External vs. Internal Filament Supply
Martin Manning
Anyone else compared results using an external filament supply?
I tested several tube types using both the u-Tracer’s internal filament supply and an external 6.3VDC supply and found a significant increase in the measured anode current using the external supply. This was especially true for 12AX7 tubes where the error was 30%!
A switch and banana jacks have been added to my u-Tracer for the external filament supply connection, so it is easy to run a test using the external supply and then quickly switch over to the internal supply to compare results. An internal supply voltage setting can then be found such that the measured anode current matches the result using the external supply.
When the switch and jacks were added, I removed two ferrite beads from the filament supply wiring (these were the five-pass type that came with the kit) to reduce filament circuit inductance, and this significantly reduced the required voltage offset. For example, the internal supply originally had to be set to 7.0V in order to match the external supply result for the 12AX7. With the two ferrites removed, the required voltage input is reduced to ~6.6V. My tracer has just three sockets, with a small (~5 x 10mm) single-pass cylindrical ferrite at each pin on each socket, so each loop has three ferrites.
The voltage offset required to match the external supply result is well correlated with filament current for the tube types tested (n = 8, all 6.3V), so this data can be used to find an internal supply voltage setting using the nominal filament current for the device under test:
300mA types 6.65V
450mA types 6.70V
900mA types 6.85V
1500mA types 7.20V
This offset will be slightly different for each u-Tracer, since they will all have different socket wiring configurations and therefore different filament circuit inductances. The adjusted filament voltage can be saved in the set-up file for each tube type when using the internal supply, so this is a relatively easy thing to deal with, but it does require some extra work to determine what that voltage should be. Alternatively, one could just use an external supply.
mks...@gmail.com
My uTracer setup can be switched between the uTracer supply and an auxiliary 4.5A DC supply (with digital readout of V and mA), or external heater via jacks. The heater supply cathode connection can also be center tapped. A number of sockets are hardwired and use no ferrite beads in the heater wiring I have 11 sockets in my uTracer, the wiring is about 2 meters in a continuous loop. Usually the differential between the uTracer heater and the aux. DC heater for 6.3V tubes usually requires the uTracer heater to be increased by ~0.2-0.4V to get equivalent readings. In my case this included 12AX7 and 6L6 tubes, mine wasn't as linear relative to the filament load. The big problem is this seems to vary a bit between different tubes and also depending on the wiring and load, so it is not a simple issue of having a fixed multiplier. Another individual with a uTracer had consistently low readings on his TAD - 7025WA tubes, when he switched to an external heater supply the Gm increased about ~200. It may be that the filament voltage uTracer fudge factor is not always consistent, and may even vary between the same tube series with different constructions.
I did test a Good NOS RCA 12AX7
Tests at 12.6V
Auxiliary Heater
Filament voltage = 12.6V Plate 1 Gm = 1740 Current = 1.51mA
Filament Current = 0.15A Plate
2 Gm = 1660 Current = 1.45mA
uTracer Heater
Filament voltage = 12.6V Plate 1 Gm = 1730 Current = 1.49mA
Plate 2 Gm = 1630 Current = 1.42mA
Tests at 6.3V
Auxiliary Heater
Filament voltage = 6.3V Plate 1 Gm = 1730 Current = 1.50mA
Filament Current = 0.3A Plate 2 Gm = 1670 Current = 1.46mA
uTracer Heater
Filament voltage = 6.3V Plate 1 Gm = 1680 Current = 1.46mA
Plate 2 Gm = 1610 Current = 1.38mA
uTracer Heater
Filament voltage = 6.5V Plate 1 Gm = 1720 Current = 1.48mA
Plate 2 Gm = 1650 Current = 1.45mA
There is no measurable difference in test results using a separate heater supply . As expected, the uTracer heater results suggest that the higher the heater supply the less or no correction is required (results are with a uTracer power supply set at 18.5V), at 6.3V, a uTracer heater voltage of 6.5V gave the same results as an external supply of 6.3V. Older/used tubes would be more sensitive to heater supply adjustments, so there is probably no set adjustment to the uTracer that adjusts for all the different parameters.
