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Old 26th February 2013, 04:38 PM   #1
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Default rebuilding 3tube zenith amp

hi guys,
new here, this is my first post,,,, i just rebuilt my first amp,a little zenith 3L03, new caps, resistors, polarized cordset & stdp switch.... i redressed the filament leads, and put in a 3rd stage filter, to help get rid of teh hum,,, it works pretty well, the amp has crisp highs, but not a lot of bass, and it has some distortion, at high volume levels...
the distortion is what i'm trying to reduce now,,,the tubes all test very good ... i modded teh amp a little,,, replaced teh .01 coupling caps with .068, in hope of getting more bass, it helped a little, but seems to have maxed out there...
i replaced teh 150 ohm K resistors with 180's,,, and dropped teh 270K coupling resistors, to 220K,,,tried as low as 120K, to get more negative bias, but it didn't seem to change after 220...
i would appreciate any help i can get to get rid of teh distortion, or at least to understand why its there,,,
i posted teh original schematic,,, and teh chassis..
thanks for reading,
regards,
john

(schematic removed by moderators for blatant safety issues)
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Old 26th February 2013, 05:18 PM   #2
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From my quick look at the schematic and not knowing even 1/10th as much as the other regular posters here at diyaudio, I can say that it is pretty underpowered but nonetheless pretty cool.

You are getting all your power from 110 V from the wall, with no power transformer required (the tubes are at 117V according to the schematic), so I wouldn't think that you would be able to get massive amounts of volume from this setup because of small tubes, low B+ voltage, SE set up, all usually means low power.

You might not get much bass response because this looks like it was set up for playing records -- I was told that there is something about records not having very low frequency range because of something or another, (I am not very knowledgeable when it comes to this stuff, I can build it, and I am still alive) only as I don't see an auxillary in and only the phono cartridge in.

The only thing I can think of is upgrading to beefier output transformers (but that requires math and money to figure out what would be appropriate). It might not be worth it, but see what the experts here have to say.

What kind of speakers are you using? Are you using other than the original ones which are rated for 9 ohms and probably as old as the hills?
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Old 26th February 2013, 05:28 PM   #3
SY is offline SY  United States
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FIRST THING- install an isolation transformer. This setup is dangerous enough that we have a specific rule on this forum about showing anything with a direct mains connection.
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Old 26th February 2013, 05:47 PM   #4
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Quote:
Originally Posted by SY View Post
FIRST THING- install an isolation transformer. This setup is dangerous enough that we have a specific rule on this forum about showing anything with a direct mains connection.
i am running it thru an isolation transformer... plugged into a gfi receptacle... i have been a licensed electrician for over 40 years, and am aware of teh hot chassis design,, same as most AA5 radios.... this is the only tube amp i have, and am using it to learn about tube circuits,,, i have plans to restore, or build my own 5 tube amp, i found a schematic to use....
if i can't discuss it on your forum, please let me know... i will remove the info in the post...
thank you,
john
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Old 26th February 2013, 05:52 PM   #5
kevinkr is offline kevinkr  United States
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Since you've clearly stated that you use an isolation transformer this discussion can continue.
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Old 26th February 2013, 06:00 PM   #6
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hi thanks for your reply,

Quote:
Originally Posted by overtheairbroadcast View Post
From my quick look at the schematic and not knowing even 1/10th as much as the other regular posters here at diyaudio, I can say that it is pretty underpowered but nonetheless pretty cool.

You are getting all your power from 110 V from the wall, with no power transformer required (the tubes are at 117V according to the schematic), so I wouldn't think that you would be able to get massive amounts of volume from this setup because of small tubes, low B+ voltage, SE set up, all usually means low power.

You might not get much bass response because this looks like it was set up for playing records -- I was told that there is something about records not having very low frequency range because of something or another, (I am not very knowledgeable when it comes to this stuff, I can build it, and I am still alive) only as I don't see an auxillary in and only the phono cartridge in.

yes, it is from a zenith portable phono,,, i have it connected thru a cd palyer, now,,, and used it with a casstte player, also...

The only thing I can think of is upgrading to beefier output transformers (but that requires math and money to figure out what would be appropriate). It might not be worth it, but see what the experts here have to say.

What kind of speakers are you using? Are you using other than the original ones which are rated for 9 ohms and probably as old as the hills?
hi thanks for your reply,
yes, it is from a zenith portable phono,,, i have it connected thru a cd palyer, now,,, and used it with a casstte player, also...
i was told that different OT's may help teh bass, but i don't know how to calculate what i would need...
i'm running thru my main speakers(12"-4" tweeter, in a bass reflex enclosure)... it came with 3x5" but sounds much better pushing teh bigger speakers,,, also, teh output of teh amp is 3.2 ohms,,, my mains are 8 ohm...

regards,
john
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Old 26th February 2013, 06:08 PM   #7
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Originally Posted by kevinkr View Post
Since you've clearly stated that you use an isolation transformer this discussion can continue.
ok,,, well, i see teh schematic was taken down,,, am i allowed to pm it to someone who may offer to help me out?
tahnks,
john
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Old 26th February 2013, 06:10 PM   #8
kevinkr is offline kevinkr  United States
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Might help to post a hand drawn schematic of the amp which must clearly show the isolation transformer as part of the circuit.

Edcor makes some nice output transformers, but I suspect even their small ones might be a bit too large for this tiny chassis.

I would enjoy as is, and if you really like tube sound consider this: Simple SE
George is quite active here and a friend to many - I can highly recommend this as a project for a tube newb.
Performance will be immeasurably better, providing real bass, dynamics and detail with enough power to get some decent levels out of your speaker system.
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Old 26th February 2013, 06:13 PM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by knockbill View Post
ok,,, well, i see teh schematic was taken down,,, am i allowed to pm it to someone who may offer to help me out?
tahnks,
john
No problem there, but you could draw it yourself and snap a picture with your digital camera, not brilliant but it would work to get the information across.
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Old 26th February 2013, 06:49 PM   #10
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Originally Posted by kevinkr View Post
No problem there, but you could draw it yourself and snap a picture with your digital camera, not brilliant but it would work to get the information across.
ok,,, heres a pic of the amp and teh iso trans is under the table...
i tried to post schematic pics i took with my camera, they came out to blurry to read...

thanks, also for teh link to teh "simple se",,, i have a 6v6 amp in mind, i have the tubes, and a chassis i built, i'm not really interested in pc board construction...

i sure hope this thread gets pointed to teh distortion problem i first asked about!!!!!

regards,
john
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