6SN7/6SL7 and DC Heaters

[bal704]

I'm getting ready to start a build with 6SN7/6SL7 pre-amp tubes. From reading here and other places, it seems one theory is these older octal tubes need DC heaters to minimize hum, and another theory that a well-designed AC heater system will work.

I'm still in the planning stages for this build, so I'm trying to determine if I need to acommodate DC heaters.

Any words of wisdom on this, or better yet, any hard-earned experience?

[Structo]

Good luck!

I will be following this as I would someday like to build a all octal amp.

I'd like to build an early Bassman some day or other Tweed.

I don't think they should get DC heaters automatically.
Fortunately, you can build the amp with AC heaters and if you can't conquer a hum problem,
then by just adding a couple components you can go with DC heaters for the preamp, at least.

Not sure of the amp you want to build but the early Fender single ended amps can sound great but tend to be
noisier than their push pull cousins.

As with all amps, heater wiring technique and layout can be critical.

The old Fender combos had pretty small chassis so lead separation was dictated by the space available.

It's cool to trace the evolution of Fender tube amps as technology developed ( tube manuals that showed designs)
as well as did Leo Fenders knowledge.

Best thing to do is look at schematics, try to find sound clips of the amp then consider how it would fill your needs.

[bal704]

Thanks for the encouraging words.

I'm planning on this being a 2 6V6 version of a Blond Twin PA (with that 2.5 tube tremolo). The front end is going to have a pair of 6SN7's ( or 6SL7's, haven't decided). 2 channels with clean and crunch.

I"ve got a PT that has a 5V/3A tap I won't be using, which I could double and use a 7806 (or something beefier) to get 6V DC to use with the 6SL7/6SN7 tubes. If I go that route, I'll be stuck using DC heaters on the octals. Also, I never use tube rectifiers, so I won't have any future use for the 5V tap.

I've got a PT with two 6.3V heater taps, which would allow me to try AC heaters, then switch to DC if needed.

I have a small 12V/2A DC power supply that I could use a voltage divider with to get 6V DC, which would mean I could use either PT.

One PT is a stand-up type, the other is a toroidal, and neither fit perfectly on my chassis. I'm going to have to fit whichever one I choose, so taking it off and replacing it with the other will be a major PITA that I would like to avoid.

Hence my request for input.

[jjman]

A friend had Sovtek 6SL7's in his ampeg and they were the root cause of big hum. NOS resolved it.

[Phil_S]

If you haunt eBait for a while, you can buy good USA made 6SN7 and 6SL7 for very reasonable prices. Look for auctions of multiple tubes so the shipping gets diluted. I've used them a few times and never did anything unusual about the heater wiring.

[RWood]

I've built 3 PTP 5B6 Bassmans with Russian octal preamp tubes 6SL7/6SN7. No problem with noise whatsoever. I use standard twisted heaters, ground right to the tube socket mounting screws, and keep grids short and away from others as possible.
I got 3 each SUPPOSEDLY NOS tubes which were all microphonic or noisy to some degree, so it was a big waste of money.
All new Russian replacements were good with no problems reported after more than a year.

[bal704]

I've got some russian 6SL7 tubes (6N9S) on order. I've also got some NOS 6SN7 tubes on hand.

[Stevem]

These big structure octal tubes are more likely to pick up 60 hz hum from the PT than from the heater string, so in that regard the layout may need to though out better!

These tubes where made into the smaller 9 pin package back in the day for improved microphonics and to save space, not due to a AC heater hum issue, that being said where ever I do a build with Octals I drill out the tube socket mounting holes big enough to take rubber grommets to bolt the sockets to.
With this set up I have never had a microphonic issue, and in my own amp I have been able to use a great sounding but microphonic Nos 6SN7 that I love!

[Andy Le Blanc]

yes, try a DC heater string, they are polarity sensitive too, depending upon how the tube is utilized.

12sl7 have the same heater character as 12ax7 wired series.
12/6 sn7 have a different requirement.

I sometimes use a series string after a simple cascade voltage doubler, a couple diodes and a couple large value caps, to get 12v.

the trick is to make the R of the heaters match by paralleling them with a resistor then the heaters will have equal current over each heater.

if you reference the dc supply to ground, the ground end of the heater string will be the quietest.

[dorrisant]

6SL7 has 0.3A filament current draw... 6SN7 pulls 0.6A... two different animals. Just saying be sure your PT will handle it. I have used the DC filament rectifier from a Mesa DC-3, (look for the switching matrix page and reverse the top right diode). If you convert to DC for the filaments this way consider that the currents drawn are about double.

Also, I have built an all octal 5E3 that is as quiet as anything I've built and it is straight AC on the filaments. Of course it is a supplied by a dedicated filament TX rated at 4.5A. There is something to be said about over spec ratings for TXs. The less the TX strains, the less EMF gets thrown around.

[renshen1957]

Good luck!

I will be following this as I would someday like to build a all octal amp.

I'd like to build an early Bassman some day or other Tweed.

I don't think they should get DC heaters automatically.
Fortunately, you can build the amp with AC heaters and if you can't conquer a hum problem,
then by just adding a couple components you can go with DC heaters for the preamp, at least.

Not sure of the amp you want to build but the early Fender single ended amps can sound great but tend to be
noisier than their push pull cousins.

As with all amps, heater wiring technique and layout can be critical.

The old Fender combos had pretty small chassis so lead separation was dictated by the space available.

It's cool to trace the evolution of Fender tube amps as technology developed ( tube manuals that showed designs)
as well as did Leo Fenders knowledge.

Best thing to do is look at schematics, try to find sound clips of the amp then consider how it would fill your needs.

Hi Structo,

DC Stand-off for AC heaters works as well as DC Filaments (and I only use DC stand off on DC heaters for Hi-Fi) without the current increase penalty.

Best regards,

Steve

[gingertube]

http://www.diyaudio.com/forums/instrume ... ing-2.html

On my own amp I used PP 6V6 with 6SL7 in preamp and concertina phase splitter. Preamp is a London Power Standard but with 6SL7 in lieu of 12AX7. Used AC heating with pseudo centre tap, DC elevated.

Hand drawn schematics link above for reference. See Post#14

Cheers,
Ian

[renshen1957]

http://www.diyaudio.com/forums/instrume ... ing-2.html

On my own amp I used PP 6V6 with 6SL7 in preamp and concertina phase splitter. Preamp is a London Power Standard but with 6SL7 in lieu of 12AX7. Used AC heating with pseudo centre tap, DC elevated.

Hand drawn schematics link above for reference. See Post#14

Cheers,
Ian

Hi Ian,

Thanks for the link, looks great as always.

Best regards,

Steve,

PS How are things in Oz?

[Stevem]

One thing you got to love about those big NOS octals is how long they last due to there big cathode and plate structure!
If these two items get any smaller in some AX7s made now for the sake of microphonics where going to be lucky to get 250 hours out of one!

[bal704]

Thanks for the links!

My plan as of right now is to have a dedicated transformer for the 2 6SL7/6Sn7 tubes. I'll start out with AC heaters, maybe play with referencing it to a higher (or lower) DC voltage if there is hum. The backup plan is to use the transformer and convert to DC if needed.