300B PSE Hum

[Cyprus Martin]

I have hum on one of my monoblocks. It was noticeable but not too intrusive when playing music. I did the mod on the 6AU6 heater (schottkys etc) in the hope of curing the hum but since the mod it has got worse. I have compared the wiring on both monoblocks (the other one is perfect) and they are the same. Taking out the 6AU6 the hum remains. Removing the 6SN7 ( I changed from the ECC82) kills the hum totally.
Anuone give me some pointers short of coming out for a free holiday.
Martin

[Les Reed]

Using a diode bridge for the 6AU6 heater will change the the hum from 50Hz to 100Hz which could sound worse depending on the response of your speakers. If measured on a scope though the hum should be at a lower level. The CRC pi filter will not give smooth DC.

From your evidence it appears that the 6SN7 is contributing most hum. First step would be to try the 6SN7 from the good monobloc and see what happens.

[Cyprus Martin]

Thanks Les,
You were quite right in recognising the problem was centred around the 6SN7, I had tried swapping the 6SN7 s over but the problem remained the same. In the end I tracked it down to the BR1. I had changed the WO4 to a WO8 after about changing the WO4 four times but I have now changed the WO8 twice, I think I will get some STT512D diodes and replace the WO8 with those.
On that point, does anyone know if the body of the STT512D is live?
I have to say that the 6AU6 heater mod was worthwhile, not thebig difference the 6AU6 battery bias made but nticeable nevertheless.
Martin

[John Caswell]

Hi Martin,
The WO* series are not really suitable for two heaters such as a 6AU6 and a 6SN7 together as the total current draw when HOT is 0.9A, so when cold the surges could easily be 10 times that ie 9A.
A better choice is the 1N5401 series and make a bridge up from them.
See the FAQ for other various types

John aka Dr John

[Cyprus Martin]

HI John,
The BR1 (WO4) isn't on the 6AU6 heater circuit, its on the power supply circuit that supplies +195V, -195V and -40V (all DC). I had several of these fail before upgrading to a WO8 but this is I think the second of these to fail.
I was planning on replacing them with a bridge of STTA512D diodes.
I have already upgraded the 6AU6 heater as per the tweak on this forum (good mod too)
Best regards from a gloriously sunny Cyprus
Martin

[John Caswell]

Hi Martin,
That's what happens when you answer something without referring to the cct diag, sorry.
I am very surprised at the WO4 failing as you are probably only drawing a few mA throught the 6SN7, are you sure there is not anything else wrong ie a leaky decoupling C or one reversed.

John aka Dr John

[Cyprus Martin]

Hi John,
The WO4/8 isn't on the 6SN7 heater circuit, it's on the power supply circuit for delivering + and - 195VDC etc.
It's this that I'm intending to change to a bridge of STTA512D, by the way is the body live on these?
Martin

[John Caswell]

Hi Martin,
Yes I appreciate that the WO4/8 is the HT rec for the 6SN7/ECC82, which is why I am querying its recurring failure, as it only supplies the +ve & -ve to the 6SN7 and the -ve bias to the 300Bs. The current draw is very small ie about 20-30mA at the most so it is safely rated.
This is why I am checking as to whether you think there may be any other faults such as leaky capacitors.

John aka Dr John

[Cyprus Martin]

Hi John,
Sorry about my confusion - it's the heat out here getting to me again! Sorry couldn't resist that after seeing that a football match in the UK was called off on Saturday due to a snowstorm!
I checked all the voltages and they were way out around the WO8 but after I had changed it all the voltages were once again correct and the humming which was quite intrusive had stopped. In fact since I changed BR2 to the diode bridge there is no hum at all. I've now run it for around 100 hours and everything is sounding right, in fact it's not just right it's superb. I don't think it's a problem with the capacitors, I have got some spare 100uF 250V ones and if anything goes awry again I'll try changing them but I'm certainly going to change the WO8 to a STTA512D bridge as soon as I find anyone who is coming out to Cyprus and can bring them.
Best regards
Martin