Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Boone on November 16, 2013, 08:54:14 pm
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I have 5F2-A per Fender layout. I have been messing with switchable NFB. A few weeks ago I started with a 22K in there and it sounded less highs little less vol and lower distortion. I have had a 68K in there for awhile and recently went back to 22 to hear diff. With everything cranked the amp was doing a medium speed motor boating type noise. If I backed off T or V the motor boating stopped. With the 22K the amp now sounds literally 1/3 the vol, highs and distortion. Also, I don't know if this is normal for tweed prince but amp has no sound until vol is on 3. Any thoughts?
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... With everything cranked the amp was doing a medium speed motor boating type noise. ...
You could have OT wires swapped, or you could have a poor-quality ground somewhere.
Only other way motorboating is likely to happen is to have a too-small value series decoupling resistor in the power supply. For this amp, those are the 10kΩ and 22kΩ connected to the filter caps.
Also, I don't know if this is normal for tweed prince but amp has no sound until vol is on 3. Any thoughts?
Could just be the pot. Some brands and types are finicky.
I usually never worry about that in my amps, because I'm unlikely to play them with the volume almost all the way off. I also noticed my first tube amp didn't have the volume kick on until about "2" on the knob (it was a blackface Princeton Reverb, and obviously not broken).
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I tried the OT lead swap. Sounded like an electric razor when switched on without sending signal into amp. The amp sounds like it is under wage with the NFB switched on. Any ideas? Forget the vol issue I will live with it.....I thought it might be a pc to the NFB puzzle.
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Sounds like its under water not wage.......
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Did the amp have this behavior originally when it had the 22K NFB in there? and the noise only appeared after playing with a 68K R and the returning it to the original 22K value? Or did it always make this noise with the 22K?
I have 5F2-A per Fender layout.
So that means a 22Kohm and a 1500ohm cathode resistor under the 2nd 12AX7 stage.
What kind of OT are you using, and what is the OT's secondary load?
*I think* the original 5F2-A used a 4ohm sec. OT. If so, and you are using an 8ohm sec. OT, then 22K is too low., you'd need at least 32K to match the original NFB. Of course, those are just starting points if you are tuning (tuning= replace the entire NFB resistor with a 100K or 250K pot, dial in the NFB you like).
RE: motorboat sound...
Is the medium speed motorboat 120Hz or 60Hz or faster?
a gut shot of the components might help.
Also, the rolling off tone or volume reminded me of a problem I had with a Premier that I sorted out last summer, I had a thread on the subject in this forum. The problem ended up being a coupling capacitor that was picking up a parasitic oscillation at high volume. cutting the tone and/or vol dropped it out. that was high freq. though, motorboat=low freq?
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Terminal, I can't remember if it did it or not when originally in there. I never had it in the amp and tried it years later. I have an OT from Silvertone 1481. The schem says 8 Ohms. I am running 8 Ohm spk. The boating is a very low mid type tone. If I turn amp on with guitar plugged in and vol/tone maxed there is no motor. As soon as I brush the strings in the slightest way it boats. If I back down the tone it still boats. If I back vol to 10 it stops. If I turn back to 12 it is absent ( until I brush the strings) . It does it with a 33K as well. I have not tried the 56-47K range yet. I have a 39K in there now. I prefer the NFB tone for the higher distortion stuff but the amp sounds dead with NFB engaged.The 68 is only slightly diff tone than no NFB . How can I take some readings on the OT to give you some idea on what I have? The original owner of the 1481 said the spk was changed 2 wks after they bought it due to shipping damage. It may have had 4 Ohm spk in it when I bought it . But sticking to the schem I figured it was 8. I will ck later and post back. Any other info I can get for you? Btw - how do I place pot there to dial in res then hard wire permanent res in there? If you will try to explain let me know from back of pot perspective .....I do not know what whipers etc.....are. Thanks