Harley Benton GA15

[flintan]

Have still not found a way to get rid of the resonance/ghost notes. It's appears only on certain tones.

I have tried with different preamp tubes, removed C16 and reduced C17+C18 to 22n. I have also tried with another speaker and also played with another amp trough the internal speaker which sounded fine.

What can it be? It sounds really good except for this resonance thing.

 

[leningrado]

Hey,
(I'd like to say from the very start that I don't own any tube amp (yet), but've been digging the matter about the mods of cheap amps for a while, 'cause I'm gonna get one soon.)
The ghost notes, according to this guy "sound like a second note following your playing, and can get really annoying.  This is closely related to something called "intermodulation distortion" which occurs in all tube amps, regardless of pentode/triode mode, as a byproduct of the overdrive/distortion generation itself, but not at that great a level." He suggests that the power supply needs better filtering.

 

[jimmyjoe]

Hello, everybody, I'm new to this forum.
Not long ago I bought a HB GA15 and of course want to do some mods.
flintan, I read all your posts (and everything I could find on the net about modding GA15). Your modifications seem the best. They sound great! Could you give me some more info?
Is everything OK with heating the input valves from 12V meant for the ICs?
What guitar did you use to record the samples?
Didn't you think about leaving the effect loop sockets and connecting them between the preamp and PI?
About the resonant sounds - I have heard it in my GA15 (no mods so far) and in some samples I found on the net. I am pretty sure it's some mechanical resonance. Valves, valve cap springs or some other loose part vibrating. I would try taking the chassis out of the enclosure, putting the back in place, connecting (maybe with a longer cable) to the speaker and then playing to see what happens. If it's something on chassis, the resonance should stop.
Thank you for the whole idea and so many details and photos. Congrats for the great mod!

 

[jimmyjoe]

And some more questions (after peeking inside the amp): what tool did you use to cut the PCB?
Where is the 500 Ohm resistor you added to the power supply unit? I can't locate it in your photos.

 

[flintan]

Is everything OK with heating the input valves from 12V meant for the ICs?

Yes that's what i did. I think you have to feed these tubes with dc filaments to cure the humming or you would have to rebuild the whole amp point-to-point with better filament wiring (away from the audio wires).

What guitar did you use to record the samples?

A "Bach" telecaster copy.

Didn't you think about leaving the effect loop sockets and connecting them between the preamp and PI?

Mybe you could but I don't need it.

About the resonant sounds - I have heard it in my GA15 (no mods so far) and in some samples I found on the net. I am pretty sure it's some mechanical resonance. Valves, valve cap springs or some other loose part vibrating. I would try taking the chassis out of the enclosure, putting the back in place, connecting (maybe with a longer cable) to the speaker and then playing to see what happens. If it's something on chassis, the resonance should stop.

Yes if i remember correctly now the noise is only there when the amp is inside the speaker cabinet. With a different amp it's all fine. So it has to be some kind of microphony. Please let me know what you find out!

And some more questions (after peeking inside the amp): what tool did you use to cut the PCB?

Where is the 500 Ohm resistor you added to the power supply unit? I can't locate it in your photos.

I think it's this one (in the picture) two parallel 1K 3W resistors.

 

[jimmyjoe]

Wow! I wouldn't dare to cut the PCB with a saw. Left the "low voltage part" where it was but disconnected it completely. Did all the mods by means of cutting paths and adding jumpers and screened wires when needed.

As to the resonance problem: I think I resolved it. First, I ditched the springs with rings holding EL34s in their sockets. The tubes sit firmly in the place, these springs were an overkill. Second, I noticed the springs inside ECC83s screens were too loose and they resonated as I knocked on the cabinet. Stretched the springs a little bit, now they extrude outside the screens about 1cm. Now it's harder to put the screens on the tubes, but the resonance disappeared.

Decided to heat ECC83s from separate supply voltages, one from +12V, the other from -12V. Some faint hum is still heard. I will probably lead EL34s heating with a twisted wire pair and cut off the printed paths. Will let you know whether it works or not.
Added LINE OUT the way it was done in some Fender amps: from the 8Ohm output, voltage divider 2.2k/270Ohm.
Tested also Post Phase Inverter Master Volume for a while. It sounds great (extreme overdrive at low volume) but I prefer more "classical" sound, so left MV as is (before PI, 1Meg in my version).
Added push-pull 500k potentiometer for gain with 220pF bright capacitor. Works great.
I am quite happy with the overall result, tomorrow night will test the amp with the band.
Thank you again for so much information

 

[flintan]

Wow! I wouldn't dare to cut the PCB with a saw.

How else would you cut it?  You could of course use a

but i didn't find mine when i was at it..

As to the resonance problem: I think I resolved it. First, I ditched the springs with rings holding EL34s in their sockets. The tubes sit firmly in the place, these springs were an overkill. Second, I noticed the springs inside ECC83s screens were too loose and they resonated as I knocked on the cabinet. Stretched the springs a little bit, now they extrude outside the screens about 1cm. Now it's harder to put the screens on the tubes, but the resonance disappeared.

