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Schematic for a small SE EL84 amp

RMS

Member
2002-04-13 10:38 pm
MA
I have a 269JX Hammond PT, only has 60mA so I wanted to build a amp with one pre-amp tube in the 12AX7 family and one EL84 hooked up in Class A, SE.

Has anyone come accross a schematic like this that I can copy?

Also I am fairly new to the audio world using tubes and I have a few questions :
Because of the pre-amp tube does that make this amp intergrated?
Why is the volume pot located just after the RCA jack input and not after a pre-amp stage?
I have built tube guitar amps before and these audio amps look so darn close to each other what would you say the difference is?

Thanks!
 
Last edited by a moderator:
Just so you know, if you run an EL84 single-ended you'll only get about 1W out of it. There are tons of schematics on the web, but maybe not for that tube in SE. Just copy another design and re-scale for this tube. I suggest you browse www.duncanamps.com and their tube data sheet locator. Search 6BQ5 and it should point you to data and schematics.

The preamp tube you mention is a necessary driver for the output tube. You can call it an integrated amp if it has a volume control, but there are no rules. Not all designs have unbufferd volume pots.

The difference with guitar amps is one of tone and distortion. In audio we try to eliminate distortion. For guitars, the more the better. And their are many ways to do it, each one sounding different. Also, guitar amps are severely rolled off in the treble. They commonly use "tone" controls which have a big midrange "suckout". If you played a guitar through a straight transistor audio amp your ears would bleed.
 

RMS

Member
2002-04-13 10:38 pm
MA
One watt is that all? Is it because the other 6 watts or so will be distorted?? Also if I understand you right one of the basic difference between audio and guitar amps is that audio amps are designed to put out more highs?

Thanks for the info!
 
Re: Schemi for a small SE EL84 amp

RMS said:
12AX7/EL84 in Class A, SE.

Has anyone come accross a schematic like this that I can copy?

Because of the pre-amp tube does that make this amp intergrated?

There are tons of EL84 SE maps out there... i scanned thru what is on my HD and this one comes closest to what you want. You might need to juggle the PS for whatever trafo & rectifiers you have handy (or want to use).

el84-AX7se.gif


Wired in Pentode SE it should put out somewhere between 3 & 4 W. Wire it in triode and it will drop to less than 2. I am currently running a similar amp (but with a 12AU7) and there is some real synergy between it and my BD-Pipes.

dave
 
Oh sorry, I should have mentioned TRIODE mode. It's a lot more linear. You can get about 5W with a pair of push-pull EL84 in triode mode.

The schematic from planet10 is nice but I don't like it (just my opinion). What's the point of trying to go SE with a pentode? And feedback? It will kill all the magic of SE. Go push-pull and it will be cheaper (you save on OPT).

I also don't like the 680pF feedback cap. This is CURRENT FEEDBACK, not voltage. A capacitor in the feedback loop only makes the amplifier more unstable. The loop compensation cap should be on the plate of the first stage.

RMS: if you realy want to go SE, then you must get a REALLY linear tube. And that's why the 300B, 2A3, 45, etc. have achieved cult status. Nothing else comes even close. Measure the EL84 or EL34 or 6L6, they just don't have the linearity. That's why they're used with feedback.

There's more to a guitar amp than bandwidth < 5kHz. They need distortion, and lots of it. And tone controls that make it far from flat. It's a whole different way of looking at things.

jh
 
hagtech said:
The schematic from planet10 is nice but I don't like it (just my opinion). What's the point of trying to go SE with a pentode? And feedback? It will kill all the magic of SE. Go push-pull and it will be cheaper (you save on OPT).

Don't discourage him. It is his 1st amp, let him get his feet wet -- the journey can be as important as the destination...

The EL84 makes a nice little SE amp -- in pentode or in triode. Maybe not in the same league as some of the vaunted DHTs (i have heard some of those that sound like s^%$). And a SE EL84 amp can probably be built for the price of the DHT output tubes -- certainly Frugal-phile(tm) approved.

And why not SE. Way simpler to get working well than a PP amp that would need close to twice as many parts. He already has the OPTs...

It isn't important to have the best amp possible, just one that lets you sit down and enjoy the music -- and a well executed EL84 (in SET or SEP) can certainly do that (assuming you get or have some synergy with the speakers).

And i doubt it will be his last amp...

dave

PS I'd probably leave the feedback out too -- the map was just something close that was on my drive -- one of probably 20 different amps. Go do a Sherlock or Google search and spend some time exploring.
 
Here is another

This is what the amp i'm currently using would look like (approximately) if wired in triode. (i am just enjoying the system too much to bother pulling it down to fiddle with it at the moment)

SE-EL84-AU7.gif


With suitable anode & cathode loads the 12AU7 could be a 12AX7 or AT7 or 6H1Pi (6N1P) or 6922 or ... with enuff drive off a pre or CD player the EL84 could even be used by itself in a spud amp.

dave
 
Hi all,

actually 2 watts will be just fine for normal listeing levels in a small room... so a SE EL84 can indeed work ... I've build a EL84 SE guitar amplifier which is much to loud indeed to crank up in my room.... I remember a article about ' the most simple amp' to be build with tube.. it was in tubecad.com I think two years ago or somethiong like that....you should read it, is gives comments on output power, input level etc..

goodluck..
Thijs
 

RMS

Member
2002-04-13 10:38 pm
MA
Now I can start building!

