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Author Topic: Small Push-Pull Amplifier  (Read 13717 times)

blackcorvo

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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.

J M Fahey

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Re: Small Push-Pull Amplifier
« Reply #1 on: November 23, 2012, 05:59:08 AM »
As a general idea, it's very interesting, specially that one of driving a bridged SS amp from a tube inverter  :tu:
Now, in practice, you'll have unmatched voltages problems.
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.
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 repeat, yours is a very original idea and may work, with some tweaking. :tu:

Roly

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Re: Small Push-Pull Amplifier
« Reply #2 on: November 23, 2012, 08:23:19 AM »
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'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.

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.

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
If you say theory and practice don't agree you haven't applied enough theory.

blackcorvo

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Re: Small Push-Pull Amplifier
« Reply #3 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.

J M Fahey

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Re: Small Push-Pull Amplifier
« Reply #4 on: November 23, 2012, 11:34:02 AM »
Oi amigo, eu vi que você é Brasileiro.
De onde?
Ou só fala o Time e já basta ;)

blackcorvo

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Re: Small Push-Pull Amplifier
« Reply #5 on: November 23, 2012, 01:05:21 PM »
Oi amigo, eu vi que você é Brasileiro.
De onde?
Ou só fala o Time e já basta ;)

Sou de Ribeirão Preto, SP. E não torço pra time nenhum!
Só recomendo você mandar uma PM da próxima vez, pra não poluir o tópico.  :tu:

Roly

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Re: Small Push-Pull Amplifier
« Reply #6 on: November 24, 2012, 07:52:13 AM »
Okay, that covers most of the points, however a couple remain, plus a new one.

I certainly think that having a resistor hard wired across the OPT secondary is a wise protective measure, we suggest that in the "Lamington" series, however my concern is with the value.  While 100 ohms is pretty typical I have seen reports lately of people suffering OPT failure when using a brand of dummy load that presents a 30 ohm load to the 8 ohm output.  From this experience I'd now be inclined to go for no more than a 47 ohm in this function.

Amplifier mains switching and fusing is a bit of a variable area, and I personally favor putting the switching on the active side of the fuse for exactly the sort of reason you suggest, which is not "standard", HOWEVER if you employ neutral side fusing you have no protection against failure of the mains transformer primary to the secondary or ground, and preventing your PT from catching fire is a large part of why the fuse is in circuit at all.

After looking at your cunning Hi/Lo HT supply I would suggest that you add a 150k resistor across each of the 47uF caps.  These will serve both as safety bleeders and to equalise the voltage across the electros in the low voltage position.

HTH
If you say theory and practice don't agree you haven't applied enough theory.

blackcorvo

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Re: Small Push-Pull Amplifier
« Reply #7 on: November 25, 2012, 03:09:41 PM »
Okay, that covers most of the points, however a couple remain, plus a new one.

I certainly think that having a resistor hard wired across the OPT secondary is a wise protective measure, we suggest that in the "Lamington" series, however my concern is with the value.  While 100 ohms is pretty typical I have seen reports lately of people suffering OPT failure when using a brand of dummy load that presents a 30 ohm load to the 8 ohm output.  From this experience I'd now be inclined to go for no more than a 47 ohm in this function.

47R for the dummy load. Got it!

Amplifier mains switching and fusing is a bit of a variable area, and I personally favor putting the switching on the active side of the fuse for exactly the sort of reason you suggest, which is not "standard", HOWEVER if you employ neutral side fusing you have no protection against failure of the mains transformer primary to the secondary or ground, and preventing your PT from catching fire is a large part of why the fuse is in circuit at all.

I don't see how that makes sense.... if there's some sort of current overload, it won't matter if the fuse is on the phase or neutral side. At least that's what I learned in tech school.

After looking at your cunning Hi/Lo HT supply I would suggest that you add a 150k resistor across each of the 47uF caps.  These will serve both as safety bleeders and to equalise the voltage across the electros in the low voltage position.

HTH

I see no need for those since I'm using 400v rated caps, but also I see no reason why not to do it.
The closest value I have here (of 1w rated resistors) is 220k.

J M Fahey

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Re: Small Push-Pull Amplifier
« Reply #8 on: November 26, 2012, 11:23:24 AM »
Quote
I don't see how that makes sense.... if there's some sort of current overload, it won't matter if the fuse is on the phase or neutral side. At least that's what I learned in tech school.
And who mentioned current overload?
In that case, a fuse will catch it in any end of the primary, since they are in series, *but* Roly speaks about
Quote
if you employ neutral side fusing you have no protection against failure of the mains transformer primary to the secondary or ground, and preventing your PT from catching fire is a large part of why the fuse is in circuit at all.
Ground and neutral are *not* the same wire or plug leg.
If you fuse Neutral (brown wire in Argentina/Blue in UK-Australia/white USA) and the winding shorts to Ground, either directly or to Chassis, which is directly connected to Ground (green/yellow wire), a fuse on Neutral won't blow, different paths.
Besides, if the fuse blows for any reason, and it's on Neutral, the amplifier is *still* connected to "hot/live" and any hot terminals inside are still dangerous, even if the amp "looks" unplugged.
While fusing the Hot wire solves all these problems in a safer way.
So ... it *does* make sense, after all. ;)

