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Schematic for a small SE EL84 amp
Schematic for a small SE EL84 amp
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Old 14th April 2002, 08:57 PM   #11
RMS is offline RMS  United States
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Question Pentode/Triode

Dave, I printed out the two maps and I have been studying them am I right that the first one is wired in Pentode? and I think the second one is wired in Triode am I right??

Rob
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Old 14th April 2002, 10:03 PM   #12
planet10 is online now planet10  Canada
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Schematic for a small SE EL84 amp
Default Re: Pentode/Triode

Quote:
Originally posted by RMS
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:
Quote:
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
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Old 14th April 2002, 10:10 PM   #13
zobsky is offline zobsky  India
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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.
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Old 14th April 2002, 10:50 PM   #14
ezmeralda11 is offline ezmeralda11
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Default tube amp

I believe www.decware.com has a little 5 watt amp that uses the Svetlane SV83 tube, but one of the earlier versions of the amp, if not current one can work with the EL84. They sell it as a raw kit for $400 last I looked. I haven't looked at the site in a long time.
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Old 15th April 2002, 12:34 AM   #15
RMS is offline RMS  United States
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Thumbs up 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?
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Old 15th April 2002, 02:59 AM   #16
yves is offline yves
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Default EL84 amp

Or try this:

http://www.triodeel.com/compact.html

It's a great pp topology. You can replace R6
with a constant current source and get
a differential pair driving the opt.
Opt pair and phase splitter all in one.
You probably need a 100 ohm resistor from
screen to plate though.

rgds
Yv
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Old 16th April 2002, 04:33 AM   #17
planet10 is online now planet10  Canada
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Schematic for a small SE EL84 amp
Default Re: Pentode/Triode

Quote:
Originally posted by RMS
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.

Quote:
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
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Old 17th April 2002, 04:57 PM   #18
RMS is offline RMS  United States
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Question 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
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Old 19th April 2002, 12:46 PM   #19
FlMike is offline FlMike
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could r3 and r5 be omited in this amp? if so what size R would be used in their place?
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Old 19th April 2002, 03:59 PM   #20
planet10 is online now planet10  Canada
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Schematic for a small SE EL84 amp
Default Re: about input signals

Quote:
Originally posted by RMS
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
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