SE El84 amp how to get the 'right kind' of distortion and a good clean tone ?

[walkman]

Hi

I have a simple single ended amp I've been mucking around with. It is 12ax7 with el84 and 7K OT.
El84
Anode = 283v
Screen = 272v 470 ohm 1 watt
Cathode 8.3v 220 ohm 1 watt 100 uf cap

12ax7 HT = 271v ( both halves)
Anode =130v
Cathode 1.3v 1k5 resistor + bypass caps on each half

Its clear bright and loud, and quick sounding wich is great, however when the guitar volume ( strat ) is on 10, the amps overdrive sounds a bit harsh, when guitar volume is on about 5 ( with treble bleed ) the overall tone for any amp volume setting is great.

The Gain is reduced between the 12ax7 halves with a 1m in series and 1m to ground combination. .022 caps decoupling caps.

What I'd like to achieve is a tone that gets more compressed and warmer as I roll the volume pot up on the guitar from 5.

First though was screen resistor, as i mistakenly put in a 470k screen resistor on the el84 ....nice compression but low output and almost no headroom.
I'm wondering about 5k to 10k 5 watt?

However reading different posts on forums perhaps suggests that reducing the screen grid down to say 100 ohm may 'improve' the tone.

Where should i look at to tweak this amp?

Any suggestions?

Cheers

Walkman

[Dave H]

Single ended EL84 does sound a bit ragged in overdrive to me compared with push pull. It's only running at about 9W so you could try warming it up a bit by changing the cathode resistor to 150ohms say (that's what I use).

[Jazz P Bass]

Here is a nice SE EL84 kit amp from an Australian company.
http://www.atps.net/Projects/SET%201A%201.jpg
It's interesting to see a gain & a drive control.

A 1K/5W screen resistor is fine. (that 470K was probably 'choking' the output)

(Quote:" 022 caps decoupling caps." the 0.02 caps are 'coupling caps')

[Gingertube]

C13 in Jazz's linked schematic is the "usual" method for controlling harshness at high levels. TRy something like 1nF to 10nF/1kV ceramic.

Also look at AX84 SE EL84 Projects to see what worked for others.
http://www.ax84.com/static/p1/AX84_P1_101004.pdf

Cheers,
Ian

[eschertron]

C13 in Jazz's linked schematic

C11?

[Gingertube]

Sorry,
C11 is correct.
Cheers,
Ian

[olddawg]

I gigged with a stock Epiphone VJ for a couple of years. Here is what I found helped the most rather than a bunch of circuit mods:
1) A good 12" speaker. (I used an Eminence in an open backed cab)
2) Try some different tubes. I liked a NOS RCA 12AX7. The output tube wasn't that different except make sure it's fresh.
3) A good OD pedal ( I used a cheap Bad Monkey)
4) Use a Compressor ( I used a Rocktron Big Crush)
5) Expect it to be a one trick pony. Don't expect real clean at any real volume. Expect some hiss.

Hope that helps. You can't make an amp something it isn't. But if you do what I said it will sing and sustain for days...

[eschertron]

Sorry,
C11 is correct.
Cheers,
Ian

No need for sorry

I'd be interested in seeing what the author thought was a good value for C11. Without a largish resistor to set the knee of the filter up a ways, I imagine the effect is, well, more than audible.

[walkman]

Thanks for the advice.

I did some more reading around the net as well during the previous day and decided that I should put a resistor in their too.
So I ended up with a 4k7 5w ceramic and 3k3 5w ceramic inseried with a 0.0039uf 600v poly? Cap.

This makes quite a change to the amp much smoother distortion with out any change in gain . However it took too much treble off not wooly or covered with a blanket but now the strats tone controls are best up full and so with the volume too . Where as before the position of the tone and volume seemed to give quite slot of subtle variations.

I purchased some .001 3kv caps the blue circular ones and will swap one instead of the .0039 cap.

Are thes sort of caps suitable for audio work or will it not matter in this function of a corrective filter.

Also can the filter be from plate to ground I have seen that on several old radio and radiogram schematics.

I wired the resistor cap combination across the

[walkman]

The primaries as suggested.

Forgive the typos silly iPhone auto spell.
Would be nice if there were a forum app for this forum!

[eschertron]

The primaries as suggested.

Forgive the typos silly iPhone auto spell.
Would be nice if there were a forum app for this forum!

in the lower LH corner of the window there is a desktop/mobile display option. I was directed to that some time ago. probably should be a sticky!

[walkman]

in the lower LH corner of the window there is a desktop/mobile display option. I was directed to that some time ago. probably should be a sticky!

Thanks, the mobile view is so much easier!

[Gingertube]

The 4K7 + 3K3 + 0.0039uF (3.9nF) gives a corner frequency of 5100 Hz. To raise that cutoff frequency a bit (less treble loss) then try 0.0022uF (2.2nF) for 9042 Hz, or 2 of the 1nF in parallel for 2nF for 9947 Hz.
1nF + the 8K of resistors puts the corner frequency up around 20kHz and so would probably not have noticeable effect on the harsh distortion.

For info, other options:
3 of the 1nF in parallel (3nF) + the 8K would give 6600 Hz
1 off 0.0027uF (2.7nF) would give 7368Hz

Armed with these figures you should be able to tune the cap value to suit.

The ceramics are perfectly OK for this "Conjunctive Filter" duty.
Cheers,
Ian

[ronh]

If your amp is a EVJ, then you should increase the value of resistors R12 and R13. These are in the B+ string. Make them twice as big. 4.7K to 10K, 10K to 22K. Fender Se amp values.
This will make the amp cleaner with your guitar volume turned down a bit. Wont help with distortion though.

[walkman]

Thanks Gingertube.

As it turns out I mistakenly bought a .01 caps instead of .001 bit found some old .001 400v poly caps in my bits and pieces.

With 8k and .001 the amp breathes a lot more and the guitar tone controls and volume are a lot more useful. There doesn't seem much of a noticeable affect on the distortion character as you explained.

Will put the other .001 400v in parallel tomorrow night as it is too late tonight. Would then be as a safer voltage rating too. Hopefully there is enough lead left on the old cap to wire it in without damaging the cap.

The amp is not a VJ but is not dissimilar to most other 12ax7 el84 SE amps on the planet.

Will post a full schematic when I get a chance.

The strat copy that I am using with it has modified tone controls so that neck and middle share a tone and bridge has its own tone.

Also they are setup so that they are useable all the way down to about 1 on the control.

With this configuration I can easily play with volume and or tone down low and still have a very clear and clean tone or on the edge of breakup as desired.

The pickups are tonerider pure vintage and are excellent sounding pickups IMHO.