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OPT design for guitar amplifier
OPT design for guitar amplifier
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Old 1st June 2018, 09:21 PM   #1
50AE is offline 50AE  Bulgaria
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OPT design for guitar amplifier
Default OPT design for guitar amplifier

Here I'm really wondering.

1. Should OPTs used in a guitar amplifier be deliberately distorting, including a severe HF and/or LF roll-off - in the end, cheaply made?

OR

2. Be as transparent as possible, which means working at a low flux density, high enough bandwidth, low distortion, leaving the later to the driving stages only?

The second makes especially sense, if the amp would be used for a clean channel.

But I would like to hear your opinions, even subjective ones are welcome.
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Old 1st June 2018, 09:27 PM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
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I wish I knew how Marshall got their sound. I have designed and build guitar valve amplifiers and they all sound too clean.
I can only guess something Marshall use something being driven hard somewhere. I gave up and used a soft limiter instead which gives a good approximation to an overdriven valve sound for much less money.
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Old 1st June 2018, 10:46 PM   #3
Wavebourn is offline Wavebourn  United States
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OPT design for guitar amplifier
I used Ada Depot 80W transformers in 40W hi-fi amps. Fantastic transformers, wide band! Made by Schumakher.
"Clean channel" in guitar amps mean less amplification in preamp than in "Dirty channel".
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Old 1st June 2018, 11:57 PM   #4
kevinkr is offline kevinkr  United States
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OPT design for guitar amplifier
Relocated to Instruments & Amps forum per policy.
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Old 2nd June 2018, 12:02 AM   #5
kevinkr is offline kevinkr  United States
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OPT design for guitar amplifier
Guess it depends on the sort of amp you are trying to make. I had a friend some time ago who designed and built a lot of guitar amplifiers from scratch and he was looking for specific sound qualities which he said were hard to achieve with transformers approaching hifi quality.


I am not aware of all of the things he tried, but under sizing the core, and reducing primary inductance, along with finding the right trade off between leakage inductance and stray capacitance were definitely some of the things he experimented with. Core materials I think also played a role.
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Old 2nd June 2018, 12:33 AM   #6
TonyTecson is offline TonyTecson  Philippines
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OPT design for guitar amplifier
i use H50 cores for guitar amp duty, they are the lowest grade of laminations,
and guitarists that i made head amps for seem to like them...
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Old 2nd June 2018, 02:46 AM   #7
JMFahey is offline JMFahey  Argentina
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No need to deliberately make OTs distort, which in any way would happen at lower frequencies, not the main part of guitar sound by any means, distortion is left to tubes themselves.

OTs need simply to be "adequate", and Guitar duty is easy: lowest frequency is about 80 Hz but more important, itīs attenuated in most "good" amplifiers, sometimes to the extreme, from 6 db down@80/160Hz in Marshall amps to 450Hz in Trainwreck to 650/700Hz in VOX, go figure.

And at the "high" end (what Hi Fi calls high mids), speakers typically have a 6 to 12dB peak somewhere between 2200 and 3500Hz and then drop like a brick at none less than 24dB/oct .

Click the image to open in full size.

Even a center tapped Power Transformer is "usable", if impedance ratio matches, although thatīs an extreme case: it will lack Bass because of lack of primary inductance and will lack treble because of excessive self capacitance and highish leakage inductance ... but even so it will "work".

And with a minimum amount of care, say adequate inductance @ 80 Hz and simple interleaving, say half primary - secondary - half primary you will have a very competent OT.

That said, some famous OT suppliers, such as Schumacher, Drake, Dagnall, Hammond Canada, and above all, Partridge, the "king of OTs" , used only by Hiwatt and Orange because of higher cost, have steadily supplied "better than necessary" OTs simply because they were (still are) suppliers of good quality Hi Fi OTs, they know their job, are fully experienced, know how to do things "right" and they prefer to do things that way.

