What tube

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

Tue Mar 24, 2015 9:28 pm

Hi
What tubes have the Blackstar HT -5?
BIAS needs adjustment ?
And in case you need it , where I have to measure?
Thanks!

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thephantum
Posts: 1160
Joined: Wed Jan 29, 2014 7:42 pm
Location: Virginia, United States

Wed Mar 25, 2015 2:00 pm

What tubes have the Blackstar HT -5?
The HT-5 uses a 12AX7 in the preamp and a 12BH7 in the power amp
BIAS needs adjustment ?
Yes.
And in case you need it , where I have to measure?
https://www.blackstaramps.com/forum/vie ... 120#p22870

Franck1001
Posts: 3
Joined: Tue Mar 24, 2015 9:22 pm

Mon Apr 13, 2015 8:42 pm

Thank's a lot
But I saw in this video that I only need a tester, and in the post that you post he uses other thing.
https://www.youtube.com/watch?v=PtvsRn9H29Q

What is the resistor that I need to mesure and what value is the corrent?
Thank's!

PD: What tubes i need to sound more british? :D

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thephantum
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Location: Virginia, United States

Tue Apr 14, 2015 2:41 pm

I have seen this video before (as well as this method posted in multiple places) and IT IS NOT CORRECT. It will set the bias way to cool. I know this from personal experience as I own both an HT-20 and an HT-5R and have swapped tubes/re-biased multiple times in both.

Using the HT-20 as an example, it is a straight up, fixed bias amplifier. There is no "one size fits all" bias setting. Every amp is a little different and every tube is a little different. You need to measure the plate voltage, do the math to calculate desired bias current, measure and set the bias/balance, then rinse and repeat. The reason for this is as you adjust bias, the plate voltage will change slightly. So each time you adjust, you have to measure and adjust again until you get it dialed in.

Like most manufacturers, Blackstar sets the bias of the HT-20 amps cool from the factory. They ARE NOT setting them to 11.5v across R221, but they aren't measuring either. They are assuming a plate voltage of 300V and setting them to 60% of maximum plate dissipation, which is exactly 50mV at each tube. This is a very conservative setting. If plate voltage went as high as 350V it would be closer to the target...but it doesn't. It tends to live between 305V and 325V, depending on the tubes used.

These are from my own experience biasing my own HT-20H (and debunking the method detailed in that video):
  • - From the factory bias was set at exactly 50mV (out to 2 decimal points...exactly 50.00mV) per tube when measured across a 1ohm resistor at pin 8 with a plate voltage of 314V at pin 3. That's 63% of max plate dissipation which is cool. That setting, when measured at R221, was 11.7V...so setting it to 11.5 is going even colder. :shock:

    - With the stock (Ruby) tubes, rebiasing it to 70% of max dissipation was 56mV at each tube, with plate voltages of 308V. This measured 13.35V at R221.

    - With winged C's, biasing it to the same 70% yielded 54.7mV at each tube with 320 plate voltage and 12.25V at R221.
You can clearly see that different tubes have different plate voltages and bias settings. You can also see that, when setting the bias correctly on the HT-20 and then taking the measurement detailed in that video, it's not even close.

The HT-5 is even more sensitive to bias adjustment. As I said earlier in this thread, part of the reason the amp sounds so good is that they run the power tube on the edge of it's specs. There is a very narrow sweet spot where the amp absolutely shines (80-85% max plate dissipation). Go too high (hot) and the tube will redplate. Go too low (cool) and it will sound like a fart machine.

You are better off biasing the amp correctly. The tubes will live longer and the amp will sound better. You don't have to have a probe....you can use the OT shunt method and measure/set that way. A probe is just safer since you don't have to poke around at live components in the amp.

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thephantum
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Location: Virginia, United States

Tue Apr 14, 2015 2:59 pm

As far as tubes, in terms of new production tubes you have two choices: Electro-Harmonix (EH) or Tube Amp Doctor (TAD). In my opinion, both are mediocre at best. You are better off going NOS. Most of the online dealers (TubeStore, TubeDepot, etc) will have some NOS 12BH7's in stock and resonable priced (under $40 usually). The main ones out there are RCA, GE, Sylvania, TungSol and Raytheon. I personally prefer TungSol's or RCA's.

To really change the sound of the amp to be more British, you'll get the most bang for the buck by swapping the speaker. A Celestion, WGS British Invasion series, Eminence Red Coat Series...something like that.

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