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 Post subject: Power supply redesign
PostPosted: Mar Mon 04, 2019 5:13 am 
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I have the following power supply I built for my BC-455A receiver and 6BQ5 push pull amp out of parts I had on hand.

Attachment:
Power supply.png
Power supply.png [ 27.15 KiB | Viewed 1037 times ]


I would like to redesign the HV section so that there is less heat.

Also I don't like the choke because it is either run right at or exceeds its current rating.

B+ does need to be well filtered.

So I'd like to ditch T3 with T2 having a 12.6Vac 3 amp CT winding or a 12.6Vac 1A winding and a 6.3vac 2A winding. I'd even be interested in a transformer just for the HV and one for the LV as that would give me a standby switch option where I can keep just the tube heaters turned on.

The 392 ohm resistor needs to be gone as it is two 196 ohm 30 watt resistors in series under the chassis and does put out quite a bit of heat.

The B+ for the inverter is taken from the amp B+ right after the choke. That stage has no other filtering as I determined it not necessary given the already good filtering provided by the first filter cap and choke.

For the choke the idle current of the radio and amp based on the voltage across the choke divided by the DC resistance of the choke is 125.79mA which is a bit over its 100mA spec. It was what I had on hand and it fixed a problem I had.

I'm thinking perhaps of keeping a choke in that position that can handle at least 150mA unless it simply is not needed and I can get just as good filtering without it. The ripple voltage before the choke is 3.48Vrms. The ripple voltage after the choke is 21.2mVrms.

What HV will the transformer need to put out to give me the DC voltages I show on the schematic?

I'd prefer to go with an Edcor power transformer if at all possible.

EDIT:

I found the following Edcor choke.

https://www.edcorusa.com/cxc125-10h-200ma

Its downside is its size.

I found the following Edcor chokes.

https://www.edcorusa.com/xc63-0_5h-250ma

https://www.edcorusa.com/xc63-1h-200ma

https://www.edcorusa.com/xc75-1_5h-250ma

https://www.edcorusa.com/xc75-2h-200ma

Would any of those provide the same or better filtering?

Or is the 100mA choke being run at 125.79mA idle current ok?


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 Post subject: Re: Power supply redesign
PostPosted: Mar Mon 04, 2019 5:10 pm 
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Quote:
I would like to redesign the HV section so that there is less heat.

You don't have any internal DC loads in there, so the power dissapation is determined mainly by the loads that you use The only way to reduce that is to make the raw output of the transformer lower. (IOW, use a different transformer)

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Power supply redesign
PostPosted: Mar Mon 04, 2019 6:54 pm 
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That's the idea, but I'm not sure what voltage the transformer would need to put out to get me the voltages I require.

I've never been real good at figuring that out.

basically the amp is on the power supply chassis and the receiver plugs into the amp/power supply chassis.


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 Post subject: Re: Power supply redesign
PostPosted: Mar Mon 04, 2019 7:06 pm 
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Have you ever used the Duncan power supply design tool?
Regardless, the key is rectification efficency, which is the ratio of the raw DC voltage (before any dropping resistors) to the peak (loaded) voltage from the transformer. If nothing else, see what you are getting in the current design, and then use that to design your improvement

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Power supply redesign
PostPosted: Mar Mon 04, 2019 7:22 pm 
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Think I've heard about the tool but never used it. Will download it when I get home.

One issue is I cannot get a correct measurement of the DC at the rectifiers given there is no filter cap there. If I could take a proper measurement I'd know better what voltage I would need.

Oh I failed to mention that running the Hammond transformer's 115Vac primary on 120Vac I get something more like 332-0-332 Vrms for the HV.

EDIT:

I tried that software, but it does me no real good as it only seems to cover a transformer operated at its specified primary voltage and gives nothing in the way of calculating the changes when operating the transformer at higher than its specified primary voltage.

I've tried various things and so far cannot get the voltages I am measuring as they are all a little lower.

I could try running the primary at 115Vac then retaking the measurements and currents.

Now the current in the choke is assuming the resistance is indeed 190 ohms.

EDIT 2:

If the source resistance of the HV wining on the transformer is 50 ohms I will need a transformer that can put out 250-0-250Vrms in order to get the proper voltages.

For the power supply I used two current sinks to simulate the two loads.

I had forgotten I added a 1 ohm resistor between the HV CT lead and ground for current measurements so I took that voltage reading and subtracted it from the receiver B+ current to get the current the amp draws.


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 Post subject: Re: Power supply redesign
PostPosted: Mar Tue 05, 2019 7:07 pm 
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I'm not able to follow all the nuances of this....

measuring the DC voltage: With a competent meter, you can measure the DC and AC components of just about anything. But--if there is any doubt, get a scope on it.

