Dear All,
I understand that considerably more power can be had by allowing triodes in push-pull to go into positive g1 voltage and draw grid current (class AB2). For instance, Norman Koren did this with triode-strapped 6550s in "The Emperor's New Amplifier." However, I've never seen class AB2 advocated for triode-strapped EL34s. Is there some specific reason for this, or is it just something that no-one's ever bothered to do?
I understand that considerably more power can be had by allowing triodes in push-pull to go into positive g1 voltage and draw grid current (class AB2). For instance, Norman Koren did this with triode-strapped 6550s in "The Emperor's New Amplifier." However, I've never seen class AB2 advocated for triode-strapped EL34s. Is there some specific reason for this, or is it just something that no-one's ever bothered to do?
No reply yet to this question but I'm still interested in people's opinions. I have the necessary components to insert a direct-coupled cathode follower between the driver and OP stages of my amp but I don't want to do all that work just, to find out that the EL34 (triode-strapped) isn't capable of operatmg like this.
You need to first look at the triode plate curves and see whats going on...
I believe you should have no problem doing this with EL34...
Just keep in mind that to properly extend up into AB2 region, you need to readjust the load-line that you were using in AB1...
Make sure to figure your peak grid current and properly calculate the drive impedance needed for good bandwidth...
I have had good success using 6V6 as a cathode follower as a driver tube.... 1/gm
Chris
I believe you should have no problem doing this with EL34...
Just keep in mind that to properly extend up into AB2 region, you need to readjust the load-line that you were using in AB1...
Make sure to figure your peak grid current and properly calculate the drive impedance needed for good bandwidth...
I have had good success using 6V6 as a cathode follower as a driver tube.... 1/gm
Chris
Hello ,
Why on earth would you want to use AB2 with EL34 ? You would be far better off using EL34 in class A1 or even AB1 where a good 15 watts is available into a 5.5k a-a load . AB2 will get you another 3dB but distortion will rise significantly when grid current starts being drawn , also your driver would probably need to be another EL34 . Not worth it really , if you need more power use the EL34 strapped as UL or pentode
cheers
316a
Why on earth would you want to use AB2 with EL34 ? You would be far better off using EL34 in class A1 or even AB1 where a good 15 watts is available into a 5.5k a-a load . AB2 will get you another 3dB but distortion will rise significantly when grid current starts being drawn , also your driver would probably need to be another EL34 . Not worth it really , if you need more power use the EL34 strapped as UL or pentode
cheers
316a
There is method in my madness and it's based on Norman Koren's "The Emorer's New Amplifier" article. He built a PP amp using 4x6650 in triode mode, class AB2. He claims it is sonically excellent and more linear than UL. His enthusiasm has rubbed off on me to a certain extent, because I would like to get the extra power without incurring the pentode's higher order harmonic distortion and high OP impedance.
Modelled in LTspice, the AB2 arrangement seems to be capable of delivering up to 30w before cutoff, which is twice the power of AB1. I don't intend driving it to high such levels on a continuous basis but I would like to have the headroom. It wouldn't cost me anything except effort, since I already have the necessary parts to insert a cf driver where needed. I reckon a 6SN7 would have enough grunt for this.
I have already tried a pure pentode arrangement but I couldn't get satisfactory bass (too boomy), which I suspect is because I couldn't pile on enough NFB to increase the damping factor to a realistic level. UL is out, unless I invest in a new pair of OPTs, which I'd like to do when I can afford it (maybe Lundahl).
I'll attempt to attach the LTspice schematic that I used to model the planned amp, which is only for one channel. Please note that a number of current sinks are included to represent the load impose by the other channel.
Modelled in LTspice, the AB2 arrangement seems to be capable of delivering up to 30w before cutoff, which is twice the power of AB1. I don't intend driving it to high such levels on a continuous basis but I would like to have the headroom. It wouldn't cost me anything except effort, since I already have the necessary parts to insert a cf driver where needed. I reckon a 6SN7 would have enough grunt for this.
I have already tried a pure pentode arrangement but I couldn't get satisfactory bass (too boomy), which I suspect is because I couldn't pile on enough NFB to increase the damping factor to a realistic level. UL is out, unless I invest in a new pair of OPTs, which I'd like to do when I can afford it (maybe Lundahl).
I'll attempt to attach the LTspice schematic that I used to model the planned amp, which is only for one channel. Please note that a number of current sinks are included to represent the load impose by the other channel.
