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Author Topic: HW A-7 Amplifier to Speach amp  (Read 33642 times)
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ke7trp
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« Reply #50 on: October 23, 2011, 09:26:05 PM »

I hooked it up the way you told me and it works 100%.  Smooth zero to full blast. Thanks alot Fred! 


C
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KA2DZT
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« Reply #51 on: October 23, 2011, 09:45:05 PM »

Ok. That worked Fred.  Its to touchy still thought. The usefull range of the Pot is only 1 to 1/4.  I need a half value pot so I can get a wider range. Or just leave it.  This stage is super hot with sensitivity

Power supply tests:

At 126volts mains:

450 DC out of rectifiier.
First supply 450dc
Second supply is 420
Third supply is 171

A 2K only got this down to 400

I have a bunch of 10K power resistors.  These got the volts down to 190.  But the rest of the voltages where low and what concerned me was that the volts on the rectifier dropped all the way down to 350.  So I had a 100 volt Drop OUT of the Power supply. This is telling me that the reistor (which runs hot) is pulling down the supply.  Is this expected and how can this be easier on the transformer? The tubes ran cool at 190 volts. The amp seemed fine.  Maybe put two of these 10Ks together to get a 5K and leave it?  Or I can try on the other side of the Filter?

C


The resistor in between the rectifier and the first cap is NOT pulling down the supply.  It is doing two things. One, it is dropping some voltage across it, depends on the current and the value of the resistor, Vdrop = RxI.  The second thing it's doing is changing your supply from a cap input filter to a resistor input filter.  This second thing brings down the voltage on the first cap.  To explain,  without a choke or resistor before the 1st cap, the 1st cap tries to charge all the way to the peak ac voltage of the xfmr.  This can be 1.414 times the ac voltage from the xfmr secondary.  When you start your filter with a choke or resistor the DC voltage will be about 0.9 times the ac voltage from the xfmr secondary.

The cap input filter puts a heavier VA (volt amp) load on the xfmr.  The choke input filter puts a lighter VA load on the xfmr.   Choke or resistor input filter allow xfmrs to run cooler.

When you look at ratings for a power xfmr,  it will say how much current the xfmr will handle.  BUT with what type of filter?  Example,  a xfmr might be rated at 100ma with a choke input filter. Now, if you use the xfmr with a cap input filter you will get a higher DC voltage output.  BUT, you must reduce the current drain down from the rated 100ma.  Usually, this current reduction will be about 30%.  So, you should only draw about 70ma from the supply.  This reduction keeps the VA load on the xfmr within its ratings.  Likewise, a xfmr rated at 100ma with a cap input filter can be used with a choke input filter and yield about 30% more current than the rated 100ma.

I'll get back to the resistors on my next post, I've got to yield the computer for awhile.

Fred
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ke7trp
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« Reply #52 on: October 23, 2011, 09:59:41 PM »

I follow 100%.  I just reread the PS chapter in my 1950s handbook. 

For a learning experience, I am just going to redo the entire supply.  There are three voltages needed:

390
360
180


Right now, I have:
425
400
211

Not far off.  I would like to ditch the 5y3. Its just running hot and if I am going to redo this, why keep it? I would like to use a choke. I can rip out the old can cap (not used).  Then mount any choke there on the deck.  Otherwise, I can just use a resistor input, THen adjust the resistors after this one to reach the other voltages.

Parts on hand:

Chokes:
70/350vdc/500ma (golf ball size)
15/1500/1amp (way to big for this but its free.

Caps, 100s,47s,22 at 450, new stock

Lots of 10K, 1500 and 2k  power resistors,

This is lots of fun!

Clark
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KA2DZT
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« Reply #53 on: October 24, 2011, 02:38:45 AM »

Clark,

Glad you're having fun,  FB on your parts supply.

Let me know when you get up to more than 1000 xfmrs and chokes, about 250,000 resistors and maybe 100,000 caps.

I'll look at the PS in a bit.

Fred
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KA2DZT
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« Reply #54 on: October 24, 2011, 04:29:07 AM »

You may want to keep the 5Y3 for a few reasons.  Your voltage is very near the voltage rating of the filter caps (450V).  When you use SS rectifiers the B+ comes up as soon as turned on.  The tubes are not yet hot enough to draw current.  So, there is very little or no load on the PS.  With little load, the B+ can go way over the 450V and short the caps.  The 5Y3 brings up the B+ in step with the other tubes heating up.  This keeps a load on the supply.

The choke or resistor input filter keeps the caps from charging to the full ac peak voltage, PROVIDED there is a minimum amount of current being drawn from the supply.  That is why bleeder resistors are used in power supplies.

