Speaking of transformers: instrument amplifier output transformers

Brinkman

So I’m considering building a clone of an old fender guitar amplifier which has me visiting other forums. I’m noticing this peculiar habit people who work on/collect guitar amplifiers have of characterizing an output transformer by its secondary winding alone. In other words, they will speak of it as having an “8-ohm output” or 4 or 16, whatever is the case. Now, my more technical understanding is that transformers are characterized by their impedances which are determined by their turns/winding ratios; ie a 4000 ohm to 8 ohm output transformer has a winding ratio of roughly 22.4:1.

Anyhow, this got me thinking: are there instances where a 4000 ohm to 8 ohm is not also a 8000 ohm to 16 ohm transformer? I mean, I understand other requirements such as bandwidth and power handling are going to dictate core size and winding technique. What I’m asking is is there any such thing as a “16 ohm output”?

AllenB

Paid Member
Yes, you can do that if you account for the band over which you want it to be used, for example by noting the primary inductance. In any case, instrument transformers may already be low in this area.

WhiteDragon

What I’m asking is is there any such thing as a “16 ohm output”?
Yep, and rather common.

Transformer are wound to a ratio.
So changing the impedance on one side
is reflected on the other.
Aside from losses the impedance is not set.
The ratio is. change one side the other side changes too
according to the winding ratio.

Current is more a issue, so trying to make a 4 ohms tap
2 ohms is possible.
But 2 ohms is very high current so usually more
thicker wire is needed

Far as using 8 ohm tap as 16 ohms.
16 ohms less current so wont be issue.

You just have to calculate the ratio
to see what the primary / tubes will see
if you running different load on secondary.

basic math if you doubling or halving the load.

incredible basic common guitar transformer.

8 ohm output tap, and primary would be 3k

4 ohm load, primary be 1.5 K
8 ohm load , primary be 3 K
16 ohm load, primary be 6 K

impedance not set, the ratio is.

as mentioned a 4 ohm load = more current.
so usually the winding be designed to handle
more current.

So better idea to change 8 ohm tap to 16 ohm.

Why many many old Fenders where just a 4 ohm
output. The windings where designed for biggest load.

No problem to run 8 ohm or 4 ohm load.
Of course the primary changed.
common 6L6 pair for Fenders.
6L6 pair does fine on 3.3 to 6.8K
least for Guitar bandwidth/ distortion levels.

So weasel cheap transformer be 3.3k primary 4 ohm tap
be fine with 8 or 4 ohm load.

Of course there is plenty of after markets with multiple taps.
Easiest off the shelf transformer is " Marshall " style
should have 16 , 8 and 4 ohm taps

or just a dual tap transformer. with 8 and 4 ohms.
You could use 8 ohms as 16 ohm. and 4 ohm as 8 ohm.

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NareshBrd

The above post by 'White Dragon' is comprehensive.

But please state your real issue, or was it a theoretical question?

Also, note that impedance and resistance are two different things, the transformers can be wound to have a tuned frequency response, if required.

WhiteDragon

Well with "HiFi"
most are shooting for bandwidth / low distortion.

Guitar amp usually shooting
for max power...sometimes
Usually just playing with plate voltage
and impedance to weasel as much power
as possible.

Far as tube life and not blasting tubes
at max plate.
Easy case is examples of " 30 watt"
Amps with 6L6 pair. Or " 50 watt"
getting blasted max plate.
Rather have " 30 watt " ( 23 to 18 watts) 6L6
and if you want 50 watts go up to 6550 pair
for 60 to 80 watts. And be well under
max plate/dissipation. AKA Sunn figured it
out long ago and didn't blast 4 tubes.
6550 pair and all done.

And yes 32 or 16 ohms not common
with MI applications.
But 32 and 16 ohms tap wire gauge
much easier to deal with at lower
current. 8 and especially 4 ohm
taps more wire gauge. 2 ohm

At least marshall figured it out
make 16 ohm 4x12
and 2 cabinets or 8 speakers
on a pretty cheap 8 ohm
transformer.

Brinkman

Hopefully this clears some things up:

My question came about because a particular Fender output transformer (the 45550) was developed to couple a nominal 16-ohm driver (JBL D130) to one pair of 6L6 push-pull power tubes seeing 4K anode-to-anode; so a 4K::16 output transformer.

That same transformer was used to couple a pair of those same nominal 16-ohm drivers (in parallel, so a nominal 8-ohm load) to two pairs of 6L6 power tubes, seeing approximately 2K anodes-to-anodes (2K::8 output transformer). Sounds good so far.

The trouble is: JBL was lying about the 16-ohm D130 rating; it was the same exact voice coil as the 8-ohm D130. I own a “16 ohm” D130 and verified this. This means what’s being referred to in guitar circles as a “16-ohm output transformer” was actually a 1k::4 ohm and 2K::8 ohm output transformer. Strikes me as strange to call it simply a “16 ohm output transformer” when that doesn’t tell you anything and in this case isn’t really how it was even used.

