How would you design transformers in such a way so that you can run a tube amp without a load ?

[Cathode Ray]

A guy in Australia is doing this with amps he demonstrates on his YouTube channel.

He's rather cagey about how it's being done.

After watching several hours of him talk about it, here's what he has said..

The pre-amp section of the amp (Marshall 1959 circuit) is stock.
There are no dummy load resistors/inductors inside the amp.
The amp has only a line-out jack on the rear.. no speaker out jacks.

All he will say is that the transformers in the amp are "custom made".

In his latest video he alludes to it having something to do with the phase inverter being overdriven.

[TUBEDUDE]

If there's no power section it's easy. Phase inverter output into a balanced to single ended line level transformer to output jack. Jensen makes good ones for this.

[Cathode Ray]

If there's no power section it's easy. Phase inverter output into a balanced to single ended line level transformer to output jack. Jensen makes good ones for this.

Holy SMokes..

That never occurred to me..

So, he's not even using any power tubes or an OT in the damn thing ?!?!

[Stevem]

Correct.
The output stage and output tubes in particular have very little gain as compared to a preamp section, and the reason for this is that a speaker needs high current but a low voltage to drive it.

Also a output transformer is a step down type transformer and whenever you transform a higher voltage down to a lower one you gain current.

By the way there are tube type home stereo amps that have no output transformer to worry about killing, but even to produce 20 watts of power it takes a lot of output tube to develop the needed current to drive a speaker.

should a output in tube in this type of amp short out you get to hear the pleasure and see the smoke of burning up your speaker on that channel with the full B+ of the amps power supply.

[R.G.]

Yep to that. No output section with feedback, no problem.

It's not really a transformer thing. The reason you should not run a tube amp without a load is that having a high load resistance (including especially an open circuit) runs the effective voltage gain of the output stage up, and if there is any feedback this can cause the output stage to oscillate. Massive oscillation can cause voltage spikes on the primary side of the OT and kill the output tubes, arc their sockets, and puncture the insulation of the OT. No feedback, probably safe to run without a loading resistor.

I can't picture a way to design an OT that would not give this issue in a typical output stage with feedback. So the fellow has deleted the output stage. No problem.

[Cathode Ray]

Awesome guys ! Thank you !

It was misleading the way he's doing the videos, because you're looking at him playing thru (what looks like) a Marshall 100w head..

And he's being intentionally misleading dropping these little "hints" as to what he's doing.. "custom-wound transformers", "distorting phase inverter tube", yadda, yadda..

If he had just said, "It's a pre-amp" that would have been that.

[Cathode Ray]

linked at time stamp.. 11:40 in video incase that isn't working.

guy's saying he has output tubes in the amp.

[maxkracht]

Couldn't handle listening to that guy for more than a couple minutes, but I don't think your timestamp said he was using that power tube. He just said, "here's like a tube" and "the hotter the tube gets, the easier it is for all the coatings on the plates right, going through grids and cathodes, all the different elements of the tube, they become more reactive, right?" Just sounds like word salad to me. Guy is trying to sell stuff. I don't think his mystery load is adding all that reverb, so there is clearly some external processing involved.

Assuming it made sense to use a Variac in this way in the first place, it doesn't make sense to continue using a Variac after you find a setting you like. Why not use his custom wound transformers to emulate where the Variac was set?

Apologies to that guy if I am misunderstanding something, but seems fishy to me.

[Cathode Ray]

Couldn't handle listening to that guy for more than a couple minutes, but I don't think your timestamp said he was using that power tube. He just said, "here's like a tube" and "the hotter the tube gets, the easier it is for all the coatings on the plates right, going through grids and cathodes, all the different elements of the tube, they become more reactive, right?" Just sounds like word salad to me. Guy is trying to sell stuff. I don't think his mystery load is adding all that reverb, so there is clearly some external processing involved.

Assuming it made sense to use a Variac in this way in the first place, it doesn't make sense to continue using a Variac after you find a setting you like. Why not use his custom wound transformers to emulate where the Variac was set?

Apologies to that guy if I am misunderstanding something, but seems fishy to me.

It's got me intrigued.

He's running the amp's line out into his DAW and using impulse responses.. according to what he's saying.

Same questions about using the variac and "custom transformers", although I have seen that done before (custom wound PT & variac).

I had to search around thru several videos to find him actually saying he's using output tubes.. but that's his assertion here.

[maxkracht]

Using impulse responses for a custom reverb, or some kind of amp modeling?

If I thought he was being honest, and it really is a stock Marshall preamp, and there is actually something special in the amp, I would suggest he might be doing something like running the Marshall preamp into some small signal pentodes. More likely, he isn't the keeper of some esoteric information but is good at using the right buzz words to keep people interested in the mystery box. Just my first impression, don't mind me.

[Cathode Ray]

Using impulse responses for a custom reverb, or some kind of amp modeling?

If I thought he was being honest, and it really is a stock Marshall preamp, and there is actually something special in the amp, I would suggest he might be doing something like running the Marshall preamp into some small signal pentodes. More likely, he isn't the keeper of some esoteric information but is good at using the right buzz words to keep people interested in the mystery box. Just my first impression, don't mind me.

There's nothin' new under the sun.

I don't think for a minute, he's doing something Ken Fischer or Chris Merren (or whoever) hadn't figured out decades ago..

I just want to understand what he *is* doing here.

How is he running a 100w amp with no load ?

[maxkracht]

If there isn't any signal going to the power tubes, you can run them without a load without changing anything. Maybe he kept them in so he didn't need to change the power supply. Or, maybe he just renamed the Van Halen patch on his POD "stock Marshall preamp with custom transformer".

[Cathode Ray]

If there isn't any signal going to the power tubes, you can run them without a load without changing anything. Maybe he kept them in so he didn't need to change the power supply. Or, maybe he just renamed the Van Halen patch on his POD "stock Marshall preamp with custom transformer".

He's selling the amps. He actually builds a lot of pre-amps and audio devices, so it's not my contention that he's being deceptive about this..

I'm just trying to edify myself as to how it's possible to run a tube amp with no load (speakers or resistive/reactive).

[maxkracht]

Take a stock Marshall, install a transformer after the PI going to an output jack, leave the power tubes on but not connected to an audio signal. I think that fits all the criteria.

[R.G.]

I'm just trying to edify myself as to how it's possible to run a tube amp with no load (speakers or resistive/reactive).

Open the feedback loop, if there is one. The AC30, for instance, does not have output stage feedback.