Using a Tube Amp from a "Hi-Fi" set?

[Joe Hart]

I don't know if this is really a stompbox question or not, but I'll give it a shot here (I think it qualifies somewhat??).

I picked up an old "Hi-Fi" -- or at least that's what my grandmother used to call it. It's one of those things that looks like a long wooden piece of furniture, but it's really a record player/stereo. Anyway, inside it is a tube amplifier and four speakers (two per side). The amp is in a completely self-contained chassis (even the volume and tone knobs are mounted in the chassis with the shafts sticking out) and it looks like all the wires are pretty easy to figure out, so I was wondering about rehousing it to use as an amp head and/or using it as a tube preamp to push a tube amp to more distortion (that's where I think it may qualify as a stompbox!).

So, I clicked the panel selector to "phono" plugged a guitar into one of the "phono" inputs (the record player plugs into two RCA jacks on the chassis -- see, it should be a very easy rehouse) and got sound out of one side! Not great sound, but sound.

Here are my questions:

1 - the sound was fairly weak, is this due to an impedance mismatch or maybe because the "phono" inputs are expecting a higher level? I use fairly high output humbucking pickups (Seymour Duncan JB's). Or is it because of the RIAA Equalization thingy (where record albums are recorded with extra high end, then the record players are designed to roll off tons of high end to even the sound back out)? This may create a flat response when playing records, but probably wrecks havoc with my guitar signal.

2 - there was an incredibly weird sound coming out. It was a puttering motorboat sound that seemed to create an octave-down pitch that was out of tune (the whole sound was kind of like when you have two pitches that are a few cents off from each other and they "beat"). I couldn't figure it out until I took my hands off the guitar and it went away. Does this sound like a simple grounding issue? The power cord is only two prongs.

3 - I would assume that once I get the sound situation sorted out, "building" an amp out of this would be a simple matter of rehousing the chassis and speakers. Is there a simple way to turn this "amp" into a tube preamp that I could patch between my guitar and amp like any other pedal? Joe Stump (if anyone knows who he is) used to use a big thing that was made by Bob Gjika (I believe) that had a few tubes and a big transformer. I don't really know anything about it because I only saw it a few times and this was many years ago.

Any ideas? Thank you!
-Joe Hart

[Ronsonic]

Use as a pedal is pretty much pointless dumb bughumping crazy impractical. There's nothing in there that would make it a good pedal in any way other than say, running a guitar amp into a dummy load and then into another qualifies as an effect. The preamp section, which is what you'd want gets its power from the power amp and hacking that would be a large bulky pita.

On the other hand, you've got all the parts you need to build at least one guitar amp. Compare the parts to various existing designs that interest you and see what looks practical. Evidently it is a stereo amp, you can use that if you like. Hell, you could just build a guitar pre into the front end and have the ultimate living room guitar experience. Make sure you can get a stereo echo in the loop.

I wouldn't be disappointed by the poor results from your test. You usually have better luck with an aux input, maybe modding it for more gain.  I'd look more at working with the amp chassis, creating a front end for it and building a guitar amp with the parts.

I've built a lot of stuff out of singing furniture.

What's the tube complement on the power amp?

[Philippe]

Interesting concept. I've got an old Dynaco ST-70 lying around. At 35/35 watts per channel, could this thing be converted into a practical guitar amplifier? Maybe incorporating a switchable 35/70 watt output?

Decades ago, I recall some shows where the Grateful Dead were running a series of McIntosh mono-blocks for PA amplification...that Wall of Sound thing. Sounded pretty cool but I sure wouldn't have wanted to be a roadie. Then again, there were probably some pretty good 'fringe benefits'.

[Dan N]

The motor boating and overtone are signs bad filter caps. Old unused tube gear likes to be slowly brought up with a variac or some kind of current limiter so the old electrolytic caps can have a chance to reform. Even then, I don't trust old filter caps.

Ronsonic gives good advise. You have the most expensive tube amp parts (transformers and chassis) covered. Now, a few caps and resistors and you can have a fun little amp.

[a soBer Newt]

Interesting concept. I've got an old Dynaco ST-70 lying around. At 35/35 watts per channel, could this thing be converted into a practical guitar amplifier? Maybe incorporating a switchable 35/70 watt output?

I used one of those for a bass amp for a bit it work quite well.

[petemoore]

Interesting concept. I've got an old Dynaco ST-70 lying around. At 35/35 watts per channel, could this thing be converted into a practical guitar amplifier? Maybe incorporating a switchable 35/70 watt output?
  Converted?...this is state of the art Dyna stuff, hea we did a couple mods...but tubes with this quality of trannys'..very fancy pinnacle-like performers here ! Totally worth rebuilding, look how expensive..
  Mains what mine was made for were 115, and 'modern day' 125vac overbiased the whole shebang...sure wish I had those A-440 trannys and that Dyna now !
  Any of these...but note that the tubes, sockets, capacitors...may all be less than reliable at this point, change caps, use tubes to test tube, you can wiggle-test tube/socket connections.