SE power trans hookup

[funkgang49]

I'm a little confused about the heater wire hookup on my SE build that I hope someone can help clear up.

According to the tech sheet diagram from Mercury Magnetics (see attachment) the heater has 2 Green wires which I have attached from the Transformer to the jewel light receptacle and then a Yellow/Black pair of twisted wires from the receptacle on to the tube sockets Heater connection points starting with the power tube then PI and onto the preamp tubes. The diagram illustrates a Green/White wire which I would have thought should be attached to the Main Ground point however below there is a warning: 'DO NOT GROUND GREEN AND GREEN/YELLOW WIRE!'.
There is NO Green/Yellow wire so I'm assuming they mean the Green/White wire filament tap.

Also there is NO Red/Yellow wire. There is a Red/White wire which is what I assume is the 0v Secondary tap.

I was under the impression that both the Green/White and Red/White should be connected at the Main Ground point. Would this be correct?

[Phil_S]

I think you should call MM and they will clear up the wiring colors for you if you aren't sure.

According to the diagram, the 6.3V feed is a pair of green wires. The green/white is a center tap that should be grounded unless you choose to use a faux CT with a pair of 100Ω resistors. It's either/or, not both. I'd ground the green/white CT.

It is possible MM uses green/yellow for other products. Maybe one of the solid green should have been green/yellow? We can only guess. I'd ignore that as it doesn't seem to apply.

If the HT CT is red/white instead of red/yellow, that's not a big deal. You can easily determine if it is the CT with your DMM set to ohms. The two halves of the winding won't be perfectly identical, but they should be close. IOW, the reading for either red to the red/whilte should be about the same. The same goes for either green to the green/white.

If you can't get comfortable with this. make the phone call.

[Firestorm]

The warning is not to ground one end of the 6.3V winding AND the CT. It's common practice in SE amps to connect one end of the heater winding to the chassis. This is not just laziness {although Fender did it in P-P amps in the Tweed era). Since SE amps don't phase cancel hum in the output, having one end of the heater on the chassis can make the amp quieter. We would not do this in modern designs. SE heater hum can be controlled in better ways.

[funkgang49]

Previously I had both the green/white and red/white wires connected to the main chassis ground. I followed the hookup example of this amps original incarnation which was built by a much more knowledgeable & experienced amp builder than myself. I have since considerably altered and rebuilt this amp on 2 different chassis. This is version 2 of my circuit. What brought up this question is that the amp exhibits some hum. It is low at moderate volumes but increases as the volume is increased and is noticeable past 12 0'clock. I was hoping to minimize the hum as much as possible.

Phil,

According to the diagram, the 6.3V feed is a pair of green wires. The green/white is a center tap that should be grounded unless you choose to use a faux CT with a pair of 100Ω resistors. It's either/or, not both. I'd ground the green/white CT.

If I were to dissconnect the Green/White filament heater center tap and use a faux CT - I should connect a pair of 100ohm 2W power resistors from each heater wire attached at the power tube heater connections then connected to the main chassis ground OR can I attach at power tube pin#8 Cathode resistor(cathode biased) which is also, in this case, connected to pin#1?

I will, on occasion, be using EL34 or KT-77 power tubes in this amp.

Would this be considered DC elevated heaters?

[funkgang49]

firestorm:

It's common practice in SE amps to connect one end of the heater winding to the chassis. This is not just laziness {although Fender did it in P-P amps in the Tweed era). Since SE amps don't phase cancel hum in the output, having one end of the heater on the chassis can make the amp quieter. We would not do this in modern designs. SE heater hum can be controlled in better ways.

What better ways?

[Phil_S]

First determine if your hum is 60Hz or 120Hz. If hum is 60Hz, it might be worthwhile to pursue your intentions on the heater CT/faux CT. Figuring this out is relatively easy. The 5th string A at pitch is 110Hz. 120Hz is a bit above that, but not as high as Bb. To get 55Hz, tune your low E down one octave from the A string. Same deal there, make it a bit sharp but not quite Bb. Compare to your amp.

Or find a tone generator you can use with your computer.

I am hard pressed to think a hard wired CT on the filament circuit is a problem. You might consider elevating the real CT rather than fiddle with a faux CT. Use pin 8 of the power tubes as that is what the data sheets show as the cathode. I don't know about pin 1, as data sheets indicate N/C for no connection. It is possible some tubes are internally connected. Heater elevation needs consistent and reliable. That's pin 8. You should see if you can find the H-K spec for your tubes. You don't want to exceed that on account of elevating the heaters. I had no luck finding that.

