PT with separate bias winding

[Ed_Chambley]

This Hammond (attached) has a bias winding at 60v.  I am not familiar with this arrangement.  What is the purpose if having this and how would it be used?

[sluckey]

That PT will produce about 600V unloaded. It is meant for a BIGGG amp. Big means expensive too. These amps warrant the extra cost and reliability of a totally separate bias supply that is independent of the B+ supply winding. Probably wants a full wave bridge on the bias tap. See page 5 of my amp scrapbook for an example.

[stratele52]

IMO this Power Transformer is made for multitask job , too expensive( ? )  and too many features for guitar amps use.

[kagliostro]

Seems a muscle PT devoted to Very BIG Bass Amps

K

[HotBluePlates]

The 1A rating of the high voltage winding oughta tell you something.

[J Rindt]

I thought he was asking about the 60V bias winding...??
The one that  is for building a negative bias potential for a fixed bias amp.
How can I misread that badly.?

[sluckey]

You didn't misread. It's a $350 transformer. I'd expect a separate bias winding, electronic key entry with remote start, seat warmers, etc.  

[Ed_Chambley]

I am just looking at different PT's and ran across this one and had never seen one like it.  I am really trying to find one where I can run 2, KT120's and a couple three of preamp tubes.  I would like to run about 600 loaded plate voltage, but I have concerns investing in a PT like this and New Sensor stop production on the tube.

I just ran across this one while looking at specs.  Also, most who are building around the KT120 are using dual adjustable fixed bias.  If anyone has a suggestion on a suitable PT, I am all ears.

[J Rindt]

You didn't misread. It's a $350 transformer. I'd expect a separate bias winding, electronic key entry with remote start, seat warmers, etc.  

Oh.....OK.
Another one of those "budget" offerings from Hammond...

[tubeswell]

I've gotten several PTs made with separate bias windings. One advantage I can think of for doing this is that you can zener down the B+ using reverse biased zener(s) on the HT centre tap without it affecting the bias supply voltage.

[stratele52]

I've gotten several PTs made with separate bias windings. One advantage I can think of for doing this is that you can zener down the B+ using reverse biased zener(s) on the HT centre tap without it affecting the bias supply voltage.

This is not a problem , just change value of some resistor in bias circuit.

[DummyLoad]

If anyone has a suggestion on a suitable PT, I am all ears.

insufficient data. idle current/valve? max. current at max. output/valve?

however, off the cuff, you need about 900VrmsCT to get close to 600V with a reasonably loaded PT. the 800V @ 1A hammond heavy hitter will get about 565Vdc. i suspect it is the PT of choice to lite up 3 or 4 pairs of KT88 to mate with the 280W OT they sell.

lastly and respectfully, but if the tubes are out of production, why bother, unless of course you have some? why not design for KT88 and just use KT120 in the same hole? that way when/if you deplete your KT120 stock, you'd still have a sustainable platform.

other bias PS options... use a 120V/12.6V small filament transformer. connect the 12.6V secondary to the 6.3V secondary of the PT - you get about 60Vrms at the primary and can use FWB. i think that's in slucky's scrapbook too... :-)

happy rosin smokin'

--pete

[stratele52]

No need bias winding .

 From one side of the high voltage winding , connect 0.1 MFD with couple resistors to drop voltage to 60 volts. Go to rectifier diode ( cathode side )  , filter caps , pots .

That it is you have your bias circuit.

Many Marshall have used this.

[Ed_Chambley]

If anyone has a suggestion on a suitable PT, I am all ears.

insufficient data. idle current/valve? max. current at max. output/valve?

however, off the cuff, you need about 900VrmsCT to get close to 600V with a reasonably loaded PT. the 800V @ 1A hammond heavy hitter will get about 565Vdc. i suspect it is the PT of choice to lite up 3 or 4 pairs of KT88 to mate with the 280W OT they sell.

lastly and respectfully, but if the tubes are out of production, why bother, unless of course you have some? why not design for KT88 and just use KT120 in the same hole? that way when/if you deplete your KT120 stock, you'd still have a sustainable platform.

other bias PS options... use a 120V/12.6V small filament transformer. connect the 12.6V secondary to the 6.3V secondary of the PT - you get about 60Vrms at the primary and can use FWB. i think that's in slucky's scrapbook too... :-)

happy rosin smokin'

--pete

Pardon me, sometimes I think everyone knows what is in my head. I am building 2 amps which are very similar.  One will run KT88 and use a Hammond UL OT 1650RA, 5,000 ct , 318 ma., 4-8-16 and has a PT with SS rectifier and is running about 530 v on the plates.  Dynaco PT  amp is running -42 bias.  It is basically a dynaco power section.

