Hi
I have a Harley Benton GA5 which is basically an Epiphone Valve Jr. amp with a tone knob attached.
I decided to change some stuff around to make it into an AX84 P1.
I did most of the modifications, but not the tone stack or the power supply.
Now the amp is very very quiet and has some strange behavior.
When I turn the amp on and it warms up a little, it kind of works; I can hear my guitar (not very loud, but not very quiet either), but then after 20 seconds or so, it let's out a small pop, and the sound goes really quiet and I have to turn the amp to full to be able to hear it at all. The sound i very distorted when turned to full, but... it's quiet.
I have tried both new a pre amp tube(12ax7) and 3 different power tubes (EL84), no difference with other tubes.
Do you guys have any idea what could be wrong?
Bonus question:
What's the purpose of C11 on the Harley Benton power supply circuit? Can/should I remove it?
The plans for the AX84 P1 can be found here:
AX84.com - The Cooperative Tube Guitar Amp Project
Here are the plans for the Harley Benton GA5:
http://4.bp.blogspot.com/-ElQP_uSdAdI/T6rV5PvRQEI/AAAAAAAAAnc/r3KI_BS4vSk/s1600/GA5+stock.JPG
I have a Harley Benton GA5 which is basically an Epiphone Valve Jr. amp with a tone knob attached.
I decided to change some stuff around to make it into an AX84 P1.
I did most of the modifications, but not the tone stack or the power supply.
Now the amp is very very quiet and has some strange behavior.
When I turn the amp on and it warms up a little, it kind of works; I can hear my guitar (not very loud, but not very quiet either), but then after 20 seconds or so, it let's out a small pop, and the sound goes really quiet and I have to turn the amp to full to be able to hear it at all. The sound i very distorted when turned to full, but... it's quiet.
I have tried both new a pre amp tube(12ax7) and 3 different power tubes (EL84), no difference with other tubes.
Do you guys have any idea what could be wrong?
Bonus question:
What's the purpose of C11 on the Harley Benton power supply circuit? Can/should I remove it?
The plans for the AX84 P1 can be found here:
AX84.com - The Cooperative Tube Guitar Amp Project
Here are the plans for the Harley Benton GA5:
http://4.bp.blogspot.com/-ElQP_uSdAdI/T6rV5PvRQEI/AAAAAAAAAnc/r3KI_BS4vSk/s1600/GA5+stock.JPG
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I changed the bias resistors and caps on both tubes. I tried with different caps to ensure they weren't faulty. I measured the resistors, they were as they should be.
I changed the signal path, removing some components, adding other components.
It's nothing major really. It's just supposed to be a bit more gritty after the mods.
Any specific places I should check voltages? Or just everywhere more or less?
Crystal ball is fogged up.
All I can suggest is check the voltage readings against the schematic.
If they measure +-20% OK, I would suspect the output transformer or output valve related issues.
I wouldn't like to run my RL84 with only 6.77volts on the cathode. The bias should be around 8 - 10volts, depending how clean or muddy you want the sound to be.
All I can suggest is check the voltage readings against the schematic.
If they measure +-20% OK, I would suspect the output transformer or output valve related issues.
I wouldn't like to run my RL84 with only 6.77volts on the cathode. The bias should be around 8 - 10volts, depending how clean or muddy you want the sound to be.
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EL84?
Thanks, I'll check the voltages.
I'm suspecting the powersupply myself. But ran out of time in the weekend to properly check it.
Damn kids and their stupid birthdays.
This sounds like something improperly biased somewhere, perhaps a valve with a missing grid bias resistor to ground. As power is applied, the leakage current through a cap is slowly shifting the bias, so it works briefly as bias goes through a reasonable range of values, then ceases to work and distorts heavily as the bias continues to drift to either saturation or cutoff.
This could be caused by a damaged PCB trace, a bad solder joint, or a missing component.
You can find the fault by narrowing down its location, until you eventually pin it down to only one component or connection, and so find it.
For example, you can inject an audio signal into the guitar input, then use a 'scope to look at the signal at the input triode's grid, cathode, and anode. If that first stage is amplifying as it should, move on to the second triode. And so on, until you find a stage that isn't functioning as it should.
DC checks are useful too, and if there is a missing bias resistor as I suspect, you will find voltages around that valve are abnormal.
Good luck!
