Dealing with High B+

[Leo_Gnardo]

Heater voltages up around 6.8V or more do increase noise and decrease lifespan of the tubes.

Good point and all the more reason to choose the bucking xfmr. As others have mentioned, looks like your power company may have done you the "favor" of boosting your AC from say 117 to 123 VAC somewhere along the line, not unusual. Did you move to a new location since you last measured B+? AC power varies practically from one block to another. And of course sags somethin' awful in midsummer when everyone's got the air conditioner going full blast plus hot wires are less conductive. Something to look forward to. Just like every year. Keep cool!

[Mick Bailey]

Something I've read, but never done, is putting zeners in the primary to drop a minimal voltage. See;
Antique Radio Forums ? View topic - Low cost alternative to a bucking transformer

[Leo_Gnardo]

Something I've read, but never done, is putting zeners in the primary to drop a minimal voltage.

Yikes! Incoming power waveform is already hacked-up enough in many locations without sticking zeners in the AC line. I'd opt for "inrush limiter" NTC resistors instead. Many commercially made amps already have one (Fender, Peavey, old tube McIntosh) and I've seen a recent Peavey model that strings about 8 of 'em in series for a deliberate line voltage knockdown effect. I guess this last is to simulate the line voltage that was available from the TVA in the "good old days."

[chinrest]

Did you move to a new location since you last measured B+?

I've moved several times, and picked up a few volts along the way. I'm reading 121vac out of the wall now.

[chinrest]

Suggest that you take comparitive readings of line, heater and B+.
An issue with very high VB+ like this is that EL34 may be pushed over their limiting value of plate dissipation.
Pete

Stand back, I'm holding a calculator. I've taken some comparative readings and calculated my desired bucking. My current transformer is a 115v primary, 375-0-375 secondary.

Current readings
121vac primary
416-0-416 secondary
381vdc rectified (SS)
533vdc B+ loaded
Heater 7.0v

-10v bucking transformer
111vac primary
381-0-381 secondary
349vdc rectified
489vdc B+ loaded
Heater 6.4v


It seems to work on paper, right? It also seems that this tranny's 115v primary spec might be a little wonky. If I bucked 6v ( to bring it down to the 115v primary), it looks like I'd get 391-0-391 secondaries, not 375. Hmmm.

This is a great thread. Well done everyone.

[g1]

The HV spec. (375V) will be at a specified amount of current for that winding, which may not be what the amp uses. Bias will affect it.
What are those readings you have called vdc rectified? A vdc reading after the diodes is B+ isn't it?

[chinrest]

The HV spec. (375V) will be at a specified amount of current for that winding, which may not be what the amp uses. Bias will affect it.
What are those readings you have called vdc rectified? A vdc reading after the diodes is B+ isn't it?

That makes sense. This tranny is beefy (200ma?). You are correct about the "rectified" voltage reading--in standby, directly after the diodes.

I whipped up a ghetto bucking transformer rig, with a handy 12v transformer, fuse, strain relief, outlet, junction box. It certainly does the trick. I'm down to 470vdc B+ and the heaters are a little soft at 6.2vac. I may build a nice one, with a lighted switch and 6v switch in case I ever want to run it closer to meltdown. I figure a 6v buck will give me right around 500vdc on the plates. Those F&T cap cans will have to sweat a little.

[J M Fahey]

More than the HV voltage, I'm worried about the filament one and find it a better indicator.
7 VAC? .... OUCH!!!
6.4V ? better.
Bucking is the way to go.
Get ready for the 127V you'll have in a couple years and plan accordingly.

[Leo_Gnardo]

More than the HV voltage, I'm worried about the filament one and find it a better indicator.
7 VAC? .... OUCH!!!
6.4V ? better.
Bucking is the way to go.
Get ready for the 127V you'll have in a couple years and plan accordingly.

ghetto bucking transformer

HA! Love it!

100% in agreement with J M Fahey.

Cue Elvis : "In the Ghetto"

Juan, do you think those ever-helpful power companies will dial up our power to 127V? Hm, the more voltage they send us, the more power they charge us for, so...

FWIW my college apartment had 128V in 1972. We complained that the light bulbs were going bad, fast. Cant' remember they ever fixed it. Also had mushrooms growing out of the carpet. Brand new building. Circus of mismanagement.

"Old" Italian: geto = furnace. IOW the part of town where scrap collectors melted down scrap metal for re-use in the Middle Ages. Spelling changed as time went on.

[trem]

Yikes! Incoming power waveform is already hacked-up enough in many locations without sticking zeners in the AC line. I'd opt for "inrush limiter" NTC resistors instead. Many commercially made amps already have one (Fender, Peavey, old tube McIntosh) and I've seen a recent Peavey model that strings about 8 of 'em in series for a deliberate line voltage knockdown effect. I guess this last is to simulate the line voltage that was available from the TVA in the "good old days."

