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Convert Hifi Amp To Guitar Amp


Jozer99

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I'm considering building a tube amp as a project. I'm a decent woodworker, and have SOME electronics experience. I wire my own guitars, and have built a few effects pedals. Seeing as I'm no electrical engineer, and since tube amps have a tendency to store large amounts of electricity laying in wait for the uneducated technician's fingers, I thought it might be a good idea to buy an enclosure-less amp from eBay and build my own cabinet. Looking at the prices, I see that tube amps from record players or PA systems are much cheaper than dedicated guitar amps. After doing some research online, I have found many websites with different opinions on what is necessary to convert such an amp for guitar use, and how the resulting amp will sound.

Anyway:

1. Is it a realistic goal to convert a "HiFi" tube amp into a guitar amp?

2. Is it necessary to modify the amplifier's circuitry to achieve a decent sound?

3. If it is worth it to do this conversion project, what features should I look for in a "HiFi" or PA amp that would make it good for this project?

4. I'm looking to build a practice/very small gig amp, I am assuming that means 15W - 30W. Is this a good power level for that goal?

5. How difficult would it be to add a 3 band EQ to the amp?

Thanks!

Edited by Jozer99
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The preamp for a guitar amp is different than for a hi-fi amp. There is no reason why you cannot convert one to the other, but one would need to work with the circuit to see what to change. I think it would be easier to build a guitar amp from a kit than to convert a hi-fi amp. Have you looked at the AX84.com site? Or search for guiatar amp kits.

I'm considering building a tube amp as a project. I'm a decent woodworker, and have SOME electronics experience. I wire my own guitars, and have built a few effects pedals. Seeing as I'm no electrical engineer, and since tube amps have a tendency to store large amounts of electricity laying in wait for the uneducated technician's fingers, I thought it might be a good idea to buy an enclosure-less amp from eBay and build my own cabinet. Looking at the prices, I see that tube amps from record players or PA systems are much cheaper than dedicated guitar amps. After doing some research online, I have found many websites with different opinions on what is necessary to convert such an amp for guitar use, and how the resulting amp will sound.

Anyway:

1. Is it a realistic goal to convert a "HiFi" tube amp into a guitar amp?

2. Is it necessary to modify the amplifier's circuitry to achieve a decent sound?

3. If it is worth it to do this conversion project, what features should I look for in a "HiFi" or PA amp that would make it good for this project?

4. I'm looking to build a practice/very small gig amp, I am assuming that means 15W - 30W. Is this a good power level for that goal?

5. How difficult would it be to add a 3 band EQ to the amp?

Thanks!

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Hifi amplifiers amplify sound with high fidelity - guitar amps are designed (in general) to produce desirable colouration of the original input source (guitar). You would find a Hifi amp probably as far from being a decent guitar amp as you can get unless you prefer ultra-clean sound "true", which to be fair is probably as far from what guitar amps will really give you!

In general, guitar amps alter the dynamic quality and harmonic content of the source material as part of their general function - Hifi amps do the opposite otherwise we'd find record producers and sound engineers out of jobs. :D

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I am aware of this. However, after looking at the circuit diagrams, and reading around on the website, it appears the main difference between an amp capable of amplifying a magnetic phonograph input (similar level to guitar input), and a guitar amp is the tone circuit. The hifi amp uses RIAA equalization and a tone circuit that covers the whole audible spectrum (i.e. 40Hz - 20,000Hz), whereas a guitar amp uses a flat equalization and a tone circuit that only covers the guitar's range of sounds (80Hz - 2,000Hz or so). So in theory, replacing the tone circuit and the input jack should be all I need to do to change the amplifier into a guitar amp (given that I don't need crazy mesa boogie distortion or anything fancy like that).

As for buying a cheap guitar amp, the cheapest tube amp I have seen is the $149 Epiphone Valve Junior at 5W Class A. I can get a quite decent 14W mono "Hifi" amp for $30-$60. A tube amp of equivalent power is about $300 (Epiphone Blues Something-or-other). Sadly, the prices for used guitar amps aren't that much better than for new amps, and the older the amp, the higher the price! With the $100 I save with a hifi amp, I can buy myself all the materials I need to build a nice little speaker cab, and the components for a new tone circuit.

