Re: 7603 HV oscillator doesn’t start up


 

Hi Christian,

Rollyn is absolutely right about the 2N3055. The original hometaxial process was switched to an epitaxial process. Unfortunately the side effects of this change were not considered important enough to include in the data sheets.
The most significant changes were the increase in Ft - the transition frequency - from 0.8MHz to 2.5MHz and greater risk from secondary breakdown.
I found this out the hard way when a 2N3055 (they were all hometaxial) used in one channel of the Dynaco Stereo 120 Amplifier I built from a kit in 1968 failed. It failed in the mid-1970s and by then the 2N3055s were epitaxial. A few months after I replaced it I noticed the high frequency hiss (noise from the input transistors in the Dynaco PAT-4 preamp) was gone in that channel. At first I thought this was an improvement. When I investigated further the hiss was still there at the output of the power amplifier on both channels. I didn't hear it because the tweeter was blown out. I got the tweeter repaired and a few weeks later the tweeter blew out again.
The gain of the hometaxial 2N3055s began rolling off around 10KHz which was fine for audio amplifiers. Because the Ft of the epitaxial 2N3055 was 3X greater the small inductor Dynaco used at the output of their Stereo 120 to prevent oscillations was no longer sufficient. That was what blew out my tweeters. I only found this out years later.

There is still one more thing and this may be something you need to be aware of: The Wikipedia article on the 2N3055 at
https://en.wikipedia.org/wiki/2N3055
warns against erroneous readings you may get with a multi-meter when checking the gain of the epitaxial versions of the 2N3055. It says: "[Beta] may be 100 to 200 when testing using a multimeter."

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of ROLYNN PRECHTL K7DFW
Sent: Saturday, July 04, 2020 7:49 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7603 HV oscillator doesn’t start up

It isn't that they were selected, the 2N3055 devices at the time were all homotaxial and had low beta compared to more modern versions.

Even with this in play some HV circuits were cranky and wouldn't do well with very low and very high betas.

Look for a 150-0140-xx with a beta in the range of approx 20-50.

We fought this in the B Phase of the 611 and I'm familiar with the problem.

Rolynn
Tek Bvtn and Sunset 1966-1971





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Dennis Tillman W7pF
TekScopes Moderator

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