5E8A Tweed Twin Mods for Harp

[Diablo]

My buddy bought himself a used 57 Twin - Fender'sreissue 5E8A, for a guitar amp. He's been playing guitar forever, but recently decided to play harp too. He has some basic questions that perhaps the forum wizards can answer better than me.

He is having issues with feedback at gigging volumes. Some folks recommend using lower gain preamp tubes. His amp has all 12AX7 tubes now. Wouldn't the amp still feedback at the same sound pressure level despite the lower gain tubes?

Any other good proven methods to minimize feedback?

Any good mods to improve tone for harp?

[Jazz P Bass]

Fender 5E8A

I thought the original 5E8A had 12AY7 tubes in the preamp & a 12AX7 in the phase inverter.
12AY7 EH - (6072 ) Hints on using the "Tweed tone tube"

[Diablo]

I thought the original 5E8A had 12AY7 tubes in the preamp & a 12AX7 in the phase inverter.
12AY7 EH - (6072 ) Hints on using the "Tweed tone tube"

Yes, the original 5E8A uses three 12AY7 preamp tubes. Fender's reissue uses all 12AX7 tubes. The question is: Does using 12AY7 tubes increase the sound pressure level before feedback becomes a problem with a harp?

[ThermionicScott]

This is mostly an aside, but since Electro-Harmonix makes a new 12AY7, I don't understand why Fender doesn't ship their tweed reissues with them.

- Scott

[MWJB]

"The question is: Does using 12AY7 tubes increase the sound pressure level before feedback becomes a problem with a harp? " It CAN do, but not always, depends on the circuit & type of mic being used. Even if it does, the softer sound might not cut so well, but you may well get an easier life when adjusting the controls. It can be swings & roundabouts, there's no right/wrong, experiment & whatever works...works.

I'd say 12AYs in the preamp stages is the first port of call. In fact get a bunch of preamp tubes (5751, 12AT7, 12AU7) and try different values in the PI too, you have an unnattenuated gain stage there that might benefit from a lower mu tube? Rebias to 15-20mA per power tube, this will help curb feedback. A 2x12" is always going to be more feedback prone than many other configurations, perhaps explore getting some 12" to 10" baffle converter plates (maybe even one might help - there have been a couple of harp specific 1012 amps built) & trying some proven 10"s (Eminence 102, Kendrick Blackframes, Fane Medusa 75, Eminence Alphas can be good for harp, but probably too dark for guitar). If going the 2x10" route the bias can go back up to around 30mA per tube for harp.

[MWJB]

"This is mostly an aside, but since Electro-Harmonix makes a new 12AY7, I don't understand why Fender doesn't ship their tweed reissues with them. - Scott"

2 reasons - guitar players, especially Strat & Tele owners, often prefer the higher gain with 12AX7s. Secondly, the 12AY7s cost more.

[Diablo]

Thanks MJWB. Can you explain why a lower power tube bias helps control feedback? From the guitar side, all I heard was recommendations to bias hot and hotter. I'd like to understand why the harp amp benefits from a cold bias.

[MWJB]

Guitars need a typical bias setting (30-35mA) to help generate fat, smooth harmonics to properly propagate the sustain of a note. Most of what you hear of a guitar note is sustain, after the string has been plucked. Also, the amp will typically be faced with a smaller input signal from a guitar, in turn it is turned up higher. At the volumes that some folks might like to play, the amp is verging on developing a crossover notch and a little crossover distortion can make it's way into the overal sound, heard as a little crunch? In the old days (& even commonly today in the UK), techs might bias an amp by observing the sine wave with the amp vol set to say 7/10 & rebias until the crossover notch just disappears at that setting. Typical bias currents provide a bigger signal, with more high end.

Harp has no sustain, so does not need quite as much plate current. Some preferred harp sounds (cathode biased, budget PA 50'S PA vibe) still require regular, or even high currents (2x10" also sound thin & harsh with lower currents). The volume control will typically be set far lower with a hi-z harp mic, so to benefit from a little crunch we might set the bias so that the crossover notch apears in the region of volume control sweep that we might actually be using. Harp mics can put out whole volts, compared to a couple of hundred millivolts from a guitar pick up. The bigger signal from high currents makes the amp more feedbak prone, effectively increasing gain & high end. Using cold, or "just enough" current can make an amp used for harp feedback noticably less & be easier to deal with on stage.

Speaker configuration plays a part, I have typically found 2x10" to require at least as much current as a typical guitar set up, 1x12"/2X12"/1X15"/3X10"/4X10" can all get away with "just enough" current....which may be anywhere from 7mA to 20mA depending on the amp.

Before anyone pipes up quoting the evils of crossover notch distortion, in numerous ear tests, you have to go very low indeed on the plate current for experienced folk to recognise it in the context of harp. Some tubes (if your amp will take them like EHKT90) don't actually sound "cold" when run at <10mA per tube. You will know when the current is too cold, the amp can drag & lose note separation, long drawn out notes can fade out, the amp tone might generally be described as "dry" or "Shusshy". I often find that in say a typical 4x10" Fender the harp tone will sound the most "normal" (e.g. not dry/short/cold, or bright/greasy hot) at around 20mA per tube.

[ThermionicScott]

Pentodes and beam power tubes also have less gain at lower bias settings. Does that play into it?

- Scott

[MWJB]

Yes, essentially, lowering the bias reduces gain at the output.