6AQ5 SE amp build from Ampro parts

[shaun]

Hi All. I am converting an old Ampro reel-to-reel into an SE guitar amp. The tape mechanisms were all rusted to hell and non-functioning, but the basic amp seems nicely intact, as is the wee cabinet that I'll re-use to house the amp.

My question concerns possible schematics to follow during my rebuild. I've seen schematics for 6AQ5 SE amps, which have a single preamp tube, usually a 12AX7. But if you take a look at the attached pics, you'll see there is a 12AX7 and a 6UA6 in the preamp stage of the Ampro design, which also has a volume pot and a tone pot.

I've heard that 6UA6 is an okay tube for guitar amp building, so I'd like to find a 6AQ5 SE schematic that would use all four tubes in the current chassis: 12AX7, 6UA6, 6AQ5, and 6X4 rec. I might have to trade the 6UA6 for another 12AX7, or I may simply have to scrap the 6UA6 altogether if I can't find the right schemtaic, and just have a volume pot.

The transformers seem fine, so it seems like a nice challenge for a novice such as myself to convert the current tube line-up into a guitar amp. However, I would like to start with a tried and true schematic, if anyone knows of one.
Thanks for your help.

[PRR]

You neglected a key detail: what model number.

The attached is not your machine but is likely 70% the same, shows what Ampro was thinking those years.

Guitar pickup is much hotter than tape-head. Also we know many groovy g-amplifiers (even the Valve Jr) managed with just one 12AX7 and one power tube. Using the pentode will be just Too Much for guitar. The 12AX7 and power bottle are wired nearly the same as a Champ or Jr or small Gibson. That is, would-be except there are eighty lugs on the function switch to convert the electronics for record or play. I suppose you could cut those wires, use g-amp typical fixed values, and jumper power to any stage that gets power through the switching. That's really half-way to a scrape-and-refill job, so maybe that would be a way to go.

Some things trouble me. At first I thought the 755 was Hot Chassis. Heater transformer but where does plate power come from? There is a hot-chassis DC supply but that seems to trace to the solenoids (not shown). It looks like plate power comes from the motor! (not shown) I have seen that done on small transistor phonos, never on a tube tape recorder. And you sure don't want to drag a whirrring motor to gigs. Yet it looks like *your* model has an actual power transformer (and also output transformer). Verify that before you go too far.

[shooter]

fwiw,
you're money ahead converting to a 9 pin'r. 
I did a build with the 6AU6, 3 actually as a 3SE.  pretty gungy amp, the guys that play guitar liked it, but the tubes were pretty microphonic and finicky

 

[PRR]

The stock tone-control is interesting. Not sure just what it does, other than it gives NFB (speaker damping) around the amplifier with variable frequency curves. I assume it worked for Ampro and it may be worth keeping in your build. (The 5k tone-pot is not real suitable for other tone schemes, and other pots may not fit your knobs.)

[sluckey]

You see that RF coil on the chassis? That's part of the bias oscillator. This means that one of those tubes (probably 6AU6) is being used as the oscillator. So, only two tubes left for the audio amp. With that in mind, take a look at this...

     http://sluckeyamps.com/smoky/smoky.htm

[Tony Bones]

IMO, little single ended amps sound best when they're kept simple. The simplest is something like a Champ 5F1. You can add a tone control like the Princeton 5F2. Both of those circuits use a 6V6, but a 6AQ5 is pretty much a direct replacement. Likewise the rectifier tube, so just copy the Fender circuits and you'll be fine.

If you really insist on using the 6AU6 then figure out how to use it to make a LFO for trem (ala VibroChamp.)

[shooter]

also early Gibson versions of fenders 
use the lfo trem into a GA5-TEDIT Kalamzoo, somehow; you'll have a fender 5F2-ish 

[shaun]

Yep, that's what I thought - I'll keep it simple with just a 12AX7 and the PT. I like the GA 5 and will probably go in that direction.

One of the reasons I didn't post the Ampro model # is because any time I try to work with what's there already, I find myself spending too many hours in a fog of misunderstanding - I'm just not that good. Yet:). So I didn't look too closely at the original schematic. But it is an Ampro 731. Schematic attached.

