Isolation Transformer question..

[PRR]

> Power Transformers are expensive.

Not as expensive as Output Transformers.

Well, on balance, OT$ ~~= PT$. The low-freq limits are similar, OT handles about 1/4th the power, but PTs can distort grossly while we like the OT to stay well below gross distortion.

You *NEED* a PT or similar function if you are going to put your finger on it! (Unlike a radio, a guitar-amp user is in contact with the circuit.)

> how few parts and dollars are required to build a tube amp.

One cheap path is a 20VA 120+120V:6VAC (dual primary), two 6AU6, and a Fender Reverb transformer. About 1/3rd of a Watt, which with a good speaker can be loud in a small bedroom.

[tubegeek]

Hey.. if I didn't know any better, I'd think you guys were trying to deter me.

Aw HELL no! Anybody wants to build a tube amp, no matter how foolishly, has my full cooperation. I'm just trying to help you avoid doing something that seems like a better idea now than it probably will after you try it.

So:
building your own amp = GOOD CLEAN FUN LIKE WE USED TO HAVE BACK IN THE OLD COUNTRY!
building a heater-string version of a Champ = sort of a wrong turn

OK, OK.. I got to this point because I thought I'd see how few parts and dollars are required to build a tube amp.  Power Transformers are expensive.

Yeah, I hear ya there. One really cheap way I had LOTS of fun was to get a couple of very cheap transformers as surplus: one was 120:12.6 and the other was 240:12.6.

The first one connected to the mains on the 120 side and ran the heaters on the 12.6 side. The second connected to the 12.6 heater supply on ITS 12.6 winding and - voila! I had a nice 240V B+ transformer on the other side of THAT one. Put a voltage doubler on the 240VAC, you can get into some real trouble/fun. I only mention it because I know you can get yourself health insurance as of this week.

..thanks for the motivation...

I hope it's motivating and not discouraging, that's my intent. Next time I hit the basement I'll look and see what I've got down there, you never know. Postage on any power transformer that fits into a Priority Mail box isn't too bad, and if I find something along the right lines I'd be happy to send it out to you. There are all sorts of Stupid Transformer Tricks we can help you with, don't worry.

[chptunes]

Thanks for the dual 6AU6 amp suggestion Paul R.  About .3 Watts, eh.  I've already built your sub-watt Darlington Transistor Amp, which drives a Line Transformer into an 8 Ohm speaker.  Love it.. especially with the adjusted Negative Feedback Loop.

But, I would like to go bigger than 1 Watt with this build.  While I was researching the 6Y6 tube designs that you suggested earlier, I ran across the 6AQ5 tube [a small bottle 6V6, no?]  That tube looks like a great candidate for a lower Voltage Champ build.  I could use a Voltage Doubler circuit (2 Diodes and 2 Capacitors) after a 120:120 Transformer, and have a 250V Champ with about 2 Watts of output.  But, I'd need to spend $10 more for a 6.3v Filament Transformer [Hammond 166L6, perhaps].

tubegeek- You described a handy way to achieve 240v..

"One really cheap way I had LOTS of fun was to get a couple of very cheap transformers as surplus: one was 120:12.6 and the other was 240:12.6.

The first one connected to the mains on the 120 side and ran the heaters on the 12.6 side. The second connected to the 12.6 heater supply on ITS 12.6 winding and - voila! I had a nice 240V B+ transformer on the other side of THAT one. Put a voltage doubler on the 240VAC, you can get into some real trouble/fun. I only mention it because I know you can get yourself health insurance as of this week."

I need to draw something up for you guys to pick over.. if you don't mind...

[chptunes]

hey tubegeek.. is this what you did?


image hosting 20mb

[chptunes]

What about this little guy right here.. including all 3 Hammond Transformers, parts would be an estimated $140 USD from the popular on-line shops, if you bring your own chassis/enclosure (Hammond 1590DD or 1590R, perhaps).

Wonder how close the B+ would be to 250v?


how to screenshot on windows

[tubegeek]

hey tubegeek.. is this what you did?

Pretty much exactly.

Note that there is nothing special about 12.6 - if you can find the same setup with 6.3 go for it!

What about this little guy right here. Wonder how close the B+ would be to 250v?

It would sort of depend on how much current the transformer is rated for.

Have you had a look at PSUD on the Duncanamps web site? That could really help you narrow it down, I think. (PSUD = Power Supply Unit Designer)

[davent]

Wattkins folks came up with a design they called the Savage Croaker, the goal of which was to do it all inclusive using new parts (where applicable) for under a $100. Uses a 6aq5 output tube with a 6u8 pentode/triode for the preamp. Haven't finished mine yet but purchased a couple flat pack Hammonds for the HV and heaters, under $20 for the two and a small OT from AES for ~ $15. Some people were using an Edcor PT which are pretty inexpensive but a hassle/expensive to get to Canada.

May have to register at Wattkins to see the schematic, i hope not.

http://www.wattkins.com/files/wattkins/savagecroaker_final_1.pdf

[gmoon]

...and let's not forget Tino Zottola's MA-1, which used a single 120V isolation transformer. The primary coils are in series which drops the secondary to 60V (for the 12ax7 & 50L6 filaments), followed by a halfwave voltage doubler for the plate voltage...

