How to figure out OT primary for a market amp when increasing tubes?

[Balticnoiseforge]

Hallo el34world.

I have a question about OT primary if you add power tubes to a design that is already out there, since the primarys ohm helps to set the sound of a amp, even if it would work within margins.

In my case iam thinking about the Ampeg V4 that runs x4 7027 tubes into a 120W OT transformer with a primary of 2.9K, while their 60w version has a primary of 5.9K.

If i would like to build a Ampeg with x6 7027 tubes i would need a 180-200w OT - but how do i figure out the primary so it is set within the Ampeg "style"?

Or am i missing it totally? Like the Hiwatt dr201 is running x6 el34 is 250w 1.2k primary.

And if doing a x6 ampeg it is also stuff like primary induction that sets the sound? ( Which i havent been able to dug out except from posts where people measure old transformers. )

Such like this post measuring a old ampeg V4b
"This transformer has been tested for inductance. See below Input section Purple to Blue = 29 Henries Orange to purple = 7.5 Henries Orange to Blue = 7.5 Henries. Output section Brown to Red 20 millihys 2 ohm Brown to Yellow 46 Millihys 4 ohm Brown to White 83 Millihys 8 ohm."

If anyone can try suggest me into a path where i could figure it out, or just know the answer and would be willing to share that would be also as appreciated - iam no designer, i simply like to build.

Thanks for taking time reading!

[sluckey]

In my case iam thinking about the Ampeg V4 that runs x4 7027 tubes into a 120W OT transformer with a primary of 2.9K, while their 60w version has a primary of 5.9K.

If i would like to build a Ampeg with x6 7027 tubes i would need a 180-200w OT - but how do i figure out the primary so it is set within the Ampeg "style"?

If 2x7027 is 5.9K then 4x7027 would be half that or 2.9K and 6x7027 would be one third of the 2x value or 2K.

[Balticnoiseforge]

Oh didnt know it was that simple, so basically 1.9k or 2k would be suitable, or to be exact 1.95K - is there anything else that you would need to think of when it comes to the other stuff like henries and such if ordering one?

[sluckey]

Surely the inductance affects the OT sound. That's probably part of why this OT sounds different from that OT. But to be honest, I've never been concerned with inductance. I just look at impedance ratio and power ratings.

[Balticnoiseforge]

Thanks for your time!

[HotBluePlates]

... is there anything else that you would need to think of when it comes to the other stuff like henries and such if ordering one?

Surely the inductance affects the OT sound. That's probably part of why this OT sounds different from that OT. But to be honest, I've never been concerned with inductance. I just look at impedance ratio and power ratings.

To Sluckey's point, the Transformer Maker knows the prices & parts & specs their product needs to have.  So for output transformers, we normally look just at Rated Power, Primary Impedance and Secondary Impedance.


    The primary inductance of the transformer has less to do with "sound" than making sure the transformer actually behaves like a transformer in the desired frequency range.  Mainly, if a transformer only needs to "behave like a transformer" down to 1kHz, that's easier and takes less primary inductance than if it needs to "behave like a transformer" down to 35Hz.  That latter is a much bigger part, with much higher primary inductance.

There are other transformer characteristics, and some will directly impact the sound.  But almost no one publishes all their specs (Hammond is one of the few who do).  Therefore, no sense worrying about those other specs, since they're almost never available.

... I have a question about OT primary if you add power tubes to a design that is already out there, since the primarys ohm helps to set the sound of a amp, even if it would work within margins.

... the Ampeg V4 that runs x4 7027 tubes into a 120W OT transformer with a primary of 2.9K, while their 60w version has a primary of 5.9K.

If i would like to build a Ampeg with x6 7027 tubes i would need a 180-200w OT - but how do i figure out the primary ...

Designers start with "make it work."

A tube is just a "voltage-controlled resistor":  we manipulate the voltage on the control-grid, and the tube respond by acting like more/less resistance.  Along the way, the tube will pass less/more current if the rest of the circuit will allow.

The OT primary impedance is between the tube & power supply:  there will be a voltage-drop across the primary impedance, and that Voltage = Plate Current x Primary Impedance.

If the supply voltage stays unchanged and we want to Double Power, then we need to Double Current.  The same voltage is available to drop, so Same Volts / Double Current = Half Impedance.  And tubes pull but so much current per-tube, so we double-up on tubes.

If the supply voltage stays unchanged and we want to Triple Power, then we need to Triple Current.  The same voltage is available to drop, so Same Volts / Triple Current = Third Impedance.  And we triple-up on tubes.

[PRR]

If i would like to build a Ampeg with x6 7027 tubes i would need a 180-200w OT

There's no free lunch. Don't you also need a 50% bigger power transformer?

Isn't a six bottle Ampeg an SVT? (OK, that's 300W.)

I've run an SVT once. That was plenty.

I've worked with the two-7027 VT-40 combo in small and medium venues and it was TOO DARN LOUD. And burned itself up. And was TOO HEAVY. I rebuilt it from 60W to 25W and it was a lot more useful (true, this was for Jazz not headbanger).

Also, since golden-age 7027s are extinct, I adapted the VT-40 to take the more common tubes (6L6GC, EL34, KT66, etc.) At 25W it was just a bit too wild for 6V6, but I knew good 6V6 would survive a gig.

[Balticnoiseforge]

Indeed PRR - 15w with a mic can do almost all gigs, a 40-50w can do for sure, i would not want any more than a small 50w to carry atleast.

About 50% bigger power transformer, guy i talked to said it would just be slightly bigger - ( 133 laminations ) for a 400vac 700mA 6v3 10a transformer.

And about burning itself up, all V/VT series amps had really high voltages - but wasnt the VT40 especially punishing using a 410vac transformer? So compared to V4 running 550 plate/540 screen the VT was almost close to 600vdc?

And yes a 150w+ is just to loud for pretty much any gig - but hey i like to blast alot of volume when jamming at rehearsal, i often use a 150w x4 kt88 amp paired with a slave amp running 400w solid state power into a huge cabinet.

Is it necessary? No, is it practical? No i cant even move the cab by myself - is it fun? Yes 😅

Also building a amp for myself there is also the enjoyment of building it, and i have never done a x6 amp ( except modifying old sound citys )

For those who mentioned 7027 tubes, i have taken a look at Tung Sols 7027a which seems to be a reliable replica that could do the job.