Bucking transformer

[dude]

I have a 66 Vibro Champ all original, except the PT, swapped out the original with a vintage SE that's a little larger, sounds much better. But the original PT puts out 356-0-356, schematic says 315-0-315.


Could this large A/C be caused by 121v wall current? And if so, will a bucking transformer lower the voltage to around 315-0-315? I have a radio shack, (PRI 120v, Sec 6.3-0-6.3  @ 3A) part 273-1511B can I used this PT to drop either 6 or 12 volts and how much would that lower the Champ's A/C from 356-0-356? If I can use this Radio Shack PT, I saw a layout here of a bucking transformer but can't remember what post.


al

[sluckey]

That transformer will drop the primary voltage by 12.6v, or approx. 10%. All the secondaries will also drop by approx. 10%. So, your 356 will become about 320V.

See page 2...

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[shooter]

the original PT puts out 356-0-356

the vintage SE PT you're using is the same as original?

[dude]

the original PT puts out 356-0-356

the vintage SE PT you're using is the same as original?

Never heard of a vintage SE PT, 


Sorry, I meant OT...


The OT I used is 10K, using 4 ohm spk and a 6L6, matched? 

[jjasilli]

Let's untangle this.  Please confirm or correct each of the following:

*  You are using the stock PT.
*  The stock PT is outputting 356 -0- 356.
*  Is the 356 -0- 356 loaded or unloaded?
*  You're using a 6L6 (not a 6V6).
*  You installed a "new" (actually vintage) OT.
*  The primary of the "new" OT is 10K. 
*  What is (are) the OT's secondary tap(s)?
*  What is the speaker impedance?

With this information we can mystically divine the correct answer without ever having heard or seen the question. 

[dude]

Stock PT original
Yes 356-0-356
Loaded with US 5Y3, two 12Ax7s,
RCA 6L6GC, (bias R 3W 470 that drifted to 360)
OT install is old, from early 60's, SE, slightly bigger in size (1 1/2 times bigger) sounds way better all the mud is gone with distortion
Replacement OT PRI is 10,000 ohms (was unknown, did the math, 99% sure it's right, nothings 100% but death)
Don't know the Sec. (but there were two of these OT's in a (guess [2] EL84) stereo something?
Using a 4 ohm replace Champ speaker from Weber


My guess: But not sure...?
with a 6L6, (wants to see 4K) so use an 8 ohm speaker (plays well on the 8" 4 ohms spk).
with a 6V6 use the 4ohm (wants to see 7K-8K, 10K close enough)


extra info: Plate voltage (US 5Y3) with 6L6 plate 348v (750 screen R), K resistor is 460, 27v across K resistor, didn't do the math but probably running hot. Up the cathode R..?

al

[PRR]

> didn't do the math but probably running hot.

Maybe you should do the math?

[sluckey]

Replacement OT PRI is 10,000 ohms (was unknown, did the math, 99% sure it's right, nothings 100% but death)

Show us the process and math you used to determine 10,000.

Don't know the Sec.

Why not? If you did the math above you must know the secondary.

My guess: But not sure...?
with a 6L6, (wants to see 4K) so use an 8 ohm speaker (plays well on the 8" 4 ohms spk).
with a 6V6 use the 4ohm (wants to see 7K-8K, 10K close enough)

Quit guessing and get it right. Hint... If a 4Ω speaker reflects a certain impedance to the primary, then doubling that speaker to 8Ω will double the impedance on the primary. It's as simple as that. The primary impedance is directly proportional to the secondary. Increase one, the other increases. Decrease one, the other decreases.

[shooter]

bias R 3W 470 that drifted to 360

that IS a significant drift, is that because of different tubes you're playing with, or you woke up and walla 110 ohms left the game

[mresistor]

bias R 3W 470 that drifted to 360

that IS a significant drift, is that because of different tubes you're playing with, or you woke up and walla 110 ohms left the game

Methinks that is not a drift but a result of over current and heat. Guess you could call it drifting but don't cc resistors tend to "drift" up in value?

[jjasilli]

Good progress.  At this point I suggest ignoring everything, except do what PRR & sluckey advised.  Sluckey is right that you can't know the OT's primary impedance without also knowing its secondary impedance.  This is determined by applying voltage to figure out the turns ratio.  If want help spec'ing out the OT, please post.

