Amps Blackstar HT5RH

[CapnDenny1]

The next in the line of amps from the bonepile, a cute little Blackstar HT5RH head amp. 5 watts of tube power! A single 12ax7 preamp tune and a 12BH7 power amp tube. The 12BH7 is the same pinout as the 12ax7, with 2 separate tube amps inside the glass. They use both sides in the 12BH7 to do a regular class AB push-pull power amp.

It's an interesting little big amp. The cab weighs a ton. I have had combo amps that were lighter. Built like a brick schmidthouse. I bought it off ebay too long ago to go back and read what was wrong. But it had the tubes and knobs which is always nice. It basically had no sound. It wasn't blowing fuses or drawing too much power, but no output.

I tried hooking into the Send/Return input, and that seemed to be dead as well. So I went about looking for a schematic. It is so much easier with a schematic. Blackstar won't share them, but somebody else did. There are apparently several versions of the HT5, and the one I have with the reverb is somewhat newer. Here's a few pics of the guts of the amp. The amp is pretty complicated, with a lot of circuits fro such a simple seeming amp.

3FAE323E-3549-4468-8EF6-0CF4DC03539F by Dennis Kelley, on Flickr

I traced the signal from the signal generator through the preamp all the way to pin 1 (black wire) of the 14 pin bunch of mostly white wires going from the main pcb to the back panel pcb. The black wire is the send and the next one, pin 2, is the return. It was making it there, but not coming back on pin 2 to get to the power amp.

I followed it to the return jack, which has the usual shorting switch, if no plug is inserted it shorts the Send right back to the Return. The Return jack is the second from the top.

DD455DAA-B81A-47FF-8A96-CBFC3AF7854C by Dennis Kelley, on Flickr

When I probed this with my ohmmeter it read as zero ohms, but if I probed on the pcb it was open. I tried cleaning it and that made no difference. If I had one of these on hand I would have replaced it, but I didn't. So I figured it just wasn't pressing down hard enough. So when you put in a 1/4" phone jack it lift the spring tab up. So I pressed down on the tip to bend it back a little bit. After that it was carrying the signal back through.

No big deal, simple fix, right?

 

[CapnDenny1]

Except I still had no sound at the speaker? Unless I turned the volume way up to almost full blast, and set the signal generator to a pretty high signal as well. And when it did start working the waveform was just little tips of the signal coming thru.

I had to do some searching about how to set the bias on one of these. Basically you monitor the cathode bias current thru a 4.7 ohm resistor. They say to set the bias current at 0.046V across the 4.7 ohm resistor. The problem was with the adjustment all the way up, with a bias voltage of -16V there was no voltage and hence no current across the 4.7 ohm resistor. I wasn't sure if perhaps I needed a lower (less negative) bias voltage, or if something else was wrong. So using Pspice I did a simulation of the bias circuit.

Here is the schematic, first off. The bias voltage is generated using a voltage doubler circuit off the low voltage AC from the power transformer. That then goes to the cicuit that mixes the bias voltage and the signal voltage that gets applied to the power amp tube, the 12BH7 at the top of the page. interesting to note the phase inverter circuit is a couple of MOSFET transistors, which are also used as the phase inverter on both the Sonzera 20 and Sonzera 50 from PRS that I have to fix. I wonder who stole from who?

bias sch by Dennis Kelley, on Flickr

Here is my simulation circuit, and the output result.

HT5 bias by Dennis Kelley, on Flickr

HT5 bias sim by Dennis Kelley, on Flickr

This shows a bias voltage of -16.8V with the bias pot all the way down, R10 on the pspice ckt, and PR1 on the real schematic. So this shows that the circuit is actually working as designed. But the tube just isn't working right, and isn't showing enough bias current at the correct voltage. I guess?

 

[CapnDenny1]

So in order to determine if the tube is just bad or what I looked at the tube itself. I did a little light reading to look about load lines for the vacuum tubes and stuff. In fact I found a really cool online book that covers some light tube theory.

https://www.vtadiy.com/book/chapter-1-introduction/

Here is the tube chart for the 12BH7 tube with the operating point and the load line on it. So from this it shows where I have the spot, that with a bias voltage of -21.5V I should see a bias current of about 5mA.

IMG_4099 by Dennis Kelley, on Flickr

Just when I was feeling pretty good about cypherin' the graphs and all I found the above book also has a bias calculator program included.

https://www.vtadiy.com/loadline-calculators/loadline-calculator/

Here is the output from the loadline calculator. They even had the 12BH7 tube in their models?

bias calc by Dennis Kelley, on Flickr

Prettier than my pencil and paper sketch, but it kind of agrees with mine.

 

[CapnDenny1]

So after all this work, and deciding the tube should be OK, and the bias circuit is working, I decided that the problem must be the tube itself. It must be a 12BH7 tube with a weak response. So the next day I went down to test the tube in my tube tester. My tester works for some tubes, and doesn't work for some others! It doesn't work on the octal power tubes, or basically any power tubes. Preamp tubes seem to be tested OK.

The amp only has two tubes, a 12ax7 for the preamp, and a 12BH7 for the power amp. So I pulled the power tube out and read it and it said, 12ax7. So I thought, oh that's the preamp tube, I [pulled the wrong one out. No problem, I went a pulled out the other tube. It was also a 12ax7 tube? So I think I know why the 12BH7 tube doesn't have enough power gain? It's a 12ax7 instead!

I just had to laugh at myself, all the work I did to validate the circuit design, and analyze the tube circuit, and Pspice it, and stuff. Then it's just the wrong tube installed! I've seen some pretty "interesting" user errors before, but this is the first time I've found a preamp tube in place of a power amp tube. It's only bcause they have the same pinout it didn't blow up I guess?

I usually test the tubes first, but I seldom find tubes to be the problem. But perhaos I should at least make sure they are the correct tubes?

 

[GeeDub]

Interesting read. On the bright side, because of your researching on the issue(s), you found a really cool online book that covers some light tube theory.and an apparently good bias calculator program that may prove useful for other amp repairs.

 

[CapnDenny1]

Well, it’s here!

IMG_4112 by Dennis Kelley, on Flickr

Popped it in. Set the bias current. Plug in and play.

Nice sounding amp. A little weak on the lean channel, but the gain channel sound a lot like thee JCM800 that I fixed. If you like to play with the amp providing the distortion, lots of harmonics,then the gain channel is good. Unless you want it loud too.

I will probably start at $200 and see what happens.

2FE86B27-DD85-4188-AE51-9AB58E1DBC78 by Dennis Kelley, on Flickr


1C1728F0-06BB-4673-B9BC-7A7AF48A21D1 by Dennis Kelley, on Flickr

 

[gpower]

A good working amp for some time invested and one tube...

 

[CapnDenny1]

I paid $133 for it, but I’m not sure if that included shipping or not. So I will need to sell for $200 to break even.

 

[Al Holloway]

I have the Non reverb HT5 combo (paid about £200 new for it). Nice amp loud enough for quiet jams (Like the BGUUK bash) not sur it would keep up with a drummer. However I also have an HT1R combo with an 8" speaker. That is much better for home. With the gain button engaged and use the guitar vol to go from cleanish to heavy crunch without upsetting the neighbours.

cheers

Al.

 

[CapnDenny1]

I sold it for $200 a couple days back. Guy is 50 something, just starting out learning guitar. he was so delighted he had to email me after he got home to let me know how much he loved the sound. I used to have trouble selling amps, especially ones that I really liked. But when the buyer enjoys the amp as much as that, then I am really happy I sold it to him. That's the payoff! Cool freakin' beans!