Where do I start?

[paull]

Hello everyone.

I would like to build a guitar amp for my first guitar that is on its way. I play bass so guitar is somewhat new. I have a basic knowledge of electric circuitry but I am clueless about all the tube-amp-speak I see online

I have a couple reel to reels from the 50's and 60's, a tube testing module, and a big box of tubes. I got all of this after the relative of mine( a radio nut ) who owned it all passed away. Unfortunately he can't be here to guide me so I need a place to start.

I don't want a hand out, but a little guidance

So I have a knight kn4110 and an adt(something from 70's i think) set of reel to reels with tubes in them! They are both record-play units, one of them having two tape heads

How do I make this a guitar amp?

I have not tested the units because I was told these were made to take less than standard wall voltage.

thank you in advance for helping a noob

[Chuck H]

First, scour the internet for a schematic of those units. Then post the schems here with the same request.

[paull]

Looks like I can get a manual for $10... don't know If it will contain a schematic. Is there anything I can see from the amp itself that will be a sign this conversion will NOT work. just so I don't buy a useless 10 dollar document? thank you!

[Chuck H]

Well, something to consider is that however loud the unit is is how loud it will be as a guitar amp. Most old RTR's don't put out much. Most conversion projects start with something more like an old organ amp or a tube PA. If all the volume you need is bedroom practice levels then an old RTR might work if it actually has a power amp. Many were just pre amplifiers intended to plugged into a power amp.

The manual probably won't include a schematic. But you could email the seller and ask.

A basic book on tube guitar amps is a good idea. There are many out there aimed at the DIYer.

I started with simple preamp mods and moved further into design as I learned more about the circuits, There's a lot to know. If I were a novice considering a build I don't think I would do a conversion, I would do a kit. With a conversion, even if you have a very specific amp in mind with a layout and schematic for that amp that you can copy other things will come up having to do with specific voltages, transformer impedance, layout/grounding scheme, safe lead wire size and insulation, etc. A book on the basics and a kit built could take you a long way. A conversion can cause a lot of frustration and lessons by error. Ebay nearly always has some half baked conversion project amp up for sale.

[Jag]

Also, a starter clue on their usefulness for guitar would be to list what tubes are in the units.

[paull]

well the ADT says it has 30 watt stereo output . It also has volume and tone pots on it. I am trying to find the manual online. it sure is difficult. I'm going to try and grab a book this weekend.Thanks everyone!

[Chuck H]

Without a schematic it's a lot tougher. Since you know nothing about the unit and we know nothing about the unit what can we possibly offer. Actually, we could offer something to someone who knows something about cathodes, output transformer primary impedances, rectifier circuits, etc... With you as a sheer novice there's no platform to work from. We need to know what your dealing with and you need to be able to understand the correspondance.

[paull]

Ok so inside the reel to reel there are 4 tubes visable. Possibly one more within an aluminum cylinder i see on the chassisThe larger one is labeled EZ81 the next is EL84 There two other smaller ones are labeled 6AT6 and 12AX7. What are we lookin at here? Thanks!

[Csquare4]

Ok so inside the reel to reel there are 4 tubes visable. Possibly one more within an aluminum cylinder i see on the chassisThe larger one is labeled EZ81 the next is EL84 There two other smaller ones are labeled 6AT6 and 12AX7. What are we lookin at here? Thanks!

Sounds like you have a 6AT6 Preamp feeding a 12AX7 gain stage which feeds the EL84 power section. Pretty much has to be a Single Ended design if there is only one EL84. Is there another section? If it is stereo, it should have at minimum two signal paths that resemble each other, i.e., two 6AT6's and two EL84's. The 12AX7 could possibly pull double duty depending on the circuit design. I have a hard time believing 30W stereo coming from a single EL84...is this the same amp you mentioned earlier?

Can you post some photos of the guts? From what it sounds like, I would be looking at possibly a Fender Harvard or Princeton with an EL84 driver...that might be a start. But there are a ton of single ended EL84 DIY amps on the internet...google "Single Ended EL84 Amp design" or something similar.

What does the output transformer look like? -- this should be the smaller one connected to the pin 7 of the EL84 on one side and the output side connected to the speaker jacks. What is the speaker out configuration? Does it say 8 ohm or anything there?

In lieu of schematics there is quite a lot you can do, but you need to figure out a few things. For one, if the output transformer is for a single ended amp, you are pretty much stuck with that kind of design, which can really lend itself to some pretty desirable low power designs. But you need to determine how the EL84 is being driven, and what the pre-amp/tone stack look like.

What I like to do if I have an unknown amp is draw out the schematic myself. It should not be that difficult on a small single ended amp like this...just start on one side (I like to start on the Output Transformer side) and draw your way across the circuit. Make sure every component has a connection on each end, and figure out what is connected to it. It should be pretty straightforward if you are willing to invest the time. Once you draw it up, post it here or look at single ended designs, low power Fenders/Vox/etc. and try to find something similar to work from as a basis.

[Csquare4]

A couple more things -

The EZ81 is small full wave rectifier, this is what converts the AC coming from the Power Transformer to a pseudo DC for the other tubes to operate with. You may be limited in your design by this. It is a pretty low power rectifier compared to most.

And - I would *not* invest in the owner's manual. I think you will likely find it next to useless in what you are trying to do here.

[Chuck H]

And - I would *not* invest in the owner's manual. I think you will likely find it next to useless in what you are trying to do here.

Unless it has a schematic in it.

If this is the ADT I can't imagine how they got "30 watts per channel" from a single ended EL84??? Must be a misunderstanding. A single ended EL84 guitar amp is typically good for about three or four watts. Players love them for their ability to be run balls out without blowing your head off. Always a nice little amp to own.

[paull]

Here are some pictures of the amplification set up I extracted from the adt reel to reel. The second to last picture with me holding the tube that came from the socket that reads el84 however the tube says 6bq5. Don't know if that means anything. In the photo of the control panel you can see a large black transformer that may power the reel motor. but it is somehow wired Into the amp circuit( at least the fuse from what i can see). Thanks for your comments!!

[loudthud]

The 6BQ5 is the American version of the EL84. No problem there. The big can near the 12AX7 is the filter capacitor. It might be hard to find a replacement, but there is plenty of room under the chassis for axial or radial replacement caps. The power transformer might have a winding to power the motors, you can just cut and insulate the wires.

[paull]

So is this thing good to go as is? I would guess these old paper capacitors should be replaced. About this large cylinder... is it assumed to be broken due to its age?

Thanks!

[loudthud]

About this large cylinder... is it assumed to be broken due to its age?

Old elecrtolytics are unpredictable. It might work for a while and then explode or fail and damage the rectifier tube. The smart thing to do is replace it and then you will have no worries.

[greekie]

This schematic results in a really fun amp. I've built one myself, and it's just a whole lotta fun to play!

Just for good measure, I'd make R9 a 5Watt or 10Watt-resistor, and slap an electrolythic capacitor 100uF / 50V or more across it with the negative terminal to ground. That way you'll get plenty of volume from the little champ!

[greekie]

... and just some info you'd might like: The 6AT6 can replace the half 12AX7 in the tremolo department (if you need that sort of thing - or else just scrap it!) and the EZ81 rectifier you have is plenty to support this circuit.

[paull]

Thank you for the help. SO I am going to try and take what I have apart. and see how closely it lines up with the schematic you provided!

thanks again!