Hallicrafters Model HT-45 Mark II A Amplifier Help

[KD8ENK]

HI
I have a Hallicrafters Model HT-45 Mark II A Amplifier
I would like to start with the fact that I am a sort of a new ham and do have some electrical experience but nothing that relates to amplifiers. The amp does have a 3-400Z tube. I am not getting close what I think should be full output and would like to see if I can get some help with this. The amp is in good shape and very clean. I have taken some reading like what the manual shows. I have some photos of the amp and of the manual on my website http://www.projectgm.com/html/hallicrafters_ht-45_mark_ii_a.html and http://www.projectgm.com/html/ht-45_manual.html
According to the book it should be.
Plate V.X10   Grid MA   Plate MA   
300      333      150
250      400      165
200      400      168

8-21-07
40 Meters   Idle      Keyed      
Frequency   7.235      7.235      
Plate V.X10   190      185      
Grid MA   0      80      
Plate MA   10      140      
With watt meter      190 Watts   

20 Meters   Idle      Keyed      
Frequency   14.235      14.235      
Plate V.X10   190      185      
Grid MA   0      70   
Plate MA   10      140
With watt meter      210 Watts

15 Meters   Idle      Keyed      
Frequency   21.275      21.275   
Plate V.X10   190      185      
Grid MA   0      150      
Plate MA   10      240      
With watt meter      310 Watts   

10 Meters   Idle      Keyed   
Frequency   28.400      28.400   
Plate V.X10   190      185   
Grid MA   0      140   
Plate MA   10      170
With watt meter      250 Watts

Like I said I am new to this and would like to see if anyone could guide me on how to test and hopefully get this working better. I am driving this with a Kenwood TS-140S. I am putting in 45 watts. I also have a Heathkit SB-102 that I thought about hooking up to this and see if it would do any better. I am open for suggestion and thoughts.
Thanks
GERALD
KD8ENK

[KA5N]

Your indicated plate voltage is 1850 volts instead of close to 3000 volts.  This could very well be due to changed values in the meter circuit.  The series string of resistors uses high value 2 watt resistors which are likely to change value over the years.  So we don't know what the high voltage really is.
Your input power (plate volts times plate amps) is low but again we can't be sure of the exact value since the plate voltage reading is suspect.  However you appear to be getting around 70% efficiency which seems too high.
You don't say whether you are using the HT-45 on 115 VAC or 220 VAC.  The 866 mercury vapor rectifiers may need replacement. The 3-400 final may need replacement.  I suspect it is a little soft.
More than likely the meter resistors R3,R4,R5,R6,and R7 all 1 megohm two watt resistors should be replaced as well as R8 a 100Kohm 1 watt resistor.  This should give a more accurate indication of plate voltage and a more reasonable number for efficiency.  The rectifier tubes may well be OK and the other parts are probably OK.  Probably you will need a new final amp tube.
Since you don't have much experience with amplifiers I can't caution you enough about the dangers of high voltage which can be lethal.  Such voltages can remain after the amp is turned off and unplugged.
The HT-45 is a straight forward simple amplifier and should not take a lot of effort to be it back in good running order.
Allen

[KI4NX]

First, if, as you say, you are not that familiar with amplifiers, you should seek the assistance of another local ham who feels comfortable and is experienced with high power amps.

Then, if you are going to replace the resistors, I would select high precision, as in 1% tolerance, resistors for the meter circuit to get better meter readings.  "Regular" resistors may be within 5 or 10% of there labeled values.  This means that you might be within 5 or 10 percent, (or more), of what you should be reading.  

GL
Ken
KZ4Y

[KD8ENK]

The power supply is running on 115 VAC. It has been converted and no longer has the rectifier tubes in it. I am at work now but when I get home tonight I will upload some more photos of the Power supply. I can’t remember what the name was on the solid state unit that is replacing the tubes. I will see about getting the resistors. From what I have read the resistors must be a common problem with the meter reading. The manual also shows a chart with readings for AM which I tested and the output jumped to 350 watts on 15 meters with 40 watts input (290 40 meter, 290 20 meters, and 310 10 meter).
Thanks
KD8ENK
Gerald

[WB2WIK]

If the plate voltage is really 1850-1900v, that's way too low for a 3-400Z to develop any real power, or anything close to its ratings.

I'd first check the actual high voltage using a known accurate DC voltmeter (and being careful!) to see what it is.  If it's only 1900v, that's the problem, right there: You need to determine why it's so low.  It should be 3000v.

WB2WIK/6

[KD8ENK]

I now have a High Voltage probe I barrowed from work (Yes I signed it out). Where is the best place to check it at should it be at the Power supply or the top of the tube? Or is there a better place? And the negative should be the Negative High Voltage return not to the case correct? I don’t mean to sound stupid just want to do this the right way.

