Integrated Tube question

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Well-known member
Jan 1, 2005
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Huntington Beach, CA

This week I finally replaced my old 1988 Onkyo 45W/CH receiver that drove my ReQuests. I'm now using an integrated tube amp to power them, but my speakers are now reproducing a low frequency hum and a high frequency rapsody kind of hiss. The Onkyo was Dead-Silent. Any suggestion to remedy this?

When I switch to the other source selection (no cables are connected there) the hum is barely audioable. After switching cables to the other inputs, I determined that there the hum only comes from the input selection that has a connection (that is a good thing).

I heard a while back that when using passive pre-amps, the cables should be no longer than 6 feet. So today I replaced the 6 footer with a 4 footer I had lying around. With the amp volume knob turned set to 12 o'clock (it gets louder as the volume position increases), and the source turned off, the low frequency hum measurement dropped from 77 DBs to 68 DBs (w/the mic almost touching the woofer grille). So, I am assuming that if I got a 2' footer or shorter, things should get even better huh?

I've heard the better interconnects can improve the sound quality and can prevent the RF noise from being picked up, but will it reduce the negative conditions I am experiencing? If so, what is an very inexpensive model to get?

In addition, I am using my Onkyo as a tuner for the tube amp (from the tape outs). My CD player, coming from a 12' interconnect, is also connected to the Onkyo. If I connected the 12 footer to the tube amp directly, the hum is even worse.

I'm new with tube amps, so this final observation may seem ridiculous to you guys, but are they suppose to smell like burnt toast? When I first turned it on, it was at night with the lights off, and I literally felt like Dr. Frankenstien: "Igor, pull the switch!" I hear the tubes spark to life, begin to glow brighter, while the sounds of electricty and a power generator come out of the MLs....
When the 12AX7 tubes in the phono stage of my preamp started to die in a serious way, they made the exact sounds you are describing - low frequency burping sounds and high frequency microphonics. I assume that the rhapsody hiss that you describe in the highs is not high frequency pink noise, but actual dsicernable high pitches among the hash (the discernable pitch would imply a tube issue)? It sounds to me like you need to replace the tubes in either the preamp or amplifier section (or both). The smell you describe could be dust in or around the tubes (or other heat sources inside the unit), or manufacturing materials that are still curing if the unit is new. In either case, a smell is not typical of my experience with tube preamps or hybrid power amps. Tube replacement is easy, and retailers like thetubestore make purchasing a piece of cake. Have you explored the possibility that the tubes are going?
Thanks for the reply Sky Saw!!

The amp I got was the actual PrimaLuna Prolugue One that was used in a review (forgot which magazine). It's in mint condition and I just got it this week.

So I don't understand why the tubes would already need to be replaced, but from what I've read in the owners manual, that definetly sounds like what the problem could be.

I'm waiting to hear what the guy who sold it to me has to say (from Upscale Audio). In the meantime, I'm going to try to get shorter cables and see what happends.

Any other suggestions or possible explenation?

Nevermind, I got it figured out.

The CD player is connected to the receiver by a long cable, and then the receiver (to be used as a tuner) is connected to the int via a short cable. I disconnected the long CD player cable and now the hum/hiss is completely gone!