blackcorvo

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Re: Small Push-Pull Amplifier
« Reply #9 on: November 26, 2012, 06:05:30 PM »
Ground and neutral are *not* the same wire or plug leg.
If you fuse Neutral (brown wire in Argentina/Blue in UK-Australia/white USA) and the winding shorts to Ground, either directly or to Chassis, which is directly connected to Ground (green/yellow wire), a fuse on Neutral won't blow, different paths.
Besides, if the fuse blows for any reason, and it's on Neutral, the amplifier is *still* connected to "hot/live" and any hot terminals inside are still dangerous, even if the amp "looks" unplugged.
While fusing the Hot wire solves all these problems in a safer way.
So ... it *does* make sense, after all. ;)

Ahhh, I get it now.
I'll put the fuse on the Phase side, no problem!

J M Fahey

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Re: Small Push-Pull Amplifier
« Reply #10 on: November 26, 2012, 07:21:30 PM »
Cool. :tu:
It's safer  :dbtu:

sim0n

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Re: Small Push-Pull Amplifier
« Reply #11 on: November 27, 2012, 02:50:28 AM »
Does it matter that much? I'm in euroland with the reversable AC plugs where you can happily switch live and neutral... Or even if you have non-symmetrical ones, the wiring in the socket itself may be hooked up the other way. And if a fault does occur between the live and ground it should be caught by the residual-current circuit breaker (but then again...one may be plugging into 30 year old wiring without such amenities)

J M Fahey

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Re: Small Push-Pull Amplifier
« Reply #12 on: November 27, 2012, 07:20:49 AM »
Quote
Does it matter that much? I'm in euroland with the reversable AC plugs where you can happily switch live and neutral... Or even if you have non-symmetrical ones, the wiring in the socket itself may be hooked up the other way.
Of course it matters.
At least in 2012.
Whatever was acceptable before 1985 does not apply today.
By today's European Electrical Safety Rules, the CEE 7/16 Europlug you mention is allowed *only* as follows:
Quote
The Europlug itself is used in class II applications throughout continental Europe.
And what does class II mean?:
Quote
Class II
Class II symbol

A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require a safety connection to electrical earth (ground).

The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that this is achieved without relying on an earthed metal casing. This is usually achieved at least in part by having two layers of insulating material surrounding live parts or by using reinforced insulation.

In Europe, a double insulated appliance must be labelled Class II, double insulated, or bear the double insulation symbol (a square inside another square).
This applies to small home use electrical appliances, such as a fan, food processor, desk lamp, clock radio, etc. , which is visibly "all plastic" , including knobs and buttons, and there is *no way* you can touch any metallic part inside.
This does not apply to guitar amps, where you are directly connected to the metallic amp chassis through the strings and guitar cable.

As of
Quote
wiring in the socket itself may be hooked up the other way
you can go to jail for that.

I suggest reading:
http://en.wikipedia.org/wiki/Domestic_AC_power_plugs_and_sockets

http://en.wikipedia.org/wiki/Appliance_classes

http://en.wikipedia.org/wiki/Electrical_wiring

sim0n

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Re: Small Push-Pull Amplifier
« Reply #13 on: November 27, 2012, 07:50:48 AM »
I was referring to this plug here http://en.wikipedia.org/wiki/CEE_7/4 where you can easily flip it reversing neutral/live (with ground is present at all times). Since the design itself allows this I'm assuming the people that drafted the spec allowed for it, with the third prong convering the safety portion.

phatt

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Re: Small Push-Pull Amplifier
« Reply #14 on: November 27, 2012, 08:47:35 AM »
I was referring to this plug here http://en.wikipedia.org/wiki/CEE_7/4 where you can easily flip it reversing neutral/live (with ground is present at all times). Since the design itself allows this I'm assuming the people that drafted the spec allowed for it, with the third prong convering the safety portion.

Never make assumptions with electricity :grr
It can run a lot faster than You 8|

As to the link,,Then make Damn sure you use a plug with the Earth notch ,, which cannot be reversed.

That would make the metal chassis safe.

And just in case You (or anyone reading this) Missed the big print from Mr Fahey;
The Electric Guitar is one of the rare situations where the metal parts on the guitar are Directly connected to the Chassis of the equipment.

Which is why it's unwise to play in bare feet. Hint.

Some brand name equipment actually states; *This equipment Must be Earthed*

I strongly suggest to all those who are unsure go find a qualified electrical person in the country you live and seek advice.
I think both Roly and Jaun have gone above the call of duty to inform us of the fatal flaws in assuming stuff just by observation.

I have seen  repair jobs by backyard dodgy brothers where two wire power cord was used as a quick fix on Amplifiers that required 3 pin plugs and it is just inviting trouble.

I guess if you kill yourself it's sad but if you kill someone else because you just did not bother to check about safety is unforgivable.
Phil.

 

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