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Old 5th June 2008, 12:27 PM   #1
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Default Zenith Console find, amp suggestions?

I just picked up a Zenith phono console. The amp inside is a single-ended 6BQ5, one each 12AX7, 12AU7 and 5Y3. Chassis is marked Zenith 5B30. Because it is phono only, I figure on removing the iron and starting with a new circuit. I'm looking for ideas for a rebuild and found the Zen Triode amp, with very similar specs in regards to the iron. I also have some 6DJ8s to work with the different driver circuit.

Any other ideas for a cheap SE implementation?
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Old 5th June 2008, 12:44 PM   #2
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It would be very nice to have a schematic of the present amplifier circuit. Could be that you could use most of it as-is, but removing the 12AU7 and tone controls. That may be the simplest thing to do. If you want to start from scratch there are lots of circuits to choose from. Are you wanting a no NFB triode circuit like the Decware Zen or a pentode circuit with some feedback? Output will realistically be about 2 watts triode versus 4 or 5 watts pentode.

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Old 5th June 2008, 01:03 PM   #3
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Haven't found a schematic online. Our library here has SAMS schematics for copying. I guess I haven't decided on final design. Going simple is my first choice. Definitely for removing the tone controls and the phono pre. As it is, it might be enough to play with it to see how it behaves w/o a full rebuild.
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Old 5th June 2008, 01:29 PM   #4
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Sams folder 523-13 covers this amp. 5B29 is online here (same tubes): http://techpreservation.dyndns.org/s...ics/Zenith.htm
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Old 5th June 2008, 02:54 PM   #5
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Thank you for the schematic. Most helpful. Looks like I can just cut out the front end up to the 12AU7s, remove the tone controls and put in a power switch.

Quick guess, but is this a pentode SE, giving about 5w?

About the feedback loop: should I keep it, reduce it or eliminate it with this circuit?

Most of this is an intellectual exercise for me. I just ordered a Bottlehead SEX (about a week ago) and think it would make an interesting side by side comparison.
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Old 5th June 2008, 08:13 PM   #6
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Yes, single ended pentode presenting 4 or 5 watts. You need the feedback with a pentode circuit, otherwise output impedance will be too high for proper speaker damping.

Cut out the front end, remove the tone controls, remove the 12AU7 and replace it with the 12AX7, and change the 47k plate resistors to 150k and the 2.7k cathode resistors to 1.2k. You'll have to tinker with the value of the feedback resistors to see what sounds best to you. I wouldn't use any capacitors across the feedback resistors until you are ready to start tuning the square wave response with a 'scope.

I have one that is very similar, also a Zenith, and I think it sounds fantastic.

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Old 6th June 2008, 12:41 PM   #7
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Wow! The more I look at the Zenith design, the more I see how it is a spitting image of the Decware Zen design (except for the triode conversion.)

Much of the circuit is so identical, it has the same resistor values for the 12AU7 (on the Zenith) and 6DJ8 (on the Zen) positions. I see the possibilities of different driver tubes that could be used. Maybe change it's sonic 'profile.'

One more question: the volume pot is a megohm value. The Zen uses a 100k instead. Do I need to get the lower value volume pot?
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Old 6th June 2008, 02:13 PM   #8
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Do I need to get the lower value volume pot?

Yes, you do. That MegOhm value works with a piezoelectric phono cart., which doesn't have much HF extension. Miller capacitance interaction with that value can/will roll the top octave off.

Mouser carries Alpha brand Carbon controls that are inexpensive and quite decent sounding. If you want something better, buy PEC hot molded Carbon stuff from DigiKey.
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Old 28th August 2008, 06:20 PM   #9
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I just finished with the circuit refit of my Zenith console find. Right now listening to it through my poor man's OBs and rolled in a pair of Ultratone EL84s (made in Great Britain markings) and a Telefunken 12AX7. Sounds wonderful with only a hint of hum. Base is a bamboo cutting board.

Click the image to open in full size.

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Thank you tubewade, Tom and Eli for your suggestions and support.
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