DC heaters in Magnatone 280

[tictac]

I have heater CT and HT CT internally connected inside PT

Was your PT made that way or did you make this connection yourself?

Probably not related but could this be a possible source of your noise

problem?

In any event, if you made this connection yourself you could separate the

grounds and elevate the heaters as shown.... This is also a good practice if you have any cathode followers in the preamp as it lowers the grid to cathode potential... (or is it plate to cathode?) well... it's a good idea....

TT

[dinkotom]

Well, I didn't know better at the time (6 years ago), I just told the guy to make the trannies same as on schematic.
BTW, I just found a very cold joint on 4.7M res. at the input!

[dinkotom]

ok, did few tests: disconnected .047 caps, got just a buzz, after about 5 seconds the hum starts, at the same level as w/o preamp tubes, if it would stay the same with the tubes in, I could live with it, not too terrible. Also tried aluminum shield, no change. Back to new ground wiring...

[Structo]

E string, third fret G is 98Hz ( close enough to call it 100Hz)

A string, second fret B is 123Hz.

See if it is close to that.

check power supply wiring (lead dress) and grounds.

[dinkotom]

Thanks Tom, will do, I'm rewiring the whole amp today.

[dinkotom]

Just to give this thread some kind of closure: rewiring the grounds didn't help, so I kept searching the forums and stumbled upon a suggestion to add (in a BF amp) a bias filter cap from the bias pot wiper to ground. I added another 20uF to ground after the 12K resistor in the bias supply, hum gone, amp is quiet now. Don't know, might help another noobie like me, who didn't bother to measure the bias ripple...

[Stevem]

So many times it's such a simple cheap fix, but in the end what matters is that we have learned something to pass on and help others with!

[Leo_Gnardo]

Just to give this thread some kind of closure: rewiring the grounds didn't help, so I kept searching the forums and stumbled upon a suggestion to add (in a BF amp) a bias filter cap from the bias pot wiper to ground. I added another 20uF to ground after the 12K resistor in the bias supply, hum gone, amp is quiet now. Don't know, might help another noobie like me, who didn't bother to measure the bias ripple...

Good solution and bias supply an overlooked source of hum. Sometimes I've done similar to squeeze out the last pinch of hum/buzz for those who need low noise amps for studio recording. I put mine from the low end of the bias adjust pot to ground, 4.7 to 22 uF, hum-be-gone.

[Structo]

Interesting amp.

Did you use 6CZ5 power tubes?

As for the bias filtering, a lot amps use a 100uF cap there.

What did you use for the varistors?

What is house voltage and frequency there? 220v/ 50Hz?

Thanks

[Structo]

Wait, did you build a 280 or a 260?

[dinkotom]

Both
I used 6V6 power tubes in 280, was more familiar with them and also wished that I have an amp with four of them
For the life of me, I can't find which varistors model I used in 280. I think blue Zenith 1225, not sure, give me a couple of days, I'll open the amp and tell you, but I know they are subsitute for the original ones (brown). I thought at the time that the vibrato should be stronger so I bought and installed the 8 real ones, but it sounded the same, so I returned the blue ones back.
In 260 amp I used original varistors.
Yes, voltage is 220v/ 50Hz.

Edit: Haha, Tom, look here how that 280 amp started :
https://tubeamparchive.com/viewtopic.ph ... natone+280

[dinkotom]

found it- varistors in 280 amp are Workman FS 1226