Date: Tue, 17 Dec 2024 16:59:06 +0100
From: Lukasz <sp4it.mail@gmail.com>
To: amps@contesting.com
Subject: [Amps] Acceptable ripple on high voltage plate power supply
in tetrode amplifiers.
Hi,
I have a number of parts of a mobile (mounted on a 7 ton truck) Warsaw pact
HF military radio. Parts include 1500W HF amplifier (modifiable up to
3.5kW), 3 phase power supplies (HV:3kV,350kV, LV: - 27,-110V,250V) and a
huge 3.5kW tuner.
When I used this HV 3kV psu with my DIY amplifier on 5ye same tube (gu-43b)
I added 16uF smoothing capacitance (13x 220uF 450V electrolyte caps in
series each with a discharge resistor).
This meant the ripple was about 14V at 300mA (this is 3 phase rectified).
With only 10nF it was ~200V or so. It is so, because the PSU has a big
inductor to lower the ripple.
Now I'm restoring the actual military amp and the manual says the smoothing
capacitance is supposed to be 2uF 4kV oil filled capacitor because higher
capacitance will make the arc protection ineffective (it has a relay based
overcurrent protection)
2uF seems awfully small. From the calculations I did (200hz, 200mH
inductance) it seems there will ve 300V of ripple at max current of 1A
(10%!). Isn't that too much and how could the military transmitter work
like this?
Many thanks for your help,
?ukasz, SP4IT
## 300 volts P-P ripple on a 3000 vdc supply (loaded to 1 amp) = 3.5%
ripple or -29 db,
You will easily see 3.5% ripple on any RF monitor Scope when a dead cxr is
applied. Dunno if folks will hear the ripple (6F or 300 hz) or not. IMO,
3.5% ripple is way too much.
## Ripple is proportional to capacitance. With 16 uf, your ripple will
drop to just .4375%.....or 37.5 V P-P or -47db. Much, much better.
## to get around the arc suppression problem, here is what I would do. I
install a FAST HV (1 amp rated) fuse in series with the B+, followed by a
50 ohm, 50 watt resistor. Under a fault condition, it's then just 3000 vdc
/ 50 ohms = 60 amps of fault current.
## 60 amps of fault current will open off the HV fuse in < 1 msec
everytime...event over.
## You can use microwave oven fuses for this application. They are typ
rated for 5 kv..... and come in 750 ma - 850ma - 1000ma - 1100 ma ratings.
Jim VE7RF
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