Getting to the resistors,  Use a value that will bring the voltage on the first filter cap down to the 390V or maybe a little lower than 390V  If you go lower like 325-350V, you can adjust the value of the other two series resistors to bring up the two other voltages.  Probably the second voltage can be left at whatever it ends up being, a little lower than the first voltage.  It's the last voltage that you'll probably have to bring up.  I think that resistor was a 100K,  this resistor can be dropped to whatever value will bring up the voltage to the right amount (180V).

After you find the right value series resistor to put between the 5Y3 and the 1st cap, you then must figure what wattage you need.  Two ways,  watts = current through the resistor, squared, times the resistance.  But it may be hard to measure the current.  So, measure the voltage drop across the series resistor.  Wattage = voltage drop, squared,  divided by the resistance.

Once you find how much power is being dissipated in the resistor, you need to use a power resistor 2-4 times that wattage.  You may have to use a number of resistors in parallel to get the right value and wattage.  (2) 10K 10W in parallel will give you 5K 20W.  (3) 10K 10W in parallel will give you 3.33K 30W.  These resistors will get HOT.

Hope this helps,

Fred
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ke7trp
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« Reply #55 on: October 24, 2011, 11:06:37 AM »

Ok. Sounds like a plan. I should have some time later to redo the supply. I need some terminal strips. I am going to hack everything out and start over.  Mount the 5K resistor and then mount the caps, 47/47/22.  Any reason to up this to 100, 100 47?  I have the caps.

The other thing I could do is add a Standby switch.  I noticed yesterday that the B+is 550!!! when you first turn the amp on, then it slowly settles down over about 5 seconds.

I can do this a couple ways, 

1.  Add in a switch (I have about 200 brand new 50s style toggles) that turns the fils on independant of the main, on and off switch.  This would allow me to warm up before turning the volumn knob/on and off switch.  I have LEDs and neon lamps for fil indicator.  Simple and with an indicator lamp would look nice.

2. I could have the new switch operate the B+ stand by/operate and use an indicator for B+

3. The last thought is that I have a really neat looking amperite vac relay(looks like a tube).  I could install this where the old can cap was and wire it up so after the time runs down, it would click on the B+. I think its 5 seconds, Alos with a indicator for B+

C


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KA2DZT
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« Reply #56 on: October 24, 2011, 12:28:28 PM »

Are you sure your volt meter is reading correctly??

The 47/47/22 is just right, don't use the larger cap.

No need for a stand-by switch, or any delay relays.

Surprised the 550 volts hasn't shorted the filter cap.

You may need to add a bleeder resistor to keep the peak voltage down.

The best thing to do is put the correct resistor from the 5Y3 to the first filter cap.

Fred
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ke7trp
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« Reply #57 on: October 24, 2011, 12:41:52 PM »

Yes. Even with that resistor installed, The voltage is 470 before the rectifier warms up and volts lower. Its right on the endge. The meter is a fluke.  The voltage is really high before the tube warms up

C
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KA2DZT
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« Reply #58 on: October 24, 2011, 01:09:31 PM »

Yes. Even with that resistor installed, The voltage is 470 before the rectifier warms up and volts lower. Its right on the endge. The meter is a fluke.  The voltage is really high before the tube warms up

C

You are measuring the voltage from the negative lead on the filter cap to the first positive on the cap.

Seems a little strange to see that much voltage even before the 5Y3 warms up.  Not sure if that's normal,  nobody ever measures the voltage on the first cap on start up.  Maybe the 5Y3 was still hot from a previous turn on??

If you have the resistors, try adding a bleeder resistor.  Use about 100K 20W.  Connect the bleeder resistor from the SECOND filter cap to ground.  Just clip lead it in for testing.  See if the voltage on start up is somewhat lower.

Fred
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ke7trp
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« Reply #59 on: October 24, 2011, 02:40:16 PM »

I measured right at the Rectifier on the way to the first filter cap.  Voltage is super high at turn on, Then settles down as the tube warms up. 

Here is the plan,

Rip the old crap out.

1. Mount a terminal strip.
2. Run a wire from the rectifier Pin 4 to terminal strip.
3. Frist terminal to 5k 10 watt resistor, then other side of resistor to terminal 2
4. Terminal two is where the first + cap goes.  Other side of cap to ground.
5. Then to a resistor and next cap +, Minus to ground.
6. Then to a resistor and last cap +  Minues to ground.
7. Tap this PS for all three wires leading to the tube stages.

Once that is done, I will adjust the resistors to get  300 volts for the push pull tubes, Then 275 for the second supply and 160 for the last supply.

If the B+ still soars, i will add a Standby switch on the output of the rectifier after the resistor but before the first cap with a Neon indicator lamp.  This is common on Guitar amps to stop cathode stripping and blowing filters out.  Some guitar amps will have 500 to 600 volts before the tube warms up according to google.

