Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Dec Tue 07, 2021 11:28 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 73 posts ]  Go to page Previous  1, 2, 3, 4  Next
Author Message
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Fri 03, 2021 12:30 am 
Member

Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
Looks like adjusting the line adjust R5 just a hair higher (not even a tick) was enough to get exact same test results in the 6L6 tube and another tube i tested also. Brought plate voltage to 146v which is within spec the other voltage still low at 62v. Screen voltages are still within spec also slightly higher at 134 instead of 133 before.


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Fri 03, 2021 2:49 am 
Member
User avatar

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 11325
Location: Sunbury, Ohio 43074
.....all of which just proves, again, that neither tubes, nor tube testers, are precision instruments ;-)

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Fri 03, 2021 3:42 am 
Member

Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
yup. The low plate voltage being 62v is that a concern or should i be using 5.1v zener diode instead of 5.6v


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Fri 03, 2021 11:09 am 
Member
User avatar

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 11325
Location: Sunbury, Ohio 43074
tysman wrote:
Looks like adjusting the line adjust R5 just a hair higher (not even a tick) was enough to get exact same test results in the 6L6 tube and another tube i tested also. Brought plate voltage to 146v which is within spec the other voltage still low at 62v. Screen voltages are still within spec also slightly higher at 134 instead of 133 before.

When you increase the line voltage, even by a small fraction, you increase ALL the voltages in the instrument. That includes of course the filament voltage, the signal levels, everything, so it's not entirely straightforward what the "proper" readings would be when you do that, or if your new readings were correct, or the old readings were correct.

You can make just about every tube read higher that way ;-)

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Fri 03, 2021 3:27 pm 
Member

Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
Ah that makes sense so I think best thing would be is to try and get the much needed 150v so that way don't have to change my line calibration from regular 83 tube. In calibration instructions it says low plate voltage is a defective 83 tube so I did order some 5.1v zener diodes to see if that will get me closer to 150v


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Fri 03, 2021 5:20 pm 
Member

Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
i flipped the zener diodes around so silver bands of diodes are going same direction and i now got 149v on plate and low plate is 66v :) Not sure why had to flip the diodes against what all the instructions i see online which say to make the silver bands of the diodes face each other


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Sat 04, 2021 4:42 am 
Member

Joined: Dec Sat 28, 2019 4:18 pm
Posts: 1124
Location: Corinth, TX
Maybe because a Zener needs to be reverse biased to drop the required voltage. Forward biased (up to its PIV rating), it just drops the usual .7 vDC.

John


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Sat 04, 2021 5:33 am 
Member

Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
So its ok to run a zener in either direction? the way it is now i can measure the zener and the 1n4007 without flipping the voltmeter leads


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Sat 04, 2021 5:53 am 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 2916
Correct. A zener diode is pretty much an ordinary diode but with a specific reverse breakdown voltage.


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Sat 04, 2021 1:22 pm 
Member

Joined: Dec Sat 28, 2019 4:18 pm
Posts: 1124
Location: Corinth, TX
No. A Zener's voltage rating is its reverse biased, breakdown voltage.


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Sat 04, 2021 2:29 pm 
Member

Joined: Aug Wed 31, 2011 11:23 pm
Posts: 547
Location: Tucson, AZ
The Zener diode direction does make a difference in how it operates, in this application/direction it provides a fixed voltage drop similar to what is seen in the 83 tube rectifier. That is why the diagrams show the orientation.


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Sat 04, 2021 2:35 pm 
Member
User avatar

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 11325
Location: Sunbury, Ohio 43074
Interestingly enough, an 83 tube only plugs in one way ..... :mrgreen:

Sorry, I had to :lol:

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Sat 04, 2021 4:21 pm 
Member

Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
So its ok to run it the way i have it which is not like the diagram? test results are exactly the same, power tubes are slightly weaker but by only 1 tick the 6L6 still reads 5500 the needle just starts closer to it before it drops


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Sun 05, 2021 5:36 pm 
Member

Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
The filament voltage for 83 and 5y3 are below 5v maybe that's why couldn't run zener diode the right way


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Mon 06, 2021 12:12 am 
Member

Joined: Oct Thu 29, 2020 10:17 pm
Posts: 189
Location: Centennial, Colorado
I find it amazing that knowing the plate voltage is not critical its still used as the reference voltage for the line test.

I deviate from procedures that I have seen to calibrate the line test. Rather than use the plate voltage of 150 VDC I use the 2.5 signal voltage. It’s the most critical voltage and now that we have accurate digital multimers its logical to use it as the reference.

