18WPC Schematic Help

[Blair]

Hello, I am pretty new to this, but I have a friend who designed this amplifier unit using 2 EL84s per channel. The amplifier has some grounding issues, and seems to lack some volume. We changed the resistance on the rectified power feeding the 100 uf cap to about 750 ohms leaving a B+ of about 250V. Before, with this schematic it was giving overdrive symptoms with harsh charachteristice. If anyone could help us to troubleshoot this issue It would be greatly appreciated.

I have never tried to post an image here, so here goes nothing:

Thank you

Blair


http://img.villagephotos.com/p/2005-12/ ... EMATIC.JPG

[erichayes]

Hi All,

Blair, what you're doing here is the electrical design equivalent of getting your car rolling by starting out in third gear. You're gonna scrub your clutch until it burns up.

What you need to do is start out with a conventional, non ultralinear 6BQ5 circuit that has cathode bias and a phase inverting stage and get that working. Then hook up the UL taps on the output transformer to the screens and make that work. Then, replace the cathode resistor/capacitor with the 317 network and see what happens. Finally, disconnect the phase inverter and wire the outputs as shown in your diagram, with one grid grounded and the other receiving input.

I believe that you will answer every question you had yourself. Theoretically, there's nothing wrong with the circuit you have. From a practical perspective, there are better ways to make an amplifier We'll all be here to help you figure out why.

Suggested schematics would be the Eico HF-14 and the output stage of a Heathkit AA-151. I also suggest you prolong your sanity by building only one channel; that way you'll only have one set of mistakes to correct as you go along.

Good luck, and remember: if you can't figure it out yourself, holler.

[Blair]

We rewired the amplifier with better grounds and added sheilded cable to the input lines. This cleaned the sound up drastically. Very nice smooth sounding midrange. The only issue is that I am driving it with a DECWARE single tube line stage as a pre amp, and it does not seem to deliver enough power to drive the amplifier to higher volume levels. It may be a matter of adjusting the plate voltage a bit. Any suggestions, or should this line stage drive this circuit fine? Thank you for your response, and information.

Blair

[TomMcNally]

Blair -

What Eric was trying to say using metaphors, is that the circuit
you built isn't complete. There is no (conventional) driver
stage, or phase splitter. The designer seemed to use op-amp
technology as a basis for the design.

I would suggest you and your friend that designed the amp
take a look at some other circuits, like the amps that Eric
suggested, and it will become more obvious.

Basically, you are missing the first and second stages of the
amp, and you are feeding the the third stage (gear) directly.

... tom

[mesherm]

The lack of gain is normal and not surprising. You never explained your reasoning in building this amp. I would not consider this design to be a "real" amplifier but only an experiment in minimalism. You have combined all three (or four) stages of a typical PP tube amp into only one. The only reason that this design works at all is due to the relatively high gain of the EL84 tube. You are going to have to live with the shortcomings of this design or else build a line stage with increased gain to compensate. If you wanted a "real" amplifier you should have designed in at least a simple triode gain stage to drive the differental/push-pull output.

[Blair]

What we are thinking of doing is adding a 12AX7 circuit to each channel on mine to make it an integrated amplifier, and he has a preamplifier that will deliver similar results on his unit. Whould this do the trick in yor opinion. Again, thank you guys for your informative suggestions. I was not born yet when tubes went out of mass market production for common use.

Thank you

Blair

[Blair]

I am really new to this, and am not really able to do all this myself. You guys mention 3-4 stages. I realize this is only the final amplification stage, but other than the line stage/preamplification stage, what stage is there? Sorry for all the ignorant questions, but I am really trying to get to a point where I am self sufficient enoough to do this on my own. What would a small line stage circuit like the one you guys mention look like so I can identify it in these other amp circuits?

Thank you

Blair

The lack of gain is normal and not surprising. You never explained your reasoning in building this amp. I would not consider this design to be a "real" amplifier but only an experiment in minimalism. You have combined all three (or four) stages of a typical tube amp into only one. The only reason that this design works at all is due to the relatively high gain of the EL84 tube. You are going to have to live with the shortcomings of this design or else build a line stage with increased gain to compensate. If you wanted a "real" amplifier you should have designed in at least a simple triode gain stage to drive the differental/push-pull output.

