Ideas For EL34 Pentode Push-Pull Amplifier
- Cressy Snr
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#1 Ideas For EL34 Pentode Push-Pull Amplifier
OK, so I went inside the KT120 amplifier this morning and connected pin 1 to pin 8 on the underside of each power tube socket, so that I could drop in EL34s; just to see what they would do you understand.
Here is the op point the EL34s settled at with the existing cathode resistors.
Of course this was all worked out before any attempt was made to simply plug in different valves.
Our Ant brought round his set of CVC EL34s to use as a test quad.
The EL34s biased up beautifully at the op point shown on the graph.
only difference is I put 225V on the screens rather than 250, just to nudge the curves down a little, but you get the general idea.
Turns out the KT120 amp as I built it, can take anything from the 6L6/KT family as well as EL34/6CA7 varieties, so it's a pretty universal design.
Here is the op point the EL34s settled at with the existing cathode resistors.
Of course this was all worked out before any attempt was made to simply plug in different valves.
Our Ant brought round his set of CVC EL34s to use as a test quad.
The EL34s biased up beautifully at the op point shown on the graph.
only difference is I put 225V on the screens rather than 250, just to nudge the curves down a little, but you get the general idea.
Turns out the KT120 amp as I built it, can take anything from the 6L6/KT family as well as EL34/6CA7 varieties, so it's a pretty universal design.
Last edited by Cressy Snr on Sun Jan 12, 2014 3:22 pm, edited 1 time in total.
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- Cressy Snr
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#2
I'm currently enjoying the amp in EL34 mode, before returning the valves to our Ant tomorrow.
I had forgotten what a nice valve, the EL34 is.
In straight pentode mode, with regulated screens and plate-to-grid feedback, it makes a great sound.
Earlier this evening, we had it running on a diet of rock'n roll and Blues, in the form of Van Halen, Canned Heat Deep Purple and John Mayall and the Bluesbreakers, played at high levels. Lovely stuff; what rock should sound like.
Ultralinear? phooey! pentodes rule baby
I've ordered a matched quad off Langrex, which should be here by Wednesday as an early Christmas prezzie to myself.
In the new year, I'll build a nice pair of pentode monoblocks using EL34s, which is one of the reasons I have opened a new thread. The KT120 remains one of my all time fave amps, along with this EL34 variation.
they both do things differently, but EL34s seem to work some magic in this application, yet the sheer kick-arse stomp of KT120s remains an intoxicating experience. For once, it is nice to be spoiled for choice.
I had forgotten what a nice valve, the EL34 is.
In straight pentode mode, with regulated screens and plate-to-grid feedback, it makes a great sound.
Earlier this evening, we had it running on a diet of rock'n roll and Blues, in the form of Van Halen, Canned Heat Deep Purple and John Mayall and the Bluesbreakers, played at high levels. Lovely stuff; what rock should sound like.
Ultralinear? phooey! pentodes rule baby
I've ordered a matched quad off Langrex, which should be here by Wednesday as an early Christmas prezzie to myself.
In the new year, I'll build a nice pair of pentode monoblocks using EL34s, which is one of the reasons I have opened a new thread. The KT120 remains one of my all time fave amps, along with this EL34 variation.
they both do things differently, but EL34s seem to work some magic in this application, yet the sheer kick-arse stomp of KT120s remains an intoxicating experience. For once, it is nice to be spoiled for choice.
Last edited by Cressy Snr on Mon Nov 25, 2013 11:29 pm, edited 3 times in total.
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- Paul Barker
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#3
Agreed, my first amp was an EL34 push pull pentode.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
#4
You have no idea how fortunate you are to be able to do that sort of thing domestically. These days, I do most of my listening when ''er indoors' is out of the house, just to ensure relative harmony when she is indoorsSteveTheShadow wrote:Earlier this evening, we had it running on a diet of rock'n roll and Blues, in the form of Van Halen, Canned Heat Deep Purple and John Mayall and the Bluesbreakers, played at high levels. Lovely stuff; what rock should sound like.
Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?
Douglas Adams (HHGTTG)
Douglas Adams (HHGTTG)
- Cressy Snr
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#5
Yes I suppose I am relatively fortunate.