Upon looking at KT120 (attached), the tube is capable of 60 watts and I have read where some are pushing a pair into 150 watt territory.  Just trying to find out how.  I think 150watts would soon destroy the tubes, so I was looking to get about 100 watts from 2 tubes.  When I ran across the transformer I posted, I simply wanted to find out more about it.  I have, and thanks.

Now if using this tube with A431S Triode OT
Transformer specs are: 4300 ohms CT primary with screen taps, 4,8 and 16 ohm secondary. 20 Hz to 20 KHz response at 60W, within 1 db, 30 Hz to 15 KHz at 120 watts. Max DC per side (suggested) 120 ma.
I was shopping to see if I could find the correct PT.  I have no reason to believe the tube will cease production, it just seems odd New Sensor is the only one making it.  I only have 2 of them and they are running is an actual Dyanco stereo.  They sound nice and I believe them to be a replacement for current production kt88 since people are not getting many hours out of KT88.

Basically just random thinking.

[PRR]

> capable of 60 watts and I have read where some are pushing a pair into 150 watt territory.  Just trying to find out how.

Dissipation is not the same as output!

Ideally dissipation could be zero even at high output.

Using Sine waves (the way we customarily measure audio), in push-pull linear amplifier, the ideal efficiency is 78%, or 22W in tubes and 78W to load, or output 3.5 times dissipation.

Tubes are far from ideal. 60% efficiency is good. So 60W out for 40W dissipation. Or output 1.5 times dissipation.

With stock 6550/KT88 at 35W, 70W per pair, we may expect 105W output. Indeed both sheets show 100W output conditions.

So two 60W dissipation tubes, 120W diss, can support 180W of output.

However the KT120 sheet does not show any new conditions showing how-to-use the higher ratings. Do we bump the current (lower loads Z), or raise the voltage?

It may be conservative to stick with the customary load, raise both voltage and current just enough to meet a 150W output goal. Square-root of 150/100 is 1.225.

KT88 makes 100W in UL with 560V, 4.5K load, 360mA total cathode current. Apply 1.225 factor to V and I: 686V(p&g2) 441mA cathode current. Compare to ratings: 850Vp, but 600Vg2 in UL. So this condition exceeds a rating.

6550 is KT88 in a shorter bottle with less brave ratings. A pair of 6550 in pentode at 600Vp 300Vg2 5K load 303mA will deliver 100W output. Factor 1.225: 735Vp, 367Vg2, same 5K load, 339mA cathode current. Well under voltage ratings, under current rating (250mA per tube or 500mA/pair). Grid bias is likely -40V which is hardly heroic in terms of big amps.

In fact it looks like we can go further. Raise plate voltage to 848V, G2 at 424V, keep 5K load, expect 429mA cathode current, and expect 100W*1.414^2 = 200 Watts output. Inded the KT120 sheest says "up to 200W".

> Transformer specs are: 4300 ohms CT

OK, we trade V and I to get 4300/5000. Or factor 0.86. Using the 200W output numbers: Plate voltage becomes 730V, cathode current now 499mA. Cathode current is right at rating. G2 voltage is about 470V and G1 voltage may be -51V.

Using a 70W-100W Hi-Fi OT, the higher voltage swings will distort the deep bass. I'm guessing the bent-bass point moves up 1.5 in frequency or about a half octave. Fine for guitar.

> I have no reason to believe the tube will cease production
> it just seems odd New Sensor is the only one making it.

New Sensor doesn't make tubes, they just brand and distribute. I thought KT120 was from Tesla factory. The Tesla factory moved recently. Another tube factory has stopped making small tubes (by their standards KT120 would be a small tube). If demand becomes huge, Shantung could make them; I suspect sales have not been spectacular enough to divert production from the gravy-train 6L6 and EL34 with KT88/6550 on thursdays.

