12 VAC heater circuit question

[Lee_ranaldo]

I got this secondary winding on a tube amp that feeds a rectifier to provide DC heater voltage to the pre-amp tubes

The dilema is.

- The transformer has a center tap but is not grounded
- How do i power 4 6L6GC's safely without having issues.? Can i wire 2 6l6 heaters in series?

This is a preliminary drawing of the circuit.

Does anyone has experience with this ?

Thanks in advance!

[R.G.]

The questions all involve current. 6L6 heaters take a lot more current than 12AX7 heaters. Do you know the current rating of that 12V winding? And what is the voltage/current of that third winding that you show?

You are going to have to juggle the opposing needs for a lot of heater current for the 6L6s at 6V each, a little current for the 12AX7s at either 6V or 12V, and the current rating of the 12V winding. Cutting the connection between these opposing needs with a second heater transformer for either the 6L6s or the 12AX7s may help.

The biggest question is this: how much current can the 12V center tapped heater winding provide? If it's a small number, then use it for the 12AX7 heaters and add another small transformer to power just the 6L6 heaters. If it's a large number, ground the center tap and connect the 6L6 heaters in series across it, and add another tiny transformer to power the 12AX7s.

You can power the 6L6 heaters in series, as long as they are ground referenced. However, powering both 12AX7 and 6L6 heaters in series from the single winding will involve tricky circuits to prevent hum. One possibility is to use the 12V winding CT grounded, then a diode from each end of the winding to make about 9Vdc, and then regulate to 6Vdc to power the 6L6s AND the 12AX7s from 6Vdc. This is wasteful, as you're having to make lots of high-quality DC to power the 6L6s as well as the 12AX7s.

These are only a few of the many, many possible solutions.

[Lee_ranaldo]

The questions all involve current. 6L6 heaters take a lot more current than 12AX7 heaters. Do you know the current rating of that 12V winding? And what is the voltage/current of that third winding that you show?

You are going to have to juggle the opposing needs for a lot of heater current for the 6L6s at 6V each, a little current for the 12AX7s at either 6V or 12V, and the current rating of the 12V winding. Cutting the connection between these opposing needs with a second heater transformer for either the 6L6s or the 12AX7s may help.

The biggest question is this: how much current can the 12V center tapped heater winding provide? If it's a small number, then use it for the 12AX7 heaters and add another small transformer to power just the 6L6 heaters. If it's a large number, ground the center tap and connect the 6L6 heaters in series across it, and add another tiny transformer to power the 12AX7s.

You can power the 6L6 heaters in series, as long as they are ground referenced. However, powering both 12AX7 and 6L6 heaters in series from the single winding will involve tricky circuits to prevent hum. One possibility is to use the 12V winding CT grounded, then a diode from each end of the winding to make about 9Vdc, and then regulate to 6Vdc to power the 6L6s AND the 12AX7s from 6Vdc. This is wasteful, as you're having to make lots of high-quality DC to power the 6L6s as well as the 12AX7s.

These are only a few of the many, many possible solutions.

I am guessing the heater winding can provide a lot of current since its a BIG transformer and also the wire used in the winding can provide around 10A easily. Of course i am just making assumptions. i'll hook up the DVM and get some real numbers

The other winding shown in that diagram is the winding used for the bias circuit. its around 45 volts AC or more if i remember correctly.

This amp had the 6L6 heaters referenced to ground via 2 330ohm resistors. from pin 2-to ground and pin 7 to ground installed on power tubes 1 and 4 respectively.

I started replacing the bad wiring and getting everything tidy and properly done when i realized about the unusual heater arrangement. The amp worked "fine" for years

[Mick Bailey]

I've found that powering output tube heaters in series can result in uneven heating during warm-up - one will glow way too brightly while the other 'catches up'. Whilst tube characteristics are matched, heater resistance characteristics aren't. This may not be a problem in every case, but I found this out the hard way.

[Steve Conner]

First of all, presumably the amp was working before? Its power tube heaters must have been getting power from somewhere. So it should work again. If it ain't broke don't mess with it.

With the heater supply wired like you showed in your schematic, it should be OK to just connect the 6L6 heaters in two series pairs across the 12V winding, so they receive 12V AC while the preamp tube heaters get 12V DC (probably more like 14 in practice) from the rectifier. The two 330 ohm balancing resistors shown in the schematic help to balance out the AC side too. You can put capacitors in parallel with the resistors to reduce noise on the preamp heaters further. The power tube heaters don't care about noise.

This system was used in some high gain Marshalls except the heater winding was 6.3V. The rectifier got extremely hot. Going to 12V would be a big help by halving the current.

The downside is that at 6.3V the diode drops of a bridge rectifier cancel out the factor of 1.4 that comes from rectifying a sine wave, so you get 6.3V DC from 6.3V AC. At 12V this is no longer the case so the DC voltage will be a bit too high.

Mick's concern is somewhat valid, tube heaters are non-linear so they're not guaranteed to share voltage evenly at warm-up, especially if the tubes are unmatched or different brands. I bet you would get away with it though

[Lee_ranaldo]

Thank you all for your useful replies.

That was exactly my main concern. This amp was made by a builder who passed away years ago (sadly). So its mostly DIY. They changed the Output transformer with a Classictone one and the amp began to sound like a machine gun, (yea, like an ak47 being fired). Since its mostly DIY the huge mess of random wiress going everywhere was a pain in the eyes. I decided to re-build everything in order to avoid failures in the future.

I've seen the DC heater circuit on many marshalls (DSL/TSL series) and they are known for going bad in a few years . Thats exactly why i decided to rebuild the whole wiring in this amp also.

I was thinking if i could get away with connecting the CT to ground but i've found that the heater voltage feeds a 12V relay. I'll try the heater circuit just like it was wired before and then evaluate the possibility of reducing the relay voltage by replacing it (5V) and drop the voltage received by the relay's coil. Then connect the center tap to ground to have 6,3 - 0 - 6,3 volts from the heater secondary. This will allow me to connect 2 pairs of 6L6's to each winding referenced to ground (or virtual ground) and still provide enough DC voltage for the rectifier and pre-amp tubes

I'll test this out eventually and let you guys know..

Thanks for your useful info btw!

[Mick Bailey]

The DC circuits in Marshalls do fail, but that shouldn't condemn DC supplies to heaters so long as the components are sized accordingly. There are a number of reasons some circuits fail - I've seen quite a few Marshalls where the CF tube has arced through to the heater and put HT onto the DC heater supply side, causing the heater circuit to fail. That's a tube problem specific to CF circuits. Also, the caps are marginally rated for the current draw and can short.