Blackstar HT-5 Tube Swap

Discussion - HT-5 amplifiers.
squicker
Posts: 4
Joined: Mon Aug 01, 2016 1:13 pm

Tue Aug 02, 2016 10:36 pm

thephantum wrote: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.
Well, quite. I merely asked them to confirm which pot was balance and which is bias, I'd already worked out the biasing information using standard protocol. I also read on here that I'd under-egged my figures as BS run their valves right on the limit so asked for confirmation on that. Plus been told by two seperate BS reps on different occasions that the HT5 was self-biasing, so a certain amount of clarity was needed from BS.

In the end I downloaded a circuit diagram and worked it all out anyway.

Biasing is not exactly rocket science, so don't know why they get so iffy about it. Christ, I just rebuilt the engine and uprated the brake system in my track car, which most certainly has the ability to kill me, and Lotus told me everything I needed to know, no patronising fluff like, "sorry sir we can't tell you how to torque this nut in case you drive your stupid self into a brick wall at 150mph". So I don't buy the 'if you ask questions you don't understand stuff' mentality, generally it's people who don't ask questions that never improve their understanding. The whole thing is merely a sign of a litigious world where people don't like to be accountable.

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

Wed Aug 03, 2016 12:07 am

The problem is most people don't even understand how electricity works....much less how a push-pull pair of electron tubes amplifies a signal. Couple that with the fact that there are lethal voltages involved and, yea...it makes sense that B* isn't going to layout a how to on biasing an amp.

There's always been a lot of confusion around the HT-5 being cathode biased because the HT-1 is. The problem is, most reps are not engineers. Hell, for a long time, the reps were propagating the myth that it and the HT-1 were Class A amps. Why? Because "they have one power tube". :roll:

squicker
Posts: 4
Joined: Mon Aug 01, 2016 1:13 pm

Wed Aug 03, 2016 5:36 pm

thephantum wrote:The problem is most people don't even understand how electricity works....much less how a push-pull pair of electron tubes amplifies a signal. Couple that with the fact that there are lethal voltages involved and, yea...it makes sense that B* isn't going to layout a how to on biasing an amp.

There's always been a lot of confusion around the HT-5 being cathode biased because the HT-1 is. The problem is, most reps are not engineers. Hell, for a long time, the reps were propagating the myth that it and the HT-1 were Class A amps. Why? Because "they have one power tube". :roll:
Oh well, wasn't that anyway! Replaced the tubes, quick test and same fault, which is very similar sounding to a dead power tube fault. So off to a repairman and then he can fix and bias.

Yeah, I was a bit confused by BS telling me it didn't need biasing, which is why I checked on here. To my mind, even swapping the precise same tube you may as well bias the amp, due to manufacturer tolerances.

Anyway, thanks for your input, guess I'll be on Amplitube and the PC for a few days! :)

Surecatch
Posts: 2
Joined: Thu Aug 25, 2016 7:44 pm

Fri Aug 26, 2016 3:14 pm

Hi folks. I'm new here and looking for some guidance on changing an HT-5R power tube. I'm pretty sure the power tube is blown and I need to replace it. Looking to order one, I came across this "Tube Complement for Blackstar HT-5" set. Is this sufficient or should I take the opportunity to put something better in there? I'm not really very knowledgeable on the differences in tone etc. that can be gotten with different tubes, etc.

Thanks in advance for the heads up.

https://www.amazon.com/Tube-Complement- ... ords=12BH7
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thephantum
Posts: 1160
Joined: Wed Jan 29, 2014 7:42 pm
Location: Virginia, United States

Fri Aug 26, 2016 7:56 pm

HT-5R's currently being manufactured/sold come with that output tube (TAD). Previous versions came with an Electro-Harmonix. Those are about the only choices in 12BH7 tubes currently being manufactured. If you were happy with how the amp sounded with the stock tube, then you'll be happy with the TAD.

However, and I have said this many times before, NOS (or even gently used) 12BH7's will sound much better and are relatively cheap. They can be bought for under $40 USD from many retailers and you will be amazed at how much better the amp sounds.

Regardless of which option you choose, just make sure you have the amp biased correctly after replacing the output tube.

Surecatch
Posts: 2
Joined: Thu Aug 25, 2016 7:44 pm

Fri Aug 26, 2016 8:52 pm

Thank you for the response, that helps. I think I might try and find something like that.

Pilliwinks
Posts: 1
Joined: Fri Nov 18, 2016 11:23 pm

Sat Nov 19, 2016 12:13 am

I just want to thank the contributors to this forum for their assistance in helping me understand my options in replacing tubes on my HT-5.

I found the stock 12AX7 too harsh, with little usable range on the gain control. Because of that I have been using pedals for distortion, and going into the FX return. That means my sound is primarily colored by the 12BH7, so I wanted to see whether replacing it with NOS would be of any benefit. But that meant re-biasing, about which I knew nothing.

Anyway, with much reading, and some hesitation, I got myself a nice AWA NOS 12BH7 and got ready to electrocute myself. I thought I would post my readings just for the sake of any others who might be interested in the numbers.

My amp is a HT-5, completely stock, unchanged from the factory, and before doing anything I measured the plate and cathode voltages. That is, I opened up, plugged in the speaker, guitar and power leads, powered on, standby off, and measured the voltage with the black probe on the chassis, and red probe on Pin 1 (Plate 1) or Pin 6 (Plate 2) of the 12BH7. The shared cathode voltage I got from either probing Pin 3 or measuring across R33 - they are identical.

So initially:
Plate 1: 442V
Plate 2: 438V
Cathode: 60.8mV

To get the cathode current, that's V=IR, I=V/R, so I=0.0608/4.7 (as R33 is a 4.7 ohm resistor). That's a shared current of 12.9mA, so about 6.5mA on each side. Power output is VI, so at 442V that is 442*0.0065=2.85W. With a max output of 3.5W, that is running at 2.85/3.5= 82%. About what I expected.

So replacing the 12BH7 with a NOS tube gave:
Plate 1: 443V
Plate 2: 442V
Cathode: 46.6mV

That's running way colder! Crunching the same numbers gives just under 5mA each side or 2.2W, so 62%.
So I turned PR1 clockwise about a quarter turn until I got up to 60mA cathode voltage. That gave plate voltages about 439V, giving me about 2.75W or 78% dissipation. I was happy with that.

I twiddled the bias balance pot but was not able to either hear any change in hum or see any difference, so I left it where it was.

Can I tell the difference in sound? Yes. Certainly on the bottom end. Is it way better? Not really. I think I will need to replace the speaker for that. The Celestion G10N seems to me "middle-of-the-road" trying not to offend anyone, but not actually doing any particular genre that well. I am after a more vintage growl with none of the harsh stuff, and I am happy to give up spanky Fender Twin sounds and Mesa sounds to have that. I have a 12AT7 on the way, and looking at an Eminence Ramrod or maybe Jensen Jet series replacement speaker. The eternal quest goes on...

Anyway, I am happy that I have been able to replace my 12BH7 without damaging anything, and I think it was a positive step for the amp. If you are a cheapskate like me and would rather be electrocuted that pay someone else to do it, I recommend the swap.

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