nc-183d hot and low negative bias

[AB3FL]

I have an nc-183d that runs very hot.  If I keep the lid down after an hour, the transformer is too hot to keep my hand on.  I have replaced all electrolytics, paper caps.  I even popped in a 5r4 rectumfier sintead of the 5u4 to lower the B+ and take a few watts of heater current away.  The HV does measure 260V as it should.  The heater voltage is closer to 6.5V instead of 6.3V.

The bias is very high(low).  It should be around -39V, but I am getting -55V.  Is this too much?  If so, how can I decrease(increase) it to -39V.  The two 1K resistors do read 510 ohms together.

Otherwise it works great.  Also the 6v6GTs read a little weak on my tester, but it gets plenty loud

thanks

Tom - AB3FL

[The Slab Bacon]

Tom,
       after having several 183s and 183Ds They all seem to do that. Apparently the power transformer National used was a bit undersized for the job at hand.
If you have recapped it and dont have any leaky caps sucking the life out of the power supply, I'd say this is somewhat normal for them.

the increased biass could just be from slightly aged 6V6s. Or it could be caused by excessive draw on the B+ line. IIRC they have a voltage divider on the B- line to create the biass voltage. If it has been recapped and the values of the critical resistors checked. (that generation of Nationals is known for crappy, out of tollerance resistors) If all else is good, consider it normal.

The 2 that I have now (1 183 and 1 183D) both have replacement power transfomas in them. Noticably larger than the originals. So have the majority of them that I have repaired for other people. IIRC, this is a rather common problem.

Just my $.02 worth

[WQ9E]

My 183D also has a replacement transformer.  As long as your are sure C98,99, and 100 have all been replaced I would also consider it normal.

[AB3FL]

Thanks!   I will try new 6V6s and maybe lower the resistors from 500 ohm to maybe 400 ohm and see what I get.

Here is the transformer

Tom - AB3FL

[The Slab Bacon]

How uncanny! ! ! ! !  That looks exactly like the transfoma in my 183D!

[AB3FL]

Another thing I noticed on the schizmatic is that the screen grids are about 5 volts higher that the plate.  I though it was bad to have the screen at a more positive potential than the plate.  Also on the voltages page on the manual there are a lot of voltages that are positive but when I measure them, they are negative.  For example the diagram says 19V on the cathodes but I have more like -29V on them (6V6s).  IS there a proper voltage diagram anywhere?

Tom

[The Slab Bacon]

The screens being a few volts higher could just be a product of low emission in the tubes. (the screens are just not drawing enough current to get enough drop out of the screen dropping resistors.)

But the cathodes being negative is a bit odd........ IIRC they have cathode biass resistors which should place the cathodes at a positive voltage. (making the grids more negative in respect to the chassis ground and the cathodes as well.) If you have a negative voltage on the cathodes this makes the rest of the tube's elements more positive.

First, I would verify your meter and the pins that you are checking the voltage on. If all is good so far, I would closely examine the wiring for the "new" power transfoma and make sure that the wiring isn't "hamboned".

[WQ9E]

The bias scheme for the NC-173, 183, and 183D will provide a negative output tube cathode bias reading when you read from terminal to ground.  I think they just left out the minus sign by accident.  In this area, the NC-183 is very similar and they specify -19 volts between tube terminal and ground.  The NC-173 uses a similar circuit but with a single output tube and specifies a reading of -21.5 to ground.

Measure the actual resistance and then the voltage drop across R-66 and this will let you calculate the actual idling current for the 6V6 stage to see if it is normal.

The screen voltage is going to be a bit higher than the plate; full B+ is fed directly to the screens and the center tap of the output transformer so the plate voltage is going to be reduced by the resistance of the transformer winding.

I would have to look but that power transformer looks mighty familiar.  I thought mine was a replacement but maybe National built some with this style transformer.  Carl???

[K1ZJH]

If the caps have all been changed, I'd consider adding a line bucking transformer.
Since you have 6.5VAC on the filaments adding a line-bucking transformer is a good
avenue to consider since you have a bit of headroom!

I'd buck the voltage down using a 2-amp 12-volt filament transformer and go from there.  The radio and transformer will run a lot cooler, and you will notice no
difference in performance.

Peter

[The Slab Bacon]

If the caps have all been changed, I'd consider adding a line bucking transformer.
Since you have 6.5VAC on the filaments adding a line-bucking transformer is a good
avenue to consider since you have a bit of headroom!

I'd buck the voltage down using a 2-amp 12-volt filament transformer and go from there.  The radio and transformer will run a lot cooler, and you will notice no
difference in performance.

