Apache TX-1 Solid State Rectifiers Produce 20% More Voltage: What to do?

[oldsalt]

With a handful of 1N5408 I began to replace tube and selenium bias rectification in the Apache.  Got RF output for awhile, but soon gassed the 6146Bs.

There is appox. 20% more voltage on plates as well as bias voltages.  Schematic calls for 750, yet I have 950 on the plates of 6146Bs.  Other voltages on the LV B+ are high but not as extreme.  I do notice that the 50W/30K bleeder and 10W/15K bleeders are hot to the touch after loading up the dummy load and shutting down the AC.

As some of you may recall, I've been working on the rig this winter and have had an array of JN troubleshooting queries that resulted in smoking the filter choke and burning up resistors.  However, I'm just about there in revamping the rig, so I figured I'd get back in here to update the journey.

Is there a way to drop the voltage with more resistance after the solid state diodes?

Would 6293s take 950VDC?

73
Russ
WA1JFX

[W3SLK]

Russ, the Apache was primarily designed for use with 6146's. The A variant is ok to drop in but the 6146B is a slightly different beast. The bias requirements are different. There is a website out there that documents the evolution of the 6146 and all of its cousins. That would be a good starting point. I thought in some of the slop-bucket rigs, they put ~900 VDC on them. Google is your friend. Gud Luck.

[WQ9E]

Russ,

Was the filter choke you smoked the HV one?  If so are you sure that the replacement is the same?  How much does the voltage drop under load (I am assuming you don't have 950 under load, if you do your line voltage must be well on the high side). 

You can reduce the HV some by using the now unneeded 5 volt rectifier winding on the LV transformer to buck the primary voltage on the HV transformer.  But first make sure your choke and bleed values are correct so that your HV supply is truly acting as a choke input filter.  If there is a major drop from no load (choose SB mode with proper bias) to full load (loaded to near rated input on CW) voltage then it sounds like you don't have sufficient choke inductance or bleeder load.

Rodger WQ9E

[kb3ouk]

I've looked at a few data sheets for the 6146B on the internet, and they all say that the max plate voltage is 750. I'm not sure what the plate voltage in my FT-901DM is, since it also uses 6146Bs.

[AF9J]

Hi Russ

As mentioned previously, the problem may very well be the 6146Bs that are in the the Apache.  If I recall correctly, the Apache was designed before the 6146B came out (which was in 1963 - the year I was born).  Some rigs can handle the 6146B without any major fuss biasingwise. My Globe Scout had no problems with the 6146B, nor does my Johnson Viking II (which at one time or another, had its finals replaced with 6146Bs).  But Collins and Heathkit radios from the the early 60s and earlier, were known to have issues with the 6146B.  Many times the 6146Bs would not neutralize,and in some cases, would go into runaway parasitics.  At the very least, you might have to rebias your Apache for 6146Bs, or else, go back to the 6146 or 6146As it was originally designed around.

BTW, I've attached the Word file of Glenn Zook, K9STH's, article on the 6146 family (I didn't know which section of the forum to download it to) to this post.  It's considered to be one of the best descriptions of this family of tubes.  

73,
Ellen - AF9J  

[AF9J]

I've looked at a few data sheets for the 6146B on the internet, and they all say that the max plate voltage is 750. I'm not sure what the plate voltage in my FT-901DM is, since it also uses 6146Bs.

That's surprising.  My Kenwood TS-820 runs at least 800, and probably closer to 900V on the plates.  When Gary, W2INF got bit by his FT-102, it was with 900V.  If you're curious aout your plate voltage, switch the meter over to HV. That will show your plate voltage.

73,
Ellen - AF9J

[WBear2GCR]

The thing that killed virtually all  of my 6146 toobes is/was the grid.

Too much grid drive, a parasitic that involved the grid, loss of grid bias all being the same as too much grid current in the end. That's what seems to have killed all of my 6146s in the past...

