#1
Im building a 50 watt jcm 800 (2204) and have a question about a couple of 5k6 resistors that go into pin 5 of the 2 output tubes (V4 and V5). Some schematics/diagrams have them and some do not such as:

These have:
http://mhuss.com/MyJCM/JCM800_2204.gif
http://ceriatone.com/images/layoutPic/marshallLayout/JCM800_2204Ceriatone.jpg

Theses do not:
http://www.drtube.com/schematics/marshall/jcm800pw.gif (the top one of the two)
http://metroamp.com/wiki/images/e/eb/2204_50W_MV.jpg

I think this has to do with the mod where you add resistors to the screens of tubes in order to get more gain, but I am not sure. If someone could clear this up for me I would appreciate it.
Gibson SG Standard
Ibanez S2170FB
Peavey JSX
Marshall 1960A
TEXAS A&M
Last edited by sacamano79 at Aug 12, 2010,
#2
The screen resistors deal with gain, but more importantly the way the amp distorts. 1500 ohm resistors are screen resistors. The 5600 ohm resistors are grid stoppers, and can influence treble response, as well as prevent blocking distortion. Larger grid stopper values can help smooth out a power amp. To understand this better purchase Merlin Blencowes guitar preamp book, it touches upon the topic pertaining to power amps iirc in addition to explaining the effect.

The third link, DRtube, includes both screen and grid stopper resistors.

Here's a link to merlin's site:
http://www.freewebs.com/valvewizard/

Edit: In the last link, it looks like the screens are fed from the output of the choke. I'm not sure what the circles to the bottom left are about, I've never seen them before, the one marked mains filter is just a filter to assist in removing hum caused by rectified AC.
Last edited by blandguitar at Aug 12, 2010,
#3
Either you were looking at the bottom of the two schematics on the third link or I just cant see the 5600 ohm, are you sure its there?

And the 5600 is necessary isnt it?

edit: The circles in the bottom left are just caps
Gibson SG Standard
Ibanez S2170FB
Peavey JSX
Marshall 1960A
TEXAS A&M
Last edited by sacamano79 at Aug 12, 2010,
#4
I didn't differentiate them the first time around. The top one is just the power amp, or so it looks, the grid stoppers may have been included on the part of that schematic that had the preamp. But grid stoppers are typically essential, you may be able to get along without them for the power amp, but I wouldn't recommend it at all.
#5
bottom left circles are the filter caps, in a can.

5k6 resistors are grid stoppers. all tubes should have them on each control grid. it's very nice. if the schematic doesn't have it, add it? anything from 1k5 to 10k would work.

screen resistors drop voltage into the screen grid. what this does, is increase/decrease the threshold of input before signal goes into clipping. higher screen voltage = more signal swing input before the signal starts clipping. so if you have lower screen voltage, you have more headroom. you'd use 100R if you want more headroom from the power section (although 100R is pretty low, you need a decent resistor) and you can put up to 1k5 (maybe more, but i go for 470R or 1k in my amp) in there.

there are something called VVR (Variable Voltage Regulators) that controls the screen voltage from max to minimum to control the amount of poweramp headroom. turning it down (less voltage) distorts the amplifier more, but reduces the volume too.
Call me "Shot".

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#6
This is going to sound very ignorant but what is with the 'R' at the end of resistor values?

edit: and just to make sure, when you say grid resistors, you are talking about the ones going into pin 4 correct?
Gibson SG Standard
Ibanez S2170FB
Peavey JSX
Marshall 1960A
TEXAS A&M
Last edited by sacamano79 at Aug 12, 2010,
#7
R at the end means ohms. at least in text, 100R = 100 ohms. 10k = 10k ohms. ya... i think R looks like the ohm (omega) greek letter, so i guess somebody used it?


control grid is where the signal enters the tube. this one is generally called "grid". it's there in pentodes, and triodes like 12AX7.

screen grid is between the plate and the control grid, it gets higher voltage, and it controls the headroom. this one's called "screen" most of the time.

supressor grid is usually connected to the cathode by an internal connection, or just connected in the wiring. this are present in pentodes, and tetrodes that call themselves tetrodes because they have an internal connection between the supressor grid and the cathode.


pin 4 is the screen. grid (control grid) is pin 5. and btw, did i say grid resistors? i try not to say it because it could mean two things; a screen grid resistor, or a grid stopper (resistor)... sorry if i did.
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#8
R is short for resistance, which is measured in ohms.

The control grid is called that because its voltage widely controls the output voltage, which is also dependent on the load line chosen.

The supressor grid is a grid that collects secondary electrons, electrons that bounce off of the plate, and should never be at a higher voltage than the plate (anode.)

The screen grid squeezes the electrons into a smaller, more concentrated, path. This allows for higher amplification.

Tetrodes aren't really used in audio because of the high distortion they naturally have. A pentode allows much more control over harmonic distortion than a tetrode.
http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm
Last edited by blandguitar at Aug 12, 2010,
#9
100R is too small for a JCM800, your tube's wont last long with 100R. You want 1K 5W.

Modern production EL34s are incredibly weak, you need to be careful with them.