Sounds good Fred?   

C




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KA2DZT
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« Reply #60 on: October 24, 2011, 03:23:01 PM »

Clark,

Sounds like a good way to be able to adjust the resistor values.  Go ahead with the plan and you can then work forward from that.

Fred
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ke7trp
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« Reply #61 on: October 24, 2011, 05:10:11 PM »

Ok. New PS is working.

411 volts on the rectifier
305 on the first
285 on the second
110 on the last one. 

Need to remove the 100K I have on the last one.  110 is to low.  I would like 190 here.  Amp sounds smooth and there is no hum at all. 

I used 47,47 and 22 caps at 450.    It bleeds down in 5 seconds.   I need to test again to see if the voltage soars up. I forgot to do that.

Neat little amp.  Sounds really nice for a complete piece of trash that was free. 

Really... Thank you for the guidance Fred.

C

   

 

C
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KA2DZT
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« Reply #62 on: October 24, 2011, 05:21:29 PM »

Clark,

Those voltages look good.  Just drop the 100K resistor between 2nd and 3rd filters. 
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ke7trp
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« Reply #63 on: October 24, 2011, 06:37:02 PM »

33K got it right on the money.   

Thinking of putting in an VR150 in the open hole in the chassis for the LV supply just for fun. MIght try that later on today. 

C
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KA2DZT
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« Reply #64 on: October 24, 2011, 06:49:18 PM »

33K got it right on the money.  

Thinking of putting in an VR150 in the open hole in the chassis for the LV supply just for fun. MIght try that later on today.  

C

Clark,

Don't get too creative,  you don't need any VR tubes in this amp.

F
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ke7trp
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« Reply #65 on: October 24, 2011, 08:27:08 PM »

VR tubes look cool though  Shocked


Its been running for about 45 minutes now in a 75 degree room.    The power trans is at 89 degrees.  The 12a6 tubes are 135f.  The Rectifier is smokin hot at 200f.

The rest of the map is cool.  I will check temps again in an hour just for fun.  The original setup ran the Trans very very hot.  I could not touch the trans without pain.  So as of right now, Its running much cooler.  But it may be heating up still. Its a potted sealed type transformer.

I have to work tomorrow on location so I wont be able to hook the amp up to the rig until tomorrow night at the earliest.  If it sounds good, great, If not, I will hook it to the diode load on an SP600 and let it play 160 meter AM.


C
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« Reply #66 on: October 24, 2011, 08:45:39 PM »

Is there a possibility that the power transformer has a 105/120V tap and the thing is accidentally running on the 105V tap? What is the filament voltage running during all this super high voltage business?
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ke7trp
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« Reply #67 on: October 24, 2011, 09:17:43 PM »

No the trans has only one primary Pat.  All of the voltages where correct and by design.  Just slightly high due to line voltage.  They spec;d 390 DC.  It had that at 110 volts input. It was a cheap kit amp and did they really ran the crap out of the tubes.


Ok. Its fully warm now and not getting hotter.

Trans was 105F.  Warm.  Remember, Its surrounded by the 12A6s and the 5y3.  So some of that heat is likely radiant heating. 

The 12A6s are running at 135F
The 5y3 is at 162. I think the 200F reading was false.
The preamp tubes are cool.

The 5K power resistor on the output of the rectifier is at 130F. 

I checked the Voltage spike on turn on and its 426DC(at the rectifier tube but before the resistor and caps) the instant you hit the power switch and then fades down to 365 over about 5 seconds.  The big 550 volt spike is now gone.  I would guess this was the resistor input fixing that problem. No need for any stand by switch as my caps are 450 DC rated.

Short of a SS rectifier and a buck boost or lower voltage transformer, I dont think we can get better then this!

Fred your idea worked perfectly!

C

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KA2DZT
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« Reply #68 on: October 25, 2011, 01:23:17 AM »

Clark

I would leave in the 5Y3.  You'll only raise the voltages again.  If your voltages were too low you might do the SS mod.

I built (HB) an amp very similar to your amp with 6V6s back in 1964.  Worked great and still works.  My voltages were a little on the low side so I SS the rectifiers many years ago.  Still working but I may put the 5Y3 back in.

OK on the temp measurements.  I never did any myself except for touch measurements.

Small xfmrs will run warm but shouldn't be hot.  Tubes run hot, especially any of the power tubes and rectifiers.

Fred
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ke7trp
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« Reply #69 on: October 25, 2011, 12:52:07 PM »

I agree.  We took 75F off the Transformer.  So it has to be happier.  I will leave the amp as is. Time for on air testing tonight.

C
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