Attachments
Starting with the "obvious": When the grid goes positive, its impedance drops from infinity to about 1K ohms very abruptly. If the driver's gain falls off at 1K load, distortion is large. As a first approximation, driver source impedance should be zero. Actually, since Gm is rising and Rg falls somewhat slowly, lowest THD may be with a source of 50Ω-100Ω.
Another way to look at it: negative-grid tube is an FET. But positive-grid, it is a BJT: plate current is approximately proportional to grid current, the "Beta" starts at not much more than 10 and soon falls close to unity. It is entirely possible to push a tube with more grid current than plate current. That actually works in RF where tuned circuits transform impedances and store energy. In untuned amplifiers, big grid current is wasteful and there is usually a better way.
The implication of a low "Beta" is that for maximum power and maximum efficiency you should run the output stage at HIGH voltage, and the driver from a lower separate supply.
BTW: most SPICE tube models have very poor modeling of grid current. Some omit it entirely. Some put in a silicon diode and a 1K resistor: this gives you a flat-topped hint if you thought you were not running grid current, but is not a good approximation of the real performance.
It is very hard to make a 100Ω source with tubes. (And you sure need to lose the 1K grid resistors.
I've seen it done with cathode follower 6SN7 but with both sides coupled with a center-tapped choke, which doubles the drive current and halves the output impedance (since only one of the grids is low-Z at a time). And that was a high-power P.A. application where a little mud on peaks was a fair trade for a lower price (or higher profit).
A very traditional way to go AB2, back when we HAD to slap grid current to get to a Watt, was to use a third "output tube" as the driver, working through a 3:1CT step-down transformer.
Trying to drive 1K loads to 10V-30V, i.e. 10mA-30mA peak drive current, from a 300V supply, through tubes of ~6K plate impedance, is doing things the hard way.
If an electrode passes current it has a dissipation. If you exceed rated dissipation, the electrode gets in trouble: often it warps, possibly shorting against another electrode. By omission, the G1 dissipation on all the 5W-40W audio power tubes is "zero" (exception: 6L6 has an AB2 suggested condition, but no explicit G1 rating validating a warranty claim if it melts). Obviously the true capacity is more than zero. Geometry suggests something like 1/1,000 of the plate rating. For a pentode, the G2 rating divided by the G2/G1 Mu might be the gross-overheat point, but the close G1-K spacing says that G1 might short to the cathode before that (G2 has more space to warp without hitting anything).
> Norman Koren's ... claims it is sonically excellent
He may be right. It is hard to know what another person hears. Though I do respect Koren's writings, I take all such descriptions with a grain of salt.
> I would like to have the headroom.
I've made some money with audio. Usually if X is not enough power, X+3dB is still not enough power.
+3dB means double the amplifier, double the weight, nearly double the cost. Running AB2 mostly does not change that. You don't need double the output tubes, but you nearly double the power and output core weights, plus you need that super-driver.
Ah, but what is the worst that can happen? You build it, it isn't sweet or isn't reliable, you re-make it as a conventional amp. No big deal. And the best-case is: it works sweet and well, you come back here and rub our noses in it.
Just don't put much faith in the models. You have to build it and see. While I doubt you have any worry about grid-melt with unclipped speech/music, be cautious in full-power testing. Have strategic burn-out resistors to protect the transformers against tube shorts.
Another way to look at it: negative-grid tube is an FET. But positive-grid, it is a BJT: plate current is approximately proportional to grid current, the "Beta" starts at not much more than 10 and soon falls close to unity. It is entirely possible to push a tube with more grid current than plate current. That actually works in RF where tuned circuits transform impedances and store energy. In untuned amplifiers, big grid current is wasteful and there is usually a better way.
The implication of a low "Beta" is that for maximum power and maximum efficiency you should run the output stage at HIGH voltage, and the driver from a lower separate supply.
BTW: most SPICE tube models have very poor modeling of grid current. Some omit it entirely. Some put in a silicon diode and a 1K resistor: this gives you a flat-topped hint if you thought you were not running grid current, but is not a good approximation of the real performance.
It is very hard to make a 100Ω source with tubes. (And you sure need to lose the 1K grid resistors.
I've seen it done with cathode follower 6SN7 but with both sides coupled with a center-tapped choke, which doubles the drive current and halves the output impedance (since only one of the grids is low-Z at a time). And that was a high-power P.A. application where a little mud on peaks was a fair trade for a lower price (or higher profit).
