,V5: 0, -, 482, 477, -32, 481

Are this voltages correct or should I change something?

Second question: I tried lowering the 10k cathode R6 resistor to increase gain (I would like to have more distortion), but just by lowering it a bit (shunting it with a 20k or 30k resistor) I get a highly clipped signal, something like a buzz tone, could this be due to wrong voltages?
Thanks,
Frank

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Re: Jcm 800 2204 build voltage question

Post by SDM » Tue Jul 14, 2009 9:05 am

2203/4 preamps drop less voltage on down to V1 due to the colder biased 10K cathode stage. Most amps here are 1987/59 amps so that's a small part of it. The larger part is that (going by your voltages) you went by the latter 2204 schem with 4 filter stages. That along with your starting B+ of 480V would produce voltages like what you are seeing.

So looks like nothing is wrong exactly (though a couple readings are a bit outside of expected, but not much), just a high starting B+ and the latter filtering circuit basically.

Some options that would be worth considering: If you added the fifth filter stage between the PI node and V2, as earlier 2204's had (for which you'd need another 10K 2 watt resistor and 50uf filter cap) , your B+ voltages on V1 and V2 will drop around 40ish volts.

In addition to that or instead, you could add a second 10K 2 watt resistor in series with the first one between the screen B+ node and the PI node. This alone would drop your V3, V2 and V1 B+ nodes down about 60V.

If you do both the above, V3 node would drop around 50 some volts, V2 and V1 nodes down about 90 volts from what you have now.

Would be a good idea to do one of the above here at least as V2 is living a hard life right now. 12AX7s have a max spec of heater to cathode voltage which you are exceeding right now (well exceeding for some brands). Easiest thing to do would be just to add the second 10K in series with the first after the screen node for starters (or just use a single 20K 4-5 watt resistor instead to = same thing), would put V2's cathode in a safer spot. See how it sounds from there.

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Re: Jcm 800 2204 build voltage question

Post by demonufo » Tue Jul 14, 2009 5:43 pm

franksl wrote: Second question: I tried lowering the 10k cathode R6 resistor to increase gain (I would like to have more distortion), but just by lowering it a bit (shunting it with a 20k or 30k resistor) I get a highly clipped signal, something like a buzz tone, could this be due to wrong voltages?
Thanks,
Frank
Nah, that's more likely down to the resistance. These things can get ugly below 4.7K. And bearing in mind that paralleling the 10K with another 10K will give you 5K, you've gone way below that by using a 20K or 30K.
I've got a 5.1K installed in mine currently - mainly 'cause I happened to have a nice NOS Pihers lying around!!!
So I like purple, okay!!!!!!

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Re: Jcm 800 2204 build voltage question

Post by franksl » Wed Jul 15, 2009 4:46 am

Steve,
Really thanks for the good info, the schems I used are these:
http://www.drtube.com/schematics/marshall/2204prem.gif
http://www.drtube.com/schematics/marshall/2204pwrm.gif

Are there any better schems you suggest?
Just to know if I understood well, you suggest to put another 10k in series with R29 right?
Do you know what the correct power transformer secondary voltage should be?

@demonufo
Pardon me if I'm wrong but paralleling the 10k with a 20k or 30k should give more than 5k total, am I wrong?

Thanks again,
Frank

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Re: Jcm 800 2204 build voltage question

Post by franksl » Wed Jul 15, 2009 9:07 am

Ok, I had a little free time to try putting a 10k in series with the other 10k between the power tubes b+ and the phase inverter, these are my new readings:
V1: 233,0,2.05,-,-,271,0,3.01,- (B+ = 323)
V2: 177,0,1.28,-,-,337,177,185,- (B+ = 337)
V3: 224,11.5,36.3,-,-,215,12.3,36.3,- (B+ = 336)
V4 and V5: same as before
Do these look better?
Thanks,
Frank

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Re: Jcm 800 2204 build voltage question

Post by demonufo » Wed Jul 15, 2009 5:46 pm

franksl wrote:Steve,
Really thanks for the good info, the schems I used are these:
http://www.drtube.com/schematics/marshall/2204prem.gif
http://www.drtube.com/schematics/marshall/2204pwrm.gif

Are there any better schems you suggest?
Just to know if I understood well, you suggest to put another 10k in series with R29 right?
Do you know what the correct power transformer secondary voltage should be?

@demonufo
Pardon me if I'm wrong but paralleling the 10k with a 20k or 30k should give more than 5k total, am I wrong?

Thanks again,
Frank
Yep, you're right. I really shouldn't give advice late at night! :lol:
So I like purple, okay!!!!!!

83.7% of all statistics are made up on the spot!

