Jcm800 Downscale

[Enzo]

AH, threads are conversations - they go where they go. You can't herd them. If you want to discuss something differnt or more specific, threads are also free. Go ahead and start a new one. This may be going in a cocckeyed direction but if people are interested in the direction, it keeps going on. WWhen people lose interest in participating in a screwy thread, the thread grinds to a stop and fades down the menu list...all by itself.


About those thousand views. Ever google something and up pops some forum thread, perhaps even one on this forum? Google and its competitotrs have robots going all over the internet all the time. Down at the bottom of the page it says we have 9 members and 33 guests (or whatever) currently viewing the forum. Well a lot of those guests are bots. Those bots are how google even knows we had the discussion. A LOT of those views are from that.

[jazbo8]

AH, threads are conversations - they go where they go. You can't herd them. If you want to discuss something differnt or more specific, threads are also free. Go ahead and start a new one. This may be going in a cocckeyed direction but if people are interested in the direction, it keeps going on. WWhen people lose interest in participating in a screwy thread, the thread grinds to a stop and fades down the menu list...all by itself.


About those thousand views. Ever google something and up pops some forum thread, perhaps even one on this forum? Google and its competitotrs have robots going all over the internet all the time. Down at the bottom of the page it says we have 9 members and 33 guests (or whatever) currently viewing the forum. Well a lot of those guests are bots. Those bots are how google even knows we had the discussion. A LOT of those views are from that.

Ah, I see, learn something new, cool...

Jaz

[printer2]

I was thinking AB2 also for more output. How about a couple of Mosfets in source follower? A little messier than a transformer though. Any idea how much current will go into the grid?

I was thinking of properly researching the the JCM800 stages for the distortion then try to achive the same distortions with the little ecc86 tubes. I know that i will not get the output power, but i don't really care for that too much. I think it would be a neat practice amp that i can play ad a moderate volume.

So it was to end up as a practice amp at low voltage.

[Austin]

I was thinking AB2 also for more output. How about a couple of Mosfets in source follower? A little messier than a transformer though. Any idea how much current will go into the grid?




So it was to end up as a practice amp at low voltage.

Good question about the current, I may be wrong here but
I think the gist of it is that the current required to drive the grid ab2 increases pretty linearly the further positive the grid goes. So to get a ballpark figure on a tube with no info about it, one way could be to use 18vdc and briefly apply it to the grid with 40v on the plate and measure the current going into the grid with the volt meter. With two volt meters you could measure the change in current at the plate at the same time.

[Austin]

What about a circlotron with dual independant floating 40v power supplies as a way to cheat a voltage limit?

[soundguruman]

The more Austin talks about cyclotrons, the more I wanna make one.
That would b cool.
But I have to have, basically two power transformers. There's not enough current in the current power supply.
I wondering if I should stack em' vertically?

A. Two power amps. One is just to overdrive for tone, The second power amp is for PA, to drive the speaker.
B. A variable DC power supply, to regulate the field winding of a field coil speaker. The magnet on the speaker is adjustable to change the output volume.
Thanks to my old buddy Glen Kuykendall, for the suggestion...good one Glen.
C. Gotta have a 6J7 preamp tube. Grinds like a chainsaw.

http://www.angelfire.com/electronic/...irclotron.html

Does anybody still make 6J7? Now, I'm sad to see that one retired.

[J M Fahey]

1) from what I understood, the OP wanted (still wants) to build a real JCM800 preamp or something as close as.
***BIG*** limitation is 40V tops.
No "no more than 40V from ground" which would allow for +/-40V rails but no more than 40V between any 2 points in the circuit.
Which also forbids adding a little switcher just by a 12AX7 to feed it some proper voltage. Oh well.
Funny thing is, an amplification stage *can** be designed around a 12AX7.
But of course ..... you have to *design* it.
No need to go for tubes used in 1950's MIG fighters in Korean skies or stuff like that.
I'm amazed nobody in 108 posts printed a 12AX7 datasheet , took a pencil, ruler, a calculator, and designed a gain stage within the limitations.

2) Well, Florian should be doing this but just to kickstart this:

a) This is the 12AX7 plate curves graph.
No detailed explanation, I'm in a hurry, yet it should be self evident:

From datasheet:


I look at the 40V line (not my choice but, hey!, it's what we have)
Choose (going by the book to get reasonable voltage swing) half that as idle plate voltage: 20V.
Mark that too.
Look up (that's what graphs are for) what current will pass @20V plate voltage and with a reasonable bias voltage (my choice) which must be somewhat negative to allow for positive and negative signal swing.
Choose -0.5V . Among other things, because it's drawn and the first one below 0V.
Look up plate current under those conditions: -0.5V grid and 20V plate= 0.17mA (interpolated).
Now we know under which conditions our 12AX7 is working.

Let's see if we can build a useful gain stage around that:

b) load line inclination= DeltaV/DeltaI=20000mV/0.17mA=120000 ohms (closest standard value)
So we need a 120K plate resistor
Delta means variation or difference bertween two working points.

c) cathode biasing resistor: 500 mV/0.17mA=2941 ohms=2K7.

d) Gain: Delta Plate Voltage/Delta grid voltage
Grid can swing from 0V to approx -1V (extrapolated)= 1Vpp (peak to peak)
Plate will swing from 40V to approx. 4V (36Vpp)
So gain becomes: 36/1=36. Not bad, eh!!!

