I recently tried to modify this amp for a friend of mine. The sound was very dull and there was almost no treble and highs coming out of this little combo. The main problem was the hybrid (tubes+opamps) circuit configuration and the 10" stock speaker. The 12AX7 tubes were no-name Chinese made ones and I changed them to EH and Groove Tubes.
I was asked to modify the GA-15 and turn it into a british style amp like an Orange Tinyterror or a Vox Nighttrain and that was a real challenge. So, how do we turn this "muddy" jazzy sounding amp into a high-gain british little monster? The preamp plays a major role in the overall tone of the amp. So I changed the chinese tubes, reformed the preamp gain structure and installed two gain pots. Also, the 3 band tonestack was replaced by a simple and pure treble cut/boost control. All the IC opamps were fully bypassed in order to obtain a full tube signal path. The fx loop is now tube buffered. Also, I placed the master volume control after the second 12AX7 driver tube to get as much preamp gain as possible.
Furthermore, I decided to improve the power supply of the ga15 to reduce hum and buzz. I put a 330ohm 15w resistor and 2 x 47uF/450V capacitors for the B+ supply. This reduced 50hz hum a lot but I wanted silence! So I put a 5A bridge diode rectifier and a 10000uF capacitor into the 6.3V filament supply. The heaters are now fed with DC voltage and the 50Hz hum is almost gone.
The final touch is the replacement of the muddy 10" HB stock speaker with a 10" VHT chrome back. The improvement is fantastic. Now the amp sounds bright and open and the tone is really close to the tiny terror which costs much more than the HB GA-15.
The total cost for the modification was about 50euros and it was worth it! The price of the ga-15 was 140euros at the time.
To conclude, if your looking for a nice little tube combo on a budget or if you're into tube amp modifications, Harley benton ga-15 is for you!
And since this amp is no longer produced by Benton, check out their latest Fender-style Tube amp monsters!
www.thomann.de/gb/harley_benton_vt_30_rd.htm?partner_id=73786
www.thomann.de/gb/harley_benton_vt_30_bk.htm?partner_id=73786
UPDATE! (20/2/2012) Ultra-GAIN mod!
Here is an update concerning the HB GA-15 mod. I wanted a little bit more gain out of this amp and so I decided to create yet another modified ga-15 circuit. I added two Red LEDs as clipping diodes to get JCM Marshall style distortion and it worked! Now I can turn the gain all the way up and get all the overdrive and dynamics the preamp tubes can give without having trouble with the police... Also, the clipping is switchable in case you need less gain and more clean headroom. I kept one gain pot and decided to add a master presence knob to reduce any nasty highs. The fx loop was relocated to work in a better and more flexible way. Also, the speaker was upgraded to a Celestion BLO-100, 100W guitar/bass speaker which sounds amazing: more bass, more headroom, less nasty highs, less "quaky" mids. See the new ultra-gain schematic below!
Here is the original schematic for the GA-15: (click to enlarge)
And here is my modified schematic for the ga-15:
(*note: it is best to wire two 100k resistors in series with the C18,C17 caps before the master volume pot to reduce bias instability and "loose" bass, see second diagram).
*note:
UPDATE! (6/5/2012): VIDEO REVIEW of the the latest JCM mod for the GA-15!
Sounds Awesome! Thank you Alex!
Some pics of the ga-15 mod process:
Γειά σου φίλε, πολύ ωραίο το προτζεκτ γιά τον Χάρλεϋ, θα το επιχειρήσω με τη βοήθεια φίλου ηλεκτρονικού σύντομα.
ΑπάντησηΔιαγραφήΤο μόνο που μέ ξένισε λίγο είναι η θέση που έβαλες τη λούπα: Δεν θα ήταν καλύτερα να είναι μετά το ΤΟΝΕ;
Και μιά ερώτηση: Το rectifier γιά τούς heaters που πρόσθεσες είναι το κλασσικό 4 διόδων; Το "5Α" σημαίνει 5 Αμπέρ ή είναι τύπος φιξ rectifier; μπορώ να χρησιμοποιήσω το rectifier από το κύκλωμα τών 12V που έτσι κι αλλιώς είναι άχρηστο μετά την παράκαμψη τών τελεστικών;
Ευχαριστώ.
Hello,
ΔιαγραφήDen eida to sxolio sou tote. Sorry gia auto. To sxedio auto exei merika themata kai gi auto to logo ebala kai mia diorthosi(note).
To 5A rectifier simainei 5 Ampere revma. Den mporeis na valeis auto pou exei sta chip giati einai 1-2A.
Cheers!
Wow! It's a very very great mod!
ΑπάντησηΔιαγραφήKindly, can I ask you a info?
It be possible to apply this mod but maintaining all tone controls?
Please, can you indicate me how?
Thank you in advance!
Lorenzo - Italy
Hello, this mod was revised recently as it had an issue with the power amp overdrive, see "note".
ΔιαγραφήThe eq on this amp is an Active tonestack(using IC chips). Maybe you could use the tonestack(input= R8 , out= R12) with the chip, placing it between the 2 gain stages of the 12AX7 but that would be a totally different mod/circuit.
Hi Thanos,
ΑπάντησηΔιαγραφήI recently did a mod on my own HB GA15 but it didn't include an IC OpAmp bypass/replacement. What would be an analogue replacement for the TL072's?
Great blog too.
Phil
Hello Phil!
ΔιαγραφήThe TL072 opamp IC could be easily replaced with another dual opamp such as the TL082 or the NE5532. You should place an 8-pin IC base on the PCB of your amp to test out different opamps without soldering all the time. You should aim for a lower-noise opamps but that's a general tip. Check out a few more specific reviews of audio opamps:
http://www.diyaudio.com/forums/chip-amps/82610-best-sounding-dual-opamps.html
http://www.tangentsoft.net/audio/opamps.html
Cheers,
Thanos
Thanks Thanos. One other query. I'm interested in having a pure/electrical signal path in my amp. Do the TL072's or replacement IC OpAmps make the signal path digital? And if they do, is there a way around that?
ΔιαγραφήHello Phil,
ΔιαγραφήThe TL072 or any other dual opamp is NOT a digital device. It's an Analog audio device. So, the signal path is all pure analog. There is no digital circuitry inside these amps.
Still, if you need this amp to have an all-tube signal path, you could bypass the opamps and modify the amp. I've posted a few mods that bypass the tl072 opamp. The result is different so I wouldn't suggest doing any mods if you like the sound of your amp as it is. Clearly, you should take the amp to a qualified technician if you would like to modify it. There are lethal voltages stored in electrolytic capacitors
inside this amp.
Cheers,
Thanos
Hi there. Did you manage to eliminate the hum and buzz?
ΑπάντησηΔιαγραφήHello!
ΔιαγραφήYes I did. This is how I modified the amp to achieve this: I installed a 330ohm/15w resistor and 2 x 47uF/450V capacitors on the B+ side of the power supply. This reduced the 50hz hum a lot but I wanted silence! So I also installed a 5A bridge rectifier and a 10000uF smoothing capacitor for the 6.3Vdc tube filament power supply. The heaters are now fed with DC voltage and the 50Hz hum is gone.
Cheers,
Thanos