Peter

Peter,
         that is a very good idea, but.................. He has a problem somewhere if his voltages are that far out of whack. Once he figgers out what is wrong with it, that will be a viable life-insurence policy.

Tom,
       While I'm thinking about it, take a real good lookie at where they tied the center tap of the transformer secondary to. They could have tied it to one of the cathodes of the outpoots thinking it was grounded.

[AB3FL]

Looking on the schizmatic, The B+ goes to the screens with no resistor then to the center tap of the outpoot xformer then to the plates.  This would cause the plates to be slightly lower than the screens due to DC resistance in the outpoot xformer.  If you follow the B- line which comes from the center tap of the power transformer, it does go to the cathode and the control grids.  I also notice that if I rotate the RF Gain, the amount of B- changes from about -54 to -24.  The RF gain pot measures fine

Tom - AB3FL

[The Slab Bacon]

I dont have one here at work, but look closely at the skizmatic and take notice of what the CT of the powa transfoma is supposed to be hooked to. That doesn't sound right, but it's been some years since I have looked at the underside of a 183 or D. I just dont remember how the biass divider was hooked up.

The biass changing with the adjustment of the RF gain is being caused by the increasing / decreasing current flow through the biass divider as you vary the biass on the RF tubes. This could be an indication of something not wired properly or a defective part in the biass divider string.

[K1ZJH]

The screens running a bit higher than the plates is normal in many sets. The plate voltage
is lower because of the voltage drop is due to the resistance in  primary winding off
the audio transformer. Usually the screens and the B+ side of the audio primary
are tied together.
 
Higher than normal bias when using back bias or cathode resistance bias usually
indicates weak tubes. Swap in a NOS pair and see what happens.

Pete

[AB3FL]

I found some 1614s and popped them in.  The voltages are better.  I guess I need to get new 6V6s


Tom

[The Slab Bacon]

I found some 1614s and popped them in.  The voltages are better.  I guess I need to get new 6V6s
Tom

1614s Ought to rock and roll! They are basically 6L6s..............

Weak tubes will cause all of the voltages to be off some, but not the grid and cathode being off that far. Unless, maybe the tubes are totally piss-beat and spent.
The negative voltage on the cathodes still doesnt sound quite right to me. But I just dont remember the audio outpoot circuit on the 183 that well, it's been a while since I worked on one.

Tom,
       through all of this, how does the audio sound? ? is it distorted or clean?
that might just tell the tale.

[AB3FL]

The audio has always sounded good.  I am just trying to find a way so it doesn't run so hot


Tom - AB3FL

[W1DAN]

Tom:

This is not to the letter of "hollow state", but on one of my HRO-60's, I unplugged the 6V6's and use an external solid state power amp. I also plugged in a solid state rectifier. This cuts the heat way down and allows the power transformer to loaf. As a rule I always pull the 2a fuse from the rear and install a 1a fuse. I then add an inline 1/4a fuses on the HV secondary leads to save the stock transformer.

The image of the transformer above does not have the 11.5v tap. Wonder if this was a replacement and the filament current regulation was discarded?

Finally the potted transformers that look like the image above have about 1/2 the HV secondary resistance as the older ones with the end bell caps. This means the newer transformers can take the load of an HRO-60 (the NC-183D's are similar).

Dan

[AB3FL]

Thanks all.  I think I am going to get a 12V transformer to buck the primary to bring the line voltage down.  I don't want to kill a transformer made out of unobtainium


thx de AB3FL

[K1ZJH]

I'd measure the bias right from cathode to the control grid, just be sure.

Pete

[KA2DZT]

I completely redesigned the power supply in my 183-D.  I got rid of the choke and replaced it with a 7hy 200ma choke.  I solid stated the rectifiers and change the filter from a cap input to a choke input filter.  I run the 6V6s of the first choke.  I then added a second 6hy 125ma choke and cap.  The rest of the radio runs off the second choke.  So, the filter went from CLC to a LCLC.   NC-183-Ds run so hot you can fry eggs on them.

The power supply with a choke input filter allows the xfmr to run much cooler.  Getting rid of the 5U4, which runs very hot, helps to cool down the whole receiver and also reduces the VA load on the xfmr.

I made other changes as well, I added a regulated 105V screen supply (using a VR tube in the 5U4 socket)) for the RF and IF tubes and got rid of the screen resistors in those stages as most always those resistor go way high in resistance.  Doing this mod is more involved and may be beyond what the OP is able to do.

