A friend of mine brought me this new blackstar HT-1 amp head. It's a tiny little hybrid tube amp that produces enough volume for practice and gig purposes. There are other ht-models now available as well. The Ht-1 (and ht-1r with added reverb) has one channel and a boost/od switch. The boost circuit is solid state and it's meant to work as a gain channel but it shares the same gain and level controls with the clean mode... So it's not entirely practical or versatile. Moreover, there's no external foot-switchable boost control. We decide to fix that!
The steps to locate how the "boost" is activated were not easy as I didn't have a schematic for the ht-1 preamp at hand. So, I traced the pins of the on board switch that activates the boost and tested every pin combination. I finally spotted the 2 pins than activate the boost circuit! I wired a coaxial cable on these pins and connected a Marshall footswitch(f/s) via a mono jack. The result was quite interesting: when the amp was on the clean mode/channel, I turned the boost ON via the footswitch(f/s) and it gave me a gain and a volume boost! The on board switch gives you only a gain/OD boost. So I decided to experiment a bit and wired a 20Kohm log pot in series with the hot f/s lead.
With the pot set at max, I got the same reaction(od and vol boost). But when I turned the pot down, I got a clean boost and just a little OD! The pot could regulate the amount of OD boost you get from the footswitch even if the amp's gain is set to max.! It's a way to get even more sounds out of this little amp, it's brilliant!
So I drilled two holes on the back of the metal chassis and placed the pot and jack for the external f/s. The amp has now a 3rd sound mode remotely controlled via a simple one-button f/s (any Marshall, fender, Vht, DIY dpdt f/s will do the trick!).
HT-1 schematic:
http://schems.com/bmampscom/blackstar/Blackstar_HT-1_schematic.pdf
Here is a simple layout for the mod. You can see the twelve on-board switch pins at the bottom of the pcb. I used pins no 2 and 3 to connect the external f/s circuit. You can just solder a coaxial cable on these pins (Attention! There's a hot and a ground for these two pins, don't mix it up!).
Here are a few more pics taken during the modification:
So, now you can put a channel select footswitch on your ht-1 amp!
Cheers,
-Thanos
This is very interesting!
ΑπάντησηΔιαγραφήI'm curious: what does the built in od button do after this mod? Is it the same as the foot switch?
The onboard switch selects the channels. I set it on the clean mode to use the f/s. You can get various sounds using the onbaord switch and the f/s. You could try it out and see for yourself. Just be ultra careful when working on tube amps!
ΔιαγραφήCheers,
Thanos
Hi Mate
ΔιαγραφήHave you got any samples ?
Hello! Unfortunatey I gave the amp without recording any samples. However, the tone you get by doing this mod has the classic HT-1 character. Only the volume and gain levels change when you use the external footswitch. It's an easy and interesting mod. Just make sure the amp is fully discharged before you open it up.
ΔιαγραφήCheers,
Thanos
Hello! What if i put a 47k ohm pot instead of the 20k?
ΑπάντησηΔιαγραφήThanks
Hello! You can use it without any problems. The 47k pot will provide you with a slightly different boost adjustment range. Just pick a Log pot. Linear pots will work as well but it will be more difficult to dial in the right amount of boost.
ΔιαγραφήHi and a very special thanks for you work , do you think it's possible to add an loop effect ports on this amp ?
ΑπάντησηΔιαγραφήHello! Yes, it's possible to add an fx loop in many amps and the HT-1 is no exception. First of all, DO NOT modify your tube amp unless you are familiar with tube amps and DANGEROUS HIGH VOLTAGES!!! If not, take the amp to a professional and show him/her this article.
ΔιαγραφήYou have to check the classic fx loop circuit which consists of two stereo 1/4" Female Jacks (chassis-mount) and a couple of wires. http://cdn.instructables.com/FXZ/3LEH/GPVF23WK/FXZ3LEHGPVF23WK.LARGE.jpg
Once you build the fx loop circuit with those two jacks, you have to decide where to place it on the chassis and were to connect on the PCB of the amp. I would suggest looking at the schematic of the HT-1 (make sure you have the exact same model as the names/values of the components may differ): http://bmamps.com/Schematics/Blackstar/Blackstar_HT-1_schematic.pdf
I would personally unsolder the C21 (22uF/16V) Capacitor and connect the fx loop send/return hot cables in its place. I would certainly add two 47uF/25V capacitors in series with the send/return hot wires to make up for the substitution of the 22uF/16V capacitor (when two 47uF caps are wired in series, the total capacitance is about 23.5uF which is fine).
