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I built this Single 45 amp and it sounds great. I was measuring the plate voltage and it seems unusually low. I'm measuring 128v on the 6SN7 plate and 280V on the 45 plate. Is this right? Is there a way to optmize this circuit for 2A3 instead of 45 ? Here is a link to the schematic.
http://gallery.AudioAsylum.com/cgi/gi.mpl?u=33876&f=se_glorya.jpg
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Congratulations, that's a very handsome amp. The $100 Glory was my first project and I still like It. There are quite a few alternate drivers for 45s out there. JE Labs has a couple and there's a pentode driver at the link below that I've used in another amp and like. For sentimental reasons, I haven't changed my amp, but I'm curious about the other driver possibilities for other 45 projects.
Doug
Sorry about having to repost - I'm having "one of those mornings." Going by the photo, there are two drivers. Is that accurate? How did you hook them up?
I'm using 1/2 of each driver tube. I used opposite pins on left and right channel so I can swap left with right after a couple of years.
I have 2 options for the 2.5v supply.
First option: A 5vct 3A which only allows me to use 45 tube.
Second option : 2- 2.5v 3a each but no center tap. This allows me to try 2A3 tubes. The problem here is that I don't know how to reference to ground without the center tap. Can anyone help with this ?
At Joseph Esmilla's excellent site - je labs - he shows a 75 ohm 2 watt pot across the filaments of his Simple 45/2A3. The ground is from the wiper. I believe you can do the same with a pair of low value, high watt resistors, again with the cathode resistor and cap taken from the junction between the resistors to ground. Maybe someone else can confirm this - or not. The RCA Tube Application section of their tube characteristics books show the same thing, but without values.
Thanks, you steered me in the right direction. I actually used a couple 100 ohm half watt resistors with a 1.5K 10watt paralleled with 100uf 100v cap to ground. This worked like a charm. I found this method searching google.
After a couple of mods this amp sounds so good now (with 45s )it brought tears to my eyes. It now has so much volume with my horns that I have no interest in using the 2A3. This is good because I stuck a 2A3 in it and the plate voltages dropped much lower ,too low. It still sounded good though, It probably would get very hot. Thanks again for your help.
Les Hudson
Hey LesGreat looking amp. Puts my bud box design to shame. I've got something very similar, but it uses a 6SL7 in SRPP as the driver as my preamp is low gain. I use no electros anywhere (PIOs in the power supply and films as bypasses/coupling caps). AC really does sound better than DC (if done properly there is no hum).
Low parts count, low power designs like this offer incredible clarity, however, they are also easy to screw up. Cheap electros as cathode bypasses will instantly destroy the sound (my my 2 cents).
Just as the 2A3 is a major step up from the 300B, the 45 is a step up from the 2A3. I don't think it gets better than the 45, but I do think the 10/10Y is in the same league.
Regards. Doug
Gorgeous amp! Ditto the questions about the case construction details. I'm curious if you have experienced any hum problems - it looks from the photo like the power and output transformers are all oriented the same way; usually they're rotated 90 degrees relative to each other to minimize hum (though they appear to have a fair distance between them in your case).
No hum! Sounds great!
The spec sheet shows a 440 v CT transformer rated at 50 mA, or an Antique Elec Supply PT 142 which is 325-0-325 at 50mA. Those seem like pipsqueaks for a "power" amp, and going to 2A3s makes matters worse.As mentioned below, 2A3s draw 2.5V at 2.5 amps, vs 2.5 v at 1.5 amps. Do you have adequate power to the filaments for a 2A3?
6SN7 can be a good tube at 128v. 32k ohms is a relatively high resistance load on 6SN7, but your gain might be only 15 - 17 at a guess, which is not enough to fully drive 45 unless your source is giving you more than 3 volts peak in. With 280v B+ on the 45, I bet it would need 45 v peak to drive it to full power, and I believe a driver stage ought to have more headroom (Lynn Olson suggests 50% extra, he also suggests some ability to deliver current to the grid, which makes a switch to say a 6SL7 problematic).
You could try a current source or choke for the driver stage. I would suggest the fabulously linear and undiscovered 6C8G (octal, grid cap would look good on your beautiful amp) with the little Hammond 90H chokes, running at the same 5 mA you have now. Your gain would be more like 32. May need to fiddle with the cathode resistor.
I would also look for a honkin' power trnasformer, if yours is currenlty only a 50ma one.
I'm using a 450vct 65ma. I do have adequate power for the filiments to run 2A3. I have 2- 2.5v taps that are 3amps ea.
I'm using a preamp with a 6H6P tube with lots of gain.
I did not say it before, but your amp is beautiful. I love the glass idea in particular.For a 225-0-225 transformer, full wave CT into 5V4G, in a CLC arrangement with 10uF as C1, 280v is what you would get (I would have guessed lower). With that transformer, you cannot optimize the circuit for 2A3 because a 65 mA transformer is only enough for one 2A3 at normal operating points (IMHO).
In my view, you could drop 2A3s in and see what the sound like, with no risk. They will only be running at half power (given the 750 ohm shared cathode resistor and ~235 plate to cathode voltage).
I would add that I agree with the concept in DIY amps of using the gain from the preamp to help solve driver voltage issues. Commercial stuff may need to get to full power from a 1v peak source, but my stuff can have 3v in if I say so.
One other thing re the schematic. I would include a bleeder resistor to discharge the power supply caps.
. . . so maybe you have already beefed that up vs. the schematic.If you are thinking 2A3 for more oomph out of your amp, I suggest first trying to eek more power out of your 45s, through more voltage gain (and perhaps more drive current).
If 280v is all you have at the 45 plate, perhaps you could apply some negatve bias to the 45 grid, with less resistance in the cathode, so that through combination bias you would have a full 250v from the 45 plate to cathode.
You are right about the beefy tranny in the pic. (good eye for detail). This chassis I had origionally built for a PP 6L6 amp.I realized that I didn't need so much power and changed it to a 45 amp. I already had the tranny mounted there and it looked so good that I left it and only used the 6.3v and 5v taps. The actual hi voltage tranny is down below.
and would make it appear that you don't really need a lot more power.
Yes my 45 will reach live music level and the 2A3 will reach concert level. More power would be rediculious.
Nice looking amp, great job on putting it all together.Those voltages are fine, though looking at the schematic I would expect it to be a bit higher if you built exactly to that schematic. Have you checked the plate current through each tube? I expect your 6SN7 should be 10ma or less, and the 45 should be around 35ma. If you are in the ball park then you are okay. You may want to look up data sheets for both tubes to see where you are in the plate curves.
At that B+ voltage and cathode resistor you can drop a 2A3 in, but always come up slowly with a variac if you are not comfortable and keep an eye on plate voltage and current. Since the filament requirement is different and the 2A3 pulls more juice you need to think about whether the winding or transformer you are using has the current headroom. “Optimize”? Well that’s were the fun is. Since the 2A3 plate resistance is lower the 45 a lower impedance output transformer could be used. But tweaking a driver circuit to your ear and speakers is what this is all about. I don’t think optimize translates between people, but experiment and listening does. And there are plenty of alternative driver schematics on the net for ideas.
Matt
That is a great looking amp. Congrats on a job well done. Did you do the case work yourself, it looks superb, what is made of?Also, can you tell me where you got the analog dial. I've been looking for something similar and have come up empty-handed?
The chassis is a Hammond. I made side handles out of 1.5" thick mdf and painted it all with Marantz Hammertone paint.I made slots to insert glass panels on the top and front. The meter is an automotive 12v from Auto Meter.
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