Is this possible?

[daz]

No, but i DID use a matchless design feature that now has the amp soundingb pretty much perfect. The cut control. Seems to sound similar to NFB with a presence pot, but between the cut pot and the tone section i can dial in all sorts of tone and the bass flab and hard mids are gone gone gone ! Amp sounds phenomenal.

[Paul P]

Well, all I can say is congratulations !

I've been following your efforts with interest. I was beginning to lose hope
there for a bit. Having myself been chasing a few things that I had trouble
putting my finger on I can sympathize with what you must have been going
through. It's hard to sleep when something is unresolved.

I hope your newfound tone remains as good as you currently think it is.

Paul P

[daz]

Thanks Paul. And yes, it will ! I've been playing the thing for hours and i'm just blown away. I've owned a number of what i felt were very good sounding marshalls, but this thing blows them all away....literally. The only thing is the volume from 2 EL84's just isn't enough for live use because the cleans i get when i turn the guitar down start to go away very soon as i roll the master up toward gig levels. so i'm about to start a new build with the same pre and PI but a 36 watt/4 EL84 PA. I also may consider EL34's which i prefer, but then the volume of every 50 watt marshall i've ever owned was the opposite....too loud. But i love those tubes more than any other output tube.

What just amazes me to no end is how that cut control alone changed the amp completely, giving it the sound i wanted and a lot more. I don't really understand the theory of a pot and cap between the grids, but it's incredible how many tones i can get between that and the tone stack. add to the ability to fine tune the tone to exactly what is in your head the most harmonically complex distortion i've ever had in an amp, and you can see why i'm so thrilled. (not to mention relieved after all those tedious hours !)
But what puzzles me is how unlike all the filters and other tweaks i've tried, this is the only one that totally changed the amp's character. it didn't just add and subtract highs. thats the least of what it does. It totally smoothes out the amp even at high enough treble levels to be quite bright. removed the pervasive loud harsh mids and tightened the lows. Makes it breathe, and the harmonics just drip from it unlike any amp i've owned. All from one pot and one cap. Pretty interesting stuff........just wish i understood why.

Oh, and this I think really shows that the amp is now *right*........i was finally able to remove that 220k on the gain pot and like it better w/o !

[Paul P]

That's an interesting way of doing things that seems to be present in most Matchless
amps. Some only have one pot for a Master volume, some only have the
pot and the cap for a Brilliant control, and some have both Master and Brilliant.
The Master shorts more or less of the two sides of the push-pull together
which will act like a master volume and the Brilliant does the same but only
at certain frequencies determined by the RC of the pot and cap. Pretty neat.

Is this unique to Matchless amps ?

Paul P

[daz]

That makes sense. Now it has me wondering about the master. I may have to try that. I tried a PPIM but at that time the circuit was seriously flawed and it was slaughtering the PI w/o a pre PIM. This one is so simple i may as well give it a go.

For the first time i fired it up after a nites sleep and was still thrilled with the tone. Thats the true test. Fresh ears tell no lies !

[Paul P]

I have a feeling that the cap + pot is only targeting one particular slope
because they're in series and the pot is only adjusting the amount of
reduction ? Matchless must have been going after the same sort of
tone as you are.

The master volume is really neat in its simplicity. It works by cancelling
the two opposite signals more or less by how much you let them reach
each other. I wonder why other companies didn't do this. You save a
wire if nothing else.

Glad you still like your tone.

Paul P

[daz]

i was wondering the same thing....why bother with dual pots and more complicated wiring when a simple pot connected to to places does the same job. I'll know soon tho, as i'm gonna try it today.

[bluefoxicy]

i have tried various cathode resistors and i'm sure went too low at one point. (120 to 150 R) But i'm unsure as to how to choose the right resistors for the cathode and screen and whether my plates are too high. (355v)

Biasing tubes and setting up plate voltage is no problem, just deadly as hell if you mess up.

http://www.diycustomamps.com/valvejunior.htm

This tells you how to bias an EL84 (Epiphone Valve Junior output); it's treated like a specific problem but really it's pretty general. Read it, find the data sheet for your tubes, and adjust. i.e.

http://www.firebottle.com/ampage/td/vtd6ca7.html

EL34, 25 watt plate dissipation, bias it to 25W instead of 12W. Notice the pin-out is different; look at the EL84 pin-out and match it to the EL34, then change the pins in the VJ biasing instructions to match, calculate your numbers. Using a different tube? find its data sheet.

