Turning a Tube-Guitar Amp Chassis into Amp for Turntable, Stereo, etc.

[NeverTheMachine]

I was wondering how this could/would be done. I am not the most electrically handy guy, but I figure this would be a cool project to undertake. I love the look and feel of those vintage tube amps running through a simple turntable and some speakers.

 

They also seem to be on the high-end of price in a niche market, $2000+ ... but could it actually be possible to build for a lot cheaper?

 

I have a few guitar amps I don't use much that I want to sell, but am thinking about turning one of them into a stereo system.

 

Thoughts?

[Danocoustic]

it could certainly be done, with some mods...but guitar amps arent exacfly hi-fi, probably wouldnt sound so good

[WRGKMC]

It is not something an amature is going to be able to do without reengineering the circuits

which requires extensive electronics knowlege or some serious guidence.

 

For stereo you have to mod two amps not one. Its totally impractical to make one guitar amp into a stereo

because everything has to be doubbled. Stereo is two single amps in one chassis and guitar amps wouldnt have the space

in a single chassis to doubble everything.

 

So my first question would be, do you have a matching pair of tube guitar amps to convert to a stereo amp?

 

You may be able to find so paint by numbers mods that will let you turn a HiFi into a guitar amp

but its highly unlikely you would have any info on doing it the other way around.

 

You would have to completely re-do the preamp section and increase the fidelity

and frequency responce. You'd have to change the input impedance from instrument level

to line level and add another stage for phono level. Guitar amps roll off at 5~6K so you would need to tripple that responce.

Then you'd need to replace just about every coupleing cap in the thing so it would pass that higher frequency.

The phono section would need to EQ the signal to RIAA standards if you expect the turntable cartridge to play back

a record sounding anywheres near HiFi standards. http://en.wikipedia.org/wiki/RIAA_equalization

 

To get started you would need to find HiFi schematic that are close enough to the guitar amps you want to modify.

That alone is a tough task comparing guitar amp circuits to HiFi amps so you dont have to completely gut everything at a

high cost of replacement. If its a high powered guitar amp something like Mackintosh used high powered mono block power

amps with large transformers. You're going to have to do allot of digging to find something close enough to be worth doing.

Otherwise its cheaper to build from scratch if you cant use most of the more expensive parts in a guitar amp to get you there.

 

In my gestimation, A pair of guitar amps isnt cheap and would cost you just as much or more to mod up a pair to act as a stereo

then it costs to buy a stereo tube amp kit. There are many out there that are very reasonable cost and it would be my best suggestion.

If you already have two tube guitar amps, sell them and buy a botique tube amp kit. You can get them from china without tubes for

a decent cost. and when it comes down to it you can build the Power amp and preamp separately.

[Danocoustic]

not only that, but all yer gozintas and gozouttas would hafta be changed too

[yanktar]

WRGKMC is more pessimistic than I am but his points are well-taken. My take is a little different:

1) Your speakers on an instrument amp (assuming it's a combo) are not appropriate for music. You'll have to get stereo speakers.

2) You CAN route both channels of a turntable into a single amp with a simple y-connector but will only get mono sound, not stereo so the fullness will be lost. Otherwise you need matching amps.

3) The schematics and layouts I've seen of tube guitar amps, at least the power amp sections, are not that different from other, or at least older tube amps. But the pre-amp sections are QUITE different. I've noticed a lot of amps now have a regular line-level input, but I don't know if the older tube amps do. IF you have the line-level input (used for output from a CD player or iPad or some other component) on both amps, you are half-way home. If not, WRGKMC is probably right--you have a lot of work ahead.

4) You WILL need a phono pre-amp. These raise the low-gain phono output up to the line level above. They can be very simple boxes that run $30-$50, or be insanely expensive for the audiophile nuts.

 

5) Or you can go the eBay route and find an older audio tube amp for a couple of hundred bucks. I found (and fixed up) a Dynaco 17.5wpch integrated tube amp (with phone pre-amp built in) for less than $200. It's not loud, but OH! is the sound sweet! You can also find all over the place working Dynaco ST70 stereo amps--35wpch, and PAT-4 and PAS-3 pre-amps (again with built-in phono pre-amps). That PAT-4 is solid state, the PAS-3 is tube (and commands a premium), usually pretty cheap.

