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Old 9th January 2017, 07:03 PM   #11
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Quote:
Originally Posted by SirDuctTape View Post
Would I be able to just put the dummy load in the place of output transformer, with maybe an inductor along with it?
You can't do a push-pull output this way, because the transformer also does the job of combining the two push-pull anode currents with the proper phasing.

You can certainly make a single-ended stage with a resistive load - exactly like a normal preamp stage. There are, however, some quantifiable differences from a transformer or choke load - you only get about half as much output voltage swing (which may be plenty, since all you need is to drive a subsequent power amp).

There may be other differences as well. The transformer nonlinearity can add distortion to the sound, and its interaction with the loudspeaker may or may not be audible. Some guitarists think these things are very important, others (myself included) are not so sure. It probably depends a lot on your personal preferences, as well as the type of guitar sounds you're going for.

I've done a little experimenting along these lines, and I think I can get some good guitar sounds out of a small-signal pentode (with a resistor as anode load, no transformer) used as the final stage in a guitar preamp. Your ears may or may not agree.

It's probably worth a mention that there are a few uber-expensive boutique guitar amps out there which supposedly derive all their "tone" from the preamp, with the power amp being essentially distortionless. If true, these designs are getting all their much-coveted tone without an output transformer entering the picture. And if they can do it, so can you!

Quote:
Originally Posted by SirDuctTape View Post
I'd rather avoid the transformer for cost reasons, but if needed I can spend the money.
For low power valve output stages, it's possible to use a 70 volt audio line transformer as an output transformer. These transformers are very inexpensive - they're normally used to distribute background music, voice announcements, et cetera, over a wide indoor area. Here is an example: Quam TBLU 5W-25/70V Speaker Line Matching Transformer Multi-Tap

If you decide to go this route, I can provide more information on how to use these types of transformers. There is also information out there on the 'web, particularly on a few Australian websites. This particular innovation seems to have come from that most amazing continent.

-Gnobuddy
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Old 9th January 2017, 09:09 PM   #12
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Quote:
Originally Posted by Gnobuddy View Post
I've done a little experimenting along these lines, and I think I can get some good guitar sounds out of a small-signal pentode (with a resistor as anode load, no transformer) used as the final stage in a guitar preamp. Your ears may or may not agree.
-Gnobuddy
Ooh that's appealing! I might try this out, except with a tube power stage with it, hopefully resulting in some power tube distortion.

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Originally Posted by Gnobuddy View Post
For low power valve output stages, it's possible to use a 70 volt audio line transformer as an output transformer. These transformers are very inexpensive - they're normally used to distribute background music, voice announcements, et cetera, over a wide indoor area. Here is an example: Quam TBLU 5W-25/70V Speaker Line Matching Transformer Multi-Tap
I would like to try out those little audio transformers, so any information would be really great.
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Old 10th January 2017, 12:39 AM   #13
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Originally Posted by SirDuctTape View Post
Ooh that's appealing! I might try this out, except with a tube power stage with it, hopefully resulting in some power tube distortion.
My logic went like this: power tubes are either true pentodes (EL84, EL34, etc) or beam tetrodes (6L6, 6V6, etc).

Using an actual power tube with a resistive load is a bit of a pain. To get these tubes biased anywhere near their usual operating regime, you need a high-wattage resistor, high B+, etc.

But pentodes are pentodes, and beam tetrodes are beam tetrodes. It doesn't really matter too much how beefy / big they are - their characteristic curves have a lot of similarity.

If you look around, you can find small-signal true pentodes (eg 6AK5), and also small-signal beam tetrodes. The latter are often called "pentodes" on the data sheet, but if you take a magnifying glass and look at the actual tube, you can see there are only two grids. Bingo, it's actually a beam tetrode!

These little pentodes and beam tetrodes are easier to experiment with - you can run them with normal 1/2 watt anode loads, and many are happy with B+ of 150 volts or so, half to one-third of what you find in the typical 6V6 or 6L6 amp. (My Princeton Reverb has an appalling 440 V on the 6V6 anodes, according to the Fender schematic. )

Best of all, little tubes like the 6AK5 tend to be unwanted, and therefore, dirt-cheap.