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Re: Jcm 800 2204 build voltage question

Post by Structo » Thu Jul 16, 2009 1:03 pm

10K and 20K give you 6,666.666 :shock:

10K and 30K give you 7,500 Ohms or 7K5 or 7.5K
Tom

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Re: Jcm 800 2204 build voltage question

Post by franksl » Fri Jul 17, 2009 5:50 am

I tried putting both a 15k and a 20k in parallel with that 10k but still get buzz and high frequency whistle (oscillation?) even at low gains, any tip?
Thanks,
Frank

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Re: Jcm 800 2204 build voltage question

Post by demonufo » Fri Jul 17, 2009 2:27 pm

Aaah, that may be a lead dress issue. A lot of JCM800's had a litle ceramic cap on V1 to combat oscillations On the later JCM800's (horizontal inputs) the cap is on the PCB.
Try a 100pF 1 Kv rated across pins 6 and 8 on V1. Or just poke the wires around with a chopstick to see if it goes away.
You may want to change the input leads for shielded cable too (grounded at one end only)
So I like purple, okay!!!!!!

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Re: Jcm 800 2204 build voltage question

Post by Structo » Sat Jul 18, 2009 12:41 pm

Does it squeal at all volume settings?

Do you have your grid stoppers on the board or at the socket?

Try them at the socket with shielded cable grounded at the jack end only.

15K and 20K give you 8,571 ohms.

Here is a handy calculator.

http://www.calculatoredge.com/electroni ... sistor.htm" onclick="window.open(this.href);return false;
Tom

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Re: Jcm 800 2204 build voltage question

Post by franksl » Sat Jul 18, 2009 1:12 pm

Really thanks guys!
I actually solved the problem like you suggested, using shielded cable for the connections between v1b grid and the gain pot and between r6 and v1b cathode.
I also had too long cables from board to v1a, so I made them as short as possible.
I didn't put grid stoppers on socket because they're now very close to the socket itself and I actually don't have squeal anymore, should I put them on socket anyway?
I'm actually doing some mods because I want this channel to be high distortion, now v1b handles correctly 5k as cathode resistor. I removed C4, C5 snf R5 and put a 68k resistor in place, I like the sound, it's a little darker and less harsh.
I also changed R15 to 68k and that gave a really good tone, what's the purpose of that resistor?
Thanks again!
Frank

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Re: Jcm 800 2204 build voltage question

Post by Structo » Sat Jul 18, 2009 1:56 pm

You don't need to move the grid stoppers if you have solved your instability problems on V1.

I don't know the component numbering system on your amp so somebody else will have to help you with that.
Tom

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Re: Jcm 800 2204 build voltage question

Post by demonufo » Sun Jul 19, 2009 7:56 am

franksl wrote: I removed C4, C5 snf R5 and put a 68k resistor in place, I like the sound, it's a little darker and less harsh.
Personally I got tired of this one after a while. I did like the sound, but anytime I ever set the gain above 8, the sound would turn to buzzy fuzzy mush with little definition and absolutely no dynamics and headroom.
So I like purple, okay!!!!!!

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Re: Jcm 800 2204 build voltage question

Post by franksl » Mon Jul 20, 2009 3:43 am

Demonufo, are there any mods you suggest me? Consider that I'm looking for distorted sound as I play metal with detuned guitar (2 tones down).
I'm trying some of the mods on 'the ultimate jcm 800' article, are there any better?

@Tom
I'm referring to the schems I posted in a previous post on this thread (those are the standard 2204 schems).

Thanks,
Frank

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Re: Jcm 800 2204 build voltage question

Post by demonufo » Mon Jul 20, 2009 3:15 pm

I'd stay away from Tone Lizards suggestions myself. They only make it worse after you've done the first one.
His suggestion for extra bass is an absolute no no. If you need more bottom end that doesn't go all loose and flubby (like his suggestion) then lift the purple NFB wire, and put a .0033uF cap in line between the wire, and the point it was attached to.
There is another point on these where many people like to piggyback one of the resistors for a .68uF, but for the life of me I can't remember which. The point of this one is to create more distortion in the mid-range, but for de-tuned metal and doomy stuff, this might not be where you want to go with it.
I've tried a few more, but didn't like the results. Any other mods I've tried mess the amp up too much for my liking, but I'm more classic rock without hi-gain. Strictly a guitar-lead-amp kind of guy.
Try putting R5 back to stock and use a 250pF on C5 if you can't get by with the 68K on R5. I'm sure you'll discover the mushiness of that one after a while.

Most importantly, any mods you try, try to live with them for a while and truly get to know the effect they have. It helps to be real familiar with an amp before you start changing things.

All in all, great amps though. Good luck with it.
So I like purple, okay!!!!!!

83.7% of all statistics are made up on the spot!

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