Tube Gone?

ball ox

hey,
My Vox AC30 started doing bad things at rehearsals the other day. The best way to describe it is..... if i hit a note and let it ring out the sound begins to crackle or "fizzle" as the note rings (only a few secs after hitting the note).
2 Questions- 1) does it sound like a tube is gone
2) If so, is it bad for the amp if I use it while waiting on a new tube

Also, havnt a breeze about tubes, can anyone recommend a decent brand for an AC30 (assuming it's the tube that's the problem)

Eoin Madsen

1) It could be a failing signal valve. But other failing components might generate the same effect.
2) Valves fail destructively. If a power stage valve goes it could potentially end your amp, so playing if there's any kind of undiagnosed valve problem is inadvisable.

How old is the amp, and when was it last revalved? An AC30 has a quad of (matched) EL84s in the powerstage, an ECC81 in the tremolo circuit, and a whole bunch of ECC83s (number depending on model). You kinda need to know what's wrong before you can replace it.

Glassheart

I'm having a similar problem with my AC30 only i'm getting it when i hit the note.
I only bought mine back in April so i'm wondering if it's a tube problem...

Eoin Madsen

Tbh, lots of amp problems are characterised by crackling or fizzling, so it's pretty hard to narrow something down to a valve purely on that.

Stock amp valves are junk, period. They're cheap, you don't know how old or abused they are prior to purchase, and frequently they've been knocked around in transit. The golden rule is, if you buy a valve amp, buy a full set of (better) valves at the same time. Change power valves annually after that, and preamp valves every two years (at least) if the amp is getting use. Rectifiers usually operate to full spec a while longer, I think, but also need to be changed periodically.

There are some AC30 forums about where people could suggest replacements. Plexipalace is a good place to start. JJ/Teslas in an AC30 seem to be pretty modern "indie" sounding to me. I think something cleaner would be implied if one was going for a more classic AC30 sound. Eg, the "harma" valves from Watford Valves in the UK. I have no experience with any other kinds of valves in an AC30, but I'm sure there are a lot of options out there. Last I checked (a couple of years ago, admittedly), the Sovteks and EH valves you get in the shops here were expensive but barely better than the Chinese made generic valves that come as stock in most amps. I would shop internet only to be honest.

ball ox

Eoin Madsen wrote: »

Tbh, lots of amp problems are characterised by crackling or fizzling, so it's pretty hard to narrow something down to a valve purely on that.

Stock amp valves are junk, period. They're cheap, you don't know how old or abused they are prior to purchase, and frequently they've been knocked around in transit. The golden rule is, if you buy a valve amp, buy a full set of (better) valves at the same time. Change power valves annually after that, and preamp valves every two years (at least) if the amp is getting use. Rectifiers usually operate to full spec a while longer, I think, but also need to be changed periodically.

There are some AC30 forums about where people could suggest replacements. Plexipalace is a good place to start. JJ/Teslas in an AC30 seem to be pretty modern "indie" sounding to me. I think something cleaner would be implied if one was going for a more classic AC30 sound. Eg, the "harma" valves from Watford Valves in the UK. I have no experience with any other kinds of valves in an AC30, but I'm sure there are a lot of options out there. Last I checked (a couple of years ago, admittedly), the Sovteks and EH valves you get in the shops here were expensive but barely better than the Chinese made generic valves that come as stock in most amps. I would shop internet only to be honest.

Thanks for the info Eoin, sheds a bit of light on things. I am still using the stock valves so might pop over to thomann. Do you know if its a big job to change them by yourself?
I was testing the amp at home yesterday and it seemed grand
Could be a patch lead in my effects chain. Could also be that it only occurs when the amp is cranked (couldnt try that at home ).
Think I'll change out the valves anyway as you have suggested.
cheers

Eoin Madsen

The AC30 is cathode biased which means that you don't have to make a bias adjustment when changing valves in the power stage as you would in a grid biased amp. So basically, the replacements just need to be dropped in. Be careful if you remove the chassis from the cab, as the insides of the amp can contain stored voltages which are potentially fatal. Don't forget to change the 5AR4 rectifier valve as well.

