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Old 26th October 2006, 09:21 AM   #1
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Default First Guitar Amp Help

I am looking to put together a guitar amp as I sold my last one off (which was a big 100W thing that I never really liked) a while back and now I need one again. I am looking, I think, for low watts, probably single ended, and would like more of a vintage sound, but also the ability to overdrive a bit.

To this end, I am thinking of starting with the 5F2 at Angela.com, but maybe making some mods to this as I'd like to build it with stuff i have around, and I am an audiophile, so I feel like the PS needs some more filtering. But, since I m totally green w/r/t/ what a guitar amp needs, I could use some friendly suggestions.

First, here's the schematic.
Click the image to open in full size.

The project can be found at http://www.angela.com/

For the output iron, I have some edcor 10K:8 transformers that I'll start with. Angela recommends the Fender iron, so I may try that as well. I am also thinking about using a 300b as the output tube as I have them, but if this is a bad idea then I won't.

Aside from those things, I have a few other questions. The first is, could anyone explain what goes on between the two sides of the 12ax7? I take it it is a tone control and a volume control, but I don't really understand the circuit. Also, (and I know this is a dangerous thing to ask) is there a better tube to use than the 12ax7? i do have them, so it is good for that, but if something else is better then I am happy to experiment.

Second, what is the connection from the transformer's secondary to the 12ax7's cathode?

Third, how would I add reverb to the amp?

Oh, and last, any speaker suggestions? Cheap and easy to find would be the criteria -- nothing vintage unless you have a source.

Thanks,

-d
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Old 26th October 2006, 10:17 AM   #2
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Default Re: First Guitar Amp Help

Quote:
Originally posted by dsavitsk

To this end, I am thinking of starting with the 5F2 at Angela.com, but maybe making some mods to this as I'd like to build it with stuff i have around, and I am an audiophile, so I feel like the PS needs some more filtering. But, since I m totally green w/r/t/ what a guitar amp needs, I could use some friendly suggestions.



The whole point of the 'valve sound' is that the power supply is crappy, and it 'sags' under load, introducing lots of distortion and colouration.

Quote:

For the output iron, I have some edcor 10K:8 transformers that I'll start with. Angela recommends the Fender iron, so I may try that as well. I am also thinking about using a 300b as the output tube as I have them, but if this is a bad idea then I won't.

Aside from those things, I have a few other questions. The first is, could anyone explain what goes on between the two sides of the 12ax7? I take it it is a tone control and a volume control, but I don't really understand the circuit.
The 1meg is the volume control, the 250K is a crude tone control, remember quality isn't an issue here - cheap and nasty is what people want!.

Quote:

Also, (and I know this is a dangerous thing to ask) is there a better tube to use than the 12ax7? i do have them, so it is good for that, but if something else is better then I am happy to experiment.
They plug in, and generally double triodes all have the same connections - so you can try different ones.

Quote:

Second, what is the connection from the transformer's secondary to the 12ax7's cathode?
It's negative feedback.

Quote:

Third, how would I add reverb to the amp?
The usual way is to use opamps to drive the reverb (no point using valves) - but reverb will probably be more complex than the entire existing circuit.

Quote:

Oh, and last, any speaker suggestions? Cheap and easy to find would be the criteria -- nothing vintage unless you have a source.
There are plenty of 1x12's intended for guitar - from that it's just a question of picking one you like the sound of, it's a VERY subjective choose - not a question of 'quality', but of 'the sound'.
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Old 26th October 2006, 12:23 PM   #3
bembel is offline bembel  Europe
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Hi all,

I share nigel ideas on almost any points.

For the speaker I recently tried the new Eminence series (blue & red).
and found them good for guitar use. And they come at a very fair price. They are available in many sizes & Z & responses.

PS: sure, a tube/spring reverb is almost the same effort as the whole amp.
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Old 26th October 2006, 06:41 PM   #4
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Take a peek at ax84.com. There are lots of projects there of varying topologies, and a great support network on the forum.... which is a great place for a crash course in guitar amp building.
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Old 26th October 2006, 11:42 PM   #5
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Default Re: Re: First Guitar Amp Help

Quote:
Originally posted by aletheian
Take a peek at ax84.com.
Thanks for the link, that's quite helpful.