A very traditional way to go AB2, back when we HAD to slap grid current to get to a Watt, was to use a third "output tube" as the driver, working through a 3:1CT step-down transformer.
Trying to drive 1K loads to 10V-30V, i.e. 10mA-30mA peak drive current, from a 300V supply, through tubes of ~6K plate impedance, is doing things the hard way.
If an electrode passes current it has a dissipation. If you exceed rated dissipation, the electrode gets in trouble: often it warps, possibly shorting against another electrode. By omission, the G1 dissipation on all the 5W-40W audio power tubes is "zero" (exception: 6L6 has an AB2 suggested condition, but no explicit G1 rating validating a warranty claim if it melts). Obviously the true capacity is more than zero. Geometry suggests something like 1/1,000 of the plate rating. For a pentode, the G2 rating divided by the G2/G1 Mu might be the gross-overheat point, but the close G1-K spacing says that G1 might short to the cathode before that (G2 has more space to warp without hitting anything).
> Norman Koren's ... claims it is sonically excellent
He may be right. It is hard to know what another person hears. Though I do respect Koren's writings, I take all such descriptions with a grain of salt.
> I would like to have the headroom.
I've made some money with audio. Usually if X is not enough power, X+3dB is still not enough power.
+3dB means double the amplifier, double the weight, nearly double the cost. Running AB2 mostly does not change that. You don't need double the output tubes, but you nearly double the power and output core weights, plus you need that super-driver.
Ah, but what is the worst that can happen? You build it, it isn't sweet or isn't reliable, you re-make it as a conventional amp. No big deal. And the best-case is: it works sweet and well, you come back here and rub our noses in it.
Just don't put much faith in the models. You have to build it and see. While I doubt you have any worry about grid-melt with unclipped speech/music, be cautious in full-power testing. Have strategic burn-out resistors to protect the transformers against tube shorts.
Help - Please explain
I'm confused.
Its stated by a few folks above (whose opinions I've come to respect) that runing a triode into grid current significantly increases distortion and that accords with what I've read in Morgan Jones and similar texts.
Then we look at some of the SET designs (2A3, 300B, 845 etc) around the place and the common theme seems to be that you need a decent driver stage to push a bit of grid current into the DHT - so why don't they sound rubish - or do they.
Are we to conclude that any decent SET is strictly Class A1 and doesn't transit into A2 at higher levels?
I'm with Ray on this. If a Class A2 SET sound OK why not Class AB2 PP?
Can anyone enlighten me? I feel my education is incomplete. If I had to take a WAG based upon the above I'd have to conclude that Class A2 SETs probably DON'T sound OK - any comments????.
I had a similar problem to Ray - but I tackled it by using parallel pairs of triode strapped EL34s to get O/P Power up to 40W without recourse to grid current.
Cheers,
Ian
I'm confused.
Its stated by a few folks above (whose opinions I've come to respect) that runing a triode into grid current significantly increases distortion and that accords with what I've read in Morgan Jones and similar texts.
Then we look at some of the SET designs (2A3, 300B, 845 etc) around the place and the common theme seems to be that you need a decent driver stage to push a bit of grid current into the DHT - so why don't they sound rubish - or do they.
Are we to conclude that any decent SET is strictly Class A1 and doesn't transit into A2 at higher levels?
I'm with Ray on this. If a Class A2 SET sound OK why not Class AB2 PP?
Can anyone enlighten me? I feel my education is incomplete. If I had to take a WAG based upon the above I'd have to conclude that Class A2 SETs probably DON'T sound OK - any comments????.
I had a similar problem to Ray - but I tackled it by using parallel pairs of triode strapped EL34s to get O/P Power up to 40W without recourse to grid current.
Cheers,
Ian
I'll throw one more example into your experience-pile: the Fisher 50A, which used a very similar configuration, CT choke, but in a high-quality amp for home use. There were a few different versions- the ones I used were EL37-based. Matched with Quads, they produced a stunning sound.
As a system, that's usually the case- as PRR pointed out, it's a tough load for a driver. BUT... if you drive the grids correctly, and the output tubes will take it, you can achieve reasonably low distortion. Take a look, for instance, at the data sheets for 6L6. The trick is driving the grids correctly and that will require a tube with some grunt. Or (as I use) a source follower.
edit: Direct-coupling is just about mandatory. The other possibility is using a step-down transformer with a high-voltage driver.
100 ohms isn't that hard, 10mmhos will do it. 6AU6 even, at enough Ip. 6CL6 or 12BY7 (or any number of similar frame grid tubes *COUGH*7KY6*cough cough*) will provide more oomph.
