K4040 (Blown tube and resistor)

Hi Velleman,

Today I’ve finalized my K4040 and started to measure and tune to ensure the amplifier works as expected.

I’ve measured all the voltages and I noticed most of them were dead-on, only the 335V on both C35 and C36 are a bit low (305V) and the 110V on R28 and R22 as well (94V and 102V). Nevertheless I concluded it was close enough to continue fitting the tubes.

Then I fitted all the tubes (6.3V was ok on all 2 and 7 tube connectors) and started to adjust all the variable resistors. Then I noticed instead of tuning them up (from 0.275v up to the level LD4/LD5 are burning), I was tuning them down from 5v back to LD4/LD5 are burning (although all my var. resistors started anti-clockwise). I tuned all valves down up to valve nr. 7 and then a large flash and a bang at valve 7 occurred. R102 was blown, so was the 2.5A fuse and I think also the tube itself.

Then I started studying what was wrong with my assembly (no shorts, all connected and assembled as intended). I couldn’t find anything but then I start wondering about positioning the var. resistors. I noticed it is possible to fit them in with the adjustment to the front and to the back. I’m now thinking I have them all fitted 180 degrees out (with the adjustment towards the front panel). This would generate a tuning method opposite to the one suggested (starting position being clockwise instead of anti-clockwise). Could this explain why the tube and resistor R102 toasted and if so how could I get a new tube (LE34, the resistor is quite easy to purchase at a local store). All other components look ok (I’ve measured all resistors and they work out as expected).

Is it also possible a faulty valve was delivered (e.g. leaking air in) causing this behaviour???

Please advice,
Rob Musquetier.

Is there anyone at Velleman who is willing to answer my question???


Certain trimmers turn the other way round. In this case, the tube was set to full conduction, which probably caused it to fail.
As a one time courtesy, we can send a spare tube, if you provide an address.

So if I understand correcly it should indeed been trimmed up from .275V to .4V instead of trimmed down from .5V to .4V?

Thanks for offering me a new tube. I’ve sent my address via mail. Good service!!!


That is correct, the bias should be adjusted from zero anode current up to the required level.

Hi Velleman,

I’ve received and fitted the new tubes and adjusted as suggested. The amplifier works fine now, audio is very good as expected. I only experience the fuse to blow sometimes when the main switch is operated rapidly (from on to standby or standby to off). Is this normal behavior?

The delayed starting on circuits do seem to work, the led is first blinking before it is switched to the on mode…

please advice,

replace original fuse with 5A slow blow type

Can I ask Vel417 to clarify what you suggested earlier Rob, that in fact the trimmers operate the opposite way round to the instructions?

I am about to apply power to my new K4040 kit for the first time and suspect the trimmers provided by Velleman as standard in the kit (see tinyurl.com/6dblpk for a picture) do operate this way, i.e. opposite to the instructions, and would like to get it right!

Rather than just “Certain” trimmers turning the other way round, in fact I would expect all common makes of vertical mount trimmers to operate this way, as it’s the right-hand of the three pins (viewed from the front) that connects to the common -50V bias supply, giving the minimum tube bias current at the full clockwise position of the slider. Vel417, are the instructions wrong?

Thanks for any clarification.


The instructions are correct, however, we have discovered that we have received a batch of trimmers that behave in the opposite way.
It is best to measure the bias before you insert the tubes. Turn the trimmers in such a way that you get approx. -50V bias (tubes not conducting).

Thanks for a quick response. Appreciated. Dave

[quote=“VEL417”]The instructions are correct, however, we have discovered that we have received a batch of trimmers that behave in the opposite way.quote]

Mmmm, that is surely surprising… At least it explains the matter.

One last remark. I think the amplifier is lacking a bit in the bass area (compared to my Sony high-end amplifier), the high end is beautifull on my Marrtin Logan electrostats. Are there any capacitors which could be ‘upgraded’ to boost up the lower frequencies a bit???


PS I have also bought a K8010 to power my Martin Logan Cinema electrostat center speaker.

The amp has been designed for max. bass response. Compared to solid state devices, it will alway have less bass, this is just a property of tube amps. Solid state amps do not have an output transformer, which results in far better damping, hence a tighter bass response.

Ok, thanks for the response… Can I use the tube amplifier for bi-wiring (e.g. only use it for mid ad high considering they should always be hooked onto speakers)?