Neil Young's "Magic" 5E3

[Mike K]

Chuck - I think this particular OT was very susceptible to any damage. It had a an incredibly high impedance, it had 3 secondary winding taps, and the absolutely smallest core imaginable. Think a champ OT, but a bit shorter and wider. All this winding packed into a very small space with a core that could have easily been overloaded. I think somewhat of a perfect storm should something shift the balance slightly.

Most OTs are probably not in this level of danger zone.

[Chuck H]

Chuck - I think this particular OT was very susceptible to any damage. It had a an incredibly high impedance, it had 3 secondary winding taps, and the absolutely smallest core imaginable. Think a champ OT, but a bit shorter and wider. All this winding packed into a very small space with a core that could have easily been overloaded. I think somewhat of a perfect storm should something shift the balance slightly.

Most OTs are probably not in this level of danger zone.

I once built an amp using parts from a VOM (Voice of Music) phonograph that I picked up left for curbside garbage pickup. The unit had two 12ax7 tubes, a pair of 6V6's, a 5y3 rectifier and a P12R speaker. After building a cabinet and making it into a small 1x12 single channel Fener-ish combo it had only 285Vp and sounded "good" but sort of plain and workman like. I made a few circuit mods to compensate for the small cabinet and bumped the rectifier to a 5V4. Now it had a much better tone both clean and overdriven. The P12R was an unusual model with a convex, bell shaped cone like a brass instrument and with the new voicing and the higher plate volts (now 305) the lows would undulate like a water in the ears effect and the high end chimed like bells. The overdrive tone squeezed up nicely and sounded like a raucous cello with smooth (though not particularly defined) lows and a nice, bristly high end with no uglies that dipped enough into the high mids to keep good definition on the low notes. Overall not exactly "my" tone, but I knew what I was hearing. It was really good. The amps future story is unimportant now, but...

In the end I saved the OT and I still have it. It's tiny! Only noticeably bigger than a Champ OT. But it delivered the goods without failing. So I kept it around with the idea to have it reverse engineered if it ever came to that. This would be an example of a small OT that has all the frequency response it needs for clean tones but tightens up a little when the amp is pushed. Sort of what I was thinking about WRT the standard 5E3 OT. That is, improving it may not improve the amp as a whole.

[Mike K]

I once built an amp using parts from a VOM (Voice of Music) phonograph that I picked up left for curbside garbage pickup. The unit had two 12ax7 tubes, a pair of 6V6's, a 5y3 rectifier and a P12R speaker. After building a cabinet and making it into a small 1x12 single channel Fener-ish combo it had only 285Vp and sounded "good" but sort of plain and workman like. I made a few circuit mods to compensate for the small cabinet and bumped the rectifier to a 5V4. Now it had a much better tone both clean and overdriven. The P12R was an unusual model with a convex, bell shaped cone like a brass instrument and with the new voicing and the higher plate volts (now 305) the lows would undulate like a water in the ears effect and the high end chimed like bells. The overdrive tone squeezed up nicely and sounded like a raucous cello with smooth (though not particularly defined) lows and a nice, bristly high end with no uglies that dipped enough into the high mids to keep good definition on the low notes. Overall not exactly "my" tone, but I knew what I was hearing. It was really good. The amps future story is unimportant now, but...

In the end I saved the OT and I still have it. It's tiny! Only noticeably bigger than a Champ OT. But it delivered the goods without failing. So I kept it around with the idea to have it reverse engineered if it ever came to that. This would be an example of a small OT that has all the frequency response it needs for clean tones but tightens up a little when the amp is pushed. Sort of what I was thinking about WRT the standard 5E3 OT. That is, improving it may not improve the amp as a whole.

Yes - that IS entirely true, and true of this particular OT in my experience. It had such a presence built into that it sounded quite nice when pushed, although it did have a limit where it fizzled out and kind of lost composure.

There likely is some affect of that nature with the 5E3 but IME it still has far too much bass to really tighten up. It sounds like a broken amp if you wail on it - at least mine always did and every report I've read says the same about originals. They require a certain amount of finesse.

Perhaps my thought is not to emulate the 5E3 which I don't love though but rather tighten up the whole thing from the preamp and let the mids and highs compress and the bass come through with some clarity. I know that's not Neil's thing, but perhaps the practical way to make this circuit sound good.