So my question isn’t can a 2K:8 ohm output transformer be used as 4K:16 ohm? I know it can (and was).

My question is when is it more appropriate to characterize an output transformer with a 22.4:1 turns ratio as a 16-ohm OT rather than a 4- or 8-ohm OT?

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WhiteDragon

Speaker coil be wound to stated impedance.

More than likely a 8 ohm re cone used.
And the original 16 ohm label wasn't changed.

16 ohm speaker just DC measurement might
be around 12 to 10 ohms at lowest.

Doubt fender would run a pair at 2k
more like 3k

Fender kept current rating/ wire gauge
at good levels.

As mentioned at the least , that transformer
be designed for highest current load.
8 ohm.

And would be run at 16 or 8 ohm
for the usual 3k / 6k you can use a 6L6 pair
for.

Would have to look at schematic of amp.
might even be multi taps been changed with
switching jack on output.
Some later amps in 80's would have series/parallel
switching jacks to get 2x 4 ohm cabinets on a 8 ohm
tap.
Easier to do as a " quasi" 2 ohm tap.
Which nobody wants to deal with wire gauges
for actual 2 ohm tap. Easier to do 8 ohm

WhiteDragon

As mentioned
aside from reflected impedance change.

Majority of the concern is current
and wire gauge.

8 ohm tap being run at 16 ohms
is fine. 16 ohms is less current.

If you tried to run 8 ohm tap
at 4 ohms. Could be a issue
4 ohms is more current.
So wire gauge usually need to be larger.

lots of mistakes. tried to weasel transformers
with to much voltage and current.
fried often. Not to mention the amp
was very unstable as well.
They just want to die, worst amp ever.
Big oscillator with garbage transformers
and too much voltage.

Brinkman

My source regarding JBL lying about the nominal impedance rating is the designer himself, Harvey Gerst:

Back then, we lied.

Even though they're marked as 16 Ohms, they were all really 8 Ohm speakers. The DC resistance should be around 70% of the nominal rated impedance, so an 8 Ohm speaker should read about 6 Ohms (or so) DC. Ignore the 16 Ohm decal; it was really an 8 Ohm speaker. It hasn't been reconed.

WhiteDragon

Woah !!

Wow. interesting.

Suspected same thing with some Ampeg " special design" speakers
rated at some magical 6 ohms.
Possibly just plain old 8 ohms and using DC rated 6 ohm.

Fender engineers might have been aware or unaware
of the 16 / 8 ohm JBL thing.

Great thing about what i'm guessing heritage forums.
There is many old posts from actual JBL engineers.
Finally settled old myths about " F " series drivers
and what was actually done.

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Brinkman

Yes, you can do that if you account for the band over which you want it to be used, for example by noting the primary inductance. In any case, instrument transformers may already be low in this area.
The inductance of the primary determines the low frequency -3dB of the primary’s bandwidth. Is the bandwidth of the primary fixed with regard to the load on the secondary?

AllenB

Paid Member
Yes, in the sense that the impedance transformation is fixed by the turns ratio.

Rikaro

The inductance of the primary determines the low frequency -3dB of the primary’s bandwidth. Is the bandwidth of the primary fixed with regard to the load on the secondary?
No, the lower corner frequency is the frequency at which the reactance of the primary inductance equals the reflected impedance at the primary.
So if reflected impedance Raa is doubled, you would need twice the primary inductance for the same corner frequency.
Otherwise you will lose one octave of bass.
This is because changing the secondary load resistance doesn't change primary inductance.

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Brinkman

So other considerations aside, coupling an 8-ohm load to a 16-ohm secondary technically extends the primary lower corner frequency an octave?

Rikaro

So other considerations aside, coupling an 8-ohm load to a 16-ohm secondary technically extends the primary lower corner frequency an octave?
Yes, but transformer copper losses will increase by a factor 4 because of doubled currents.
OTs are optimised for their rated load impedance.

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Brinkman

So it sounds like we can characterize a transformer by its secondary impedance if the intention is to preserve a specified bandwidth with the lowest losses.

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Yep.

Brinkman

Fender engineers might have been aware or unaware
of the 16 / 8 ohm JBL thing.
Hard to tell either way. We don’t know have the paperwork to know how the transformer was intended to designed. All we have in people calling it a “16-ohm output transformer” possibly under the mistaken belief that’s what it was because it was intended for use with a deceptively labelled 16-ohm driver. If Fender knew the truth of that matter, then why would they disastrously use it with a dual Showman? Seems like they would want to have avoided all those headaches.

Rikaro

Acc. to Dave Funk's book the 45550 was used in the Showman 6G14 with 4x5881s and 8 Ohm output as well as with the Vibrasonic 5G13 using 2x5881s and a 16 Ohm output.
Seems to make sense.

Brinkman

And both guitar amplifiers used the D130. One in the Vibrasonic - an 8-ohm load - and two in parallel for the dual showman, a 4-ohm load. Hence all the confusion. Doesn’t seem like Fender was aware of what JBL was up to which would explain why the Showman OPT required a new design after problems occurred when using the 45550.