The curves don't lie, class 2 isn't a bad thing. Look at 6SN7 for instance, the GE curves show some +35V (it would seem I drew on my copy, extrapolating curves out to +60V ), to which it draws 150mA at a mere 45 plate volts! (And grid current is only one third of the cathode current, about 75mA, not bad for such low Vp and high Vg!) Well anyway, look at the spacing of the curves in this graph: very slight upward curve (indicating decrease of Rp, usually coincident with increasing Gm), greater spacing at higher currents indicating higher Gm (peaking around 200mA; it appears to fall off ever so slightly at the end of the Vg = 35V curve, suggesting full emission (saturation) at maybe Ik = 0.5 to 1A, and at a guess, Vg = 125V, Vp = 500V, for 45W grid dissipation and 350W plate dissipation 
).
Since you've got that knee there, you load it exactly like a tetrode, sliding the low-Vp end right of the loadline right into that pocket. RL = 3Rp goes out the window; we have no use of such archaic rules of thumb. You're welcome to draw a line and see just how linear it is! (While you're at it, check 6V6 (triode or pentode mode) at +160Vp, zero bias class A2...)
So no, the problem isn't the output tube; it must be driving it solidly. Tubes aren't good because they have low Gm, you could use a FET I suppose, but weshallnotspeakanymoreofthis!
You can use a transformer, but that sucks. You can use CF triodes, but those suck. You can use SS, but that sucks!
FYI, one way around the drive Zo problem might be local shunt feedback to a driving amp stage. Plus everything PRR mentioned...
Have fun engineering the circuit conditions, if you wish to do it from scratch.
Tim
The curves don't lie, class 2 isn't a bad thing. Look at 6SN7 for instance, the GE curves show some +35V (it would seem I drew on my copy, extrapolating curves out to +60V
), to which it draws 150mA at a mere 45 plate volts! (And grid current is only one third of the cathode current, about 75mA, not bad for such low Vp and high Vg!) Well anyway, look at the spacing of the curves in this graph: very slight upward curve (indicating decrease of Rp, usually coincident with increasing Gm), greater spacing at higher currents indicating higher Gm (peaking around 200mA; it appears to fall off ever so slightly at the end of the Vg = 35V curve, suggesting full emission (saturation) at maybe Ik = 0.5 to 1A, and at a guess, Vg = 125V, Vp = 500V, for 45W grid dissipation and 350W plate dissipation ).Since you've got that knee there, you load it exactly like a tetrode, sliding the low-Vp end right of the loadline right into that pocket. RL = 3Rp goes out the window; we have no use of such archaic rules of thumb. You're welcome to draw a line and see just how linear it is! (While you're at it, check 6V6 (triode or pentode mode) at +160Vp, zero bias class A2...)
So no, the problem isn't the output tube; it must be driving it solidly. Tubes aren't good because they have low Gm, you could use a FET I suppose, but weshallnotspeakanymoreofthis!
You can use a transformer, but that sucks. You can use CF triodes, but those suck. You can use SS, but that sucks!
FYI, one way around the drive Zo problem might be local shunt feedback to a driving amp stage. Plus everything PRR mentioned...
Have fun engineering the circuit conditions, if you wish to do it from scratch.
Tim
> runing a triode into grid current significantly increases distortion
The output versus grid voltage IS linear.
As a rough approximation: the Cathode-Plate circuit only cares what the grid-cathode voltage is, not how much current flows in the grid-cathode cuircuit. Tubes are very linear in the positive grid range. (Actually, grid current steals cathode current away from the plate, but up to a point this is a small effect.)
The problem is in driving the sometimes-low grid impedance with a steady voltage. You won't do it with the plate of any tube smaller than your outputs. You can't do it very well with old-time cathode followers (though there is a tube that is an A2 output plus a cathode follower). The late-era high-Gm tubes can drive home-size output tubes into grid current if you wish.
Note that with conventional outputs and cathode follower drivers, you need a BIG negative rail to pull-down the cathodes and grids on the negative side. (We can't capacitor-couple because of the asymmetric current waveform.) Between high driver standing current and deep negative rail, driver power consumption can be as expensive as just tossing a couple more output tubes in. Transformer coupling has real advantages: transforming a high-Rp tube down to low impedance, and kicking negative without a negative supply.
