Got parts but what to make with them?

[Tyrannocaster]

I have a Fisher X-100 here which is available as a donor for PT and OTs as well as tubes. I’m stumped when it comes to figuring out what to make of it, though. First of all, it’s a stereo amp, so there are two OT’s. Second, it uses 7189s, not EL84s, and according to the schematic it puts about 390 volts on them. There’s no tube rectifier (and no 5 volt circuit on the PT) so I have to use a ss rectifier or add a separate transformer, which I’d rather not do. Because the 7189s are so expensive to get and because they are happy with so much higher voltages than EL84s I’m reluctant to build something that relies on them - I don’t want to pay the bill when I have to replace them. But how to lower that voltage? I’ve used zener diodes in the power supply to do this but I don’t like it; they overheat and if you have a failure it's awful.

Anybody have any ideas? I was really attracted to the old Gibson stereo amps until I started looking at the schematics and realized I would be getting into more than I can handle as there are no layouts available; plus, they use EL84s and not 7189s and the tremolo on those things wobbles the power tube bias - different tubes, different values needed and I wouldn’t know what they are. I’ve built about nine amps from scratch but I haven’t done any in the last several years and I’m feeling very rusty.

I guess the main problem for me is trying to get that voltage down so I could use EL84s. Then I could still use the 7189s (they’d probably last forever), too.

All ideas welcome!

[sluckey]

I guess the main problem for me is trying to get that voltage down so I could use EL84s.

Maybe build a 6V6 amp instead?

[Tyrannocaster]

Yeah, that's the conclusion I came to, too. That's one reason why I was looking at the Gibson stereo amps. They are supposed to sound cool, but since only some 300 of the 83 were built they aren't that common and I've never played one. But I know that JJ 6V6s can tolerate that kind of voltage so I think that's probably the way I will have to go. But the tremolo circuit is intimidating to me so I'm still looking around. If it weren't for the stereo aspect I'd be happy to make something like a 4 tube Deluxe (I have a '55 5D3 that is a killer amp and I cloned it once (the clone was wonderful too, but I figured "why do I need two of these" and turned it into something else) but I wouldn't mind a more powerful version. The 5D3 never gets any love but it's more interesting than the 5E3 IMO. Its distortion is different with that PI but everybody builds the 5E3. But I don't think a stereo 5D3 would be that useful...

[kagliostro]

Why not Doug's Stereo Guitar Amp with 6v6 instead of el84 ?

It is well documented, there are schematics, layout and photo documentation






http://www.el34world.com/projects/StereoPreAmp1.htm
http://www.el34world.com/projects/StereoPowerAmp1.htm

Kagliostro

[Dave]

Look at how many commercial amps there are out there that are squeezing "50" watts out of a quad of modern EL84's. I don't hear people complaining that there Peavey Classic 50, or their Carvin BelAir, or their Mesa Nomad 45 (I could go on and on) is having trouble because it is running 400+ volts B+.

Data sheets are great, but rules are meant to be broken.

Dave

[Tyrannocaster]

I didn't realize those amps ran the EL84s that hot! I thought you had to stay down in the low 300s with those tubes. That's interesting.

I'm going to look at Doug's stereo amp; that's interesting, too. Thanks; I've been out of the loop for a while and i didn't know about that one.

[Tyrannocaster]

Why not Doug's Stereo Guitar Amp with 6v6 instead of el84 ?

It is well documented, there are schematics, layout and photo documentation

Kagliostro

Is there actually a layout? Doug's page says not, because it was wired point to point.

[kagliostro]

The preamp has a layout



about power amp you can use the photo as a layout



Kagliostro

[PRR]

How many Fisher X-100s are there?

With and without vacuum rectifier, 7591 and 7189....

Leave the poor X-100 mostly as-is.

It is perhaps "too good for guitar" (look at the size of those OTs!) and you should check that there is no Hi-Fi market, but I know it may have no gold-ear aura.

Run modern EL84 in it. Since 1959, everybody has been cheating-up the EL84's rating, and the tube makers had to use the better stuff or drown in complaints. I didn't think thrice before stuffing new EL84 in my 410V Traynor.

Mod the NFB loop around the power stage for less-polite more-raw guitar sound.

If it has 7247 near the power tubes, drop in 12AU7 for instant less-polite sound (and much easier retubing).

Everything to the left of the power-stage probably should be re-worked from scratch as guitar preamp.

Get a schematic for YOUR amp and we can figure this out.

[DummyLoad]

How many Fisher X-100s are there?

With and without vacuum rectifier, 7591 and 7189....

his has 7189 so that should narrow things down a bit; X-100 is likely. X-100B/C used 7591 & 7868 respectively.

-DL

* flip it on fleabay and use funds to procure guitar amp iron.

[Tyrannocaster]

Ebay - no thanks. I've had bad experiences with that. I tried listing it on Craigslist a couple of times at different prices and got zero emails; I think the three hefty trannies are worth around a hundred bucks to me easily and I couldn't even get offers at lower prices than that - it's staying here, I guess. I have not decided what to do with it at all; there are a lot of possibilities but nothing leaps out at me. You're right - a buyer who actually wants an X-100 would be the best, but there don't seem to be any of those in my area who are willing to spend cash. As mentioned, this is not a Golden Ear fave. I haven't touched it and may not - I'm just trying to figure out what to do with it. It works and sounds good, but the selector switch is totally hosed and a stereo tech I trust says that unless you can find another one of those it's going to be hard to sell since you have to use vise grips to change the input. He tried heating it gently to see if it would break loose but no dice. And that switch is totally proprietary so chances of finding another one are slim to none.

[HotBluePlates]

If everything is present on that Fisher, I'd suggest getting it in working order and using for your home stereo. It will probably sound as-good or better than what you already have.

I had a Fisher integrated unit, with 7591's, that somehow disappeared during a move. Never got a chance to listen to it. However, the McIntosh MC-30's I have (yeah, they're a step up) sound better than any solid-state stereo I've heard (I haven't listened to the modern $$$$$ ones, though).

[Tyrannocaster]

The Fisher doesn't sound nearly as good as my old Marantz that I use for hi-fi. I tried it out as soon as I got it; it's okay, that's about all I can say for it.

[Tyrannocaster]

Also, that messed up selector switch makes the amp unusable in practical terms if you have more than one audio source and I have many. Well, nothing will happen overnight and perhaps I'll find just the right person for it.