Big Iron or Oversized Transformer

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idnotbe
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Big Iron or Oversized Transformer

Post by idnotbe »

i am really a newbie in DIY world.

1. when someone says "big iron" or "oversized transformer" is better, is it a power transformer or a output transformer?

2. if i want 50w loudness / headroom but 100w big iron effect, there seems a few alternatives.
for ex, (1) 4 * EL34 but 50w actual output, (2) 2* EL34 with a 100w output transformer, (3) 2 * EL34 with 100w output & power transformers.
what are the pros & cons?

3. what else should i consider when implementing the mismatched transformers?
for ex, can i just order to a transformer company and say "hey, i am making a 50w but will install oversized 100w irons. give me the proper ones." ?
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xtian
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Re: Big Iron or Oversized Transformer

Post by xtian »

The only reason you might want an over spec'd PT is to eliminate DC supply sag. Bass players don't want sag. Some guitarists do. Are you looking for a super stiff power supply with no sag?

But still. I built a 50-watt Marshall 1987 circuit ("four holer", "plexi"). SS rectified, two EL34s, ClassicTone PT meant for two EL34s. No noticable sag. Unless, of course, if you dime the thing and play hard, which I can't, because ouch that's loud. There's no point in using a 100-watt PT in this design; you'd never know the difference.

So, before we proceed, are you barking up the right tree?


What kind of amps have you played and liked? Disliked? Hiwatt DR104 (stiff)? Tweed Deluxe (saggy)?
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Bombacaototal
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Re: Big Iron or Oversized Transformer

Post by Bombacaototal »

And for a 2x 6L6, 50W amp which oversized OT would you recommend?
100w twin? Or something on the 75W like a musicman?

I used a Vibroverb 40w OT on my 25W amp and loved the result! So I am thinking about replicating this on the 50W
ChopSauce
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Re: Big Iron or Oversized Transformer

Post by ChopSauce »

On a related matter:

- any opinion(s) about the Hammond audio 1650_ series?

These also seem to be big iron(s). At least they weight as such.
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Plexified
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Re: Big Iron or Oversized Transformer

Post by Plexified »

Ok , to generalize small iron power transformer = a spongey feel , add a tube recto and its more so.
Go with small iron output and you get saturation as you max it out = blurrrr and compression and that means a high freq cut and a low end cut with character. Big iron in general means more reserve for power and frequency response accross the board. Its a big tuning meat and potato thing. We can talk more , knowing more. Spicey kitchen stuff. :wink:
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romberg
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Re: Big Iron or Oversized Transformer

Post by romberg »

idnotbe wrote: Thu Jun 15, 2017 11:59 pm 3. what else should i consider when implementing the mismatched transformers?
Randall Aiken has probably forgotten more about amp building than I will ever know:

http://blueguitar.org/new/text/threads/ ... ormers.txt

Also, the power rating is only one factor of many that needs to be chosen correctly or you risk creating a smoke generator instead of a guitar amp. You can't just willy nilly swap say a marshall 100w OT for a 50w if you are driving it with two el34s. The 100w OT will have a primary impedance that would make four el34s happy. If you drive it with just two then the load line of the power section get tilted. One would need to adjust for this by changing the load on the secondary side. Merlin Blencowe has a couple of nice articles that helped me out quite a bit in understanding how to "spec" a transformer:

http://www.valvewizard.co.uk/se.html

http://www.valvewizard.co.uk/pp.html
R.G.
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Re: Big Iron or Oversized Transformer

Post by R.G. »

Plexified wrote: Fri Jun 16, 2017 9:07 am Go with small iron output and you get saturation as you max it out
Actually, no. Or, certainly, not necessarily. Saturation isn't what most guitar world people think it is.

A transformer saturates (= quits transferring energy into the secondary, among other things) when you put all the volt-second integral across it that the iron can stand. That means that a transformer saturates at the lowest voltage drive only at the lowest possible frequency. Low frequency means the longest time spent with the signal on one polarity. The ability to withstand more voltage drive increases linearly with frequency as the time per cycle goes down.

So if you had a transformer that saturated with, say, 300V peak sine drive on its primary at 40Hz, then the same transformer would need 600V of drive at 80Hz to saturate, and 1200V of drive to saturate it at 160Hz. As you can see, things get rapidly out of hand for saturation. I know that people have this whole mythos of the sound of saturation, but really you almost can't saturate an OT with anything but the lowest notes you can generate. I'm sure that somewhere long ago and far away, someone did saturate an output transformer, but it's so rare that it would be really interesting to actually catch that on some instruments.
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Malcolm Irving
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Re: Big Iron or Oversized Transformer

Post by Malcolm Irving »

I completely agree with R.G.

Another factor that makes saturation difficult to reach in practice is that the load on the output tubes is a parallel combination of the OT primary inductance and the referred load from the speaker. The impedance of the OT primary inductance falls away as frequency drops (towards zero at 0Hz). As frequency reduces, the output tubes are running into a lower impedance load and may be unable to provide enough voltage across the OT primary to cause saturation.

This is particularly true with beam tetrode and pentode output tubes, which have a relatively high internal resistance.
Last edited by Malcolm Irving on Sat Jun 17, 2017 9:53 pm, edited 1 time in total.
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xtian
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Re: Big Iron or Oversized Transformer

Post by xtian »

So let me turn this around and ask, is there something special about the tweed deluxe 5E3 and its small OT that lead to this idea of saturated OTs?
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Malcolm Irving
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Re: Big Iron or Oversized Transformer

Post by Malcolm Irving »

A big iron OT will give more output at low frequencies (because the OT primary inductance is increased). This can be desirable for baritone guitars or down-tuned playing.

A smaller OT gives less output at very low frequencies. For many styles of guitar music (IMHO) that can be a good thing, as too much bass can muddy up the overdrive sound.
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Malcolm Irving
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Re: Big Iron or Oversized Transformer

Post by Malcolm Irving »

Another way of looking at it is that a small OT does not produce so much output power at the fundamental frequencies of the low guitar notes (low E to A let’s say). When the output tubes are overdriven, the added harmonics for these low notes stand out (without too much fundamental to muddy them).
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roberto
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Re: Big Iron or Oversized Transformer

Post by roberto »

With oversized transformers you don't get less sag. Sag doesn't come from the OT. Normally the resistance from center tap to anode(s) is around 40 Ω for 50 Watters and 20 or less for 100 Watters, so with the involved currents, the voltage drop is minimal, and mainly due to the rest of the PSU.

If you don't play overdropped guitars, there's not a big impact on saturation itself, as there could be on hi-fi or bass amps.
Often the improvements (may we talk about improvement "per se" in sound reproduction?) are due to a better overall design of the transformer rather than the Gauss the OT is actually working at.

I have to say that bigger is not always better.
Once I substitued the OT of a Laney VC30, and the Customer insisted to install a WAY bigger one, keeping the same pri/sec ratio.
In fact the original one is quite small, but after that substitution the sound became definite in a way that was somehow unpleasant to my hears. Anyhow the Customer preferred the amp that way, so if the Customer is happy, everyone is.
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