BTW: the Macintosh 6L6 amp runs slightly AB2 to meet its power rating. Not a lot of grid current: it only uses a 12AX7 as driver. The original Mac 50 not only runs past the old-6L6 AB2 rating of 47 watts, it uses a transformer driver and takes feedback around two transformers.
The output versus grid voltage IS linear.
As a rough approximation: the Cathode-Plate circuit only cares what the grid-cathode voltage is, not how much current flows in the grid-cathode cuircuit. Tubes are very linear in the positive grid range. (Actually, grid current steals cathode current away from the plate, but up to a point this is a small effect.)
The problem is in driving the sometimes-low grid impedance with a steady voltage. You won't do it with the plate of any tube smaller than your outputs. You can't do it very well with old-time cathode followers (though there is a tube that is an A2 output plus a cathode follower). The late-era high-Gm tubes can drive home-size output tubes into grid current if you wish.
Note that with conventional outputs and cathode follower drivers, you need a BIG negative rail to pull-down the cathodes and grids on the negative side. (We can't capacitor-couple because of the asymmetric current waveform.) Between high driver standing current and deep negative rail, driver power consumption can be as expensive as just tossing a couple more output tubes in. Transformer coupling has real advantages: transforming a high-Rp tube down to low impedance, and kicking negative without a negative supply.
BTW: the Macintosh 6L6 amp runs slightly AB2 to meet its power rating. Not a lot of grid current: it only uses a 12AX7 as driver. The original Mac 50 not only runs past the old-6L6 AB2 rating of 47 watts, it uses a transformer driver and takes feedback around two transformers.
An externally hosted image should be here but it was not working when we last tested it.
Taking a quick look...here are some usable numbers I came up with...
A pair of triode strapped EL34 valves in AB2...
Operating at 400V....use a 4.6K plate load you can extend up to the +20V curve in a reasonably linear region, and the load line hits the +20 curve roughly at .320 mA....
This comes out to roughly 30W Power Output....
Figure biasing somewhere abouts -35.....
Make sure you have the full needed AMPLITUDE prior to your driver stage..
Or if you operate at 350 plate volt....use a 4K plate load and you hit the same point at +20V curve which is .320mA ....
This is roughly 26 Watts Power Output.....
I am using plate curves I made a number of years back using a Mullard El34 "bogie" valve.....
The plate load is CRITICAL is AB2 ...if it ain't right...then you ain't going to extend into the + region......unless you LUCKY....
Getting to the driver valve..... There are lots of options..
YOu can use the 6SN7 follower with both sides in parallel, this will give you roughly a 180 ohms for a driver.... or my personal favorite a 6V6 tied as Triode but used as a follower..this is roughly a 160 ohm driver, very linear....Take your pick....
I don't have the magic number for grid current at +20V ...But figure your grid will come crashing down to roughly 1K as pointed out by another poster.....this is a rough but good guestimate....
Chris
A pair of triode strapped EL34 valves in AB2...
Operating at 400V....use a 4.6K plate load you can extend up to the +20V curve in a reasonably linear region, and the load line hits the +20 curve roughly at .320 mA....
This comes out to roughly 30W Power Output....
Figure biasing somewhere abouts -35.....
Make sure you have the full needed AMPLITUDE prior to your driver stage..
Or if you operate at 350 plate volt....use a 4K plate load and you hit the same point at +20V curve which is .320mA ....
This is roughly 26 Watts Power Output.....
I am using plate curves I made a number of years back using a Mullard El34 "bogie" valve.....
The plate load is CRITICAL is AB2 ...if it ain't right...then you ain't going to extend into the + region......unless you LUCKY....
Getting to the driver valve..... There are lots of options..
YOu can use the 6SN7 follower with both sides in parallel, this will give you roughly a 180 ohms for a driver.... or my personal favorite a 6V6 tied as Triode but used as a follower..this is roughly a 160 ohm driver, very linear....Take your pick....
I don't have the magic number for grid current at +20V ...But figure your grid will come crashing down to roughly 1K as pointed out by another poster.....this is a rough but good guestimate....
Chris
Wow! Thanks very much for all the responses! My original question was mainly concerned with whether the EL34 could tolerate some grid current, but all the other advice is gratefully received too.
On balance, having read what people have said, I think I'll give AB2 a miss. (Chicken!!) The modeling with LTspice has been fun but, if it's not going to give me a credible result for grid current, then the only way to find the real answer would be to build it. That would involve making extra holes in the chassis for the additional 6SN7, plus the wiring. That's a lot of work for little or no gain (no pun intended).