Edited by Jozer99
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It's an interesting project, but it may be difficult depending on your skills and the amp you buy. The input stage will definitely need to be reworked. Guitars need to see a higher impedance than the input stage of a Hi-Fi amp will provide. Tone controls may or may not be difficult, depending on whether or not the amp you buy includes them, and how they are voiced. You may be able to revoice the tone circuit to your liking, or you might have to completely rework the tone stage. Any RIAA equalization will have to go, too. You may also want to play with the gain and biasing for both the input and output stages. As Prostheta said, perfectly linear operation is not necessarily desirable in a guitar amp.

You will probably need to trace out the schematic, and figure out how the amp works before you do much of anything. I would track down a small (15W max, maybe less) Class A tube amp to start with. A Class A circuit will have less components, and it should be easier to figure out how to bias the output stage to your liking. If you want to be guaranteed good sound, you're probably best off just buying an amp. Otherwise, you're putting time and money into something that may turn out to be a huge failure (not saying that's a bad thing, but you should keep it in mind).

I miss Lovekraft :D

Also: Safety first! Tube get hot, and they often contain high voltage electricity. You really should buy a book on tube amps and read up on safe practices for working on them!

Another thought: you should try running your guitar though a simple preamp (like the Tillman) straight into a non-RIAA input on whatever tube amp you buy. Hook the amp up to a guitar cab, and play with it for a while. See if you like the overall tonality, check out the tone controls, et cetera. That will give you a good idea of what sort of work needs to be done before you start ripping and tearing at the circuit.

Edited by fookgub
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Yes, you can change the input impedance, throw out the RIAA, and put in a bass-mid-hi tone circuit. You might also want to change the values of some coupling capacitors. A guitar output is quite a bit higher than a good phono mag cartridge, but that is OK since you want the extra overload capability. You might also want to add a gain control (that is, have two "volume " controls, one early, the other late, in the preamp, so that you have independent control over how much preamp distortion you have.) But if you feel confident in making those changes, go for it. Guitar amps are intended to survive when played overloaded forever. A cheap hi-fi amp might not last, but it might have a good enough power supply.

I am aware of this. However, after looking at the circuit diagrams, and reading around on the website, it appears the main difference between an amp capable of amplifying a magnetic phonograph input (similar level to guitar input), and a guitar amp is the tone circuit. The hifi amp uses RIAA equalization and a tone circuit that covers the whole audible spectrum (i.e. 40Hz - 20,000Hz), whereas a guitar amp uses a flat equalization and a tone circuit that only covers the guitar's range of sounds (80Hz - 2,000Hz or so). So in theory, replacing the tone circuit and the input jack should be all I need to do to change the amplifier into a guitar amp (given that I don't need crazy mesa boogie distortion or anything fancy like that).

As for buying a cheap guitar amp, the cheapest tube amp I have seen is the $149 Epiphone Valve Junior at 5W Class A. I can get a quite decent 14W mono "Hifi" amp for $30-$60. A tube amp of equivalent power is about $300 (Epiphone Blues Something-or-other). Sadly, the prices for used guitar amps aren't that much better than for new amps, and the older the amp, the higher the price! With the $100 I save with a hifi amp, I can buy myself all the materials I need to build a nice little speaker cab, and the components for a new tone circuit.

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I bought an amp! The one I bought has "Tuner", "Mag" and "XTAL" inputs. I assume these mean "Line Level", "Magnetic Phonograph", and "Crystal Phonograph" respectively, although I am not quite sure of the last one. I figure having both line level and magnetic phonograph inputs would be good, as in the worst case, I can always use a separate preamp strait into the amp.

Here is a schematic of the amp model:

http://www.miduho.com/jp/st/amp-kairo/Heat...%20amp/ea-3.gif

The knob layout on the amp is as follows:

Source Selection

Volume

Bass Tone

Treble Tone/Power OFf

I would like to rewire it slightly as follows:

Volume

Gain

Bass Tone

Treble Tone

Then bring the input from the back to the front, as a 1/4" jack, and remove the on/off from the treble pot, giving it a dedicated switch on the front panel. I have heard that there are some decent 2-knob tone control circuits for guitar amps out there, seeing as this is a rather small amp, it might be better to stick with the same number of knobs. Of course I will rewire for a modern grounded 3 prong AC plug.

Also, do you think I should have/need a standby switch on the amp? What advantages does that provide, and how hard is it to add?

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That's a nice circuit to modify. No, you don't need a standby switch, and I wouldn't waste the space - well, that's an assumption. How about a pic of the front?

Yes, crystal refers to a phonograph with a crystal cartridge. These were quite high output.

I'd put the gain where the source selector is... Leave the volume control where it is, put the gain on the output of V1 feeding V2. Might want to reduce the value of C8 to .022 microfarad, lose C6 and C7, putting the gain control where R5 is now... Hmm, lots of possibilities.

