ac30 sounds grainy/fizzy

Vox and Hiwatt Discussion

Moderators: pompeiisneaks, Colossal

User avatar
JoshBernstein
Posts: 289
Joined: Tue Jan 27, 2015 1:31 am
Location: Key Largo, FL USA

ac30 sounds grainy/fizzy

Post by JoshBernstein »

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?
You do not have the required permissions to view the files attached to this post.
Not really
User avatar
JoshBernstein
Posts: 289
Joined: Tue Jan 27, 2015 1:31 am
Location: Key Largo, FL USA

Re: ac30 sounds grainy/fizzy

Post by JoshBernstein »

Pics
You do not have the required permissions to view the files attached to this post.
Not really
User avatar
JoshBernstein
Posts: 289
Joined: Tue Jan 27, 2015 1:31 am
Location: Key Largo, FL USA

Re: ac30 sounds grainy/fizzy

Post by JoshBernstein »

Pics
You do not have the required permissions to view the files attached to this post.
Not really
Stevem
Posts: 4551
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

....

Post by Stevem »

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.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
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!
User avatar
JoshBernstein
Posts: 289
Joined: Tue Jan 27, 2015 1:31 am
Location: Key Largo, FL USA

Re: ac30 sounds grainy/fizzy

Post by JoshBernstein »

Ill check the bias when i get home tonight. Do you have any recommendations for different outputs?
Not really
User avatar
JoshBernstein
Posts: 289
Joined: Tue Jan 27, 2015 1:31 am
Location: Key Largo, FL USA

Re: ac30 sounds grainy/fizzy

Post by JoshBernstein »

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.
Not really
User avatar
JoshBernstein
Posts: 289
Joined: Tue Jan 27, 2015 1:31 am
Location: Key Largo, FL USA

Re: ac30 sounds grainy/fizzy

Post by JoshBernstein »

Correction: i really my scribbles incorrectly, I've actually got 314 on the plates, putting me closer to 16w per tube.
Not really
User avatar
JoshBernstein
Posts: 289
Joined: Tue Jan 27, 2015 1:31 am
Location: Key Largo, FL USA

Re: ac30 sounds grainy/fizzy

Post by JoshBernstein »

One more question, could a conductive filter a ccross the primaries help witht the fizz? If so what values should i use?
Not really
User avatar
lord preset
Posts: 447
Joined: Tue Aug 27, 2013 3:06 am
Location: San Diego

Re: ac30 sounds grainy/fizzy

Post by lord preset »

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.
User avatar
xtian
Posts: 6990
Joined: Mon Apr 19, 2010 8:15 pm
Location: NorCal
Contact:

Re: ac30 sounds grainy/fizzy

Post by xtian »

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:

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%
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.
I build and repair tube amps. http://amps.monkeymatic.com
User avatar
JoshBernstein
Posts: 289
Joined: Tue Jan 27, 2015 1:31 am
Location: Key Largo, FL USA

Re: ac30 sounds grainy/fizzy

Post by JoshBernstein »

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?
Not really
User avatar
lord preset
Posts: 447
Joined: Tue Aug 27, 2013 3:06 am
Location: San Diego

Re: ac30 sounds grainy/fizzy

Post by lord preset »

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?
User avatar
JoshBernstein
Posts: 289
Joined: Tue Jan 27, 2015 1:31 am
Location: Key Largo, FL USA

Re: ac30 sounds grainy/fizzy

Post by JoshBernstein »

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.
Not really
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: ac30 sounds grainy/fizzy

Post by martin manning »

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.
Your calculated dissipation per tube is correct.

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.
User avatar
JoshBernstein
Posts: 289
Joined: Tue Jan 27, 2015 1:31 am
Location: Key Largo, FL USA

Re: ac30 sounds grainy/fizzy

Post by JoshBernstein »

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.
Not really
Post Reply