-Gnobuddy
Can you mark up the 'stock' amp schematic showing your changes, and post a scan or picture?
That's exactly where all of us started.
Can you tell us what sort of diagnostic tools you have? I believe you have a DMM (digital multimeter). Anything else?
Knowing what tools you have will allow people to make better suggestions for finding the fault. (For example, I suggested using an audio signal and an oscilloscope, but that suggestion is only useful to you if you have access to a 'scope.)
I agree with the suggestion to show us a schematic of what you built - without that, all we can give is general guidelines. With the schematic, someone may be able to guide you more precisely to the most likely problem areas.
-Gnobuddy
That's exactly where all of us started.
Can you tell us what sort of diagnostic tools you have? I believe you have a DMM (digital multimeter). Anything else?
Knowing what tools you have will allow people to make better suggestions for finding the fault. (For example, I suggested using an audio signal and an oscilloscope, but that suggestion is only useful to you if you have access to a 'scope.)
I agree with the suggestion to show us a schematic of what you built - without that, all we can give is general guidelines. With the schematic, someone may be able to guide you more precisely to the most likely problem areas.
-Gnobuddy
I have a DMM, nothing fancy, but it can at least check the smallest caps
, soldering iron, extra hands with a magnifier and solder suction tool. No scope or proper cap tester.The schematic of what I built is the AX84 P1, without the tone stack and powersupply. I modified the print of the original with jumpers, a bent pin on a connector and cutting a path on the circuit board.
I will draw the changes on the original schematic and post here once I get back home and can check that I actually wired everything correctly
I will definitely check the bias resistors and caps again.
Excellent! A DMM is an extremely useful tool. If you measure the DC voltage on every pin of every valve, and provide that list, along with the schematic (showing exactly what you built), there is a good chance of finding the fault.
There can also be a different kind of fault, in which the DC voltages are all correct, but the AC signal doesn't go through the amp as it should. In those cases, it really helps to have some test equipment that looks at the AC voltages in the amp, rather than DC. (This includes simple equipment such as an audio signal injector, which you can make yourself, or the much more complicated and expensive deluxe solution, which is an audio signal generator and an oscilloscope.)
But let's not get ahead of ourselves. With a little bit of luck, the problem you're dealing with will be one that reveals itself in wrong DC voltages.
-Gnobuddy
Thanks for the schematic. Most of it looks fine, but I am concerned about the value of the cathode bias resistor for the EL84. My quick estimate is that you will have over 360 volts DC (B+), and with such a high B+, I suspect a 120 ohm resistor is too small, and will cook the EL84.
But we will know for sure once you measure the DC voltage on every pin of every valve. (You can skip the heater pins, assuming the heaters are lighting up.)
According to Merlin Blencow's book, DC heater power done the way your schematic shows it, can cause a noisy (buzzy) amp. I guess we'll have to wait till your amp is working to find out.
Nothing wrong with a 1k resistor there, in fact it's a good thing, as it protects the fragile screen grid from overheating. But if it's not wired properly, that could certainly cause the amp not to work.
Again, if you measure the DC voltage on every pin of every valve, that will reveal if there's a problem with the 1k screen resistor or not.
I'm a little confused - why did you suspect this resistor earlier, and why do you now think it is not the source of the problem?
-Gnobuddy
Because I don't know what I'm doing
I changed that back to 220, the voltages below are measured with that.
So I measured the voltages, they are as follows:
12ax7 pins 1-9
165
0 (buzz when measuring)
1.3
5.8
5.8
26.5 (this looks like the problem? should be higher?)
-0.2
0
0
EL84 pins 1-9
0
23.4
21.6
0
5.9
0
248.7
0
236.8
The voltages were drifting slightly as the amp was getting warmer.
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Ok, I found out the amp must have been modified before. There's an unaccounted for resistor in the circuit. And the tone circuit is moved, attached directly to the volume pot... I've done my best to update the schematic. I really need to turn on my brain when I am using my eyes.
Also, I had missed some of my own mods.
I will double check after I post this.
I've attached an updated schematic that shows what and where.
Also, I had missed some of my own mods.
I will double check after I post this.
I've attached an updated schematic that shows what and where.
Attachments
Aaah!
That's what I get for trying to mix 2 schematics without knowing what I am doing.
I will install that again asap and see if that's it.
Still too quiet
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Will this have fried my EL84 tubes? or will they only have died with prolonged used in that configuration?
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