EL84 Blues Amp

[10thTx]

I thought about converting one of my amps to this but decided against it. I thought I'd go ahead and post it thinking someone might be looking for an alternative to the 18w idea.

It has some nice features to the idea:

- sort of a Dumblish tone stack (with diff values) and a pre-amp boost
PAB
- cathode switch on V1 for a bright/full/fat tone
- NFB switch
- Mstr vol
- reverb and a passive effects loop for either digital delay or tremolo
pedals

This is a concept amp. I have NOT built it. I do think it would be a very versatile amp with lots of different tones. Look for errors.

With respect, 10thtx

[10thTx]

I should mention that I am currenting playing a 6V6 version of this with a paralleled V1 and like it alot. Using the PAB and disconnecting the NFB gives it a nice overdrive, IMO. However this EL84 version is simple a concept that I thought someone might want to consider?

There is an editable version of this schematic and layout here:

http://www.el34world.com/Forum/index.php?topic=4845.0

10thtx

[sliberty]

Two questions:

On V1, your cathode switch has 5uF on both sides. Perhaps you meant to have different values?

On the power tubes, I don't really understand your cathode switch. It seems that the 270R resistor is going to be the shortest path to ground regardless of the switch position. Please explain.

[10thTx]

Yes, you are correct the schematic should've had a 5uf & 10uf as is indicated on the layout. I'll try to get that corrected.


I don't know why the cathode attenuation switch works? However, I know it does work because I installed one on a friends amp. I used a DPDT mini-toggle and set it up so it was 1/2 power and 1/4 power. What amazed me about the switch is that the tone of the amp stayed pretty much the same regardless. I was skeptical it would work until I actually tried it. It was on a 6V6 cathode biased amp. The switch disengages or engages the paralleled 22uf caps to ground.

There is some information about it here along with more schematics of variations of it.

http://www.el34world.com/Forum/index.php?topic=4437.0

With respect, 10thtx

[sliberty]

I understand better now:

In his diagram, the switch is a dpdt center off. The switch either connects one, two or none of the bpass caps to ground. A bypassed cathode should provide more gain on the tubes. I amnot sure it is a good idea to bypass one however as it throws the tubes out of balance - in a push-pull circuit thay may not be a good thing.

Your switch is not configured this way though. Yours either lifts the caps, or connects them. I am not expert enough to know how the 100k resistors impact this, but you are lifting the caps together so it should be in balance. Your other switch position is not connected, to no bypass caps are in circuit.

[Andy Le Blanc]

I think your on the wrong path...... if you look back to the early electric blues
it was pre war, depression era...... thru the early post war years.....
an authentic tone would not come from.....

- sort of a Dumblish tone stack (with diff values) and a pre-amp boost
PAB
- cathode switch on V1 for a bright/full/fat tone
- NFB switch
- Mstr vol
- reverb and a passive effects loop for either digital delay or tremolo
pedals
- and a switch to go from full power to 1/4th power by switching off
individual cathode caps on the p/p power tubes.

you might want to explore much simpler and much less elaborate designs
and explore the application of inverters other than the split load or long-tailed cathode coupled...
a simple non-master volume amp with NO global feedback......

Ive fallen in to the same trap that you might be about to.... it wont give you what you want..... you think it will but it wont....

just ask yourself.. "what did Hubert Sumlin get down at the local pawn shop" ?
when he first was makeing the scene.....

[10thTx]

Andy,

Regarding being on the "wrong path" .........

I can appreciate your comments and that you're making a reasonable arguement regarding traditional old school blues. I have over 200 blues records, tapes, CD's with many of the old sounds in them back to Robert Johnson, Lightning Hopkins and others.

Having said that, I've been listening to blues for some time (since 60's) and saw live ........ Muddy Waters, early Johnny Winter, Freddie King, Albert King, Albert Collins, BB King, Buddy Guy ...... and ........ Tab Benoit, Smokin Joe Kubick, Sonny Landreth, Larry Carlton's Blue Sapphire band, Ronnie Earl, Legendary Blues band, Anson Funderburg ........... and many many others.

I think there is more than one blues sound. I've played tweed clones and think they're great. I've also built 16 different amps and think my 6V6 version of this amp has some great blues tones. Mostly what I play is blues. This is the type sound I like.

http://www.soundclick.com/player/single ... i&newref=1

I don't care if anyone builds the amp or not ....... each to their own. I was just simply trying to share an idea with the amp building community since so many others have generously shared their knowledge with me.

With respect, 10thtx

[sliberty]

Without passing any value judgements, I'll just say that all of the following players qualify as blues players, and none sound like the other tonewise.

Robert Johnson
Buddy Guy
Robin Ford
Stevie Ray Vaughn
Joe Bonomassa

There are many ways to skin that cat.

[El_Martin]

Hi there!

Some interesting ideas. I just happen to be on the verge of a new amp with a similar concept. Thought about a 2xEl84 pp circuit w. fixed bias and some special feedback loop. Preamp might be Dumble inspired.
There should be two different feedback configurations:
1. 2 gain stages ->PI ->El84 without NFB or a bit of NFB for a Vox/Marshall flavour.
2. more NFB to tame the nasty EL84s for the dumble sounds.
Probably NFB w. presence and deep. Maybe only internal trimmers for presence and deep? Go HRM? Ooops, should I post it in the Dumble board ...

@ 10thTx: You're right. More than one blues sound

Ciao
Martin

[El_Martin]

double post due to connection failure...
sorry!
Martin

[Andy Le Blanc]

I appreciate your responce...... shared passions leading toward opposing
arguments for the same thing....

with the proposed amp I might suggest a 10K tail and a 470 bias for the inverter
and a internal pot. to make the feedback adjustable for variations in gain
when differnt tubes are used for the inverter.......
you might want bleeder resisters somewhere in the power supply too.....

[10thTx]

Andy,

Thanks for the response! I hope I'm understanding what you're saying about the pot? I'm thinking you're meaning for the NFB ....... or are you
talking about a pot in series with the bias resistor?

With respect, 10thtx

EDIT: per Andy's response below .......... I increased the value of the NFB pot to 100kl.

10thtx

[Andy Le Blanc]

Yes.....the series resister in the feedback loop ....
though I might make it a bit larger..... the inverter will make
a bit more gain but sacrifice balance which means more harmonic distortion
or tone color...... all in the most general terms..... now most make the common
mistake of applying feed back to an amp after the fact..... an internal trim or
pot. say 250k will give a more than sufficient range to play with....
what your doing is makeing a ratio of the series resistor (pot.) and the
resistor to ground at the end of the tail ( the 1.8k) any where from 20:1
to say 8:1 ........ so 20:1 with the 1.8k you'd set the pot. to 36K.....
but with a 5k presence pot ....it be set at 100k....... what ever you settle on
keep track of the ratio that you used so that if you find a preference you can
apply it to other amps

[10thTx]

This is my 6V6 version of the schematic I posted. It has a paralleled V1 and does not have the 1/4th power switch or bright/full/fat switch. And it has a local negative feedback around the second gain stage that is switchable and offers a more compressed sound when engaged.

Of the amps I currently own it's by far my favorite.


http://s28.photobucket.com/albums/c216/ ... CBScab.jpg


With respect, 10thtx

[Andy Le Blanc]

that looks amazing......... love the color.....
whats the sound board material and speaker?.....