The "Smoky" amp is way cool - in a toaster oven! Easy to read schematic, too. Thanks, Sluckey.

Thank you for the guidance. I'll post when it's completed. Or when I screw up, whichever comes first.

[shooter]

too many hours in a fog of misunderstanding

the reason for "understanding" the original design is keeping the "new" design within reasonably close criteria to what the old parts are capable of.  More than once someone has an old SE 3 tube'r and wants a new twin, or Marshall  100!

[shooter]

FWIW
thinking (always a problem:) if you're still considering pentode pre, schematics with the EF86 should be reasonably close to "replacement" for your 6AU6.  If you wanna keep the AU, I would suggest having a new socket and 1 or 2 spare tubes.  When I did my 3SE I went through 3-4 before I got 3 that were acceptable

[PRR]

> one of those tubes (probably 6AU6) is being used as the oscillator.

Now we see they used half 12AX7 as bias. Never saw such a weak bias. Don't see an erase function either.

Also there is no microphone input (what I thought the 6AU6 would do), only line input. High sensitivity: maybe 100mV. So you could play e-guitar that way. Would be hard work.

At a glance, you could rig V1 123 with maybe 5k unbypassed cathode resistor, disconnect red wire to head, feed to C7. Have to work out the switching.

[kagliostro]

6AQ5 has a Micromhos of 4800 and 5881 that has a Micromhos of 5100 -- 6100 (depending on B+ level)

so ...... I think I really would like to give a try to this schematic

(Redoux of an Electar Tube10 - posted by our friend Dummyload)

(Unfortunately I'm not able to find the original post, but I've a copy of the schematic in my archive)



in your case use the 6AU6 in place of the 6267 and the 6AQ5 in place of the 5881

---

@ PRR

It looks like plate power comes from the motor! (not shown) I have seen that done on small transistor phonos, never on a tube tape recorder.

I've a small Geloso (tube) recorder that use such a configuration

I've seen some guy that dismanted the rotor from the motor and used it as a simple transformer



Franco

[ALBATROS1234]

I was kinda thinking like you when I first started a few years ago and wanted to try and make a se amp sound like a marshal stack. I ended up making squealing oscillations instead of sweet guitar tone . I do like pentode preamp but they usually have a lot of gain but are a completely different animal than a dual triode to set up. I have not used a 6au6 yet but have used 6sj7, 6u8a, ef86 and 7199 for preamp. I have two favorites one uses the 6sj7 and the other the ef86 along with a 12ax7 se. But I set it up like a 5e3 deluxe whe r the ef86 was one channel, half the 12ax7 the other channel each with a volume and single tone like the 5e3 fender . Next it goes thru the other half of the 12ax7 then to a 6v6. The volumes blend the 12ax7 and ef86 so if you plug in the 12ax7 channel and turn the ef86 channel down it's basically a champ. But blend in that ef86 and it crunches up really nice. The ef86 by itself makes a decent clean chimney sound. Good luck

[murrayatuptown]

http://sluckeyamps.com/smoky/smoky.htm

Toasty! (nice)

Murray

[1blueheron]

Shaun,

Airline liked the 6UA6 and used it as a preamp and as a tremolo

Airline 8502 used the 6AU6 as preamp and 6V6 as power output with a 6X5GT rectifier.  Its a usable stock circuit with your tube list from what I can tell.  You could use your 12AX7 as a trem, a reverb, or an FX loop or as a gain stage later.



My Airline 8504 used a similar tube compliment 6X4 rectifier, 12AX7 preamp, 6AU6 Trem, 12AX7 PI, 6v6 output power, albeit it was a PP rather than an SE. 

Pretty simple circuit if you make it a SE and substitute the 6AQ5 for the 6v6 everything should pretty much match your lineup.  Also the Kalamazoo as mentioned, and the Gibson GA-5  use a similar setup without the 6AU6.

Just a couple ideas to get the gerbil wheels turning.

[shaun]

Thanks for the info, 1blueheron! That's great food for thought. Much appreciated.