Both the filament and power transformer characteristics are well known BUT
the On Off switch is unobtainium and the HV transformer will still thump at
times when turned on depending on where in the AC cycle it is at.
To further add to switch injury is the tendency to increase filter cap
values beyond reason.
I build my own soft starts since the kits with cement style resistors were
known failure points, dont know if that has been changed....but nothing
beats an old style vitreous enameled wire wound resistor.
Carl
----- Original Message -----
From: "David Gow" <d.gow@frontier.com>
To: <ka4inm@gmail.com>; "'amps'" <amps@contesting.com>
Sent: Tuesday, July 04, 2017 11:49 PM
Subject: Re: [Amps] SB220 step start blows fuses
There is little to be gained by adding a soft start to an SB-220. I don't
have the original articles handy any more but it was common knowledge back
during the SB-220 production life and confirmed by Heathkit that the
design of the plate transformer actually does create a soft start effect
and that it was purposely designed that way. The 3-500Z tubes were
designed as instant heating tubes and although a filament soft start my
have some slight thermal shock advantage it would be minimal and not worth
the trouble and added complexity and one more thing to fail.
Dave W7VM
Past owner of at least (5) SB-220s over the decades but now I love my
solid state 1200 watt amp instead.
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Ron Youvan
Sent: Tuesday, July 4, 2017 2:51 PM
To: amps
Subject: Re: [Amps] SB220 step start blows fuses
Warren Volz wrote:
Thank you for the replies. I had some time to circle back to this today
and checked a few things.
- With the tubes removed, it looks like they have no shorts but the grids
measure ~0 ohms (maybe it’s too low for my meter?). Is that expected?
- Checking the filament transformer I see (All are while in circuit with
the power switch off)
BLK-BLK/YEL = 3.2 ohms
BLK/GRN-BLK/RED = 4.1 ohms
RED-RED = 10 ohms
GRN-GRN = 0 ohms
GRN/YEL-GRN = 0 ohms
Tube pins 2, 3 & 4 should all measure zero Ohms in any direction.
Am I correct in assuming that 0 ohms across the GRN/GRN winding is not a
good sign?
- Checking the anode to chassis resistance without tubes and the HV
interlock defeated I see ~750 Kohm, is this normal?
Zero Ohms is a good reading across a 5 Volt 30 Ampere transformer
winding.
When I rebuilt this, I did the following modifications:
- W7RY board with fuse option installed
- ES4SB220 filter capacitor board installed with new electrolytics
- RM220 replacement metering board installed
- Grids grounded
- Molex connector on the HV transformer primary windings
As I mentioned previously I think the fan isn’t working, but I haven’t
had a chance to verify that yet. I only have one more replacement set of
fuses so I’m trying to be smart about my testing until I can get
replacements (likely later this week). I can easily disconnect the HV
transformer. Am I correct in assuming that if the fuses blow after
disconnecting the hV primary I can assume the filament transformer is
bad?
Thanks again for your help!
You do not say if the filaments light or start to light when you turn on
the power. You do not say if you have the unit in the high power or low
power mode.
I do not know anything about the step start circuit, but it is suspect.
On Jun 25, 2017, at 11:54 PM, Warren Volz <warren@warrenvolz.com> wrote:
All,
I’ve been restoring a SB220 and finally got to the testing stage this
weekend. When I turned it on, the HV went up to ~1kV and then started to
drop. Successive cycles of the power switch did nothing. Oddly, the
meter lights and fan never started running. I looked at the bottom of
the amp tonight and the 3A fuses I added to the W7RY step start board
blew. What areas should I look at now to determine what is going wrong?
I have both tubes in, and I believe I previously checked them for
filament shorts. I’m worried the hV supply has an issue.
--
Ron KA4INM - Youvan's corollary:
Every action results in unwanted side effects.
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