Quote:
Originally Posted by SirDuctTape View Post
I would like to try out those little audio transformers, so any information would be really great.
The key is that these are honest-to-goodness audio step down transformers. In normal use, as intended by the manufacturer, you connect the speaker to the secondary, and drive the primary with a "70 volt audio line", which is exactly what it sounds like.

There are various taps on the primary, which control how much power gets to the speaker when driven by 70V on the primary. The idea is that you can drive the ceiling speaker in the bathroom off the same audio amp as the speakers in the machine shop; you just use the appropriate low-power tap to keep the bathroom speaker volume low, and the machine-shop speaker loud.

So, what we have, is a cheap audio step-down transformer, with multiple taps on the primary. The various taps are labeled in watts - for example, if you connect your 70V audio line to the tap marked "2.5 W", the speaker will be driven with a nominal 2.5 watts of power.

The really clever work was done by an Australian called Paul Cambie, and then fleshed out by two other Australians, Roly Roper, and Grant Wills.

Cambie and Roly figured out how to decipher, say, "0.25 W" on a tap, and translate that to what a guitar amp designer wants to see: primary impedance, in ohms. Grant Wills proved the concept by building actual guitar amps using the math Roly and Cambie had come up with.

Note that, in Australia, they use 100V audio lines. In the USA, 100V is considered dangerous, so the standard is 70V instead. This is a pity, as it makes some American-standard audio line transformers a bit less suitable for use as valve amp output transformers, because the stepdown ratios are smaller, i.e., the primary impedances are smaller.

Still, some of these 70V transformers have primary impedances that are usable with some output valves. More on that in a second.

Here's the relevant page from Roly's website. If you can't make sense of this, let me know here, and I'll attempt to clarify: Cheap Output Transformers

Just to be clear: if you go with small-signal pentodes like the 6AK5, you'll want to use a resistor as the anode load; you won't find a transformer with a big enough primary impedance.

If, on the other hand, you go with typical (low wattage) power pentodes or beam tetrodes, then the 70V audio line transformers might work for you.

-Gnobuddy
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Old 10th January 2017, 04:52 PM   #14
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So I think I've got this straight. If I go the route of a small power tube, the 6AK5 for example, I would just match the plate resistance in the schematic, right? Would I just use a 500k ohm resistor for a 6AK5 without shields? (I'm looking at this datasheet).

Quote:
Originally Posted by Gnobuddy View Post
Still, some of these 70V transformers have primary impedances that are usable with some output valves. More on that in a second.
To calculate if it would work, I would just do the calculations detailed by Paul Cambie, then see if it matches with the specs that the tube needs, right? If that is true, would I only have to match the impedance ratio? Also I am confused as to what the Zpri measurements are, could you me out with it?

Thanks!
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Old 10th January 2017, 06:41 PM   #15
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Quote:
Originally Posted by SirDuctTape View Post
If I go the route of a small power tube, the 6AK5 for example, I would just match the plate resistance in the schematic, right? Would I just use a 500k ohm resistor for a 6AK5 without shields? (I'm looking at this datasheet).
Not quite - that plate resistance is extremely high for all pentodes (and beam tetrodes). The actual load resistance is usually much smaller, and it's actually the "corner" of the curves in the top left area of the characteristics that is more important.

If you want a bit of theory, Merlin Blencowe has described a simplified semi-mathematical method here: The Valve Wizard -Small Signal Pentode

But the super-simple way to get something working with almost no math goes something like this:

1) Pick a B+ around 150 volts (works for most small-signal pentodes I've tinkered with.)

2) Pick an anode (plate) resistor that will flow a milliamp or two with about half of B+ (or a bit less) across it. Anything from 68k to 33k should work here.

3) Use a 5k pot for the cathode resistor, initially set to maximum resistance.

4) Use a 1M pot for the screen resistor, initially set to maximum resistance.

