#1
Hi all,

I have been sitting on a simple Volume and EQ upgrade to my Harley Benton GA-5 that I bought for £10 about a year ago, but finally found the courage to make the mod today.

The mod replaces the existing volume and EQ potentiometers with higher quality, quieter versions and replaces the EQ mini circuit board with two capacitors and just one fixed resistor.

Although I was sceptical about the effect this mod would have, I have to say that I am amazed at the tonal improvement it has made. The amp is clearer, brighter, more dynamic and I would say, a little louder too.

Although taking a Valve amp apart with charged Capacitors and high voltages is a little risky if you're not careful, it was really worth the effort. The total mod took about 3 hours, allowing for multiple checks at each stage.

Anyway, a few photos of the mod are attached, or a full set of photos can be found here: http://www.photobox.co.uk/album/275729881

Regards,
Colin (Net-Hed)
Attachments:
29122009015c.jpg
29122009017c.jpg
29122009026c.jpg
29122009028c.jpg
29122009030c.jpg
Last edited by net-hed at Dec 30, 2009,
#2
Did you buy the amp or the mod for 10£?
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#3
Hi,

Sorry, bad grammer there. It was the Mod that was £10 The Amp was only £85 though, still great value for money

Colin
#4
Just ordered a JJ/TESLA EL84 Valve to replace the SOVTEK Valve in the power amp stage, Hoping for a much softer 'bluesy' break-up and a little more headroom (due to the slightly lower gain of the JJ) after installation.

http://www.mikehillservices.com/el84-jj-56-p.asp

I'm not able to re-bias the valve/amp after swaping them over so I am hoping all works OK.

Hopefully I'll know in a day or two Will post my feelings about the Valve swap once complete
#5
Quote by net-hed
Just ordered a JJ/TESLA EL84 Valve to replace the SOVTEK Valve in the power amp stage, Hoping for a much softer 'bluesy' break-up and a little more headroom (due to the slightly lower gain of the JJ) after installation.

http://www.mikehillservices.com/el84-jj-56-p.asp

I'm not able to re-bias the valve/amp after swaping them over so I am hoping all works OK.

Hopefully I'll know in a day or two Will post my feelings about the Valve swap once complete

These are cathode biased, so no biasing necessary.
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#6
iirc, the ga5 is identical to epiphones valve junior, right?
The only difference being the ga5 has an added tone control

cool

I've got a modded vj that I really enjoy, with the jj el84 / tungsol 12ax7 tubes you're looking at
I think you'll really like them..they're my favourite combo out of all the different tubes I've tried in mine
#7
Thanks Tubetime86, I read somewhere that the HB GA-5's voltages can be a little wild. I'm just planning on swapping the original for the JJ and hoping for the best

James4, thanks for the info on the valve combination, it's reassuring to know that it should be an improvement. I can't wait for mine to arrive now

Thanks,
Colin (Net-hed)
#8
yay, my JJ EL84 arrived this afternoon Removed the back of the combo, released the spring valve retainer, gently pulled the SOVTEK out and popped the JJ in, then replaced the retainer.

One thing I noticed once I removed the Sovtek, the Sovtek has much thicker Glass, thinker pins and wiring and is much, much heavier. The JJ seems delicate in comparison, noticably thinner glass and lighter construction all round.

Here are some photos: http://www.photobox.co.uk/album/280500281 I've uploaded these directly too.

Played for around an hour, initially sounded a little harsh and bright with earlier brake-up. Seems to have settled down after a hour or so, already smoother, softer, still bright though, you can certainly hear each individual string more clearly than with the Sovtek.

Not sure if its better at the moment, but certainly different. I'll leave it in until I've upgraded the stock speaker to the Celestion Super 8 which should arrive within the next few days, I think this may make things even brighter though, so may replace the Sovtek and give it another try.

Before I can install the Celestion (which is 8ohm) I need to install a toggle switch and to locate the 8ohm secondary tap off the output transformer so that I can switch between the standard stock 4ohm output and 8ohm for the new speaker.

Will post some photos of the speaker mod and installation as soon as its complete.
Attachments:
JJandSovtek.jpg
Valveclip.jpg
JJinplace.jpg
JJon.jpg
#9
Quote by net-hed
Before I can install the Celestion (which is 8ohm) I need to install a toggle switch and to locate the 8ohm secondary tap off the output transformer so that I can switch between the standard stock 4ohm output and 8ohm for the new speaker.

I recommend that you install a second jack for the 8ohm tap. If you do it this way, then you plug the speaker into the appropriate tap. It is easier to verify that the correct impedance is selected. With a switch you also run the risk of it accidentally getting flipped to the wrong impedance without your knowledge. If you do use a switch, I would suggest a slider over a toggle.
#10
Thanks, You have a good point there, also, the switch would just add additional connections and routing of the signal. I know its just the speaker output, but I guess it all adds up. I have a 1/4 Mono Socket so will probably go with your recommendation.