Pardon my ignorance.....
Are you saying that this Zener Method will change, clip, distort the Sine Wave as it comes from the wall source.?
That is to say...will this add "noise" to the wall source.?
It seemed like such an easy solution.
Thank You

[Steve Conner]

If you want to knock 10V off with zeners, it'll take a 10 volt bite out of the zero crossing. I imagine the effect would be a buzz similar to interference from a lamp dimmer.

The zeners will have to be bigger than you think to withstand the inrush current when the power switch is flipped. (FWIW: I believe the series string of NTCs inside new tube amps is for limiting inrush and giving the heaters a soft start, not knocking down the wall voltage.)

So by the time you factor in the cost of a box and some plugs and sockets, it probably ends up costing the same as the transformer while running hotter, being less robust and possibly making your amp buzz. Go ahead and try it if you want, but I'd use the bucking transformer. A toroidal transformer might be worth considering as they are smaller and lighter than the EI type for a given rating.

RG's site shows a "super Zener" made from a small Zener and a power MOSFET. This would probably be cheaper than an array of real Zener diodes of the same rating. It can be used on AC by connecting it to the + and - terminals of a bridge rectifier, and connecting the AC terminals of the rectifier in series with the line.

[trobbins]

Normally, the secondary HT will only be conducting at the peaks of the mains voltage, so that winding isn't an issue per se.

If the heaters are AC then there could be a slight amount of higher frequency noise that could couple through, but probably not an issue.

There would also be some noise from residual magnetising current being started and stopped. I guess shunt capacitors on the secondaries would knock some of the edge off any very high frequency transients.

[chinrest]

Looks good R.G.! Any plans for a production stand alone unit? I think it would be a hit, especially with antique radio guys.

"Lil' Buckaroo"

[J M Fahey]

Juan, do you think those ever-helpful power companies will dial up our power to 127V?

I don't "think", I'm *seeing* it happen.
Problem is, power consumption keeps going up steadily , I laugh (sort of ) at the Government "worrying about a Green Planet" and forcing nasty CFLs on us, when filament bulbs are only a small fraction of household power consumption, and so on.
Copper is becoming more expensive too, I guess soon we'll start seeing homes wired with aluminum , etc.
So going up in voltage is the sensible solution.
Problem is, instead of taking the bold step of going straight from 110V to 220V , they are creeping up bit by bit.
Just look at *old* Gibson and Fender schematics: those from the 40's showed straight "110V" , the original Edison DC standard .
He meant 100V actually , lamps *were* 100V , but the generators made 110V at the power plant so inefficient DC transmission guaranteed 100V for the furthest consumers, not many blocks away.
The race started when AC power meant the 110V could actually reach everybody.
I remember "115V" blackface Fender schematics, then "117V" (the UL 135W era), now 120V is being specified.

I sell amps in Brazil, and they have a maddening dual voltage standard, depending on "which World Power" was running their economy.
1/2 the Country runs on "110V", 1/2 in 220V, sometimes in the same city
And their 110V now is becoming 127V everywhere.
Old Giannini amps (Fender clones made in the 60's and 70's) die like flies .
*All* modern amps have a front panel 110/220V switch.
I had to add the same to mine, to be sold there.
I already mentioned here that every year I go to their Expomúsica, the Brazilian NAMM, and last time a friend of mine was invited to play.
He carried his trusty VOX VT100, plugged it into the wall .... and burnt it.
*All* of São Paulo is 110V, but the "Expo Center", smack in the middle of the City .... is wired 220V.
Worst thing is that they *should* have a BIG, red letters on a white background poster stating "YOU ARE ENTERING A 220V WIRED BUILDING"
People told me he was not the only one, that droves of Musicians had the exact same problem.
When he complained, they showed him a tiny homemade paper label glued to the wall socket, stating "220V"
Oh well, I had to go buy a soldering iron and a cheap multimeter and repair his amp ... which uses a hard to find Sanyo or Sanken power module.
Oh well.

[Leo_Gnardo]

Copper is becoming more expensive too, I guess soon we'll start seeing homes wired with aluminum , etc.

Tried that in the USA starting @ mid 60's. Wasn't working out so well (house fires) so the "fix" was splicing copper ends onto the aluminum. That didn't work either so by 70-71 it was back to copper 100%. My father was a house builder & pointed this out to me, just in case "in the future" aluminum was back in the picture, insist on copper for your house wiring.

Thanks for all the good answers!

If I'm not mistaken there's parts of Brazil on 50 as well as 60 Hz power. When I first did any work there 1982 I was warned some zones even had 25 Hz. Japan also has 50 and 60 Hz zones.