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Old 06-10-2021, 05:49 PM   #1
Schoenradt
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Default ECH81/6AJ8 Vacuum Tube Question

Recently I acquired an Eico HFT 92 AM/FM radio made back in 1958. The unit has had some decent modifications done such as replacing the rectifier tube with silicon diodes and replacing the main filter capacitor with electrolytic capacitors. When I powered it up FM wouldn't tune past 105Mhz. I checked the FM tuner front end assembly and found a capacitor not soldered to the coil from the Plate B+ to and ground on the other side of the coil. Some calibration was needed on the trimmer capacitor on the tuner. My question is AM is extremely weak. When I tap on the tube I get all kinds of snapping and popping noises on AM. Also the magic eye doesn't peak well when centered. I have a cool Seco model 1100 tube tester and tested the 6AJ8 in the correct socket and set the selector to "B" and the load to"10%" I let the tube warm up for a minute and the meter doesn't move out of the replace range. When I hit the test button it even gets worse and shows about 38%. This tube also shows a second test and that test just stays in the replace area with no movement at all when testing. It's my understanding radio tube failure is very rare. Any suggestions? Should I check all the DC voltages on the pins while the unit is running? Should I observe the wave forms from the AM tuner with an oscilloscope? All the other tubes test is the good range so I'm assuming my tube tester works. I am attaching a block of the schematic below.
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Old 06-10-2021, 06:36 PM   #2
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Default Re: ECH81/6AJ8 Vacuum Tube Question

It looks like the tube is used for both FM and AM I suspect the tube is ok but might test low in the tube tester. What does it use for an AM antenna? It might require an external AM antenna.
If you tap the tube when it's in the tester, does the short light flash? or the needle jump?
One thing to note is when the rectifier tube is replaced with solid state diodes the B+ will be a lot higher as there is a higher voltage drop across the tube than the new diodes.

Last edited by R_J; 06-10-2021 at 06:40 PM..
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Old 06-10-2021, 07:40 PM   #3
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Default Re: ECH81/6AJ8 Vacuum Tube Question

The Seco 1100 is an emissivity only tester, so it doesn't do very much for gain. So it might not completely test the tube. So it still could be weak. That ECH81/6AJ8 is not completely used in FM mode, it appears to be just another IF amplifier stage in FM mode, but in AM it's a lot of stuff.

Kind of surprised. This is like the first radio I saw with something not a 12BE6 or 12SA7 as the AM first stage tube...
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Old 06-11-2021, 11:03 AM   #4
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Default Re: ECH81/6AJ8 Vacuum Tube Question

Quote:
It's my understanding radio tube failure is very rare.
Actually, tube failures were somewhat common back in the 50s (and before) and into the 60s. Among other things, the heat needed for the tube to operate is also a long-term self-destruction mechanism. Having worked for a company that produces travelling wave tubes and uses them in its microwave power modules, I saw quite a few RMAs whose failure was an open or high resistance heater.
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Old 06-15-2021, 05:38 PM   #5
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Default Re: ECH81/6AJ8 Vacuum Tube Question

Going back to the rectifier tube section. You mentioned that the rectifier tube drops more voltage than the standard 0.6Vdc drop of a silicon diode. So it would be a wise assumption to first measure the voltage drop across the main power resistor in series with the B+ to establish the current through it under normal operation. Once this known simple ohms law will determine a new value using 2 silicon diodes as a 1/2 wave rectifiers off each leg of the power transformer. Also could I just use 1/2 the transformer for a full wave bridge rectifier? If I remember my electronics education voltage out would be V-RMS AC from transformer secondary times 1.414 - 1.2Vdc for the diode drop.
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Old 06-15-2021, 06:12 PM   #6
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Default Re: ECH81/6AJ8 Vacuum Tube Question

It's not that simple, ultimately nonlinear devices are needed to maintain a constant voltage because of that resistor. Current draw changes over time - mainly due to speaker volume changes.

Sometimes things are "good enough" and that's why these rectifier drops are baked into the regulation system, assuming that wall power voltage is the constant that can be depended upon.
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Old 06-15-2021, 06:23 PM   #7
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Default Re: ECH81/6AJ8 Vacuum Tube Question

Quote:
Originally Posted by Schoenradt View Post
Going back to the rectifier tube section. You mentioned that the rectifier tube drops more voltage than the standard 0.6Vdc drop of a silicon diode. So it would be a wise assumption to first measure the voltage drop across the main power resistor in series with the B+ to establish the current through it under normal operation. Once this known simple ohms law will determine a new value using 2 silicon diodes as a 1/2 wave rectifiers off each leg of the power transformer. Also could I just use 1/2 the transformer for a full wave bridge rectifier? If I remember my electronics education voltage out would be V-RMS AC from transformer secondary times 1.414 - 1.2Vdc for the diode drop.
The 6X4 tube has a voltage drop of 22 volts per plate at 70ma per plate. Measure the voltage across the main filter to make sure the voltage is not too high for the cap and also the other tubes.
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Old 06-15-2021, 06:31 PM   #8
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Default Re: ECH81/6AJ8 Vacuum Tube Question

first rule out corroded valve pins .
also check heater voltages .
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Old 06-16-2021, 07:02 PM   #9
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Default Re: ECH81/6AJ8 Vacuum Tube Question

