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PostPosted: Sat, May 12, 2018 6:53pm 
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Supah Stah

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I have an 20 watt amp that uses 2 x EL84 power tubes. The gain side has an insane amount of gain available, but it sometimes sounds a little fizzy to me. Is this the nature of EL84's when used with a high-gain preamp? The clean channel doesn't sound fizzy to me when pushed hard, so I'm thinking it could be the gain side preamp? I tried installing mod board that I built to clean this up, but it didn't make any difference. I can clean the fizz out with an EQ in the gain side loop, but I'd like to be able to remove the fizz without using an EQ.

I know a lot of guys here have experience with the Jet City 20 watt, so maybe you can enlighten me as to what I may be dealing with. This is the first high-gain EL84 amp I've ever owned. The amp uses a single tone knob, so I have limited control in terms of tone adjustments. It does have selectable slope and dampening, along with a built-in attenuator. Oddly enough, the fizz is reduced when I attenuate the signal, one notch down.


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PostPosted: Sat, May 12, 2018 7:16pm 
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Supah Stah

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No, but I know what you mean. I have an Engl Ironball and it CAN get fizzy, but re-EQing or turning down the gain usually dials it out. Years ago I had a tiny HiWatt that used 2 EL84s that wasn’t fizzy at all, it was actually awesome. This goes back 20 or so years ago, that amp was pure awesome, way ahead of its time, I sold it like an idiot...wish that one didn’t get away lol...live and learn.


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PostPosted: Sat, May 12, 2018 7:20pm 
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Supah Stah

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Interesting. Okay, I'll throw a set of JJ EL84 tube in it tomorrow to see if maybe it's the EH power tubes. The EH tubes are new, so...


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PostPosted: Sat, May 12, 2018 10:44pm 
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Immortal Rock God

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Could be that they're not biased out of crossover distortion. Have you attempted to measure and verify the bias is correct?

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PostPosted: Sun, May 13, 2018 12:03am 
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Supah Stah

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glpg80 wrote:
Could be that they're not biased out of crossover distortion. Have you attempted to measure and verify the bias is correct?

I'll post some pics of the board next time I open it. It has 2 x pots for balancing the bias between the tubes, but it is a cathode bias amplifier. Graydon (Guytron) said that they were biasing to something like 42ma. I took a plate voltage reading, and from what I recall, the recommended bias was something like 23ma. It's been a while, so that number may not be accurate. You can't set each tube bisas individually... It's balancing act between the two tubes. I'm trying to track-down Guy Hedrick, but no one seems to know where he is. There was a sighting in Jacksonville, FL after he quite working for Suhr, but nothing since. He's like bigfoot!


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PostPosted: Sun, May 13, 2018 4:07pm 
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Immortal Rock God

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Run a 1KHz sine wave from a signal generator into the amplifier and measure the output voltage into a speaker load. If it's crossover distortion, you'd see it regardless of how it was built.

I'll look into my datasheets and see what the nominal bias is for EL84s. I'll need you to share the B+ voltage though?

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6505MH 20W

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2012 Fender Select
1988 Charvel 475XL Special
2018 Gibson Les Paul Traditional
1999 Wolfgang Standard (Emerald Green)

Effects:
ADA 2FX
Lexicon PCM 81
TC Electronic 2240
Damage Control Timeline
Dunlop 535Q


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PostPosted: Mon, May 14, 2018 8:47am 
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Plank Cranker Wanker

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I don’t get any fizz using EL84’s in my high gain modded Jet city 20. I use Chinese, JJ and NOS Russian EL84’s.

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PostPosted: Fri, Aug 17, 2018 6:31pm 
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Plank Cranker Trainee
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Some builders have referred to the high-gain EL84 sound as "a can of buzzing bees". Now I can hear that in every high-gain 84 amp. Some solutions for minimizing the angry fizz:

- filter out high end. Something like a Chupa/Fortin/Cameron (between first and second gain stages)
- lower the preamp signal with voltage dividers. The 84s need very little signal to drive them into distortion.

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