[dude]

OK, I can't find the exact things I did to find the rating on the OT I used. I had it all written down from a forum somewhere, saved it for years and now I can't find it. Seems the older I get the more this stuff (shit) happens.   What I can remember: I had a small PT 12.6v sec, 120 pri, hook up wall current to the OT on one sec, 6.3v and it's center tap, took a reading on the primary to find the ratio, then determine the OT's impedance.  Can't remember how I determined the ratio, I don't remember how the sec on the OT came into play but I was trying to find that out. If somebody can clue me in, I promise to "remember" how to do this. If I don't don't know the the impedance and sec of the OT, how am I supposed to use it correctly .


I'm sorry, the K resistor is 460, not 360, I screwed up typing, if it was 360 probably would have blown the 6L6 by now. Still it's probably running hot. Once I get the correct speaker for the tube I used, I'll bias the amp correctly.


I need to write things down, take notes and make diagrams as I learn. Because in two years, I might not remember my name,  , yeah that's funny but I get pissed when I forget stuff, guess it's old age. I'm a college graduate with a BS, whatever that's worth, nothing now at my age. Been a Cabinetmaker for over twenty five years, I can do math but have no electronics education, never had a teacher, everything I learned on my own and that's not much, ha.


As far as the correct speaker to use at the correct impedance, double one, double the other, decrease one decrease the other, an example:
OT with 8K OHMS PRI with three taps: 4 OHMS, 8 OHMS and 16 OHMs, use the matching speakers you get 8K to the PRI.


Use a 4 OHMS speaker load on this OT (on the 8 OHMs tap) the PRI impedances changes to  4K?  Use an 8 OHMS speaker (on the 16 OHM tap) the PRI impedance stays at 4K? Lowering the wanted spk impedance, lowers the PRI impedance? I got this right?


If so, next step how do I determine the PRI and SEC impedance of an unknown OT. Get this down, then I can "know for sure" what speaker to use with what tube I pick.


thanks   

[jjasilli]

Let's focus on the OT for now. Note that an OT has no impedance per se.  It has a turns ratio.  Once we measure the turns ratio we can see that a certain impedance on the secondary side reflects back an impedance on the primary side.  This site explains it, and is basically a tutorial:  http://www.radioremembered.org/outimp.htm


Please go through this process with your OT.  It's critical, because if the impedances from tube plate > speaker are too far off, you may be causing an over-current condition in the power tube, it's cathode resistor and the OT which are harmful. 

[sluckey]

So how did we get from bucking transformer to OT??? Your Cadillac has a wheel in the ditch...

[dude]

So how did we get from bucking transformer to OT??? Your Cadillac has a wheel in the ditch...

I'm a Southern Man    Am I right with the impedance selection.

[sluckey]

No

[shooter]

take notes and make diagrams as I learn

get the PA sorted out,
here's a fwiw on how I started documenting my builds, I consider it "bare minimum", go to Sluckey's page for the Gold standard

[dude]

No

Guess I'm out to lunch

[sluckey]

Read the link jj posted. Tomorrow I'll describe how I deal with an unknown OT.

[sluckey]

My method for determing the impedance ratio of an unknown OT is basically the same as the method described in the link jj posted. The only difference is that I apply a small known voltage to the secondary and measure the voltage across the primary (plate to plate leads if push/pull OT). Goes like this...

Connect a variac to the primary of a filament transformer and connect the 6.3VAC secondary to the secondary of the unknown OT. Adjust the variac to provide exactly 1VAC on the secondary winding of the OT. Now, using the same meter measure the voltage across the primary of the unknown OT. Let's say it measures 30VAC. That's all the testing you need to do.  We have measured a voltage ratio of 30:1. The voltage ratio is the same as the turns ratio. Now break out the calculator. The impedance ratio is the square of the voltage (turns) ratio, so calculate the square of the voltage ratio, 30² = 900. So the calculated impedance ratio is 900:1. This means that if we connect a 1Ω speaker to the secondary, the primary would reflect 900Ω back to the tube(s). Nobody uses 1Ω speakers. Let's connect an 8Ω speaker instead. So, multiply the 900 impedance ratio by 8Ω and the OT primary now reflects 8x900=7200Ω to the tube(s). Most people would say this OT is a 7200Ω to 8Ω impedance. That's not quite right. Remember, an OT has no impedance. It has an impedance ratio. If you connect a 4Ω speaker the primary will now reflect 3600Ω to the tube(s).

Get this simple process nailed down. Don't confuse the issue by stating this tube or that tube, or saying if I put a 4Ω speaker on the 8Ω tap. Once you are comfortable with determining the IMPEDANCE RATIO, then you can easily determine if the OT is suitable for a particular tube by consulting a tube manual.