There's a few different valve stores around, so check prices etc. I didn't find Thomann to be especially good in terms of selection or even price. I imagine a dedicated valve shop will have better valve testing and matching procedures and equipment as well. Personally, I usually go with Watford. They can also make a recommendation, and they'll know which valves an AC30 needs if you tell them which year it's from (there's a couple of small variations). It costs a little more, but as I see it, if you have a top end valve amp it doesn't make any sense to skimp on the valves just to save the price of a few pints.

But then, I quit drinking 5 years ago. :pac:

Glassheart

Sorry for hijacking your thread ball ox...:)

Thanks Eoin you have been very helpful.

I got an AC30cc2x.On the vox website the tube complement is:
4 x EL84/6BQ5
3 x 12AX7/ECC83
1 x GZ34

Would i have to replace my tubes with these exact model numbers?
For example,they have a lot of options for EL84's but only 3 options for EL84/6BQ5 which are Phillips and GE.

I've been recommended Harma's a few times and was hoping to replace the whole lot with Harma's...

Eoin Madsen

Yes, but not exactly. Just to clarify:

ECC83 = 12AX7
ECC81 = 12AT7
GZ34 = 5AR4
EL84 = 6BQ5
Valve = Tube

The valves are the same. The different names mostly arose out of differing naming conventions between US and UK manufacturers. An ECC83 (UK) is the same spec valve as a 12AX7 (US). Likewise for EL84/6BQ5. I presume 6BQ5 is a much less commonly used name, as EL84s have always been regarded as a British valve, and US made versions were probably not popular.

Glassheart

Eoin Madsen wrote: »

Yes, but not exactly. Just to clarify:

ECC83 = 12AX7
ECC81 = 12AT7
GZ34 = 5AR4
EL84 = 6BQ5
Valve = Tube

The valves are the same. The different names mostly arose out of differing naming conventions between US and UK manufacturers. An ECC83 (UK) is the same spec valve as a 12AX7 (US). Likewise for EL84/6BQ5. I presume 6BQ5 is a much less commonly used name, as EL84s have always been regarded as a British valve, and US made versions were probably not popular.

Thanks smartarse! :pac:

What about mixing brands?
Would i get away with 4 Harma EL84's and 3 Mullard 12AX7's for example?

Voodu Child

Glassheart wrote: »

Thanks smartarse! :pac:

What about mixing brands?
Would i get away with 4 Harma EL84's and 3 Mullard 12AX7's for example?

Your power-valves (the big ones ie EL84, EL34, 6L6 etc) should all be the same brand and age as each other.

The preamp valves are pretty much up to taste. They can be different brand to the power-valves, and different brands compared to each other. Guys who are tweakers will often mix and match different preamp valves because they know certain ones will do certain things. For the average (non-tweaker) guy, its simpler just to buy them at once and all the same.

Eoin Madsen

Glassheart wrote: »

Thanks smartarse! :pac:

What about mixing brands?
Would i get away with 4 Harma EL84's and 3 Mullard 12AX7's for example?

Yes. As Voodoo said, lots of people mix brands for various tonal reasons. The first ECC83 in your AC30 is input gain stage for the brilliant and normal channels (volume pots follow this, and then the signals are summed), the second ECC83 is the second gain stage and the feed for the tone stack, the third ECC83 is the phase inverter. The phase inverter is what feeds the two halves of the power stage.

Since the number of subsequent gain stages is highest from the first ECC83, people often make a point of putting a valve with low microphonics in there. Selecting a phase inverter with a lower gain value can be done to shift the gain structure in favour of more preamp distortion over power stage distortion. And so on. For starters, I'd just get a set of the Harma cryos or something like that and then maybe pick up a few different valves to experiment with later. There isn't much that can go wrong here anyway.

The power stage works in opposing pairs of valves (push/pull) so they have to be roughly balanced. Definitely don't mix and match here unless you know what you're doing. Only use matched quads (they will be tested and matched already by Watford or whoever you buy from) or span matched pairs (one from each on either side). Matching is done by measuring the electronic characteristics of the valves and is not directly related to manufacture or age - two valves manufactured by the same company at the same time could have as widely varying characteristics as any given random pair of valves.