[mboxler]

I'm building a SET amp from a kit.  I hope that I will learn something about electronic circuits.  If it works, icing on the cake!

First, is the output signal from the plate AC, or DC?  I think it is DC with the varying amplified voltage of the input signal that was applied to the grid.  Most pictures make it look like it's AC (output from A, in the following picture, with the voltage varying from plus to minus...the line equaling zero volts).



If I'm right, then there is the function of the coupling capacitor.  I always read that it blocks DC...passes AC.  I hope this (to me) is a poor description of what the coupling capacitor is actually doing.  ANY varying voltage on the input side of the capacitor will cause the capacitor to charge, then discharge an AC signal on the output.  The capacitor does not block DC, it blocks constant voltage DC, and "passes" varying voltages (AC or DC) as AC.
 
Sorry for the rambling (and ignorance).  If someone could post a link to a better explanation of what is really going on, I'd appreciate it.  Again, I'm trying learn something from this project. 

Thanks in advance!

Mike

[cheap-Jack]

Hi.

I'm building a SET amp from a kit.


Mike

This "cathodyne" phase splitter is to drive a PP power stage. But you are building a SET which does not need any phase splitter.

My question is: why you bring this PP phase splitter into yr SET discussion??

c-J

[mboxler]

Hi.
This "cathodyne" phase splitter is to drive a PP power stage. But you are building a SET which does not need any phase splitter.

My question is: why you bring this PP phase splitter into yr SET discussion??

c-J

I just grabbed the first picture I found showing a signal from a plate (left tube). 

Mike

[DaveC113]

Let's see the schematic from the amp you plan to build....

[mboxler]

Let's see the schematic from the amp you plan to build....

Here is the schematic of the amps I'm building. 



Getting back to my original question, are the signals from pins 1 and 6 of the triode and pin 7 the pentodes AC or DC?  Do the coupling caps pass varying DC as AC?  Do output transformers pass varying DC as AC?  I do not like the term "pass", but using to simplify the questions.

Again...sorry for my ignorance.  I'm trying to learn as I build.

Thanks, Mike

[Davey]

Your original thoughts were fairly accurate.

If you were probing a variety of points in a typical tube circuit you'd see a DC voltage with an AC signal superimposed on it.

In the schematic you've shown, there would be a high voltage DC present on the plates (pins 1/6 on the 6922 and pin 7 on the output tubes) provided via the "B" and "A" connections respectively.

Capacitors are actually two plates with an insulator in between.  This provides DC blockage but allows AC to "seemingly" pass through because of the rapid charging/discharging action taking place.

The 0.1uF coupling capacitors prevent that DC voltage from being applied to the grids of the output tubes and leave just the AC voltage to be amplified.

On the output stage, DC is prevented from making it the speakers by the transformer.  The transformer "transforms" the high voltage (low current) DC in its primary to lower voltage (higher current) AC in its secondary suitable to drive the speakers.

There are plenty of good books to explain vacuum tube amplifier circuits much better than I.

Hope that helps.

Cheers,

Dave.

[mboxler]

Thanks, Dave 

Everything I've read has either been over simplified, or way over the top.  Just needed something in the middle that I can build on (sorry for the pun).

Mike

[DaveC113]

Yup, Davey's post is very good, I can't do better. 


I also just remembered you are looking into building an Aikido preamp as well? I just built one and discovered it is way better than a single triode driver I was using in my SET amp. I built a Tubelab Simple SE and used a 12BH7 w/ CCS load as a driver, and after my Aikido was built tried driving the EL34s directly. This was a HUGE leap in performance. And it turns out input sensitivity is right at 2V, so gain is about perfect. Anyway... if you build the Decware amp you're considering you could likely leave out the driver section if you're going for an Aikido preamp, or build it as-is and try it out both ways, you can input the signal at the EL84's gridstoppers, remove 1 leg of the coupling caps, remove one leg of the 10k 1W resistors right where B+ comes into the amp...

[cheap-Jack]

Hi.

Here is the schematic of the amps I'm building. 

