Dual-Amp project, single PT?

[Don Moose]

Wife wants us to start playing together in the living room - she on piano and me & boy on bass & guitar, respectively. The hiccup is here requirement of amp(s) that are 'living room compatible'. I got hold of this Magnavox end-table stereo (solid state, but useful cabinet) to build into - the cabinet is mostly plastic, as I learned after we got it home.

I'm thinking something like a Princeton/Deluxe (15-20W) size circuit for guitar and an Ampeg B15 (50W) for bass, using the left-hand speaker cavity for one and the right for the other.

The existing circuit enclosure is about 12.5 x 12.5 x 6", with free space both above and below. My question for you lot is whether a single Twin Reverb sized PT to power the whole mess has any significant disadvantages over fully isolated circuits.

The single-PT setup would have one set of rectifiers and main filters, followed by separate chokes and divider strings.

Happy new year, y'all!

[clyde]

I guess my first thought would be whether the pt can handle the current requirements for all the tubes. with that aside, it sounds like you are wanting to build a stereo amp with each channel run mono and voiced for the specific instrument. Don't see why it can't be done.

[Don Moose]

I was thinking something aimed at a twin reverb would be good because that's 4x 6L6 + 5 x 12AX7 for heaters. I'm thinking that to get the B+ I want for the deluxe side, I'll have to use separate rectifiers and filters - maybe a really lossy tube recto for the guitar side (2x 6V6) and solid state for the bass side (2x 5881).

Since I'm given to understand that I need a physically larger OT for bass, I'm thinking that a Twin/DualShowman Reverb sized OT would be preferable.

Once again, though - I want half the output tubes that OT is aimed at, so my amplifier output impedance is doubled, so I need to connect the N-ohm speaker to the N/2-ohm tap to stay even? ... or is it the 2N tap?

Lemme see if I can work this out:

2T-ohm output connects to T-ohm primary of T:N transformer, reflecting 2N ohms to the secondary. That makes the "8-ohm" tap want 16. What did I forget or misunderstand?

[Enzo]

Just a thought. Considering it is a piece of furniture that makes music in your living room, why not just make or buy a small solid state power amp and a preamp or two? Then take the transformers and stuff and build a nice real guitar amp for yourself to use on gigs. 50w per channel is nothing in the world of power amps, a likely candidate should be simple to find. Or find 100w a channel and put a pad at the input jack.

SOmething like a Samson Servo 120 or Servo 200 can be had for $200 new with a warranty. And an Alesis RA100 or something also cheap. And used, geez, the sky is the limit. AN old PV CS400. How about a Crown D65 or similar?

Those are pro audio amps, but a common stereo receiver would also work. I use to know a guy who had a 40w per channel Pioneer receiver he used for his PA. (FOlksinger)

Then you wouldn't have something full of tubes getting hot in the living room as well.


And another thought. Take the thing to gigs. Set up the stage with your Magnavox front and center.

[Don Moose]

Hmm. A couple of preamps does sound easier.

The thing is solid-state so there are no transformers to repurpose.

[6267]

We took all the tube stuff *out* of a magnavox cabinet, and cut two more 12" holes in it. Ended up with a 4x12" + 1x15" + misc tweeters open-backed speaker cab that was easy to move to gigs. The tube stuff unfortunately had a blown PT (open B+ primary ) so I gutted it for parts. We used to have another amp that was in a stand-up 40's radio, and another one in a fisher speaker cabinet...shoulda had a couch amp!

I currently have an amp I'm working on that's similar to what you are talking about, where it was originally an organ amp with two dissimilar channels (seperate preamps and poweramps, seperate rectifier, same HUGE power transformer...it's like 6" cubed ). Each speaker is tapped through resistors to drive a common 6sn7 p-p for the reverb. Reverb return comes in through the 6v6 channel, the 6L6 channel is fairly clean. I cut the feedbacks, and realized that these bastards didn't really design it with no feedback and add it, though...they just added feedback till it stopped howling. Bad layout, still in the process of making do. I built a cab for it that has attachable casters, because it's heavy. Probably heavier than each amp separately. You have to make sure you have enough capacitance in the PSU to keep the amps seperate if they share any PSU components (6L6 screens run on the 6v6 B+ in mine) to keep them from interacting in strange ways. It's cool to have both amps in one box though, since it does probably take up less space than equivalent discrete amps.

[Don Moose]

Hmm. A couple of preamps does sound easier.

The thing is solid-state so there are no transformers to repurpose.

Oh. NOW I get it - the iron I was proposing to buy, use for giggable amps.

I might, but the kid's 11, and I will have to put in a solid year's practice to get back to merely sucking, so I have time to build it both ways.

[Don Moose]

This brings up another question, based on old rumor.

If I knock a couple of preamps together and plug them into the tape player inputs on this HiFi, will I need to roll off the highs in order not to cook the tweeters?

I ask because, back in the late Pleistocene, I plugged a microphone into a Sunn Concert head and fed that output to a pair of HiFi speakers and did that very thing.

[Enzo]

Lemme see, 300 watt head, home stereo speakers with 10 watt tweeters, and... you say the tweeters blew?

[Don Moose]

C'mon, it's not like I had it dimed.
Still - headroom, transient response. Yeah. Makes sense.