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Image of the amp as it is now:

e9be_1_sbl.JPG

Eventually, I will put the amp into a custom combo enclosure. I also plan to make a new control plate for it to deal with the extra knobs and jacks.

I would be eternally grateful to anyone who happened to want to mark up the circuit diagram to show me how to modify the gain and tone control parts. From what I have heard, this amp is somewhat similar to a Marshal 18 watt. I think I want to go with a vintage Marshal style tone circuit.

Also, what kind of speaker should I be using with this amp? I was planning on getting a 12" 8 Ohm guitar speaker with plenty of power handling (say 30W). I have heard some rumors that a Hifi amp shouldn't use guitar speakers, and visa versa. Is this true, and does it apply to a hifi amp being used as a guitar amp?

Edited by Jozer99
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didn't a company like traynor produce at one point hi-fi guitar amps and they got taken off the market because they sounded "too natural"? That's what someone told me once.

Interesting subject though. Wouldn't mind trying one out.

-Jamie

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Its quite possible they did. The goal of this project is to turn a hifi amp into a lo-fi guitar amp, not to play guitar though a hifi amp (which would just mean plugging my guitar into the amp as it is now). From what I can tell, after some minor modifications to the tone controls, the only difference between this amp and a guitar amp would be the output transformer. Hopefully, a hifi output transformer won't change the sound too much, especially if I use a guitar speaker.

Edited by Jozer99
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Here is a link to the original schematic:

Original

Here is a link to my first draft of modifications:

Modified Draft 1

Changes I made:

Removed input selector switch (mag phono input only)

Added volume control before 1st gain stage

Added Marshall EQ circuit

Added volume between 2nd and 3rd gain stages

Will this circuit work? Is it a good circuit? What other changes would you make? I haven't figured out where the RIAA equalization is. How do I get rid of this?

Thanks, J

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By "lose" C6 and C7, do you mean cut them out and replace them with strait wire, or just cut them out and leave the leads on either side disconnected? I put up a 1st draft of the amp schematic (before RIAA stuff) does that look good?

That's a nice circuit to modify. No, you don't need a standby switch, and I wouldn't waste the space - well, that's an assumption. How about a pic of the front?

Yes, crystal refers to a phonograph with a crystal cartridge. These were quite high output.

I'd put the gain where the source selector is... Leave the volume control where it is, put the gain on the output of V1 feeding V2. Might want to reduce the value of C8 to .022 microfarad, lose C6 and C7, putting the gain control where R5 is now... Hmm, lots of possibilities.

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Hey, good pickup. That circuit has a lot of potential. The expensive parts of an amp are the transformers, so you've got those and some tubes and a chassis if nothing else. :D

Here's something you could do with the amp. I find the " .005uf-500k-5M-500k pot" part confusing. I would simplify it the first time it appears and dump it the second time. But, I might be wrong... :D It looks like some kind of tone filter.

http://i134.photobucket.com/albums/q119/Ge.../Modified1a.png

The only doubtful thing, I think, is the tone stack placement. I know the Fender-style tone stack (basically what youv'e got but with Marshall values) is high-loss. But you have a lot of gain before the tonestack, so it might still be enough drive into the PI.

Or you could totally change it... 6AU6 as input gain stage > volume > 1/2 12AX7> 1/2 12AX7 as cathode follower > tone stack (driving the tone stack that way reduces loss) > 1/2 6AN8 and resuming the original schematic. You could throw in a master volume if you like that sort of thing.

By the way, this amp will be probly be too loud to distort at practice volumes. I have a 12watt amp. I wear earplugs when I play it loud. Still, this is a great starting point for an amp.

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Thanks for the help! Heres draft 2:

Draft 2

Right now, for the sake of simplicity, I think I will keep the tone stack as is (as you said, cutting the RIAA stuff will lead to a good gain boost). Eventually of course, I would like to completely redo the amp, keeping only the tubes and chasis, and turning it into a Marshall 18 Watt. For now, though, I will keep it simple. With my schoolwork, these mods should keep me busy until Christmas!

Let me know what you think of my plan of attack, I've tried to balance easiness with usefulness in selecting the order:

0. Check voltages and test amp

1. Change input jack and speaker outs to 1/4" jacks

2. Remove input selector switch

3. Remove RIAA EQ and change misc cap and resistor values

4. Change over tone circuit

5. New enclosure

I got the 14 watt as opposed to a 5 watt because my band tends to practice loud. I quite often have my solid state practice amp, a 25W Fender, up around 8 or 9 on the volume knob. I figure with a good speaker, this amp might be able to keep up with my other practice amp. Don't worry, I do wear ear plugs (although I can't convince the rest of my band to...)

