question on HT Club 40 "topology"

[rschleicher]

I know that Blackstar doesn't divulge schematics, but I'd still like to understand the basic circuit topology of my new HT Club 40. Part of this has been prompted by a book that I've been reading, that describes the basic arrangment used in a variety of historic/vintage amplifier designs.

My basic understanding is the there is an ECC83 tube/valve acting as the preamp for each channel. These are dual triode tubes, though, so is it two gain stages in each channel? Or is there some other function for the "other half" of each ECC83?

The two EL34 power amp tubes are presumably set up in a class AB push-pull configuration, with fixed biasing (adjustable, but fixed). But in the clean channel's "class A" voice (with the voice switch out), are they actually using just one of the EL34's, in more of a class A configuration? Or is this mostly just some sort of adjustment(s) of gain?

I understand that my amp uses solid-state (diode) rectification (as do quite a few of the old classic Marshall's, and some Fenders), but how about the phase inverter function? Is this done via solid-state circuitry, or is this part of what the ECC83(s) are doing?

The tone controls and ISF function are presumably all being done using op-amps and other solid-state circuitry, and the reverb is apparently done with some form of digital signal processing, which maybe some traditionalists think is not "all tube", but since (or is it if?) all of the gain stages are via tubes, then to me this is still an "all tube" amp, or at least "essentially all tube". In any case, I love the results!

Sorry if this is getting too "into the woods". But my electrical engineering background is causing me to want to know more!

[thephantum]

Based on running used ECC83's through a tube tester, my HT-20 only uses one triode for each tube. However, the HT-40 has the voice buttons...which may or may not (I'm honestly not sure) engage the other triode to help modify the tone.

The power amp itself is straight up Class AB push-pull and is fixed bias (meaning that the bias needs to be adjusted or "fixed" into place). The fact that there is a voice switch for each channel is the clue that they are preamp controls and not power amp. At a minimum, they are clever EQ and gain adjustments.

You are correct that the rectifier, PI, reverb and tone stack (the ISF is just a glorified tone control and part of that stack) are all solid state.

Technically speaking, some of the coloring going on in the preamp (clean and OD) does come from some solid state circuitry...but the majority comes from the ECC83's. The power amp is all tube...assuming you don't consider the PI. Depending on who you ask, the PI is part of the power amp. In my own opinion, it's not. It's part of the bridge between the preamp and power amp.

Yes, purists will say it's not all tube. My response to most purists is "Does your 'all tube amp' have any resistors in it? Oh it does? Because by definition those are solid state components, so tell me again what 'all tube' really means?" Bottom line, when I crank my Blackstars I get everything I want to hear in a tube amp. That's really all that matters.

[rschleicher]

phantum,

Thanks for the reply. Interesting that your HT 20 only uses half of each ECC83. I think you are right about the voice switches being used to add in the "other half" of each tube, to get more gain.

The voice switch for the OD channel is pretty clearly adding more gain. For the clean channel, I am guessing that the voice switch is more or less doing the opposite - with the switch out, the "pseudo-class A" sound with reduced headroom is with both halves of the ECC83 used, and the "more headroom" clean sound (switch pushed in) is with only half of the ECC83 used. (But maybe I am not thinking about this in the right way....)

So far I am liking having both voice switches in the out position. Just a little bit of crunch on the clean channel, and then the ability to have more crunch on the OD channel, via the foot-switch. And with both remaining pretty responsive to the guitar's volume knob.

At some point I might put in a different speaker, but I want to see how the Seventy 80 breaks in, and I also don't want to incur any loss of warranty coverage. (I guess one question is whether there are any sort of no-tamper seals that get torn by removing the back, that would let an authorized service center know that the amp had been opened up....).

[thephantum]

There aren't any tamper seals that I have found, but it's pretty easy to tell if the screws on the back panel have been removed and replaced due to the ferrules. So maintaining your warranty is a pretty good idea.

The whole Venue series is the same basic platform. The main differences are in the components needed to allow for higher plate voltages (giving more output power) as well as the additional bells and whistles added to the higher wattage models.

I personally don't like the Seventy 80. Even after it breaks in, it sounds like the "budget Celestion" it is...flat and lifeless. If you think the amp sounds good now, wait until you change the speaker. The amp really comes to life.

Depending on how hard (loud) you play it, by the time the warranty expires, you'll most likely need new output tubes as well. I have found that the Venue's really respond well to Winged C tubes.

The Venues also respond very well to warmer bias settings. Just as some additional information, the bias is set pretty cool from the factory. They assume a theoretical plate voltage and set bias at 60% of maximum plate dissipation. So for example, on my HT-20, the factory tubes were set to exactly 50mA. So they assumed a plate voltage of 300V and set it to 60%. The actual (measured) plate voltage with those tubes at 50 mA was 315V. Re-doing the calculation at 60% is around 48mA...but at 70% (a warmer setting), it should be closer to 55mA. Re-biasing the amp actually caused the plate voltage to drop to under 310V, which caused the bias setting to be around 57mA. So you can see that factory bias was cool.