Blackstar HT-5R Mk2 MkII Cracking Noise Issue / New valves + Bias on Mk2?

[Maciek_Brist]

Hi guys

I'm having some issues with the otherwise great Blackstar HT-5R Mk2. I've seen a lot of topic related to somehow troublesome mk1, overheating, dead capacitor etc, but not much about the recent Mk2.

I bought it second hand couple of months ago and it was all good until two weeks ago when it has developed cracking/poping noise and the regular hum seems to be louder as well. Theres a link to sound sample, hopely showing clearly what I mean. I've recorded start up of the amp, then in 5-10 second that noise shows up and come back every 20-30 sec. https://soundcloud.com/nooneknowme/b...ht5r-mk2-noise

Does it sounds to you like a valve issue?

I think I will order new set of valves but then I'm quite hopeless when it comes to biasing this thing... can't find anything useful on Mk2. Does anyone know the procedure for those? I guess it may be somehow similar to Mk1 but the PCB looks way different.

I've made some detailed photos of the whole thing.
See the whole album here
https://flic.kr/s/aHsmUGpLoT

Help much appreciated!
Cheers
Maciek

[The Dude]

Got a schematic? Upload if so.
I'd first try the effects send to another amp and see if the noise is there. Also try going directly into the return with signal. The point is to see if you can narrow it down to a preamp or power amp problem.

[Mick Bailey]

I haven't yet seen a Mk2 but the Mk1 is a frequent amp for repair. I notice one of the main problems with the earlier amp is still present with this one - the proximity of those two electrolytic caps to the MOSFETs which causes them to dry out. Valves would be the easiest thing to check, though I'm also unfamiliar with the biasing of this amp. TP13 marked at 46mV appears to coincide with the bias setting for the previous model but could be unrelated.

There are many possible component failures that could give rise to these symptoms and these are complex amps. If the problem isn't a valve issue then it's going to be more involved tracking down what's wrong, especially without a schematic.

[Maciek_Brist]

Hey The Dude, thanks for the reply!
I dont have a schematic im afraid.
I did a test with going directly into the return and there was no cracking/poping noise whatsoever.
Given that, Im assuming its the preamp valve that might have gone bad, am I right?

Many thanks

[Mick Bailey]

Now you've eliminated the output section, replacing the preamp valve would be the next step as it's the easiest part to replace and noisy valves are a common problem.

[Maciek_Brist]

Hey guys, many thanks for help. I finally had my hands on new valves and have replaced the preamp one. Aaaaand the cracking/poping noise is gone!
Also some nice high end came back and generally the amp sound more lively! Having said that, I've seen a lot on different forums/pages that you don't have to bias while swaping only preamp tube as its self-biasing. I've swaped TAD's ECC83WA with JJ's ECC83S.

I have also a new matched power tube (bought full revalve kit) but if I can get away with no biasing while chaning only preamp one than I guess i'll leave it as is. Otherwise I will need to dig deeper to have some idea on biasing mk2, which apparently is not well known.

Anyhow. Many thanks for pointing me towards right direciton! Sorted!

Cheers

[Mick Bailey]

Excellent. Yes, the preamp tubes don't have any requirement for bias adjustment. Until you find out how the biasing operates, I'd leave swapping the output valve for another time. My hunch is the 46mA at TP13 is the bias test point and TP8 is the voltage reading for high and low power respectively, but I could be completely wrong.

As a real outside chance, you could try emailing Blackstar and asking them. I've never had any luck whatsoever, though. YMMV, as they say.

[g1]

My hunch is the 46mA at TP13 is the bias test point and TP8 is the voltage reading for high and low power respectively, but I could be completely wrong.

I don't think it could be anything else. There is a 4R7 with parallel diode coming off the cathode in the pictures. That is exactly what the MK I used, and it also set for 46mV (10mA with around 450V at plate).

[Mick Bailey]

Agreed - I did say in post #3 "TP13 marked at 46mV appears to coincide with the bias setting for the previous model...." My caveat is just down to not having worked on one of the MkII amps - I'd like to know what the relationship is between TP8 and TP13 - if the bias changes, does TP8 and If so, what's the significance of TP8?

[g1]

I'd like to know what the relationship is between TP8 and TP13 - if the bias changes, does TP8 and If so, what's the significance of TP8?

Pure speculation, but I think TP8 must be raw bias after switch but before 'set bias' pot. The TP8 appears to be connected to the 'bias balance' pot so that should agree with it being raw bias if comparing to HT5C schematic (attached). I guess it would be a spot to check raw bias levels are good and power level switch working.