Trust this transformer any more or less?

[nsubulysses]

https://www.ebay.com/itm/Power-Trans...EAAOSwkcFbEFPO


I am considering buying this PT to make a 2 power tube 100W bass amp (roughly)

So, say you have a PT made for a guitar or bass amp from Hammond, Heyboer, Classictone, etc, It seems like in this audio application you know the transformer is gonna be working kind of hard because in the music realm most people likes to crank things up to high volume. Is this part of the design per se, as far as current handling or power supply sag/regulation in rock amp application vs a transformer for another application?

The transformer in the ebay listing is from a ham radio project. Its an Elenco that will do 850V CT at 230mA for the high voltage winding. But the thing is, no one EVER cranks a radio up to full volume, plus distortion.

Bascially, do transformers that are pulls from old TV, oscilloscope, radios, etc work just as good as a transformer that is made for what is known will be a rock amp application?

[Tom Phillips]

Good transformers from good manufacturers are good for many applications.
HOWEVER, every manufacturer makes various grades of products so there is really not sufficient information supplied by the seller on eBay. Specifications are given but they are just written in the description by the seller so I would consider those specs unverified. If the stated specs are correct we still don't know if the transformer would achieve that performance at a 100% duty cycle within a reasonable temperature rise. For $70 plus ~$40 S&H I would buy a known new and guaranteed product if I was going to put the effort into an amp build.

[olddawg]

Good transformers from good manufacturers are good for many applications.
HOWEVER, every manufacturer makes various grades of products so there is really not sufficient information supplied by the seller on eBay. Specifications are given but they are just written in the description by the seller so I would consider those specs unverified. If the stated specs are correct we also don't know if the transformer would achieve that performance at a 100% duty cycle within a reasonable temperature rise. For $70 plus ~$40 S&H I would buy a known new and guaranteed product if I was going to put the effort into an amp build.

If it's just a "hobby amp" or personal build, I would consider looking on Craig's List for a big old Carvin, Peavey, etc for $150 or so and get 2 transformers, a cabinet, and maybe some good tubes. I see them all of the time. Maybe not in your area though. Especially the old 160 watt 6 output tube monsters from the 70s.

[nsubulysses]

Part of the reason this has me thinking is I have two transformers that are close to what I need, but both might be slightly low if I was actually aiming for 100W.

I would think if I had 600-620V B+ or so and two KT88 I could do 100w amp


I have sitting around a stancor 8412 PT which is 800VCT at 200mA and a Hammond 278X PT 800VCT at 230mA.

I have used the big brother of Hammond 278X, the Hammmond 278CX, a bunch which is 800VCT at 535mA. With 4 power tubes I get a loaded B+ of about 580V.

I was guessing with Hammond 278X and 2 power tubes I would get similar B+, about 580V.

I recently saw this 850V CT transformer on ebay and thought it would be perfect, except for it's an Elenco

[nsubulysses]

If it's just a "hobby amp" or personal build, I would consider looking on Craig's List for a big old Carvin, Peavey, etc for $150 or so and get 2 transformers, a cabinet, and maybe some good tubes. I see them all of the time. Maybe not in your area though. Especially the old 160 watt 6 output tube monsters from the 70s.

It's gonna be a custom build for a friend. I am down for some cheap (cost-wise) transformers but not for reusing a headcase or chassis because is has to look good too.

I LOVE finding sweet deals on transformers on ebay and what not. Actually that Stancor 8412 is one I saw on ebay and I just bought it on a whim becuase it super cheap like $30 or something. Still haven't used it and it's been about two years whoops.

I recently looked on reverb which I never have before. Someone was selling a hammond PT/OT set that was for Marshall JMP/JCM800. So it's Hammond 290HX PT and Hammond 1750U OT. I got both for $150 including shipping.

[loudthud]

100W with two tubes usually means a split rail design, 600V for the Plates, 300V for the screens. 850VCT ain't the way to go unless you can design a Voltage Regulator.

Read this page about transformer duty cycle. Link: http://www.ab4oj.com/quadra/icas.html

[cerrem]

Following up on loudthud's suggestion.... It is common to use a split rail for High B+ plate voltage on the 6550 and KT88 type tubes...
One method is to use a Full-Wave Voltage Doubler arrangement in the power supply ...then use the Center of the Doubler as the screen supply...therefore you get roughly 600V 300V supplies...
Another cool trick with split rails is to go Ultra-Linear .... The screen has it's own separate primary winding in the transformer ....Acrosound did this OT with 6146 tubes...

