Small noise using "bold" setting on Triple

[vitriol666]

I have a small noise problem with my 2 channel Triple Recto, bought recently. When I use the "bold" setting a very low noise appears. The amp seems a little bit more noisy in general on this setting but this sound does not seem right/healthy to me. I could best describe it as the "scraping" sound you get when using pots that are dirty or have bad connection. The noise does not get any louder if I turn up the volume and could not be heard through playing loud volumes, but when I stop playing it´s there. It does not seem to matter which channel I use. I was wondering if one of the tubes could have suffered from the one month long shipping from the states to Norway? The amp came with brand new 6L6 tubes. The sound sometimes goes away by tapping lightly on the top of the amp, and can be provoked that way as well. Any suggestions to what this may be?

 

[Boogiebabies]

Sounds like a microphonic preamp tube. They can manifest a problem in many other ways than sqealing. They motor boat, pop, tick, product white noise and can also just go dead. With the Bold switch on, all the preamp tubes are seeing full voltage. One of the lead stage tubes has over 400 Volts to the plate. At full power you can expect a high gain amp to chew up a few tubes, especially the DR with the high gain stages and loop. At these voltages, the tubes will manifest there problems and all but a strictly dead tube can be intermittent. In some instances, putting the amp in standby will make the problem go away. It may or may not reappear until later, but will come back. When you switch to Spongy, the power transformer is tapped to lower the entire amps voltage and thus reducing the voltage to the preamp and power tubes. In most cases, you can run nearly bad tubes without knowing because they behave normally under the lower voltage. Generally a 100 Volt drop in plate voltages runs the amp around 340-360 Volts and basically runs all the tubes well below their maximum voltage.
The variac/browning effect is the byproduct of the reduced voltage. Lowering the voltage however reduces the tubes natural plate current draw and the power tubes draws less bias. With some tubes, you may be drawing -8ma, but the amp still sounds pretty good.
The effect is less dramatic on the preamp tubes, but does lower the preamp plate voltages. The Tube rectifiers have a similar effect on power, but this is due to their inefficiency to covert AC voltage. Diodes are much more efficient, but do not have the same feeling to the power section sag when driving the amp. Again, all the voltages are lowered and are similar to Spongy's voltage, but it has a slightly differernt result in the tone.

Enjoy.

 

[vitriol666]

Thanks a lot! I guess you can say I got my question answered :wink: I´ll try to locate the bad tube now.

 

[triple rectumfrier]

awesome answer, i have a question, I am running an OD808 in front, no gain, 100% volume and gain around 1 o clock on red channel modern. Is it normal for there to be some noise? I run a wireless so there is never any feedback, but there is a slight hum that is diminished when i flip it to the green channel.

Could this be one of my tubes? I have changed V1 and V2 only. V1 is a tungsol i think...that i have been hearing nothing but good things about...The amp is only used in band practices and shows. volume usually at 2 oclock (on output) and on the red channel, the master is around 8 oclock. Should I just re do all of my V spots?

Thanks!!