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Post subject: Archeology of the Alamo Futura with schematic
Posted: Sun Oct 02, 2016 5:23 pm
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As a bench tech, I see a lot of amps come across my bench. Most are copies of each other. Whether factory or boutique or homemade, they are all pretty much the same circuits. Then there is the Alamo Futura.
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The layout is typical for amps of the day. A simple plate chassis and no reverb transformer made it cheap to produce.
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The amp runs a 16 ohm speaker, but the ext speaker circuit is a bit ambiguous. I couldn’t dissect it further.

It all seemed normal when I first fired it up, an old 60s tube amp, Made in San Antonio. Defunct company. No schematics. It was a simple fix, burnt resistor in the power supply. Replace that and start measuring tube voltages. Hey, that’s not a 12AX7 in V1. What the heck is a 6U8. Then there’s no voltage on V3. It glows, but no plate voltage. Pull it. Still no plate voltage. Hmmm. The amp works… what does that tube do?

Well, I went in deep. I drew up the schematics and specs. It was an interesting journey for the weeks I had the amp. There are tricks that are still relevant to amp tinkerers.
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The numbers in red are voltages

Alamos were built for Country/Western music.
They were very clean due to their simple tone controls. Just one cap to ground.
The Tremolo in the Futura is based on patent 2835870. It claims a stable slow speed lower than most designs and somehow mentions echo. Now this tremolo is based on a balanced modulator circuit. In my experience, this is normally only found in CB sideband radio. But somehow he makes it work for guitar.
The balanced modulator was used in the early days of music synthesizers. Its tone is shaped by the low frequency oscillations that are mixed into it. In the Futura, a simple low frequency oscillator feeds the BM. a sawtooth wave at 4-5 Hz to produce tremolo. Reviews said it was very musical, even at high speed.
While a Fender tremolo bug will do tremolo, the RF balanced modulators is more agile and is designed to impress voice into the CB signal. I would expect the Alamo circuit could do the same. A staccato or complex envelope should be easy to recreate.
I have never seen a tube powered BM before, let alone one suitable for guitar. This circuit could be a gateway to modding new guitar tones. The V3 tube with no voltage is the key to deriving an isolated mixed signal.
Also, the modulator, when used in radio applications, creates what are called upper and lower sidebands. These significantly alter the tone and can be mixed in creative ways.
The output of the modulator is low gain, so a boost is in order for modern guitar amps.

As interesting as the balanced modulator is, another circuit caught my eye that could be very useful is how the reverb return is designed.
This is an all tube amp, but there is one small transistor in it. I knew it was hooked up to the reverb, but was baffled about how they got a safe, low voltage to power the 2N3708 transistor.
Turns out, it’s powered off the cathode voltage of the power tubes.. Just a 10K series resistor and a 50uF cap for a filter and it provides enough gain for the sensitive reverb return amp. This innovative idea can be harnessed for all sorts of SS mods..
The reverb driver tube is a Pentode /triode pair. It is an obsolete 6U8. The tank is directly driven off the cathode of the triode section. This gives a high quality signal to the tank without the usual distortion in the drive signal.
The lack of an isolating transformer requires the tank be insulated from the consumer to avoid the possibility of shock. The tank is mounted under a wooden box. There is a piece of fiberglass insulation between the tank and box to create a spring damping force against the tank body. Further, the inner, spring loaded chassis is cradled in foam carriers to minimize reverb banging.
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These innovations were produced by the last generation of tube designers. While Leo gave me great insights with his designs. It's been valuable to learn from his competitors as well. The current crop of boutique builders may look at these schematics and take note of what actual innovation looks like.


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Post subject: Re: Archeology of the Alamo Futura with schematic
Posted: Sun Oct 02, 2016 6:29 pm
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Nice write up! Good, quiet NOS triode-pentode 6U8 and 7199 getting very hard to find. No direct substitutes, that I know of. Dynaco ppl have DIY to convert the 7199 to 6AN8A or 6GH8. With associated lost of rich tone.

6U8 mil spec are the best. Raytheon or Sylvania. RCA blackplate 7199 is the best, no contest.

From your schemo, it looks like the 7199 pentode is a drive and the triode is setup like a Concertina phase splitter. My experience with the Concertina --- is that the coupling caps between the PI and the output (6V6GT) make more of a tonal difference than in long-tailed paired phase splitters. I like PIO caps, for these PI to output stage. Russian K40Y-9 are most awesome! :)
Coupled with the fact, this amp doesn't appear to have any global feedback. This makes coupling between stages and proper bias levels in those stages more important. You may even want to put screen resistors on each of the 6V6GT screen grids. An extra 470-ohm, 2-watt metal oxide off B-point in the PSU to each screen.

