6AU6 ac heaters ?

[early]

Maybe a daft question , I don't know ? .

But I have been poking around inside of my 300b PSE trying to find out how it's been built , and checking voltages as it doesn't look quite the same as the diagram , a bit of ad libbing has happened .

Voltages are there or there abouts except the 6AU6 heater is in ac , is there a reason for this ? , was it another mod ? , it's had the battery bias mod done if that matters .

Cheer's , Phil .

[Richard]

HI Phil,

DC was the original spec. Does it hum? If not then it doesn't matter, some actually prefer AC and think it sounds better so that may be the reason for the mod. If it hums put DC back in.

Many like battery bias (on the 6au6 I take it?) but I'm not so keen. It may suit different amps though and I've not heard it on yours. Check around that first stage and see if feedback is connected or if all the cathode components have been removed when the battery was fitted. I'd be very tempted to restore cathode bias without fb or any cathode bypass just a 680R from cathode to ground for a listen. Then connect in the LF bypass network R4 C1 and have a listen. Or a 220uF or 470uF bypass cap by itself.

Rich

[early]

Thanks Richard ,

It looks like the feedback circuit and it's ancillaries have been removed .

They do hum a little 5mV on one and 6mV on the other , however removing
the 6AU6 has no effect on the hum , so I guessing the hum is related to something else , although through my 87dB speakers it's only noticeable when you stick your ears next to the drivers .

However last night I decided to strip the amps and rebuild them as standard .

[Ali Tait]

That level of hum is what you would expect normally.Not much to be done except change heating to dc if it's a problem.

[Ali Tait]

Forgot to say,most people with this amp preferred it with battery bias.I did on mine.Rebuilding it stock may mean it doesn't sound as good!

[early]

Hums not a problem , as I say my speakers are insensitive enough not to hear it.

Problem is there seems to be lots of changes to the circuit , lots of extra bits in the psu , and bits missing from the signal path . Maybe this makes it much better than standard , I just don't know .

But if I put it back as standard I know what it should be like , before I start modding myself .

Anyway I've past the point of no return , first amp is gutted and I'm working on a list parts to bring it back to life ( will it ever live again ) .

Cheer's , Phil .

[Ali Tait]

Ok,best of luck! Did you trace the circuit before gutting it?

[early]

No I didn't trace it , but I have left one monoblock intact so I can compare them
and if need be I can copy it .