I also deviate in testing the plate voltage where the norm is 150 VDC +- 5 volts using a 1000 ohms per volt analog meter or simulate one with a resistor. Strange they gave it a 5 volt tolerance when that’s the reference voltage used to set the line test. So I use the 10K 10 watt resistor to load the plate while testing with a digital multimeter. The screen voltage is loaded with close to 10k in the bias circuit, so that puts about the same load on the plate supply. Now these voltages have a more realistic relation.

It sometimes pays to think about what's going on rather than blindly following what we may find.


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Mon 06, 2021 12:42 am 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 2916
Bill, your comments, however valid, have ignored the importance of filament voltage. This is more important for some types of tubes, probably early directly heated types.

Many tube testers took short cuts regarding filament voltage. In many cases, accuracy simply wasn't possible due to the combination of one-turn transformer resolution and varying loads. A 5U4 would draw 3 A from the 5V tap, while a 1S5 would draw 50 mA from the 1.4 V tap. I have pondered that anomaly for many years, and it haunts me each time I plug a tube in and watch the line adj pointer sag as I respond by readjusting. Some heater voltages would be intentionally wrong to save the cost of yet another transformer tap and switch position. And the line adj meter was set to the same reading regardless of heater load, which had a more direct effect on the winding supplying the tube than it did on the winding supplying the meter.

On the other hand, swinging the meter reading on line adj while watching the reading of the tube (impossible at the same time) can help determine the effect of heater voltage, as well as identify 'tired' cathodes.

I was also taught (it's not in any manual I have read) that it's unwise to hold in the the Test button for too long, lest something (transformer?) overheats.


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Mon 06, 2021 1:16 am 
Member

Joined: Oct Thu 29, 2020 10:17 pm
Posts: 189
Location: Centennial, Colorado
Bob, I understand what your saying, but with The Hickok's the line adjust is reset after the tube has warmed up, which corrects most of the filament sag, however the plate current load from power tubes do reduce it as well. The tube charts did compensate for what looks like a issue. I really don't look at this as a problem, since most of the Hickok's like the 752 have a life test that reduces the filament voltage by 10% to give an idea of the cathode activity. This is true for other manufacturers with higher quality testers. Bill


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Mon 06, 2021 2:12 am 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 2916
The whole thing becomes academic if you are consistent. Test all tubes at the same wrong voltages and you can sort out the good and bad with appropriate calibration. There is no sudden loss of emission at, say, 85% heater voltage or loss of plate current at 80% voltage.


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Mon 06, 2021 6:28 am 
Member

Joined: Aug Wed 31, 2011 11:23 pm
Posts: 547
Location: Tucson, AZ
It all depends on the Hickok model as to how you "calibrate" it and in many cases other than the line voltage, there are limited adjustment options. The reason to use of the zener diodes (per the schematic orientation) was that without it, on many Hickoks the line set voltage could not be set because the plate voltage was too high. Even if you could set the line voltage and then set the plate voltage then then all the other voltages would read incorrect. It appears that Tysman does not seem to understand the function of the Zener in the reverse orientation in that it provides the proper voltage drop (around 8-12V) similar to what was seen with an 83 tube rectifier, if you do not use it in the reverse orientation it operates like a diode (with a voltage drop of less than 1V), so you do not have the proper voltage drop. Yes it will work, but readings will be off. This will throw off other voltages if you bring the line voltage down so your plate voltage is ~150V, you are talking about maybe a 6-8% difference in plate voltage w/wo a reverse zener diodes in the circuit. You could just leave the plate voltage higher and this has less of an impact on Gm, but does significantly effect plate current if you are measuring that. The bigger impact is the signal voltage, which I agree with Bill that this probably has the most significant impact on Gm. Filament voltage can vary quite significantly, but even if it were +/-5% the impact in Gm is less significant then signal voltage. I have AVO tube testers which can read 10% higher or lower on the filament taps, which is pretty insane, fortunately I pulled another voltage tap that feeds the filament transformer and can now get all filament voltages within +/-0.1. I did a similar mod. to a Hickok 580A which has a separate filament transformer. Between tube testers, if using the same operating points/voltages, the readings can be quite consistent across all of them.


Top
 Profile  
 
 Post subject: Re: Hickok 752 Calibration
PostPosted: Sep Mon 06, 2021 5:30 pm 
Member

Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
In my application with zener diode 1 way its 144v and when i set line to 150v all other voltages are to high. When flip zener diode the other way its 149v and other voltages are perfect. sounds to me like in my application wouldnt need to drop volts with a zener. cause 8-12v would be the v144 and 1v would be the 149v. Or is the way I have it now the correct way and the diagrams are wrong?


Top
 Profile  
 
Post New Topic Post Reply  [ 73 posts ]  Go to page Previous  1, 2, 3, 4  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: bob91343, PaulAm and 7 guests



Search for:
Jump to:  




































Privacy Policy :: Powered by phpBB