If a tube gets hot but doesnt glow?

[BowerR64]

I have an HT-5 that appears to be newer then the versions with issues. When i fire it up the pre-amp tube glows fine but the power tube you can see the element glowing. It gets hot but it doesnt appear to be working correctly.

Has the same issues of no sound.

I ordered another 12BH7 to get it on the way but i thought while i wait ide see if maybe there is a way to check it first.

I know all about being zapped by these amps, ill be careful.

What voltage should i be getting on the legs of the output tube?

[thephantum]

Depending on the construction of the tube, you might not see it glow. The fact that you have no sound however makes it a pretty good bet that it's a bad tube. It sounds like the heaters are working, but the main filaments are not glowing.

You can measure plate voltage but it will tell you very little about the tube (unless it's shorted). It will tell you that the amp is providing a voltage though. Plate voltage measurements should be taken as follows:

• Triode 1 - Between pins 6 & 8
  Triode 2- Between pins 1 & 3

Both should read somewhere in the neighborhood of 450 VDC.

If you haven't done so, check all the stupid/silly things: the speaker and cables, test all the fuses (don't just look, put an ohmmeter on them)....make sure they are good. After that, just replace the output tube and see if the filament glows. If that fixes it, then just bias the amp and you're good to go. If not (or if it glows but the amp still has no sound), then you have to start checking things like the PT, choke resistor, drop resistors, OT, etc.

[BowerR64]

Well the guy i got it from said he took it some where and they told him it had a bad OT

What should the voltages be for that? in the 300-450AC?

I read that if you hook up and amp and turn it on with no speaker plugged in it could fry the OT, but im not sure what was done to it.

I figured since the out put tube was getting hot that the trannys were working fine but maybe not?

I ordered a new output tube, but i wont get it for a week. I thought i could poke around in it for a week and see if i can figure anything else out before the new tube gets here.

I have plenty of 12AX7s, 12BH7 are kinda rare had to order that one. It only has 2 tubes in it.

My amp has 2 little onboard trim pots. Why are there 2? which is the bias one? PR1 or PR2?

One is over near the cap that looks like its been hot, the other is over near the 2 FETs

[Toneconsultant]

Did you ever find out what each of those trim pots were for? I'm assuming one is for bias, but which one?

[thephantum]

Technically, they both are. PR1 is the BIAS trimpot and is used to set the overall bias current. PR2 is the BAL trimpot and it is used to set the balance of that bias current across both triodes of the 12BH7.

[Toneconsultant]

Technically, they both are. PR1 is the BIAS trimpot and is used to set the overall bias current. PR2 is the BAL trimpot and it is used to set the balance of that bias current across both triodes of the 12BH7.

Oh boy. I have to set the bias on my amp. I was hoping it was just one trim pot, which it sounds like the 1st pot that adjust the overal is what I was thinking of. I wasn't aware of the 2nd. O.K. More homework for me before I can address the bias.

Thanks Thephantum! We might be close buddies before this is all said and done. I'm having way too much fun with what I plan to do with this HT-5R amp. Lots of changes, but lots to learn.