I already have a negative supply in place. However, it has to supply two channels and wouldn't tolerate more than about 28mA, since it consists of a 50v 100mA winding with a voltage doubler, to give ~110v; from people's comments, I would probably need more than this, especially if I have to use a beefier driver than the 6SN7.
Perhaps the best approach for me, if I need more headroom and don't want to use the EL34s as nature intended (pentode or UL), would be to do what Gingertube has done, i.e. stay with AB1 and add another pair of EL34, triode-strapped, in parallel. My OPTs are big enough to tolerate this, I believe, and I could use the 8 ohm tap for my 8 ohm speakers instead of the 4 ohm tap (which I'm currently doing, to get 7k plate-to-plate load). Just have to find another power tranny!
For anyone interested, I attach the schematic of my present amp, which sounds OK to me. ( Any comments in this circuit would also be gratefully received).
Thanks again for your valued opinions.
Regards,
Ray
On balance, having read what people have said, I think I'll give AB2 a miss. (Chicken!!) The modeling with LTspice has been fun but, if it's not going to give me a credible result for grid current, then the only way to find the real answer would be to build it. That would involve making extra holes in the chassis for the additional 6SN7, plus the wiring. That's a lot of work for little or no gain (no pun intended).
I already have a negative supply in place. However, it has to supply two channels and wouldn't tolerate more than about 28mA, since it consists of a 50v 100mA winding with a voltage doubler, to give ~110v; from people's comments, I would probably need more than this, especially if I have to use a beefier driver than the 6SN7.
Perhaps the best approach for me, if I need more headroom and don't want to use the EL34s as nature intended (pentode or UL), would be to do what Gingertube has done, i.e. stay with AB1 and add another pair of EL34, triode-strapped, in parallel. My OPTs are big enough to tolerate this, I believe, and I could use the 8 ohm tap for my 8 ohm speakers instead of the 4 ohm tap (which I'm currently doing, to get 7k plate-to-plate load). Just have to find another power tranny!
For anyone interested, I attach the schematic of my present amp, which sounds OK to me. ( Any comments in this circuit would also be gratefully received).
Thanks again for your valued opinions.
Regards,
Ray
Attachments
Ray,
If you want to have a quick and dirty trial of AB2 then use SY's trick and use a IRF820 as a source follower. No hacking up your chassis for a new tube hole or heater supplies to worry about. Sonically it will be VERY close to what you'd achieve with a tube cathode follower, so if you liked it you could convert to tube cathode follower afterward. You've already got a suitable negative rail so its really just one MOSFET and one resistor to add to each channel.
BTW my 4 x EL34 Triode Strapped parallel push pull is into about 2400 Ohms Raa (Plitron VDV-2100 with nominal 6 Ohm speakers on the 5 Ohms secondary). These monoblocks are working quite well with a wimpy front end - ECC99 as common cathode with direct coupled concertina. I'm getting -3dB top at 105kHz at 10 Watts out, despite "inadequate?" drive of the output tube grid capacitances.
Cheers,
Ian
If you want to have a quick and dirty trial of AB2 then use SY's trick and use a IRF820 as a source follower. No hacking up your chassis for a new tube hole or heater supplies to worry about. Sonically it will be VERY close to what you'd achieve with a tube cathode follower, so if you liked it you could convert to tube cathode follower afterward. You've already got a suitable negative rail so its really just one MOSFET and one resistor to add to each channel.
BTW my 4 x EL34 Triode Strapped parallel push pull is into about 2400 Ohms Raa (Plitron VDV-2100 with nominal 6 Ohm speakers on the 5 Ohms secondary). These monoblocks are working quite well with a wimpy front end - ECC99 as common cathode with direct coupled concertina. I'm getting -3dB top at 105kHz at 10 Watts out, despite "inadequate?" drive of the output tube grid capacitances.
Cheers,
Ian
> if I need more headroom .... stay with AB1 and add another pair of EL34
Or stop fooling with the small-beer EL34, and use the big-bottle 6550. It is a very powerful pentode and a mighty fine triode. While the EL34 can do amazing things, you have to be heroic with a pair of EL34 to make half what a pair of 6550 will do. I think heater consumption will be less.
Or stop fooling with the small-beer EL34, and use the big-bottle 6550. It is a very powerful pentode and a mighty fine triode. While the EL34 can do amazing things, you have to be heroic with a pair of EL34 to make half what a pair of 6550 will do. I think heater consumption will be less.
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