Blackstar HT-5 Mk2 - the 0.5 Watt mode

jaymenon · July 2023

I'm curious about the Blackstar HT-5 Mk2 where it says it is

'Inspired by our patented Dynamic Power Reduction circuit, simply switch down to 0.5 Watts at the push of a button without any loss of tone.'

The phrase: 'Inspired by...'

...is interesting. It makes me wonder if this involve power-scaling with actual reduction of voltages on the power amp tube as with the DPR circuit? Or is this a 10% power switch with some resistors involved? (details below)

A 90% cut in power can be attained by using a 16 ohm 25W resistor for the upper resistor and a 12 ohm 25W resistor for the lower resistor (8 ohm output transformer secondary).

Image: https://tfb-user-images.eu-central-1.linodeobjects.com/7b827d64c842ad3874a3bd43d62badf22c515851a3228a28e7

Image: https://tfb-user-images.eu-central-1.linodeobjects.com/4a4cb4663eede459f3099f82798df4a066aeff047fe5d004a2

ecc83 · July 2023

You can never quite trust advertising copy, not even Blackstar's but if they REALLY mean "no loss of tone" then the power reduction cannot I suggest be achieved with a simple speaker level attenuator no a crude HT reduction.

DPR works (as wus told to me years ago anyway!) by reducing the continuous power (the really noisy bit) but allowing a very short initial "attack" at close to full power. This is achieved by some clever circuitry manipulating both G2 and bias voltages.

Of course, 12BH7s don't have screens so maybe anode volts and bias are jiggled? Since we are only dealing with two conditions, 5W and 0.5W and not continuous control I would expect the circuit to have been greatly simplified.

But! I am only guessing and they ain't gonna tell me now!

Dave.

impmann · July 2023

As I said to Jay on PM - its definitely not done with a couple of big heat soak resistors. Its a lot more sophisticated than that - and does involve circuitry that links back to the DPR on the Series One, but obviously its not the same due to the different topology of the amp.

As I'm not an "official mouthpiece" for Blackstar on here, its not for me to go any further than that, guys...

ecc83 · July 2023

impmann said:

As I said to Jay on PM - its definitely not done with a couple of big heat soak resistors. Its a lot more sophisticated than that - and does involve circuitry that links back to the DPR on the Series One, but obviously its not the same due to the different topology of the amp.

As I'm not an "official mouthpiece" for Blackstar on here, its not for me to go any further than that, guys...

Ah so.

Dave.

jaymenon · July 2023

From the website:

Blackstar’s patented DPR feature controls the output power level by dynamically adjusting the output stage voltages, allowing power valve distortion and compression at any volume level.

Is this the same thing on the HT5 MKii I wonder…

nacnudnai · July 2023

If you are interested, here is the patent! https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=GB&NR=2446614A&KC=A&FT=D&ND=3&date=20080820&DB=EPODOC&locale=en_EP

I haven't read through it, but it will set out the basic principles I guess.

I'm not familiar with the schematic of the HT5, but as pointed out above the 12BH7 is a dual triode rather than a tetrode/pentode. It's also set up as push/pull, I did wonder whether it was a 12BH7 set up as a cascode - which then would have something possibly resembling a screen - but alas, no. "Inspired by" insinuates to me it's doing the same "thing" as in that patent, but they've found another way to adjust the continuous power to the triodes without lowering B+ supply, etc.

jaymenon · July 2023

nacnudnai said:

If you are interested, here is the patent! https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=GB&NR=2446614A&KC=A&FT=D&ND=3&date=20080820&DB=EPODOC&locale=en_EP

I haven't read through it, but it will set out the basic principles I guess.

I'm not familiar with the schematic of the HT5, but as pointed out above the 12BH7 is a dual triode rather than a tetrode/pentode. It's also set up as push/pull, I did wonder whether it was a 12BH7 set up as a cascode - which then would have something possibly resembling a screen - but alas, no. "Inspired by" insinuates to me it's doing the same "thing" as in that patent, but they've found another way to adjust the continuous power to the triodes without lowering B+ supply, etc.

Thanks @nacnudnai ;

The Blackstar HT5 does involve a form of power scaling / reduction of voltage to the output section.

Incidentally, the Marshall DSL1 which has a 0.1W (10%) mode does not but uses a bunch of resistors to do it...
2 x 22Ω in parallel = 11Ω
Then an 8.2 Ω in series for a total of something like 19.2Ω total resistance
...constituing a 10% power switch

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Blackstar HT-5 Mk2 - the 0.5 Watt mode

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