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Alamo amp

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11/18/1999 3:34 AM

I recently acquired this Alamo Electronics Paragon 7+75 Super Bass amp head. Basically a two 12ax7 to one 7199 to two 6L6GC amp. I picked it up cheap and thought I would check it out to see what makes it tick In the process I came up with a few questions that seemd like this was the place to get the answers.

This is the first amp I have actually put my hands on that uses the 7199 tube. I only know the most remedial data on this tube (combo pentode/triode.) What would motivate a design using this tube instead of a typical 12ax7 style tube?

The output transformer is unlike what I have seen (not that I have seen a whole lot, mind you.) There seem to be quite a few lines on the primary side (about 5 if I remember correctly) and it is huge! Larger than the power transformer. Any hints what this thing is all about?

All in all, everything seems original, sort of. The main speaker leads go to a jack on the _side_ of the head. Maybe it was moved from somewhere else. Maybe this was a combo made into a head. Any idea where I might dig up a schematic on this thing? I tried tracing things out, but it gets real confusing around the transformer leads. I was originally thinking I might futze with it some to make it more Fenderesque, but I would prefer to understand more of what I have (the explorer in me I suppose.) At the very least I wouldn't mind brightening it up some and changing it from a s.s. rectifier to a tube.

Thanks for any info or help,
Joe

11/18/1999 9:52 PM

I'll try to help you with the first part of your question. The 7199 was popular for a time because the triode part was employed as a split-load phase inverter (which had a gain slightly less than unity), and the pentode part was used as a high gain voltage amplifier just ahead of it. So this one tube and a pair of 6L6s made up the entire power amp portion of the circuit. It's just another way to do it, and typical of amps from the '60s, both guitar and home audio. Other circuit designers chose to use a matched pair of triodes in a Schmitt/differential/long tail type PI, which had a higher voltage swing capability.

I don't have a schematic for that Alamo, but you could take a look at the Ampeg B25 schematic over on Joe Piazza's site. That shows a similar circuit with the 7199 and a p-p pair of 7027s (big 6L6s). It should at least help you in reverse engineering your circuit to make up a diagram.

11/20/1999 5:55 PM

Thanks for the 7199 info. This is probably a dumb question so please excuse my inexperience. Looking at some tech specs, would it be possible to use the 7199 as a low power output tube?

About this transformer. The additional primary leads connect to pin 4 of the 6L6 tubes. Looking at some old schematics I found some old Magnatone amps and one or two Marshalls, if I remember right, doing the same thing. What is this about and can the transformer still be used in the typical Fender wiring?

Thanks,
Joe

11/20/1999 10:53 PM

Joe, the extra taps on the primary of your OT are ultralinear taps. This is a special OT that is designed to reduce distortion and provide a cleaner signal. Basically you feed the B+ in the center tap, and the plates hook up to the start and finish windings. The screens of the output valves hook up to the UL taps, which are usually wound 40% in from either end of the winding. There was (might still be) a massive thread involving this idea here on AMPAGE... Look for it first, then call Ken Gilbert. He's the resident UL guru (by default.) Many 70's Fender amps used UL outputs, so did some Marshalls (Bass amps mostly I can't remember...) and possibly Magnatones, too. Since it is a bass amp, it's not a bad idea - you can get cleaner output at low frequencies.

As for the 7199, it's mostly used as a cathodyne phase inverter and a voltage amp. for same.

I'd plug the thing into a bass cab and listen to it before I went monkeying around with it. Our bass player uses a stock '73 Marshall Super Bass 100W and it sounds AMAZING.

Farrow
http://surf.to/pharaohamps
http://www.skybolt6.com

11/21/1999 3:12 AM
Joseph Futral	Thanks for the info. I did plug it up to a 4x12 cab and it sounded ok. Mostly it lacks a bit on the top end. and I was also interested in swapping to a tube rectifier. Joe

11/21/1999 4:20 PM

A few things to try - cut the UL tap wires off at the tube sockets and feed the screens from the output tranny CT with a couple of 1K 5W screen resistors (or whatever's appropriate to the screen voltage) and that'l gnarly up the sound a bit. Try biasing the 6L6s to aboout 38ma a tube - that helps also (but watch the plates and make sure they don't glow red!) Also the tone controls can probably be re-voiced closer to your liking.

Farrow
http://surf.to/pharaohamps
http://www.skybolt6.com

11/22/1999 3:02 PM
Doc	I just ran across a copy of an RCA applications notes AN 183 article for the "new" 7199 tube. There's a suggested circuit from the tube manual, also. It's on Triode Electronics website. http://triodeel.com/7199.htm

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