[Mike Sulzer]

If a ham project, it would be for a transmitter. It would probably be used close to full rating with a duty cycle about 50%, maybe not that different from a bass amp. I think the current rating is too low if you want 100 watts average power all night long, but I might be too conservative.

https://www.ebay.com/itm/Power-Trans...EAAOSwkcFbEFPO


I am considering buying this PT to make a 2 power tube 100W bass amp (roughly)

So, say you have a PT made for a guitar or bass amp from Hammond, Heyboer, Classictone, etc, It seems like in this audio application you know the transformer is gonna be working kind of hard because in the music realm most people likes to crank things up to high volume. Is this part of the design per se, as far as current handling or power supply sag/regulation in rock amp application vs a transformer for another application?

The transformer in the ebay listing is from a ham radio project. Its an Elenco that will do 850V CT at 230mA for the high voltage winding. But the thing is, no one EVER cranks a radio up to full volume, plus distortion.

Bascially, do transformers that are pulls from old TV, oscilloscope, radios, etc work just as good as a transformer that is made for what is known will be a rock amp application?

[R.G.]

Transformers are tricky. Their specs often don't say what you think they say unless there are a number of conditions listed with the specs.

First, the power rating of a transformer is a variable. That 850Vct at 230ma carries an implied "at the temperature limit of the insulation inside this thing" which is not specified. Copper melts at 1085C, and iron goes non-magnetic at the Curie point of the specific alloy, generally 500C to 750C. Below those temperatures, the stuff still works as a transformer PROVIDED THAT the insulation keeping the turns from shorting stays good. Those temperatures are high enough for the thing to literally glow in the dark.

Transformers get hotter from internal heat generated by the resistance of the copper and by eddy currents set up by the magnetic field in the iron. The eddy currents are set by the M-field intensity and don't change much in the limited temperature range that insulations can stand. The heating in the copper goes up with the square of the output current. So you can simply get more power out of a transformer by running it hotter. As long as the insulation doesn't melt or decompose, the transformer can put out more power.

Transformer makers generally don't tell you the temperature rating of the transformer, except very indirectly in a footnote. But that's very important. It's also true that most transformers are running in a benign environment and cooled by ambient air around 25-35C. So it's understandable that the makers don't go to a lot of trouble to specify the details.

The transformer as described will put out 850Vrms times 0.23Arms, or 195VA, plus 36VA and 15VA on the heater windings.

The thing will deliver less than 195VA to a DC load. It is the nature of rectification to make constant (-ish!) DC out of AC. Normal rectifier circuits make the heating (rms) current in the transformer's copper be higher than the DC current delivered. For full wave rectifiers, this is a factor of about 1.6 to 1.8, depending on how big the first filter cap is. For full-wave-center-tap rectification, it's about 1.2. So if the transformer can deliver 230ma to a resistor load, it can deliver .230/1.2 = 192ma to a DC load in a FWCT and .23/1,6 = 144ma to a DC load in a FWB. For a nominally 600V supply, that's 600*0.192 = 115W of DC.

Tube amps are not highly efficient. If you want 100Wrms out of a tube amp output stage, count on feeding it three to four times that much power as DC. So you need 300-400W of DC to get 100Wrms out to the speakers. There are a pile of footnotes and caveats behind that statement, but as a broad brush, it's generall true.

So that transformer, in my estimation, will not successfully power a 100W tube output stage without getting MUCH hotter than may or may not be stated for the transformer (if you can find that info); getting hotter also means more copper wire resistance losses, so the output voltage will sag a lot. My best guess: that's not what you're looking for.

[nsubulysses]

Cerrem and LT are beating me to my next question about the voltage doubler arrangement.

Does this arrangement actually double the voltage of the HV secondary or is it just a way to use the half voltage at the first power supply node for the screens (assuming you have two series capacitors) in a way where both B+ and screen supply are full wave rectified?

Also, do you need a dual high voltage secondary to do this like how SVT and Sound City 200+ are, or is it possible to do it with only a single high voltage secondary

[nsubulysses]

Also, is it worth messing around with doing it NOT ultralinear. Or is ultralinear just a no-brainer for a bass amp application and should be done.

I guess I need to know this before I source my OT. Well, guesss I still have to figure out this power supply arrangement so I source the PT too.