Why do you want maximum power from a minimum number of single-source tubes? Nobody drives 2-cylinder cars. Twenty 6V6 would give you 150 Watts (yes, the 1KCT load is awkward). A six-pack of 6L6/EL34 gets you there, with ample redundancy (one tube down, nobody will know), and you can get a replacement on a saturday night in a strange town. Four-pack of 6550/KT88 will do 150W E-Z, and you can use four KT120 in those holes if you like the look.

[Ed_Chambley]

> capable of 60 watts and I have read where some are pushing a pair into 150 watt territory.  Just trying to find out how.

Dissipation is not the same as output!

Ideally dissipation could be zero even at high output.

Using Sine waves (the way we customarily measure audio), in push-pull linear amplifier, the ideal efficiency is 78%, or 22W in tubes and 78W to load, or output 3.5 times dissipation.

Tubes are far from ideal. 60% efficiency is good. So 60W out for 40W dissipation. Or output 1.5 times dissipation.

With stock 6550/KT88 at 35W, 70W per pair, we may expect 105W output. Indeed both sheets show 100W output conditions.

So two 60W dissipation tubes, 120W diss, can support 180W of output.

However the KT120 sheet does not show any new conditions showing how-to-use the higher ratings. Do we bump the current (lower loads Z), or raise the voltage?

It may be conservative to stick with the customary load, raise both voltage and current just enough to meet a 150W output goal. Square-root of 150/100 is 1.225.

KT88 makes 100W in UL with 560V, 4.5K load, 360mA total cathode current. Apply 1.225 factor to V and I: 686V(p&g2) 441mA cathode current. Compare to ratings: 850Vp, but 600Vg2 in UL. So this condition exceeds a rating.

6550 is KT88 in a shorter bottle with less brave ratings. A pair of 6550 in pentode at 600Vp 300Vg2 5K load 303mA will deliver 100W output. Factor 1.225: 735Vp, 367Vg2, same 5K load, 339mA cathode current. Well under voltage ratings, under current rating (250mA per tube or 500mA/pair). Grid bias is likely -40V which is hardly heroic in terms of big amps.

In fact it looks like we can go further. Raise plate voltage to 848V, G2 at 424V, keep 5K load, expect 429mA cathode current, and expect 100W*1.414^2 = 200 Watts output. Inded the KT120 sheest says "up to 200W".

> Transformer specs are: 4300 ohms CT

OK, we trade V and I to get 4300/5000. Or factor 0.86. Using the 200W output numbers: Plate voltage becomes 730V, cathode current now 499mA. Cathode current is right at rating. G2 voltage is about 470V and G1 voltage may be -51V.

Using a 70W-100W Hi-Fi OT, the higher voltage swings will distort the deep bass. I'm guessing the bent-bass point moves up 1.5 in frequency or about a half octave. Fine for guitar.

> I have no reason to believe the tube will cease production
> it just seems odd New Sensor is the only one making it.

New Sensor doesn't make tubes, they just brand and distribute. I thought KT120 was from Tesla factory. The Tesla factory moved recently. Another tube factory has stopped making small tubes (by their standards KT120 would be a small tube). If demand becomes huge, Shantung could make them; I suspect sales have not been spectacular enough to divert production from the gravy-train 6L6 and EL34 with KT88/6550 on thursdays.

Why do you want maximum power from a minimum number of single-source tubes? Nobody drives 2-cylinder cars. Twenty 6V6 would give you 150 Watts (yes, the 1KCT load is awkward). A six-pack of 6L6/EL34 gets you there, with ample redundancy (one tube down, nobody will know), and you can get a replacement on a saturday night in a strange town. Four-pack of 6550/KT88 will do 150W E-Z, and you can use four KT120 in those holes if you like the look.

Appreciate your reply. Why? Hell I don't know.  I guess mainly because I have built lots of 4 tube power sections using the Saturday Night Specials, but never built anything like this.  Really has nothing to do with appearance.  I completed the KT88 and really like the forceful bottom end.  Clear and strong with a sort of hit you in the chest clean.  Great tone from quite to louder than you want to play it.  It goes BOOM, but I am not going to keep this one as it is built for someone else.  I am going to build one for me and since I am driving a 2 cylinder, it might as well be 2 stroke, however the emission police may get me.  This is the first UL designed amp I have ever built and I guess I have gone a little off the deep end which is normal for me.  It is the thought if this is good, then more of the same has to be better.  It is usually a bad idea, and the same reason I no longer drink.