>> From: Rich Measures <measures@vc.net>
>> Subject: Re: [AMPS] SB-220 C4 failure
To: <amps@contesting.com>
>> Date: Wed, 9 Jul 97 02:06:09 +0000
>
>I wrote:
>> >The current limits because of the components internal ESR, which was
>> >a natural result of the low power levels (1 kW ICAS INPUT power) and
>> >duty cycle (less than 50%) the 220 was designed for. Same with the
>> >filament transformer, any properly sized transformer combined
>> >with the resistance of the filament choke, wiring, power line
>> >ESR, and so on would limit filament inrush. There was no special
>> >design, it was a conventional transformer.
>
>Rich replied:
>> I disagree. The TL-922 has a conventional-core filament transformer that
>> delivers 48A of filament inrush current to each 3-500Z. OTOH, the
>> filament inrush current in a SB-220 is 23.2A per tube. This amount of
>> current limiting can not be explained by the ESR in the filament circuit.
>> (according to Eimac, the max. allowable filament inrush current for the
>> 3-500Z is 29A)
>
>Really? Doesn't the TL-922 use a series filament string??
>
- Did you measure the filament inrush current in each tube in both
amplifiers, Mr. Rauch?
- Does Mr. Rauch's ESR 'explanation' for the 220's minimal fil. inrush
current wash? It can only be explained by ohm's-law if the total ESR is
around 70milli-ohms, which would necessarily mean that the open circuit
voltage from the filament transformer was around 8V---which is
measureably not the case.
>> - The filament transformer in the SB-220 appears to be similar to the
>> design of current-limiting transformers used in neon signs.
>> Rich---
>
>I worked for Heath, and was involved in modernization of the
>SB-220 series. ... ...
To the best of my knowledge, the design of the SB-220/SB-221/HL-2200
filament transformer core was never changed. It uses a thin, wide E/I
lamination stack---like all of the neon-sign transformers I've seen.
During startup, the 220's filament transformer delivers about 60% of the
max. allowable inrush current to the 3-500Zs. OTOH, the 220's HV
transformer uses a thick E/I lamination stack---like all of the
spot-welder transformers I've seen in sheet metal shops. Spot welder
transformers are designed to deliver high peak current. Neon-sign
transformers are designed to limit current. A transformer for a C input
filter supply must be designed to be capable of producing high peak
current.
Mr. Rauch: Were you responsible for replacing the SB-221's original tune
C with the wider-spaced tune C?
Rich---
R. L. Measures, 805-386-3734, AG6K
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