Question about Alamo Montclair

[AlNewman]

I was looking for some new build ideas and came across this amp.

I'm curious as to how V2 works?  It doesn't seem to have any high voltage on either triode, it doesn't look like it's conducting at all?
What's the point?


https://el34world.com/charts/Schematics/files/Alamo/Alamo_montclair.pdf

[Latole]

I was looking for some new build ideas and came across this amp.

I'm curious as to how V2 works?  It doesn't seem to have any high voltage on either triode, it doesn't look like it's conducting at all?
What's the point?


https://el34world.com/charts/Schematics/files/Alamo/Alamo_montclair.pdf

IMO resistor from B+ is missing.

A sketch from Tim Galand from another schematic........

[AlNewman]

IMO resistor from B+ is missing.

A sketch from Tim Galand from another schematic........

I don't see a link...?

[Latole]

IMO resistor from B+ is missing.

A sketch from Tim Galand from another schematic........

I don't see a link...?

There are no link, I read that on you schematic;

[Willabe]

Gerald Webber used to tell a story that a friend of his was in the studio when ZZ Top was recording Tres Hombres.
 
His friend said that Billy used an old Alamo tube combo amp that was similar to a tweed Deluxe/2x6V6, with a weak power supply (lots of sag) for most of that album.   

[moosetone]

His friend said that Billy used an old Alamo tube combo amp that was similar to a tweed Deluxe/2x6V6, with a weak power supply (lots of sag) for most of that album.   

I've got an Alamo Titan that sortofkindof resembles that... 

[Carlsoti]

This might not apply to the Alamo, but I think I saw a similar drawing recently. IIRC they were talking about a CCS, which I thought to mean constant current source, but I still don't understand how it works or what it does, exactly.

[AlNewman]

It is strange, for sure... 

Maybe there could be a missed connection to B+, but doesn't seem likely.  There are voltages measured at all connections.  I just don't see how it can work as drawn, but it'd be interesting to find out.

[AlNewman]

I've been looking at this, and maybe have some ideas?

I think the schematic is drawn correctly, there are 3 different drawings of essentially the same circuit, 2 of the Montclair, and one of the Electra, all have measured voltages of different values with the same basic circuit, and somewhat varying values of capacitors and resistors.

It seems to me like it's some sort of positive feedback loop, mixing wet/dry signals through the second gain stage.  The signal splits after the 1st cathode stage, then goes through 2 capacitors, and then the plate of the 2nd gain stage, changing the phase by 300 degrees.  Then with the addition of the 3 resistors in the feedback loop, it could return to the 1st cathode stage (somewhat) in phase.

So perhaps it would act like a tubescreamer type circuit which preserves some of the dry signal before mixing in the overdrive.  It could also maybe create a swirling effect, or perhaps a slight delay, like a poor man's reverb?  Maybe it would make the response more touch sensitive?  I dunno...  it must be that way for a reason. 

It would be interesting to try I guess.  Perhaps there's also opportunity to put a pot in to replace one of the resistors (the 220k mixing resistor?) to control the amount of phase shift and wet signal?

Another strange anomaly with this amp, is there isn't a cathode resistor at the 1st gain stage of the non trem channel.  Again, it's the same in all drawings, and looking at the 12ax7 datasheet, it should work, but is nearly biased to saturation, about .25 V at the grid.

Strange amp, any more input is definitely welcome.

[sluckey]

Have you noticed that V2 triodes are connected to operate like a diode, ie, plate and grid tied together?

Another strange anomaly with this amp, is there isn't a cathode resistor at the 1st gain stage of the non trem channel.  Again, it's the same in all drawings, and looking at the 12ax7 datasheet, it should work, but is nearly biased to saturation, about .25 V at the grid.

It's called grid leak bias. The 220K grid resistor is usually something more like 5.6M or even higher. Never seen a grid leak bias with 220K grid leak. This Magnatone 10A is an example.

[AlNewman]

Have you noticed that V2 triodes are connected to operate like a diode, ie, plate and grid tied together?


It's called grid leak bias. The 220K grid resistor is usually something more like 5.6M or even higher. Never seen a grid leak bias with 220K grid leak. This Magnatone 10A is an example.

