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Old 15th October 2013, 10:17 AM   #1
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Unhappy EL84 "typical" amplifier

I have built the amplifier in the schematic below. After a great many troubleshootings including blown poor quality caps i got it to be noise free and working. Just a little pop sound every now and again at least until the tubes heat up. I noticed the pops are at the same time when the tubes chime as elements move due to normal heating.
The real problem is, it works, but it's rather dull, not chimy as some suggest EL84's would usually be, and the bass notes are a bit undefined and monotonous. Below is also part of the power supply. The EL84's are at 23mA plate current (almost 70% dissipation) while the phase splitter tubes run at about 1.2mA current (seems little but at that setting when i was using them as preamps they were very lively sounding). Below is also the filtering part of the power supply. Before you ask, no, that is not a regulator, it's just meant to drop voltage while still being stiffer than a plain resistor. (it also has a bonus of added filtering of ripple).
Perhaps you people can help me improve it, especially since i only built this for personal enjoyment no commercially...

http://s16.postimg.org/noqy09f8l/RAMP_schem.jpg

http://s14.postimg.org/81cwi35xd/sursarad.jpg
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Old 15th October 2013, 10:39 AM   #2
SY is offline SY  United States
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Is this supposed to be a guitar amp? If it's meant for hifi, I'd wait until the rookie has a few years in the league before trying to build another one of his designs. Zero feedback pentode is unlikely to get you good sound with conventional speakers- the source Z is probably in the neighborhood of 50 ohms, and bass distortion from driving the transformer with a high impedance could be north of 10%. The power supply is likely to cause noise issues from the extremely high charging currents at the input cap. The inverter has its tail bypassed.for reasons that are unclear.

As a guitar amp, it may work OK, but building in some power supply sag could be beneficial.
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Old 15th October 2013, 10:46 AM   #3
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Hi,
This is not very typical... I hope you are using it as guitar amp, not for audio...
You could get more sparkle with some "bright caps" - 200 pf or so over the tonestack and "gain" potentiometers, and reduce the 100 cathode bypass on the first stage to 2-5.

edit: I posted before seeing SY's answer.
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Old 15th October 2013, 10:51 AM   #4
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GMTA
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Old 15th October 2013, 10:52 AM   #5
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Thanks for the reply. So i am to understand supply sag IS actually bad for a general use amp (not instrument). The tone control would suggest guitar amp but at the lowest position of the pots it's actually almost completely linear. It is as if the correction has only boost and no cut along the entire pot span. It's a modified Marshall type control, as you can see.
I opted for a no feedback amp since i read many people do not use feedback, they prefer to let the tubes do what they want... as it turns out it may be bad after all (at least in pentode mode). I can easily add feedback and i think i'll try it today and see what it does if i'll love it or hate it even more. Although with paraphase it can be tricky, there all sorts of phase shifts due to capacitors etc. Of course i could try something different, adding feedback through the volume potentiometer "ground" pins as it was done a great many years ago.
I just noticed the inverter tail bypass issue. I will remove the bypass as it provides very little extra gain. I had done that since as far as i could find about this inverter type, not having bypassed could insert a minute amount of positive feedback,,, which is always a bad thing and i've never seen so far a controlled positive feedback.

Actually the problem was designing it around what i had available, not as it should be.

Last edited by Bloodmist; 15th October 2013 at 11:16 AM.
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Old 15th October 2013, 11:17 AM   #6
cjkpkg is offline cjkpkg  United States
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There are many tried and true el84 pp amps an this site.
Red light district
Oddwatt el84 pp
Baby huey
Tubelab simple pp
Tubecad journal (john broskie)

I built one that is basically the broskie circuit with a few tweaks from other amps like a CCS on the el84's cathodes for fixed bias current and using a non bypassed red led on the 6n1p. It is a fine little amp with big sound and strong bass.

Anyone ever made the EL84 P-P circuit referenced on the Tubecad Journal site?. Here is part of my journey.

Last edited by cjkpkg; 15th October 2013 at 11:32 AM.
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Old 15th October 2013, 12:22 PM   #7
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Thanks for the simple and nice schematics. I suppose sound quality comes first and i must confess i had been infected by a more experienced builder who managed as much as 40 (!) watts out of a pair of russian EL84 and that was the recipe, high plate voltage, low-ish screen voltage to prevent burning, moderate plate current leaving room for more swing. As i can tell that was a clas AB2, a different beast altogether so it was pointless incorporating any of those ideas into my amplifier. Although he does say he used a simple split load inverter (don't know how that could drive the output tubes that far) i'm not so sure...
Power and pleasant sound i guess one can't have both, so i'll just settle for quality sound and use one of your schematics, still deciding which one.
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Old 15th October 2013, 12:26 PM   #8
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Quote:
Originally Posted by Bloodmist View Post
Power and pleasant sound i guess one can't have both...
Au contraire.

With 450V anode, 250V (approximately) screen on JJ EL84, approximately 25mA/tube idle current, I'm getting about 25 watts out at low distortion, and the amp runs quite cool. Perhaps it could be pushed to about 2dB more output (hardly noticeable), but for 40W, there are much better output tube choices.
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Old 15th October 2013, 12:35 PM   #9
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So that basically means... you are operating in class A? That 25mA is just about there. I do think you wouldn't be using it if it sounded bad so i believe you. Others did suggest my quiescent current is way too low then i asked "way too low for what?". I'm a little confused as to what is considered safe area of operation for these tubes, since if 70% dissipation (in theory that's where they are currently) is way too cold. They sure can take some abuse, they're not new manufacture, they're 1970 Tungsram, but that doesn't mean they'll have their usual lifespan. I suppose going to class A and adding global NFB would greatly improve things, perhaps i'll do just that.
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Old 15th October 2013, 01:55 PM   #10
SY is offline SY  United States
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No, very much AB1. AB2 requires a source or cathode follower before the output tubes, and in this case the extra complication might get me 1-2dB more level. Class A would require much lower plate and screen voltage and higher current- figure 5-6 watts on a good day.

Remember, idle current, power, and class of operation also depend strongly on the anode-to-anode impedance!
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