Output transformer impedance

[navdave]

Wanting to to build a 100 watt bass amp using two KT-120's or KT-88"s.
Would using a OT with a 10K primary give me output impedance for 2, 4, and 8Ω when using two tubes?

[kagliostro]

the transformer is rated 10Kohm primary - 4-8-16ohm secondary

if you connect 2-4-8ohm speakers (2ohm to the 4ohm intake ond so on) you halve the primary impedance

the impedance depends on a turn ratio and on the impedance of the speaker connected to the secondary

K

[ernie_jr]

with 2 kt 88 or 120's you need an o/p tranny with around 4000 ohm and over 500 volts on the plates.
Sunn got 60-70 watts using 2 kt 88's with 500 + volts in their bass amps from the late 60's. Their o/p trannies
were 4300 ohm. If you connected an 8 ohm speaker to the 4 ohm tap, you would have 5000 ohm primary with  your tranny.
ernie

[navdave]

I know if I have a 4.2k primary with four tubes I have a secondary's for 2,4, and 8Ω and taps for 4,8, and 16Ω when using two tubes.

the transformer is rated 10Kohm primary - 4-8-16ohm secondary

if you connect 2-4-8ohm speakers (2ohm to the 4ohm intake ond so on) you halve the primary impedance

the impedance depends on a turn ratio and on the impedance of the speaker connected to the secondary

K

[tubeswell]

The reflected load in an output transformer comes from the impedance ratio of primary winding to the secondary winding. So if you change the speaker load on the secondary winding, that automatically changes the reflected load seen at the primary. So if you want to change the primary reflected impedance, you have to think in terms of using a secondary tap that was intended for a different impedance.

If the OT is rated to reflect 10k with 2R, 4R and 8R secondary taps, then if you hook a 2R speaker load to the 2R tap you'll get 10k, and if you hook a 4R speaker load to the 4R tap you'll still get 10k, and so on.

But if you hook a 2R speaker load to a 4R tap, (or a 4R speaker load to an 8R tap) you'll get 5k reflected load.

[navdave]

Thanks Tubeswell the OT I have in mind has a 10k primary with secondary taps for 16,8,and 4ohms.
If I only use two tubes with this OT would the secondary impedance stay the same or would they be 2,4, and 8ohms?

[HotBluePlates]

Thanks Tubeswell the OT I have in mind has a 10k primary with secondary taps for 16,8,and 4ohms.
If I only use two tubes with this OT would the secondary impedance stay the same or would they be 2,4, and 8ohms?

No. In your case, if you want a 10k primary impedance, then attach an 8Ω load to the 8Ω tap or a 4Ω load to the 4Ω tap, etc.

What everyone is telling you is that it does not matter what you attach to the primary. 1 tube, 18 tubes... the primary impedance with be unchanged.

So they're telling you you use the impedance and turns ratios of the transformer to determine which load you should attach to the secondary to arrive at the desired primary impedance.

[navdave]

So it would be SAFE to run an OT with a 10K primary impedance this way because the two power tubes will be seeing a 5K load right?
I'm sorry for all the questions I'm just having a hard time getting my head around this one.

the transformer is rated 10Kohm primary - 4-8-16ohm secondary

if you connect 2-4-8ohm speakers (2ohm to the 4ohm intake and so on) you halve the primary impedance

the impedance depends on a turn ratio and on the impedance of the speaker connected to the secondary

K

[tubeswell]

The way you figure getting a 5k load resistance with the speaker being run on the next 'knotch up' (in terms of the OT secondary taps) is right, but your understanding of how output transformers provide load resistance appears to need a bit more work.

OTs don't 'have a primary impedance' so-to-speak as such; rather they have an impedance ratio (Primary-to-Secondary) which determines the load that will be reflected to the output tubes, depending on the load you hook up to the secondary.

As it happens, this Primary:Secondary impedance ratio is the square of the Primary:Secondary VAC ratio.  And, what's more, the Pr:Sec VAC ratio = the Pr:Sec Turns ratio.

So an OT with 31.62 turns on the primary winding for every one turn on the secondary winding has (a Pr:Sec turns ratio (or a Pr:Sec VAC ratio) of 31.62:1, and therefore) a Pri:Sec impedance ratio of 1000:1. So if you hook an 8R speaker to the secondary, you'll get 8k reflected load on the Primary. Or if you hook up a 4R speaker to the secondary, you'll get a 4k reflected load on the primary, etc

[HotBluePlates]

So it would be SAFE to run an OT with a 10K primary impedance this way because the two power tubes will be seeing a 5K load right?

Let's go at it a different way.

