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Unread 05-13-2012, 05:08 AM   #1 (permalink)
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Output Transformer Primary Impedance and Tonal Differences Marshall vs Fender

Anyone here have any experience in the tonal differences between Marshall and Fender output transformers?

I know there's a primary impedance difference between the two makes. I was thinking about changing the stock OT in my '73 40 watt Bandmaster Reverb, which is smaller than my Fender Blackface Bassman 40 watt OT, to something a bit beefier. I was even considering throwing in one of the JCM800 50W Drake OT's I have on hand. The Drake OT would also give me the switchable impedance options. Now I would still like to run 6L6's in the amp if I installed the Drake OT. Then there's the slight difference in OT primary impedance between the two (Fender has 4.2K and Marshall/Drake I am figuring is about 3.4K).

The Drake OT is actually huge in size compared to the stock Bandmaster Reverb OT. The Bassman OT is somewhere in between the two. So is it correct to ASSume that the Drake OT would provide my Fender with a bolder bottom end punch as well as more clean headroom? I know that swapping in a Blackface Bassman OT would do this somewhat. I was even considering looking for a Twin Reverb OT and running my Bandmaster at 8 ohms (two 6L6's). I want to retain the Fender character somewhat as well, so that is why I am wondering what a Drake/JCM800 OT would do tonally to a Fender?

Would the OT with a slightly lower primary impedance of 3.4K give off a bit more punch and volume than the higher 4.2K impedance as well?

I would really appreciate some good insight on this subject from anyone that is willing to explain it with some good theory.

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Unread 05-13-2012, 06:08 PM   #2 (permalink)
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Re: Output Transformer Primary Impedance and Tonal Differences Marshall vs Fender

It might be worth experimenting. I also have a Bandmaster but I like its lack of headroom and dirty sound. I think changing the primary impedance will affect the sound but I have no personal experience as to how. My quess this will be more apparent on dirty tones. Primary inductance is the major factor that will affect the bottom end though.
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Unread 05-13-2012, 08:20 PM   #3 (permalink)
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Re: Output Transformer Primary Impedance and Tonal Differences Marshall vs Fender

I once designed a few transformers with an older transformer guru who I discovered was a musician as well. We started talking guitar amps, and he told me of a output transformer he designed for a Fender Bassman. The OT was designed to have a flat response down to 20Hz. It ended up being larger than a Marshall 100W OT.

That story shows that size matters. The Bandmaster's unique tone comes from the relatively small OT and its saturation. OTOH, Marshall OT's are much larger, and I have always wondered how much OT saturation you really get with them. Most of a NMV Marshall's distortion comes from the tone circuit driver and the phase inverter anyhow. Fender amps usually stay very clean through the circuit, and the power tubes will distort much earlier than in a Marshall, so your OT construction will have more of an effect.

I would guess that a Marshall OT in a Bandmaster would give it more of a hi-fi clean tone, with more highs and lows. It would probably get a lot crunchier sounding with less smoothness, and less sustain as well. Might be worth a try if that is what you are looking for.
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Unread 05-14-2012, 02:44 AM   #4 (permalink)
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Re: Output Transformer Primary Impedance and Tonal Differences Marshall vs Fender

I'm on the fence with wanting to keep the smaller OT or going to a beefier OT. I dug out my Blackface Bassman the other day and fell in love with all over again LOL. The bottom end was so huge compared to my Bandmaster. But then I did do quite a few tweaks to the Bassman's circuit. I would have to find my notes to remember all that I did to it, but a few main things were increased filtering and global and local NFB tweaks/removal, revoicing of one of the channels, removal of most of the usual Fender flabby bass response ect.

I really do not want to shell out $$ for another OT at this time, but I do have a few extra early 80's 50W Drake OT's on hand. That is where that idea stemmed from.

I will keep the stock OT for awhile and see how it goes.
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Unread 10-02-2012, 10:03 PM   #5 (permalink)
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Re: Output Transformer Primary Impedance and Tonal Differences Marshall vs Fender

I am thinking real hard on this OT swap again. I have not been playing this Bandmaster, as I have been tinkering on the Traynor YSR-1 as well as getting my 2204 dialed in and tweaked a bit more.

The big plus is having the 4/8/16 cab options.
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Unread 10-03-2012, 01:38 AM   #6 (permalink)
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Re: Output Transformer Primary Impedance and Tonal Differences Marshall vs Fender

6L6 valves are of lower transconductance. While they can safely run into a lower impedance such as that offered by a Marshall OT, the power transfer will suffer, which may reduce the punch.

A lot of the reason Fender transformers seem to lose the low end when pushed is due to OT saturation. This is due to the smaller core size...less iron means the less it can be magnetizes without saturating, which makes them easier to saturate.

If it were my amp, I'd do some measuring to determine the impedance ratio of the stock OT, then have a company like Heyboer wind up a Marshall style output transformer but with an identical impedance ratio to the stock OT. The Hammond 1650N is also a viable option as it features a larger core, a 4.3K primary (usually what is used in most 40-50 watt Fenders) with 4-8-16 ohm secondary wiring options.
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Unread 10-03-2012, 04:01 PM   #7 (permalink)
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Re: Output Transformer Primary Impedance and Tonal Differences Marshall vs Fender

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Originally Posted by Wilder Amplification View Post
6L6 valves are of lower transconductance. While they can safely run into a lower impedance such as that offered by a Marshall OT, the power transfer will suffer, which may reduce the punch.

A lot of the reason Fender transformers seem to lose the low end when pushed is due to OT saturation. This is due to the smaller core size...less iron means the less it can be magnetizes without saturating, which makes them easier to saturate.

If it were my amp, I'd do some measuring to determine the impedance ratio of the stock OT, then have a company like Heyboer wind up a Marshall style output transformer but with an identical impedance ratio to the stock OT. The Hammond 1650N is also a viable option as it features a larger core, a 4.3K primary (usually what is used in most 40-50 watt Fenders) with 4-8-16 ohm secondary wiring options.
Sounds like my best option would be to find a Bassman or Super Reverb - or maybe a Twin OT (figuring in the impedance load factoring), if I want to keep my Fender sounding close to the original tone.

I know Mercury Magnetics also makes a Bassman OT with multiple impedance taps. But even looking into the Hammond stuff, I am spending $$. I already have a few early 80's 50W Drakes on hand, as was my reasoning.
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Unread 10-12-2012, 08:03 PM   #8 (permalink)
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Re: Output Transformer Primary Impedance and Tonal Differences Marshall vs Fender

Check out Classic Tone Transformers they have an ad on here they have a lot of 40W Fender Transformers and their prices seem very reasonable I was thinking of getting them for my RI Tweed Bassman
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