AC15C1 Noisy as F@*K!

ElectricXII · August 2023

At soundcheck last night, my 2016 AC15C1 was much hissier and generally noisier than usual. I suspected a duff preamp valve but undoing umpteen screws on the back to get at the valves wasn't an option, so I played the gig, having to deal with bad buzzing when not actually playing.

This morning I changed out the three AX7s and also fitted some brand new JJ EL84s. This hiss is a bit better, but still much worse than it used to be.

Any ideas what could be causing this? If it was humming too, I'd suspect the electrolytic caps.

ElectricXII · August 2023

No one?

ICBM · August 2023

It's not a good idea to change all the valves at once - you have much less chance of being able to identify where the fault is.

First, put the old valves back in, making sure they go back in the correct locations - it matters with these amps, V2 is a different spec from the other two because it's a cathode-follower. If they're the original valves, V2 should be a Chinese-made one, with two bright chrome strips on each side of the plate structure. Don't put either of the other valves (most likely Tung-Sols) in this position.

Is the hiss dependent on the master volume and tone cut settings? If no, it's coming from the power amp. If yes, from the preamp. Is it on just the Top Boost channel, or both? Is it affected by the reverb setting?

ElectricXII · August 2023

@ICBM Thanks for the detailed response! I'm just back from work but I'll have a look at it again tomorrow. It was hissing with nothing plugged in, i.e the whole noise floor was raised, and the reverb control had no effect on the noise. Turning up the master volume made it worse, and gain pedals were an order of magnitude noisier, especially my Hot Cake clone.

Dave_Mc · August 2023

ICBM said:
First, put the old valves back in, making sure they go back in the correct locations - it matters with these amps, V2 is a different spec from the other two because it's a cathode-follower. If they're the original valves, V2 should be a Chinese-made one, with two bright chrome strips on each side of the plate structure. Don't put either of the other valves (most likely Tung-Sols) in this position.

Are Tung-Sols not ok in cathode-follower positions? I thought it was JJs- is it Tung-Sol too?

ICBM · August 2023

Dave_Mc said:

Are Tung-Sols not ok in cathode-follower positions? I thought it was JJs- is it Tung-Sol too?

Tung-Sols - like all Russian-made ones - are the most prone to failure. JJs aren’t so bad but I’ve still had a few. Of the modern ones, only Chinese are definitely safe.

Vox used to ship the CC series with Tung-Sols in V1 and 3, and a Shuguang in V2 - not sure if they still do, or why they didn’t just use Shuguangs throughout.

ElectricXII · August 2023

There are no Tung-Sols, or any branded valves, in my amp. They're all unbranded Made in China 12AX7 valves. The three preamp valves all look identical. Unfortunately I don't remember which valve was in each position, so I'm thinking of testing them in my Deluxe reverb to see if I can identify which if any are dodgy, as the spares i used to change them out are not new and could be noisy too.

ICBM · August 2023

ElectricXII said:

There are no Tung-Sols, or any branded valves, in my amp. They're all unbranded Made in China 12AX7 valves. The three preamp valves all look identical.

Ah, in that case they've just gone for the sensible option, and it doesn't matter which positions they go in.

Yes, if you test them in V1 or V2 in the Deluxe it should show up any that are obviously noisy. The Deluxe doesn't have any cathode follower positions other than the tremolo oscillator (V5).

Modulus_Amps · August 2023

Dave_Mc said:

ICBM said:
First, put the old valves back in, making sure they go back in the correct locations - it matters with these amps, V2 is a different spec from the other two because it's a cathode-follower. If they're the original valves, V2 should be a Chinese-made one, with two bright chrome strips on each side of the plate structure. Don't put either of the other valves (most likely Tung-Sols) in this position.

Are Tung-Sols not ok in cathode-follower positions? I thought it was JJs- is it Tung-Sol too?

Tungsols have a max heater to cathode voltage on the data sheet of 180v, some amps with cathode followers will be above this. Some manufactures elevate the heaters to reduce the risk of this and induced noise. Best to avoid them in cathode followers unless you know for sure.

I have used JJ in cathode followers for years without issues, ICBM has maybe seen more in for failure, but it doesn't take into account the number in use I guess. Saying that, it is not my favourite valve for that position.

ElectricXII · August 2023

@ICBM Thanks for your comments, as ever. I tested all the Made in China 12ax7s in my DR and they are all in the same ballpark noise wise. I've ended up with two Chinese 12ax7s in V1 and V2, and a JJ 12ax7 in V3. The amp is very slightly less hissy with the JJ in the phase inverter rather than a Chinese valve. The amp is better than on Saturday night, but still hissier than I would like.

I'm going to rehearse with it next week and see if it warrants further investigation.

ICBM · August 2023

In that case I suspect it's not valve related and may be an IC that's gone noisy - there are a couple in the signal path, one is the channel mixer stage and the other after the reverb and before the phase inverter - both are after V1 and V2, and before V3, so you can't identify which it might be by pulling valves. The bad news is that this is often a prelude to failure. JJs have less high-end response than Shuguangs so that would fit with the noise being generated before V3 as well.

ElectricXII · August 2023

Thanks @ICBM . Damn, that would be a board-out fix I suppose. I don't rate the techs near to me in Fife/Edinburgh that I've used in the past, so I might have to tackle it myself if a chip fails. Depends on collateral damage.

Dave_Mc · August 2023

ICBM said:

Dave_Mc said:

Are Tung-Sols not ok in cathode-follower positions? I thought it was JJs- is it Tung-Sol too?

Tung-Sols - like all Russian-made ones - are the most prone to failure. JJs aren’t so bad but I’ve still had a few. Of the modern ones, only Chinese are definitely safe.

Vox used to ship the CC series with Tung-Sols in V1 and 3, and a Shuguang in V2 - not sure if they still do, or why they didn’t just use Shuguangs throughout.

Ah ok, thanks. Sounds like I got it backwards! No real harm done- I bought a Tung-Sol a good while back for my (broken) Genz Benz Black Pearl and stuck it in the cathode-follower position, but as it's been broken it hasn't really been used at all. I'll swap it out for a Chinese one when I get round to getting it fixed (and I'll be able to use the Tung-Sol elsewhere where it's safe).

Modulus_Amps said:
Tungsols have a max heater to cathode voltage on the data sheet of 180v, some amps with cathode followers will be above this. Some manufactures elevate the heaters to reduce the risk of this and induced noise. Best to avoid them in cathode followers unless you know for sure.

I have used JJ in cathode followers for years without issues, ICBM has maybe seen more in for failure, but it doesn't take into account the number in use I guess. Saying that, it is not my favourite valve for that position.

Brilliant info, thanks

Howdy, Stranger!

Categories

In this Discussion

Become a Subscriber!

AC15C1 Noisy as F@*K!

Comments