The graph details are here:


Well, Florian, you can pick it up from here and fill in the blanks.

Of course, I will be happy to be invited to your graduation Party, specially if you introduce me to Gretchen and her friends.

[Alan0354]

AH, threads are conversations - they go where they go. You can't herd them. If you want to discuss something differnt or more specific, threads are also free. Go ahead and start a new one. This may be going in a cocckeyed direction but if people are interested in the direction, it keeps going on. WWhen people lose interest in participating in a screwy thread, the thread grinds to a stop and fades down the menu list...all by itself.


About those thousand views. Ever google something and up pops some forum thread, perhaps even one on this forum? Google and its competitotrs have robots going all over the internet all the time. Down at the bottom of the page it says we have 9 members and 33 guests (or whatever) currently viewing the forum. Well a lot of those guests are bots. Those bots are how google even knows we had the discussion. A LOT of those views are from that.

[printer2]

Breadboarded a 12AX7 running up to 60V supply. Sounded real thin and buzzy to me so I did not suggest going that route. Mind you there are lot of valve casters out there with people happy with them.


German girls have nice arms (need them to hold up their beer).

[Austin]

Breadboarded a 12AX7 running up to 60V supply. Sounded real thin and buzzy to me so I did not suggest going that route. Mind you there are lot of valve casters out there with people happy with them.


German girls have nice arms (need them to hold up their beer).

Wouldn't a gain stage with a resistor plate load waste half of the already too low voltage and a choke keep the plate at nearly the b+? Seems much better given the low volts.

[J M Fahey]

@ austin. If you want to see it that way.
But the idea is to build something as close as possible to a JCM800 preamp, so I designed *working* conventional RC gain stage, within the limitations, of course.
@ printer 2. Agree.
But a JCM800 preamp, even fed 250V, *is* thin and buzzy
Just build one and feed it straight into a flat chipamp, no EQ tricks allowed, and then into a reasonably flat speaker system.

[jazbo8]

But the idea is to build something as close as possible to a JCM800 preamp, so I designed *working* conventional RC gain stage, within the limitations, of course.

I think we are being duped, why isn't the OP doing the design himself or even contributing to the discussion? Get someone else to do the homework for free, why not... Since he has not been back for awhile, perhaps he decided to do something else, so as far as I am concerned, it's time to move on, nothing to see here...

But a JCM800 preamp, even fed 250V, *is* thin and buzzy
Just build one and feed it straight into a flat chipamp, no EQ tricks allowed, and then into a reasonably flat speaker system.

The bright caps certainly add to the fizziness...

Jaz

[J M Fahey]

Well, the OP isn't asking us to do his homework, he simply placed here a legitimate doubt and asked for suggestions.
That's what this forum is for, and what most of us do.
Hope he keeps on, unless he has changed his project idea.
He *has* to submit a working one anyway, this one or any other, it's required by his University.
And it's difficult, but at least technically possible.
Let's say it's just on the cliff's edge.

As of the many bright caps, they are basically there to increase note definition and cut low frequency "mud", even with heavy distortion.
Edge rounding is provided by guitar speakers cutting frequency at 24 dB/oct (or more) starting between 3000 and 4500 Hz, OT transformer adding bandpass filtering, power amp clipping smoothing unbearable waveform peaks, 4x12" resonance peak providing thump and venue acoustics (and audience) providing further filtering.
That's why many *killer* (live) amps and pedals are dissapointing or buzzy at home.
Have you ever tried one of Jimi Hendrix's "weapons" , the Fuzz Face, straight into a Mixer or PC input interface?
UGH!!! is the understatement of the year.
Customers many times ask me "what is the secret of XXXX's sound?" or "which pedal gives me that sound?" or 100 other variations of the same.
The answer is **no single element**; results come from an interaction or careful tweaking of a 1000 factors, many of which are ignored by the very Musician who unconsciously uses them to his advantage.

[jazbo8]

JM,

Perhaps I am being too cynical... but the dialogue seemed to be rather one-way, good samaritans -> OP, not much the other way around, anyway... While we are on the subject, you are absolutely right about all the variables involved in making those iconic guitar amp sound, especially the big ones (imo), the OPT and speakers - those have great influences on the sound, yet much more time are spent on tweaking everything that comes before them, my guess it come down to economics, i.e., it is much cheaper to change some resistors, capacitors, may be even tubes than swapping OPT's and speakers as they are usually much more expensive and difficult to do on the whim.

Saw a video recently where Bruce Egnater was demonstrating some of his amps, where he mentioned that during the design process, all the amps get mate up to a switchable speaker bank, like playing with a very big tone control - once the best sounding speaker for the particular amp gets picked, it is then put into the production (I am sure after the bean counters crunch some numbers as well). That makes perfect sense for me, but admittedly very hard for a DIY'er or even small custom builders to do, since you need to have at least a dozen speakers of different sizes, cone material, timbre, impedance, etc. in order to provide a wide enough palette to select from. So most of us resort to Internet recommendations, which is often hit or miss - well you get what you paid for...

Jaz

[J M Fahey]

That's why I design and make my own speakers
Not to mention that the amps also get designed and made considering the available speakers.