Building a 300B PSE

[A Stuart]

Are you saying the amp itself makes audible "sings/whistles" - the valves/transormers/chassis?

Luckily I have never had to troubleshoot a problem like this so can't help a lot. I am most surprised you are no flooded with offers of a diagnostic plan yet.

There is an FAQ page, and also at the top of the page, a "Search" function.

[Greg.]

Originally Posted by A Stuart

I am most surprised you are no flooded with offers of a diagnostic plan yet.

Diagnostic advice typically comes from Richard and John Caswell and I see both of them have not visited the forum for several days. It’s highly likely they are on holiday. Be patient. I’m sure the help will come in due course.

[MGCRead]

The setting is 0.1 amplitudes/division. I have checked the wiring and quality of solderjoints etc, it all looks OK. I supose it could be a faulty component but they are all new. What I will do is rebuild the signal amplifying circuits and tidy up the wiring as it rather resembles a bowl of spaghetti.

[Greg.]

I am no expert but I seem to recall from previous threads on this forum that the squealing symptoms you describe have been a result of incorrectly connected output transformers. Worth checking.

[John Caswell]

Hi,
Well looking at the two pictures you have a classic case of 50hz hum superimposed on the signal.
So first off, have you got a "hum bucking" control on one of the 300B heaters?
If not - fit one.
If you have - then make sure your amp input is s/circuit, a phono with a bit of wire soldered centre pin to outer will be perfect, then powering up, display the output on a scope set initially to say 50mv/div and adjust the "hum bucker' for minimum trace distortion, upping the gain of the 'scope ie 10mV/div - 5mV/div accordingly. You should be able to get it fairly low - how much will depend on wiring/construction etc. Report back here.
With regard to the whistling/squealing that could be the negative feed back the wrong way around ie positive feedback. The solution is to swap over the wiring from the secondary of the optx ie if you have purple to output and green to ground change them over green to output and purple to ground. Do not make the fundamental error of changing both the primary and the secondary over - only the secondary. Once again report back here

John

[MGCRead]

Thank you John and Greg,

I have had a look at the o/p transformer and now see I completely forgot to connect the speaker wires ( purple and green ) to the feedback switch and 0V. I was so involved in checking all the circuitry I didn't think of checking there - very silly of me. I will make the changes tomorrow and report back but it is bound to make a big improvement !!

Fingers crossed...

Once again very manks thanks for pointing me to the OPT, Malcolm

[John Caswell]

Hi Malcolm,
That's a good start, now don't forget the hum bucker pot.

John

[MGCRead]

Bad news. I had wired the o/p transformer correctly but via the speaker terminals which I have just spotted. I swapped the green and purple leads over but it makes no difference.

My understanding of the circuit is everything is connected to the 0V signal earth ( including the power supply ) and the star earth is then connected to the chassis earth via a 10R resistor ( I am using a 10W ceramic wirewound ).

However for ease of construction I have taken the amp circuit to one earth point and the supply to another earth ( neither connected to the chassis obviously); they are a few inches apart connected by a beefy cable. I suppose that might make a difference but the singing o/p transformer seems to imply a bigger problem than I would expect.

I have the two transformers and choke orientated as per the instructions but the choke is very close to the mains input.

Twiddling the humbucker makes no difference.

It is definitely working as an amp tho: I get 0.8V out from a 0.14V input ( a factor of 5.7 ). The specs don't tell me what to expect.

The dummy load is 6.8 ohm 200W they don't get warm. The valves get warm but I can't actually see any glow from the heaters which is odd. I get the correct voltages from the p/s - a bit high if anything ( eg 428, 376, 195, -202, -47 for stated 400, 350, 195, -195, -40 ), I also checked the circuit voltages and they are within 10% of stated ).

So I think I will rebuild the amp circuit and tidy up the wiring, especially the earth.

I ran it for about 15min and no component got very hot although R11 was over 40C. The glass envelope of the 300B reached 70C is that normal

Many thanks for your sugestions but I think rebuiding with neater wiring ( especially the earthing ) is my best bet, but the singing ( which is very loud with an input aproaching 0.3V or so ) is what really worries me.

I will keep you informed,

Cheers Malcolm

[Greg.]

Malcolm, do you have the build instructions? I can send you a copy if you need them. Just PM me your email address.

[MGCRead]

Yes I have them thanks Greg. But I wanted to design a rather more attractive case so I have to fit the parts a bit differently.