 

I did just the opposite: I took a Dynaco Mark IV (35-40watts) mono power amp, with a Dynaco mono PAS pre-amp (tube), hooked it to an instrument speaker a friend gave me (had been a combo amp but the amp died and was discarded), and, with a couple of connectors to convert a 1/4" plug to RCA plugs, have a really nice blues bar amp sound with lots of volume. Really clean!

[WRGKMC]

I wasnt trying top be pesimistic, I was trying to point out the practicality both

technically and cost wise.

 

 

Most tube guitar amps do not have line level inputs. Many have effects loops that are

at line level but connecting anything to that bypasses the preamp and ties directly into the power amp

so there would be no control over the EQ or gain so you'd still need a separate preamp with those controls.

 

Using a separate low cost phono preamp kind of defeats the purpose of having a tube amp.

The solid state radio shack type ones work well enough but a solid state phono preamp into a tube amp

doesnt quite get you there.

 

Digging up something to restore is a good option if you know what you're looking at.

If its vintage you often have to do a complete recap. not only will the power caps be leakey, the

smaller caps that couple and EQ will have likely drifted badly and make the two channels sound different.

 

My first job out of school back in the late 70's as a tech I repaired 50/50 Cunsumer and pro audio.

There were plenty of tube amps still around then. The only Stereo tube amp I have left is my old Eico amp.

It sounds good and all when i connect them up to my high end Cerwin Vegas, but its doesnt come close to being

any good for my studio work. I'd still be hard pressed to find a tube amp now that can beat a Crown power head

with a high end solid state preamp for the same cost.

 

There are some decent kits though as I mentioned.

 

This is probibly what the OP was thinking of. Bit over the top for most in cost.

http://www.electronickits.com/kit/complete/ampl/VEK4040.htm

 

A tube amp doesnt have to be real expensive to sound good. This ones 16 W

8W per side and would fill a typical room quite well.

http://tubedepot.com/diy-k12g.html

 

Heres something close. you'd still need the transformers.

 

http://www.ebay.com/itm/2A3-6SN7-6SJ7-Push-Pull-Tube-Amplifier-Kit-Stereo-/220724057066?pt=LH_DefaultDomain_0&hash=item33642dd3ea

 

You mentioned Dynaco builds which are excelent.

 

http://www.ebay.com/itm/DYNACO-ST-70-STYLE-DIY-STEREO-TUBE-AMPLIFIER-KIT-NEW-DIY-AMP-KIT-USA-/200688085815?pt=Vintage_Electronics_R2&hash=item2eb9f12b37

http://www.ebay.com/itm/DYNACO-ST-70-STYLE-DIY-STEREO-TUBE-AMPLIFIER-KIT-240v-DIY-AMP-KIT-/110795234116?pt=Vintage_Electronics_R2&hash=item19cbe91b44

 

10W per channel in a simple build.

http://www.ebay.com/itm/6L6-SE-Tube-Amplifier-Kit-12AX7-and-6L6GCR-Stereo-/220710459980?pt=LH_DefaultDomain_0&hash=item33635e5a4c

 

 

Another 35W dynaco build at a very reasonable cost.

you could get the whle thing here for what it would likely cost to buy a guitar amp head.

Http://www.ebay.com/itm/DIYTube-Dynaco-ST35-Tube-Amplifier-Kit-Great-Amp-4-iPod-/200499593000?pt=Vintage_Electronics_R2&hash=item2eaeb4ff28

 

Heres the Eico I have. I paid $50 for mine. This rare NOS kit is $1300

A bit of a markup there when you consider the cost of the individual parts.

 

Theres a bunch in the $300 range I've looked at but dont have time to find them right now.

[cx04332]

I had a stereo consisting of 2 Traynor tube bass heads, 2 V4 Cabs, a tuner/amp stereo and a 2 channel equalizer. It kicked ass. Everyone except the neighbors loved it. There was a nunnery next door and they gave me dirty looks every time I passed by.

 

All you need is a pre amp. If you can find an old tuner/amp stereo combo with external connections from the pre amp to the power amp, you can use the pre amp of the combo. Otherwise you can buy one or make a pre amp. Plenty of plans, kits, whatever on the internet.

 

The folks with the oscilloscopes will not be impressed but If you like loud music, it will work.

 

Nowadays, I use headphones and a computer for listening to music.

[NeverTheMachine]

Thanks for the advice and information guys. It's good to get familiar with stuff at the least, even if it doesn't work out for me.