I have been searching the web like crazy with all kinds of different search engines looking for what Planet10 just posted. Thanks many times over! I just ordered a 12AU7 Jan-Phillips mill. spec tube. I will be comparing the two schemi's above with each other and I will be working with them for this small project I have in mind.

Yes those 300B SE amps are very neat just can't afford it at this time but until I get some better experience I'll stick with this basic project.

Once again thanks to all!
 
All,



the EL84 is one of the best tubes ever made, IMO, 3 EL84 triode-wired in parallel substitute and sonically outperform a WE300B in the same circuit (circuit originally made for 300B).
And you all probably know i am not a fan of paralleling tubes at all.


I would be very eager to learn about the success of an EL84 SET amp. :)

i consider building one myself. For SET mode an output transformer of 7 to 8 kOhm primary impedance is needed.
 
Re: Pentode/Triode

RMS said:
am I right that the first one is wired in Pentode? and I think the second one is wired in Triode am I right??

Yes. Yes.

In pentode the screens are wired to a B+ (sometimes the same one as the plate). In triode the screens are wired to the plates with a series R (100-1k).

Here is another:
Quote Bill Perkins, PEARL:
I don't care to keep this any great secret . .  I just worked hard to get the theory into my head - much digging thru old texts on tetrode theory - and I didn't feel like giving it away . . . just wanted to have some fun is all, and maybe stimulate a little thinking and re-evaluation of some tired old "standard procedures."

   The way I do this little trick - as a couple of folks already knew - is to bypass a zener with a cap. But, not just any zener and not just any cap.

   My favorite zener is a 1N821, a 15cent, 1/2 watt, 6.2 volt part that is both very quiet and of nearly zero tempco. I forget why 6.2V is the "magic" voltage for zeners but it is, the noise falls thru the floor and the low tempco yields a very stable voltage source . Maybe someone could post on this 6.2V business . . .

   To build this:

   - string 5 of the 1N821s in series and set them aside.
   - take two, 16V, 470uF Sanyo OSCon caps and put a 10k resistor across
      each one, then hook the two RC combos in series; then connect that
      assy. across the zener string being sure to observe the polarity.
   - then put a ten ohm, one quarter watt resistor on the cathode end of
      the string and a resistive ferrite bead on the other.
    - connect the other end of the ten ohm resistor to the plate of the
      tube to be "triode connected" and the ferrite end of the assembly
      to the screen, and there you have it.

    Note that if you are using true, three grid EL34s that the suppressor is best connected to about -5V or so. Just voltage divide off your bias supply.

   Biasing that grid slightly negative reduces its electron interception to
essentially zero. Have fun discovering what a triode connected tetrode/pentode really
sounds like.

dave
 
on the same topic, but digressing a bit

can anyone gude me to a schematic for a simple single ended HIFI amp using EL84 tubes. I previously had one (see a thread a few weeks ago about "should a newbie cannibalize ......". that amp worked fine in one channel for a few days before it quit on me. it was supposedly single ended, and the one working channel sounded really nice while it worked. i have cannibalized it and want to start out from scratch using what I can (and buying what I need).

ref. the tube amp book by rozenblitz, the internet


specifically , i have these (naive) questions since i'm still reading up on the theory ...

1. the old amp i had sounded ok, (not loud but enough volume) with my 90dB speakers rated at 6ohm. (although when it quit, i was running them off a set of junk 6ohm test speakers). am i to assume from the thread that el84 tubes are not suitable at all for hifi applications (i have a SS reciever for loud music, if i need)

2. any recommended sites / pages on tube amp chassis construction or anyone knows any sources in the dallas, TX area.

3. if i wish to use the cannibalized old output transformers, how would i find out their specs, since there are no markings on them. if i get this amp going and am satisfied withi ts performance, i would gladly invest in quality output transformers, but right now i'm on a budget

4. any recommended SIMPLE and relatively inexpensive schematics for this tube. there is a wealth of info out there but it can be confusing to the newbie

thanks for a great site.
 

RMS

Member
2002-04-13 10:38 pm
MA
Pentode/Triode

Thanks Dave for straighting me out on the Pentode/Triode bit! but about the "zeners" I need time to absorb that one, anyways what is the purpose of that device?

now I'm geting the idear to put a switch in so I can go from Pentode to Triode should I use a SPDT or a DPDT type?
 
Re: Pentode/Triode

RMS said:
Thanks Dave for straighting me out on the Pentode/Triode bit! but about the "zeners" I need time to absorb that one, anyways what is the purpose of that device?

I just quoted it, i haven't figured out exactly how it works. I do have a lot of respect for Bill's brain.

now I'm geting the idear to put a switch in so I can go from Pentode to Triode should I use a SPDT or a DPDT type?

DPDT for a stereo amp

dave
 

RMS

Member
2002-04-13 10:38 pm
MA
about input signals

Dave: I'm curious about if there is a restriction to the type of singnal this amp can work with? Can I plug in a walkman CD player into it; talking about the above EL84 map you scaned for me. I'm just wondering what I will use to try it out with, since I don't own a pre-amp, or a seperate casset deck yet. I just felt I had to build a power amp!

Wen't out and bought some DPDT switches!

Thanks,
Rob
 
Re: about input signals

RMS said:
Can I plug in a walkman CD player into it; talking about the above EL84 map you scaned for me. I'm just wondering what I will use to try it out with, since I don't own a pre-amp, or a seperate casset deck yet. I just felt I had to build a power amp!

That should work fine.

dave