[PRR]

Third-Watt:



Look close: the hand-drawn stuff IS an early Champ, the FotoChop is my mods for a 1/10th-scale model.

The pot on V2 G2 is optional for even lower power (when parents/babies are sleeping), you can wire G2 to the nearest B+ point for "full" power.

[chptunes]

Thanks a bunch for posting circuit links/names .. cool stuff!

[tubegeek]

Third-Watt:

Cool! Thanks!

Why break the connection to the 1st cathode bypass cap? Doesn't need that much gain? Or switchable for high/low gain?

Look close: the hand-drawn stuff IS an early Champ

I LOVE the early Fender schematics that look like that, and of course, they are some of the first schematics I studied and understood so that's part of it. Do you suppose that's Leo's draftsmanship?

[chptunes]

Third-Watt:

The pot on V2 G2 is optional for even lower power (when parents/babies are sleeping), you can wire G2 to the nearest B+ point for "full" power.

That little scaled down Champ has a lot of appeal.  Looks like about $70-$80 USD, if bringing your own chassis.

Does that 100k Pot act like an Attenuator, by rolling off some frequency specific signal?

Also, concerning the Reverb Tranny, I see two models available here:

http://triodeelectronics.com/tf160.html
http://triodeelectronics.com/40-18034.html

..the first option has a 15k Primary, and the second option has a 25k Primary.. I don't see a 'Load Resistance' figure on the 6AU6 Datasheet:

http://pdf.datasheetarchive.com/indexerfiles/Scans-008/Scans-00175646.pdf

[chptunes]

..oh ya, and.. how is the tone of this little Third Watt?  Do you like it's tone when overdriven?

Thanks again...

[tubegeek]

Does that 100k Pot act like an Attenuator, by rolling off some frequency specific signal?

The pot on V2 G2 is optional for even lower power (when parents/babies are sleeping), you can wire G2 to the nearest B+ point for "full" power.

The .2 UF capacitor bypasses the pot for all AC signals (there is only a small impedance to ground for AC through the cap.) The pot sets the DC voltage for the 2nd grid "G2" and this will allow adjustment of output power level.

Also, concerning the Reverb Tranny, I see two models available here:

http://triodeelectronics.com/tf160.html
http://triodeelectronics.com/40-18034.html

..the first option has a 15k Primary, and the second option has a 25k Primary.. I don't see a 'Load Resistance' figure on the 6AU6 Datasheet:

http://pdf.datasheetarchive.com/indexerfiles/Scans-008/Scans-00175646.pdf

Hmm. This is the one I am most familiar with but I confess I don't know its ratio:

Angela Instruments Fender Reverb transformer

I suspect any of these choices will get you where you're going.

[PRR]

> first option has a 15k Primary, and the second option has a 25k Primary..

Whatever you can get, cheap.

You "optimize impedance" to get the "very most Power (maybe low THD)". If you wanted POWER you wouldn't use the itty-bitty 6AU6, you got your EL84 6V6 6V6 EL34 KT88 to rock the ballroom. This amp is for when you DON'T want "Power!", you just want an electronic tone louder than a naked solid-body.

> I don't see a 'Load Resistance' figure on the 6AU6 Datasheet

First: use a good tube-sheet source http://www.mif.pg.gda.pl/homepages/frank/vs.html

Second: 6AU6 is a "high gain" tube so would not have Power Amp numbers.

For "good power pentodes" in SE use, a happy load is near Vp/Ip. Plot a loadline: a "perfect device" will swing 100% of B+ peak signal, and will swing 100% of idle current peak signal. A good pentode can swing about 90% of B+ peak signal, and can swing about 90% of idle current peak signal. Either way, optimum load is Vp/Ip. We may go 10%-30% either way looking for better THD numbers, or to suit available standard iron. We can go way-way off if we are not obsessed with Advertised Watts numbers.

149V/10mA is exactly 15K or 25K for all practical purposes.

Yes, the $16 part says "5mA" and this amp pushes 10mA. But we are so far below the nominal 3.5 Watt core-size that it won't hurt.

[PRR]

> Why break the connection to the 1st cathode bypass cap?

The original used a "power" output tube (6V6) that needs 15V-20V signal for full output; also a higher B+ at the preamp.

This 6AU6 "output bottle" only needs like 1.2V signal for full "power". Since the original had "useful gain", this revision may have "too much gain".

Also the lower preamp B+ means the preamp may overload sooner than the original.

OTOH, tastes/expectations of gain and overload have changed since Ike was in office.

Try it both ways, see what works.

You can also snip the "power stage" cathode cap, for less gain and distortion; but I think you *want* that stage to phatten-up rather than abruptly clip.

I have not built this.

[tubegeek]

> first option has a 15k Primary, and the second option has a 25k Primary..

Whatever you can get, cheap.

On that subject, note that the Angela Instrument transformer is EVEN CHEAPER!

(I always hate calling transformers - or transistors - "trannys." Me, I save that for gender-changing turnarounds like MXLR-MXLR.)

And while I have gotten great service out of both vendors many times, I'll also note that I heard that Uncle Ned is no longer at triode electronics and he was the main reason I started using them. Anybody know what happened there?