What’s the secret to biasing a Blackstar ht40?

[g1]

Without a dummy load you can't really measure power. Speaker impedance varies greatly with frequency so you don't know the real number for the calculations. Full power output test would be a bit painful with a speaker as well.
Curious as to how you check for full power with big solid state power amps?

As far as cold bias for this amp goes, here are a couple quotes I pulled from another thread about the Blackstar HT stage 60 in particular:

Anyway - unless this thing is Class B - never heard an amp that overbiased that sounded ok/good as this...really strange.

Isn't that the truth. Schematics say set bias by achieving 50mV across D27, which makes crossover notch on the output wave look awful. I set the bias to achieve zero crossover notch (which put the reading at D27 close to 175mV!), had a good looking wave but within minutes of guitar playing test, the output tubes got that rumbling noise effect.

After killing a second set of EL34, I left the bias set to read 50mV across D27 and left it. Much to my surprise, it didn't sound crappy. It sounded good.

And for those unfamiliar with the term 'over-biased', it means bias voltage more negative, resulting in 'colder' tube operation.

[JZRepair]

It shouldn't cost more than 20 to 40 USD (depending on max. power) to build a decent dummy load.

From what I quoted out previously, it was closer to $100 to have a 200W setup, plus my time to put it all together. Not trying to make excuses, but I'd love to know what you're using to get the components so inexpensively.

[Helmholtz]

And for those unfamiliar with the term 'over-biased', it means bias voltage more negative, resulting in 'colder' tube operation.

Thanks for that explanation of a rather ambiguous term.

[JZRepair]

Without a dummy load you can't really measure power. Speaker impedance varies greatly with frequency so you don't know the real number for the calculations. Full power output test would be a bit painful with a speaker as well.
Curious as to how you check for full power with big solid state power amps?

I don't check for full power. I check the rails to ensure I'm getting the voltage I should, +/- 20% and if I'm questioning the output transistors, I test them. If both are working properly, I shouldn't have any issues with output power.

[Helmholtz]

From what I quoted out previously, it was closer to $100 to have a 200W setup, plus my time to put it all together. Not trying to make excuses, but I'd love to know what you're using to get the components so inexpensively.

E.g. get four 8.2R /50W Dale power resistors (around 4USD each) and mount them to a suitable heatsink.

Or combine other resistance values like 12R(= 3x4R) or 33R(= 2x16.5R=4x8.25R= 8x4.125R) for desired resistance and power.

Dummy load resistance doesn't need to be very precise.

[JZRepair]

E.g. get four 8.2R /50W Dale power resistors (around 4USD each) and mount them to a suitable heatsink.

Or combine other resistance values like 12R(= 3x4R) or 33R(= 2x16.5R=4x8.25R= 8x4.125R) for desired resistance and power.

Dummy load resistance doesn't need to be very precise.

It's the heatsink that gets me. Heatsinks are not cheap. I don't have one that would be suitable for mounting these resistors to them. If I'm not mistaken though, these resistors still have inductance. Aren't they wire wound?

[Helmholtz]

It's the heatsink that gets me. Heatsinks are not cheap. I don't have one that would be suitable for mounting these resistors to them.

You can use a large aluminum plate or amp chassis as heatsink.

If I'm not mistaken though, these resistors still have inductance. Aren't they wire wound?

Yes, they are wirewound but inductance is very low, totally insignificant here.
OT leakage inductance is way larger.

Don't let get perfection get in your way. Get creative if you want to save cost.

[pdf64]

I don't check for full power. I check the rails to ensure I'm getting the voltage I should, +/- 20% and if I'm questioning the output transistors, I test them. If both are working properly, I shouldn't have any issues with output power.

Amps need checking at full power, overdriven if only for a few seconds, instrument amps especially, before they leave the shop. How else can you be sure they’re not going to go wonky when cranked?

[JZRepair]

[QUOTE=Helmholtz;n969564]

You can use a large aluminum plate or amp chassis as heatsink.[/QOUTE]

I'll have to keep my eyes open for one then. I don't even have a spare chassis laying around. I might be able to scrounge something up from a friend who works at a scrap yard though. I'd be interested in trying it. Originally, I was using a Bugera PS1 Power Soak as a dummy load, but I ran into problems with it on a solid state amp. Then it started acting up on me on a tube amp. Switched over to straight speakers after that. Have been contemplating getting another attenuator, but have been leary after the issues with the last one. Main thing is, I need to be able to hear it too. I can hear a lot of the issues with amps, be it capacitor, diode, whatever, I then verify what I'm hearing with a scope and it has served me well so far.

Don't let get perfection get in your way. Get creative if you want to save cost.

Now that's asking for too much! I was the guy in school that worried about millivolts and still catch myself with the +/- 20% tolerance in voltage levels throwing me off.

[JZRepair]

Amps need checking at full power, overdriven if only for a few seconds, instrument amps especially, before they leave the shop. How else can you be sure they’re not going to go wonky when cranked?

I test every amp under normal conditions, including cranking the volume up to at max, 75%. Not to mention, I guarantee my work for 90 days. If I missed something, I cover the cost of correcting it. That's how confident I am in how I do things.

Most amps I work on are never cranked up full bore. Sound guys can't work with it and most of the amps I work on never see volumes higher than 5-6 on the master volume, if they even make it that high.

Driving any amp for extended periods of time at high levels will eventually destroy something. Most people know and understand this to a point that they know better not to push their equipment too hard. Not that it can't handle it, but because they understand that even though you can, doesn't mean you should. If someone is driving their amp full bore constantly, they are begging for failures. Engineers don't even recommend running things full bore, because they understand the stress this puts on it. Yes, it's designed to be able to go that loud, but it comes with consequences. Look at any test conditions from a factory. They don't test under high power for extended periods of time, if at all, because they don't intend for the equipment to be run that hard for long periods. When you drive your car, do you floor it every time you accelerate? Why? Because you know things will wear out quicker. It's the same concept with any device. The harder you push something, the shorter it's lifespan.

[Helmholtz]

Now that sounds a bit unworldly. Are you a guitar player?
Many tube amps sound best when at least some power stage distortion is involved.
One of the reasons why there are power attenuators and many players like cranked lower power amps.
Do you really tell your customers to avoid power amp distortion??
And trust them to do as advised?

Apart from that, vol pots don't really limit power (except when at zero).
Most vintage amps didn't even have a master volume.

A guitar tube amp must be able to stand sustained periods of power amp clipping, period.

[mozz]

8 OHM 50 WATT METAL RESISTOR

CAT # 8-50

---

$2.75 each


2 ohm 4 ohm same price.
https://www.allelectronics.com/categ...0-watts/1.html

[Helmholtz]

I bought a few of those ^°.
Don't quite trust the power rating. Had to level the mounting seat for better heat transfer.
Not really comparable to Dale's.

[The Dude]

I never understood the %70 (or pick your own %) internet lore. As long as the amp is biased enough to eliminate crossover distortion, things will be just fine.