[loudthud]

A Voltage doubler does give you double the peak Voltage, but it does sag a little more when loaded. MusicMan used it in some of their 130W and 150W amps. Another option is used on some 100W Marshalls. You have a Full Wave Bridge and a center tap that just hooks to the junction between the two series connected filter caps. The center tap Voltage isn't used for anything in the Marshall design, though it could be. You could use one of those transformers except for the fact that the B+ would be on the low (~500V) side.

I built a bass amp for my own use with a UL-nonUL switch. Never liked it in UL mode so never used it there, but I never disconnected the feedback because I had Presence and Resonance controls.

Edit: Duncan's PSU designer does have a setting for a Voltage doubler configuration.

[cerrem]

OK...Now I see your intention is a bass amp... I would not use Ultra-Linear for a bass amp...
The classical set-up for the 6550 / KT88 is 700V on the plate and 350V on the screens.... this gives around 100W to 120W range...
A voltage doubler of course sags ...no worse than any other supply.... It's all about calculating the proper value of capacitance for the doubler as well designing or selecting a transformer with the right regulation...ie losses...
Keep in mind that all power supplies have some percent of sag...and that the designer is concerned with the full power output voltage, thus the idle voltage will obviously be higher than this full power output voltages..
When I design transformers..I do not play any marketing games or hide any data...all my PT's and OT's are designed for 40C rise at the full rated "advertised" load currents ....indefinitely at continuous operation....

[Mike Sulzer]

I suppose voltage doublers are more attractive than they used to be because capacitors are relatively cheaper. But the circuit loudthud mentioned looks best. With the full wave bridge and the center tapped transformer you charge the full capacitance on both half cycles and also get half voltage for the screens. All the copper is used all the time and so the caps recharge quickly, and you have a very solid supply. The i^2R loss is high as RG mentioned, and this is a disadvantage. Of course this circuit is universal for the split supplies for a solid state amp. I suppose that in that case the high cost of very high value capacitors drives its use, but RG might have a better answer.

[R.G.]

A voltage doubler of course sags ...no worse than any other supply.... It's all about calculating the proper value of capacitance for the doubler as well designing or selecting a transformer with the right regulation...ie losses...
Keep in mind that all power supplies have some percent of sag...and that the designer is concerned with the full power output voltage, thus the idle voltage will obviously be higher than this full power output voltages..

Power supply sag: Sag comes from the losses from the AC wall socket all the way through the output at the first DC filter cap. The losses come as heating losses in the copper wire resistance, as series inductance losses which drop the output voltage without causing heating. The small parasitic losses in power transformers can usually be ignored, only coming into play with really high voltage stuff, thousands of volts.

In general, voltage doublers give you twice the voltage (minus rectifier losses) and twice the sag. But this is worse in half-wave setups, where you have to make the first filter cap be at least twice the size of the caps for a full wave rectified supply because it's twice as long between charge pulses. Full wave rectification is nearly always a winner where both transformer and filter cap costs are concerned, so if you have to do a doubler, it makes sense to use the full-wave variant of doubling.

When I design transformers..I do not play any marketing games or hide any data...

Kudos. That's not all that common. Gudonya.

all my PT's and OT's are designed for 40C rise at the full rated "advertised" load currents ....indefinitely at continuous operation....

Hmmm. That's good, it leads to long life. I'd be very interested in the temperature calcs. If you're using Class A/105C insulations and a 40C rise, you get 65C for an exterior temp, about 150F. You're about 10C conservative on the insulation rise as NEMA recommends Class A for a maximum rise of 50C. That's good for the "normal" abusive use of guitar amps.

On the other hand, if you changed to Class B insulation, you'd be able to offer the same lifetime with a 25C/45F higher temp rise on the inside of the transformer, and save significant copper and iron for your customers. If the cost of a higher insulation class is less than the material savings, it would be a win for either you or your customers or both. I suspect that Class B materials are only trivially more expensive than Class A these days, and you may already be using them, especially in your magnet wire.

I'm guessing that OT designs will not usually be temperature rise limited; the primary inductance requirements usually mean that the iron and copper are worked much less than in PT designs. more than temperature rise (unless they're from Marshall amps ).

I'm also guessing that your primary costs are in labor much more than materials as a one-off kind of supplier. Higher volume manufacturers would concentrate massively on the material costs.