I'm attaching the schematic and will note the 22V voltage drop for each anode of the rectifier tube. The schematic shows a 300 ohm @ 5watt resistor. Once I measure first the actual resistance and then the voltage across R26 I will know the actual current value. The schematic shows B+ at 200Vdc, but I'm using an isolation transformer dialed in at 117Vac. It seems that back then the engineers weren't too concerned about tolerances. I measure 190Vdc for B+1. So I will shoot for 200Vdc set for 120Vac line voltage on the primary of the transformer.
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Old 06-16-2021, 07:27 PM   #10
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Default Re: ECH81/6AJ8 Vacuum Tube Question

The B+ seems to be fine, I would have been concerned if it was around 250 volts.
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Old 06-16-2021, 11:17 PM   #11
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Default Re: ECH81/6AJ8 Vacuum Tube Question

actually n/m it did handle both halves of the 6AJ8, next thing to try, can you bump the filament voltage up and see if it works better in the tester? I suspect this would be a bit harder to do in the real circuit...
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Old 06-17-2021, 02:30 AM   #12
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Default Re: ECH81/6AJ8 Vacuum Tube Question

check the resistors.
i recently had some equipment that had a type of resistor with a moulded case - not the resin/paint dipped type.
most of the resistors had drifted way off what they should have been.
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Old 06-17-2021, 06:58 PM   #13
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Default Re: ECH81/6AJ8 Vacuum Tube Question

My replacement ECH81/6AJ8 arrived today and yes AM is now superhetrodyning again. Getting very good sensitivity from 1600Khz to about 630Khz with just the loop antenna. Magic eye now shows center tuning lock on. Still not happy with FM sensitivity so I am going through the pain staking of removing the front end assembly and checking each component with my Sencore LC103 tester. Would like to tighten up the tank circuits and increase gain on first RF amplifier. It appears this tuner uses a tuning inductor rather than a tuning capacitor. I am assuming after 60 years the components have drifted out of tolerance. This receiver was really made to last. The schematic doesn't show capacitor values, but when I had it apart I could see numbers on the capacitors they look like resistors.
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Old 06-17-2021, 09:20 PM   #14
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Default Re: ECH81/6AJ8 Vacuum Tube Question

Quote:
Originally Posted by Schoenradt View Post
This receiver was really made to last.
Ehh... could you repeat that? I missed what you said while listening to my transistor radio...

I have two tube radios (one is AM/FM Westinghouse from the 1960s, other is MW/SW Heathkit from the 1950s), but lots of transistor radios that never needed any parts changed. Granted I've had some dud transistor radios that have failed, but transistors have been way more reliable and not need any servicing at all.

The Westinghouse AM/FM seems to be working fine after being repaired from being dropped and plagued with caps, swapped 12BE6 as AM was a little weak too. The MW/SW Heathkit is having some sensitivity issues in the 10 meter SW region that I haven't figured out, but works well in MW and longer SW regions.

Course both are superhet designs, but at least for FM, ratio? Noo... I can't stand anything but PLL tuning/detection these days.

For reference:
Westinghouse: 14GT8(FM Detector/AF) 50C5(AF PO) 12BA6(IF) 12BA6(IF) 12BE6(AM RF/Converter) 12DT8(FM RF/MIX). AM detector and power rectifiers are semiconductors.
Heathkit: 12BE6(RF/Converter) 12BA6(IF) 12AV6(Detector/AF) 12V6 (AF PO) 5Y3GT (Power). No semiconductors.

Last edited by eccerr0r; 06-17-2021 at 09:40 PM..
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Old 06-19-2021, 02:53 PM   #15
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Default Re: ECH81/6AJ8 Vacuum Tube Question

Well I took the painstaking task of disassembly of the FM front end and boy oh boy every capacitor is well out of tolerance and value. Using reactance formula it seems that the impedance increases as the tank circuits are in the FM band. I have calculated that at 100 Mhz XL = 62 ohms and XC = 79 ohms. It seems that the first stage of the triode is exactly like a mosfet J-fet used as an RF amplifier. The two tank circuits must generate the beat frequency. which is then passed to the first IF transformer. I'm guessing that this frequency will be about 10.7Mhz above the center tuned frequency. The amplitude of the tank circuits must increase when in the FM band range increasing the grid voltage on V1B the 2nd half of the triode tube. attaching a picture of disassembled unit. Note dog bone ceramic capacitors.
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Old 06-19-2021, 04:43 PM   #16
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Default Re: ECH81/6AJ8 Vacuum Tube Question

Ceramic capacitors are reliable over the years, you're probably measuring incorrectly with the capacitances in the picofarads. I'd not try to touch the FM frontend as they may have bent devices a certain way to get the correct frequency response and it's not just the values of the components to get it working right.

Yes vacuum tubes and depletion mode FETs (which includes junction FETs) work very similarly.
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Old 06-20-2021, 07:02 PM   #17
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Default Re: ECH81/6AJ8 Vacuum Tube Question

The tuner front end is straight forward the components are soldered point to point. I am de soldering all the capacitors, but my Sencore LC103 does test capacitors in circuit. It has never given errors in values. I have removed some capacitors and they are over 20% out of tolerance for example the 10pF cap in series with the Antenna matching transformer measured 24Ppf that's not acceptable at over 100% out of specs. That will trash my 1st RF amplifiers gain and sensitivity. Also using the measured values my tank circuits are Resonant Frequency (f) = 31.579356303 MHz they should be Resonant Frequency (f) = 95.11327065 MHz at mid band 100MHz. Also not acceptable. It is a wonder it was able to tune any FM stations. I am going with ceramic mutilayer axial capacitors rated at 200V. Will keep you posted on progress and improvement. Also may substitute (2) 2sk161 J-Fets and test tuner with lower VDD supply.
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