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](https://tfb-user-images.eu-central-1.linodeobjects.com/7b827d64c842ad3874a3bd43d62badf22c515851a3228a28e7)
![Image: https://tfb-user-images.eu-central-1.linodeobjects.com/4a4cb4663eede459f3099f82798df4a066aeff047fe5d004a2](https://tfb-user-images.eu-central-1.linodeobjects.com/4a4cb4663eede459f3099f82798df4a066aeff047fe5d004a2)
Comments
As I'm not an "official mouthpiece" for Blackstar on here, its not for me to go any further than that, guys...
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.
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.
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