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Philuk
Junior Member
Username: Philuk
Post Number: 20
Registered: 4-2009
| | Posted on Saturday, December 18, 2010 - 6:03 am: | 
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Hi, visited this forum before for information and got some very usefull replies so I thought I would pick your brains again.
This time on the subject of old sweep tube linear amplifiers.
Firstly I have to old non working amps both using sweep tubes one uses 2 x EL509 with a blown transformer (only for 10/11m)
the other is a very old huge home brew HF using 3 x PL519 tubes for the 80/40/20/10M bands but has pi network problems and far too big and heavy for my needs, time to put them to use!
Decided to look into the therory and build 1 small qrp driven 2 tube passive-grid amp for 40/20/17/15/10M with some newer developments
(50 ohm swamping input, overdrive sensing and non relay tx/rx changeover) with an external psu I can keep under the bench nice and tidy to keep
the xyl happy if that is at all possible hihi.
I dont use amp power a lot and mainy TX qrp so my feed will only be 5W max.
Looking into the basics of these old TV sweep tubes I can easily build a psu that delivers a seperate feed either 6.3v or 40v for
the filament voltage depending on which of the old tubes I use but the grid voltage/curent has got me confused due to most the details I have found are published by hams who have English as
their second langage and some details seem to have been lost in translation.
Am I correct in thinking the screen grid voltage is not fixed at all but dependent and proportional to the RF drive?
The screen grid current is a mystery to me as the sweep tubes plate current seems to be published as 500mA Max. is this 500mA max feed or
draw per tube? both the old transformers I have are very low curent on the HV tap and a new HV smaller modern replacement transformer
will have to be purchased.
The hew HV transformer will be set up with a voltage doubler/quadrupler circuit switchable from 300 or 400v up to 600/800v then 1200/1600v for soft start and lower V testing, this is not a problem I have repaired this type of HV system before in the older big amp but for the newer
smaller amp what type of current should I be looking for when using two sweep tubes?
Any help or advice please.
Phil
M6MRP
Yaesu FT817nd, Kenwood TS 140s, Origonal Midland Alan 8001 plus.
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Tech833
Moderator
Username: Tech833
Post Number: 1896
Registered: 8-2002
| | Posted on Saturday, December 18, 2010 - 11:04 am: |
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Phil,
Lots of questions!
The reason some TV tubes have large or odd filament voltages is because they were usually run all in series, sometimes called a "series filament set". This eliminated the need for filament windings on transformers, and greatly reduced the cost and weight of the TV set. 120 VAC was applied directly to the filament string with all filaments added together rated somewhere near that value.
Screen grid voltage will determine how much amplification the tubes perform. It's like a "gain control" for them. Too much gain and their lifespan is reduced greatly. Too little gain and they do not perform to their expectations. Although, for the longest tube life, almost no manufacturer ran their tubes anywhere near their maximum specs.
Adjust screen supply voltage for a plate dissipation under the tube's maximum rating, and even lower if you want them to last longer.
The screen voltage is determined by the supply, not the RF drive. The screen current will fluctuate slightly on an amplitude modulated signal. Sometimes, due to supply sag, the voltage will drop slightly as the current rises.
Step starting the plate supply isn't for the benefit of the tubes, it is for the benefit of the power supply. When the capacitors are uncharged, they look like a practical DC short. The inrush of current can damage diode stacks and transformers on some supplies. So, the step-start is usually a brief (fraction of a second to several seconds) delay in which a resistor is switched in series with the supply B+ between the diode stacks and the capacitors. After the time is expired, the resistors are bypassed and B+ is connected directly to the capacitors, now that they are charged and no longer low resistance.
Any step-start designed to protect the tubes would be installed in the filament supply for essentially similar reasons (cold filament = dead short).
Good luck!
Your radio 'Mythbuster' since 1998
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Philuk
Junior Member
Username: Philuk
Post Number: 21
Registered: 4-2009
| | Posted on Saturday, December 18, 2010 - 2:19 pm: |
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Update: Ive just stripped out the transformers from the big old HF tube amp and I think they may do everything I need, details:-
Filament transformer HB 605 with primary 0-117-240v (set for 240v) secondary 20-0-20v @ 0.1A and 0-6v @ 2A
HV Gardeners radio ltd 50-400VA Primary (multiple) but set for 0-240v and screen
secondary 600-0-600V (did not think it was that high) will this realy need to be doubled?
If these load test OK will they do for a twin sweep tube linear when looking at heater voltage and current also HV voltage and current?
cheers
Phil
M6MRP Yaesu FT817nd, Kenwood TS 140s, Origonal Midland Alan 8001 plus.
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Philuk
Junior Member
Username: Philuk
Post Number: 22
Registered: 4-2009
| | Posted on Sunday, December 19, 2010 - 4:21 am: |
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Cheers tech833 that clears a few things up, I have tested both working transformers under load and they are fine so I may not have to go down the step voltage route on the HV feed, going to try and externally HV feed the old zetagi BV135 first to see if the two EL519 tubes are ok, the BV135 was fed on the HV side with 300v but from what you have told me and what I have read about this tube it should be fine with 600v on the screen grid as they can handle a lot more.
The old gardners radio ltd transformer did feed the huge 3 tube linear at 600v - 600v (far too high) so if I tap the 0v-600v rectify and smooth as a feed for the BV135 by-passing the internal HV input and feed direct into the screen grid via the relay switched Hv rail it should be fine yes?
The filamet voltage is not a problem as I have already introduced 6.3v to the LV rail on the BV135 and that all checks out fine.
If I can get the external HV feed sorted for this linear I think I may be able to modify the board with parts I have lying about so it covers more bands rather than building complete new modules, the physical size and PO is just about perfect for my needs it just needs better coverage.
thanks again
Phil
M6MRP Yaesu FT817nd, Kenwood TS 140s, Origonal Midland Alan 8001 plus.
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