Schematic help - Blackstar HT-5

[jamesg3373]

Hi guys,

Having a few issues with this schematic, been staring at it for a while now, hope someone can shed some light on it for me.

You can download the schematic as a PDF from this link: FBM BLACKSTAR HT5 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics



It's probably really simple and I'm missing something blatantly obvious, but there are a few connections that don't seem to go anywhere. For example on page 2 the first diagram (top left) it says "HT_PRE" out, but I can't find anywhere that it goes into. Also on the 3rd page the schematic for connector 1 says +15V out and -15V out, but I can't see where the 240AC changes into +/-15V DC and is sent into the connector...

Also (yes there's more) around and about the schematic it says "+15V", or "-15V" or "0V" next to part of the schematic, but it doesn't look like a connection. I'm guessing this is something like a reference voltage?

Thanks for any help guys.

Jamesg

[jamesg3373]

Bump for help!

Sorry to be a bother, if it's a no go then can anyone point me in the right direction to figure it out?

Thanks again guys!

[Mick Bailey]

HT_PRE connects to the tube V1A & V1B (also labelled HT_PRE)

The 240v gets transformed to LT AC/LT_CT/LT AC off the secondary side of the transformer. This is rectified and regulated to +/- 15V DC off REG 1 & REG 2. Whilst it isn't clear from the schematic, this is picked off by CON1 and other points that are labelled +/- 15V.

Not all schematics expressly define where voltages or signals are connected - sometimes the relationship is implicit/obscure/not defined. Some are better than others.

It also appears to have a sheet missing for the 12BH7 output stage.

[jamesg3373]

I see now, so even though it's not labelled as a connection the HT_PRE on the ECC83 is a connection.

I found another schematic that seems to have some of the 12BH7 bits. By red and black it means pos and neg DC right?

http://music-electronics-forum.com/a...amp-supply.gif

http://music-electronics-forum.com/a...reamp-pt-1.gif

http://music-electronics-forum.com/a...reamp-pt-2.gif


So I'll be doing some guess work when I put it together.

Thank you for the help!

[Mick Bailey]

Black and red can signify anything - they're just lead colours and shouldn't necessarily be interpreted as positive and negative. For instance, the output transformer leads are shown as black/red but they're both positive with respect to ground (they're the DC positive anode voltages fed via connection A to the transformer centre tap) They just happen to be that colour. In a Fender they're usually blue/brown (though not always).

[jamesg3373]

Thanks for the info.

Realising that I've missed a fairly crucial part to this. I need a transformer to change the 230AC into ~18DC before it goes into the 7815 (+15DC) and the 7915 (-15DC) and I need an output transformer to go from the 12BH7 to the 16 and 8 ohm speaker outputs. I understand how the input transformer fits into the schematic (the two "HT AC" are the ~18DC running in), just having trouble with where the output transformer goes (between the 12BH7 and the speaker outputs I imagine).

As you might guess this is my first amp. Trying to learn as I go

[Mick Bailey]

Are you building one of these?

The HT AC is the high voltage feed which is rectified for the tubes (also known as B+). LT AC is the 18v.

[52 Bill]

Realising that I've missed a fairly crucial part to this. I need a transformer to change the 230AC into ~18DC before it goes into the 7815 (+15DC) and the 7915 (-15DC) and I need an output transformer to go from the 12BH7 to the 16 and 8 ohm speaker outputs. I understand how the input transformer fits into the schematic (the two "HT AC" are the ~18DC running in), just having trouble with where the output transformer goes (between the 12BH7 and the speaker outputs I imagine).

As you might guess this is my first amp. Trying to learn as I go

Your power transformer will also need to supply the high voltages for the tube power supply.

Are you trying to build this amp from the schematics? If you are, you may need more information than what is published, as there seems to be quite a few parts that are not marked with actual values.

[jamesg3373]

I was trying to build the amp from the schematics, I've made a bunch of changes to the equalizer etc, but the power setup is still the same.

I've got 2 transformers, one is 1x230 to 2x24 (power in) and the other is an audio transformer with COM, 10W, 5W and 2.5W on one side and 16 ohm, 8 ohm, and 0 ohm (which I assume is COM?) on the other side.

I ordered these the other day, but I've no idea if they're right now. If not does anyone know a website where I can find the right ones?

The question I need answering is what voltages should a 12AX7 and a 12BH7 need to run? On the schematics (the ones I linked in the first post) it looks like the 12AX7 is running from the +15V line. Is that right? And if so, can I run the 12BH7 on 24 or 15 volts too?

Surely for the power circuit I can use the schematics from another amp and slot them in where they fit. Looks to me like a push/pull on the 12BH7.

Thanks again guys!

[jamesg3373]

By the way, the power in was 1x230 to 2x24 because the nominal voltages for the 7815 and the 7915 were 24V.

[Mick Bailey]

Take a look at the filter caps C37 & C38 - two 350v caps in series. This tells you that the B+ voltage is going to be high - probably over 350v.

This isn't really a first amp to build, especially if you're learning along the way. Have a look at some guts shots of the amp and count the number of components. Also, they're not randomly scattered to make them all fit - there's reasoning behind track spacing, voltage and signal separation and many other factors such as safety. After a whole lot of work you may just end up with a howling banshee (and that may be you!)

A starting point to understand building an amp would be to take a look at the AX84 site P1 theory document.

AX84.com - The Cooperative Tube Guitar Amp Project

Your best bet is to find a supported project that has a good following and comprehensive community guidance, correct part numbers, component ratings and layouts - there are quite a few around.

[J M Fahey]

and the other is an audio transformer with COM, 10W, 5W and 2.5W on one side and 16 ohm, 8 ohm, and 0 ohm (which I assume is COM?) on the other side.

Sorry but that transformer is useless for you.

By the specs you post, it's a 70V line distribution transformer, the kind used in Hotels and such to distribute music and announcements all over the building.

And that amp runs on about +400V DC, plus it's *very* complex.

Having built thousands of amps, I still wouldn't want to build one of them on demand, go figure.

Plus a lot of values are missing.

You'll be *very* happy if you build some of the AX84 suggestions.

Best of all: they are very well documented, lots of people built them, you have links to parts suppliers, lots of pictures of finished amps (it helps a lot), a Forum to clear doubts, the works.

Did I mention they have a *killer* sound?