Wanting to build a GU-50 SE amp

[DeathRex]

I came up with this.

[kheper]

I like your design better than the others, especially the regulation for the screens. Looks like you have abandoned 500V B+.

I wonder what the Po really is for 370V.

Regarding transformers, there are plenty of edcors in the range of 2.5-5k @ 8ohms.

http://www.edcorusa.com/products/534-gxse15-8-2_5k.aspx

http://www.edcorusa.com/products/469-cxse25-8-2_5k.aspx

http://www.edcorusa.com/products/471-cxse25-8-3_2k.aspx

http://www.edcorusa.com/products/470-cxse25-8-3k.aspx

etc.

Below is a Tomiko toroid. Good for 50W of SE power. No screen taps but 4 and 8 ohm secondaries. I have a variant of this model, used for a while with the 813 until the ~940V B+ scared me.

http://www.ebay.com/itm/Pair-2-300B-SE- ... 0568541828

[DeathRex]

Yep, maybe next time I can move up to 500V+. I read somewhere that lower B+ with higher current for the GU-50 means lower Ri. I have a bit of time before I have enough money to buy transformers. I'm still looking at the Tomikos and Edcor's GXSE. Tomiko's bigger, 25 watt transformer ($60) looks to be as big the the GXSE ($37). I wish Edcor would improve the GXSE to 35Hz to 18KHz, put in some extra taps and charge $55. Going from a GXSE to the CXSE is more than double for a little bit of bass that I'm probably never gonna use.

[EWBrown]

Another way to get the (approx) 250V for the screens is to "stack" a VR-150 and a VR-105, this would give 255V +/- the normal tolerance errors.
That gets those nasty transistors out of the circuit
Of course SS rectiffiers, LEDs and even Zeners are OK, Two legs good, Three legs bad (reminds me of "Animal Farm")...

Generally , a lower B+ and a commensurately higher plate current results in a lower Ri and of course, OPT primary impedance. There may be a trade-off in somewhat reduced maximum output power but that isn't all bad

/ed B

[DeathRex]

I updated the picture a bit, because it's an integrated and I forgot the volume control. It also has a bass and treble adjustment, but both are increase only. As such, with the controls fully CCW, there are no additional capacitors in the circuit, just 4 extra resistors. don't know if that 6SN7 will have enough gain. The bass and treble circuits came from Angela's fun with tone controls, but have less 'action'. Instead of 15db bass and treble gain, they are more like 7.

EWBrown; I do have a 0D3, 0B3, 0A2, but I don't think I'll have enough room. I was thinking of making this one very compact (because the others have been so big). This one was also going to be all octal, except for the GU-50. I heard that tightly regulated screens to be a good thing. I still have to get the zeners. On one schematic it showed a 10W zener. Might be hard to find that. Worst thing about the transistor is mounting the three legs to something other than a circuit board.

So far I have the power transformer (an old but pretty TV transformer that has 9.5A of 6.3V filament that I'll only be using .9A ), the GU-50s, 6N9S, 6N8S, choke, 12V filament transformer, sockets, power cord, RCA connectors, and lots of capacitors and resistors from other builds.

I need output transformers, speaker connectors (I have a post saved from Tom with the ones he likes), and wood for the frame. We have a True Value and Ace hardware here and neither have any hardwood except oak. I've gotten tired of the oak look. Like to get some maple, walnut, cherry or some other hardwood.

[EWBrown]

If the transistor is the TO-220 package , then it's easy to splay the two outer "legs" angled outward and then use a terminal strip to mount it and its associated components, and don't forget to put a heatsink on the transistor or it may release all of its magic blue smoke

I always liked the funky glowy look of the gaseous regulator tubes, whether octal or 7 pin miniatures. Some are purple, some glow orange.

Or just copy the 6GF7 voltage regulator out of the 50Watts amplifier in the schematics sectin of the RCA tube manual, and just recalc the resistor values to output 250V instead of 400V. A 6EM7 or 6FD7 would also work in this circuit. That would be a very "tight" regulator, and shouldn't take up too much chassis space.

There are a lot of ways to "skin this cat"

/ed B

[DeathRex]

I went to our only local cabinet maker and got some hard maple; 2 1/2 X 3/4 X 7 feet for $5. OK now I need to see what size my aluminum plate is going to be to see if a 7th tube will fit. I can put it beside the 5AR4 to move the GU-50s closer to the output transformers. With 8 tubes the GU-50s will be in the middle.

[kheper]

In the old tetrode and pentode manuals, the schematics have the screens supplied from a separate transformer winding and filtered with CLCR network. Another way to skin the cat without transistors or diode tubes.

[DeathRex]

Here's the finished schematic. I built the amp and had to tweak the schematic a bit. Pentode mode sounds awful, like the midrange, around 800-1KHz is very pronounced, I guess because I have no feedback. UL and triode mode sound good and very similar. I can switch from triode to UL with the unit on and running. With my B+ at 340 I'm getting about 9W in UL and 6W in triode at 1KHz. With music going, the most I read is 5W in UL on my Fluke 87.

I had to replace the 6SN7 with a 6SL7 to get enough through the tone controls. With 1VPP input I'm getting 50VPP on the anode of the 6SL7, and 2.5VPP at the grid of the SRPP 6SL7, and about 70VPP to the GU-50.

Both of the tone controls do only a little. With the controls all the way up they little more than double the bass and treble. And with the controls all the way down, they do nothing to the signal.

If I have UL taps I'm never going to try pentode again.

When I make a bottom cover I'll post a picture.

[DeathRex]

Here's the pics. I ended up replacing the GZ34 with 1N4007s and a 5C4S for slow turn on. I get 355 B+ and now using all Russian tubes. The GZ34 gave me a bit more B+ around 363. I also put DC on the 6N9S filaments, too much hum with AC.

I can switch from triode to UL and pentode without turning it off, but in pentode with no feedback it sounds kinda like a bad SS amp. Also had to add a 22K to the screen grid in UL mode, was getting 344VDC on the screen witch is rated for 250, but they never did glow. In triode mode the tubes are running at 80 ma, in UL 67ma, in pentode 57ma. Need to add another cathode resistor in parallel in pentode mode to get the current up to 80ma. Already have the screen up to 290 in pentode.

[kheper]

Nice amp. To tame the pentode mode nastiness, you could put in a switchable feedback loop.

[TomMcNally]

Very nice looking amp indeed.

[DeathRex]

Nice amp. To tame the pentode mode nastiness, you could put in a switchable feedback loop.

Ah man did that fix it, it sounds great now. Before, in pentode mode, it sounded like the midrange speaker was in a glass jar.

Well the switch was kind far away so I just added very little feedback. Also added a switch for a bit more current in pentode mode. Now the current is about the same for pentode and UL.

Updated the schematic.

[LinuxManiac]

SE Gu-50 in triode mode (g2 & g3 coupled to anode), fixed bias (about -52 V), Ea 350 V, Ia 100 mA, Ra 3.5 kOms (DIY transformers 18 sq. cm core)

EF80 driver
Ea 350 V, Ua 225 V, Ug2 250 V, Ia 8.5 mA, Ig2 2.5 mA, Ug1 -3.3 V, Ra 15 kOms, Rg2 47 kOm, Rcath 300 Oms

Russian 5Tz3S (5Ц3С) x 2 anode rectifier
Russian 6H6S (6Х6С) bias rectifier
Russian SG-3S (СГ-3С) bias stabilizer