Ralph Power amplifier

[Mohan Varkey]

Stig,

Last paragraph in my previous post is a general comment for DHT applications. I should not have used the word “Constant”. A current source limits the inrush current to cold heater and thereby prolongs tube life. Yes, I recommend it for Single Ended designs.

Yes, there is no substitute to DC coupling and Yes bypass caps ruin the quality of sound output. Unfortunately, we have to have a compromise and I believe in minimalism. In a DC coupled design, the amplified signal is delivered to the following stage in a manner that could be described as “Less modified “ than with other options using coupling capacitor and/or interstage transformers. I simply minimise the “evil” i.e. the value of cathode bypass caps. Without the bypass cap, the tube’s operating point wanders with the input signal.

By the way, I have a personal prejudice towards Directly Heated Tubes in general. I look at the cathode as a series of resistors that oppose the signal current through the tube. The end result is compression, though not as much as that is applied in some recording studios! Sometimes, I wonder as to how much of this signal energy is used up in heating the filament!!!

Mohan

[Mohan Varkey]

Stig,

Any progress with Ralph?

Mohan

[stigla]

hey...

Well, haven't started building yet, still collecting parts. I have re-designed the schematic quite a bit, as can be seen below.
Any comments?




I have completely removed the BP-cap at the input. I have finally understood that there is no need for it.
The PowerTranformer is a custom-wound toroid and oversized quite a bit...

And yes, I will run dual mono!

Is it a matter of getting the PSU caps as small as possible too? In SS equip., it's always a matter of having as many farads as possible.

Tube amps rarely use anything bigger than 220uF...

-stig

[yves]

Hello.

Great thread, and great people

Stig, you may want to try the ultrapath
connection from IS psu end to the cathode
of the first tube. You can see the
circuit in Electra-print transformer
website. It is a simple matter to
move the psu cap from B+ ground to
cathode. I think there is no harm
at all done.

Oh, and where can I see comments of the
ECC99 (positive or otherwise) on the net?

rgds
Yv

[dice45]

CCS: do you mean for the output tubes? Never seen that. I've only heard talk of using CCS's with DHT's on DC....(in SE)

Stig,







exactly this is what i am planning with my PP AD1 amp.



Hint: Have two power tubes and a CCS forming a differntial pair, a "long-tailed pair". If your CCS does't make the voltage across it alone, put it in series with a resistor biggenuff to dissipate the power.







Kirchhoff's law #1 applies here: both cathodes are linked to the same node: all current going into a node and all currents goiing out of that node sum up to total zero current. This means that the currents going into both cathodes are equal to the current fed into the node by the CCS which is constant !! (assuming real electron flow, not technical current direction). Accordingly both output tubes have to act exactly inverse instead of sloppily so as usual in common PP amps with both cathodes referenced to ground separately.





I heard an amp having this feature, it combines SE and PP virtues, best of both worlds.

[stigla]

|you may want to try the ultrapath connection from IS psu end to the cathode of the first tube|

I think maybe the 0C3's would have a problem with capacitors like this in the load, but I might be wrong...
Is the ultrapath like this supposed to shorten the signal path to ground or is it to reduce PSU ripple? Should the value of this cap be as if it where a normal BP-cap? Could it be smaller than that?
This cap would have to be of high quality, yes?

Don't know of any "offical" test/reports on the ECC99, just have read about it here and there... try a goggle search.

Dice45;

I'm sorry, but I don't think I got it strait... But it sound interesting! (your amp too!) Got any schematics?

[Mohan Varkey]

Stig,

Choke input filter is great. My personal preference is for a two stage LC filter(5Hy-100uF followed by 5Hy-470uF). Signal to noise ratio should then be around 68-70dB at full power output. I use Panasonic (marked UP 450V) caps.

Yes, if you wish to have very good transient response, then you could consider large value chokes and film foil (not metallised) capacitors. Because it is Class A-PP, we cannot expect significant changes in current demand and so a reservoir may not be required.

