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Thread: Don't Understand Schematic, A Butchers rebuild.

  1. #1
    Member HaroldBrooks's Avatar
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    Don't Understand Schematic, A Butchers rebuild.

    I recently bought this old Zenith(labeled a Stereo amp) Hi-Fi amp and so far have successfully converted it to a guitar amp, and it sounds pretty good !
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    But... I essentially did a hatchet job and ignored all the input circuit filters / Loudness, etc. as per the schematic. To make it work I connected the 56k grid resistor that leads to V1 pin #2 directly to a 12A shorted Switchcraft female guitar jack, but I can't help feeling a bit guilty about not investigating and understanding more how the second half of the V1 12AX7 tube works, and if it contributes to the signal on this chassis. I couldn't see how that works, because I was "born" into this club just a short while ago. The Treble, Bass, and presence pots work fine, but the multi switch now does nothing, and the same with the dual pot loundness control. I'd be willing to rewire this if it would help in any way, or even just to see what happens as an experiment.

    Again, as a guitar amp it works very well now, and I've modified only a couple of things i.e. smaller power tube coupling caps (from .1uf to .047uf), a smaller coupling cap between the 12AX7 and the 12AU7 (from .047uf to .01uf), and I jumped out the 270k resistor leading to the grid of the first half of the 12AU7, as it increased gain a lot just on a hunch. All in all it sounds great even at high gain. But I butchered the amp because of a lack of knowledge and patience !

    Can anyone tell me how the preamp and PI sections work on this amp ? Specifically the loudness and allusion to a "Stereo" amp, that I can't for the life of me see.

    I still need to learn a lot, so I have some big holes in my understanding. As Always, Thanks a lot for your help !!!

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    Last edited by HaroldBrooks; 12-31-2019 at 05:58 AM.

  2. #2
    Supporting Member Jazz P Bass's Avatar
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    That may be a stereo input but the output is mono.

    The circuit hanging off of the input tubes looks like an RIAA circuit for a phonograph.

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    Member HaroldBrooks's Avatar
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    Thanks. I never thought the amp was "real" stereo, but there are two signal paths running out from the V1 tube and combining later on so that would be a mono amp as you stated, still it starts off with two independent inputs to the V1 tube.

    I noticed the plates on V1 pins #1 and #6 are connected via a 100k resistor, and I will assume the 100k resistor is a load resistor for plate #1, but I'm not sure of the signal arrangement between both plates and the phase invertor works. Plate #1 goes to the first half of the 12AU7 and that appears to be a gain stage, and that's easy to trace, Plate #6 seems to walk around the first half of the 12AU7 and go straight to the grid of the phase invertor and not adding any extra gain, and to me it would appear the balance would be off between both sides of the preamp tube, so I am not understanding that arrangement coupled with the PI tube. Perhaps it's just a difference in voltages or resistance I don't get.

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    Last edited by HaroldBrooks; 12-31-2019 at 06:38 PM.

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    Lifetime Member Enzo's Avatar
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    My dad had a "record player" in the 1950s. This was before "stereo" was anything more than an oddity. But the RCA record player was set up for expansion. In the event you wanted to go stereo, there was a line out jack on the back. RCA made a companion piece that was essentially a powered speaker that looked like a record player. You could sit each unit on the end tables in your living room say.

    Idly looking at this schematic here, that is more or less what I see, the second half of the input tube looks like a cathode follower to me, and while I didn't figure out all the switching choices, I do see the pair of jacks just to the right, and I suspect that would send half of a stereo input to some other amp and speaker.

    DOn't think of pins 1 and 6 as having a 100k between them, think of a 100k plate resistor to B+, and the cathode follower plate directly to B+ as we expect. They just drew it to look like a resistor between halves.

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    Plate #6 seems to walk around the first half of the 12AU7 and go straight to the grid of the phase invertor
    That is no signal path but preamp supply voltage.

    (When analyzing a circuit it is always a good idea to start with identifying ground and DC supply wiring. All DC supplies are AC/signal ground.)

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    Last edited by Helmholtz; 12-31-2019 at 07:55 PM.
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    Member HaroldBrooks's Avatar
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    Thanks Jazz P Bass, Enzo, and Helmholtz. I thought it might be just the power for the plate pin #6, but I wasn't sure, owning to my self taught status, up to this point.

    So the plate #6 and that side of the triode is not contributing to the amplification of this power section of the amp ? It seems not. I was afraid that not incorporating it into the circuit would perhaps either dash part of the gain, or even worse, upset the function of the Phase invertor in some way. So there's more to the story and I think Enzo pointed it out. Somewhere in my distant memory I recall setups like this when I was a small child (I think). Along with the real stereos that had already taken hold by that point.

    How about the "Loudness" circuits ? Correct me if I am wrong, but presently I don't really think they would be of use in my goal to create a rock amp, because they are at the front of the amp, and while I play clean from time to time, 95% of the time I run amps hard and dump bass and the highest treble to clean things up, the exact reverse of a front end loudness circuit.

    Interesting amp for me, and I LIKE EL84 TUBES ! While I know they only contribute a small amount to the tone of a guitar amp, I can hear a definite difference from the 6V6s, 6973s, and 6L6s I've come to know and love. The EL84 seems to me (at least in these Zenith Hi-Fi circuits to sound clearer with some really dissonant chords. It helps that there is a presence control I am sure, but I also have there 6V6 incantations of the same basic amps, and the EL84 ones make better hard rock amps, no questions for me, and I went in to my exploration thinking the exact opposite. I hate having presumptions proven 100% wrong, but it happens all the time now !

    One thing I noticed on all the Zenith Hi-Fi amps I modified to guitar amps is that the Bass control becomes limited to about a 2/3 setting or so, with too high a gain that destroys the whole signal. This is even after reducing the PI to power tube coupling caps, and rolling back the input low bass with an EQ. Still no resulting hardship, I just back off the Bass to around 7 or less, and all is well. Maybe I should try some smaller coupling caps. The originals were 0.1uf, and I reduced them to .047uf, but perhaps .022uf would have been more appropriate. I keep making that mistake of trying to get the most Bass out of my amps as a compensation for using a small cabinet speaker in test. I'm 100% sure the Zenith amps would work well with a pair of 12 speakers (I only have 10" Celestions or a single 12" Jensen in a small open back cabinet). So there you have it, I am challenging my own bias notions again !

    Pick speakers first, tune amp sound second, and then see what changes you may still have to make !

    Anyone else please feel free to Chime in, and I continue to learn !

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    Last edited by HaroldBrooks; 01-01-2020 at 07:09 AM.

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    Lifetime Member Enzo's Avatar
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    It looks like the main input wants a phono input. Consider how records are made. The groove is more or less like an oscilloscope trace of the sound. The larger bass notes would take up a lot of room, and the grooves would have to be spaced far apart to make room. That would limit the length of the record available. So they came up with the RIAA curve. WHen they record onto a record, they EQ the bass WAY down and the treble way up. Now on the playback end, the RIAA curve is reversed. The phono input of a amplifier has the bass WAY emphasized, and the treble rolled way off. The curve is a but more complex than that, but close enough. (I seem to recall it is a four pole equation). So ultimately the recording and playback curves compensate one another and even out.

    That is why typically the phono input of an amp sounds boomy and bassy.

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