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Messages - blackcorvo

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106
Tubes and Hybrids / Re: Small Push-Pull Amplifier
« on: November 23, 2012, 10:50:55 AM »
Thanks for the responses!

1) the TDA2005 requires a 16V 2A (unregulated) supply to provide 15/16W RMS (I use TDA2005 all the time in my "Callejero" 12V battery powered amps)  ;) )
I don't think your rectified filaments can provide that, you'll need another transformer.

Well that certainly has some unique features.  Your bridge/doubler switching is interesting and I'll be having a closer look at that.

Your valve heater arrangement has me worried though.  Thorated cathodes have to be operated within +/-5% of their rated voltage because their emission drops off rapidly either side of their design temperature.  In this case you will be operating them at 3 volts rather than their rated 4.7 volts.

I'd suggest you also have a look at a couple of possible failure modes, a) when one of your double triode heaters fails, and b) when one of them is removed from its socket (or has poor contact later in life).  I'd suggest that you use the triodes in 12 volt mode, and that you use a series resistor for the other two in series.

I won't be using that PT for the final project. I'll order one with a 15v secondary for the filaments and TDA2005. Those 15v AC will feed a bridge rectifier and then split into 3 paths:

The first one goes directly to the TDA.

The second goes to a 12v regulator.

The third goes to a 5v regulator (followed by a series diode to drop 0.7v and keep the filaments at 4.3v).

2) the signal at the PI plates can approach 100V RMS, so way beyond overdriving, it will *destroy* your TDA2005.
Plus its impedance is too high.
3) you can solve that with a couple attenuators, and add a switch which sends plate signals to the attenuators and away from the power tubes grids.

I will have a couple 10k resistors in series to ground with the 5AQ5s 220k resistors. From that point I'll take my output for the SS power amp.

I repeat, yours is a very original idea and may work, with some tweaking. :tu:

Thanks!


I'm curious as to why you have followed the split-load PI with a buffer stage.  I have only ever seen this done where the PI would have insufficient voltage swing (e.g driving KT88's) or where the output pair were to be driven into AB2 or B2, neither of which seems to apply here.  Where an extra double triode is included it is normally before the PI as extra gain for crunch, as a low impedance driver for the tonestack (which I note you don't have at all - very unusual in a guitar amp) and/or as following makeup gain.

I'm sorry, but those are gain stages.
I made it based on the power section of the Gibson Atlas Medalist amplifier. I'm a big fan of a guy called Phil X, and he uses one of those for live performances sometimes, so I decided to try out that idea and I liked it.
Also, split-load P.I.s sound HORRIBLE when overdriven by the preamp. So I prefer to have the second gain stage after that.

I didn't add a tonestack because I've tried adding those before, but couldn't find one that I liked.

With switching the speaker output it isn't the bottles that are at risk, it's the output transformer, and 100 ohms on (I assume) a nominal 8 ohm output may not give you much protection.

You should look at AX84's circuits then. They always have that resistor on their circuits. I *could* use an 8R2 resistor, but I don't really trust the switches they sell here, so I prefer to keep a load permanently connected to the OT, but that doesn't "steal" much of the signal from the speaker.

I don't know what the SOP is where you are but in 240/250 volt mains countries such as Australia and the UK it is normal practice to fit both the mains switch and fuse in the active leg of the mains, and there is also a general move to double-pole mains switching.  If you are troubled by switch-off pops you can fit an RC snubber across your switch contact(s).

HTH

Here we have both 125v and 230v mains, but in my house all wall sockets only provide 125v. We use the 230v line for the shower heaters.
I have the on/off switch and fuse like that as a "failsafe" system against stupid people and "practical technicians" (you know, those guys "who learned by doing it").
Examples:
- If I ever sell this amp, the person I sell it to might someday want to change a blown fuse while barefoot right after getting out of the pool. They won't get shocked by the neutral wire.

- If the power switch needs to be changed, the "tech" won't have any headaches trying to find the proper switch and figuring out how to wire it.
DPDT switches aren't very common down here, and "techs" don't like to waste too much time trying to figure out how something works so they can fix it.

107
Tubes and Hybrids / Small Push-Pull Amplifier
« on: November 23, 2012, 12:37:49 AM »
So, I started working on a small project to put in use some tubes I had laying around.

Here's a little clip I recorded a couple days ago:
http://www.youtube.com/watch?v=vBoPgQ0By90

And here's the schematic:



The 5AQ5 is simply a 6AQ5 with a 4.7v/600mA filament. All other specs (plate dissipation, max voltages and curves) are the same.
The switch maked with "150v" and "280v" simply changes the B+, so I can get crunchy tones at lower levels (as I've shown in the video).

I am also planning on adding a second power section using a bridged TDA2005 (that would be powered by rectifying the AC from the filament secondary), for louder clean tones.
After checking the Datasheet for it, I came up with the following circuit (STILL UNTESTED) :


I would connect each input to both sides that feed the 5AQ5s grids.

The speaker would be connected to a DPDT switch, to change it from being connected to the tube PA or the SS PA. I would also have a 100R resistor connected to the OT's secondary, so that I didn't end up blowing my power tubes when switching to SS mode.

What do you guys think? I'm sorry if it sounds confusing... but I sometimes overthink a little when coming up with ideas like this one.

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