Nick wrote: ↑Tue Nov 13, 2018 8:56 am
Have you got a shared cathode circuit. You could add a 50R unbipassed resistor below each cathode before they join. Then try a separate hum pot but the same transformer.
Yes, the cathode circuit is shared like a diff amp. You got me stumped on how I would do that on a DHP/DHT? I've read your post lots but can't figure what you mean. Sketch or link maybe?
Nick wrote: ↑Tue Nov 13, 2018 8:54 am
Is the 15mv the same sort of hum or is it another sort appearing below the filament hum (could happen to make life harder).
I didn't look as I was in the shed and the laptop scope thingy was inside and it was late. I'll look tonight.
Nick wrote: ↑Tue Nov 13, 2018 8:56 am
Have you got a shared cathode circuit. You could add a 50R unbipassed resistor below each cathode before they join. Then try a separate hum pot but the same transformer.
Yes, the cathode circuit is shared like a diff amp. You got me stumped on how I would do that on a DHP/DHT? I've read your post lots but can't figure what you mean. Sketch or link maybe?
Cheers,
Stephen
I know what he means.
Right now you've got one common cathode resistor.
Each 813 has a hum pot across its filament.
From the wipers of each hum pot, connect a 50R (47R?) resistor to the top of the common cathode resistor.
Both 813's filamants are heated by the same single winding as before.
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
Mike H wrote: ↑Tue Nov 13, 2018 3:13 pm
I know what he means.
Right now you've got one common cathode resistor.
Each 813 has a hum pot across its filament.
From the wipers of each hum pot, connect a 50R (47R?) resistor to the top of the common cathode resistor.
Both 813's filamants are heated by the same single winding as before.
OK even with the diagram I still don't see why it would work although I can try it. By connecting up a single fil tx, it puts the hum pots in parallel which looks like it defeats the purpose. I get it with two fil tx, one per 813 but not with 1 tx. I must be missing something
Nick wrote: ↑Tue Nov 13, 2018 8:54 am
Is the 15mv the same sort of hum or is it another sort appearing below the filament hum (could happen to make life harder).
I didn't look as I was in the shed and the laptop scope thingy was inside and it was late. I'll look tonight.
Cheers,
Stephen
It looks like the same type of hum just lower
I rewired with two fil tx and hum pot per 813. With 2 tx, I can get each one closer to the 813s with much shorter wiring. Now the hum pots, right by the 813s are super sensitive so will need padding resistors either side. It so touchy but I saw 3 to 5mV hum at one point so got quite excited as it was a mass of croc clips achieving this.
Will have to do some sums to see how I can pad them with the parts I have. I think this might be the way forward.
I can see why some people use multiturn pots for this and that also may be something to consider if cost isn't stupid. Any recommendations for hum pots on 10V gladly received.
Thanks. I was just looking at that very part at Farnell (more expensive than Rapid) when your post turned up - quite expensive but than it seems a quality part. I was hoping for something more affordable but maybe I don't have a choice.
Mike H wrote: ↑Tue Nov 13, 2018 11:14 pm
I have one of those as the tune knob in one of my FM tuners, (Voltage controlled tuning), they are good and seem to be well made so probably worth it.
I ordered two last night after a bit of a think. There really is no other show in town and it should make life easy to adjust. The ones I have are so cheap but then so is their performance. They are not a good fit for the role I'm using them in.
I couldn't wait for the parts to arrive so cobbled together a compromise with the 50R wirewound pot padded out by 47R either side with 33R from wiper to each side of fil. Got hum on the Fluke down to 5mV so indoors it came. I can make out some hum with my head in the speaker but only just. There's more 100Hz it sounds but yet to measure. This is very encouraging! It's the quietest 813 amp I've had so far but it is just one stage.
It goes surprisngly loud driven by the line stage and the familiy just want to hear some stuff. Sonny Rollins going house wide in one channel with very positive comments. I reckon the 46s will be enough for it to go loud enough so should go and sim it to see.
I should make the other to see if they will go as pair and not a theremin as previously.
The next thing to address is more hum/buzz that appears without the HT on. It's now the most disturbing thing.
I'm not sure where this comes from other than the obvious (the fils) but why? It's as if the fils couple into the plate and so the Opt. Quite how I fix this, I don't know. Any ideas?
All I can say is I must have been incredible lucky with mine.
Now how sensitive are the Tannoys ?
I know things like the 288's with horns ( 109 ) will pick up buzz from the speaker cables, without the amp on. Not with the TQ UB's though , one other reason I like them.
However you must have existing set-ups with same wires that don't buzz, I assume.
"Once you find out ... the Circumstances ; then you can go out"
izzy wizzy wrote: ↑Wed Nov 14, 2018 10:08 pm
The next thing to address is more hum/buzz that appears without the HT on. It's now the most disturbing thing.
I'm not sure where this comes from other than the obvious (the fils) but why? It's as if the fils couple into the plate and so the Opt. Quite how I fix this, I don't know. Any ideas?
Cheers,
Stephen
I have seen that a lot of times. Without the HT to provide current there is no anode resistance to stop the plate from picking up hum from the heater. At least that’s how I have thought of it in the past.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.