Check out my design on the Ht-1 fx loop which is totally untested! Proceed at your own risk! (if done properly, there is no real risk and it's an electronically reversible mod. Do not drill the chassis before the circuit is tested!).
http://s24.postimg.org/vlcn275jp/HT_1_Loop.png
I am looking forward to hearing the results!
Cheers, Thanos
Hi Thanos.
ΔιαγραφήI would like to try your schematic for FX LOOP, but I would also like to have a potentiometer such as send level and return level, or a parallel configuration.
Hi Guitar dreamer, inspired by your blog about the footswitch and fx loop, I desided to try it. The footswitch didn't work properly, as it generated some od soundeffects between clean and od mod. Anyway, after almost 1,5day work I finished the loop and it works. First of all, I bought a loop kit from tube town in germany. It came in pieces, so I had some soldering work. After that I began the integration. Aditionally the loop requires a dc voltagesupply of 250 to 400V. This was available on the blackstar pcb.
ΑπάντησηΔιαγραφήThere ist only one problem. Without the jacks in the loop sockets, the amp generates quite a noise through the speaker. There must be a kind of radio noise generation or whatever. To solve this, my knowledge in electronics are not sufficient. Perhaps, if you interested, you can help me?
Here the link of the loop kit:
https://www.tube-town.net/ttstore/Bausaetze/Sonstige-Bausaetze/Kit-Serieller-FX-Loop-LND150::5973.html
Greetings from Germany, Werner
Hello Werner! Thanks for reading my blog!
ΔιαγραφήI am not familiar with the fx loop circuit from TubeTown but I noticed there a trimmers/pots and two different inputs on this pcb. Did you try adjusting the trimmers for minimum noise? Also, did you use the less sensitive input (IN.A) of this circuit? The audio cable lenghths have to be as short as possible. Also, the audio cables should be coaxial type ones with proper ground shielding. The power supply B+ cable can be twisted in pair with a ground wire for less noise. Also, make sure that the hot and ground audio cables are not mixed up!
You could add a 22uf/450V electrolytic capacitor from B+ to ground for less power supply-produced hum.
Be extra careful with the B+ voltage and the polarity of the capacitor!
For more info you should definitely contact TubeTown.
Cheers,
Thanos
Hi Thanos, thanks for your advise. Meanwhile i found the problem, caused by a wrong ground. After an intensiv study of the blackstar schematics, i found a more decent ground port for the high voltage dc supply. Anyway, the noise has disapeard. The big capacitor you mention, is already part of the loop kit. Thanks a lot.
ΔιαγραφήHello! The two cables that connect to create the footswitch at what point of the match on the schematic?
ΑπάντησηΔιαγραφήI was thinking of eliminating R14 (470k) to give more boost on the clean channel.
do you think is a good idea?
Hello! Yes, you could remove the R14 completely and for some extra clean boost but I am not sure if this can affect the f/s mod. I honestly do not remember which part of the schematic is affected by the f/s. I traced out the 10/11 pins of the OD switch at the time and experimented with the pot values a bit. It's possible that the f/s shunts the R21/C43 point to ground but you'll have to trace this with a simple continuity tester/multimeter to be sure.
ΔιαγραφήCheers,
Thanos
Thanks Thanos.
ΔιαγραφήI totally removed R14 having a slight but pleasant increase in gain.
Hello, I am unable to push out the circuit board from the head enclosure. Are there any other screws which are need to be unscrewed other than the two at the back?
ΑπάντησηΔιαγραφήThanks
Hello! The washers of the pots and jacks plus one of the transformers have to be removed as well. Check the photos. Some cables and the tubes would also have to be removed. Make sure you are extra careful with high voltages stored in capacitors and don't flex the PCB as you could damage its traces and solder joints. Cheers, Thanos
ΔιαγραφήThanks for replying Thanos, what I actually meant was how to take out the plate on which the circuit board is fixed to from the wooden enclosure of the head. (Just like the 3rd picture in your blog in which your guitar rests beside the amp). I removed the two screws at the back but was unable to push the thing out of the head enclosure.
ΔιαγραφήThanks
I don't really remember if there were any other bolts but maybe the chassis is held on by a screw at the bottom of the wooden cab. This bolt may hold one of the bottom pads on the cab. I would unbolt the bottom pads and wiggle the metal chassis a bit to remove it.
ΔιαγραφήAah..Okhay I l try that. Thanks a lot :)
Διαγραφή