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Question about pre-amps

I got a pretty cool old Zenith tube Hi Fi radio with a switched RCA input that goes right to the amp. Labels say this thing is 50 watts.

I have plugged in (with an adapter) my 2 string bass, but while I get 'dirty contact' noise thru the speakers, no guitar sound.

The ham radio guy here on the ship has told me that it's very possible the pup does not have enough output (the input is for a phonograph) and said a preamp would work.

I'm looking online and all the in-body ones are for piezo...which I am not interested in.

So what can I do? Adapt the mini micro piezo plugs to 1/4" phono, or what? Build a stand alone box (cigar!) and hard wire a jack?

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Permalink Reply by Taffy Evans on November 20, 2019 at 3:55pm

if you are using a magnetic pick up it should have enough out put to drive without a pre amp as it would be high impedance. Low impedance mag pickups would need a preamp. Maybe your problem is elsewhere?

Also that 50 watts would most probably be music power and not RMS. maybe?

Taff

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Permalink Reply by James Pobog on November 20, 2019 at 4:47pm

Taff, electronics is Greek to me....

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Permalink Reply by Taffy Evans on November 20, 2019 at 5:54pm

Maybe Paul Craig will buy in with some first hand knowledge.

Taff

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Permalink Reply by BrianQ. on November 20, 2019 at 6:18pm

I play through those amps all the time, no problem? How many inputs does it have, does it have an Aux. channel? What kind of adapter are you using, stereo/mono? Pics would be nice, otherwise just shooting in the dark like everyone else?

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Permalink Reply by James Pobog on November 21, 2019 at 3:35am

I just posted the schematic in my reply to Taffy.

No aux channel

One input, although there are two screw terminals in a speaker wire config, but they are labeled (on the fiberboard back) as

Mono adapter, homemade. Male RCA to female phono, done very nicely, these are big time ham guys who work on the ship's comm gear.

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Permalink Reply by James Pobog on November 21, 2019 at 5:18am

There's a user-mod of a toggle switch on the back that looks like it is now a radio/extension speaker switch, that's the nasty taped splices.

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Permalink Reply by James Pobog on November 21, 2019 at 3:26pm

Did test #2 tonight...

The radio amp works, haven't tried the radio side yet, 'cause that's not the goal. However, signal is weak, and lots of hum that increases not decreases when I touch the strings. So I count it as a win because I can hear the bass through the speakers.

I think next step is repair/ mod by either a shop or one of my radio guys. At this point what i would like to accomplish is:

1. Fix the funky toggle switch mods.

2. Mod the wiring so that the RCA is bypassed to a jack.

3, Make output to speakers switchable between the speakers and line out (speaker wire screws)

4. Re-cap if needed

Push comes to shove, I suppose it could be gutted and a modern amp (hopefully tube) put in.

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Permalink Reply by James Pobog on November 21, 2019 at 5:14am

Here's the little adapter...

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Permalink Reply by Timothy Hunter on November 21, 2019 at 2:10am

firstly, does the radio work? if you get no sound from radio then the amp might be faulty.

if you are going in through the record player inputs then the guitar should be more than enough i would expect nasty distortion. the record player also has RIAA equalisation on it. the bass is reduced and the treble increased to keep the groove on the record manageble.. so on playback the bass volume is boosted and the treble reduced.. might have some funky sound..

If the switched RCA bypasses the record player input and goes direct to the power amp section then yes you will need a preamp. a clean boost pedal would probably be enough. a boss ge-7 graphic equaliser pedal also has a level boost . very handy pedal to have.

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Permalink Reply by James Pobog on November 21, 2019 at 3:11am

I plugged it in on the ship last week, and since I was inside the Faraday cage that is a battleship, signal was week but there, that is to say spinning the dial I picked up some very weak signals, I could discern human voice, and since the contacts are pretty dirty, it'll crackle like a big dog. Here at home I was in that laser focus and did not switch back to 'Radio'. The switch is labeled 'Phono/Radio', and goes between the two. Would it help if I posted the schematic? I think the guy said radio is radio, and Phono is the RCA straight to the amp.

The amp is bottom right, and the RCA plug is 11 oclock from it..

Attachments:

schematic.pdf, 128 KB

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Permalink Reply by Timothy Hunter on November 22, 2019 at 2:34am

from schematic, phono/radio switch selects between the output of the radio or the input from the RCA phono input. good news id the phono input goes to the preamp tube . Either input goes to the 1Mohm volume pot then to ist audio tube 19T8 then through a tone circuit to the 35C5 poweramp tube..

hope you get it working..