Output Transformer Saturation Surpise

[nickb]

I must admit I've tended to take the line that you won't usually see saturation of the output transformer in the typical guitar amp. Looks like I may be wrong.

Case in point is a super simple Fender Custom Shop '57 Champ 5F1, SE 6V6 class A. By the by, the biggest shock to me was the list price at some $1,200 or so. Ouch! While checking the power out which I usually do at 1Khz but I also tested at 100Hz (as the owner said it distorted on low notes) into a resistive 4 ohms. The power out dropped to a rather feeble 1 watts clean(-ish) at 100Hz. I if turned the signal up the waveform got really ugly, not what I'd expect to see at all.

I subbed in a new output transformer, just in case, and got the same result. So, I hooked up a probes for the output waveform and the plate current and this is what I saw at a lowish 0.6W output.




Turn it up a bit to a nominal 2.5W and the plate current shoots up with an ugly output waveform, indicating saturation:




The manufacturer quotes -1dB @ 70Hz with 187Vrms into 7k ohms (== 5 watts out) so this was rather surprising to me.

I think there might be two things going on. One is that as the primary drive hits the limits and the voltage is no longer a sine wave so the primary volt-seconds goes up so increasing the flux in the core. Once the core starts to saturate things get rapidly worse and with the feedback not helping the situation at all. The other thing is that the transformer spec is exactly as written i.e. with no DC current present. That in effect roughly halves the SE voltage swing limit and means 1/4 of the output power. Putting it another way, below 140Hz you can expect this particular transformer to saturate.

I guess that is the secret, for good or bad, of the '57 Champ.

Possible solutions?
1) a bigger OPT
2) lower the idle current so you get a better LF response at the expense of less 1KHz output power
3) increase the corner freq of the drive to the output tube grid to make it harder to get the nastiness on lows during real world playing
4) removing the feedback should make things drop off in a more progressive way.

[Chuck H]

I've experienced what you're writing about. My analysis was a bit less technical than yours though and amounted to me surmising "Shitty little OT."

And I think the "secret" to the Champ's tone was little more than players could crank them without disrupting all life for a city block. In a studio there is typically ample opportunity to change the EQ of a track. I remember hearing EVH's Marshall when he appeared as a guest on SNL. He actually had his half stack on stage with a mic in front and, I have to assume, a flat EQ on the board. Compared to recordings and VH live shows it was strangely thin and one dimensional with more hashy static tone on the top end. Not nice. You could tell it was HIS tone, but with no makeup on as it were. I think the common reference to these old Champs as studio tools is like that since I've never heard one that was really good in person. Well... Except for the one I changed the OT on.

[Gregg]

My analysis was a bit less technical than yours though and amounted to me surmising "Shitty little OT."

I'll join your less technical analysis on this one. It's a small crappy OT on top of that it's a SE OT so it's kind of supposed to saturate.
If you want Champ tone you should use that OT. If you want cleaner and better frequency response you should get a "normal" SE OT.

[J M Fahey]

The manufacturer quotes -1dB @ 70Hz with 187Vrms into 7k ohms (== 5 watts out) so this was rather surprising to me.

From my very beginnings I fought Transformer manufacturers on similar grounds.
They called me ugly things because "you are the only one who complains" but in fact "I was the only one who actually measured and scoped stuff".
So I eventually decided on winding my own, go figure.

Something I found out early on, is that manufacturers only "know" mains frequency sinewaves, and NO , repeat NO DC component.
Oh, and they *only* know about resistive loads.

I am certain that on his workbench he´ll be able to set a Variac to 187V RMS, no DC, and the transformer will perform like he states ... so what? ... it´s a f*ck*ng SE Class A OT!!!!!!!!!

That said, your tube is badly saturating, and unsymmetrically, so even on a resistor current would be unsymmetrical ... let alone on a near-to-saturation iron core.

Well beyond the call of duty, but if you want to improve your testing, split the DC and AC components in two: on one side apply 58 mA DC (or whatever idle current you are using) fed from, say, the drain of a MosFet (which is a constant current source) with some 200/250VDC at the other end of the winding, and apply , say, 5V RMS at the secondary side from any amplifier, a chipamp is fine, then monitor (attenuate and scope) waveform at the MosFet drain .

That way you get the waveform actually "transmitted" by the transformer, with realistic voltage and current values.

Just talking aloud, you won´t go into that trouble justb to check a customer´s amp.

I did use something similar to check my own power transistor driver transformers (think early VOX/Acoustic/Standel type) which of course works at much lower (and less dangerous) levels.

PS: now that I think of it, you might repeat that test using the 6V6 itself, a pentode is a high voltage current source if I ever saw one.