Amp lacks grid stopper resistors on the 6V6GT tubes. So, you really need tough 6V6GT tubes. I'd prolly put grid stoppers. 1.5k to 2k-ohms.

Anyhow, I digress... good luck with the amp. Keep us posted!


Last edited by BMW2002Ti on Sun Oct 02, 2016 6:35 pm, edited 2 times in total.

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Post subject: Re: Archeology of the Alamo Futura with schematic
Posted: Sun Oct 02, 2016 6:31 pm
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That's a remarkably innovative design! The only real experience I have with any Alamo amps was when an 8th grade buddy's student model took a dump and he brought it over for me to look at (like I as a ninth grader was some kind of marconi, ha ha). It was a single-ended class A design with a 12AU7 (stage one and two) driving a 6AQ5 into a 6-inch alnico of unknown pedigree. Nothing at all like the complex beast you're working on.

Thanks for the tour, Tim.

Arjay

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Post subject: Re: Archeology of the Alamo Futura with schematic
Posted: Sun Oct 02, 2016 6:39 pm
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Retroverbial wrote:
That's a remarkably innovative design! The only real experience I have with any Alamo amps was when an 8th grade buddy's student model took a dump and he brought it over for me to look at (like I as a ninth grader was some kind of marconi, ha ha). It was a single-ended class A design with a 12AU7 (stage one and two) driving a 6AQ5 into a 6-inch alnico of unknown pedigree. Nothing at all like the complex beast you're working on.

Thanks for the tour, Tim.

Arjay


Yeah, I think the pentode side of that 6U8 has enough current gain to drive the tank, ala "Output Tranny-less" amp. Which prolly tends to give this section a brighter tone. The few OTL amps I've played on are clean and super crunchy, when overdriven. Like a small TR tone. :D


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Post subject: Re: Archeology of the Alamo Futura with schematic
Posted: Tue Oct 04, 2016 5:20 pm
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BTW... this OTL section driving the reverb tank limits the aftermarket replacements available. I have seen ppl put reverb tranny into OTL sections --- so they could use available stock.

Just a thought... Good luck, again with that unique amp!


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Post subject: Re: Archeology of the Alamo Futura with schematic
Posted: Thu Oct 06, 2016 7:52 pm
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Hey, I can log in again!

The Alamo has gone back up into the hills from where it came. I was only allowed to replace the burnt resistor and replace one knob. I was honored to be able to just explore it for posterity.

The OTL reverb design, I believe, was a cost cutting move. At 8 cathode volts and 175 ohms input, I would expect most tanks to take that. The low drive distortion makes it worth it to go OTL, I think.
The rest of the amp reflects that cost cutting philosophy as well. Almost all the caps are .02 ceramic disk. The chassis is minimalist and poorly shielded, but cheap to build.
Its the tremolo where they went all out.
The little trick to derive power for SS off the cathode is a useful trick as well. That could power an effects loop IC easily in an otherwise all-tube amp. I'm gonna try it.
I could dream about building a standalone balanced modulator with an intellegent LFO, but I'm busy makin a livin instead.


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Post subject: Re: Archeology of the Alamo Futura with schematic
Posted: Thu Oct 06, 2016 10:48 pm
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TimsAudio, OT... have you ever redone a stereo Gibson GA-79RVT? Will this work on non-stereo guitars? Or do you need an adapter to make pseudo-stereo?

Thanks!


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Post subject: Re: Archeology of the Alamo Futura with schematic
Posted: Fri Oct 07, 2016 9:35 am
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I worked on a lot of organs back when. The reverb on all of them was very poor. Then I heard a Hammond spinet and thought," Hey that reverb doesn't suck" Looking in, I saw a separate reverb amp and speaker.
You have that available on the Gibson.
It looks like the reverb drive is pre-volume control. So driving both channels with the CH1 volume off should give one dry channel and one reverb channel..
The reverb creates a second and third signal that can't be mixed back into the amp with any agility. The less a speaker has to do, the better it can do it. So by routing the reverb through its own system, both dry and wet signals sound better. There are no electronic cancellations or speaker buzz. The delays are mixed acoustically, which allows for more effect and greater transparency of the reverb.
Also, by using a second channel, it doesn't need a tone-choking dropping resistor in the signal path to balance the reverb signal.

The system looks low gain. A boost pedal into a Y connector into the hot input on each channel will avoid the high series resistance of the stereo jack.and pump it up properly.
Now turn off channel one volume and turn up the reverb to match Ch2.. Does that do it?.


