generating bias voltage question

[doctord02]

I'm building a variation of a 200 watt Hiwatt and using a toroid PT that does not have a bias winding on it. My PT has two, 230 v windings that I plan to stack like the Hiwatt power supply uses - please see Mark Huss' most excellent schematic here:

http://mhuss.com/Hiwatt/Schematics/DR_200wPS.gif

Can I pull a negative supply from the raw power feeding the bridge on the second coil, the one marked 280v in Mark's schematic? I'm considering something like what Amgeg does...

http://schematicheaven.com/ampegamps/v4poweramp.pdf

I've never built anything with this stacked coils/bridge power supply before, so are there potential got-ya's I should be aware of? I just need to get sufficient neg voltage for 4 KT88's.

Any suggestions welcome!

Dave

[stoo]

When there is no CT used in the power supply as with a non bridge rectifier then you MUST use a cap to isolate the bias supply as in your second diagram. It's the .047/600v cap..
I don't like that arrangement personally but you gotta work with what you got.
Stew

[Jana]

If you don't mind another transformer in the chassis, Triad makes some neat little transformers called flatpacks. They lend themselves to mounting on an eyelet board, are quite small and come in a wide range of voltages. Since you are only doing bias with it, you don't need hardly any current. I have used these before to build a separate bias supply.

[skyboltone]

Both Stoo and Jana are right on here. Make sure the cap is AC rated for at least a couple times the HT rating. I know this can be argued but if a DC cap is rated at 600VDC doesn't mean necessarily that it can handle your AC voltage. Why sacrifice a pair of output tubes for want of a $1? I've been using some Wima FKP types I got off a surplus table at a swap meet. $30 caps. Vishay makes a MKP type that will probably do. There are some Ceramics that will do also.

http://www.mouser.com/Search/ProductDet ... d1qA%3d%3d

[doctord02]

Thanks guys. I'm gonna look into Jana's suggestion on the flatpacks... I like having this bias voltage completely separate and stable from the HT.

Time to call Mouser!

[FUCHSAUDIO]

I used a cute trick a few years ago that worked out well: I took a 120 volt to 6.3 volt Mouser transformer, hung the 6.3 volt end off the amps filament line, and rectified the 120 volt side. I had more voltage than you traditionally have for raw bias (easy to knock it down really), and it worked out great !

[doctord02]

Hah! Nice idea Andy. Unfortunately, this PT is bordering on filament current challenged - it has a 4 amp 6.3 volt wind and a 3 amp 12.6 volt wind. I cant run all four KT88's off of a single heater winding, so I'm gonna slap a voltage divider on the 12.6 v wind and drop it down to 6.3 v and run one KT88 and the 4 12a_7 tubes off of it and the other three KT88's off of the existing 4 amp heater circuit.

When I bought the PT I didnt realize it had a 12.6 winding, I thought it had two 6.3 v windings... The vendor had two versions of it available and I was reading the wrong spec.

Grrr.

[skyboltone]

Hah! Nice idea Andy. Unfortunately, this PT is bordering on filament current challenged - it has a 4 amp 6.3 volt wind and a 3 amp 12.6 volt wind. I cant run all four KT88's off of a single heater winding, so I'm gonna slap a voltage divider on the 12.6 v wind and drop it down to 6.3 v and run one KT88 and the 4 12a_7 tubes off of it and the other three KT88's off of the existing 4 amp heater circuit.

When I bought the PT I didnt realize it had a 12.6 winding, I thought it had two 6.3 v windings... The vendor had two versions of it available and I was reading the wrong spec.

Grrr.

Wait! Run the KT-88 filaments in series pairs. Believe me, it seems odd to us but it was VERY common in the golden age of radio. A voltage divider will just use up current. Use the filaments themselves as voltage dividers. Each pair of tubes will use the equal of one tube at 6.3V.
You'll have 3.2 amps at 12.6 volts to fire the filaments. You'll have plenty to run the 4 KT-88 off the 12.6V winding. AND WHAT IN THE HECK ARE YOU BUILDING WITH 4 KT-88?

[doctord02]

Sky, what kind of current are the KT88's gonna pull wired up in series pairs? My winding is only rated 3A, and KT88's spec out at 1.6 amps each...

(sorry if this is obvious...)

EDIT - what am I building? A Hiwatt DR201 as mentioned in post #1



(GF is a bass player...)

[skyboltone]

Yeah, I caught the Hiwatt reference after I had asked my question.

Ok, here's the deal. A watt is a watt is a watt. The filament in your incandescent light bulb is rated at (say) 100 watts at 120V. So I=P/E right? So the light bulb draws .83A.

The winding on your PT is rated at 12.6V at 3 amps. So P=I*E right? So you have 3*12.6 or 37.8 watts of POWER available from the 12.6 volt winding.

Each KT-88 requires 6.3V at 1.6 amps or 10 watts. So if you put two KT-88 filaments in series (because they are identical) each one will drop half of the voltage available across it. The watts of power required remain the same though. So the POWER or watts required by two KT-88s is still 20 watts. So a pair of KT-88 filaments in series at 12.6 volts will require 1.6*12.6 or 20.16 watts. Do that two times and the winding will see 40.32 watts being drawn.

Now. Don't let it trouble you that you'll be drawing 40.32 watts from a 37.8 watt winding. It's just not a significant over current situation. The winding is rated nominally at 3 amps. I am quite sure that it will push the extra .2amps no trouble. In addition, the primary of your PT is almost certainly 115V. With 120V at the wall you'll be high on filament voltage anyway unless you burden it fully. Do it. It's a MUCH better solution than the one you are noodleing.

Dan

[doctord02]

Consider it done. It a much better plan than my jackleg voltage divider notion, and the PT should easily handle it.

Thanks! I'm down in San Mateo - swing by sometime and collect a free beer or two for your trouble. We could even chase down Dartanion in RWC...

Dave

[skyboltone]

Consider it done. It a much better plan than my jackleg voltage divider notion, and the PT should easily handle it.

Thanks! I'm down in San Mateo - swing by sometime and collect a free beer or two for your trouble. We could even chase down Dartanion in RWC...

Dave

Sounds like a plan. Dart and I are planning a wreckfest in the future here sometime. I'm currently varnishing everything on the boat while we have this good (for SSF) weather. I wanna run my Dumble through his Fane cabs.

[doctord02]

A Wreckfest sounds cool Dan, I can provide a few variations for comparison... I have the Spalted Maple one that Paul Ruby featured on his website, as well as a stock clone I built, plus a 6SN7 output version... I've also got some ceramic SRO's and some Scumbacks if you'd like to try them out.

Keep me in mind if it comes to pass; PM me and I'll shoot my contact info over.

The series heater trick will really save me a heap of trouble. I've already made several compromises and part changes to accommodate off the shelf parts as is.

[stoo]

I used a cute trick a few years ago that worked out well: I took a 120 volt to 6.3 volt Mouser transformer, hung the 6.3 volt end off the amps filament line, and rectified the 120 volt side. I had more voltage than you traditionally have for raw bias (easy to knock it down really), and it worked out great !

+1 !!
stew

[doctord02]

Hey Dan-

are there any polarity or phase issues to be aware of when wiring these heaters in series pairs? Keep them both the same? Reverse one from the other? Doesnt matter either way?

Seems I recall reading that there are some noise issues when wiring preamp tube heaters in series and to overcome them you wire them out of phase.

Of course, I could be smoking crack... memory aint what it used to be...