Quote:
Originally posted by Nigel Goodwin
The whole point of the 'valve sound' is that the power supply is crappy, and it 'sags' under load, introducing lots of distortion and colouration.
Sags is fine, but that is different from ripple isn't it? I was just thinking of throwing an extra pi filter in to try to kep the hum to a minimum, but I won't if that's a bad thing to do. Actually, I'll probably try both and see what happens unless this is just a terrible idea.


Quote:
They plug in, and generally double triodes all have the same connections - so you can try different ones.
By different tubes, i meant hidden things that manufacturers don't use. So, in hifi, something like subbing in a 12sn7 or 7n7 instead of the ubiquitous 6sn7 to get higher quality while saving a few bucks. It looks like the 5751 is one option here, but I am just looking for others. Also, how about using an octal?


Quote:
The usual way is to use opamps to drive the reverb (no point using valves) - but reverb will probably be more complex than the entire existing circuit.
It looks like reverb will have to wait.

So, I think I am going to add a second pot between the 12ax7 and the output tube and otherwise leave it pretty much as is and tweak once I have something that makes noise. I never found having lots of tone controls particularly useful, so I think I'll try with just the one as I like the minimalist design. But as a last thought, anyone see any reason to use, or not to use, a 300b on the output?
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Old 27th October 2006, 12:20 AM   #6
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The way that I get around ripple and still preserve sag is to use a mosfet series regulator with a voltage divider setting the gate voltage, so it will track the DC sag while still eliminating the AC ripple.
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Old 27th October 2006, 02:29 AM   #7
fredl is offline fredl  United States
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Quote:
The whole point of the 'valve sound' is that the power supply is crappy, and it 'sags' under load, introducing lots of distortion and colouration.
A single ended (or any class A) amp won't sag with the signal since it's already drawing max current at idle. I would increase the PS caps to 15 or 20 microF. You'll get less hum.

I really like this type of tone control on small amps. It's very interactive with the volume control. You can eliminate or reduce the feedback for a more lively response. I've used your master volume idea and it definitely gives you more tonal options. Experiment with bypassing the cathode resistor in all three stages. You might like something small in the first stage to control the voicing of the amp.

With that OPT either a 6V6 or EL84 would be good choices. A 6L6, EL34 or 6550 into a 4 ohm load would work well, but you would want a more current from your power supply. I'm too cheap to know anything first hand about the 300B.

Have fun!

Fred
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Old 27th October 2006, 05:53 AM   #8
SHiFTY is offline SHiFTY  New Zealand
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I like this circuit, have built it a couple of times with various mods.

The first section of 12AX7 is pure gain stage- remember the guitar output is very low level. This is where you should use shielded cables, DC filament and site the 12AX7, input sockets etc far away from the power and output transformer and connections. You don't want hum in this stage! Also star grounding.

12AX7 is great for this, nice tone, quiet, loads of gain. You *could* use 6SL7 but it doesn't have as much gain and may be noisier. you could try EF86 if you were really keen! SN7 would not have enough, you would need two of them or more to get the necessary amplification.

The tone and volume stages simply "bleed off" the signal to ground. The second 12AX7 stage is the output tube driver stage. You could use 1/2 a 6SN7 here if you really needed to.

You can use a 500K pot for the volume and tone pots, no probs. A mod to consider is using a 6L6 or even better, EL34 instead of a 6V6- you need more voltage and bigger transformers but the amp will give a fair bit more power. You could use a 300B but it would give the same power as the 6V6 (triodes are less efficient)

Reverb- use a pedal or buy a reverb tank and build another fender amp with a reverb driver (usually 12AT7??)

