Blackstar HT-5 Tube Swap

Discussion - HT-5 amplifiers.
Posts: 10
Joined: Fri Mar 16, 2012 12:00 am

Mon Mar 19, 2012 5:54 am

No thanks,my pleasure.

Yes,you got that wright,connect the black probe of your DVM to the chassis ( that is also the ground),use the red probe to measure on the points at the resistors.

On my Jvm there are also the 3 testpoints,on the blackstar they didn't make it on the pcb ,but measuring on the resistors will get you there also.

Greets Charlotte . Image

Posts: 38
Joined: Sun Mar 11, 2012 12:00 am
Location: Long Island, NY

Mon Mar 19, 2012 1:14 pm

Great thax!

Posts: 10
Joined: Fri Mar 16, 2012 12:00 am

Wed Mar 21, 2012 8:35 pm

I did get a EH12AX7 and a EH12BH7 en put them both in the amp.
The sound changes to more bluesy and much less gain.
Tried next the original Tad 12Ax7 with the EH12BH7,and there is more gain and bite to it,and still more bluesy,I do like it.
Going to get a JJ12Ax7 and try that out with the EH12BH7.

Have no info yet about the Bias value.
The EH12BH7 gets not that hot as the original TAD one.

Greets Charlotte. Image

Posts: 114
Joined: Tue Apr 20, 2010 12:00 am
Location: Canberra, Australia

Wed Mar 21, 2012 11:57 pm

I just dropped a gold pin Tung Sol 12AX7 into my HT-5 yesterday. Only a quick play with it, but I think I noticed a slight volume increase (could have been because the stock SovTek was pretty used), and a brighter, clearer tone, maybe a bit more gain too. Amp is definitely a bit quieter on the dirty channel too. Happy with the tonal upgrade from it.

I was replacing the stock one because I was experiencing fluctuating volume issues, which I could cause by tapping on the amp. I wanted to see if perhaps the stock 12AX7 might have been on its way out. Seemed like a good starting point.

About five minutes after dropping in the new gold pin Tung Sol I had a volume drop, and then it came back. On another go later on it didn't seem to happen, even with tapping on the amp. Hopefully the valve swap has remedied the situation. Can a valve act a bit funny when first installed, does it need to "wear in" or anything?

Posts: 114
Joined: Tue Apr 20, 2010 12:00 am
Location: Canberra, Australia

Fri Mar 23, 2012 12:53 am

Used my amp last night at band practice, and wow! Definite volume increase, had to turn down a bit. Tone is also clearer and heavier too. Great upgrade.

Posts: 10
Joined: Fri Mar 16, 2012 12:00 am

Sat Mar 24, 2012 5:30 pm

Pre-amp tubes normaly don't need to wear in as far as I know,maybe you had a bad contact?

Ok,I found out more of the issue of the Bias.


Above you see a part of a data sheet from a 12BH7 tube.
The part between the yellow lines is the max. -dc bias voltage for the whole tube = -50 Vdc,since the tube is a double triode,each side of the tube may have a maximum of - 25 Vdc .
So the bias has to be adjusted with this value measured on bove sides,as you can see on my drawing posted earlier on this thread.

I did also some mods on the amp,I put a accutronics reverb unit in it.

Image Image

Left you see the digital reverb unit from accutronics,there are 3 types of reverb,short,medium and long reverb time.I took the medium one.Along with the unit you see also a voltage regulator,wich gets the 15 Vdc to the required 5 Vdc,also did I put the reverb pot on the pcb.

Right you see the connector of the blackstar pcb,where I connected the wires from the reverb unit to.

Image Image

Left you see the reverb unit in place in the amp,the large piece of metal is for cooling of the voltage regulator.

Right you see the part of the schematic that shows the function of the connector pins.On the Fx send comes the input of the reverb,on the Fx return comes the output of the reverb,and the Cd input,0 V is the earth connection of the reverb,the +15 volts delivers the power for the reverb unit.


Above you see the back side of the chassis with the knob for the reverb level and the CD input.

Image Image

Left you see the backside panel,where I mounted a little computer fan onto it,also you see a voltage regulator that regulates the voltage from Dc12 to Dc 9 volt,in this way the fan makes less rpm,and makes less noise.

Right you see the panel with the fan in place,I did put the fan just befor the power transformer,so this gets the first cool air,then the air flows into the box,cools the tubes and leaves the box after that.

Also you see here the seperate power supply for the fan.

Greets Charlotte. Image

Posts: 8
Joined: Mon Jan 16, 2012 12:00 am

Tue Apr 10, 2012 9:21 pm

The HT5-R is a little different set up than the HT5 schematic that is posted. I did change the power tube and the bias stayed around 25 at both of the test point. Not real sure if it is self biasing but there is a pot there to adjust. I purchased the upgrade kit from Watford. Sounds great. I did change the speaker to a vintage 30 - 60 watt. I also have a vintage 30 - 30 watt. Does any one the difference between the 2 other than the wattage?

Posts: 192
Joined: Sat Apr 30, 2011 12:00 am
Location: West Yorkshire, England

Sun Jun 03, 2012 12:06 pm

Just put an EH 12AY7 preamp in my HT5 R Head - thats more like it!!!!! Cut down the gain and made it more usable IMHO. Cracking little amp just got a whole lot better. (Thanks to Hotrox)

Posts: 7
Joined: Sun Mar 25, 2012 12:00 am

Sat Jun 16, 2012 6:28 am


I was at this topic a few months back and there were photos and drawings of the HT5 pcb in CharlotteM's submission. I don't see the pics anymore. I see other pics in this same topic so I assume it is not a problem with my system. Have they been removed? I would like to see them again cause I need to bias a new tube.


Posts: 10
Joined: Fri Mar 16, 2012 12:00 am

Sat Jun 16, 2012 10:35 am


I had a server crash,so the pictures where lost. :(
I restored it,and you can see the pictures again. :D

I don't have the HT5 anymore, I trade it in on a Marshall Haze40 combo,wich I already modified also. :lol:

Greets Charlotte Image

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