ac30 sounds grainy/fizzy

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JoshBernstein
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Re: ac30 sounds grainy/fizzy

Post by JoshBernstein »

Could my ot be causing the dizziness? Im wondering if it is not suitable for class a operation. It is a classictone 40-18006 i had laying around. It doesnt get hot while in use, and sounds really good clean.
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martin manning
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Re: ac30 sounds grainy/fizzy

Post by martin manning »

Not to start a big argument or anything, but an AC30 is cathode biased, but not Class A. Seems like a 40-18006 OT should work, but it is much smaller than an AC30 spec OT, 40-18049 e.g., which should have more bandwidth.

What value do you have for the PI coupling caps? If they are 100 or 150nF, you might try reducing them to 47nF.
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JoshBernstein
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Re: ac30 sounds grainy/fizzy

Post by JoshBernstein »

Ah, i have been previously mislead then.
i do currently have a 47nf in there, and have thrown a couple of other values in there with no real changes.
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Post by Stevem »

Your PI may be driving the outputs too hard once they start clipping, you should only need 10 to 14 volts of audio drive signal as measured across the input grid of each tube .
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pdf64
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Re: ac30 sounds grainy/fizzy

Post by pdf64 »

JoshBernstein wrote:Im using a 3 Henry 120ma choke i got from hoffman, im not sure what the brand is.
The resistance in the screen grid supply, eg choke resistance, is very important in setting the idle and dynamic operating conditions.
Something around 500 ohms is a good idea.
Much less than that will tend to run the screen grids / plates too hard.
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JoshBernstein
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Re: ac30 sounds grainy/fizzy

Post by JoshBernstein »

Well, im currently out of town, but when i return i will try the suggested actions/measurements and report back. Ill also most likely replace the ot with an actual ac30 replacement, seeing as it is much larger and will probably sound better.
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R.G.
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Re: ac30 sounds grainy/fizzy

Post by R.G. »

Do you **know** for sure that the output section is where the problem is?

You're doing a lot of messing about with the output stage and nothing seems to affect it. Is is possible that an earlier tube is misbiased and getting into (for instance...) grid clipping at higher signal levels?

Tinkering with a transmission forever won't fix a carburetor or fuel injection problem.
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JoshBernstein
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Re: ac30 sounds grainy/fizzy

Post by JoshBernstein »

That is a possibility, the only things I've done in the preamp are tube swaps and coupling cap changes. What should i look for?
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10thTx
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Re: ac30 sounds grainy/fizzy

Post by 10thTx »

Try an "enhance" cap across the plate resistor to the entrance of the LTPI. Try 120p to 220p range. Maybe try a 12AY7 or 5751 in V1.

With respect, 10thtx
R.G.
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Re: ac30 sounds grainy/fizzy

Post by R.G. »

JoshBernstein wrote:That is a possibility, the only things I've done in the preamp are tube swaps and coupling cap changes. What should i look for?
A good start would be to make yourself up a table of plate, grid, and cathode DC voltages with no signal into the amp for all the tubes.

This tells you pretty quickly if the tubes are biased properly. Grids that are supposed to be at ground (that is, grid leak resistor to ground is how they work) will measure OK, but some grids balanced up at a positive voltage will be loaded and moved around by the loading of the meter. The plates and cathodes will still read OK on those.

The ticks and pops you hear when touching your meter probe to plates, cathodes and grids are one way of diagnosing dead tubes or broken signal path, by the way. You're disturbing the voltage by touching the meter to it, and this makes a tiny <tick> each time. This is amplified by all stages after the touch point, so as you go from output to input, the ticks get louder. When they stop, you have passed a "dead" stage. Not your problem in this case, but a handy quick debug technique.
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JoshBernstein
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Re: ac30 sounds grainy/fizzy

Post by JoshBernstein »

Well im back home and have tinkered around a little bit more. I replaced all the ceramic disks with silver mica's, and added a 220pf "enhance cap." Also tried putting a 12au7 in v1, and also tried it in the pi. I left it in the phase inverter position because i like the larger range of clean tones (i understand i dont have any more head room, i feel like i can control it better with the volume knob). All of these tweaks have helped a bit with the fizzing problem, but its still there and preventing me from getting the sweet overdrive im after. Ill try to post a voltage chart tomorrow, but any more suggestions in the mean time? Thanks!
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10thTx
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Re: ac30 sounds grainy/fizzy

Post by 10thTx »

I have built 3 different EL84 amps. Owned an old Epiphone and a Carvin amp that used EL84's.

Got rid of ALL 5 of those. I just don't like that tube. I have yet to play an amp with EL84's that I liked. As much as I've wanted to, I have never bonded with that tube. They seem inherently fizzy/buzzy/grainy to me. Just an issue of personal taste not saying my perspective is "better".

I would be inclined to change two of the tubes to 6V6's and have separate cathode resistors and caps. That might make for a nice sounding amp?

I also like 6AQ5's and 6BM8's really well. One could VVR the plates and the screens to reduce voltage to use these tubes & run the EL84's at a lower voltage which may sound less fizzy/harsh?

With respect, 10thtx
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JoshBernstein
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Re: ac30 sounds grainy/fizzy

Post by JoshBernstein »

If it comes down to it, i may end up changing to 2 6v6's, but it really dont want to do any more drilling. Im going to try a 5y3 with the amp in half power mode to see if lower voltages will help. If that does anything, ill give the vvr a try. Also, i just noticed last night, me preamp voltages seem pretty low. Ive got 150v on the plate of v1, and I've got 350 on the other side of the 220k plate resistors. Is this an issue?
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M Fowler
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Re: ac30 sounds grainy/fizzy

Post by M Fowler »

10thTx wrote:I have built 3 different EL84 amps. Owned an old Epiphone and a Carvin amp that used EL84's.

Got rid of ALL 5 of those. I just don't like that tube. I have yet to play an amp with EL84's that I liked. As much as I've wanted to, I have never bonded with that tube. They seem inherently fizzy/buzzy/grainy to me. Just an issue of personal taste not saying my perspective is "better".

I would be inclined to change two of the tubes to 6V6's and have separate cathode resistors and caps. That might make for a nice sounding amp?

I also like 6AQ5's and 6BM8's really well. One could VVR the plates and the screens to reduce voltage to use these tubes & run the EL84's at a lower voltage which may sound less fizzy/harsh?

With respect, 10thtx
I disagree the EL84's are excellent whether 18w Matchless, Vox, Trainwreck, or other various circuits I've built. The TW Rocket and LP are excellent as well. The only amp I had fizzy problems with was my 18W JCM800 which I increased the voltage by using SS recto plug replacing the 5AR4 tube. Increased total voltages took care of that annoying fizz.

Mark
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Littlewyan
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Re: ac30 sounds grainy/fizzy

Post by Littlewyan »

Well a couple of issues here that could be the cause.

1. I thought the Vox AC30 had a 20H Choke which has a 500Ohm Resistance (something that PDF64 suggested).

2. A small Fender OT is being used instead of the nice big Vox OT.

The Choke could be the cause of the EL84s running a bit hot, or one of the EL84s is way off.

The small OT could be the cause of the fizzyness. Although I will say I had an 18W Amp I built from a kit a few years ago and that was very fizzy. I hated it. But I was too inexperienced to tweak it so I sold it instead. Still regret that! Anyway perhaps try different EL84s. Could be as simple as that.
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