BTW if you put a 4 Ohm guitar speaker on that transformer, it presents a 5K impedance to the output tube. So you have some options there. For speakers just use any old thing- 95dB efficient or above- an old speaker out of an organ or even a tube radio would work ok for starters.
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Old 27th October 2006, 07:13 AM   #9
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Hi Shifty

Quote:
You *could* use 6SL7 but it doesn't have as much gain and may be noisier.
Don´t get fooled by the different mu parameters of the those tubes. Actually, with the loads as shown in the schematic, resulting stage gain of either 12AX7 or 6SL7 being dropped in will be almost the same, within a single dB or something.

Tom
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Old 27th October 2006, 03:16 PM   #10
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Most guitar players want the ability to crank it to eleven. The 12AX7 has the most gain of all of the common 12**7 tubes. Any guitar amp built commercially must use tubes that are available to musicians at a music store. That limits the designer to about 10 different tubes. A DIY guitar amp doesn't have this limitation, although it must be considered if you ever want to sell the amp. I have tried other tubes in the input stages (TV tuner tubes, pentodes) but I have always returned to the 12AX7 (or its variants like the 5751). You can always plug in the other 12A*7 tubes, for less gain and then drive the snot out of the amp with a distortion pedal for a unique sound.


Quote:
A single ended (or any class A) amp won't sag with the signal since it's already drawing max current at idle. I would increase the PS caps to 15 or 20 microF. You'll get less hum.
The above statement is technically correct, but as soon as the amp is driven into hard clip it is no longer a class A amp and the current goes all over the place. The power supply in a guitar amp has a lot to do with the sound. A zillion microfarads of filtering will give the amp a more solid sound (some call it sterile) while raising the output power. Small filter caps allow the B+ voltage to "sag" as the amp is overdriven causing the total amp gain to drop. As the note dies out the amp comes out of clip allowing the B+ to rise. The amp gain slowly rises causing the note to have a longer "sustain". A good guitar player can modulate this effect with the volume knob on the guitar or a volume pedal. All of this is a matter of personal taste, so don't be afraid to experiment. The hum is less of a problem on amps using a small speaker.

I tried a 300B based guitar amp, and I wasn't too happy with the sound. I used one of my TubelabSE amps with a guitar preamp. It did the clean sounds great, but I couldn't get it to sound right with heavy overdrive. I may have been able to if I experimented further, but I gave up early on.

Most guitar amps are pentode wired for two reasons. It gives the most power per dollar (important if you are a manufacturer) and it has a lot of inherent distortion which allows for a lot of flexibility for feedback based tone controls (the "Presense" control on a Marshall amp). I put a triode - pentode - UL (if available) switch on my amps for flexibility, but triode is mostly used by acoustic players).

I have used Edcor iron in a guitar amp, and it sounded great. It has a UL tap which is good for a loud clean sound. For my small amps I prefer the Hammond 125CSE, because it has 4 different impedance taps. I put a switch here to allow intentional load mismatching for tonal flexibility. Do not operate the output tube at or above its maximum voltage rating with a high (10k) load impedance. I have summons up the fire gods this way. It sounded cool right before it blew up though! Fender always used undersized iron. This cuts hum (and allows the 8uF caps) and is partly responsible for the Fender sound (transformer saturation).

My favorite output tubes for guitar amp are 6L6 and EL84 for small amps (bluesy tone) and 6L6 or EL34 for most amps (rock, blues and metal). For the crank it to eleven crowd I use a KT88 or 6550. I have used sweep tubes in some of my own amps (150 watt P-P), but a gigging musician wants an amp that can be fixed on the road.

A few years ago I built a few very flexible amps based on the Fender Champ. The schematic can be found here:

http://www.tubelab.com/Turbo_Champ.htm

Many of the amps that I built (probably all of them) were modified from that schematic. Most of the modifications were capacitor value changes. I had to replace the screen and cathode resistors on at least one amp. Use 3 or 5 watt resistors for these parts. The cathode cap should be at least 50 volts for 6L6 and bigger tubes.

Note the .047 cap on the grid of the output tube. Resist the temptation to put a bigger cap here. This can lead to blocking distortion (known as farting out to a guitar player) and an unstable amp.
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