I do not use the uTracer heater because it probably is not a simple factor of adjusting the heater voltage by a fixed amount, and you get a known corrected result. It will also vary by the tube socket used and associated wiring. The degree of error is both load and voltage dependent.Martin Manning
Peter Gillespie
I also found the uTracer's RMS voltage low. I don't have a 'True RMS' meter (question how true they are for PWM) but I used the RMS feature on my Tektronix TDS2004 scope. I haven't plotted the results for different tubes but for EL34's I have to set the uTracer to 7.1-7.2V. I also compared the results using an external 6.3V supply and the uTracer supply putting out 6.3 V RMS according to my scope and the results were identical. I'm going to record the results for different tubes.
IMPORTANT NOTE:
My laptop supply has the negative DC output connected to the AC ground prong - the scope has the ground connected to AC ground prong. If you're using an AC power scope or meter, depending on your equipment, you may have to float one of them.
On a different note - directly heated triodes with external AC supply.
The trouble with testing 811A and 572B etc with an external DC supply connecting one end of the heater to the Utracer cathode connection is that the grid voltage is wrong. It is 3.15V above or below the mean tube cathode voltage. I tried 2 six ohm precison power resistors in series across the external heater and connected the centre of them to the uTracer cathode connector. The result was exactly what I expected - reduced plate current because of the voltage drop across these resistors. The tube cathode mean voltage is elevated with respect to the uTracer cathode connection. The condition increases with the current and to me is negative feedback. Reducing the resistor value would help but places more load on the power supply.
I then used a 6.3V centre tapped winding on a Hammond Classic tube power transformer and connected the centre tap to the uTracer cathode connector. I got nice smooth curves! I should say that the voltage and waveforms from the centre tap to each end of the winding are identical. Unless my thinking is wrong, this places the uTracer cathode connector consistently at the mean tube cathode voltage. If the centre tap is not accurately placed then there is a sinusoidal variation between the uTracer cathode connector and the tube mean cathode voltage. I wondered about the inductance of the heater transformer affecting the measurement pulse and added a .1uF cap between the centre tap and each end but there was no difference.
The uTracer is a GREAT tool and Ronald's support is wonderful. The quick test feature gives all the relevant data in a few seconds.
Regards
Peter
April 15, 2014 7:07:36 AM UTC-4, Martin Manning wrote:
Martin Manning
MPM
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Jim Yates
Bill van Dijk
Jim, see in-line comments below:
From: utr...@googlegroups.com [mailto:utr...@googlegroups.com] On Behalf Of Jim Yates
Sent: Thursday, March 17, 2016 9:12 AM
To: uTracer
Subject: Re: External vs. Internal Filament Supply
Regarding the internal filament supply, I've got my 9 pin socket hardwired for 12AX7,12AT7 type tubes, with the filaments wired in parallel.
[Bill van Dijk] Although perfectly acceptable, hardwiring the sockets for a specific tube limits the uTracer capabilities significantly.
Is the higher current draw an issue?
[Bill van Dijk] If you used the recommended 19V, 2A power supply, than no. No issue at all for that small tube.
What's the best way to wire this before comparing with an external supply?
[Bill van Dijk] Not sure why you would with that tube, but if your sockets were wired with jumpers you could feed it directly to the tube, not using the uTracer supply. If you wanted, you can still use the uTracer regulator with an external power supply (nice for very low filament voltages) as described in the uTracer manual.
Hope that helps,
Thanks,
Jim
On Tuesday, April 15, 2014 at 7:07:36 AM UTC-4, Martin Manning wrote:
Anyone else compared results using an external filament supply?
I tested several tube types using both the u-Tracer’s internal filament supply and an external 6.3VDC supply and found a significant increase in the measured anode current using the external supply. This was especially true for 12AX7 tubes where the error was 30%!
A switch and banana jacks have been added to my u-Tracer for the external filament supply connection, so it is easy to run a test using the external supply and then quickly switch over to the internal supply to compare results. An internal supply voltage setting can then be found such that the measured anode current matches the result using the external supply.
When the switch and jacks were added, I removed two ferrite beads from the filament supply wiring (these were the five-pass type that came with the kit) to reduce filament circuit inductance, and this significantly reduced the required voltage offset. For example, the internal supply originally had to be set to 7.0V in order to match the external supply result for the 12AX7. With the two ferrites removed, the required voltage input is reduced to ~6.6V. My tracer has just three sockets, with a small (~5 x 10mm) single-pass cylindrical ferrite at each pin on each socket, so each loop has three ferrites.