[Leo_Gnardo]

I'll join your less technical analysis on this one. It's a small crappy OT on top of that it's a SE OT so it's kind of supposed to saturate.
If you want Champ tone you should use that OT. If you want cleaner and better frequency response you should get a "normal" SE OT.

That right there ^^^ what Greg says. SE OT's that have better low frequency response cost a lot more than a Champ OT, are also bigger & heavier. And as Juan points out, you could send only the AC component to the OT. I haven't tried that solution but I have seen hi fi gear, also the output section of old tube Pultec EQ's use that scheme to keep DC out of the OT's primary. Hi Fi, quite so, and they sell for gold dust money.

[Malcolm Irving]

I did some very similar tests, reported and analysed in Chapter 8 of ‘Guitar Amplifier Overdrive’, Neumann and Irving, 2015.

I don’t think the transformer is actually saturating. At low frequencies the reactance of the primary inductance (also known as the magnetising inductance) drops below the proper load impedance (reflected through the OT from the secondary). (The proper load and the magnetising reactance are in parallel.) This means that the output tube is trying to drive into a very low impedance. What is nominally ‘a 5 watt amplifier’ then becomes (say) ‘a 0.5 watt amplifier’ and it is the tube itself which is causing the distortion, when you try to drive it a bit harder.

[Malcolm Irving]

The frequency at which the OT magnetising reactance equals the 'proper' load impedance defines the -3dB point for the bass response of the output stage of the amplifier.

[trobbins]

So NickB could check the primary current waveform when driven from a mains variac stiff voltage supply and probably a stepup transformer, to see when magnetising current waveform starts getting peaky (assuming rms primary current doesn't overheat the winding) ? That could shed some more light.

[Malcolm Irving]

So NickB could check the primary current waveform when driven from a mains variac stiff voltage supply and probably a stepup transformer, to see when current waveform starts getting peaky (assuming rms primary current doesn't overheat the winding) ?

Yes, what is needed to saturate a transformer is a high enough current through the magnetising inductance. This corresponds to low frequency (so that the reactance of the magnetising inductance is low) and a high primary voltage at that low frequency. A tube is not usually capable of providing a high enough ac voltage across the primary at low frequencies (where the magnetising reactance has collapsed). Something like a mains supply (with variac etc.) can do it and you then see a saturated current waveform, which has a different shape.

[nickb]

That right there ^^^ what Greg says. SE OT's that have better low frequency response cost a lot more than a Champ OT, are also bigger & heavier. And as Juan points out, you could send only the AC component to the OT. I haven't tried that solution but I have seen hi fi gear, also the output section of old tube Pultec EQ's use that scheme to keep DC out of the OT's primary. Hi Fi, quite so, and they sell for gold dust money.

In point of fact these are these are sold as SE transformers. That's the whole point. The specs are quoted w/o any DC and so are wildly misleading

[nickb]

So NickB could check the primary current waveform when driven from a mains variac stiff voltage supply and probably a stepup transformer, to see when magnetising current waveform starts getting peaky (assuming rms primary current doesn't overheat the winding) ? That could shed some more light.

I really need to impose DC on the winding otherwise I'm just repeating the manufacturer's test. If you think of it that's exaclty what I already did using a 6V6 as the current source.

[nickb]

I did some very similar tests, reported and analysed in Chapter 8 of ‘Guitar Amplifier Overdrive’, Neumann and Irving, 2015.

I don’t think the transformer is actually saturating. At low frequencies the reactance of the primary inductance (also known as the magnetising inductance) drops below the proper load impedance (reflected through the OT from the secondary). (The proper load and the magnetising reactance are in parallel.) This means that the output tube is trying to drive into a very low impedance. What is nominally ‘a 5 watt amplifier’ then becomes (say) ‘a 0.5 watt amplifier’ and it is the tube itself which is causing the distortion, when you try to drive it a bit harder.

It is a factor, but it not the main factor in my view. The spec states 88H magnetizing inductance at 187V rms (no DC). You assert it is not saturating so the permeability will not have dropped too much so let's conservatively pick the inductance as 40H.

The reflected load is 7k/3.2*4 = 8.75k so the -3db point is 35Hz. Looking at it differently at my test frequency of 100Hz the load impedance had dropped roughly from 8.75k to 6.4k. So the tube works harder, distorts more, increases the volt-seconds and the core starts to saturate.

[nickb]

Juan - I've found it very hard to get much concerete out of them either.

[Malcolm Irving]

... so the -3db point is 35Hz.
... .

OK, fair point, the -3dB point for the OT in my tests was 80Hz. I also disconnected the feedback for my tests.
The magnetising inductance that I measured was 9H.

[Malcolm Irving]

A transformer manufacturer can obtain the desired magnetising inductance for an OT, either with a lower reluctance core (more iron) and fewer primary turns, or more turns on a higher reluctance core (less iron). The former would tolerate a higher magnetising current before saturation occurs.