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 Questions about tube substitution

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OtakuN3rd

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PostSubject: Questions about tube substitution   Fri Oct 15, 2010 4:22 pm

My Philco has a 7C7 in place of a 7B7. Nostalgia Air doesn't list 7C7 as being a suitable substitution for 7B7, but it does list 7B7 as being suitable for 7C7. Am I ok to leave it this way? It does sort of work, but I'm fairly certain that my crappy antenna is why it has problems now (I just want to double-check).

For my second question I was wondering if I should get the actual tubes that the radio calls for, or would getting a cheaper substitution be ok? (that is, in the event that I need a new tube).
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Jeff Hersh



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PostSubject: Re: Questions about tube substitution   Fri Oct 15, 2010 4:59 pm

There is no direct substitute for a 7B7. You can download a very nice substitution manual here. http://www.scottbecker.net/tube/other/Sylvania/SylTubSubstManual1953.pdf

Jeff
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Tom Bavis



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PostSubject: Re: Questions about tube substitution   Fri Oct 15, 2010 5:18 pm

The 7B7 is a remote cutoff type that is intended for use with automatic gain control - the 7C7 is a sharp cutoff type. 7A7 or 7H7 will work as a sub for 7B7 if it's a transformer set - 14A7 will work in a series string set.
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burnt fingers



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PostSubject: Re: Questions about tube substitution   Fri Oct 15, 2010 5:28 pm

It will work depending upon where the tube is used. The 7C7 is a sharp cutoff pentode so it will degrade the AVC action and overload on strong signals. It should be OK as a RF amp as long as you have manual RF gain control. The 7B7 is remote cut off.

The 7C7 is usually used as an an audio driver and a 6B7 will work OK there.

There are a few higher performance replacements for the 7B7 which is a low gain low performing tube for SW use similar to the 6SS7. Its fine for the BCB and lower SW frequencies. The 7AH7 can often be used with no changes but its not always a given; sometimes a resistor or two requires changing. There was minimal development on improving the loctals and they died off rapidly.

Carl
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OtakuN3rd

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PostSubject: Re: Questions about tube substitution   Fri Oct 15, 2010 5:43 pm

I think I'll just try and get originals for it, this has kinda got me confused....

It is a series set. The schematic lists the 7B7s as "IF AMPL" (1st and 2nd).
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Tom Bavis



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PostSubject: Re: Questions about tube substitution   Fri Oct 15, 2010 6:06 pm

I have a dozen 7B7s - send a PM.
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OtakuN3rd

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PostSubject: Re: Questions about tube substitution   Fri Oct 15, 2010 6:12 pm

PM Sent
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