Harley Benton GA5: cheap Valve Junior clone + EQ

frequencycentral · Post by **frequencycentral** » 22 May 2009, 18:42

I've had this little amp for a good while now. Nice little amp, and way cheap for what it is. It is apparently made for Thomann in the same factory that Epi Valve Juniors are made in, has the same PCB even.

I've had a couple of issues with it. The PT has three plastic fuse holders, two of which have failed, resulting in the internal spring popping them open, and the amp therefore powering down. The other issue is some sort of sympathetic vibration, which is quite slight and only occurs on some notes, really only very noticable when playing really quietly.

Today I repaired one of the fuse holders and decided to try tracking down this annoying oscillation. I took the sheild off the (unbranded Chinese) 12AX7 and the stay off the (Sovtek) EL84, no joy. Then I pulled the 12AX7 and subbed in a Penta Labs 12AU7. The overall volume of the amp about halfed, which I expected. It sounded pretty much the same toneally though - and the annoying vibration/oscillation was gone. So I'm putting it down to the cheap unbranded Chinese 12AX7. Has anyone experienced similar, and would agree which my prognosis?

I'm just wondering if anyone can recommend a replacement for it? ie which 12AX7 manufacturer would you recommend? TBH I'm pretty lost with 'big' tubes, almost all my tube experience is with subminis!

GJonesy · Post by **GJonesy** » 22 May 2009, 22:58

Ive got the Epi VJ and the first thing I did was put JJs in. So much better than the stock valves. Then I modded it with the bitmo kit (http://www.bitmomodkits.com). I think Briggs may be able to help you further, Im not sure what his VJ has evolved into at the moment.

briggs · Post by **briggs** » 22 May 2009, 23:21

Hi there, I've got a pretty crazy set up going on there at the moment.

I run JJ tubes, they are best bang for buck in my opinion

No microphonics or rattles.

OK - My mod started as a VJ upgrade and it's sort of morphed into a rebuild! I redesigned the preamp stages into a more marshallesqe arrangement, I have my own bass style control on there and a Supro style treble cut control. The two preamp stages run straight into an EL34 output stage (I have a switch set up so that I can stick a EL84 in and rebias though, makes swapping output tubes easy), biased for about 10W of power (I needed to upgrade both transformers for this - needed more current from the PT and more headroom in the OT - I went for stock Hammond models and they work great!). Your PT should only be using one of the fused leads, in mine its the orange lead. OH, and I run the simple "Light Bulb" noise reduction trick too, which is fun (Switchable, of course!).

I should really draw up a schematic for it - it's nothing like a standard VJ any more really.

culturejam · Post by **culturejam** » 23 May 2009, 06:20

Another plug for JJ here. I think they make the best new-production stuff right now. I was (and still am) a huge fan of Ei preamp tubes, but now they are defunct. Plus, JJ makes really great power tubes as well. I just got one of their E34L tubes (with blue glass, no less!!), and it kills in my Univalve. Their 6v6 and EL34 tubes rock as well. EL84s rule as well. I'm ordering a KT66 soon, so I'll report on that when I get a chance to test it properly.

What's really crazy is that the prices are reasonable.

soulsonic · Post by **soulsonic** » 23 May 2009, 06:31

My personal experiences with unbranded Chinese 12AX7s have been almost entirely negative. They are notoriously noisy, microphonic, and inconsistent. You can go through 20 of them and find a couple good ones, but you'll find many that are so bad they are unusable.
The hiss and microphonics are especially unbearable in the worst ones.

JiM · Post by **JiM** » 23 May 2009, 08:57

frequencycentral wrote:Nice little amp, and way cheap for what it is. It is apparently made for Thomann in the same factory that Epi Valve Juniors are made in, has the same PCB even.

I confirm that. On mine, the silkscreen that says 'valve junior' is still barely readable after being sanded.

frequencycentral wrote:The other issue is some sort of sympathetic vibration, which is quite slight and only occurs on some notes, really only very noticable when playing really quietly. [...] So I'm putting it down to the cheap unbranded Chinese 12AX7. Has anyone experienced similar, and would agree which my prognosis?

I had the same problem (out-of-the-box), a nasty vibration on some notes when pushed a bit. I immediately swapped the Chinese 12AX7B for a Sovtex 12AX7WXT+ that i had in my Valvesate, and voilà, microphonics is gone !

