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Instruments and Amps Everything that makes music, Especially including instrument amps.

Gibson GA20 - more power
Gibson GA20 - more power
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Old 7th August 2018, 03:47 PM   #11
Enzo is offline Enzo  United States
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You seem overly focused on power. Power is not loudness. If you doubles 12 watts to 24 watts, you would only gain about 3 decibels. Barely enough to hear.

On teh front of many Fender amps will be pairs of input jacks. One is hotter than the other. The difference between the two jacks is 6db. Doubling power makes only half that difference.

Remember there is a preamp and all the rest of the circuit, not just power tubes and B+ voltage.
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Old 7th August 2018, 04:19 PM   #12
jimijimi is offline jimijimi
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hi enzo - thanks, but i get it

my thing is that i can both hear and see that the 6V6 tubes are not anywhere near the peak of their performance capacity - and thats what i want, i want to hear this amp how it sounds if the power amp is max´ed out there is alot of overdrive when i turn it up to 5 - but it doesnt sound like overdrive - it sounds like a weird distortion from either preamp or splitter tube - boring!

dont worry i would not do anything to the amp that i could not retreive later again
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Old 7th August 2018, 05:22 PM   #13
Parafeed813 is offline Parafeed813  Netherlands
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Famous last words.... There are worst case scenarios when you start hotrodding an amp. Especially an antique like this. Say you replace the rectifier for solid state diodes: the voltage will go up, increasing maximum power a bit. The question you need to answer first, is if the caps can handle the higher voltage. If not, bad things can happen. You could blow caps, valves, transformers. Not easy to revert back to the original.

If the amp is working as designed, leave it as it is. Need more volume? Use more efficient speakers. An old AC30 with alnico bulldog speakers can blow away a 100W amp with medium sensitive speaks.

However, judging by your story, you think that the output stage is not saturating, and you are looking for different tone, not volume. If you want to hammer the 6V6s harder, you may need a different driver configuration. In other words, a different amp design. Again: leave this one as it is and build something else. Good chance you enjoy having two completely different amps at your disposal.

PS How do you see that the 6V6s are not pushed hard? I can understand hearing or measuring, but seeing?
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Old 7th August 2018, 05:47 PM   #14
Michael Bean is offline Michael Bean  United States
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Are you aware that even IF you could double the output power of the amp (you can't), it would only get you 3 dB louder, that's a just noticeable increase in level. I would heed other's advise here, don't mod the amp because you probably ruin it. If you're really set on louder while using this amp, get a multi-driver set up, each doubling of the number of speaker drivers gets you the same 3 dB that doubling the amp power gets you. A stack of eight gets you 9 dB more than a single driver, assuming equal driver efficiency.
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Last edited by Michael Bean; 7th August 2018 at 05:49 PM.
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Old 7th August 2018, 06:03 PM   #15
jimijimi is offline jimijimi
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parafeed -

you can see on the light reaction of the tubes when they overdrive 6V6 tubes usually will light blue colour (to my experience) el34 / 84 - just same colour but super bright
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Old 7th August 2018, 08:04 PM   #16
Printer2 is offline Printer2  Canada
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Have you had the amp serviced, how old are the capacitors? Are the resistors in spec? Which input are you going into, three are connected to each other and loads the signal going to the 6SJ7. The other input has a direct input. How healthy are your tubes? What voltage is the 5Y3 giving to the 6V6's? What is the cathode bias voltage on the 6V6's? These are all questions I would look for answers to before I go jacking up the voltage.

You do realize that you have a pair of 6SJ7's on the input that are configured as grid leak bias? If you do much more than light overdrive them they are not going to sound great. The parafeed PI sounds good driving the 6V6's with a little unbalance for cleaner breakup. Push an unbalanced parafeed and things may not sound that great. How does the amp sound through a different speaker? Again, without these answers I would not go jacking up the voltage. You do not have screen resistors. Overdrive the 6V6's too much and you might melt the screens.

Or to put it more simply, unless you know what you are doing you could hurt your amp.
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Old 7th August 2018, 10:01 PM   #17
PRR is offline PRR  United States
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> annoyed that i can never break it

In 1953, distortion was RUDE. Amps were low *gain*. They would barely distort even when played HARD, maximum arm effort.

Put a simple booster in front. The "LPB" would be the go-to pedal.

AND--- it may not be working as intended. Stuff goes bad. Usually it goes to crap, but it can just go to low gain (lower than designed).

If you want an amp that "breaks", most/all of the recent low-price tube amps will do that easily. Trade the GA-20 to a fan of old Gibsons.
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Old 8th August 2018, 07:11 AM   #18
jimijimi is offline jimijimi
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Printer2
wow - interesting - this is the kind of info i need. but all tubes are new, and 80% of all caps are new - unfortunately
a few wording i dont understand though - what is : grid leak bias - parafeed - screen resistors ???

PRR - there is alot of distortion in the preamp.
i have many other amps - but i want to heat THIS ONE break
the reason this amp doesnt play loud is probably because theyd bought a million of those small trafus back in the day, and wanted to keep the price down.
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Old 8th August 2018, 04:50 PM   #19
Printer2 is offline Printer2  Canada
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ga-20.gif

The cathode of the GA20 is grounded unlike the GA30 which has a resistor between the cathode and ground. There is a 10M resistor and it helps keep a charge on the grid so it is biased in a way that does not allow too much signal voltage for clean playing. It can sound good mildly distorted but a large signal turns the sound mushy. The below biasing is a little better than the above Grid Leak if you want to slam the input.

ga-30.gif

Both amps have what is called a parafeed phase inverter. The first triode amplifies the signal and the second one takes a portion of that signal and sends it through the other triode. There is a 180 degree phase inversionbetween the grid and the plate. If the first triode is distorted the second one takes that distorted signal and inverts it. It is not necessarily balanced and the two 6V6's get a distorted signal. Some people do not like this sound, it really depends on how much it is driven.

You should be able to get the amp to break up, a lot of talking at first but you can hear the amp at the end. If you can not achieve this level of distortion the amp may have a problem or your pickups are not putting out enough.

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Old 8th August 2018, 07:42 PM   #20
jimijimi is offline jimijimi
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thanks a million

so what i can make of it - i need to lower the resistance to ground, on the cathodes on all the tubes (a bit - maybe 20%??) - i dont want distortion, i want loudness - and i dont care if i have to change my tubes every 2nd year
right ?
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