Blackstar HT5-RC Tubes?

[thephantum]

First of all, have the amp biased correctly. One of the reasons that HT5 sounds so good is that it's designed to run the tube right on the edge of it's specifications. Towards that end, the amp sounds best when biased very, very warm (bordering on hot), around 80% of max plate dissipation. Any cooler and it starts to sound like a wet fart machine.

Second, get the GT 12AX7 out and put in a TungSol 12AX7. The difference will be night and day.

The speaker change isn't a bad thing. Anything is better than the stock Blackbird speaker. If you only have a single speaker, it makes no difference which way the wires are connected...it will sound exactly the same. It's only when you have multiple speakers that it's important to have them all wired in phase with one another to avoid frequency cancellation.

As far as 12BH7 vs 12BH7A: they are electrically identical and interchangeable (For the record, the "A" version was designed for a specific type of heater circuit found in old TV's). There are other tubes similar in design to a 12BH7. However, the 12BH7 is the "beefiest" one of it's cousins. So when you consider that the HT-5 runs it on the edge of those beefy specs, running one of it's lesser cousins would not bode well.

Like I said, get the amp biased (make sure they dial it in hot and balance that current across both triodes) then replace the preamp valve. The amp will scream back to life....

[Toneconsultant]

Thephantum,

I've been googling for info on this for about 9 days. Finally, an answer to my questions. Thank you. There is hope!!!

One more question.

"You wouldn't happen to know what reading on a multimeter I'd be looking for to bias the power tube, would you? I've had a horribly time locating the specs on the HT-5R.

Oh, by the way, "May the force me with you."

[thephantum]

It's not quite as simple as looking for one reading on a meter. In a Class AB push-pull amp (which the HT-5 is), the bias setting is actually based on the plate voltages of the valves and the desired plate dissipation of those valves. In the case of the HT5, we are talking about two triodes within the same valve....but the concept is exactly the same.

You need to read your plate voltages, calculate overall bias current, set that bias current and balance it, then rinse and repeat a couple of times. The reason is that as you adjust bias current, the plate voltage will change (Ohms law) thus negating that bias voltage you just set. So you need to repeat the process a few times until the bias is dialed in. All of it should be done with the amp should be powered on with a speaker plugged in. Standby should be off (as if you are going to play) with the channel volumes at zero.

See the first 4 or 5 posts on page 13 of this thread for some more details and methodology on biasing an HT5: viewtopic.php?f=32&t=92&start=120

[Toneconsultant]

Thanks again, Thephanthum.

I've been reading a lot about bias and I knew there was more to learn. I'll check out the link you shared and break out the calculator.

P.S. and thanks for not getting into the dangers of voltage. No politically correctness needed here. I'd think anyone sticking their fingers in the back of an amp should be aware of the dangers. Well, I'd hope they'd be aware.

[Toneconsultant]

Thephantum,

I have been reading and reading. Fascinating. I'm talking about tubes, in general.

Well, I've been ordering sockets, tubes, wires, etc. I wanted to make a bias probe for my 12bh7 amp. It's a bit overkill, but I'm having fun.

Anyway, as I'm doing all this homework, I wanted to run something by you, or anyone that would like to chime in. Here it goes.

-If I used a multimeter & wanted to check my plate voltage, the one lead would go to pin 3 and the other lead would go to pin 1. The differences between those amounts would be the plate voltage? That doesn't sound correct to me, but I Believe that is what I read somewhere. Can you clean up my confusion, if any? Oh, and then The same on pin 8 and pin 6.

Now I will say, to get a bias reading, I thought I could just touch pin 8 at the base with one lead, with the other lead at the top of pin 8 ( 1 ohm resistor will be soldered between the two in the probe I'll be making), but as I continue to research, I continue to run into questions. Wouldn't that be the correct way to check what my bias was set at?

FYI-The probe to be built is really for my safety. I'd like to keep my finger out of the amp, if possible. I can bought a new amp if I make a mistake. I don't necessarily want to light up myself any type me soon. Yes, I have more to learn before I dive in. Thanks in advance to everyone.

[thephantum]

First of all, have a look here: http://www.blackstaramps.com/forum/view ... 120#p22870

I think that might answer a lot of your questions. Plus, it also has some details on a bias probe that I built for my HT5RH about a year an a half ago...including a hand drawn schematic.

[Toneconsultant]

I was putting together a bias probe for my HT-5 and the basic idea all of the Google post say is to:

-place a 1ohm resistor on pin 8 and then check the top and base of pin 8.
-check pin 3 also

What the post don't say are as follows:

-where do your DMM leads check your pin 3? One on pin 3 and where does the other lead go?
-what to do with pin 9? Pin 9 is never mentioned. Do you even connect to pin 9?

I'm assuming these are obvious answers to those that know, hence the reason it's never mentioned, but if someone could clarify this for me, that would be helpful. Thanks

[thephantum]

Check the link above. There's a schematic there for the probe I built for a 12BH7 tube. The only pins you need to be concerned with is pins 8 & 3 (cathodes) as well as pins 6 & 1 (Plates/anodes).

What the probe is doing is placing a 1 ohm resistor between the Pin 8 on the tube and pin 8 on the amps socket. You are then measuring the drop across that resistor. Same thing for pin 3.

To check the plate voltages you are measuring the drop between ping 6 (plate/anode) & 8 (cathode) as well as Pins 1 (plate/anode) & 3 (cathode).

In my build, I used pairs of 1 MOhm and 100 Ohm resistors to drop the plate voltages into the millivolt range. That way 450 volts reads as 450mV. That's just because I'm lazy and don't want to have to switch ranges on my meters (I've had issues with some autoranging meters blowing fuses while trying to switch from a mV to a V reading...this just avoids that).

[Toneconsultant]

Thephantum,

I was looking for parts for my bias probe build and I came across a kit that had all the parts (https://www.tubedepot.com/products/tube ... -scout-kit).

I just had an epiphany. This bias probe is for the EL84. I'll admit, I've known about that tube, but I've never physically handled one. Therefore, my question is, "would this bias probe fit and work for the 12B7 in my HT-5?"

I always thought the EL84 was a bigger tube so I didn't think it would fit, but I thought I should ask someone that would know. Is the socket for the EL84 and 12BH7 the same size?

[thephantum]

That probe is for octal (8 pin) tubes like EL34's, KT77's etc. The EL84 is a noval (9 pin) tube. While the 12BH7 is also a noval tube, it's pinout it completely different than an EL84.

So even if that probe was for an EL84, which it's not, it would not work for a 12BH7.