Low voltage inside Rivera Rake tube amp

[alistair016]

Hi guys,

I bought a Rivera Rake head a few years ago and haven't been using it much. I've been thinking of selling it, so I thought I'd chuck in some fresh tubes and make sure it's all operating ok.
It seems to have very low output (not much louder than my champ!) and has very little headroom.
I initially thought it might be the power tube bias, but following the Rivera method (as listed here http://web.archive.org/web/200402101...rRiveraAmp.htm) I can't even get a reading in mA. As soon as I flick the standby switch off, the reading is off the scale of my multimeter, and I'm obviously not getting more than 1000vDC at the test points!

Looking for other faults I tested the marked voltage test points on the main PCB, and they all seem to be quite under voltage. For example, there is a 18v test point on V1, and it's sitting around 12v. There is a 34v test point near the bias adjust, that is sitting on around 26v.
This would seem to suggest that the amp is underpowered, hence the lack of volume and headroom.
Has anyone seen something similar and might be able to suggest where the culprit might be?

Thanks for your time! I've done a bunch of searches but haven't found something similar.

[alistair016]

Sorry, I forgot to mention plate voltage on the power tubes is sitting at around 350v too, which is pretty undervoltage.

[SatelliteAmps]

Did you let it settle in, as explained in the detailed explanation about biasing a Rivera?

[alistair016]

Hi Adam,

Yep, I left it for around 5 minutes. Perhaps this isn't long enough in this model, but I've never had to leave an amp for more than a few minutes before when setting the bias.

I actually traced the problem back, and it seems that every secondary from the power transformer is undervoltage by around 25%. The B+ is running at 300v, and the heaters are sitting at around 4.2v!

Is this likely to be the power transformer or something further on in the circuit loading it down or causing the values to be appear skewed? I know PT failure is very rare.

[SatelliteAmps]

If everything is down 25%, I would check a few things out. What is the voltage coming in from the wall? Next, I would pull tubes and check voltages. Are they normal with no tubes? If so, then try a new set of tubes. If there is no difference, it would be time to start tracing the power supply.

[alistair016]

Hi Adam,

Thanks again. Yes, the testing was done with power tubes removed. Would it make any difference to pull preamp tubes? I didn't believe so.
Wall voltage is sitting at 244v, which is within tolerance (I'm in Australia, on 240v wall supply).
Someone else has suggested that an overload on a secondary could easily cause all secondaries to have low output. He suggested filter caps as a good place to start. The amp is around 15 years old with an unknown service history, so it's probably something that wouldn't be harmful to try regardless.

[SatelliteAmps]

You should pull all of the tubes, including the preamp. I am leaning more towards a power transformer problem at this point. Next step would be to lift the secondary wires, and test them. If they are still low, then you've got a bad PT. If they are high enough, then I would start looking at filters.