ac30 sounds grainy/fizzy
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- JoshBernstein
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- Location: Key Largo, FL USA
ac30 sounds grainy/fizzy
I completed my ac30 build the other day, built off a hoffman board. It has no tremelo or reverb, and no master volume. The cleans sound really good and chimey, but when i crank it for a little bit of power tube hair, it sounds a bit fizzy. I used a couple of ceramic disks in the preamp, and I've read that some people say that they sound grainy. Could that be my issue?
The tubes im using are as follows:
Input tube- amparex bugle boy
cathode follower: Shuguang
Pi: jj
El84s are sovtek, rectifier is a jj gz34.
Classictone 4k ot
running it into a 16 ohm 2x10 cab with a celestion g10 gold and an eminence rf10c
anyone have any suggestions?
The tubes im using are as follows:
Input tube- amparex bugle boy
cathode follower: Shuguang
Pi: jj
El84s are sovtek, rectifier is a jj gz34.
Classictone 4k ot
running it into a 16 ohm 2x10 cab with a celestion g10 gold and an eminence rf10c
anyone have any suggestions?
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- JoshBernstein
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Re: ac30 sounds grainy/fizzy
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- JoshBernstein
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Re: ac30 sounds grainy/fizzy
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Two things, your output could be biased too on the cold side, or the more likely thing is that those brand outputs are known for that type of tone when the get pushed hard.
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Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
- JoshBernstein
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Re: ac30 sounds grainy/fizzy
Ill check the bias when i get home tonight. Do you have any recommendations for different outputs?
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- JoshBernstein
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Re: ac30 sounds grainy/fizzy
Based on my calculations (which could be very wrong) im getting 16.8w per tube. Ive got 10.82 volts on the cathode using a single 50 ohm cathode resistor and 322.2 volts on the plates.
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- JoshBernstein
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Re: ac30 sounds grainy/fizzy
Correction: i really my scribbles incorrectly, I've actually got 314 on the plates, putting me closer to 16w per tube.
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- JoshBernstein
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Re: ac30 sounds grainy/fizzy
One more question, could a conductive filter a ccross the primaries help witht the fizz? If so what values should i use?
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- lord preset
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Re: ac30 sounds grainy/fizzy
Before doing anything else try subbing the Bugle Boy for some other tube. They have such a stellar reputation they seem to stay in circulation past their use-by date.
Re: ac30 sounds grainy/fizzy
Let's compare maths. You said 315 plates, 10.8 cathode. You have to subtract the cathode voltage, so that's 303v for the plates. Plug 303 into this bias calculator for EL84s: http://ax84.com/biascalc.html:
Next, plug your cathode voltage and resistance into Ohm's law, and you get 216mA, divided by four tubes = 54mA per tube.
If I've done this correctly, you're biased WAY too hot.
Code: Select all
EL84 (Max disapation 12 watts)
Results: For 303 Volts
60% 70% 80% 90% 100% 110%
23.76 ma 27.72 ma 31.68 ma 35.64 ma 39.6 ma 43.56 ma
Recomended Bias point for AB1 operation is 70%
If I've done this correctly, you're biased WAY too hot.
I build and repair tube amps. http://amps.monkeymatic.com
- JoshBernstein
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Re: ac30 sounds grainy/fizzy
I did try subbing the bugle boy with no improvement. I was thinking that those numbers see high when i did it. How should i go about cooling it down?
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- lord preset
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Re: ac30 sounds grainy/fizzy
A 60-80 ohm cathode resistor will cool it down which may or may not address your fizz issue. You are biased hot but not that much hotter than AC30s run by design. They were made to eat tubes.
BTW what choke are you using?
BTW what choke are you using?
- JoshBernstein
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Re: ac30 sounds grainy/fizzy
Alright, ill pop one of those in there.
Im using a 3 Henry 120ma choke i got from hoffman, im not sure what the brand is.
Im using a 3 Henry 120ma choke i got from hoffman, im not sure what the brand is.
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- martin manning
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Re: ac30 sounds grainy/fizzy
Your calculated dissipation per tube is correct.JoshBernstein wrote:Based on my calculations (which could be very wrong) im getting 16.8w per tube. Ive got 10.82 volts on the cathode using a single 50 ohm cathode resistor and 322.2 volts on the plates.
EL84's are 12W at 100% plate dissipation, and you can add about 10% for the screen, so let's say 13.2W is 100%. You are at 16.8/13.2 = 127%. Something closer to 100% would be good.
Last edited by martin manning on Sat Dec 19, 2015 8:02 pm, edited 1 time in total.
- JoshBernstein
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Re: ac30 sounds grainy/fizzy
I put a new 10 watt resistor that reads at about 66 ohms, which brought the cathode voltage up to 11.3 volts. Im now at 41ma and about 13 watts. That issue seems to be fixed, but the fizz is still present. I added a conjunctive filter across the primaries using a 10k 10 watt resistor and a .0022 1000v cap, which helped a bit but its still there.
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