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Re: [Amps] GM3SEK Tetrode and Triode Boards - updates

To: "'Paul Christensen'" <>, <>
Subject: Re: [Amps] GM3SEK Tetrode and Triode Boards - updates
From: "Ian White" <>
Date: Wed, 5 Jul 2017 18:57:27 +0100
List-post: <>
Hi Paul

There are several reasons for needing the 748 op-amp and no other:

1. The big one is the need for access to the on-chip gain/frequency
compensation terminals. This greatly reduces the range of possible

2. The circuit uses all of the 30V rail-to-rail supply voltage rating.
(Input voltage range is not a major issue.) This further reduces the
range of alternatives.

3. A different op-amp would have different gain/frequency
characteristics, and would almost certainly require different component
values for the external compensation. 

4. The 748 was an industry-wide standard for about 40 years and was
manufactured in countless millions (some of which are still around). But
a different modern replacement would be quite a niche product, with
attendant risks of disappearing from the market. 

5. Product support is very important. My ability to provide online
troubleshooting depends heavily on a lot of accumulated experience, so I
have a strong incentive to keep the circuit exactly the same. 

For all of those reasons, I'd rather put my efforts into sourcing
further supplies of the device that we already know. I have good stocks
at the moment, and several lines on alternative suppliers.

73 from Ian GM3SEK

>-----Original Message-----
>From: Amps [] On Behalf Of Paul
>Sent: 05 July 2017 13:43
>Subject: Re: [Amps] GM3SEK Tetrode and Triode Boards - updates
>>"The uA748 op-amp IC used in the screen regulator is becoming obsolete
>there is no direct replacement."
>Is that due to the 748's +30V input spec as a comparator?  Tayda
>presently have an inventory of 127K devices for USD $1.00 ea.
>Tayda claims theirs are authentic National Semiconductor devices, but I
>wouldn't bet on it.  I don't normally source semiconductors from
>Asian suppliers, but a 748 is probably fine.
>Paul, W9AC
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