Remember this, if speaker impedance go down, primary impedance go down. If speaker impedance go up, primary impedance go up.

So, what does this have to do with a bucking transformer? Absolutely nothing. How did we stray so far from the original question? 

[jjasilli]

I think I forced this issue to document the actual operating specs of Dude's amp as modded.  It also appears to me that the voltage issue is a red herring; and there may not be any problems at all (see below).  But we still need the actual facts of the amp's operation. 


BTW: my hotrodded VibroChamp runs fine as follows:  Stock PT > 5U4GT rectifier > 375 plate volts > 6L6 with 320 Ohm cathode resistor & 25uF bypass cap > Hammond 125DSE OT


375 - 25 Cathode Volts = 350 Volts  X .0782 Amps = 27 Watts Plate Dissipation X 40% (estimate) = 10 Watts Output

[dude]

Sluckey, I apologize for my lack of knowledge, I know you're only trying to help me. Bare with me, at times I guess I can get a little thick,  . You have helped me in the past and I appreciate that.


I'm reading the info jjasilli posted on output impedance and your take on identifying the ratio turns of an unknown OT, thank you both. I don't have a variac so the math will be a little more involved but I have a 6.3v filament PT so I should be able to find the ratio using 120v wall current, or whatever it measures. I'll get back with what I came up with.


As far as biasing the amp, I did what jjasilli posted on his Vibro Champ. My readings: 329 - 24.15 = 304.85v x 0.069 A = 21.03 watts dissipation
with a 6L6GC 30 watts rating. I don't understand the 40% X the dissipation to estimate of amps wattage output..?


The whole thing started with my concern of high voltage from the original PT, so I was thinking a bucking transformer but everything evolved to possible other issues, which is good. I want to get my amps working as best I can, I admit I'm a tone freak, not that I keep "tinkering" but once I get there, I stop. My 6V6 Plexi is sounding greater than I ever could expect thanks to you guys.


I assume 21 watts Dissipation is on the money, that's 70% but I liked the tone better a little hotter, I'll lower that K resistor from 350 to maybe 320 and check again, in jjasilli's vibro champ he is running at 27 watts, that 90% and he's happy.


I'll get back on the OT ratio but I would like to match the ratio impedance to the tubes I'm using in all my amps.


al     

[sluckey]

The only reason I use a variac is because there's always one sitting on my bench. You certainly don't need one. Just connect 6.3VAC to the secondary. Measure it to be exactly accurate. Then measure the primary voltage. Now divide the primary voltage by the secondary voltage. That's the voltage (and turns) ratio. Square that to get the impedance ratio.

A single ended amp such as what you have typically runs at about 90% to 100% max dissipation.

[jjasilli]

I don't understand the 40% X the dissipation to estimate of amps wattage output..?
I understand this to be a common & accepted way of estimating the output in Watts of an SE power amp.  SE is pretty inefficient, so output in Watts is a rather small fraction of the power consumed, or "dissipated" by the power tube.  Of course it could be measured rather than estimated.

[shooter]

Of course it could be measured

I've measured a bunch and 40% is good for cleanish, full out square get pretty close to 50%

once your OT is worked out, fine tuning the cathode R is fun. play as is til you got the rif, change R, re-calculate, < 30W, play, better?, keep going til ya can't.  When you rise the R value, you can drive a bigger signal in, but rising R lowers I, so power drops, you don't have to be in tune with the whole band 

[sluckey]

As the cathode R value rises, it pushes up the plate voltage on the other end of the circuit.  The higher plate voltage pulls more current through the tube circuitry including the larger cathode R.  The same holds true in reverse for a lower cathode R value.  The plate voltage drops, so less current is pulled through the smaller cathode R

You've got that twisted around. A larger cathode resistor causes less current to flow through it (and the tube). Simple ohms law. This lower current causes the plate voltage to rise because the reduced current flow has less voltage drop across the OT. The higher plate voltage does not pull more current through the tube. The bias between the grid and cathode control the flow of current. Just the opposite is true if you decrease the cathode resistor.

[dude]

My method for determing the impedance ratio of an unknown OT is basically the same as the method described in the link jj posted. The only difference is that I apply a small known voltage to the secondary and measure the voltage across the primary (plate to plate leads if push/pull OT). Goes like this...

