Overdriven Fizz Sound

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Littlewyan
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Re: Overdriven Fizz Sound

Post by Littlewyan »

Its not fizz, its overdriving as it should, but its not overdriving the right frequencies to sound nice. At the moment its overdriving a lot of bottom end and losing a lot of top end, so it will sound like that. My JTM50 sounds similar if I lower the treble and raise the bass.

500pF bleeder cap from grids to ground will again dump a LOT of high end to ground.

Interstage attenuation is not an issue as you have gain and volume controls.

I don't know much about how you've got your effects loop set up but you are already attenuating a lot before that gain stage. You have a 220K/1Meg voltage divider before the stage and then a 220k grid stopper.

Small poly caps normally bypass the filter caps, not the decoupling resistors. Either way I doubt this is necessary. You could do with increasing the resistance between stages a bit to help with filtering.

Stick with just trying the tone stack first and see what happens :)
Theashe
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Re: Overdriven Fizz Sound

Post by Theashe »

Littlewyan wrote: Small poly caps normally bypass the filter caps, not the decoupling resistors. Either way I doubt this is necessary. You could do with increasing the resistance between stages a bit to help with filtering.
Oops! :oops: I meant to say caps. I'll go make a note of that on the old post.

I'll give the EQ a shot before any other changes then! It's good to know that you can get a similar reproduction of my problem just through EQing.

If you've got some time on your hands Littlewyan, can I hear a sound clip of your JTM50 with a tone you like, and then the comparable bass up/treble down?
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Littlewyan
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Re: Overdriven Fizz Sound

Post by Littlewyan »

I'll try but it definitely won't be today. I always get shouted at for playing it at home. Its my loudest amp!
Theashe
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Re: Overdriven Fizz Sound

Post by Theashe »

Spent the day working on the amp, and I'm gonna ramble for a bit. Since I moved into a house with good wiring, MOST of the amplifier's hum disappeared without me doing anything to change the amp. The overdriven tone actually sounds nicer now that there's less 60Hz and 120Hz to create IM distortion. Amazing!

Nevertheless, knowing that I won't be here forever, I've been tackling hum throughout the amp. My original grounding scheme was to ground everything related to one stage at the local cap ground for that stage, and run all of the local cap ground wires to a single star at the input of the amp. I tried splitting off the power section grounds and putting them with the earth connection, but that made no difference to the level of hum in the amp.

However, I had a huge success taming the hum by checking for wires that pick up hum easily with my finger and replacing them with shielded wires. This thing is DEAD silent now, until all volume controls AND the master are above 90% rotation. You can't even tell it's on otherwise.

Now, how does that relate to the overdriven fizz?

Amid all the shielded wiring today, two pots failed in my amp at different times. The pre volume, and the post volume. On the pre-volume pot, the top signal terminal failed open, but would intermittently work properly. I replaced the pot, and even at high volume and heavily overdriven... NO FIZZ.

Then, later in the day after re-wiring the post volume pot, the fizz came back. Sure enough, the wiper connection failed on that pot in the same way that terminal 1 failed on the pre-volume pot. I have a suspicion the fizz was caused by bad pots. When I get the post-volume pot replaced tomorrow, I'll report back with my findings.

Oh, and Littlewyan, I'm still going to tear out the EQ and fix that. I'll need a couple more days for that one.
bluesfendermanblues
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Re: Overdriven Fizz Sound

Post by bluesfendermanblues »

dbl
Last edited by bluesfendermanblues on Mon May 02, 2016 7:04 am, edited 1 time in total.
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bluesfendermanblues
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Re: Overdriven Fizz Sound

Post by bluesfendermanblues »

Sounds like you're using Alpha pots. :-)

IME they sound and work fine, but dont like too much soldering.
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Littlewyan
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Re: Overdriven Fizz Sound

Post by Littlewyan »

Yeah pots don't like too much heat.
guitarmike2107
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Re: Overdriven Fizz Sound

Post by guitarmike2107 »

grief three pages and not one gut shot photo to admire!

Hope you have it sorted.
Theashe
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Re: Overdriven Fizz Sound

Post by Theashe »

You're right! Here's a couple. Bear in mind, this was the first tube amp I've ever built, and there are a lot of things I'll do differently for tube amp #2 from what I've learned here.

I put together the circuit with the help of books, forums, and articles, and designed the physical layout from scratch, and hand drilled the entire thing from a blank chassis. While it was really tidy to begin with, one million modifications later I've bent a lot of wires to work around and it's become a bit messy. Most of my long-running signal wires are shielded now (thick black wires).
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Theashe
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Location: Canada

Re: Overdriven Fizz Sound

Post by Theashe »

Sorry about the late follow-up. I got a job after graduating, so I had to move again and just started this week.

When I replaced all of the problematic pots, the strange "fizz" sound all but disappeared. I can barely hear it anymore, and I'm certainly pleased with the hum level after replacing a LOT of wires with shielded ones. I'll try to get a friend who's a much better guitarist than myself to record the sound.

Also - Littlewyan - I corrected the EQ to match the Dumble one you provided, and it certainly changed the way the controls interact with the sound of the amp. The amp sounds far less bassy too.

On to project #2 - a much simpler 12W 6V6 amp with an active transistor EQ that I designed and used very successfully in a headphone amplifier. Will update in another thread as I build.
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Littlewyan
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Re: Overdriven Fizz Sound

Post by Littlewyan »

That active transistor EQ sounds like its going to be interesting. Can't wait to read about it :)
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