I did this quite a lot with Tweed Princetons (I have much more experience with them, and it's kind of the same deal except single-ended). I like the stock OT but I found later on that it sounded better to me with a slightly larger core and some treble boost up front. You could still coax the original tone by tweaking the tone knob but to me it much less limitation. And my goal with those amps was always to make them as versatile as possible without making them complicated.

[Chuck H]

I actually think you're on the right track here. And an even higher primary might not be a bad thing. Like 10k. Helmholts is going to warn against how that could compromise the screens (bless him) but while researching emulation of the original 5e3 OT I came across one after market vendor (Pacific Audio) that claimed to have reverse engineered the OT from a particularly good sounding original and their primary spec was 12k!?! Screen circuit modifications could be employed to compensate if there was any reason for concern in testing. 12k seems absurd to me but maybe 10k? I mean, if the reverse engineered OT from a particularly good sounding example measured 12k then...?

This may seem arbitrary but in my own experience even guitar amps, with the propensity to crank them, seem to sound best when the ideal audiophile primary Z is used. An exception might be the Trainwreck Express which I've read has a 6.6k primary on a pair of EL34's. I haven't built one of these either, shame on me

Also FWIW that little OT in the VOM build I mentioned above spent A LOT of time in the amp fully cranked. I wasn't going to go into the history, but... It was revoiced as a harp amp for a good friend that used it off and on for years. Always cranking it for some grind and blowing old, vintage mic's into it. Would I do the same with an attenuator? Well, no. Though I don't believe it would blow it up I wouldn't take the chance. Just what is it about THAT OT? Maybe someday I'll know. Right now it's still right as rain sitting in a box. IIRC I did measure the primary on that OT and it was, sure enough, the ubiquitous 6.6k. But this would be for use in a relatively low voltage application.

A lot to consider. It would be easy enough to try a 5e3 at a higher primary by simply plugging it into a higher speaker load. See what happens. That sort of thing. As Enzo, member emeritus due to mortality but still SOOO important here would have said... "Never waste time thinking up reasons to NOT test something."

[Mike K]

So yeah, as you know from my 10W PP thread, I've worked with 10K OTs. The first I sourced from the GA-5 crest, which is quite small, but actually sounds good. I mentioned I had a second amp like this - well I didn't use the same OT for that one. For that I used a Supro 30W OT meant to run two Class A 6L6s at 5K into 4 ohms. Well I simply run it as a 10K into 8. That OT sounds quite good too, and would be more than worthy of 5E3 level voltages.

I'm not that worried about the screens. On my SE amps I'm running 12K and 10K for EL84 and 6V6 respectively (and could run 6V6 at 12k) and I've had no screen failures (not scientific, but I'm willing to take the risk ) . I do use screen resistors to help protect them a bit, but I can tell with the 12K I'm pulling a lot more screen current than a lower load in that I need to use a fairly large capacitor for that reservoir or the distortion will suffer (it gets a crackly harshness to it). I used a choke once too and I think that worked but I can't recall the exact details. Capacitors are cheaper these days and I only use a choke when I need to have minimal voltage drop and high regulation, like on those 10W amps that are already running super low voltage.

[Tom Phillips]

I once built an amp using parts from a VOM (Voice of Music) phonograph that I picked up left for curbside garbage pickup. The unit had two 12ax7 tubes, a pair of 6V6's, a 5y3 rectifier and a P12R speaker. ... I saved the OT and I still have it. It's tiny! Only noticeably bigger than a Champ OT. But it delivered the goods without failing...

That was quite a find considering the transformers, tubes and speaker. Those small core OTs were fine for a home phonograph of the day. They just were not designed to be "HiFi" and would really be put to the test in a guitar amp.At the other end of the spectrum there was the massive OT of the 30 Watt (rated) mono McIntosh MC-30. It took my young self quite a while to realize why the MC-30 was so much more expensive than the 30 Watt amps in the Radio Shack Catalog.

[Chuck H]

That was quite a find considering the transformers, tubes and speaker. Those small core OTs were fine for ahome phonograph of the day. They just were not designed to be "HiFi" and would really be put to the test in a guitar amp.At the other end of the spectrum there was the massive OT of the 30 Watt (rated) mono McIntosh MC-30. It took my young self quite a while to realize why the MC-30 sold for so much more expensive than the 30 Watt amps in the Radio Shack Catalog.