I noticed the plate and grid tied together, but didn't really think much of it.  Does it clip the signal or something?  Give the tube bias? 

I've seen the grid leak bias quite a bit in radios, but yeah normally with a much higher resistor.

[Willabe]

Have you noticed that V2 triodes are connected to operate like a diode, ie, plate and grid tied together?

What is going on with V2B and V2A? V2B's K is tied to V2A's Plate and both have their plates tied to their grids. And no B+dcv on either V2A/B.

[passaloutre]

Not sure if it’s relevant but that V2 also has a resistor in series with its heaters.

[Williamblake]

V2 is getting B+ depending on the oscillation circuit.

[sluckey]

V2 is getting B+ depending on the oscillation circuit.

I ain't seeing that. Please explain and be specific.

[AlNewman]

So I built this circuit to try it out on a "breadboard" (an organ chassis that I set up a couple years ago), and it is a pretty cool pre amp.  I did not build the tremolo circuit, and I ran it directly from the volume control to my LTPI in a push/pull EL84 amp. 

I also didn't notice passaloutre's comment about the dropping resistor on the heaters of V2 until it was built, so the heaters were all at 6.3V.  The way I have the tubes wired, I won't be able to drop the heaters in the same fashion, I do think they must have done it for a reason, and it must have some effect...?

B+ was 327V @ V3A, 246V @ V1B.

It's interesting, It breaks up really early, and gets really hairy, but it isn't a bad distortion, in fact for pre amp distortion it sounds really good.  I think the extra gain stage into the LTPI would help, and an actual tonestack could make a big difference. 

It is interesting because the top of the signal is clipped in different amounts at different parts in the circuit, but the signal at the end of the volume looks like a clean sine wave...  And it is basically at the same voltage going into the LTPI as it is leaving the 1st gain stage...

Anyhow, all the voltages measured in the schematic are reasonably close.  It is a cool preamp, and sounds good as is, but I do think there is some room for experimentation to tweak it to taste, and give it more versatility. 

[passaloutre]

I think the tremolo is turning the diodes on and off to alternately block or pass the signal coming out of V3A. Sort of a weird way to apply the modulation to the signal.

The tremolo modulation from V1A goes through the concertina of V1B and so comes out of the anode and cathode in alternate phases.

When the tremolo signal goes high, V1B anode goes low and reverse biases the diode at V2B. At the same time, V1B cathode goes high, reverse biasing the diode at V2A. The diodes don't conduct and no signal can pass from V3A to the volume pot.

When the tremolo signal goes low, V1B anode goes high and V1A cathode goes low. The diodes conduct, V1B passes the positive half of the signal and V1A passes the negative half of the signal from V3A to the volume pot.

Or something like that...

No idea what the resistor on the heater does, maybe that makes V2 a better diode, maybe noise reduction, etc??? In any case it’s awfully strange to use two full 12ax7s for a tremolo circuit; why not just wiggle the bias on the power tubes or a preamp cathode?

AlNewman, I wonder if your diodes are only half-conducting because you don't have the modulation to wobble them. What happens if you just bypass the diodes (V3A anode > coupling cap > volume pot)? Maybe without the modulation, your input signal could be pushing the diodes in and out of conducting, which I have to imagine could make some wacky clipping...I don't know, this is kind of above my head at this point.

Please let me know if I have this all wrong, I’m still learning.

[AlNewman]

I think if I bypassed the followers, it would just create a regular gain stage. 

It seems like just a regular princeton type oscillator circuit, although even with the oscillator grounded, the intensity would have an effect on the signal, as it would ground V1B grid...  It does seem like the oscillator would have an effect on the diodes when engaged...  Maybe the only way to truly test it is by building the oscillator circuit.

I do think that V1b is it's own animal without the trem though, seems to work with the diodes to create a unity gain where the diodes clip the signal, but V1B inverts part of the signal then remixes to create the full sine wave again.  Sorta kinda like a compressor.  Like I said, it isn't a bad sound, it's just kind of lifeless for the amount of gain on tap.  Again though, maybe the trem is another part of the puzzle.

But hey, I'm learning too, if I had all the answers I wouldn't have had to build it to see if I can find out.