Do you need to Bios amp when changing preamp tubes

[BasalGanglia]

I have a Randal RD45 guitar amp. A while ago when I switched it on I heard a pop and although the power light remained on there was no sound. I first tried chaging the two power amp tubes of exactly the same type but still didn't work. So thinking of buying a single (or a set) of preamp 12AX7 tubes to see if that was the problem. On the specifications for my amp it says one of the four tubes is balanced. Not sure what this means.

If i change any of the tubes to the same type does that mean that the amp needs re-biasing or can I just do a straight swap? I would of thought that re-biasing is needed when switching between different types such as EL34s with 6L6 for the power tubes. Online it seems to suggest that you have to re-bias when switching the preamp tubes of the same type. Thanks

[Mick Bailey]

Preamp tubes do not need rebiasing, but the output tubes may need to be rebiased or at least checked if you change tubes, even if they're the same type.

Each of the preamp tubes is effectively two tubes inside the same glass envelope - a dual triode. One of these tubes splits the preamp signal into positive and negative signals to drive the power amp. That's the phase inverter (phase splitter) tube. Dual triodes can have slightly different characteristics for each half - they aren't matched. You can get tubes where the triode halves have been tested and those that have closely matched halves are sold as such.

The claimed benefit is that the matching gives identical signals so the the power tubes are equally driven, resulting in a symmetrical waveform. In practice there are a lot of other variables that affect this and the extra cost is often not justified.

In terms of diagnosing your amp, if it has an FX loop plug your guitar directly into the return and see if you get a sound. You could have a tube problem, a blown HT fuse or something else going on. It should be possible to narrow down the fault before replacing tubes.

Is there any hum from the speaker at all?

[stokes]

As Mick said, check the HT fuse if there is one, I'm not familiar with this amp. If that fuse is OK I would do a signal trace to find where the signal is getting lost. When you turn it on, does it still make that "pop"?

[BasalGanglia]

Sorry for the late reply and thankyou for the advice. There is no hum sound whatsoever when I turn the amp on and no pop sound either. I plugged the cable straight into the Return channel. Ive never used the Return channel before so am I correct in using the Return input not the Send input? There is no sound whatsoever and no hum. Even before plugging the cable into the guitar and touching the end still don't hear anything.

I can see the four red led lights are lit up from inside and also the power led light on the front is also lit. So there maybe a HT Fuse inside to check?

[g1]

schematic attached. HT fuse would be F2 on the drawing.