Many thanks.
#11
You are welcome! I have a Valve Junior (which is almost identical to your GA-5) and I initially wired it with the switch, but then decided to change to a second jack. I like the additional jack better. If your power supply also has a 16ohm tap, why not wire a jack for that too while you are at it?

If you need any help with the wiring, I'll be happy to post pictures or a diagram of how I did it. The only part that is difficult is drilling the hole through the chassis.
#12
Hi, Thanks. Luckily, The GA-5 Combo already has holes drilled and filled with rubber grommets, I'm hoping my 1/4 Jack socket will fit straight in. I think the GA-5 combo only has 4/8ohm tappings on the OT, but I'll check while I'm in there I'll post a photo of the installation once I get around to it
#13
Quote by cedricsmods
I recommend that you install a second jack for the 8ohm tap. If you do it this way, then you plug the speaker into the appropriate tap. It is easier to verify that the correct impedance is selected. With a switch you also run the risk of it accidentally getting flipped to the wrong impedance without your knowledge. If you do use a switch, I would suggest a slider over a toggle.

I actually started typing out a very similar reply last night on my ipod, but I fell asleep before posting it


but anyway, post pics once you've done it
I'm curious about how the new speaker'll sound, too

I don't think I'd really be comfortable with anythign under 10", but maybe the celestion'll surprise me
#14
Snowed-in again yesterday, so managed to install both the 8ohm Speaker Socket and Celestion Super 8 Speaker, which although being snowed-in, the UPS Guy still managed to deliver (on foot for the last few hundred yards).

Unfortunately, my Jack Socket was not a PCB mountable version, it had solder tabs, but I managed to squeeze it into place even with the small existing circuit board left in place. The Red 8ohm output from the Output Transformer was very easy to locate, but access to the black/common wire was via the underside of the mini circuit board. With the board carefully un-gluped from the chassis and the existing socket nut removed, the pcb could be turned upside down and access to the solder point obtained. I added a short blue wire to the common wire solder point and re-attached the pcb. With the red OT tap and the blue wire soldered to the jack socket, the socket was attached to the chassis.

Here are the photos of the procedure: http://www.photobox.co.uk/album/281993293
#15
Having installed the new 1/4 8ohm Speaker Jack Socket I could now install the new speaker, a Celestion Super 8.

Installation was very simple. Having removed the back and chassis for the Speaker output jack addition, the Speaker was easily accessible. I un-soldered the two speaker wires and unscrewed the speaker. The replacement Celestion also has 4 mount holes, but these didn't align when offered up to the existing screw holes. Rotaing the speaker 90degrees magically showed all 4 holes algned! The new speaker could then be screwed in and the two wires soldered to the two tabs. You just need to ensure that the red connects to the '+' tab (left from the rear) and the black to the remaining '-'. Replace the chassis and back and you are ready to go.

The Celestion website has a good article on Running-in your new speaker. I only had 10 minutes to give it a quick try, but initial impressions were of a lower gain and volume that the old 4ohm stock speaker. Slightly smoother and clearer, not overly bright though and defined but not over-powering bottom end.

I believe it takes a while for the Speaker to bed-in. The EL84 JJ upgrade is also only 2 days old, so I think I need to give the Amp a few good 1 hour (plus) plays before I can really how the EQ circuit, Power Valve and Speaker mods improved (or not) the Amp. It certainly is Clearer and has a wider EQ range than before. The amp is more Dynamic and Reactive to playing style than when it was stock too.

I'll post my thought atfer a few days playing

Here are some photos of the mod. http://www.photobox.co.uk/album/281990472
Last edited by net-hed at Jan 8, 2010,
#16
Quote by cedricsmods
You are welcome! I have a Valve Junior (which is almost identical to your GA-5) and I initially wired it with the switch, but then decided to change to a second jack. I like the additional jack better. If your power supply also has a 16ohm tap, why not wire a jack for that too while you are at it?

If you need any help with the wiring, I'll be happy to post pictures or a diagram of how I did it. The only part that is difficult is drilling the hole through the chassis.

would you mind posting the diagram? I've wanted to add this to my v1 valve jr for a while now.
02 MiM Telecaster-----\__Digitech Whammy__TS-10__535q__DE-7__6505+ 112
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#17
Hi Jeffc,

I've attached a schematic of the Stock Harley Benton GA5 that I found on the internet. The output transformer is shown in the top-right of the diagram, The brown wire is the 4ohm output, you'll see that connected to your existing output socket. The red wire is the 8ohm tapping, just connect this to the 'tip' connection of the new socket to be added. The common side of the socket should be connected to the black/ground wire already connected to the output socket mini circuit board (you'll see that in my photos above).