If you measure the DC before and after that first resistor, what do you get?

BUT---why is that resistor even there? It would be more normal to go directly from the diodes to the first filter. (Maybe have a small resistance if required to limit the ripple current)

If all you want to do is figure out a better output voltage for the transformer, just run the existing one off a Variac to see what specs to use.

pay attention to HV ratings which are normally stated for a specified DC output current.

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 Post subject: Re: Power supply redesign
PostPosted: Mar Tue 05, 2019 7:16 pm 
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The resistor is there to drop the voltage some as I often find a lower value of resistor can be used between a solid state rectifier and filter cap to drop voltage versus a resistor after the first filter cap with that requiring a larger value resistor. Also I am thinking I get some better filtering with the resistor where it is.

You know, the thought had never occurred to me to use a variac.

That said I would need to power the 6BQ5 tubes and 6SQ7 tube separately though to keep their heaters at 6.3Vac.

Am thinking a simpler solution is to get another HV transformer just for testing the HV while disconnecting the wiring to the HV winding on the existing transformer.

Depending on the required voltage (can go a few volts over 324Vdc, but would prefer not to go lower) will determine if I can find a transformer with a 12.6Vac winding. If not I can use a separate transformer with 12.6Vac winding.

Now what I may do is try dropping the voltage to the transformer by 11Vac with the tube heaters and 12.6Vac regulator powered by an external transformer and see if that will drop the B+ enough. The reason I thought of that is because I could then put the 6.3Vac and 5vac windings in series with the primary to buck the primary.


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 Post subject: Re: Power supply redesign
PostPosted: Mar Tue 05, 2019 7:24 pm 
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One of my colleagues once coined a phrase for this kind of thing: "multi-convoluted"

I'll try to help more later....

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Power supply redesign
PostPosted: Mar Tue 05, 2019 7:43 pm 
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Yes it is a bit convoluted.

Often what I do is based on what I have and can afford to buy so sometimes things like this do get a bit convoluted, although it gets me by until I have the money to do it right.


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 Post subject: Re: Power supply redesign
PostPosted: Mar Wed 06, 2019 2:41 am 
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So I did a test using a HV transformer just for the B+ and it looks like I need a power transformer capable of 270-0-270 Vrms.

It looks like this one will work.

https://www.edcorusa.com/xpwr224


Now for the two heater circuits my plan is to connect them like this if it will work.

Attachment:
Transformer.png
Transformer.png [ 59.22 KiB | Viewed 929 times ]


I will have 11.3Vrms.

EDIT:

Did a test and that won't be quite enough B+ for the regulator to properly regulate to 12.6Vdc so I will need another transformer just for the heater B+

So I can put this transformer in series with the 5 volt winding of the other and have 15Vrms

https://www.edcorusa.com/lvp10-1-2

That will give enough B+ for the regulator to properly regulate.

Idle B+ current is 115.3mA.

Will the 100mA choke be ok?


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 Post subject: Re: Power supply redesign
PostPosted: Mar Tue 19, 2019 12:57 am 
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While I like the oval Grundig speaker I used because it has decent bass response, I do not like how it sounds when I turn the radio up as it starts to sound bad which may be because it was used with a radio having one 6BQ5 output tube and I'm using it on a push pull 6BQ5 amp.

So what I'm thinking of doing is this.

Finding a power transformer that can provide a high enough rectified voltage for the 12.6Vdc regulated supply so I can free up the 5Vac winding unless it is possible to ground one side of the 6.3Vac winding used for the 6BQ5 tubes and the 6SQ7 tube and use that in series with the 11.2Vac transformer.

I will then put the 5VAc winding in series with the 115Vac winding so that the voltage to the transformer is bucked by 5Vac. That will reduce the HV secondary to 300-0-300Vac which will give an unloaded B+ of 424.2Vdc. I will then find a field coil speaker capable of dropping the necessary voltage and possibly substitute it for the choke.

That way I can get more volume from the receiver and will still have good filtering.

I would need to drop around 100Vdc if not slightly more.

Now I don't know how to determine what current a particular field coil can handle if I do not know anything but the resistance of the field.

EDIT:

I've just proven something I've said quite often about how it's best to build an amp for a communications receiver or other LO-FI mono purpose to the particular speaker used.

While using the one Grundig speaker mounted to its original wood baffle I got one particular sound and needed the tone control I added to reduce the treble some. I then switched to another slightly larger Grundig speaker mounted in a modified Dokorder speaker box that came with one of their reel to reels and I had to tweak the tone control a bit to get the sound right. Also I found that switching between ultra linear and pentode affected the bass with ultra linear having heavier bass. I'm thinking perhaps of adding a DPDT switch so I can select between pentode and ultra linear. That way if a shortwave station is particularly bassy I can switch to pentode and if a shortwave station sounds a little lacking in the bass I can switch to ultra linear to make the bass a little better.