Over our 32 year marriage, we have always had music time in our house.
Sometimes at weekends, the music is on from 8am to 6pm. 10pm is the usual time the TV goes off and the music is switched on, during the week, unless there is something we particularly want to watch.
Music is a large part of life in our house. The missus, tired of looking at the blank faces of open baffle speakers, commissioned Colin to build the big Mets, using money she inherited, to pay for them. Since the latest amp was built, the amount of family listening time around the metaphorical gramophone has increased again, which is nice. My daughter gets in on it too with Rhianna, Lady Gaga, Jason Mraz 90s dance/house stuff etc, usually with all of us in the room together. I guess we all grew up with music on all the time, so having the hi-fi playing is a normal part of life chez us.
Over our 32 year marriage, we have always had music time in our house.
Sometimes at weekends, the music is on from 8am to 6pm. 10pm is the usual time the TV goes off and the music is switched on, during the week, unless there is something we particularly want to watch.
Music is a large part of life in our house. The missus, tired of looking at the blank faces of open baffle speakers, commissioned Colin to build the big Mets, using money she inherited, to pay for them. Since the latest amp was built, the amount of family listening time around the metaphorical gramophone has increased again, which is nice. My daughter gets in on it too with Rhianna, Lady Gaga, Jason Mraz 90s dance/house stuff etc, usually with all of us in the room together. I guess we all grew up with music on all the time, so having the hi-fi playing is a normal part of life chez us.
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- Cressy Snr
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#6
For those interested, here is the schematic:
OK so it is not much different from the KT120 amp, but there's no sense in changing something as successful as this circuit, just for the sake of it.
All it takes to switch the amp between KT120s and EL34s is to insert different gas stabiliser tubes in the screen supply shunt and you're away on either setup.
Can't get much easier than that.
OK so it is not much different from the KT120 amp, but there's no sense in changing something as successful as this circuit, just for the sake of it.
All it takes to switch the amp between KT120s and EL34s is to insert different gas stabiliser tubes in the screen supply shunt and you're away on either setup.
Can't get much easier than that.
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- Cressy Snr
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#8
I have only the one source, ie. my Mac Mini as a music server . My Musical Fidelity M1 DAC has balanced outputs and it is these, that feed the amplifier.grog wrote:I've probably missed this somewhere, but which pre amp are you running to get the balanced outs?
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- Cressy Snr
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#9
The matched quad of Langrex own branded Shuguang EL34B arrived this morning and are busy burning in as I write this.
I must say, they look to be constructed to a high standard, with thick glass, double micas at top and bottom, and welded plates.
The bulb is slightly wider than the base; they look thicker in the body than most EL34.
I find them a good looking valve. The red bases go nicely with the green and the walnut.
They go nicely with the 13D9s and the stabiliser tubes.
Need to change out the PY500 for one of my 6CL3s, to get all the valves looking harmonious and of a piece.
I must say, they look to be constructed to a high standard, with thick glass, double micas at top and bottom, and welded plates.
The bulb is slightly wider than the base; they look thicker in the body than most EL34.
I find them a good looking valve. The red bases go nicely with the green and the walnut.
They go nicely with the 13D9s and the stabiliser tubes.
Need to change out the PY500 for one of my 6CL3s, to get all the valves looking harmonious and of a piece.
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- Dave the bass
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#10
Snap! Thats what I use in the Workshop amp, a WADKit34 IIRC. They've had a hard life, on for at least 9-10hrs every weekday for the last 20months or so. Still seem OK.
The bases on ours have darkened off with the heat, they started off the same colour as yours!
DTB
The bases on ours have darkened off with the heat, they started off the same colour as yours!
DTB
"The fat bourgeois and his doppelganger"
- Cressy Snr
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#11
Nice to know they will last a while Dave.
The EL34 has somewhat fallen out of fashion these days, in favour of big bruising bottles, like my KT120s and now the KT150, but there's something about the EL34. It's a bit on the warm side of neutral, but it has soul aplenty, if you know what I mean.
BTW, who is that black, hooded figure, with the wicked looking blade, reflected above the top mica, of that EL34, in your first picture?
A concerned citizen.