5) Power up. Twiddle the 1M pot until you measure anywhere from 50V to 75V on the screen grid.

6) Twiddle the 5k pot until the anode (plate) voltage is around 75V to 100V.

7) Power off, wait for B+ to fall to zero, measure the resistance of the 1M and 5k pots, replace them with fixed resistors of about the same value.

8) Add cathode bypass cap, screen bypass cap, input and output coupling caps. You now have a working "design".

This quick-and-dirty method will get you something that works, but it may (or may not) sound particularly good.

Alternately, you can do the same thing with the pots and steps 1 through 4. But, before step 5, add screen bypass cap (0.1 uF to start), cathode bypass cap (10 uF to start), output coupling cap (value depends on what you're feeding from the output, but 0.1 uF is almost certainly usable).

Now hook up some way to feed in an input signal (for instance, your guitar, and a truly clean-boost pedal). Hook up some way to listen to the output (for instance, run it to a volume pot, then to a small clean power amp, or your guitar amp.)

Then, do steps 5 and 6 to get in the ballpark for bias and operating point. But, before you do step 7, play the guitar, listen to the sound, and tweak the 1M and/or 5k pots, a little at a time, to get closer to the sound you want.

You can also experiment with the screen bypass cap (this has a powerful effect on tone), and/or cathode bypass cap value.

When you're happy with the sound, do step 7 - power off, measure pots, replace with fixed resistors. Done.

This approach would probably make "proper" valve engineers of the 1950s beat their heads against a wall and scream. But, for a guitar amp, where tone is the most important thing, it's actually an excellent way to go about the process.

Quote:
Originally Posted by SirDuctTape View Post
To calculate if it would work, I would just do the calculations detailed by Paul Cambie, then see if it matches with the specs that the tube needs, right?
I've already done the calculations for some of those Parts Express transformers, and a couple of other models I found available in the USA. I'll attach a PDF copy of my spreadsheet, so you don't have to do the calculations over again, you can just use my results.

Quote:
Originally Posted by SirDuctTape View Post
If that is true, would I only have to match the impedance ratio? Also I am confused as to what the Zpri measurements are, could you me out with it?
Right, exactly. "Zpri" is primary impedance - exactly the thing we mean when we say "My 6V6 amp output transformer has an 8k ohm anode-to-anode primary impedance"

Here's an example. Take a look at the PDF I attached. The topmost pale yellow section lists the information for a little 4W Quam 70V audio line transformer, and a similar (but much more expensive) little Hammond, both available from Parts Express.

The "Output" column lists the wattages marked on the transformer taps. For example, there is a tap marked "0.25 watt" (first row of table). Read all the way across to the right, the "Zin (Ohms)" column, and you see the number 19,600.00 (ohms).

This (0.25W tap) is one of the two ends of the primary winding on this particular transformer. The other end is marked "common", and is not shown in my table.

What we have just found out is that (based on my calculations), this particular transformer has a primary impedance of 19.6 kilo ohms between these two end taps ("common" and "0.25W").

To use this as a push-pull output transformer, we also need a centre-tap on the primary (that's what we connect to B+). We know from transformer theory that half the windings means one-quarter of the impedance. So we look for a tap that has an impedance of one-quarter of 19,600 ohms.

As you can see, the tap marked "1.00 W" is the one we want. It has a calculated impedance of 4900 ohms, which is one quarter of 19600 ohms, so this is the centre tap.

So, if you were designing a little push-pull guitar amp (less than 4 watts, because this is a 4W transformer!), and you could use a 19.6k push-pull output transformer, you know you can use this little Quam TBL70 for the job. Wire the "1.00W" tap to B+. Wire one anode to the "common" lead. Wire the other anode to the "0.25W" lead. Wire the speaker to the secondary windings (they should be marked "Speaker" or "8 ohms" or something like that.)

You may have noticed that we wound up using the 0.25 W and 1 W taps. The 1 W tap is the centre-tap of the transformer - and 1 W is four times as much as 0.25 W. This pattern (4:1 power ratio marked on the transformer) shows up whenever you are looking for the centre tap!