[KD8ENK]

OK
I checked the voltage at the tube and I am getting 2100 VDC. The manual list 2,000, 2,500 or 3000 volts as the power requirements. If this is low how would you go about getting this up to 3000? Also I have noticed that without the Amp I get a SWR on the TS-140 of about 1.2:1 with the amp I am getting 3:1 and the TS-140 is folding back. Any suggestion on fixing this? The manual states that with 45 in it should be 1000 watts out and if only 50% efficient 500watts butt wishing for 600 +.  What would happen if I used the SB-102 Heathkit with 100 watts? Would it help or could it hurt the amp or the transmitter. I don’t think the Heathkit would fold back. Once again sorry if these question sound dumb but I am learning and having a great time doing it.
Thanks
Gerald
KD8ENK

[KA5N]

The high voltage should be close to 3000 volts.  The reference in the manual of 2000,2500 and 3000 volts operation is if an external supply (not the Hallicrafters unit) is used with the Mark II.  There is no way to change taps to get a lower voltage, unless the primary windings are bridged incorrectly (i.e. set up for 220 VAC and operated on 115VAC).
Since the supply uses a straight forward full wave rectifier there is not much that would reduce the high voltage.  I wonder about the rectifier that replaced the 866 rectifiers.  Silicon rectifiers usually fail shorted which would quickly blow a fuse or overheat the transformer.  Anything wrong with the transformer would show up as excessive heat.  The one filter capacitor is an oil filled job and they often last for years and years.
One thing is that with the lower operating voltage the effiency figures out to be about 60%, a more reasonable value.  If the transceiver is folding back, that explains part of the low plate and grid currents. The Mark II does not have a tuned input circuit so that makes it a bit harder to drive with modern transceivers often a small antenna tuner is used between the transceiver and the amp to get a better match and more drive.
In conclusion: something is amiss in the power supply.  What I am not sure, you need to check all the connections in particularly the transformer tap connections and the rectifier.  Something will have to be done to get a good match (low SWR) between the transceiver and the amp.  The tube may well be good.
Allen

[WB2WIK]

Sounds to me like something is amiss with the HT-45's power supply.  High voltage should be 3000V.  You'll never get it to run much power at 2000V.

Perhaps whoever did the "conversion" to eliminate the 866's and replace them with solid state rectifiers did something wrong, or a component's failed, etc.  

Did you confirm this amplifier ever actually worked properly (i.e., produced 3000Vdc B+) before you got it?  Is there a way to discuss this with the previous owner or whoever did the rectifier conversion?  Maybe he left a filter choke in place after changing the rectifiers...

WB2WIK/6

[KD8ENK]

Today I got the probe from work and checked the AC voltage to the rectifier. I am getting 2300 volts AC. The out put from the rectifier is only 2100 DC. Now correct me if I am wrong but the rectifier should take the AC input voltage and double or almost double it to DC output voltage. If this is the case and the rectifier is bad does anyone have a suggestion to where and what kind I should get? Also I got a antenna tuner from a friend and hooked between the transmitter and the amp. When I tuned for a low SWR I was able to get 410 watts out of the amp on 40 meters. With this I am sure what you folks have been telling me about the Power supply is true (not that I doubted any one) and now I just need to fix it. Thanks for the help in advance.
GERALD
KD8ENK

[KE3WD]

*Look for numbers or part nomenclature on the existing rectifier first.  Very often can cross reference or at least find out what it was.  

Any amplifier is nothing more than a modulated power supply, so the power supply is the heart and soul of the beast.  Can't modulate what isn't there.  


KE3WD

[KA5N]

You should check to make sure that there is 2300 VAC from ground to each side of the transformer and hence to each side of the rectifier assembly (I am assuming it is an assembly and not discrete diodes).  If so and with a good rectifier assembly you should get around 2600-2700 volts (VAC X 1.414).  Apparently half the full wave rectifier has failed (there is still the possiblity that the transformer is bad, that is the reason for checking both sides)and it is operating as a half wave rectifier.  
Steve WB2WIK will probably get back with a part number for a suitable replacement as he is big on amplifiers.
Good Luck
Allen

[KD8ENK]

I checked the AC to ground and I am getting 1098 on one side and 1096 on the other.I have added some more photos of the power supply to my website. The only markings on the rectifier is Jennings IND, INC., 2 AC marks and + - sign. I have looked all around and under it. And it is an assembly which is riveted together. Some of the photos are not the best.  
http://www.projectgm.com/html/power_supply.html Thanks Again
Gerald
KD8ENK

[KA5N]

Gerald
The pictures help a lot.  The rectifier assy. is apparently a bridge rectifier instead of a two diode full wave rectifier. That points out the rectifier unit as the most likely culprit.  
I am going to look a bit more later today and see if I can reconcile some of the differences I see in the photos compared to the schematic.
Allen

[KA5N]

Back again,
Since a bridge was used, the center tap of the transformer is not used.  So the output voltage of the transformer is 2300 VAC as you measured and the  DC voltage produced will be around 1.414 times that or
close to 3000 volts (since the filter is a choke input the voltage will be reduced somewhat).  You can measure directly at the rectfier assy. + terminaland if that is only 2100 volts then that pretty much shows that the diode assy. has a fault.
You can use a bridge or just a full wave rectifier with the correct transformer connections.  You can use a sealed assy. or just a couple of strings of 1N5408 (1000 volts PIV 3 amp)in series (5-6 diodes per string)mounted on perf board for a full wave and
twice as many strings for a bridge.
I hope you are not confused.
Allen