adding DC heaters to preamp with a center-tapped trans

[Guitarist]

In this situation the transformer is center-tapped but there is enough hum yet that I want to go DC heaters on at least the first tube of the preamp. I usually use the Bøøgie appraoch with balancing +'ve and ground with the 33 to 100 Ω resistors after the rectifier.

The transformer is a Partridge in my Hiwatt rebuild/redesign. So I just want to make sure things are wired properly. The tap is grounded now. Do I leave it grounded?

[Enzo]

Have you determined that the hum is actualy coming from the heater? That is the ONLY hum source DC heaters will prevent.

Have you considered just connecting the CT to a positive voltage instead of ground?

[Guitarist]

Yes, I am pretty sure it is the heaters. The amp is star grounded. I did elevate with some + volts via the center tap but there is no difference to having it grounded. So can one do both? I have a few cathode followers and would like the hiss to be decreased and the safety of injecting DC. Still I must put some of those preamp tubes on DC. Just wondering about the topography and if both is safe for the power transformer.

thanks

[Enzo]

Pick the tubes you want on DC, disconnect them from the 6VAC, and connect a battery to them instead. Does that kill the hum? If elevating the heaters to +DC doesn't stop the hum, I am suspicious of it coming from the heaters. Star grounds are fine, but they don;t stop hum from being picked up by unshielded wiring, for example.

Is the hum 60Hz or 120Hz? 120Hz is power supply ripple, 60Hz is grounding/radiated/heater hum.

Look at the heater circuit in a Marshall DSL100 or TSL100 for example. The power tubes and PI are on 6VAC with a grounded CT. Then the 6VAC is rectified and filtered for use in the preamp tube heaters. There are filters across that DC, and from either side of it to ground. So you can have both grounded CT and DC heaters.

[Steve Conner]

This is a minefield of gotchas. If you're not careful, you can end up with DC that has common-mode AC riding on it, or put spikes onto the AC side of the heaters, or inject capacitor charging current into your ground system, all of which will trade heater hum for another kind of hum.

Probably the best thing to do is lift the transformer's heater CT off ground with a 1k resistor, and put a "virtual centre tap" on the DC side with the two 33 ohm resistors.

Alternatively you can follow the DSL100 style approach, using two DC filter caps in series with the heater winding CT returned to the centre point of them and also grounded. However, heavy charging current pulses will flow in the wire joining CT and caps, and these have to be kept out of your ground system. If you don't understand what this means, use the other approach.

[Rhodesplyr]

As someone who builds Hi-Fi tube amps and preamps, I agree with Enzo here since hum can have many sources. You have to make a positive identification of the source.

One of the problems of having high-gain preamp stages and power amp stages on the same chassis (and I assume that this is the case) is that you can have electromagnetic coupling between the power transformer and the small-signal tubes. Some amps have Faraday shields around the power transformer and/or mount the power transformer at a specific angle to try to minimize this, but the bottom line is that it's very hard to shield radiated EM. Put another way, it's a lot easier to design around it rather than to fix it.

For example, the slow motors in Leslie speakers radiate a lot of EM, and if they are placed to the left of a Hammond Organ (from the player's POV), the organ will hum like crazy due to interaction between the motors and the organ's matching transformer.

[Enzo]

Rhodes makes a point, and reminds me, we didn;t ask if you have shields on the tubes. They make tube shields for a reason.

[tubeswell]

yep I had some effing annoying hum like that on a stand alone reverb I built last year and I tried elevating and de-elevating heaters and different grounding and chasing my tail around in circles for weeks, and it turned out to be transformers sharing their EMR too much. I initially opted for trannies without steel end covers on that occasion thinking 'if the PT has got a copper shield and the trannies are oriented so they don't interfere with each other, it'll be okay' - more fool me. (Problem solved now)

[Guitarist]

I went with the Marshall DSL approach to try it. I need to get some bigger caps but it seems to work well. Still getting a bit of the "sssshh" sound with the buffer-out that splits off V1. I ran a huge wire to the main power supply ground from those "center-tap" caps.

I will have to upgrade by star ground by soldering each wire to a lug and bolt all those down versus the solder mountain there now;yikes.

thank you gents

[Guitarist]

STILL trying to remedy this amp.

Took the OT and rotated it 90 degrees to the PT—you think Hiwatt would've done this already. But this did nothing noticeable.

Inserted 1K resistor on centertap blue wire; as per Kevin's advise.
Added 33 Ohm balancing resistors after rectification. Perhaps add 100 ohmers prior for output tubes? No 3300 uF balancing caps now.

Pivoted the Fender style choke (from New Sensor) it looks to be 90 degrees from PT and on the power side of amp. I may pull it and add a 100 Ohm 5 Watt resistor. Circuit-wise it after first capacitor bank and feeding output screens and PI/preamp. The amp seems to have a bit of hash so maybe this is the choke? I have a BIG choke if that is what it takes.

Preamp loop out is nice and quiet so it seems to be PI or output tubes or transformers. Even with no output tubes there is tiny hum.

Shielded feedback wire runs isolated speaker ground to PI ground which then goes to star ground. Preamps have separate ground close to input jack.

One cannot float DC through the center tap with the TSL-capacitor approach right? Then you'd have 6 volts and 40 volts riding through on top, ahh?... There are a number of cathode-followers that might benefit.
(side question: what size of grid resistor is good after a CF? 470, 220 or 100K?)

[Guitarist]

Update: Remove 1 Ohm bias-set resistors from output tubes and replaced with uninterrupted THICK wire. Though there are ground tabs at sockets, wire goes to star ground at power transformer bolt. Also, this wire goes to the standby switch which got a shot of Deoxit.

Also removed Fender Twin style 17C010 choke and replaced with a 5 watt 270 ohm resistor. I'm not a fan of chokes presently.

Hum noticeably reduced now and it normal-faint. There is still the "shhhsh" but the Hiwatt is useable now. This noise might be from the power supply?
250K pots work best after a cathode follower.

Time to introduce KT88s to the 4x12s!