I also would not add a bucking xfmr.  The 6.5 volts on the filaments is still within their ratings.  Could be your line voltage is a little high.  With a CLC filter and a 310vac xfmr your B+ should be somewhat higher than 260 volts.  You may have something pulling down the B+, leaky filter caps most likely.  The filter cap in my 183 was leaky and had to be replaced.

Fred

[KM1H]

There is nothing wrong with the 183D as designed and there were incremental minor changes in each run.

How uncanny! ! ! ! !  That looks exactly like the transfoma in my 183D!

That is because it is a factory original as used in later production and as a replacement for the earlier one.

The reason for the heat is most likely 6V6's with leakage which will give strange voltage readings due to the way the bias is configured.....you already found that out with 1614's which may fry the filament winding due to excessive current. Stick to the correct tubes and PS and forget hammy hambone mods.
Leakage in any of the 5 6BA6's will also cause them to draw more current as well as affect the AGC.

The bucking transformer is absolutely your best bet in most tube radios that were built when the AC line was lower; dont let anyone tell you different. The manual voltage chart CLEARLY states they were measured at 115V. That set will work fine down to 105V and I run everything here at 111-113V with a high current transformer at each operating bench plus the repair benches. Nothing runs hot including the HT-32B and lots of Hallidrifters.

Carl

[AB3FL]

I measure the current draw on the B+ line and it was 140ma which I think is close to what it should be.  This is with a 5R4WGB (Potato Masher).  The B+ V is about 260V which is about right.  I am going to get 2 new 6V6GTs and a 12V xformer to buck the AC down.  Are any 6V6s good for this as I can get a pair of Chinese ones for $20 up to over $100 for a pair for AudioPhools

thanks

Tom - AB3FL

[w3jn]

Get some NOS on eBay.  You'll never notice the difference between the audiophool brands and any other.  There were GM and Ford branded 6V6s for auto radios that the audiophools shun, they're as good as any and dirt cheap.

[KA2DZT]

Tom,

I took a look at my manual for the 183D.  The 6V6s use cathode bias in my set.  The negative bias is not used for the 6V6s even though it looks like it is being used for grid bias.  The current drop on the two 1000 ohm resistors set the bias voltage.  The current through the 6V6s does not pass through the two 1K bias resistors as the cathode resistor is returned to the neg bias line.  The current through the phase inverter (1/2 6SN7) is also returned to the neg bias line.  The bias is used for the RF GAIN control.

So, the remaining current load from the remaining circuits are returned to the chassis.  This current is returned from the chassis through the two 1K resistors back to the xfmr CT and the filter caps' negative terminal.  That's how the neg bias is developed.

The higher (more neg) the bias is, the greater the current drain from the circuits in the set (except for the 6V6s and PH INV)  The adjustment of the RF GAIN control will probably cause a change in the bias voltage as it controls the current through some of the stages.

When bias is developed in this fashion in a power supply, the bias voltage reduces the B+ voltage measured from ground.  The reason the bias is applied to the 6V6s grids is to avoid this reduction in B+ on the 6V6 output stage.  This is true as the cathode bias resistor is returned to the bias line and not to chassis ground.  So, to measure the true total B+ on the 6V6 plates, it should be measured from the bias line to the plates.

I mentioned in my last post that I thought the B+ from your supply should be more than the 260 volts you measured.  In fact it is because you have to add the B+ (measured from chassis ground) to the magnitude of the bias voltage.

I also see that my power xfmr was designed for 110 volt.  I think yours is a 115 volt primary.  The use of the bucking xfmr might not hurt, especially with my set (110 volts).   My line voltage, at times, is 125 volts probably way too much on a 110 volt xfmr.  But, I use variacs in my station to keep the line voltage under control.

Hope some of this helps.

Fred

PS,  Don't make any of the HAMMY HAMBONE mods that I did to my 183D even though it made the receiver work better than designed.

[KA2DZT]

Another thing I noticed on the schizmatic is that the screen grids are about 5 volts higher that the plate.  I though it was bad to have the screen at a more positive potential than the plate.  Also on the voltages page on the manual there are a lot of voltages that are positive but when I measure them, they are negative.  For example the diagram says 19V on the cathodes but I have more like -29V on them (6V6s).  IS there a proper voltage diagram anywhere?

Tom

Measure the 6V6 cathode voltage from the bias line to the cathode.  The bias you said was about -55 volts, cathode voltage -29.  Do the math and you get about 26 volts cathode voltage with respect to the bias line which is also connected to the grids.

Fred