The EL34 has somewhat fallen out of fashion these days, in favour of big bruising bottles, like my KT120s and now the KT150, but there's something about the EL34. It's a bit on the warm side of neutral, but it has soul aplenty, if you know what I mean.
BTW, who is that black, hooded figure, with the wicked looking blade, reflected above the top mica, of that EL34, in your first picture?
A concerned citizen.
Last edited by Cressy Snr on Wed Nov 27, 2013 4:38 pm, edited 2 times in total.
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- Dave the bass
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- Cressy Snr
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#13
Do your mates know you're a closet goth?Dave the bass wrote:Thats me. I'm having an 'off' day
DTB
PY500 top-cap damper diode has been changed out for a 6CL3 (front left):
That looks much better
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- Cressy Snr
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#14
Been having a look at the schematic for the WAD KEL34 and it sure does thrash the valves. 50mA at 500V a-k, is right on the 25W maximum dissipation at idle. So it'll probably go over max on each half cycle if a 6K6 a-a output transformer is being used, as it probably is, as the screen voltage is 255V. With that OPT impedance, the class B part of the loadline will pass correctly through the knee of the 0V grid line. This is normal for this sort of amplifier; one designed to give you maximum bang for your buck.Dave the bass wrote:Snap! Thats what I use in the Workshop amp, a WADKit34 IIRC. They've had a hard life, on for at least 9-10hrs every weekday for the last 20months or so. Still seem OK.
The bases on ours have darkened off with the heat, they started off the same colour as yours!
DTB
I'm running 375V a-k with 225V g2, which, with an educated guess, as i said earlier, ought to bring the curves down slightly, so I get the class B part of the loadline to cross the 0V knee.
TBH I could do with at least 450V of HT and a 250V screen, but it'll do for now. Dissipation is 18W per valve (36W per pair) so I'm running the EL34s at 72% of maximum.
I have two 0-375-450V mains transformers as a legacy of building the 6B4G monoblocks.
Under the circumstances, it'd be daft not to use them
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- Cressy Snr
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#15
Right!
Having had a look around my store of bits, the plan is to build a pair of 30WPC class A/B mono-blocks, using the Shuguang EL34s that are currently living in the KT120 chassis.
I'll have to purchase a pair of output transformers and four small chokes for the valve rectified, driver stage pi filter and screen supply.
As per my tried and tested, methods, there will be three power supplies per monoblock:
SS rectified hi-capacitance supply for the output stage
Valve rectified supply for the driver stage
SS rectified, pi filtered, gas tube stabilised screen grid supply.
Bias will be fixed, as it really ought to be for efficient class AB operation.
low value resistors will be placed in each output valve cathode with test points for a meter, to measure bias current.
Current finance levels, from my part time work dictate Hammond iron, from Philip@Bluebell, but that's no bad thing, as I have found their stuff generally sounds good. No longer rich enough to pay for Sowter iron
Chassis will be self bashed and bent 1.5mm steel sheet with walnut end cheeks; paint job not yet decided. Our Ant is on friendly terms with a local sheet metal supplier.
Topology, true balanced, two stage; what else
Having had a look around my store of bits, the plan is to build a pair of 30WPC class A/B mono-blocks, using the Shuguang EL34s that are currently living in the KT120 chassis.
I'll have to purchase a pair of output transformers and four small chokes for the valve rectified, driver stage pi filter and screen supply.
As per my tried and tested, methods, there will be three power supplies per monoblock:
SS rectified hi-capacitance supply for the output stage
Valve rectified supply for the driver stage
SS rectified, pi filtered, gas tube stabilised screen grid supply.
Bias will be fixed, as it really ought to be for efficient class AB operation.
low value resistors will be placed in each output valve cathode with test points for a meter, to measure bias current.
Current finance levels, from my part time work dictate Hammond iron, from Philip@Bluebell, but that's no bad thing, as I have found their stuff generally sounds good. No longer rich enough to pay for Sowter iron
Chassis will be self bashed and bent 1.5mm steel sheet with walnut end cheeks; paint job not yet decided. Our Ant is on friendly terms with a local sheet metal supplier.
Topology, true balanced, two stage; what else
Sgt. Baker started talkin’ with a Bullhorn in his hand.