Salutations! And a pic of my current SE amp project

[dividebytube]

Just joined today, but I'm no stranger to tube circuits. I've been working with tubes since the tender age of 19 when I bought my first Dynaco 70. Since that day, I've owned a number of units and got interested in fixing them. And eventually started building my own.

Anyway, I've been working on an 'old school' throwback project that is more of a 1940s approach. No current sources and no solid-state chicanery (except for the diodes in voltage doubler 12V filament supply). This is also a pentode (well, a beam-tetrode) amplifier with no ultralinear or triode connections - so something closer to a vintage radio output but with better transformers and a beefy power supply.

basic description: 12J7GT RC-coupled to a 1625 output tube. Power supply is a 5Z3 rectifier with an OC3 and OD3 in series to provide 255V regulated screen voltage.

I'm currently waiting (impatiently!) for the Edcor output iron to arrive.

[kheper]

Nice amp so far. Is it a mono? It reminds me of a PPP 807 RCA.

http://www.montagar.com/~patj/mi12188a.htm

[dividebytube]

nah, it's stereo - the gaps behind the 1625s will be the site for the Edcor CXSE transformers. I'm expecting ~10Ws.

[Alex Kitic]

nah, it's stereo - the gaps behind the 1625s will be the site for the Edcor CXSE transformers. I'm expecting ~10Ws.

I do not believe you are going to have 10W per channel output power with 1625s? But close to that, yes... if the anode dissipation is really 30W (much like a 6L6).

Anyway, take a look at the RH807 schematics, since that one can be built with the 1625 - which is very similar to the 807.

You could try to modify the schematics adding the Rfb from the RH807 schematics, as there are some who believe a pentode driver would do the job better than the triode (nah...).

Still, aside the nice chassis... we cannot see the schematics

[dividebytube]

at least in pentode mode, and with a high enough voltage, it's quite possible to get 10Ws out of a 807/1625. Of course minus transformer losses and at a rather high distortion... this is an old school design so I'll also be using loop feedback.

For a different incarnation of this same type of amplifier, check out my EL156 build. This one was connected in Ultralinear but same concept.
http://6streetbridge.blogspot.com/2008/ ... ifier.html

[dividebytube]

at least in pentode mode, and with a high enough voltage, it's quite possible to get 10Ws out of a 807/1625. Of course minus transformer losses and at a rather high distortion... this is an old school design so I'll also be using loop feedback.

For a different incarnation of this same type of amplifier, check out my EL156 build. This one was connected in Ultralinear but same concept.
http://6streetbridge.blogspot.com/2008/ ... ifier.html

[scott17]

Hi,

I'm new here also. Nice amp, interesting choice of tubes.

I checked out your blog site. I'm interested in finding out more about the use of a zener on the screen like you used on your EL156 build. Thanks for the info.

Scott

[kheper]

I'm interested in finding out more about the use of a zener on the screen like you used on your EL156 build. Thanks for the info.

Scott

A blurb from "The Classic Williamson 1993 Style" by Bill Kleronomos Sound Practices #4:

"An unusual method of applying active screen voltages to the tubes is used. Instead of the usual dropping resistors, I used a set of 5 Watt Zener Diodes for special benefits : screen voltage is always held at or below manufacturer's recommended value regardless of screen current. Distortion products are materially lower because of the stiffer screen supply provided by Zener regulation - this is especially true for the EL-34 pentode with its wider ranging screen current than the usual beam power tubes. And less screen voltage requires less bias voltage and therefore less peak signal voltage from the driver at max. output. Bench tests with EL-34s and 6550s in the output sockets confirmed an approx. 20% reduction in THD when the original dropping resistors were replaced by Zeners."

The above quote may have triggered a zener frenzy; "20% reduction in THD" seems a bit optimistic, but a zener on the screen is "stiffer" than a resistor - regulation wise.

More about screens and UL mode:
http://www.oestex.com/tubes/oes.html

[dividebytube]

yes, the Bill Kleronomos article was the original idea, and also a method to bump down the screen voltage.

Note that I did have one 5W zener fail on the EL156 amplifier. In this case, it shorts (versus opens). It drove me crazy trying to figure out why one monoblock was drawing slightly more current than the other.

The second version of this amplifier - using 6550s - had the zener gone and a reduced plate voltage.

[scott17]

Thanks for the information and the link.

Yes, 20% does sound overly optimistic.

Judging by version 2 of your amp, I'm guessing the 6550s did not benefit as much from using the zener screen technique?

[dividebytube]

Thanks for the information and the link.

Yes, 20% does sound overly optimistic.

Judging by version 2 of your amp, I'm guessing the 6550s did not benefit as much from using the zener screen technique?

well, I sold the amplifier to a friend of mine. He had a huge stash of 6550s. We opted not to include the zener since it didn't seem to make a difference.

[dividebytube]

Last night I finished the amplifier. Best part: It worked from the get go. Worst part: I had a gassy RCA 1625 that went into runaway, drawing more and more current as the amp was left on. Luckily I had some more 1625s in my stash and now have a pair of 1944 Kenrads running in there.





I still have to finalize the schematic - will post later.

[DeathRex]

Those CXSEs are monsterous.

[dividebytube]

Those CXSEs are monsterous.

yeah, I thought so too!! I wasn't expecting something so massive - I would guesstimate the size of the Edcors to be the same as my old Dynaco Mark III output transformers - maybe a tad larger.

[scott17]

Nice! Looking forward to seeing the schematic.