• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tube preamp noise

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Please help...
I bought a custom built tube preamp. The builder shipped it with the tubes in the sockets...Electro Harmonix 12AU7's and a 12X4. Sounded flat and muddy. I had 3 extra JJ 12AU7's and put 1 in each channel. Sounded better so I got one more and replaced all 4 EH's. Sounded much better. Last night I noticed a high pitched noise coming directly from the preamp..not through the speakers. Can anyone offer and suggestions what the source of this noise is?
Thanks in advance....Blake
Unfortunately I don't have the EH tubes...shipped them back for a refund. I don't have a local source for 12X4 tube either....at least I don't think so. There's a business in town that does primarily guitar amp repair and he said he has all kinds of used tubes. Perhaps I'll go there tomorrow to see if I can find some tubes there. Still waiting for a response from the builder. Damn ...I should have studies electronics instead of mechanical engineering.
You should check your amp for oscillation. It is probable a component is making the noise, but also probable an electrical problem is causing the noise.Short the input of the preamp and then take measurements at places near the noise. If you don't have an oscilloscope, you can use an analog voltmeter with a 2 VAC and 20 VAC scales. Put a capacitor in series with the meter plus probe, to block DC voltages reading as AC. Use only one hand at a time measuring tube circuits. Connect the negative of the meter or scope to the chassis ground or analog ground with an alligator clip lead before turning it on. Do not use a "PC scope", the negative is not isolated enough from ground to work tube circuits safely. Read the high voltage for newbies sticky thread a the top for more tube safety tips.
Finding exactly the right tube is something guitar amp people waste a lot of time doing, when on a new circuit there may be design or construction problems. The main effect of different tubes is in the non-linear part of the curve, where distortion is created by guitar amp circuits. In the linear, published part of the tube gain curve, they should behaive per the specification on the data sheet. Or they are defective. You can check Gmo without a tester in circuit by measuring voltage into the grid, voltage out the plate or cathode (depending on circuit) and dividing by the appropriate resistances. (Capacitors and inductors have "impedances" which act like resistances, but are frequency dependent.)
I don't have a suggestion for a good basic text for learning tube circuits. "RCA Radiotron Handbook" was way too much about radio. As was the IRRL (ham radio) text in the sixties. "Electronics for Scientists", 1968 edition, was way too much about DC circuits like PH meters and instrumentation amps. I've learned as much fixing stuff and from the dynakit build manuals until I discovered this site 2 years ago.
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Thanks for the advice but this stuff is way over my head. I'm a mechanical engineer trying to learn a little about electronics. I bought this hifi stereo preamp built by a custom builder. I'm going to wait for a response from the builder. Thank you anyway.

Incidentally the amp that I purchased from the other owner of this company did an impeccably job of building the Amp.
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Yeah, the vector arithmetic Impedance equation (see on wikipedia or someplace) for modeling capacitors and inductors as resistors (at one frequency) should be a cinch for somebody with your training.
Impeccable build is as it performs. If the builder scrimped on enclosure and input filtering, it is possible a radio frequency source like a lamp dimmer, CFL bulb, LED light string, computer, or cell phone is driving your amp into oscillation. Whereas the preamp worked fine on the builders bench. Those clunky old fashioned dynakit steel boxes, with a separate steel box around the AC power transformer & wiring, cope remarkably well with the modern environment. The input filter capacitors, and RF filtering transformer, complete the package. A lot of modern tube amps don't have cases, they don't even have steel covers over the tubes like hammond organ used to use to keep RF out.
I started repairing tube stuff with a $5 Radio Shack analog meter, fixed my amp a couple of times, then upgraded to a Simpson 260 200kohm/volt meter about 1985. Second year college physics (kirchoff's laws, vector arithmetic of RLC circuits) was a lot of help. I have bought a couple of beater scopes, but can't seem to keep them running. Something about the boards being glued in the case. Never mind, I've fixed 4 or 5 amps and preamps, a mixer, a radio, a couple of organs, blah blah, just with a meter, some clip leads and a junk box full of capacitors. I didn't even have a schematic diagram on a couple of successful repairs.
If you've got time it is fun beating the lords of import **** at the game. Other than speakers that I buy working and packaged, I hear wonderful music nearly all the time (now that I'm retired) with old junk I repaired. TV's and cars aren't nearly so much fun to repair.
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indianajo said:
it is possible a radio frequency source like a lamp dimmer, CFL bulb, LED light string, computer, or cell phone is driving your amp into oscillation.
Theoretically possible, but astonishingly unlikely. Two separate problems:
1. something in the amp is picking up external RF.
2. something in the amp is acting as an RF oscillator.
One of these may be happening. Ask the builder about grid stoppers. If he looks blank don't buy any more audio stuff from him.
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