Now, we've ignored some of the other taps on this particular transformer. Let's consider the second row of my table - the one starting with "0.50" watts. Read across to the right, and Zin is 9800 ohms, or 9.8 kilo ohms.

Is there a suitable centre-tap? Yes: one quarter of 9800 ohms is 2450 ohms. The corresponding tap is marked "2.00 W".

(Equivalently, we started with the 0.5W tap; four times 0.5W is 2W; so the 2W tap is the centre-tap we're looking for.)

So we can also use this particular transformer if we need a 10k primary impedance for our push-pull guitar amp design. Wire the 2W tap to B+. One anode goes to "common". The other anode goes to "0.5W". Secondary goes to the speaker as usual.

Does it all make sense now?

-Gnobuddy
Attached Files
File Type: pdf Audio_Line_Xformer_As_Tube_OPT.pdf (53.4 KB, 9 views)
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Old 10th January 2017, 09:49 PM   #16
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Originally Posted by Gnobuddy View Post
Does it all make sense now?
-Gnobuddy
Yep! It does!

Thank you so much for all the help!
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Old 10th January 2017, 11:00 PM   #17
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Originally Posted by SirDuctTape View Post
So I think I've got this straight. If I go the route of a small power tube, the 6AK5 for example, I would just match the plate resistance in the schematic, right? Would I just use a 500k ohm resistor for a 6AK5 without shields? (I'm looking at this ... Thanks!
See post 824 on page 83 of the 100 buck amp challenge thread for a practical example powered by a 9vac wall-wart.
More grunt available with 12vac plug pack and blue diode bias.

Last edited by djgibson51; 10th January 2017 at 11:05 PM.
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Old 11th January 2017, 06:15 AM   #18
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Originally Posted by djgibson51 View Post
See post 824 on page 83 of the 100 buck amp challenge thread for a practical example powered by a 9vac wall-wart.
I always thought your little 6AK5 amp deserved to get more attention on that thread than it did.

Cute little amp, I would like to hear it in action!

I was years late to the hundred-buck thread, but shortly before I joined diyAudio, I had been considering both 6AK5 and 6AK6 valves as possible output devices. I went with the 6AK6 because it was easier to find an output transformer with the right primary impedance.

How did you solve that issue, incidentally?

-Gnobuddy
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Old 11th January 2017, 09:05 AM   #19
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Originally Posted by Gnobuddy View Post
... I went with the 6AK6 because it was easier to find an output transformer with the right primary impedance.

How did you solve that issue, incidentally?

-Gnobuddy
Empirically. Like most of my amateur home-brews.

I take the trafo I have to hand (usually 100 v line);put in the voltage I can squeeze out of a plug pack with a multiplier or reverse wired transformer; connect up a signal generator and oscilloscope, and adjust bias for maximum clean signal.

Then connect guitar and adjust component values for clean and overdriven tones. I don't need loud.

Unscientific but fun.
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Old 12th January 2017, 04:02 AM   #20
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Originally Posted by djgibson51 View Post
Unscientific but fun.
Thanks for sharing your method! Do you also check to make sure the tubes aren't overheating, either at idle or full power?

Incidentally, I'm not so sure your method is unscientific. Non-mathematical, maybe. But you're not reading tea-leaves or looking up horoscopes to make your decision - you're performing an experiment, looking at the outcome, and trying to adjust parameters to improve the result. That sounds fairly scientific to me.

A long time ago, I read a math book by a Russian mathematician, where the author had proved that the fastest way to optimize a large number of interacting settings on a complicated piece of machine-shop apparatus was actually to use pure randomness; roll the dice, make a tweak, check to see if you were getting better results or not. If results were worse, backtrack the last change, and roll the dice again. Repeat until the results were good enough (met specifications).

At the time the book was written, this randomness-based solution was more efficient than any known purely mathematical approach. (That may have changed since then, since this book was written before the era of personal computers.)

I guess the nutshell version is this: pure math isn't always the most scientific or efficient way to solve a complicated problem.

-Gnobuddy
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