Hey, good pickup. That circuit has a lot of potential. The expensive parts of an amp are the transformers, so you've got those and some tubes and a chassis if nothing else. :D

Here's something you could do with the amp. I find the " .005uf-500k-5M-500k pot" part confusing. I would simplify it the first time it appears and dump it the second time. But, I might be wrong... :D It looks like some kind of tone filter.

http://i134.photobucket.com/albums/q119/Ge.../Modified1a.png

The only doubtful thing, I think, is the tone stack placement. I know the Fender-style tone stack (basically what youv'e got but with Marshall values) is high-loss. But you have a lot of gain before the tonestack, so it might still be enough drive into the PI.

Or you could totally change it... 6AU6 as input gain stage > volume > 1/2 12AX7> 1/2 12AX7 as cathode follower > tone stack (driving the tone stack that way reduces loss) > 1/2 6AN8 and resuming the original schematic. You could throw in a master volume if you like that sort of thing.

By the way, this amp will be probly be too loud to distort at practice volumes. I have a 12watt amp. I wear earplugs when I play it loud. Still, this is a great starting point for an amp.

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I see some things that I think you should still change.

http://i134.photobucket.com/albums/q119/Ge.../Modified2a.png

You have the signal grounded after C8. I.e., no sound. :D I noted a few other things in the schematic. I'm not sure about the "047" cap feeding the tone stack. The values of caps in series act like the values of resistors in parallel, so the "047" and 250pf in series might be creating a strange value. I don't know. I would just take the "047" out and feed the tone stack directly from the plate of the driver.

You may find that you can't get clean sounds with 1/2 12AX7 feeding 2/2 12AX7, even with the "drive" control. If this is a problem, you can add a voltage divider before the volume control. This is basically a fixed volume control, and you can tweak the value of the top resistor until you get a usable sweep to the "drive" control.

Your amp has a cool phase inverter. I believe it's called a "split load". The first half of 6AN8 is a gain stage, while the second half probably has unity gain or less but makes out-of-phase signals off its cathode and plate. (Note that the cathode and plate have equal load resistors, so a signal appears at each place.) I've read that a split-load PI contributes to a more distorted sound, which is probably a good thing. :D

Aha... by "practice amp", I thought you meant practicing alone, i.e. bedroom volume levels. This amp will probably be great for playing with an unmiked drummer. It will probably be a lot louder than your SS amp too.

Checking voltages is a good first step. Be sure you know how to do this safely (i.e. one hand in your back pocket, always aware of where the high voltage is, etc.) That should determine if the amp is healthy. After having turned it on and then off, you'll need to drain the power supply before going to work in it. The power supply caps can hold a lot of voltage. Make sure you read up on that if you aren't familiar with it. And DON'T leave it plugged in. B) (done that three times, twice in one day.)

Have fun and don't kill yourself.

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Thanks for the help! Sadly, the picture you linked me to is a bit small, and I can't really read all the recommendations on it. Could you make it a bit larger or just tell me which values to change? I can see which wires you tell me to cut, but the little red print is unreadable.

I've read up as much as I can about tube safety, but there doesn't seem to be a whole lot of good tutorials. I know that after leaving the amp unplugged for 5 minutes, I want to go in and drain the big power supply capacitors. I'm not exactly sure how to do this, though. From what I gather, I want to create a special wire with banana clips and a 2W 10k resistor, and clip one end to ground, and the other to each capacitors + terminal, and wait until it discharges completely (checking with a voltmeter). Is this correct? Also, since I am unplugging the amp to work on it, how should I ground the amp? It seems to me that if the amp isn't plugged in, and I ground a high voltage capacitor to the chassis, I am simply creating a 500v ground reference level on the chassis! Would making a special power cable with only the ground (3rd prong) attached be a good idea?

I see some things that I think you should still change.

http://i134.photobucket.com/albums/q119/Ge.../Modified2a.png

You have the signal grounded after C8. I.e., no sound. :D I noted a few other things in the schematic. I'm not sure about the "047" cap feeding the tone stack. The values of caps in series act like the values of resistors in parallel, so the "047" and 250pf in series might be creating a strange value. I don't know. I would just take the "047" out and feed the tone stack directly from the plate of the driver.

