Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mpryor224 on September 24, 2010, 02:42:33 pm
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I am new to the forum and this is my first amp build. I finally fired it up today and it has a squeal. all voltages are ok...The squeal goes away when i remove the preamp tube, it is a new JJ 12ax7. I get sound from a guitar but as volume goes up the squeal goes up with it and overpowers the sound. it will squeal with the guitar input not plugged in. any help i can get would be appreciated.
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Check lead dress with a chopstick. Some wires could be too close to each other.
Invert the leads on the OT primary. The squeal could be caused by a reversed feedack loop.
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It may be a microphonic tube,but it also might be the OT primary wires reversed.
Because the amp has negative feedback,the wire that goes to B+ and the other going to pin 3 of the 6V6 may be reversed.It will squeal.
If you can't reverse those wires because of the length,just reverse the wires on your speaker output jack.Green to ground and black to the tip.
Always try the tube first and then try the reversed wires.
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Thanks for the prompt replys!!! i removed the preamp tube and the squeal stops...tried another preamp tube from my working classic 30 and it squeals also. the ot that i bought was from mojo...the sheet that came with it showed to reverse the blue and red wire. so. i have at current time red to the tube and blue to the board b+ should i switch?
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Yes,switch those.They should be the other way.
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I switched the OT wires and no squeal just warm beautiful tone!! thanks for the help...i would never have thought to look there,,,either way I am elated with my first build and I am hooked on amp building now.. What next? maybe an 18 watt marshall?
Thanks for the prompt replies and for welcoming me in. I have lurked on here for a long time and answered A LOT of questions. either way THANKS!
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Iam having the same problem with a 5f1 build. It squeals when turned all the way up,with nothing plugged in. With a guitar plugged in I have very little volume.. I found one mistake on the board that I corrected. I also have the red wire from the ot going to the recto tube. and the blue wire over to my 6v6. pin 3. Do I need to reverse these. Would that cause low volume.
It is a mojo ot with 4 & 8 taps
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The red wire to the rec tube??????????????? :huh:
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I ran the wire from the ot over to pin 8 on the rectofier and a jumber from the B plus to pin 8 of the rectofier
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That's normal practice on SE amps: Red = hotter, to the highest B+; blue to the tube plate.
Pin 2 or 8 on the rectifier tube, we'd expect to run to the input filter cap in the filtering network, meaning it's electrically the same as taking your highest-high B+ from that cap.
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Ok so I have that right. I am a lost as to why I have no volume. Any suggestions as to what I should look at and check would be great.
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Ok so I have that right. I am a lost as to why I have no volume. Any suggestions as to what I should look at and check would be great.
Hi John,
I might recommend starting a new thread for your 5F1 issues.
It may give others a chance to view YOUR problems. Many do not revisit a thread after the OP issues are resolved.
FWIW
Ray
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Ray thanks for the advice. At any rate I found the problem. I was trying to use up parts I had sitting around and used a 470 meg 3 watt resister instead of the 470 Ohm the schematic called for. I suspect the 470 meg just choked out the signal. I had a 470 5 watter on hand, popped it in and the amp sounds great.
I would not have found the problem with out all the help I find here.
Thanks everyone again
john
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Hi John,
A 470 M resistor soldered in place isn't much different than simply cutting a wire and leaving them disconnected.
good to hear your amp is up and running
ray
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> I suspect the 470 meg just choked out the signal.
You don't have a 470 MEG resistor.... that's an extremely unlikely part.
You might have a 470 K resistor. That's one-thousand times larger than normal, and thinking simple, it might give you one-one-thousandth of the output power, which would indeed be "very little volume..."
Common resistor values span a range of 100,000,000 to 1. 2:1 errors matter somewhat, so 1,000:1 errors matter a bunch. You need to keep your ohms, K-ohms, and Meg-ohms straight; that's much more important than differences like 470 versus 500.
> normal practice on SE amps: Red = hotter, to the highest B+; blue to the tube plate.
Yes, that's what the transformer maker intended. But if you apply NFB, and the tranny designer didn't see the amp plan, there's some chance it will be just wrong (positive feedback, squeal). Flip the red and blue. (With a single secondary, it might be better to flip the secondary; but if you have 4-8-16 taps that won't work right.)
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PRR, thanks for the help. I read the meter incorrectly when I was checking the value on the resister prior to installation. The ot has 4 and 8 OHM taps. I. am assuming It is wired correctly at this point as the amp sound really good, no hum or unwanted noise. Il run the amp for a while and see how it runs.
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PRR said "there's some chance it will be just wrong (positive feedback, squeal). "
That's interesting, so would a quick check to see if you need to reverse the OT leads be to open the NFB loop?
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PRR said "there's some chance it will be just wrong (positive feedback, squeal). "
That's interesting, so would a quick check to see if you need to reverse the OT leads be to open the NFB loop?
Yes,
I have now learned that many savvy amp builders do not connect the NFB until after that amp is tweaked and proved stable.
They also do not trim the OT primary wires until After they hook up the NFB.
Was good advise given to me after the fact. What could have been knowledge is now experience :angel
Ray
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Ray, my problem is trying to remember all this stuff when you need to... LOL
BTW happy 45th :occasion16:
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BTW happy 45th :occasion16:
TYVM!