I have a couple of fat books about tube audio design from the heyday. It's remarkable the detail and trouble some companies went to to try and make things work with the tubes of the era. Trifillar wound transformers with dedicated secondaries for both local and global NFB with eleven interleaves! All for an improvement of a few per cent. But when you're THE top dog in the game you can't afford to let the competition get ahead of you. Those transformers are absolutely unobtainable now. Rendering all those designs based on them obsolete. Unless you have an original in good working order you'll never even get to know what it meant!

[g1]

I came across one after market vendor (Pacific Audio) that claimed to have reverse engineered the OT from a particularly good sounding original and their primary spec was 12k!?! Screen circuit modifications could be employed to compensate if there was any reason for concern in testing. 12k seems absurd to me but maybe 10k? I mean, if the reverse engineered OT from a particularly good sounding example measured 12k then...?

A little off topic, but I always consider all the old amps with ext.spkr. jacks on single impedance secondaries (like 5E3). In the case you mention, the primary Z would be 'wrong' in normal use but correct when using an ext.spkr of proper impedance.
How about regular amps that spent their lives being used with ext.spkrs. at 'wrong' (half) primary Z? Is this less worse than no ext.spkr but double the primary Z?

[Helmholtz]

As long as supply voltages (especially screen voltage) at full power are within or below tube limits, there is some freedom regarding load impedance.
Apart from tube type/limits optimum plate load depends on plate and screen voltage as well as operating class and biasing type.
For the amp in question an Raa of 8k to 10k seems perfect. 20% more or less shouldn't be a problem at all.

Regarding Raa measurement, calculating Raa from the voltage transfer ratio doesn't give the full Raa as seen by power tubes as winding DCR(s) add.
I prefer to directly measure primary impedance at 1kHz with correct secondary load resistor connected.
Results can be considerably higher than with the voltage method, especially with high impedance, small sized OTs.

[Mike K]

I looked at the calcs earlier for 10k, it looks fine at most voltages and as you might expect, increases the power output as it can be made to go right through the knee of the g1=0 curve. By that logic it could output more power even at higher voltages in comparison to 8 or 6.6k (at the same voltages).

It actually seems if one wants to lower the output and stay in Class AB, the lower impedance is better. Once you get low enough (on supply voltage) you stay in class A, then the higher impedance (looks) better.


As far as the other question about plugging in "incorrect" loads, not to step on any toes, but I think it depends. Surely going too high could cause higher inductive spikes and potentially an arcing failure or screen damage, particularly if it's already being run near the limit and without screen resistors.

Too low I don't think will cause any damage to the tube or OT, but it could stress the PT by asking for more current than it was designed to provide.

In these conversations on past forums such as AX84 I recall it always being advised to use a lower load than higher, if a mismatch is required. I think that works most of the time, but perhaps you can get away with higher loads depending on the specifics of the design.

When I do this, I always make sure to match the design impedance. So if I'm going to run 2 speakers into my 8 ohm out, I make sure I'm running 2x4 ohm in series or 2x16 in parallel. I do on occasion run a 6 ohm speaker into an 8ohm tube output and it's never been an issue.

[Mike K]

.
I prefer to directly measure primary impedance at 1kHz with correct secondary load resistor connected.
Results can be considerably higher than with the voltage method, especially with high impedance, small sized OTs.

Could you outline this test procedure?

[Helmholtz]

What makes things worse for the screens (at high Raa) is that speaker impedance strongly increases at higher frequencies (and there are lots in a clipped signal).

Regarding (dynamic) plate dissipation, looking at the loadline in a chart that shows the plate limit parabola easily reveals that dissipation increases at lower load impedance.

[Helmholtz]

Could you outline this test procedure?

Using my LCR meter to measure AC (or series) resistance in series inductance mode.

[Mike K]

Using my LCR meter to measure AC (or series) resistance in series inductance mode.

I do not have one, but I assume I could use my scope and signal generator and a parallel capacitance. I'm not sure how accurate this might be because I'd be assuming the nominal value of the capacitor. Or I suppose I could set up a RC network and determine the value of the cap to a higher accuracy.

[Helmholtz]

I do not have one, but I assume I could use my scope and signal generator and a parallel capacitance. I'm not sure how accurate this might be because I'd be assuming the nominal value of the capacitor. Or I suppose I could set up a RC network and determine the value of the cap to a higher accuracy.

Not sure what you mean.
If you're thinking about a resonant circuit, the effective resistance would show as the impedance minimum in a series resonant circuit.