The voltage offset required to match the external supply result is well correlated with filament current for the tube types tested (n = 8, all 6.3V), so this data can be used to find an internal supply voltage setting using the nominal filament current for the device under test:
300mA types 6.65V
450mA types 6.70V
900mA types 6.85V
1500mA types 7.20V
This offset will be slightly different for each u-Tracer, since they will all have different socket wiring configurations and therefore different filament circuit inductances. The adjusted filament voltage can be saved in the set-up file for each tube type when using the internal supply, so this is a relatively easy thing to deal with, but it does require some extra work to determine what that voltage should be. Alternatively, one could just use an external supply.
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mman...@fuse.net
To: "uTracer" <utr...@googlegroups.com>
Sent: Thursday, March 17, 2016 9:12:18 AM
Subject: Re: External vs. Internal Filament Supply
Jim Yates
On Tuesday, April 15, 2014 at 7:07:36 AM UTC-4, Martin Manning wrote:
Multi Blitz
I asked Ronald if this could be done for DHT and he said DHT needs DC heating ?
I intended as well to use some 5Ohm Resistors for CT...I do mot yet understand fully what theexternal heater setting in the software can compensate for...
By the way: My Avo style wiring makes the DhT like 4p1l not even lot up a bit...dark. Nada. So, external with DHT seems to be a must.
Multi Blitz
...so, hardware works, now I need to understand a bit better: what does theexternal heater setting in thegui exactly do ? External heating on/off does what, the speced voltage means what ?
Thx and Best Regards
(I am super excited that it works from the beginning on, was a bit nervously when I measure my first DHT with cathode connected to the external supply...
Multi Blitz
Example: 4p1l as a DH tube: Can be heated 2.1-0-2.1V. I am heating it external 4.2V and connect the cathode cable to the middle tap of the heater/cathode.
I have not used the external sweep mod, it is just externally heated. So, what would be the right GUI setting ?
A. Nothing checked like internal heater (as not the sweep mod is used and cathode is on ground when measuring) ? Or
B. external heater on and 2.1V (as cathode is connected to center tap and sees only half of the heater voltage)? Or
C. external heater on and 4.2V as this is the actual heater voltage ?
mman...@fuse.net
The correct choice is A. The external heater check box is only used for the external sweep mod.
Martin Manning
----- Original Message -----
From: "Multi Blitz" <blitz...@hotmail.com>
To: "uTracer" <utr...@googlegroups.com>
Sent: Tuesday, March 14, 2017 3:41:28 AM
Subject: Re: External vs. Internal Filament Supply
prze...@gmail.com
Hello to You all
Trying to solve the problem with the internal heating I’ve made a simple test with an old EL84 tube. According to Martin Manning’s advice I’ve omitted six hole RF suppression ferrite beads and connected the internal heating PWM 6,3V directly to the heater pins of the tube, using only single hole pass trough ferrite beads on each line. The result was Ia=26mA (Vg1=-7,3V, Va=Vs=250V) In the next test I’ve connected the external, stabilized 6,3V DC heating supply directly to the heater pins of the same tube, using also only the same single hole ferrite beads. The result was Ia=32mA (Vg1=-7,3V, Va=Vs=250V) The difference in Ia seems to be around 6mA ! Can anybody explain me the reason of such a difference? Is it the wrong value of heating setup in my uTracer ?
Regards
Przemysław Gierałtowski
Warsaw, Poland
Martin Manning
What you are seeing seems like a very large difference. I ran an EL84 today and found that setting 6.5V for the internal supply on the test form reproduced the results using an external filament supply at 6.3V. This is different from my previously developed correction curve, which suggested I would need to set it at around 6.8V. That correlation did not include an EL84, however, and a different path through the socket wiring for this power level is being used. In this test, the indicated current when using the DC supply was 0.76A, exactly as expected.
The lead wires going from the uTracer's filament supply terminals to the sockets have some self-inductance, plus that of any ferrites. The heater supply is derived from the main (~20V) power supply, which is PWM'ed at 20 kHz to regulate the energy supplied to the heater. Even one or two ohms of impedance (not hard to imagine at that frequency) is significant, since the heater resistance is quite low, on the order of 10-20 ohms. I don't know the configuration of your heater leads, but I suspect the cause is something related to the high frequency supply.
My recommendation is to use an external DC supply, which can also permit monitoring of the heater current.
Martin Manning
prze...@gmail.com
Regards
Przemysław Gierałtowski
Henning Oelkers
Von meinem iPad gesendet
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