About fuses, my unit happens to have a separate PCB inside holding them, so i had no issues with these crappy plastig fuse holders. The black and blue wires on the PT provide +12/0/-12V, it might be used for some solid-state stuff like in the GA-15 (buffered effects loop, digital reberb, etc.)

Did you mod it ? The most useful things are :

to change the input pull-down (from 68k to 1M), it's like having a 'hi' input insead of 'lo'.
to get rid of the EQ : it's only the bass pot of a fender/marshall tonestack, with mids cut. You can re-use the pot as a master volume.
to add a power attenuator, e.g. a variable L-pad, for bedroom use and/or power amp saturation

Then i tried other things : triode/pentode switch, gain/bright switch, 4-position voicing selector (including negative feedback) ... it might end as a VibroChamp.

Post by **modman** » 23 May 2009, 10:18

viewtopic.php?f=2&t=3290
viewtopic.php?f=2&t=2203
viewtopic.php?f=2&t=1920

frequencycentral · Post by **frequencycentral** » 23 May 2009, 10:45

Thank you gentlemen for the heads up, and modman, that's some great information you linked to. I have considered some mods at some point, looks like evrything I need to know is here. For now I just want to get rid of that vibration, so I'll score a JJ and sub it in.

Funny how one thing leads to another. I bought my Harley Benton in September 2007. I hadn't done any electronics for a couple of years at that point, and what I had done was all modular synth stuff. It was getting the Harley that made me get the soldering iron out to build a FF and LPB, as I wanted some pedals to use with it at home without setting up my gig rig. 18 months later I have too many pedals (is that possible?), and a complete addiction to pedal design and modding. Along the way I discovered submini tubes - ouch!

briggs · Post by **briggs** » 23 May 2009, 10:49

Addictions catch hold very quickly. One day it just a few transistors, the next you're yearning for full on EL34 output tube distortion...

I would strongly recommend the EL34 output tube mod for the VJ.

Hanglow · Post by **Hanglow** » 23 May 2009, 11:57

I've got the same amp, made a few changes to it

-125ese ot , JJ tubes, changed the bias - also it's connected to the 230v primary, there is a 240v tap in there too, so I'd change that as well, a few other typical cap changes and change the tone control to a tweed style one.

I had the same problems with the shocking fuse holders as well

Reused the orignal OT in a wee 6v6 build, it suits the 6v6 much better than the EL84 I think.

sewatt.com has loads of info on vj's etc and also has the HB ga5 schem - obviously just the same bar the tone control

frequencycentral · Post by **frequencycentral** » 06 Aug 2009, 13:23

Sorry for the necrobump, but I just had to mention that I just replaced the stock Chinese 12AX7 with a vintage Brimar 12AX7 with the square getters. My 'annoying oscillation' has completely gone and the vintage Brimar has instantly transported me back to a time when Elvis was king and Brylcreem was his queen. I was looking for a Mullard, but got a real good deal on the Brimar on the 'bay.

cezzu · Post by **cezzu** » 18 Jun 2010, 13:45

I've just bought this small amp. I like its tone, it's everything i need. The problem is its tiny speaker: with my guitar(1 high output bridge,1 lowput neck) it's starting to "fard" at high volumes, because of the bass/lows. My room is pretty big, i have plenty of space so I was thinking about buying a 1*12 /2*12 cabinet. But there's another issue: the combo has only a 4 ohm output, and I wasn't able to find any guitar cabinet with this impendance.
-" 8 ohm output : the red wire comes from the output transformer, you just have to drill a hole and solder a jack." What red wire are you talking about? I don't see it

I was able to open only the back of the amp(removable by screws) .
- there's a difference between the combo posted on this site(picture) and my combo. mine has 3 holes(output): midle one -4 ohm, for speaker and 2 more closed with a rubber removable plug.( probably 'cause the fact that ga5 would probably suffer mods is well known). so I don't have to make new holes. Please tell me how to add a 8 ohm output. sorry for my bad English, I'm not a native speaker.

JiM · Post by **JiM** » 18 Jun 2010, 20:17

cezzu wrote:-" 8 ohm output : the red wire comes from the output transformer, you just have to drill a hole and solder a jack." What red wire are you talking about? I don't see it I was able to open only the back of the amp(removable by screws) .
- there's a difference between the combo posted on this site(picture) and my combo. mine has 3 holes(output): midle one -4 ohm, for speaker and 2 more closed with a rubber removable plug.( probably 'cause the fact that ga5 would probably suffer mods is well known). so I don't have to make new holes. Please tell me how to add a 8 ohm output. sorry for my bad English, I'm not a native speaker.