I very very much doubt Guitar amp manufacturers specify or even less design their own, I bet they ask an established supplier "hi, I need a 3800 ohm to 4/8/16 ohm 100W transformer" and let him supply samples ... which will usually better than needed.
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Old 2nd June 2018, 03:27 AM   #8
Enzo is offline Enzo  United States
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Ken Bran, a MArshall designer, once related that for grins he once threw Fender transformers into one of his MArshall amps. he said it still sounded like a Marshall.
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Old 2nd June 2018, 05:41 AM   #9
TonyTecson is offline TonyTecson  Philippines
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OPT design for guitar amplifier
guitar amps are all about how it sounds.....
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Old 3rd June 2018, 01:39 PM   #10
Tubelab_com is offline Tubelab_com  United States
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Quote:
1. Should OPTs used in a guitar amplifier be deliberately distorting, including a severe HF and/or LF roll-off - in the end, cheaply made?
The reality is that the "cheaply made" part is the driving criteria. Everything else just comes along with it.

Quote:
I used Ada Depot 80W transformers in 40W hi-fi amps. Fantastic transformers, wide band! Made by Schumakher.
I bought 200 of those when they were available on the surplus market by the thousands for under $20 each. I have taken them apart. Schumaker made car battery chargers, I have one from the same time period. It is a 12 volt 6 amp battery charger. The transformer in the battery charger looks identical to these OPT's, same laminations, same paper construction.

The OPT's are wound with zero interleaving. One half primary is wound on the paper bobbin, then the entire 0-4-8-16 ohm tapped secondary, using progressively smaller wire for each impedance winding, then the second half primary winding is wound over that using a thicker wire than the first half primary to get the two DCR's close.

These "budget" winding techniques and materials will result in higher leakage inductance and capacitance than a HiFi grade OPT, but this is due to the cost constraints, and is not intentional. The spec sheet for my OPT's have lots of mechanical criteria. The only electricals are "80 VA from 80 Hz to 5 KHz, 6600 ohms to 0-4-8-16 ohms." There is just enough iron and copper to satisfy those criteria, so primary inductance is low. These work better as 3300 ohms to 0-2-4-8 ohm transformers.

I bought a bunch of these for building guitar amps. It didn't take long to find that these sounded good, often real good in some HiFi amps. In others, they were sort of lifeless......

The difference turned out to be how the OPT is driven. Drive these OPT's with a good low impedance source capable of fairly high peak currents and get good sound. Run the usual wimpy HiFi tubes and try to fix it with feedback around the OPT, no bueno!

When driven open loop by a pair of 300B's the frequency response at 10 watts is 30 Hz to 20 KHz @ 3db. Saturation effects appear around 20 watts at 30 Hz.

When driven by a pair of large TV sweep tubes with lots of local feedback wrapped around the tubes, but zero GNFB, the frequency response at 10 watts is similar. Maximum power (limited by the power supply and tubes) at 1KHz is 170 watts. Saturation sets in at 45 watts at 41 Hz.

In both cases the amps sound good. In a typical EL84 amp with GNFB, the measured data is worse, and the sound is too.

The whole Fender VS Marshall thing has too many variables to figure out completely. The main players ARE the OPT's, tubes and speakers.

Marshall used EL34's feeding a 4300 ohm OPT from about 400 volts in their "50 watt" amps. Fender used 6L6GC's feeding a 6600 OPT from a higher B+ voltage in their "50 watt" amps. Fender's OPT's were much smaller than Marshall's for the same power level. A 50 watt Marshall made way over 50 watts cranked, while Fender barely made 50 when fed high line voltage. The characteristics of the OEM speakers were grossly different as well.

The frequency range in an electric guitar with standard tuning is 82.4 Hz to 1318.4 Hz if it has 24 frets. Most guitars have 20 to 24 frets. Harmonic energy will reach up into the 5 to 6 KHz range limited by the inductance in the pickup and the capacitance in the cable connecting the guitar to the amp......hence the specs on the cheap guitar OPT's.

Most guitar specific speakers have no response past 5 KHz. Their resonant impedance peak often lies within the guitar's frequency range, usually around 80 to 200 Hz. This makes some low frequency rolloff necessary in the amp, usually undersized coupling and bypass caps compared to a HiFi amp of similar power levels. The response is FAR from flat. 20 dB peaks and valleys are common.

The interplay of the output tubes, OPT and speaker response along with the feedback circuitry (presence control on a Marshall) determine much of the amp's "tone." Super good OPT's are really not needed, since we only want to cover 80 Hz to 5 KHz, and there are so many other nonlinearities present.
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