You may find that you can't get clean sounds with 1/2 12AX7 feeding 2/2 12AX7, even with the "drive" control. If this is a problem, you can add a voltage divider before the volume control. This is basically a fixed volume control, and you can tweak the value of the top resistor until you get a usable sweep to the "drive" control.

Your amp has a cool phase inverter. I believe it's called a "split load". The first half of 6AN8 is a gain stage, while the second half probably has unity gain or less but makes out-of-phase signals off its cathode and plate. (Note that the cathode and plate have equal load resistors, so a signal appears at each place.) I've read that a split-load PI contributes to a more distorted sound, which is probably a good thing. :D

Aha... by "practice amp", I thought you meant practicing alone, i.e. bedroom volume levels. This amp will probably be great for playing with an unmiked drummer. It will probably be a lot louder than your SS amp too.

Checking voltages is a good first step. Be sure you know how to do this safely (i.e. one hand in your back pocket, always aware of where the high voltage is, etc.) That should determine if the amp is healthy. After having turned it on and then off, you'll need to drain the power supply before going to work in it. The power supply caps can hold a lot of voltage. Make sure you read up on that if you aren't familiar with it. And DON'T leave it plugged in. B) (done that three times, twice in one day.)

Have fun and don't kill yourself.

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Doh, sorry about the small text. I didn't realize the resolution went down so bad on photobucket. If you want I can email the file to you. Just send me a personal message with your address.

As to that 1meg resistor, I'll just explain what I wrote in the schematic. I don't think the 1meg at the grid of V3a is necessary. The grid needs a high impedance path to ground, but it should get that through the pots in the tone stack since the bottom of the mid pot is grounded. Try it both ways; there may not even be a difference in the sound. Your 3rd Draft looks good.

With the voltage divider before the "drive" control: the 10k will determine the amount of gain that gets through to the volume control. So, if you have too much gain, make it smaller. I imagine anything down to 1k should be fine, though 1k would probably cut too much signal and not make as much distortion. The ratio between the two resistors in the voltage divider determines how much signal is dumped to ground. So, with 10k/470k, only 1/47 of your first stage output gets through to the next stage. And once you turn the "drive" control down from there, it's even less.

1/2 of a 12AX7 has a voltage gain of roughly 50. So if you have say 20mv signal from the guitar, the first stage will put out about 1 volt of signal. (This is all very rough.) If you play twice as hard, you have 2v from the first stage. That will drive the next stage to full output, because the next stage will probably have 1-2v on its cathode at idle.

Taking a larger view... a voltage divider of 10k/470k (ratio of 50) basically elminates the voltage gain of the first stage (about 50). So, your ears may want something higher than 10k. For example, a divider of 470k/470k would only dump half of the signal before the volume pot. OR, you may prefer the sound with no voltage divider before the volume pot. That part is up to you.

This thing will probably sound killer. The preamp is pretty hot--it should be an overdrive machine! :D

Glad to help you out. When I was first figuring out tube amps (about a year and a half ago), folks on the Hoffman tube amp forum helped me out a lot. They're very welcoming of newbies.

http://www.el34world.com/Forum/yabb2/YaBB.pl

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I think the purpose of the .047 uf C before the tone stack is to keep the dc out of the pots. i would put it back in. At some point you might try removing the feedback from the secondary of the output transformer. This makes the onset off overload more gradual. It is up to you how it sounds best.

Heres Draft 3:

Draft 3

This incorporates all the fixes (I think) that you put in. However, the text on your schematic was really small, so I had to guess at some of the values, and I couldn't tell what the text at the 1Meg resistor near the 4th gain stage said.

Thanks again for all your help!

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I think the purpose of the .047 uf C before the tone stack is to keep the dc out of the pots. i would put it back in.

I believe the 250pf, .02uf, .02uf of the tonestack will do that. The tone stack in a typical blackface Fender has no cap between the plate and the tonestack input, for example here.

http://el34world.com/charts/Schematics/fen...VERB_AA1164.pdf

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Geo is right. It does not need that cap, and there is no effect on the tone. I misunderstood the schematic.

So is the tone stack cap in or out? If I leave it out, is the amp in any danger of blowing up or melting down or teaching children dirty words or something, or is it more of a tone consideration?

I think the purpose of the .047 uf C before the tone stack is to keep the dc out of the pots. i would put it back in.

I believe the 250pf, .02uf, .02uf of the tonestack will do that. The tone stack in a typical blackface Fender has no cap between the plate and the tonestack input, for example here.

http://el34world.com/charts/Schematics/fen...VERB_AA1164.pdf

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