On Firestorm's comment, by "better ways" I believe he means using twisted pair, observing the "rules" for placement of the heater wiring, and employing a CT, faux CT, or humdinger. This can be combined with heater elevation. Nothing new here. I hope Firestorm with forgive me for horning in on his comment and/or if I got it wrong.

[funkgang49]

Hey Phil,

Thanks for the reply!

I found an online tone generator and it is definitely 60hz hum. I disconnected the Heater CT from chassis ground point, connected 2-100ohm 2W power resistors to Power tube heater points and connected opposite resistor ends combined to pin# 8 (cathode point) of Power tube - faux center tap?

Not much difference - still hum!

My heater string begins at the jewel light connection - Green/Green 6.3v wires from Power transformer connected to jewel light. From jewel light connection points I have a Black/Yellow twisted pair of wires connect first to Power tube heater connections then on to PI (12ax7), V2 (6SL7), and ending at V1 (6SL7). The twisted heater wires are laid down flat to chassis (aluminum) opposite from all other tube socket connections.

Could switching the heater wire leads at one or more of the tube sockets following the power tube have an impact?

How do I elevate the real CT? I assume this is different from the faux CT that I have created.

[Phil_S]

The good news is that it's reasonable to think the heaters are the source.

I am thinking that heater elevation will not fix this. I would stop looking at that and leave it alone. Assuming it is in the wiring, it may help to confirm the 6SL7's are wired in the same phase. That is, whatever wire goes to pin 7 is the same on both tubes. If it's not change it.

I suppose it is possible the tube itself is the culprit. I'm not sure how you would address that except by tube rolling.

Can you post a picture of the heater wiring?

[funkgang49]

Based on Tungsol 6SL7GT reissue datasheet(which are currently installed) pins 7&8 are the heater wire connections. Both V1 & V2 are hooked up identical: Black wire to pin#7 and Yellow wire to pin#8.

V3 (PI-12ax7) Yellow wire connects to pins 4&5(tied together) heater grounds and Black wire is connected to pin#9. This could be the problem.

Power tube heater connections are Yellow wire pin#7 and Black wire pin#2.

I have 2 Tung-Sol 6SL7GT's in V1&V2 right now but have tried a NOS RCA 6SL7GT in either position with no discernible change in hum.

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[Phil_S]

This is more or less a shot in the dark. In the second picture, there is a black wire (lead to a cap?) that seems to be touching the twisted pair for the heaters. See if you can separate them? There isn't much to lose doing this.

I believe I have read here at TAG that the new production 6SL7's are prone to humming. I wonder if it is the tubes themselves? I have no experience with the new production 6SL7's. Old USA made stock can be bought on eBay for very reasonable prices and that's what I've used. I know others will argue that isn't the way or place to buy tubes and mostly I agree. The exception I'm willing to make is when I can get them cheap enough that I don't care if they turn out to be duds. I'm willing to pay $10/tube including shipping and have found them quantities >1 for less than that. Due to the shipping cost, I've concluded it generally doesn't pay to buy a single tube, especially if you have no idea if it's a good tube.

[funkgang49]

In the second picture, there is a black wire (lead to a cap?) that seems to be touching the twisted pair for the heaters.

That's the lead from the cap itself, covered with black shrink tubing, to a 3.9K grid resistor. It just appears in the picture to be touching the Screen grid resistor but it is not.

Last night I placed an order on Fleabay for a pair of NOS RCA 6SL7GT tubes that tested high with no leaks, shorts or microphonics. Seems like a reputable dealer with high positive feedback. When I get them I'll try 'em out and see what happens.

In the meantime...
I'll try swapping heater leads on the PI (12AX7) and see if that makes a difference.

Any other suggestions?

[funkgang49]

Firestorm:

It's common practice in SE amps to connect one end of the heater winding to the chassis. This is not just laziness {although Fender did it in P-P amps in the Tweed era). Since SE amps don't phase cancel hum in the output, having one end of the heater on the chassis can make the amp quieter. We would not do this in modern designs. SE heater hum can be controlled in better ways.

Other than a faux CT, what other ways are there to reduce hum?

[martin manning]

You can try connecting the heater CT to the cathode of the power stage to elevate the heater voltage.

[funkgang49]

Thanks Martin!

I'm gonna try that right now.