Hope this helps,
Colin
Attachments:
HB-GA5-Stock_0.pdf
#18
Replaced the unbranded chinese pre-amp tube with a JJ/TESLA ECC83S (12ax7) on Friday evening. Much less Gain, much less volume, but so much quieter. Mains AC 50hz hum drastically reduced. On lower volumes with a little treble rolled off, the amp sounds sweet, warm and smooth, but clear and dynamic. With the volume above 50% or with the treble full up, the amp delivers a nice crunch when you dig in with your pick and is easy to control and predict. Full chords on higher volumes sound a little busy and rough around the edges, but for £85 for the amp (new) plus about £35 for the mods it's a fantastic little amp. #)
#19
Well, after a few weeks of playing and listening with the recent mods in place, I made the decision to replace the original SOVTEK EL84 power amp valve. With both the JJ ECC83 pre-amp and EL84 power tube the tone was a little too bright and reduced clean headroom as a result. The combination of the JJ pre-amp tube and Sovtek EL84 presents a warmer, if slightly duller tone much more aligned for Blues & Jazz.

Next planned mods (from Steven Scott's S2 document) are to replace R1 & R2 resistors to raise the input impedance and slightly increase gain at the input stage to reduce the signal to noise levels. Then, replace C1 & C2 with better quality Sprague Capcitors and lower the EL84 bias a little by replacing the 220R R14 resistor with a 270R.

Hopefully I'll be left with a smoother, warmer tone with lots of clean headroom but some dialable Bluesy crunch with Volume and Tone dials in the higher positions.
#20
During the Easter Break I have at last been able to allocate a few hours to applying the upgrades discussed in my last update, plus a few more, These are:

1) Replace R1 & R2 to raise input impedance and reduce some of th esink of the input signal to ground.

2) Replace C1 & C2 with the same value but better quality Sprague Orange Drop Capacitors.

3) Cool the bias of the EL84 Power Stage.

4) Replace the noisy 1N4007 diodes making up the mains rectifier with ultra-fast, quiet, UF4007 versions.

5) Add a Standby Switch, leaving the main power switch just controlling the heaters.

The Amp is now so much more touch sensitive, has a smoother more predictable and controllable crunch and sounds much more 'in tune' with the new CAPS.

Here are some Photos:

http://www.photobox.co.uk/album/351069004

http://www.photobox.co.uk/album/351081735

http://www.photobox.co.uk/album/351079505

http://www.photobox.co.uk/album/351074537

http://www.photobox.co.uk/album/351085785
#21
Latest upgrade, increase electrolytic capacitor C6 in power supply from 22uf to 47uf to offer improvement to Bass by reducing 'Sag'. Should also help reduce any mains hum at higher volumes.

** Please note ** Ensure you have completely discharged all caps completely and safely before even touching the circuit board.

Here are some photos of the procedure: http://www.photobox.co.uk/album/362531648

For such a small change, I am really delighted with the improvement in bass response and 'punch'.
#22
sweet, i don't think increasing the power supply cap would reduce sag though. class A amps don't have any sag (i learned on this forum). but it DOES improve the bass response. and it helps the mains hum for single ended amps. shouldn't your standby switch control the HT, and have your main switch control all? i dunno.

where are the clips! sound clips!
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#23
+1 to soundclips!

and I agree, a filter cap upgrade really makes a nice difference

whats your impression of the before/after sound you get when you changed the rectifier diodes?
I'm moving off to college at the end of the summer and my vj's gonna be my only amp for a while, so I should get back to modding it..those diodes were something I hadn't looked into yet
#24
Hi, I must admit, I can't say whether I'm hearing 'sag' or not, in fact, sag is regarded as a good thing in valve amps. I've read that it sounds like Compression, only with volume up high and when a note is picked hard. The theory seems to be that when a loud note is suddenly played, especially a bass note, the power supply cap discharges and the on the next half-cycle has to recharge. This causes the voltage to drop slightly, causing the bias levels and volume to drop. Apparently, in severe conditions, when the voltage comes back up, the note suddenly gets louder, sounding like a second strike of the string. Not something I've heard on my little GA5 though

Some sites suggest a 100uf Cap or greater, but I've also read that they cause quite a load on the diodes at power on and also cause a increase in ripple current as they disharge and charge with greater current during heavy bass. Even with the 22uf replaced with the 47uf there is a noticable punch to the amp at lower frequencies.

The diodes were cheap, about £1.50 for 6 ultra-fast quieter versions, so I replaced them anyway. I do feel that with the diodes and upgraded capacitor the mains hum is lower when turned full volume.

Sound clips, I agree, a great idea. Problem is I should have recorded some initially so that the difference could be heard. The main effect of all the mods so far has been to make the amp much more dynamic and responsive. The R1/R2 replacement certainly upped the signal level and Orange Drop Caps do offer a much more 'in tune' smoother, fuller tone.

This is the only valve amp I've ever owned, but it's certainly given me the bug for modding and I would love to hear a serious quality high powered Valve amp in action, must sound amazing.

Thanks #)
#25
oh, sorry, the Standby switch. Yes, the main switch remains as it was, controlling the mains power into the power transformer. The standby switch just switches the T1 line on the HT output of the power transformer. So now, with the standby off, when you switch the mains switch on, just the 6.3v output of the power transformer is available, so heats the valve filaments. When you flick the standby up, the HT to the circuit board is supplied and th eamp is ready instantly.