What primary impedance do you want the tubes to see?

As for safety, I don't know anything about what you're intending to do with these tubes; i.e., B+ voltage, etc. I'm thinking it might be safe with a 10k load because you might not get full power out of them, but I'd want to look back at the data sheets since I'm not very familiar with KT88's/KT-120's.

[navdave]

I've been focused on the output impedance more than the primary.
I want the secondary taps to be 2,4, and 8 ohms like an Ampeg v-4.
But instead of running four 7027's I'm going to run two KT120's with a
B+ of 550v under load. I'm happy with the two tubes seeing 5k but Edcor
does have another OT with a 6k primary so I'm thinking the two big tubes
seeing 3k load would give me more output power.

So it would be SAFE to run an OT with a 10K primary impedance this way because the two power tubes will be seeing a 5K load right?

Let's go at it a different way.

What primary impedance do you want the tubes to see?

As for safety, I don't know anything about what you're intending to do with these tubes; i.e., B+ voltage, etc. I'm thinking it might be safe with a 10k load because you might not get full power out of them, but I'd want to look back at the data sheets since I'm not very familiar with KT88's/KT-120's.

[jojokeo]

I think the short answer for navdave is YES. I'm assuming you're running the tubes in class ab1 in ultra linear mode since that's the transformer you're referencing correct?!?! You will need a B+ of 560Vdc and your "optimum" plate to plate load is 4.5k ohms for a pair of KT88 tubes. For this you hook up a 4ohm speaker to the 8ohm tap or an 8ohm speaker to the 16ohm tap and you're there. Your wattage for the tranny is fine and the reflected impedance is close enough for your tubes to perform very well since the data sheet says optimum load under these conditions would be 4.5K impedance. Impedance is always changing depending on frequency so you don't need perfectly exact, there's no such thing really. You won't hear the difference at all.

[HotBluePlates]

Gotcha. Now I see we were missing key details in your initial question.

So... what JoJo said.

[navdave]

Big thank you to everyone that helped me on this one!

[navdave]

Would a 3.3k plate to plate load for a pair of KT88's be considered not optimum or be in danger red plating if ran this low?
Just wanting to squeeze a lil more power out of this project and Edcor does stock a 100 watt 6.6K primary OT.
Of course I will be running the secondary taps into either a 2,4, or 8Ω loads.

[PRR]

> 100 watt ... OT with a 10K primary

Do math.

100 Watts in 10K is 1,000 Volts RMS.

Across one side, 500V RMS.

500V RMS is 707V Peak.

So a perfect amp would need over 700V supply. With real tubes, over 750V, and probably over 800V at idle.

That's at the upper limit of KT88. Also many octal sockets will eventually arc-over at such voltages.

Peak current is 707V.2.5K or 283mA.

From data-curves, KT88 needs screen voltage >200V but <300V to peak 283mA. We can lean higher, but excessive screen voltage makes drive difficult and allows QUICK disaster to happen.

So you need a pretty steady 770V plate supply and also a good 250V supply.

Power supply looks ugly. We really prefer conditions where screen is a bit below half of plate voltage; or with Silicon diodes, perhaps Vs half of Vp. (Yeah, you could triple 260V-520V-780V, but triplers are crude.)

As Ernie says, a "happy" load for 6550/KT88 at 100 Watts is more like 4K or 5K (with 560V-600V Vp and 300V Vs).

While you could mis-load the "10K" as a 5K, you lose the 16 output, and there's some sacrifice of performance. Edcore should have such a part in 5K.

Yes, Edcor will do Custom; and since that part is already built-to-order, it won't take much longer.

> I want the secondary taps to be 2,4, and 8 ohms

Then, yes, it works OK; but you could get a bit better performance if you get Edcor to wind the part you really want. The 10K windings are very thin wire. Working at 5K, they increase stray resistance loss. Designed for 5K, they would use fatter wire. Also the way 4-8-16 winding is often done, the "4" winding may be lossy already, worse if used as 2.

The for-sure and for-cheap path is to adopt the PT and OT from a Fender or Marshall "100W" and a *quartet* of 6L6CG/EL34. I'm not sure if the common-commodity OTs for these models offer a 2-4-8 option.

> Would a 3.3k plate to plate

What are you doing for a Power Supply?

[Willabe]

Ritchie200 has had good success with them in his Marshall Major.


                Brad     

[clyde]

I run 2x KT88's at 605v on the plates and 590v on the screens, no problems for years on a regularly-gigged rig.  Primary impedance is 5k (Hammond 1650R).  Best to use the correct primary impedance rather than halving it with a reduced load.