Opinions on this topic idea?

Mbira · May 18, 2005

I've been working on some simple tube guitar amp stuff. I know that there are lots of forums dedicated specifically to guitar amps, but the people here are so knowlegable, that I thought I'd post this idea. As a newbee in all this and someone that treats this forum as my main source of education, I would love to go deeper into specific circuits I am working on. I think it'd be very cool to look at a schematic like this:here

and really go through it step by step to understand it. I know this may be tedious and even boring for the gurus here, but I figured I'd throw it out there as an idea.

I will start the questions out by asking how would one determine the wattage of the 100R resistors coming from the "B" into the grids of the EL84? How do you do this just from the schematic? Is there a "shortcut" method other than determining the draw of each component? Then the next step would be what is the function of those resistors?

My thought is to make this a slow process of really trying to understand this at many levels and get away from the "paint by numbers" stage that I am at now.

Cheers, and if this is a stupid idea, well I'm open to that too.

Joel

Gus · May 18, 2005

I would maybe do it a little different. A 2/3 triode pre fender,marshall or vox tone stack

Then a cathodyne PI into the 84s Some/most 84s sound like crap when over driven at the grids. Look at early fender deluxe schematics for some ideas.

You need to think about how the distortion comes up with levels.

Do you want all the stages to kind of track?
do you want the output then the PI then the pre?

the way the stages "come up" to distortion/overdrive and the filter networks between them are a BIG part of how good a guitar amp sounds.

Big HINT how the HP filters are set make BIG different.

VacuumVoodoo · May 18, 2005

Regarding the resistors from "B" to output tubes' screen grids: a primer here
Lots of more good basic tube info on that site.

Mbira · May 18, 2005

I realized I didn't mention what the amp is. It is a Matchless "Spitfire". Hey Gus-thanks for the thoughts. As for my sort of "origional" thoughts on this thread, would you suggest making this a discussion of a different schematic, or sticking with this one and going into more details about your thoughts? I'm a real newbee. I don't even know what a "PI" is. I have seen that the tonestack is quite different from the Fender, and I can say from powering this up-even though I am still tweeking it- it seems a little too bright to ma and I havn't had luck dialing in a good tone yet.

Thanks for the link vacuumVoodoo, I'll check it out.

Joel

Gus · May 18, 2005

I think I know who drew that schematic. I don't understand the matchless stuff the 2 el84 amps use a 4K primary like the 4 el84 models. Some people like the 2 el84 ones wired to refect a 8K primary load. This is from people I know.

I did not know you finished it. The cool thing with simple amp circuits like that one is you can change parts and sometimes hear the changes.

I am not saying it is not good it is just another way of using that amount of tubes.

That circuit looks like it will work well with overdrives and boosters effects very well

Try mallory caps PETs # 150s IIRC and ceramic class 3 caps and other types in the tone section and other sections. The PI is the phase inverter the two tubes before the output tube. Google Speedracer's site Obsolete electronics good article on the schmitt PI.

Hang out at Ampage.org. Check Blueguitar.com. R.G.'s site geofex.com. The ax84 site etc......

Use a Nice 1X12" or 4X12" speaker with that circuit you might be suprised

PRR · May 18, 2005

> how would one determine the wattage of the 100R resistors coming from the "B" into the grids of the EL84?

You can't.

Oh, you can take the plate and screen curves and plot all possible loadlines and integrate the power dissipation. And hope the tubes you have are reasonably close to the published curves.

For a simple pentode, the screen current tends to be a quasi-fixed fraction of the plate current. Down to fairly low plate, and then screen current soars.

In advanced power pentodes, particularly the grand-daddy 6L6, the field beam-control electrodes ideally draw no current at all. They "hide" in the shadow of the control grid or just at the edge of the beam, where electrons miss them. But reality is complicated and chaotic. On signal peaks, the screen current can go very high.

Actually... I don't see why you'd bother with 100Ω resistors on screens. They can't affect the sound or protect the tube. For bigger tubes in abusive duty, you'd use 470 or 1K resistors, not 100. 1K has little effect at 10mA-20mA typical current, but will dip the screen voltage if it starts drawing 50mA-100mA as the plate bottoms-out.

I'm not going to look-up the book-value of 6BQ5 screen-current for you. But say the book has a similar condition and shows 10mA per screen. Ohms' and Watt's laws quickly tell you the 100Ω resistor will dissipate 0.010 Watts. Since 1/2 Watt resistors are cheap, and give a factor of safety of 50, I would not bother calculating any further.