Don't use a fixed resistor, use a pot for each tube if you can. Instead of a fixed resistor, do a low (say 32 or 47 ohm) resistor into a 250 or 500 ohm pot. Before the resistor, solder the wire to a metal bolt; after the pot, solder the wire to a metal bolt. Slide these bolts through rubber grommets insulating two holes side by site, double-lock with nuts.

Calculate the resistance R for your bias. Put an ohmmeter (with amp OFF) across the two bolts. Adjust the pot until the ohmmeter reads R. You're done.

[daz]

Thanks, that looks interesting. I will have to read it later but i appreciate it. The amp is a EL84 based 18 watter.

[bluefoxicy]

Thanks, that looks interesting. I will have to read it later but i appreciate it. The amp is a EL84 based 18 watter.

Then follow the same process per tube.

EL84 puts out 5.7W max SE class A, or 17W Class AB push-pull, so I assume you're using a push-pull 2 tube design. Some people get creative and use one bias circuit for all tubes; I've seen it recommended to use individual bias pots per tube instead.

http://www.firebottle.com/ampage/td/vtd6bq5.html

[daz]

Yeah, AB. I didn't realize tho that i could get 17 max out of them. If i go past 12 will the tubes wear far quicker? According to a online calculator i was dissipating more than 12 watts, so i started trying resistors till i found one that calculated to just under 12. Guess i was wasting my time. More importantly, i would like to get as much clean power as possible out of the thing, so now i'm wondering if i should go for more than 12. If max is 17, what if i set it at 15? A few more watts might just keep it a tad cleaner at the volumes where it's now too distorted. Tho i don't want to eat tubes.

[Bruce / Mission Amps]

Yeah, AB. I didn't realize tho that i could get 17 max out of them. If i go past 12 will the tubes wear far quicker? According to a online calculator i was dissipating more than 12 watts, so i started trying resistors till i found one that calculated to just under 12. Guess i was wasting my time. More importantly, i would like to get as much clean power as possible out of the thing, so now i'm wondering if i should go for more than 12. If max is 17, what if i set it at 15? A few more watts might just keep it a tad cleaner at the volumes where it's now too distorted. Tho i don't want to eat tubes.

You might be confusing a DC rating with power output.
An single EL84 has a DC rating of 12 watts.
In class A, most wanna-be techno-whiz-bangers (like me) agree, will make a maximum of around 5 watts clean power in an idle circuit.
Two of them in push pull class A will make a little over 10 watts, but in class AB with the right B+ voltage, proper bias setting the correct output transformer and allowable distortion products, can make over 17 watts output. Typically I see about 15-16 watts clean output in a properly built and biased class AB push pull EL84 PA stage.
If you up the B+ to a very high level, bias the two correctly, so the tubes are at lest idling safely and don't care if there is tons of distortion when driven to extreme levels, you can get more then 17 watts peak power too.
Even the latest and greatest 40 watt Speedster amp I have on my bench only makes about 26 watts clean into a dummy load.
Is it a lie for Speedster to call it a 40 watt amp?
Hmmm... maybe not if you consider allowing all the distortion products to be added in and called total output power... it gets about 38 watts when I do that.

[6267]

Bruce, I think you make a good point! It almost answers the old "why do tube amps seem louder?" I've always heard the RMS vs. Peak argument, but I think you're onto something. In a tube amp, there are often lower order distortion products when clipping, so it still *sounds* somewhat like the original signal, just more harmonically complex (most P-P's aren't matched all that well, so a lot of 2nd and 4th still gets through, not to mention the even harmonics from the preamp which do not cancel). Solid state will add really high order harmonics which just sound fuzzy to listeners and not like the original signal, thus they can't use the distortion products to get seemingly louder, but just fuzzier.

[daz]

If you up the B+ to a very high level, bias the two correctly, so the tubes are at lest idling safely and don't care if there is tons of distortion when driven to extreme levels, you can get more then 17 watts peak power too.

What kind of voltage are we talking about?

[Bruce / Mission Amps]

What kind of voltage are we talking about?

I have safely run a pair of fixed bias JJ EL84s into a Hammond 1608 OT at around +390vdc to 400vdc idling around 22ma each... the amp is an EL84 twist on a standard, 4 hole Marshall 50 watt amp topology (with proper preamp attenuation for the EL84 power tubes) and it frickin' smokes.