Blackstar HT 40 Bias Tube mA Balance What do TP10, TP13 and TP18 do?

General Discussion Forum - Voice The Sound In Your Head
Post Reply
BLH
Posts: 4
Joined: Fri May 05, 2017 9:57 pm

Sun May 14, 2017 10:32 pm

I know this subject has been brought up so many times but I still don't seem to find an answer. I bought new power tubes for my HT 40, matched Mullards. I've done this with a Marshalls and other amps before and they all had trim pots for each tube, so the biggest problem I am facing here is that even though the tubes are matching pairs no matter what never seem match 100%, at least in my experience. How do you balance tubes with trim pot PR1. Second, setting both tubes using trim pot PR2 at let's say 50 mA, does that mean that each tube is going to be 25 mA each or 50 mA each, 25 mA each would seem to be to cold? One EL34 can run up to 60+ mA depending on the plate voltage. So I hate to say it but I'm confused how to bias this amp without destroying it blowing the tubes. What exactly do tp10, tp13 and tp18 do respectively. Thanks for any help!

User avatar
thephantum
Posts: 1273
Joined: Wed Jan 29, 2014 6:42 pm
Location: Virginia, United States

Mon May 15, 2017 12:09 am

The BIAS pot will adjust the combined bias current for the push pull pair. The BAL pot will allow you to balance that overall current between the tubes in the pair. I have never used test points to bias any amp...I simply don't trust them. I just measure/monitor everything at the tube pins. That way I know I am getting direct measurements with no components in between.

For an EL34:
- Plate Voltage is measured between pins 3 & 8 (Plate and Cathode)
- Bias Current is measured between pin 8 (Cathode) and chassis ground.

I recommend using bias probes as they allow you to safely make all those connections, without having to constantly connect/disconnect alligator clips. Most probes will also insert a 1 ohm resistor between pin 8 and ground, allowing you to measure the bias current in mV (Ohms law).

It's also usually easier to use two probes and at least two meters. That way you can monitor each tube as you adjust and balance the bias current between them. You can do it all with one of each, but it will take much much longer. For a push-pull pair, I usually use two probes and four meters. That way I can see bias current and Plate voltage, for both tubes, all at the same time.

The math is: Plate dissipation /Plate vdc x 0.7 = Bias per valve in amps
- The Plate dissipation of an EL34 is 25W
- Plate voltage is measured
- 0.7 is assuming you want to set maximum plate dissipation to 70%.

So lets say you measure 396v at the plate. The math would be (25W/396V) x 0.7 = 0.0474A. Take that and multiply it by 1000 to convert it to mA, which would be 47.42 mA per tube OR 94.85 mA for the pair.
WARNING: Tube amps contain LETHAL voltages, even long after they have been unplugged. If you are not familiar with how to safety work on high voltage circuits, then PLEASE don't die...take it to a tech.

BLH
Posts: 4
Joined: Fri May 05, 2017 9:57 pm

Mon May 15, 2017 12:27 am

Thanks that takes the confusion out of the test points. Since I'll be taking the readings directly from the tube socket, I can just put clip for ground (black) use red probe direct on pin 8 since I have one meter just go back and between the two while using PR1 to balance. Again, thanks so much!

BLH
Posts: 4
Joined: Fri May 05, 2017 9:57 pm

Wed May 17, 2017 2:56 pm

Again I want to say I really appreciate your help:) I do still have the amp open and I put the new tubes in it still having a little trouble getting the balance to work on pr1 seems turning trim pot left or right, it I'm not quite sure it left right what it's doing? If you could please help with balance pot if I'm moving it left or right what is it doing is it moving current to one tube on the right or the left right now the right tube is reading 44.5 and left 43.9 my plate voltage is 394. Lastly, I just wanted to make a note that the calculation that you did in the example was actually for 369 not 396 plate voltage looks like 96 in 396 was reversed to 69 ergo 369. I kept running the example numbers and I kept getting 44mA instead of 47mA finally after some time realized the calculation was off. Again this is not a criticize your help it's just I wanted to clear it up in case anybody reads this forum. Thanks!!!!

BLH
Posts: 4
Joined: Fri May 05, 2017 9:57 pm

Wed May 17, 2017 5:45 pm

So now my plate voltage is running around 400. In order to get them close to balance I had to used trim pot pr1 completely clockwise of course without ruining the pot and going over the stopgap. So my question is is it okay to leave that pr1 trim pot completely clockwise? I was reading some posts were Blackstar was saying to leave it in the middle bias the tubes at around 25 each. I'm thinking of 400 I should bias my tubes at around 37 each? That would be around 60%.

User avatar
thephantum
Posts: 1273
Joined: Wed Jan 29, 2014 6:42 pm
Location: Virginia, United States

Wed May 17, 2017 8:06 pm

Setting the pot in the middle will (or at least should) split the bias between the tubes. That's assuming both tubes are electrically identical...which is almost never the case. .

So the first thing I would do is confirm you actually have a matched set of tubes.

If the new tubes are within tolerance (+/-5%), then the amp should go in for service.
WARNING: Tube amps contain LETHAL voltages, even long after they have been unplugged. If you are not familiar with how to safety work on high voltage circuits, then PLEASE don't die...take it to a tech.

Shaysterblues
Posts: 16
Joined: Tue Dec 06, 2016 10:15 am

Tue Jun 26, 2018 11:11 pm

Hi dudes
been using your app for calculating my bias.
Question is a bought a single probe to check bias at the tube itself .
Plate voltage was 407

So my bias at 70% was 41.8mv

My question is when I checked the probe it was around 22mv so is one tube half the value of 41.8 or do I turn the boss up to 41.8mv ... little confused.

Hope you can help

James

Shaysterblues
Posts: 16
Joined: Tue Dec 06, 2016 10:15 am

Wed Jun 27, 2018 1:02 am

Sorry got it sorted .
I never checked the tubes that were in the amp from new ,I just took them out and started from scratch with the new tung sol el34s .the old tubes must have either been a bad match or the balance trimpot had drifted over time.oh and the balance is for matching the bias on both tubes.i set my bias at 42.5mv.
Big big difference in sound from the ruby's that were in her.sounds far clearer before the amp was a bit bassy now it's sweet I can get a nice Warren Haynes sound on the classic mode .result.

Post Reply
  • Information
  • Who is online

    Users browsing this forum: No registered users and 15 guests