OK, used the 6.3v filament PT, the 6.3v leg was really 7.23vac. Putting that into the sec of the OT gave me 244.7vac at the primary. Probably safer to put the 7.23v into the primary and measure the sec...? Anyway:


244.7v divided by 7.23v = 33.845 or 33:1 impedance ratio (measuring 7.23 into the primary gave me a ratio of 34.42, the same


So, the OT has a ratio of say 34:1


Now to find the impedance ratio, square the turns, 34 = 1156 so calculated impedance ratio is 1156:1


Since I have a 4 ohms speaker in the Vibro champ, I take the 1156 impedance ratio X the 4 ohms and get 4624, this is the what the OT reflects at 4 ohms. So a 6L6 likes to see about 4K the 4.6K is close, this OT seems suitable for a "single" 6L6 with the 4 ohms spk. What about, if it was A B, push and pull and had two 6L6s...? things the same...?


Back to the OT, using a 6V6 which wants so 6.6K to 8K. If I use an 8 ohm speaker with this 6V6, things change. 1156 X 8 ohms = 9.2K. probably best to use an 8 ohm speaker with the 6V6...?


I got this right?


I have more questions, what about "push bull amps with two tubes?"


You say if primary impedance goes up or down so does the speaker impedance, this is where I get thick,   


Note:  An OT with an 8K primary (impedance) and 4, 8, 16 ohm taps, or an 8K pri with 16 ohm sec and 6L6s are used, AB Push Pull amp. "Use the 16 ohm tap with a 8 ohm speaker load."  So, going "down" to 8 ohm speaker, you also have to go "down" on the OT pri from 8K to 4K. Using the 8 ohm speaker on that 16 ohm tap will change the OT impedance Primary down to 4K. Don't freak,         I'm trying to get this down. 
     
 
   

[shooter]

what about "push bull amps with two tubes?"

need a new OT for that

[jjasilli]

Yes, your math & procedure is right. 


Shooter is right that you need a PP type OT for PP, and SE PT for SE.


IF you have 2 tubes in PP, go to the Tube Chart: it will state the plate-to-plate impedance.  This is the primary impedance recommended for the OT. (It's not the internal impedance of the tube.)  If you have 4 tubes in PP, the impedance will be 1/2 that for 2 tubes (because the tubes are in parallel).

In SE typically one tube is used.  If you use 2X tubes in SE, again, the impedance is halved.  So 2X 6L6's in SE would want about a 2K Primary impedance in the OT. 

[dude]

Makes sense jj, thanks

[dude]

Sluckey, still trying to figure why you didn't answer...? Guess, I said something you didn't like....? Hope not. Thanks anyway

[sluckey]

Sluckey, still trying to figure why you didn't answer...? Guess, I said something you didn't like....? Hope not. Thanks anyway

Not at all. Didn't know you asked me something?

[dude]

Oh, I was looking for your answer on the info you posted above and my method for finding the Primary OT impedance ratios of an unknown OT, if I was thinking correctly? jj did say it looks right to him, so I assume my last statement in that post was correct, the last sentence "Note part", thanks.

[sluckey]

Note:  An OT with an 8K primary (impedance) and 4, 8, 16 ohm taps, or an 8K pri with 16 ohm sec and 6L6s are used, AB Push Pull amp. "Use the 16 ohm tap with a 8 ohm speaker load."  So, going "down" to 8 ohm speaker, you also have to go "down" on the OT pri from 8K to 4K. Using the 8 ohm speaker on that 16 ohm tap will change the OT impedance Primary down to 4K. Don't freak,         I'm trying to get this down. 

Sounds good to me. I believe you got it.

Up is up. Down is down.

Yes, I told you that the light goes up and down.
Don't you notice how the wheel goes 'round?

What goes up, must come down.
Spinnin' wheel got to go 'round.

And you better pick yourself up from the ground.
Before they bring the curtain down.

Now, how 'bout that original question? Did I answer it satisfactorily? 

[jjasilli]

As the cathode R value rises, it pushes up the plate voltage on the other end of the circuit.  The higher plate voltage pulls more current through the tube circuitry including the larger cathode R.  The same holds true in reverse for a lower cathode R value.  The plate voltage drops, so less current is pulled through the smaller cathode R

You've got that twisted around. A larger cathode resistor causes less current to flow through it (and the tube). Simple ohms law. This lower current causes the plate voltage to rise because the reduced current flow has less voltage drop across the OT. The higher plate voltage does not pull more current through the tube. The bias between the grid and cathode control the flow of current. Just the opposite is true if you decrease the cathode resistor.

Thanks for correcting that.  I deleted that Reply.