Only issue is the volume control is a little sensitive. The only place I can add any sort of resistance to lower the audio level is right after the volume control before the first audio tube as the tone control is in the plate circuit of the driver tube and one fixed tone control cap is across the volume control and I don't know how adding resistance between the detector and volume control will affect the tone.

Think I'll get a 1 meg and 2.2 meg and try them in both places just to see what works best. Now adding a 1 meg before the volume control will reduce the voltage by 1/2 since the pot is 1 meg.


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 Post subject: Re: Power supply redesign
PostPosted: May Wed 08, 2019 4:45 am 
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So I bought the two transformers a few weeks ago and they came in today.

Got them installed and here's the results.

B+ 320Vdc 25mV ripple after the choke.

Receiver B+ 275Vdc.

Things were fine until I shorted the 12Vdc line to the receiver B+ taking a measurement. Now I need a new regulator.

Here's the amp.

Attachment:
Amp.png
Amp.png [ 35.16 KiB | Viewed 542 times ]


Will the higher B+ be ok for the 6BQ5 tubes?

Also will the higher receiver B+ be ok?


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 Post subject: Re: Power supply redesign
PostPosted: May Thu 09, 2019 5:43 am 
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A spec sheet for EL84 show 250 volts on the plate.
You might find a lower B+ with the power supply loaded.
My handbook gives the Max plate volts for 6BQ5 as 300 volts, so looks like you are in the clear.

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 Post subject: Re: Power supply redesign
PostPosted: May Thu 09, 2019 1:38 pm 
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I would think it would be easier to control the bass response with a switchable coupling cap instead of switching between UL and regular in the output trans.


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 Post subject: Re: Power supply redesign
PostPosted: May Thu 09, 2019 3:41 pm 
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B+ to the output transformer is 320Vdc.

If it makes any difference I am using the Russian equivalent of the 6BQ5.

https://frank.pocnet.net/sheets/093/6/6BQ5.pdf

Specs for push pull show 300Vdc plate voltage, but I use a 135 ohm cathode resistor whereas the specs call for a 130 ohm resistor plus I use a 6600 ohm plate to plate load. The amp is based on the output section of a GEC 912 plus amp. Given the amp was originally meant for ultra linear operation (made it regular pentode as bass with ultra linear was too much for its intended use) I may get an 8K transformer which may increase the output some, although it gets plenty loud enough as is.

Also in order to get the B+ for the regulated 12.6Vdc high enough I had to take the 5Vac winding and put it in series with the 6.3Vac winding along with the 10Vac from the other transformer. Pretty sure I could use a larger filter cap there to solve that issue.

The idea is not to switch between UL and pentode. It's just that I found pentode to sound best with the speaker I used.


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 Post subject: Re: Power supply redesign
PostPosted: May Thu 09, 2019 4:10 pm 
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You've probably already done this - bring the voltage up slowly with the variac.
Many Fender amp circuits run the output tubes over their max ratings and it works. But you don't need more volume, so there's no benefit to doing that. Your best design will keep the tubes happy for decades of easy listening. 8)

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 Post subject: Re: Power supply redesign
PostPosted: May Thu 09, 2019 4:23 pm 
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I did test with a variac before ordering the transformers as I needed to see what HV the transformer had to put out to get the desired B+

Based on what I posted earlier the B+ is only a few volts higher than previously measured, although I am not sure if the receiver B+ was over 250Vdc as I either didn't measure that or I didn't write the measurement down.

So it could be that everything is as it should be.


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 Post subject: Re: Power supply redesign
PostPosted: May Thu 09, 2019 8:10 pm 
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quotes from 2 posts...

"I'm thinking perhaps of adding a DPDT switch so I can select between pentode and ultra linear."
"The idea is not to switch between UL and pentode."

Huh? :wink:


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 Post subject: Re: Power supply redesign
PostPosted: May Thu 09, 2019 10:58 pm 
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Oh yes I had wanted to do that initially, but decided against it as ultra linear simply provided too much bass boost for the oval Grundig speaker I am using.

Now when I used a vintage 12" speaker in a temporary cardboard box the ultra linear configuration was good as it provided some bass that was lacking.

I think it's neat that I am getting near console radio quality out of an oval speaker in a wood box only a little bigger than the driver.



EDIT:

I've had the receiver running for several hours and so far everything is working ok. No red plating of the output tubes, no receiver issues ETC...

Had to tweak the 12.6Vdc though which I expected to do since the regulator was replaced.

I may also need to tweak the adjustment for the tuning light I added as it tends to be full brightness when there's still some static in the audio.


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