Although you said you are just learning to build a SET amp, you are already on the SONICALLY right track -
which I am indeed impressed. Triode power O/P topology - by triode strapping the EL-84 pentode. In fact, many many veteran tube amp builders don't even know sonic superiority of triodes!

This is exactly what I am always like - triode sound! If you got the chance to click to the tube manual of a triode, e.g. 300B vs a pentode power tube, e.g. EL-34, you'll understand what I just said. The transfer curves  of any triode tube are all linear - flat & straight, vs those of any pentode All come with a right-angled 'knee'.

It is this 'knee' on the signal transfer curve of a pentode causes limitation of signal swing magnitude & less good sound than a triode. This 'knee' is caused by the internal electrode structure of a pentode.

The only way to get around it is the convert the pentode to quasi triode by triode-strapping the pentode as shown in the schematic of yr amp (EL-84 triode strapped).

Good choice of schematic, my friend.

A warning:- the HV is already powering the EL-84 plate & G2 (pins 7& as well as the plates of the 6822s
well before the tubes are heated up to conduct. This for long term can shorten the service life of all the tubes there.

I'd strongly suggest you to get a HV delay circuit or a manual switch to switch on the HV to the tube plates after the tube heaters all warm up to conduct.

c-J

[Ericus Rex]

Just to clarify Mike.  Pins 1 & 6 of the 6DJ8/6922 and pin 7 of the 6BQ5 will have both AC and DC running through them.  The DC is the plate voltage (B+ or sometimes called HT, HV or V+) that the tube needs to operate.  The AC is the amplified signal which then passes through the coupling cap while the DC is left behind.  As for 'varying DC' (I think this is usually called 'ripple'), there should be none of that after the filter caps in the PS.

[mboxler]

Thanks, Guys...

DaveC113...I was following your Aikido project.  NICE  .  I'll keep your suggestion in mind!

c-J...I'm planning on using this switch.  Manual, but should be effective. 

http://tubedepot.com/p-dpst-3way.html

Ericus Rex...your clarification is where I get confused.  Perhaps it's a terminology issue.  To me, AC means that the voltage is varying between positive and negative.  DC...always positive or negative...not both.  By varying voltage DC, I meant that the audio signal applied to the grid at, say,  + - 2 volts is amplified, and the output from the plate is DC varying between, say,  +100 and +180 volts.  The coupling cap would remove this DC bias, and an AC audio signal varying from + - 40 volts would go on the the next stage.  Am I close?

Again, thanks to everyone.  I'm sure this is basic stuff, but that's kinda where I have to start.

Mike

[Ericus Rex]

The DC doesn't actually vary.  What you're referring to is 'superimposed AC'.  If you have an AC signal that goes from 0v (ground) to 2v than down through 0v to -2v (4v peak to peak signal) and combine that with a 10v DC potential then that AC signal will go from 10v up to 12v and then down to 8v and back to 10v, all the while staying in what is commonly termed the 'positive' region.  So what you're calling varying DC is the amplified AC signal acting on the plate DC voltage.  When the amplified signal goes into its negative region, it lowers the DC potential off the plate, when it goes into its positive 1/2 cycle it adds to the dc potential.  But what goes through the cap is only the AC portion so the signal drops from 100-180v (superimposed) down to +/- 40v AC only (using your example).  Does that help?

[mboxler]

FINALLY! I found a good illustration (to me) of tube amplification!



If the output signal read +150 instead of 150, this would be a great illustration of the voltage entering the coupling capacitor.  Sorry...I referred to the output voltage as varying DC, perhaps a better description would be DC voltage varying with the input AC signal.  The output from the coupling capacitor, using the above illustration would be an AC signal of +- 50 volts peak to peak.

Okay...next question. 

I've built a speaker crossover, and understand that the capacitor's capacitance followed by a resistance (in this case, impedance, an 8 ohm driver) determines the frequency that the capacitor will pass to the driver.

Using the schematic of the amp I am building, am I correct that a very small cap (.1 uf 400v poly) can be used is because the grid circuit resistance of the SV83 is very high (the 330K resistors to ground)?  This will "pass" all audio frequencies to the SV83 grid.

I promise to stop here.  .  I find this stuff fascinating. I hope other newbies are learning something as well.

Thanks everyone

Mike