HT5 MK2 Odd Noise (Now With Circuit Photo)

Grumpyrocker

This is probably nothing and I'm being paranoid. But I thought I'd ask anyway.

So I've got a new MK2 HT-5 head and when I switch it on there's a faint crackle/buzz with a slight high pitched whistle that comes from the head itself all the time it is on. It's really quiet, and easily drowned out by even relatively quiet playing. But I do find the slight whistle a bit annoying, even though it's on the edge of hearing.

But I wonder if it is the sign of a problem somewhere?

The sound isn't coming from the speaker. And is still there if I run the amp with no speaker. It doesn't get louder when I change any controls. And as I said it's fairly quiet, but noticeable when I'm not playing.

EDIT - Sound coming from over by either the power valve or output transformer.

I've owned some very noisy amps in my time so I don't know why this thing is bothering me. But the HT5 is otherwise ridiculously quiet, so did wonder if this whistle was a sign of a potential problem. 

Grumpyrocker

Great. Just broke the fucking top of the power amp valve trying to get the ridiculous spring clip off.

FML

ecc83

Grumpyrocker said:

Great. Just broke the fucking top of the power amp valve trying to get the ridiculous spring clip off.

FML

Ha! I have "de-clip[ped  scores of those valves and the even fiddlyer one in the pedals and never broke a valve. PM me Grumps and I'll send you a new 12BH7, which must I think be the source of the noise?  Or, take it to Phil at B's? I am sure he will treat you kindly!

Dave.

Grumpyrocker

Cheers Dave. I'll have to find someone to rebias,  easier said than done in the wasteland of West Dorset. 

Grumpyrocker

I'm not having a good day. Since my last post I've tripped up the stairs (no injury), have dropped this amplifier, and have also knocked my PC over. 

ecc83

Grumpyrocker said:

Cheers Dave. I'll have to find someone to rebias,  easier said than done in the wasteland of West Dorset. 

Re-biasing a Five is not that difficult nor dangerous but I would only tell you via email. You would need a digital test meter.

Dropped it now?! House insurance job? Actually that is about the only reason for the valve clips, same for the HT pedals, they are there to survive the boat trip. Once here they are pretty well set without them.  Course! They don't expect the punter to chuck 'em down the stairs!

Dave.

impmann

Dave,

Its a Mk2... Slightly different.

Grumpyrocker, if you can’t get it sorted, pm me.

Grumpyrocker

I only dropped it about a foot. It all looks fine. 

I do have a multimeter so I may have a crack at it. Though MK2 data not easy to come by. Unless power section is same as MK1. It does have a bias adjust and balance adjust. 

Grumpyrocker

Gratuitous gut shot anyone? Lots of surface mount stuff in there. 

Grumpyrocker

For anyone interested the bias test point in the MK2 is from R279 to TP13 (I believe it was R33 in some of old HT5?). and while I've measured 5ohms across there with the amp cold and disconnected it was rather fiddly. You'd have to have much steadier hands than mine to poke at those tiny points with a multi-meter with the amp live to set the 46mV. 

So I'm going to have to take it in to my nearest Blackstar place and get them to do it. My eyes (and hands) aren't up to it.

Best to know one's own limits. I'm not very dexterous. 

ecc83

WTF?! 550 V DC? That is well over the original which was around 460 V iirc. If there is indeed 550V ish on the H7 anodes the amp should put out a tad more than 5W. I am not concerned about the OP valve but when folks drive the ***t of it there will be some serious voltage spikes around.

Yes the sense resistor is 5r7 and rarher than have a probe on the test point I would advise mini crocs or latching probes across the resistor or its parallel diode. The circuit rerference Nos are all different and I no longer have maps for the new stuff.

Balance you can set with Mk1 lug against a speaker, tune for minimum hum. Balance is somewhat interactive with bias.

OOOPS! FOUR R seven!

Dave.

Grumpyrocker

Sorted. Fiddly. But sorted. 

Miguel

Grumpyrocker said:

Sorted. Fiddly. But sorted. 

Hi,

Just bought a new HT5R MK2 head this week with the same problem as you described here, pretty annoying for a practice amp.

In comparison with the HT5R from the previous version the whistle from power of this new unit is 2 or 3 times higher, when I turn the MK2 on (nothing plugged) I can ear it in all the room when not playing, or even when playing cleans at low volume.

Was your problem solved by replacing the power tube and adjusting bias?

If yes, do you recommend some specific model/brand?

Thank you

impmann

Miguel said:

Grumpyrocker said:

Sorted. Fiddly. But sorted. 

Hi,

Just bought a new HT5R MK2 head this week with the same problem as you described here, pretty annoying for a practice amp.

In comparison with the HT5R from the previous version the whistle from power of this new unit is 2 or 3 times higher, when I turn the MK2 on (nothing plugged) I can ear it in all the room when not playing, or even when playing cleans at low volume.

Was your problem solved by replacing the power tube and adjusting bias?

If yes, do you recommend some specific model/brand?

Thank you

If it’s brand new, don’t fiddle with it yourself - you could invalidate your warranty.
Send it back to the shop you bought it from.

ecc83

Impmann, the retailer is your first port of call for a faulty product and any kind of noise from an amplifier such as you suggest IS a fault. A "whistle" is slightly worrying since it implies some form of instability and that simply should not happen. A buzzy mains traff does happen but a whistle could develop into a serious fault.

I guess Blackstar is in lockdown as a mnfctr but I would think the service dept is still working?

Dave.

Grumpyrocker

I think it's just some transformer buzz. I don't even notice it anymore. 

ecc83

Grumpyrocker said:

I think it's just some transformer buzz. I don't even notice it anymore. 

Ok Well I did have a couple that did that but they should not! Not a bloody "whistle" tho'but!

A transformer swap is less than an hour's work, all plugs iirc.

Dave.

Grumpyrocker

There's no whistle now. I don't know what that was. But it's been gone a long time. Just a quiet buzz/sizzle when the amp is on. Which you can't hear from more than a foot or two away from it. 

ecc83

Grumpyrocker said:

There's no whistle now. I don't know what that was. But it's been gone a long time. Just a quiet buzz/sizzle when the amp is on. Which you can't hear from more than a foot or two away from it. 

The "whistle" might have been a bit of RFI that has now gone away. Often a buzzy traff can be fixed by simply giving the mounting screws a bit of a tighten. I have never known an HT-5 mains transformer fail.

Dave.

impmann

ecc83 said:

Impmann, the retailer is your first port of call for a faulty product and any kind of noise from an amplifier such as you suggest IS a fault. A "whistle" is slightly worrying since it implies some form of instability and that simply should not happen. A buzzy mains traff does happen but a whistle could develop into a serious fault.

I guess Blackstar is in lockdown as a mnfctr but I would think the service dept is still working?

Dave.

The Service dept has been working throughout (in fact, the whole company has but with a lot of remote working etc)... obviously "drop ins" are not appropriate or accepted This particular unit needs to go back to the store it was bought from as it shouldn't make that sort of noise what ever the cause.

ecc83

I am obliged for that information Impman.

Dave.