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Post subject: Re: Archeology of the Alamo Futura with schematic
Posted: Mon Oct 10, 2016 6:03 pm
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Thanks! I think I'd need to have a stereo guitar, to have "real" stereo with that amp. And the $$$ for used ES-345 or such guitars... Yikes!

Does your amp have the tweed Fender Deluxe-like tone? :mrgreen:


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Post subject: Re: Archeology of the Alamo Futura with schematic
Posted: Wed Oct 12, 2016 9:02 am
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It does compare to a Tweed Deluxe in that both have clean tone controls and similar stage circuits.
The 16 ohm speaker will increase the dynamics, but the tremolo channel has much lower gain than the Fender.
The input stage is not as good as a Fender either. The input load resistors are much lower value than a Fender, reducing sensitivity. It will load down the pickups and affect the brightness of your guitar. The Input is capacitor coupled, which also reduces sensitivity, and the cheapest of disk capacitors are used
However, none of that affects my guitar. You see, it just hangs on the wall because I don't play guitar. So I can't really say how it compares with a Tweed acoustically.


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Post subject: Re: Archeology of the Alamo Futura with schematic
Posted: Wed Oct 12, 2016 7:43 pm
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Amp has 220k-ohm grid-to-ground resistors on inputs. That's putting a pretty good amount of signal voltage straight to ground. Most Fender amps use 1-meg G2G and no coupling cap with 9-pin input tubes.

Alamo is really protecting that input 12AX7. :lol:


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Post subject: Re: Archeology of the Alamo Futura with schematic
Posted: Sun Oct 16, 2016 8:55 am
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Peavey does the same thing and its the easiest mod to make a Classic 30 and other Peaveys sound better.
I'm conflicted as to its purpose. I've haven't been able to determine whether designers use the blocking cap to protect the tube from high input signals or is it because it protects the guitar from a DC failure in the tube.
Regardless of the reason, its a safety device that negatively affects tone, which negates its possible benefit, IMHO.

The low input impedance is possibly designed to match the output of their guitars.
It will reduce treble and brightness from the guitar. The resonant peak response @ 2-3K is the first to go. The dynamic gain is also reduced.
This may be intentional. Remember, these are country amps from the 60s and are designed around the C/W music of the period.
These guys knew what they were doing and what they wanted to acheive. When they struck up their guitars and amps, they knew exactly what they wanted and produced it.

The old school designers had a long history of tube innovation and a complete industry behind them.
The boutique builders of today are trying to rediscover what these guys knew like the back of their hand.
It's one of those things that I wrestle with when I see a new amp. What kind of music was the amp designed for and how did the builder express that in the circuitry?
A classic example is the Dumble clone tone of Robbin Ford. It is a special feedforward circuit that produces singing harmonics.
But the amp must be driven to a certain level to acheive that. And you have to know the circuit exists so you can play to its strengths.
59 Bassmans have a special tone setting that you have to know exists before you can access its best feature.
The old designers intent is difficult to guess, but I assume there are no mistakes. I wish I could ask each one of them, "What kind of guitar and music were you playing when you designed this amp?"
But they're all gone. So I'm just trying to answer these questions with the clues they left behind. It's a marvelous journey.


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Post subject: Re: Archeology of the Alamo Futura with schematic
Posted: Mon Oct 17, 2016 10:18 pm
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Seems that Alamo primarily used pickups with 5 to 6k-ohm range. So, nothing crazy. 1:10 ratio on input is all that is needed. P/U were non-active.

Voicing is another thing. The 0.02 mfd is there to cutoff lows. Looks like Alamo was searching for a very clean tone. Throughout the volume range. Without using local or even global feedback.

I noticed how the NORMAL input half of the 12AX7 (V4b) cathode is set directly to ground. No cathode-tied, cathode biasing resistor. That is prolly why the G2G resistance is so low, as well as the 0.02mfd cap on input. To set the idle well below overdrive point without traditional biasing.

Much more engineering went into the VIBRATO input. Looks like some type of cathode follower circuit. Maybe as impedance matching device for the tone stack. And even contributor to overdrive circuits beyond the input stage. (?)

http://www.valvewizard.co.uk/dccf.html


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Post subject: Re: Archeology of the Alamo Futura with schematic
Posted: Fri Oct 21, 2016 10:41 am
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BMW2002Ti wrote:
Thanks! I think I'd need to have a stereo guitar, to have "real" stereo with that amp. And the $$$ for used ES-345 or such guitars... Yikes!

What about ...
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Post subject: Re: Archeology of the Alamo Futura with schematic
Posted: Mon Oct 24, 2016 1:10 pm
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Thank you for your insight on the grounded cathodes.
I was wondering what the significance of that was.
I recently saw an old Gibson wired with grounded cathodes. Makes sense if you want to keep it clean.


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