Afaik, the 6146B ought to work ok in the Apache until you try to go on 15 or 10 meters... at least I'm speculating it ought to...

The 750 is probably the CCS plate voltage rating?
ICAS means you can exceed that significantly, as long as you don't key down a long time at full power...

              _-_-bear

[oldsalt]

Thanks AM gang!

Rodger, the filter choke I burnt up was the LV 7H 175ma.  May have had an improper feed of voltage due to JN miswiring.  Replaced with a Triad smoothing choke: 6H at 200 ma from Allied Elect.; only 15 bucks.  seems to work OK now.  But with 465 VDC instead of the schematics 350-370 as called for.  Filter caps are only 450 volts but no smoke yet; must be a 20% margin on rating of the caps, so I'm safe for now.

HV B+ is measured at 950 no load on an RMS reading from an Extech 505 digital meter at the top of the 30K/50 watt bleeder.  Apache plate voltage reads 990 volts respectively.  Under load the digital Extech reads in excess of 850 plate VDC and plate voltage on Apache meter reads 940.

The 15K/10W pair of resistors and the 5K 7W resistor (is this a voltage divider) are hot as is the 30K/50W HV bleeder after 1 minute of RF output to a 100W incandescent bulb.  That's when the fuse blows, about 1 minute.  BTW, I replaced most resistors with 1% tolerance ratings.

Looks like the 5763 goes kaput after several attempts to tune for max output.  Drive is way down.

Bias voltages are high, so I replaced the final bias pot and clamping bias pot with higher values.  I'm able to drop the bias on both to within a few volts per the schematic, but find if the voltage is adjusted on one it affects the other plus or minus.

I saw a recent post about the bucking scheme with the 5V supply, but do not grasp the rationale fully.

Hey, Bear, I've heard you on 75 AM and a respectable sig under good conditions.  I want to thank you and the gang here on the board for all the help along the way this past winter.

Chow for now guys. Good to hear from you all.  Maybe it will all sink in here at some point and I'll be on the air!

73

Russ
WA1JFX

[WA1GFZ]

I changed the low voltage supply to choke input. I just moved the input cap to the output. This will drop the voltage down and give you better regulation.
Stay with the high voltage on the 6146s. Just tune to lower plate current to get to the same power. You might have to increase the screen dropping resistor to the 6146s.

[WA1QHQ]

I like Frank WA1GFZs advice. You also get better plate efficiency with the 6146s when you put higher voltage on the plates, 900VDC is fine they can take that no problem. Embrace the change in voltage it will probably improve the rig's performance as long as high voltage break down precautions are taken, for instance you may want to think about floating the case of the mod transformer.

WA1QHQ

[WA1GFZ]

heck I put a bridge on my PDM V2 transformer and get 1550 volts.
Now that will make a 6146 red in the face with a DC coupled modulator.

[Pete, WA2CWA]

If the filter caps in the HV supply are original (125 mfd @ 450V), I would replaced them, either with a string of 3 caps @ 450V or 2, at a voltage rating of at least 500V. I had the experience on a friend's Apache where we solid-stated the high voltage supply (roughly 900V on the plates), and after several weeks of running, blew one of those HV caps all over the bottom of the chassis. Real loud explosion as the metal cover blew apart.

[WA1GFZ]

good point Pete! I change out my caps also.

[ve6pg]

...ac line voltage...my yak-patch-ee and dx100 both have about 800+ volts on them, some days...rf outpoot is higher, as well...sk..

[N0BST]

Russ- the reason your voltages are so high is the voltage drop across a silicon PN junction is only 0.7v vs maybe 10-15v for a fresh MV rectifier, 22v or so for an indirectly heated cathode type such as a 5V4 or 6X5, and as high as 67 volts for a directly heated type with wide element spacing such as a 5R4.  Do the math.  Not familiar with your transmitter, but before you go chopping it up and replacing screen resistors and such, consider either using tube rectifiers as it was intended (they're not THAT expensive yet) or else put some series resistors in front of those diodes. 

Scott Todd