Perhaps you could consider dropping some of the B+ voltage in case the maximum allowable Va for ECC99 is limited to 200V (I do not have the specs for this tube). I am referring to the VR tubes and the 5.1K Caddock resistor. I would rather add an RC filter here and use the R (max 1K; the lower the better) to drop the voltage as required.

Input tube is biased around 6V. Is this correct?

Mohan

[stigla]

Hmmm, yeah. I just used one LC stage as in the original schematic because obviously this is enough to reduce ripple to acceptible levels. And with the Ultrapath connection on the output tubes, hum will be even lower.
Does anyone know HOW MUCH a PP amp "reduce", or "cancels out" ripple at it own?

As for the ECC99; http://www.jj-electronic.sk/tubes.htm <--- theres the specs. 400V, 60mA, 5W in short. Well within ratings. (~4 watts)
The 5.1k resistors are paralelled. If to much ripple I will replace these resistors by a 1k and a 1,5k in series with a 47uF cap in the middle.

6V is the correct bias voltage. I like to keep the bias somewhat higher than normal. 2V RMS (from a CD player etc...) is in fact around 4-5 volts, no? And transients may also reach pretty high levels I recon...
Don't know if this has any worth doing though, but it feels right.

Stig

[dice45]

Originally posted by stigla


I'm sorry, but I don't think I got it strait... But it sound interesting! (your amp too!) Got any schematics?

Stig,


before i post a schematic not tried out and running stably, some wraith shall eat my soul!



I just try to explain how a differntial amplifier stage works. Have a look at
Allen Wright's PP1 amp : the two 1000µF electros shorten the cathodes AC-wise which means that for AC, both cathodes are connected by the 68Ohm resistor to ground. Resistors have a tendency to keep currents constant, not as much as an inductor would do, but the bigger the resistor value is, the more it tends to constant the current (470k looking to minus 500V would do a might fine job here).

How bad the two caps acting as two coupling caps in series are electrolytics! (and have to be)



Now imagine the EL34 not having this 470Ohm || 1000µF in the cathode but their cathodes being tied together and connected to ground via a constant current source. As is done with the lower cathodes of the 1st stage's differential pair of cascodes.

The CCS keeps the total current through both EL34 constant to frequencies way above the audible spectrum. So if on EL34 reduces its current, the other one has to increase its current exactly to make the total fit (the CCS will see to it ) . Result: both output tubes have to act exactly inverse. Any sloppiness is prevented by the CCS, be it DC or AC. It would be interesting how this works if th CCS is replaced by a choke which only works as a constant current device from a few Hz up and so biasing would be easier.



Biasing, good i mention it, you have to add additional bias to make both tubes have the same quiescent current and you have to check and re-adjust that regularly. Otherwise the OPT goes into saturation and you have a mess instead of good sonics.



To my amp: i am trying something very similar, but my design is spiced with danger, i decided to settle on a differntial pair of 71A driving a differntial pair of AD1, both differntial pairs having a CSS at the cathodes. The spicing danger: both 71A and AD1 are directly heated tubes and i may face more noise picked up by the virtual grounds (common cathodes) than i may find bearable. Getting the noise refencenced to ground out of the heater supplies will be really tricky. This doesn't work with line levels but it may work with signal levels and S/N ratios in a power amplifier, any stage having a gain of 3 or so.The 1st stage too is a differntial pair, it uses the indirectly heated 6900 which are choke-loaded i.e the plate load is a choke. Between 71A and AD1 i use an interstage trannie: Lundahl LL1635 .

[stigla]

Hmmmm....


I've seen at Tube CAD some ideas using an inductor at the cathode to bias a common-cathode pre-amp. But no indications or hints to how to calculate the value of such a choke, but the value may be un-critical...
I liked the Idea, and I even considered building such a linestage using the '99 and two LL1668, but I changed my mind.

So the only problem then, with CCS's are biasing of the tubes? Fixed bias comes to mind... Why the regular bias check? Because of the drifting of the SS devices in the CCS?

How critical is the coloration of the CCS?




stig