#1
Hey Everyone,

Ive just brought a blackstar ht5r combo to use for home practise. I got it second hand at a decent price and the guy who sold it to me threw in some spare valves as the original valves from the manufacturer are still in the amp.

Someone sold me that you do not have to re-bias blackstar ht-5r's but im not sure? I have only re-valved class a amps before that did not need re-bias, but i thought the ht5 was A/B meaning it would need rebias.

The valves i've got to go into it are EHX 12BH7 Gold and Tung-Sol 12AX7. I know i can switch the pre-amp tube strait over but will i need to re-bias if doing the power amp tube?

Many Thanks.
#2
Looks like its auto-biasing

but until you find out for sure, i would only put the same type of tube in it
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#3
i will be using a 12bh7 tube which is the same as the tube in there now, just a different brand. Will this be ok to do? I cant get a confirmed answer but it looks like the amp might be self biasing anyway?
#4
then you should be fine.
My Rig:
Maverick F-1, Ibanez RG1527, Schecter Omen 8
Marshall JVM 410H,
Hand built 4x12 w/ V30s
Current board:
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TU2
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MXR M132
MXR M101
TIMMY
NS2
MXR M108
XOTIC EP
TC DREAMSCAPE
DL8
#6
It's not auto biasing. The power tube - the 12bh7 - will have to be biased correctly. But many people have replaced the power tube without re-biasing and that seems to be ok, esp if you're using the same brand of tube. But if you want to be sure...

A post from the Blackstar Forum:

"I did some further investigation on te bias part of the HT5.
It has an adjustable Bias system.
The bias system has two blue trimpots,one is to set the required DC voltage,the other one is to balance the voltage even over the 2 sides of the 12BH7 tube.

WARNING !!!.

IF YOU ARE GOING TO BIAS THE AMP,THE AMP HAS TO BE POWERED UP,AND YOU WILL HAVE TO MAKE SOME MEASUREMENTS ON THE PCB,AND SO THERE ARE LETHAL VOLTAGES ACTIVE ON THE PCB,DON'T DO THIS IF YOU DON'T KNOW WHAT YOU ARE DOING,AND GET SOMEONE WHO KNOWS WHAT HE OR SHE IS DOING,TO HELP YOU OUT.
THESE VOLTAGES CAN SERIOUSLY HURT OR KILL YOU !!!!!.



^^^^
You see a close-up of the "Bias" part of the main pcb ,the blue trim pot on the right is used to set the bias voltage,the trimpot on the left is to balance the voltage between both sides of the 12BH7 tube,so that both sides get the same bias voltage.



^^^^
You see the points where a measure the bias voltages.

At the right on the picture you see the voltage set trimpot,there I measure the voltage at the black dot above resistor R6 ( I cal it TP=Testpoint),and the voltage reads -26,7 Vdc.

At the left you see the balance trimpot,and at the left and right of the trimpot the resistors R31 and R35 ,there I measure the voltage at the black dot at the bottom of the resistors ( I cal it TP=Testpoint),and the voltage reads -25,4 Vdc.

The values are the stock factory settings.
I don't know what a 12BH7 tube can handle,but I'm going to try to find that out... "
Last edited by J-E-M at Aug 20, 2014,
#7
ok thanks Jem, I went ahead and put the new tubes in and it makes the amp sound alot better! i havent had any problems and i have re biased it. Will it damage the amp if i leave it in? and will i have known by now if the bias is wrong??
#8
i was under the impression it had to be biased, but i could be wrong.
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Quote by K33nbl4d3
I'll have to put the Classic T models on my to-try list. Shame the finish options there are Anachronism Gold, Nuclear Waste and Aged Clown, because in principle the plaintop is right up my alley.

Quote by K33nbl4d3
Presumably because the CCF (Combined Corksniffing Forces) of MLP and Gibson forums would rise up against them, plunging the land into war.

Quote by T00DEEPBLUE
Et tu, br00tz?
#9
So if it does am I causing damage to the amp by just putting a new tube in their or is it just gonna make the valve not last as long?
#10
I'm actually gonna retube my black star eventually too. Let us know how it turns out.
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#11
Quote by billabob
i will be using a 12bh7 tube which is the same as the tube in there now, just a different brand. Will this be ok to do? I cant get a confirmed answer but it looks like the amp might be self biasing anyway?

Those are also known more widely as EL84
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#12
Quote by Robbgnarly
Those are also known more widely as EL84


No they are not.
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#13
Quote by godgrinder
No they are not.

good catch, I was thinking of the 6BQ5.
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#14
12bh7 are normally pre amp tubes if im correct? Well so far so good with no rebias? Ami just waiting for this thing to blow up though or is it safe?
#15
^ it's not as simple as that, unfortunately. preamp tubes (generally 12ax7s or similar) have two triodes in them which normally are in different parts of the preamp circuit and which don't need to be biased.

however in the poweramp those two triodes are used like two separate bigger power tubes would be in a bigger amp, in a push-pull type circuit, and generally amps with that style of power amp have to be biased (not always, sometimes the bias is "fixed non-adjustable", but it's safer to assume it has to be unless you're sure it's not).

I'm not sure about the damage thing. Some of the other guys can hopefully help more when it comes to that. If you're lucky it'll be bang on with the bias already (or at least close enough where it won't do any harm). If not, I dunno if it just lowers tube life or can do anything more serious...
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Quote by K33nbl4d3
I'll have to put the Classic T models on my to-try list. Shame the finish options there are Anachronism Gold, Nuclear Waste and Aged Clown, because in principle the plaintop is right up my alley.

Quote by K33nbl4d3
Presumably because the CCF (Combined Corksniffing Forces) of MLP and Gibson forums would rise up against them, plunging the land into war.

Quote by T00DEEPBLUE
Et tu, br00tz?
#16
It needs biasing. Listen to JEM.
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#17
Ok , ive been looking into biasing and i think i got this. Anyone reccomend a good biasing multimeter from the uk?? I know its dangerous but iv'e done some amp maintenance before and come out ok.
#18
Quote by billabob
Ok , ive been looking into biasing and i think i got this. Anyone reccomend a good biasing multimeter from the uk?? I know its dangerous but iv'e done some amp maintenance before and come out ok.


...find someone that can show you and that has preferably done this on a BS before. Any decent guitar Tech at a shop should let you sit in whilst doing this procedure....it´s not hard but because of the danger of electrifying yourself and or screwing up your amp it should always be done by a professional

NOTE; this will affect warranty if you still have one and all work should be performed by an authorized BS tech in that case
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#19
Quote by Blackst4r
...find someone that can show you and that has preferably done this on a BS before. Any decent guitar Tech at a shop should let you sit in whilst doing this procedure....it´s not hard but because of the danger of electrifying yourself and or screwing up your amp it should always be done by a professional

NOTE; this will affect warranty if you still have one and all work should be performed by an authorized BS tech in that case


I agree if you can find someone to help show you that would be good, but it is not a requirement at all.

Biasing is a very simple thing to do

and it will not void your warranty to do it yourself at all or to have someone other than a BS service place do it.
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#20
my ht1 metal only has preamps and you dont have to bias. really suprised with the ht5 you have to.
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#21
How did your experience with the biasing of the Ht-5 turn out?

I would be interesting in knowing exactly what you have to do after you put the new power tube in.

Thanks for the feedback.
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#22
So I put new eh tubes in, I took it apart to bias the amp I took the reading and it was 27.6 and was balanced so I just left it alone. I would say don't bother biasing just change the tubes you will be fine.
;-)