DC heaters in Magnatone 280

[dinkotom]

I made this amp:
http://www.magnatoneamps.com/schematics ... e_280B.png

I want to make a DC heater like this one on Merlin's site:

[img:421:162]http://www.valvewizard.co.uk/heater11.jpg[/img]

I have the same arrangement of the PT as on 280B schem, meaning the central taps for heaters and HT are internally connected inside the PT, so I have only one central tap for both windings.
So, my question is, is it ok if I do the DC heaters like on Merlin's schem if I have just one center tap for both windings?
Thanks.

[dinkotom]

ahh, forget it, can't do it with my PT, I've just read a bit further down on Merlin's page:
"Another important thing to be aware of is that rectifying AC to DC in this way introduces a power-factor loss of about 0.5, which loads down the transformer more. In other words, to supply a heater with 300mA of DC the transformer actually has to deliver about 600mA of AC, So be careful not to overload the transformer. "

[trobbins]

Hi Dinko,
Were you wanting to do the DC heater mod for fun, or do you have a noticeable hum that you are hoping will be reduced?
Ciao, Tim

[dinkotom]

Both, I guess. There is a loud HUMMM with all the tubes inside. Removing the tubes, one by one starting from V1, it hums less and less, but is still present w/o all preamp tubes. Haven't checked w/o power tubes, will do today. Maybe I should try the DC heater just on first two input tubes...

[trobbins]

If you have the same circuit as the maggie 280, then easiest first hum test is with the Vol pots at minimum.

There are lots of opportunities for hum to get in to that circuitry - so I'd be careful about spending effort without first doing some diagnosis.

If you remove V5 and V6 then there is only the output stage in circuit, but the ripple on 'B' supply can couple in, so best to try and temporarily unsolder one end of the 47nF coupling caps to make sure. If there is noticeable hum then it will likely be either the bias, the screen, or the B+ ripple, or all. Certainly worth checking the star grounding if that is the case, as a well laid out and wired amp should have pretty low hum for that configuration.

[dinkotom]

Thanks Tim, will try. You mean lift one end on all four 0.47 caps going from PIs to power tubes? The grounds in this amp are everywhere and anywhere (like in an original Maggie), should have done a proper grounding scheme in the first place.

[Stevem]

Tape a sheet of aluminum foil to a scrap sheet of cardboard that's big enough to cover the bottom of the amp. The sheet of foil need to be big enough to cover the area of the cardboard used.
Ground the foil with a clip lead or something and set the whole thing foil side down on the bottom of the amp, does doing this tame the hum a large amount?

[rp]

Both, I guess. :)Maybe I should try the DC heater just on first two input tubes...

I don't think anyone DCs the power tubes, in a PP they are hum cancelling. You might even skip the PI and just do the pre tubes, maybe then you might have the current you need.

Post back if you do it, I'm curious to how it works out and if your DC voltage really comes out close to 6.5V

[trobbins]

You mean lift one end on all four 0.47 caps going from PIs to power tubes?

That would be the best way to isolate the output stage from the earlier stages, and all valves could then be left in place. It's quite easy to find that bias or screen ripple is the main source of hum, after spending an hour or two working through the preamp stages trying to track the entry path!

The grounds in this amp are everywhere and anywhere (like in an original Maggie)...

Hah, an accurate clone down to the last detail

Good luck with the grounding - I can recommend initially focussing on the first main filter capacitor and the power transformer CT connection - many good tutorials and schematics explaining how best to wire up that section. That has often removed the most hum problems in many diy amps.

[dinkotom]

Guys, thanks for your suggestions, I'll try them and report. In the meantime, I disconnected the outer heater wires from PT (CT still grounded) and connected them to another PT (which has a dedicated CT heater wire), just to see if that internaly connected CT was a problem, but it still hums the same. Grounding: in few other simpler amps I connected all preamp grounds to a rail grounded at input jack, while PI, output and power grounds are near or on the PT bolts. So it's not a true star-ground (there are 3 connections to chassis) but hum in these amps is at an acceptable level. Can't decide between this scheme or true star (one point only). Any thoughts appreciated. Thanks.

[Structo]

Can you tell if it is 60Hz or 120Hz hum?

Generally if the hum is 60Hz then you should concentrate on the heater circuit.

If it is 120Hz, then focus more on the power supply and filtering.

[bal704]

Stumbled across this site while trying to solve a hum problem:

http://plasticity.szynalski.com/tone-generator.htm

Might try it to see if you can tell what frequency you're dealing with.

[tictac]

Here's another solution that's easy to implement and has been used round here now and then. Doesn't require an extra transformer either.....

TT

[dinkotom]

It seems that hum frequency is the same as the G note (3rd fret on low E string), at least to my ears. I'll send a recording. bal704: thanks for the link, cannot acces right now, I'm using a mobile phone at the moment, I'll check it later.
tictac: I have heater CT and HT CT internally connected inside PT, so I'm not sure I may do as you suggest.

[shoggoth]

That's 100 Hz, so it's power supply hum - probably not from the heaters.