Great you solved it! I'm not sure if i remember correctly but i think i've tried the amp without tube screens already. I'll take a look at it again.

It's impressive how good you can make these cheap amps sound just with some minor changes. Do you still have the stock speaker?

 

[jimmyjoe]

I replaced the stock speaker with Jensen C10Q and immediately heard the difference. Now it's a lot louder and has that Fenderish bite at the treble side.
Ouch, when I put it all together I heard some unpleasant high oscillation sound at higher volume levels. I guess I have to replace the screened wire with some better one - for now I chose the one with the lowest capacitance per length, but at the cost of poor screening.
Remember that you had some oscillation problem too at some point. How did you cure it?

 

[flintan]

More shielded wire and better wire dress. I used wire with quite high capacitance 140pF/m but with a good aluminum shield.

 

[guitarfreedom]

Hi ! I'm new in this forum !  ;D

I've read this topic because I have a GA15 with some mod, and now I'm going to change the eq. like written here !
I have a question: the caps on the pot must to be in the order of 600V or above?
Or it's enough in the order of 100 V?

Thank you so much !

 

[flintan]

I guess around 400VDC or above is ok.

 

[guitarfreedom]

Hi, sorry for the disturb...I have a doubt:

I have used 12Vdc to feed heaters of two preamp-tubes, is all ok but I noticed that the voltage regulator is a little hot...I Know It's normal but I would a confirmation...and I would to know if heat could damage the nearest cap or if in a tube amp heat is normal !

Thanks a lot 

 

[flintan]

Late reply.. I removed the neg regulator and put a heatsink on mine. A little heat is ok and it will automatically shut down if it gets too hot but i always use something to transport the heat if it's anywhere near the limits.

 

[jimmyjoe]

Hi,

that's why I thought it would be reasonable to heat each tube from the separate regulator - this way the current taken from each is halved and the heat is distributed between two chips.
By the way, flintan, how is your little amp doing? Mine is fine, sounds terrific and I haven't had - well, almost - any trouble with it. I use it every week for 4 hours. Lately one of my fellow guitarists asked me about a possibility of doing the same mods for him - he told me he had a Vox AC 15 and liked the sound of my amp better.
Eventually, I cancelled the oscillations by inserting a grounded plate of aluminum standing between the pots and the rest of the board. Had to remove the bright switch as it caused oscillations and some hum. Didn't succeed with heating the EL's with twisted wire because of soldering problems - the only reasonable way to do this would be to solder the wires directly to the tube socket, at the element side.
There is only one minor issue which I don't know how to tackle: when I switch the amp off it pops loud. I paralleled the standby switch with a .1uF capacitor but it didn't make any change. Please let me know if you have encountered this problem and how you dealt with it.

 

[Herman Munster]

Hello everyone,

I just purchased one of these Benton GA15's from thomann. This is my first valve amp; it took me a long while of deliberation to decide on it. Was originally going to get something like a VHT special 6 or a Bugera V5 as the amp is to be for bedroom playing & recording. I decided to go up to the 15watt because I like cleaner sounds, thought I needed the 'headroom' and for the FX loop/eq control and such.

It has shocked me how loud it is, and how muddy/bassy it is too; I thought it would have some bright sparkle to it.

From what I have read, everyone seems to have problems with it stock so I knew I would need to mod it to get the best out of it. But as I have such little experience with amps, I dont know what exactly to expect in terms of a 'good sound'. 'alnicomagnet' who sells mod kits on ebay thinks it needs modding right out of the box to even be SAFE to use on UK mains voltage. I have to spend a whole $51 on his kit, and I dont know if this will even result in a tone I like. I currently dislike what I am hearing. The amp is VERY resonant in the lower midrange. Nothing I play has any clarity or brightness. Top E is barely audible when the other strings are making resonant low-mid drones. Everything sounds 'samey' and muddy.

It looks like am going to be needing to learn a lot of things to make this amp work for me. So I joined here and will probably be asking for a lot of advice over the next couple months while I slowly try to 'fix it'.

 

[electron]

Hi,

I'm interested on modding GA-15 too, but the thread seems abandoned for a long time now.

Anyway, searching the net I found another mod here

http://guitar-dreamer.blogspot.com/2011/11/harley-benton-ga-15-ultra-mod.html

and the guy who wrote it suggests another fix for the tubes filament heaters supply. He converted the current to DC adding a "5A" rectifier and a 10000uF cap and says that the 60Hz noise almost disappeared. I think that's interesting to try, but the schematic he has attached is weird: The rectifier has only 3 connections and the 10000uF cap is non-polarized.

So, my questions are:
Is it a good idea at all? And is the schematic correct? If no, could somebody draw it the way it should be functional? And last, if it works, could I use the rectifier of the 12V circuit that remains unused after the opamp section bypass?

I attach the part of the schematic that seems faulty to me, thanks in advance