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Help looking for a project

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keaster

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Joined 2009
#1
HI, I looking for ideas to build a small, low power amp for someone. Its going to used mostly for classical music with bookshelf size speakers and unknown room size. I know that's not very helpful, but I was thinking something along the lines of a EL84 that can do 10-15 watts with non adjustable fixed bias ( so its idiot proof) and no solid state parts except for the rectifier in the power supply or can be tube as well to try and make it as authentic as possible. Also I was hoping to have a single power supply so I can build it as a stereo amp and keep costs down.

Is the dynaco push pull el84 any good? can the supply shown power 2 channels? or is it better to use the OddWatt - ECC802S SRPP / EL84 (6BQ5) Push-Pull Tube Amp?

Any input would be great!
 
#5
Got my hopes up but now you say there's something wrong with it

Question: how come there is so many different circuits but so little info on them, such as wattage, or transformer specs, is there a way to figure this missing info out based on the components and voltage used?
 
#6
keaster said:
...
Question: how come there is so many different circuits but so little info on them, such as wattage, or transformer specs, is there a way to figure this missing info out based on the components and voltage used?
Click to expand...

Transformers, not so much...they need to be selected with reference to the output tubes you are working with.

Resistors, yes you can calculate...do not operate at greater than 50% of rating. (IMHO)

Caps, the value is given, the type will be your preference, the voltage rating will be (IMHO) at least the supply voltage for interstage and 20% to 30% over for power filter caps.

Have fun learning.
 
#8
HI, I looking for ideas to build a small, low power amp for someone.
Click to expand...

I'll grind my own axe. 😀 Build an "El Cheapo" with UL mode "finals". Power O/P in UL mode is approx. 12 WPC. Any member of the 6V6 "clan" can be used for the "finals", without parts value changes.
 

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#9
DUG said:
Where do you think it should be?
Click to expand...

It needs a resistor at the node formed by the two 1.0MEG resistors and the grid of the right-hand 5751.

The floating paraphase is a servo device that maintains amplitude balance by means of an error signal derived at the junction of those 1.0MEG resistors. If the amplitude balance were perfect, there would be no AC voltage there at all. Any imbalance generates a signal voltage for that half of the floating paraphase.

Maybe you can get it to work without that resistor? :scratch1:

Not a big fan of the floating paraphase, and it's not something I would use. Since it requires two triodes to implement, why not go with the much better LTP? :scratch1: :scratch1:

(During the early 1950s, the floating paraphase was very much a "fad" circuit, much like the SRPP is these days.)
 
#11
DUG said:
The right hand 5751 looks to be a simple inverting amplifier with a gain of -1.

Is that not what is required of it?

How would a resistor in series with the grid (I think that is what you are suggesting) change this circuit for the better?
Click to expand...

It looks like that extra resistor might be optional. The "Valve Wizard" site doesn't mention it, but this is more for guitar amps. Maybe it adds some even order harmonics? Anyway, I've never used as floating paraphase since I can't see any benefit. It still needs two triodes, and an LTP with active tail load is no more difficult to implement, and the LTP works better (has balanced harmonics between phases).
 
#14
Miles is being civil. I'll be blunt.

The only active phase splitters worth a damn are the differential (LTP) and "concertina"! Other topologies have fallen out of favor, for very good reason. If you want "gobs" of gain from the phase splitter, set up a LTP using 6AC7 pentodes and a CCS in the tail.
 
#15
rmyauck said:
But maybe it sounds better, but not necessarily measures as clean. They must use that circuit for a reason on those expensive amps!
Click to expand...

Measure better, but the sonic performance somehow disappoints is a real possibility. The floating paraphase is seen lots in guitar amps, but this is an application where distortion isn't a problem, and can even be desirable. The floating paraphase will produce harmonic imbalance since one phase is fed directly, but the other receives an error signal, and that side will produce less harmonic distortion. Nor will it be AC balanced, since if the error signal disappears, then so does one phase. The even order harmonics might be euphonic, but this is still FX, not high fidelity amplification. Maybe you'll like that effect, or maybe not, depending on what you're using it for.

The floating paraphase has a couple of cost cutting benefits: it doesn't require either a high Vpp or a negative supply for an active tail load, that an LTP would need. Nor does it elevate the heater above DC ground, like the Cathodyne does that could stress Vhk ratings and/or leak AC into the signal chain if the heater isn't elevated above the cathode potential.This from the days before reliable transistors to make CCS tail loads, and when the only alternative was a pentode requiring some -100Vdc of negative rail, and more holes in the chassis, an extra PTX or at least another secondary on the main PTX. It was also before the days of reliable SS diodes (and selenium diodes had tons of potential and actual problems).

These unusual phase splitters, the Isodyne, Van Scoyoc, floating paraphase, and some other more esoteric topologies were all intended to solve problems we solve these days with the help of solid state, and LTPs or Cathodynes. These other designs were finicky, some requiring external alignment, and/or could possibly oscillate.
 
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