Cezzu already got a response by private e-mail, but i continue here for everyone to see.
The red wire i was taking about is visible on the left side of the second picture in the first post. It comes out of the output transformer, and goes inside the amp chassis where it's held by some ziptie and heatshrink.To get inside the chassis, you need to remove the black plastic caps on top of the amp, they hide four screws.

It seems that your amp is of a different batch, maybe they cut costs by having only one type of chassis drilled for both the combo and the head.
It would be interesting to see some pictures of it (especially inside), and check if the circuit has been changed too.

cezzu · Post by **cezzu** » 19 Jun 2010, 08:36

No problem ! I promise I'll do that in the afternoon. I'm not home yet. Yesterday I've menaged to open the chasis. and I found the " output area".whell is a "chip" with 3 groups of pins(6 each group). The 4 ohm output jack is already soldered to 6 of the pins. The main problem is that I don't know where I could find a jack like that. Promise I'l post again in a few hours,attach those photos too.. Thank you very much for your support!

Jim,unfortunately I wasn't able to find any mail from you

I've already checked cezu_das@yahoo.com, dramtfan@gmail.com, and anastastiu.florentina@yahoo.com(the mail I've registred on the forum) .

JiM · Post by **JiM** » 19 Jun 2010, 11:57

cezzu wrote:Yesterday I've menaged to open the chasis. and I found the " output area".whell is a "chip" with 3 groups of pins(6 each group). The 4 ohm output jack is already soldered to 6 of the pins. The main problem is that I don't know where I could find a jack like that.

The output jack(s) are on a separate PCB (printed circuit board, "chip" means something else)
It can be seen in the lower right corner of this blurry pic (not mine) :

Only one jack is soldered in the combo version, and the PCB is attached to the chassis with hot glue.
You can either add a jack like this : http://www.banzaimusic.com/Cliff-STE-VINT-MET.html
or remove completely the output PCB and use any jack you have for both outputs.

cezzu wrote:Jim,unfortunately I wasn't able to find any mail from you

Re-sent in PM.

cezzu · Post by **cezzu** » 20 Jun 2010, 16:12

cezzu wrote:No problem ! I promise I'll do that in the afternoon. I'm not home yet. Yesterday I've menaged to open the chasis. and I found the " output area".whell is a "chip" with 3 groups of pins(6 each group). The 4 ohm output jack is already soldered to 6 of the pins. The main problem is that I don't know where I could find a jack like that. Promise I'l post again in a few hours,attach those photos too.. Thank you very much for your support!

Jim,unfortunately I wasn't able to find any mail from you I've already checked cezu_das@yahoo.com, dramtfan@gmail.com, and anastastiu.florentina@yahoo.com(the mail I've registred on the forum) .

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Well, it seems that even the el84 is chinesse! Surprise!

I'm gonna change both of the tubes tomorrow. It seems that Thomann is spending less and less money in building these amps. hmmm
If is any other difference between your ga5 and mine, please post.

JiM · Post by **JiM** » 20 Jun 2010, 20:30

cezzu wrote:If is any other difference between your ga5 and mine, please post.

There are many, but it's my fault !

Apart from the holes in the back, the chinese EL84 and the glue type, everything looks the same as an older model. Even the component values that are readable on your pictures are identical.
All previously published mods should work.

Super Locrian · Post by **Super Locrian** » 09 Dec 2010, 13:50

Is it possible/safe to put a bypass toggle switch on the tone stack? If I put a 1,5k resistor there, it would be identical to the Valve jr. schematic, right?

JiM · Post by **JiM** » 11 Dec 2010, 13:25

Super Locrian wrote:Is it possible/safe to put a bypass toggle switch on the tone stack? If I put a 1,5k resistor there, it would be identical to the Valve jr. schematic, right?

Absolutely.
You will notice quite a boost when the tonestack is disengaged.
You may want to change the values of the tonestack while you're at it : the highs are at 50%, the mids are cut, and only the bass is controlled by the pot.
Or, as many did (including me), remove the tonestack completely and re-use the 1M pot for a master volume.

Super Locrian · Post by **Super Locrian** » 15 Dec 2010, 09:38