Blackstar HT-5 Tube Swap

Discussion - HT-5 amplifiers.
Franck1001
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Joined: Tue Mar 24, 2015 9:22 pm

Mon Apr 13, 2015 10:05 pm

https://www.youtube.com/watch?v=PtvsRn9H29Q

In this video I see that I can set bias only measuring a resistor.
Can I do the same with de HT-5? And what resistor is? And what is the correct value?
Thanks!

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thephantum
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Tue Apr 14, 2015 2:41 pm


gordoflaco
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Joined: Mon Jun 01, 2015 11:35 pm

Mon Jun 01, 2015 11:39 pm

need help

just replaced the tubes on my HT-5c and now everything is suuuper quiet.

i thought i didnt have to worry about bias with this product, am i wrong?

i replaced with a JJ ECC83 and a ehx 12bh7. did i pick bad tubes? im now noticing that this JJ tube has an S after the ECC83 name. Did i screw up and get the wrong tube?

help, im a big dumb dummy.

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thephantum
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Tue Jun 02, 2015 12:40 am

The S means nothing with the JJ tube, it's an ECC83/12AX7.

The only Blackstar tube amp that does not have to be biased is the HT-1. So yes, the HT-5 needs to be biased when changing the power tube. If you are not comfortable with poking around inside a live amp, take it to a tech. If you want to bias it yourself, have a read here on the previous page of this thread: viewtopic.php?f=32&t=92&start=120#p22870

While the amp needs to be biased, I don't think your problem is due to bias. It sounds like you might have gotten a bad tube. Try putting the old ones back in one at a time to see if the volume comes back.

jaymenon
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Thu Jul 07, 2016 9:44 am

Hi Phantom

I'm wondering - why would you make a bias probe for the HT-5, an amp that doesn't have any external adjustment points? (i.e. any adjustments need to be made with the two trimpots on the PCB? I'm obviously missing something...

And where can I get the parts to make one please? I have a TAD bias master with which it would be useful. (I live in the UK).

Kind regards
Jay

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thephantum
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Fri Jul 08, 2016 8:11 pm

I'm not sure I understand the question...the HT5 has two trimpots on the PCB: one for adjusting the overall bias and the other for adjusting the bias balance across the triodes. If you are shunting the OT, you don't need a probe...but if you are using the cathode resistor method, you need to insert the resistor(s) between the cathode and ground. To do that, you can either permanently install them or use a probe that inserts it/them for you.

Things might have changed but, when I made my probe, no one was manufacturing bias probes for the 12BH7. As long as the TAD Bias Master allows you to set the plate dissipation of the tube to 3.5W, you should be able to make a probe to plug into it. If not, then the point is moot....

You should be able to source most of the parts from any electronics supply store (online or otherwise). The toughest thing to find was the 9 pin sockets. I actually got those off eBay. Do a search on '9 pin socket saver' and you'll get a bunch of hits. Just make sure you get one that is held together with a screw and nut. I used two because they are small and it was much easier to wire it that way.

squicker
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Joined: Mon Aug 01, 2016 1:13 pm

Mon Aug 01, 2016 1:17 pm

Just to be clear on biasing.

I've got an HT5-RS, which I believe has the Electroharmonix 12BH7A power tube installed. I've just ordered a replacement (think the valves just died at 4 years old) Electroharmonix 12BH7EH Gold, which I believe is a direct replacement. Even though the valve is same brand, same code, am I correct to say I DO need to bias the amp to be sure?

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thephantum
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Mon Aug 01, 2016 5:22 pm

Yes, the amp will still need to be biased. The only exception is if you purchase a matched pair of tubes, install one and then bias the amp. In that case, you can swap in the second (matched) tube without biasing the amp.

squicker
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Tue Aug 02, 2016 5:03 pm

thephantum wrote:Yes, the amp will still need to be biased. The only exception is if you purchase a matched pair of tubes, install one and then bias the amp. In that case, you can swap in the second (matched) tube without biasing the amp.
OK thanks, that is as I thought. Turns out my amp has a TAD power tube in it anyway, so going to an EHX means a re-bias is definitely prudent.

Interestingly when I asked BS for the biasing info they refused to give it to me on reasons of safety, saying I need to go to my local service centre (as I think they have said for others). So I challenged them, explaining it was a 50 miles round trip to my nearest BS service centre, so they said they'd give it to my nearest qualified technician but they can't give it to me for safety reasons.

So they are complying with consumer law by not being protectionist and forcing me to use BS, but they clearly worry about potential litigation when people kill themselves from sticking a screwdriver in the wrong place!

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thephantum
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Tue Aug 02, 2016 7:00 pm

Here's the secret: If you ask them for biasing information, they automatically know that you don't know what your doing. :mrgreen:

The only proper way to set bias is to look up the plate dissipation rate of the tube type, then measure plate voltage, do the math and adjust bias current accordingly.

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