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Old 04-30-2018, 03:06 PM
DavGoodlin's Avatar
DavGoodlin DavGoodlin is offline
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Zenith HiFi amp making "angry tweets"

Usually when I'm stumped on a tube set, something of a high-impedance nature eventually becomes obvious. But this case its phantom feedback even though the tuner works or is disconnected.
It sounds almost right until the treble is increased past 50%, then it howls with regeneration and volume, bass and bass boost switch have no effect on it.

I fully-recapped this amp intending perfection, even the wax-dipped zenith-oem maroon ceramic disc caps looked suspect and had cracks in them. I did them all using exact replacements that are brown or black dips from JustRadios. Only the grid-leak bias resistors 680K were out of tolerance.

I have narrowed it down to feedback from the speaker winding via a series RC 39Kohm x.01mf to provide negative feedback to the 12AX7 driver's cathode. The bass-boost switch extends de-emphasis further, back to the volume control. I *may* have reversed the feedback loops coming from the speaker windings and will check that next. I did not notice an issue the other times I traced out all my work. Even when I disconnect one of the speakers, it goes in to very loud self-oscillation at about 700 hz.

On many Zenith console stereos with tube amps, single-ended 6BQ5/EL84 beam pentodes are employed when most other 6BQ5s (RCA-Motorola-Fisher-Magnavox) are push-pull pairs. I heard that Zenith used Amperex-sourced tubes for some reason. I noted this difference when I checked cathode voltages on the outputs with weaker RCA tubes and the Zenith originals "made in Holland".

With the tired RCA EL84s I use for initial power ups, cathode voltage was only 5.2 volts, BUT when the Zenith-branded tubes were used, rose to the proper 10.8 volts. It also got a great deal louder with more bass, with the treble at 30% of course.

All this makes me wonder if I did not inadvertently rewire this amp into a multi-vibrator.
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Last edited by DavGoodlin; 04-30-2018 at 03:20 PM.
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Old 04-30-2018, 03:20 PM
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Electronic M Electronic M is offline
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Try un reversing what you may have reversed before and see what it does.

Another thought is microphonic tubes...If they like to resonate at that freq and volume is high enough to stimulate them that could be the source of the oscillation.
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Zenith: The quality stays in EVEN after the name falls off!
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Old 05-01-2018, 10:31 AM
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DavGoodlin DavGoodlin is offline
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Talking

Quote:
Originally Posted by Electronic M View Post
Try un reversing what you may have reversed before and see what it does.

Another thought is microphonic tubes...If they like to resonate at that freq and volume is high enough to stimulate them that could be the source of the oscillation.
It was exactly that Tom. As I wrote this thread a week ago at VintageHiFi, I had no idea.
When I wrote it here, it became obvious what I thought I did. There is a 8+lug terminal strip between the two output tubes. This is where I replaced two .022 grid signal coupling caps. the two lugs toward the back of the chassis had both the caps on feeding the opposite tube's grid
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Old 05-01-2018, 10:36 AM
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DavGoodlin DavGoodlin is offline
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Well now I know how bad the original speakers are. I think all six could be replaced with two new 10" woofers and two horns (Foster OEM, from a Zenith SS console).

The good news is the cabinet is low and long with the speakers in separate chambers, allowing for fully enclosing the rear and adding a tuned port or insulation.

Both 10" woofers sound pretty bad when the bass boost is on, which is to say "boomy". Turning up the treble brought very little in fidelity, so either the two 4" mids and two 3.5" cone tweets are shot. It cant be as simple as a crossover cap.

This console is worth the effort mainly due to its solid-built walnut cabinet, 2g record changer and quality electronics, now restored.
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Last edited by DavGoodlin; 05-01-2018 at 10:40 AM.
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Old 05-01-2018, 04:39 PM
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Electronic M Electronic M is offline
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I'd try feeding a known good pair of book shelf speakers off of the amp to have a base line for what it is giving the speakers to work with and how it can/should sound with known speakers.
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