K4040 Voltages

Kits Audio Hi Fi Projects
1

K4040 Voltages.

Hi guys.
This is my first time post on this forum so please bear with me if I appear a little slow…

I have the K4040 power amplifier. I built it about ten years ago and it has been the best amp I have ever owned. I noticed lately that it hasn’t been sounding so good and I have to be honest to say that I probably haven’t set the bias voltages in two years…

So, I got my DMM out and set them – perfect! I prefer to set them this way so I am sure I have the correct value. While I had the covers removed I thought I’d give it the once over and measured the PSU voltages, both primary and secondary AC, and the DC values. I am concerned that the secondary are not what I expected and the high voltage DC voltages are way out.

The mains input is set at 245VAC, the actual mains input: 241VAC.

Power Transformer Values.

Heater voltages: 6.8V although they have been as high as 7.3V.
300V (red taps): 293.3V.
50V (yellow taps): 47.3V.

HVDC.

J1&J3 should read 415VDC – actual value: 395VDC.
As for the other DC high voltages they can differ by as much as –40 volts, 365VDC for example reads 325VDC. The other DC test points vary greatly, but all are way too low.

I am wondering if a couple of the power caps are on their last legs, although physically all looks good.

I would very much appreciate some input on what is the probable cause of the voltage differences.

Many thanks,

Charlotte.

2

Don’t worry, the voltages in the amp are not regulated, so they can vary a little over time/circumstances.
Indicated voltages were measured at design time. Other brands of tubes or the age of the tubes can influence these values.
Proper bias setting is more important.
Most likely there is no need to replace caps at this time, if there is no PSU related excess hum.

3

Hello,

Thank you for responding to my post, you have settled my concern with regards to the voltages. There has always been a little hum in the speakers but nothing excessive. In my original post I forgot to mention that tube six bias minimum setting is 415mV and I can not set it any lower than this. Could the pot be at fault? I also recently read that if I replace V11 with a JAN Sylvania 6189W tube it would negate this hum. Would this be money well spent?

Thanks again,

Charlotte.

4

Sorry, we have no experience with this tube.
Regarding the bias setting, you could remove all tubes and check if for every tube if the negative grid voltage is adjustable between more or less the same range.
If yes, then most likely tube is off-spec. If no, check parts for that particular part of the circuit.
Please be extremely careful when you measure on a live amp. These voltages can kill !