[Blair]

I apologize for not really answering you question about rationalization and building this amplifier. I was interested in building a tube amplifier. The designer has a 70WPC Ming Da and really has no practical reason for building this amplifier except for to have fun. I could have built a kit, but I was more interested in learning, and I have a tendency to build and play versus learn (I can read a scematic, so if it says to put A and B together, I would do it without question) This way, I can play, and troubleshoot my way through the flaws of the design. This may not make sense, and I am young so rationale may not all be there for you guys, but it has been fun so far. Again, thank you very much for your replies.

Blair

The lack of gain is normal and not surprising. You never explained your reasoning in building this amp. I would not consider this design to be a "real" amplifier but only an experiment in minimalism. You have combined all three (or four) stages of a typical PP tube amp into only one. The only reason that this design works at all is due to the relatively high gain of the EL84 tube. You are going to have to live with the shortcomings of this design or else build a line stage with increased gain to compensate. If you wanted a "real" amplifier you should have designed in at least a simple triode gain stage to drive the differental/push-pull output.

[TomMcNally]

Blair -

There is a schematic for the Heathkit AA-151, the one Eric
mentioned in his reply, on my website ...

http://mcnally.cc/amps.htm

Scroll down until you see the AA-151 and then click on
"right side" ... you'll see the phase inverter section.

Another good schematic is the Dynaco ST-35, also on
my site.

You'll learn a lot as you go ... if you can read a schematic
and follow the signal flow, look at a bunch of them - do
a google search on "phase inverter" tube and you'll
see how it works.

... tom

[Blair]

I see what you are talking about. Thank you for the schematic link. It looks like they are driving the EL84s with 360V. I believe we are running them at 280V. Would you recommend driving the plate voltage closer to 350V? Thank you again for the link.

Blair

Blair -

There is a schematic for the Heathkit AA-151, the one Eric
mentioned in his reply, on my website ...

http://mcnally.cc/amps.htm

Scroll down until you see the AA-151 and then click on
"right side" ... you'll see the phase inverter section.

Another good schematic is the Dynaco ST-35, also on
my site.

You'll learn a lot as you go ... if you can read a schematic
and follow the signal flow, look at a bunch of them - do
a google search on "phase inverter" tube and you'll
see how it works.

... tom

[mesherm]

Sorry if I appeared to be harsh but I was a bit taken back that your main complaint about this amp was exactly what one of its major flaws is: lack of gain. Designing tube amps (or most anything) is always a trade-off. That's why there are so many design variations. The advantage of your design is simplicity and low cost. It is basically a power differential amplifier with one input grounded.
This design normally functions as a phase-splitter stage in a PP amp (sometimes as the input stage).
Usually a PP amp will have a input gain stage to boost the signal to compensate for the signal loss in succesive stages. The next stage is normally a phase-splitter stage to provide inverted but equal signals for the push pull output stage. Some designs will add a driver stage after the phase inverter to drive certain tubes. Then you get to the PP output stage which actually drives the speakers.
Your amp design skips the first gain stage and combines the functions of the phase splitter and output stages into one. The main trade off is that you have low gain.
So your choices are to design in a triode input gain stage (which you have mentioned) or use this amp as a learning experience and build a more conventional amplifier for practical, enjoyable listening.
If you want to seriously get into building and designing tube amps I would suggest picking up a couple of books or some software on basic tube circuits and amplifier design. GlassWare is a good place to start. TubeCAD also.
http://www.glass-ware.com/
http://www.tubecad.com/

[TomMcNally]

Blair -

The problem with your amp is NOT the voltage, it's the
lack of a driver stage ... you need more tubes. No matter
what you do with those two EL-84's, they aren't going to
function in that simple circuit. Like mesherm says,
add a front-end (driver) and it will work great.

... tom

[Blair]

Thank you for all the information. You have all contributed to my delimma. I will add a "front end" circuit to this design, and hopefull the amp will open up and be what I am seeking. I did this as an inexpensive way to gain knowledge of tube design. For the 200 bucks, it is well worth it so far. Thank you again gentlemen. Trust me, I'll be back with more questions I'm sure. Thank you as well for the suggested reading.

Blair

Blair -

The problem with your amp is NOT the voltage, it's the
lack of a driver stage ... you need more tubes. No matter
what you do with those two EL-84's, they aren't going to
function in that simple circuit. Like mesherm says,
add a front-end (driver) and it will work great.

... tom

[Shannon Parks]

http://www.diyparadise.com/simpleel84.html

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

[Blair]

Thank you Shannon. Those are great links. Written in english too! Yellow_Light_Colorz_PDT_01

Blair

http://www.diyparadise.com/simpleel84.html

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