Use of 6528A as power tube

[Paul Joppa]

Hah! Those who persevere will inherit the earth!

[Loon]

Hmm...

Having trouble trying to find a suitable transformer with secondary of 9V @ 1.5 A for the rectifier feeding the 12AU7.

Have three reasonably close contenders:

1. Triad FS16-800-C2  [email protected] - 25% voltage regulation
2. Triad F-3152XP  7.5V @ 1.0A - voltage regulation unknown
3. Triad VPT18-1390  [email protected] - 13% voltage regulation

Is voltage regulation a factor I should be concerned about?  Any other variables apart from size I should be looking at?

I am tempted to go with option 3, which is a well-stocked toroidal ( although is $26.57, about double the price of other two options).  Is it worth exploring the possibility of learning to wind my own transformers?

Thanks again.

[Paul Birkeland]

You'd have to talk to Pete about his voltage regulator, and how much voltage you need to shove into the regulator to avoid dropout.

[Loon]

Another issue is the operating point of the cathode follower. 

I would like to experience the sonic effect of varying output impedences determined by the mutual conductance of the cathode follower.  Therefore, I would like my Crack to be able to accept the following tube types:

6AS7G
5998
7236
7802WB
6528(A)

It's one thing to be able to plug these tubes in and have sound come out.  It is quite another to plug these tubes in and have a great sound come out.

The Vp / Ip curves for these tubes are all quite different.  The effect of this is that the operating point will move depending upon the cathode load and the grid voltage.

I would like to determine the optimal, most linear operating point for each of these tubes.  I have a couple of questions in this regard:

1. Can I use the Vp / Ip curves to determine the best cathode load and grid voltage?  Would the Cathode curve be the same a the anode curve, except with reversed polarity?
2. How do I determine resistor values for the speed ball to achieve a good cathode operating point?

Implementation

I assume that changing cathode load would be easier than changing grid voltage, as the latter is determined by anode voltage of input tube and playing with this could mess up the 12AU7's operating point.

When I went under the user name "w0lfd0g" I floated the idea of putting plate load switches on the input tube on the underside of the chassis.  I plan on having variable cathode resistance to set the operating point of the cathode follower through a similar implementation, but I will probably need a switch with more throws. 

Could someone more knowledgeable than me (most people!) please chip in to answer my questions and provide feedback on my plan?

[Loon]

You'd have to talk to Pete about his voltage regulator, and how much voltage you need to shove into the regulator to avoid dropout.

I used the formulae Pete provided with his schematic to determine minimum voltage and current input.  This is determined by input AC frequency, peak voltage, value of capacitance and the requirements of the diodes used for rectification.  All three transformers listed exceed the minimum requirements.

I was mostly concerned about the rated voltage regulation of the transformers and grossly exceeding the minimum input current requirements.

[Paul Joppa]

This project really calls for more information than can fit in a forum post.

I'm sure you can learn a lot about power supply design on the web, including the meaning and effect of transformer regulation.

Less easy is to determine optimum operating points. To over-simplify, those tubes that have a higher transconductance will drive lower impedance phones. But transconductance is itself a function of the operating point. More cathode current will increase the transconductance but creates more heat and may require a new design to replace the Speedball current source. Incidentally "transconductance" is another way of saying "tendency to oscillate" - that's why I referred you to 6C33 design issues.

...

I once took a drawing class, thinking it would be cool to be able to sketch people and places I've seen. I learned two things - it takes a lot of practice and many mistakes, and a drawing takes more than 10 times as long as I had thought. But I still work on it from time to time because it's still rewarding. Same goes for electronic design - it's more work than you think, but it's also more rewarding than you think.

[Loon]

Thanks for the encouragement Paul.  This is a hobby that I would like to continue for the rest of my life and that I am only just starting.  It is really helping with my depression issues atm as a kind of diversion therapy.

You have gone above and beyond the call of duty in answering my questions. 

Warm regards and thanks

[Paul Birkeland]

I'd recommend buying a piece of plywood, some octal relay sockets, a beefy variable power supply, and Morgan Jones's book on designing tube amps.  There are high voltage supplies around that will give you 100-300V and a few hundred mA if needed.  Some even have a beefy 6.3V supply just to keep you out of trouble. 

Of course, you'll need to sketch circuits and buy the appropriate extra bits to get them running (a few caps and some very large resistors)

You can use those tools to try a whole ton of different things without committing to buying a whole bunch of parts.  If you try 20-30 different designs, odds are that there will be one or two configurations that make enough of an impression on you to move forward.

[Loon]

Awesome idea.  I imagine there would be cheap thrills and spills to be had with this set up!  The perfect balance of theory and practice.

[Doc B.]

That's how I started. Only the original gangstas will remember my frightening breadboards in the basement listening room back on the 90s. As long as you are willing to take full responsibility for the potential hazards of a bunch of clip leads carrying high voltage strung all about it can be kind of a fun way to quickly sketch out a circuit.