Fender Express Creation

Express, Liverpool, Rocket, Dirty Little Monster, etc.

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BillyBob
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Fender Express Creation

Post by BillyBob »

Hello,
I built another amp. Gut shots of my amature handywork are posted below.
I carved up a silverface Bassman and built another Express. Its my second Express. Its up and running and sounds good (great!).
Very low noise floor. No oscillation with controls dimed.
You can see I used the Fender Bassman chassis and original transformers; both power and output (output 4ohm)
I used the internet Express A0 schematic.
The Bassman layout was Fender 864. The Express A0 schematic was similar.
I have a LarMar type PPIMV and a mod that allows me to use EL34 or 6L6 output tubes by rebiasing.

Im running some great NOS 6L6 tubes at the moment. Amp sounds cool. Im playing through the Fender Bassman cab 2x12 4ohm. Les Paul and Celestion speakers.

Question: I did not use any flyback diode arrangement at the output tubes. How critical is that to do?
What might happen if I plugged the head into into a 16ohm speaker cabinet? For years beforehand I played the Bassman through 16-ohm cab with no problems.

Bill
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10thTx
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Re: Fender Express Creation

Post by 10thTx »

What might happen if I plugged the head into into a 16ohm speaker cabinet?


From Kevin O'Connor's FAQ at London Power:
Q: I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.

A: It is awfully expensive, and ridiculous that such things would be suggested. There are two issues here, though; one is the notion of "impedance matching", and the other is simple design preference.

As stated throughout the TUT-series, speaker load impedances and reflected loads to the output tubes are all "nominal". An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.

A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.

The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors.

The design issue for impedance matching comes into play when a designer takes the approach that "everything is critical". In some circuits, this may be the case. Tubes don't really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says "this is good", "this is bad" and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.

Design approaches are dealt with in TUT4.


Q: An "expert" suggested that I change my speakers to ones that match the highest impedance tap on my amp. How do I do this and still have the option of using a second cabinet when I play out? I think I would need three cabinets to achieve this.

A: Yes, and what a waste of your money.

Not too surprisingly, this is the same expert as in the tube-pulling/power reduction question. He really should stop talking about transformers.

Rest assured, the impedance taps on your amp are there for your convenience, to use as you will. Connecting the rated cabinet impedance to the identical rated tap selection will get you the rated power bandwidth of your amp into that load. As stated above, any "mismatch" reduces power and bandwidth, and that is all.

If you are using your 4-ohm cabinet and the 4-ohm tap, does it matter if the 16-ohm tap is being unused? Of course not. This subject is explained in detail in TUT3, as the "Myth of Encompassment" - a myth created purely to sell speakers and transformers. To unsuspecting players and readers of the "expert's" column, it is no more than a scare tactic.

Transformer designers take into account the loads to be connected to the device. There is limited space in the winding window for each lamination size, and the designer wants the space to be fully utilized. The percentage of space used is the "build". Ideally, all windings are used all the time, to keep parasitic effects to a minimum. When there is a tapped secondary, some of the secondary may not be loaded under certain conditions, so those "free" parts of the winding can potentially upset the parasitic balance. The amount of upset is usually so small as to be insignificant, even in hi-fi where such a thing might matter. In MI, there is no concern whatsoever.

In most amps, you can set the impedance selector to whatever sounds best. The one caveat is: NOT in English amps. Having replaced more Marshall OTs than anything else, I would advise that the impedance selector always be set to the rated load, or less.
BillyBob
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Re: Fender Express Creation

Post by BillyBob »

Thanks for that excellent and informative posting.
That confirmed some things I had vague technical notions of previously.

So the flyback diodes are a separate issue..
Are those a "must do" for Express amps?
Randy Magee
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Re: Fender Express Creation

Post by Randy Magee »

The flyback diodes in my Express build caused an edgy, fizzy distortion to the overdriven tone. I ended up removing them and all was well.
Randy Magee
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JazzGuitarGimp
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Re: Fender Express Creation

Post by JazzGuitarGimp »

I bought a Fender Custom Shop Tone Master head in the mid-90's. Used it for about 8 years, then put it in the closet. Over the ensuing years, the owner's manual got lost in the shuffle. The output impedance selector switch, a 3-position slide switch, has panel labels "Low", "Med" and "Hi" for the three positions. I put the amp back into service about 3 years ago, and I remembered that the switch position labels were non-standard, and I also had a vague recollection that the manual said to use whatever position that sounds the best for whatever situation you find yourself in - but I wasn't absolutely sure about my old-fart memory. So I called Fender and asked a tech support guy, I told him what I thought the manual said, and asked him to verify it for me. At first, he said ABSOLUTELY NOT - MAKE SURE THE IMPEDANCE SWITCH IS MATCHED TO THE SPEAKER. I then told him how the three positions are labeled, and asked him to actually look at the owner's manual. After being on hold for five or so minutes, he came back to the phone and said. Well, the manual does say to use whatever position sounds good, but your output tubes probably won't last as long with mis-matched loads. After reading KOC's excerpt posted by 10thTx, I now think the tech support guy was probably mis-informed when he warned about decreased tube life.
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pompeiisneaks
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Re: Fender Express Creation

Post by pompeiisneaks »

I also remember reading this blog by Hughes and Kettner:

http://blog.hughes-and-kettner.com/ohm- ... impedance/

and I think they're pretty much confirming 10thtx. It just changes the tone, no damage to the OT or speakers should be expected.

It seems the only time impedance matching is super important is with solid state electronics as the change in current could smoke a transistor/fet/etc.

~Phil
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BillyBob
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Re: Fender Express Creation

Post by BillyBob »

So the flyback diodes ara general safety feature... Not specifically relevant to impedance mismatching?
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JazzGuitarGimp
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Re: Fender Express Creation

Post by JazzGuitarGimp »

BillyBob wrote: Thu Apr 27, 2017 12:24 am So the flyback diodes ara general safety feature... Not specifically relevant to impedance mismatching?
Correct.
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DaveMon
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Re: Fender Express Creation

Post by DaveMon »

I have diode strings on my express.......I thought they did not affect tone and were for OT insurance only.... Is the fizzy sound described above......."diode clipping" ?? I read here on TAG, that someone put their diode strings on a switch, and they very much preferred the sound of NO diodes.........
Does anyone else recommend.that?
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pompeiisneaks
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Re: Fender Express Creation

Post by pompeiisneaks »

I'm confused too, I was under the impression that diodes had to be in the path of the signal flow to cause the clipping effect. Not from ground into the side of the circuit like used in many amp's transformer tie in to protect the system in failure modes?

There are many amps that use this. Express and Rocket I think, and Ampeg used them too.

I have heard several modern amp repair techs and builders add them to every amp to protect them as well. No note ever of tonality changes. The diode clipping is supposed to be pretty obvious and horrible. Some of the distortion pedals just use diode clipping and it's very obvious.
~Phil
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BillyBob
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Re: Fender Express Creation

Post by BillyBob »

I was wondering about the frequency of OT failures.
How many fail, how often..
It never happened to me or anybody I know.
Through hearsay and internet reading I came away thinking Marshall OTs fail the most.
Not sure why. Dont know if its true.

But diodes are always recomended for Express amps.
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