I'm working on an amp an area buddy dropped off to me and I can't quite decide which/what
route to take so I thought I'd toss a question out for some possible ideas/thoughts from others.
Here is the scenario,
It's a VTL ST-85 running EL34's in pentode with 600V on both the plate and there SCREENS!
(outputs have no UL taps)
The owner was having an issue with bias and the controls either not getting there required bias
or if they did the pots were turned pretty much all the way with any tubes ever used in this amp,
and a channel not "sounding" quite right. I fixed the bias and the channel had an issue with the
tail resistor in the LTP being 30% off in one channel, fixed that as well.
The amp has gone through tubes quite a bit I'm told an it doesn't surprise me with that voltage.
(and the owner is going to buy a new quad when I get the amp settled, any suggestions?)
---------------
So I'm trying to figure out if I want to alter anything in the amp based on the voltage, the tubes
and the fact I'm not a giant fan of push pull pentode, unless say a nice regulated screen volts.
Options I have tossed around are, (if I would be making any changes)
(sticking with EL34's)
Wiring outputs in triode as some pdf's say 600v triode is max (I doubt any new tubes like it)
(would it be better then running pentode at that voltage, humm?)
Taping from the RC stage that feeds the phase splitters has 400v then use it for the screens.
(to stick with pentode operation but still drop the screen voltage down some)
Changing cathode resistors of 10ohms for sense to say 500-700 ohms and do a combo bias,
(which would drop my plate to cathode voltage a bit)
(going to KT88's)
Again wire the outputs in triode, up the cathode resistors to make up the bias voltage needed.
(which again would drop my plate to cathode voltage a bit)
Taping from the RC stage that feeds the phase splitters has 400v then use it for the screens.
(to stick with pentode operation but still drop the screen voltage down some)
(Or combos using any of the above together)
---------------
I can modify the bias circuit to get upto about 70v, the original ran 600v B+ and 30ma per tube.
(or mod it to get anything lower as well) (I may try calling VTL, see what power tranny handles)
This thing was rated for 85wpc running a pair of EL34's, do not need anywhere near that power.
(.4a total extra on heater circuit running 4 KT88 verse the 4 EL34)
I'm assuming a .4a difference in heater from EL34 to KT88 shouldn't be a problem, also the amp
runs your typical VTL circuit, Paralell 12AT7 voltage amp cap coupled to a schmidt type LTP an
using a 12AT7 on out to the usual cap coupled outputs with the fixed bias mentioned.
(I have no schematic for the amp, just traced it out and the usual VTL stuff)
It uses 270k grid resistors for the EL34 circuit, if I go KT88 I'll lower those, also G3, pin 1 is tied
to pin 8 so it accepts a 7AC type pinout. It has 1K screen grid resistors and uses a PCB so I will
have to lift one end of the screen grid resistor to either go triode or feed another voltage source.
(nothing here difficult to do)
There isn't really any extra room inside this thing to try and say add another filter stage and drop
some B+ voltage, so I've all but given up altering the B+ (unless someone has a good idea there)
(there is plenty to feed the rest of the amp so dropping voltage wouldn't hurt anywhere though)
---------
Anywho am I off in thinking this original arrangement/voltage is just to stressful on the tubes here?
Any thoughts and or suggestions are welcome, (or any more info needed ask, I tried to give all)
Thanks in advance, (hope the post wasn't to long winded)
Keg
route to take so I thought I'd toss a question out for some possible ideas/thoughts from others.
Here is the scenario,
It's a VTL ST-85 running EL34's in pentode with 600V on both the plate and there SCREENS!
(outputs have no UL taps)
The owner was having an issue with bias and the controls either not getting there required bias
or if they did the pots were turned pretty much all the way with any tubes ever used in this amp,
and a channel not "sounding" quite right. I fixed the bias and the channel had an issue with the
tail resistor in the LTP being 30% off in one channel, fixed that as well.
The amp has gone through tubes quite a bit I'm told an it doesn't surprise me with that voltage.
(and the owner is going to buy a new quad when I get the amp settled, any suggestions?)
---------------
So I'm trying to figure out if I want to alter anything in the amp based on the voltage, the tubes
and the fact I'm not a giant fan of push pull pentode, unless say a nice regulated screen volts.
Options I have tossed around are, (if I would be making any changes)
(sticking with EL34's)
Wiring outputs in triode as some pdf's say 600v triode is max (I doubt any new tubes like it)
(would it be better then running pentode at that voltage, humm?)
Taping from the RC stage that feeds the phase splitters has 400v then use it for the screens.
(to stick with pentode operation but still drop the screen voltage down some)
Changing cathode resistors of 10ohms for sense to say 500-700 ohms and do a combo bias,
(which would drop my plate to cathode voltage a bit)
(going to KT88's)
Again wire the outputs in triode, up the cathode resistors to make up the bias voltage needed.
(which again would drop my plate to cathode voltage a bit)
Taping from the RC stage that feeds the phase splitters has 400v then use it for the screens.
(to stick with pentode operation but still drop the screen voltage down some)
(Or combos using any of the above together)
---------------
I can modify the bias circuit to get upto about 70v, the original ran 600v B+ and 30ma per tube.
(or mod it to get anything lower as well) (I may try calling VTL, see what power tranny handles)
This thing was rated for 85wpc running a pair of EL34's, do not need anywhere near that power.
(.4a total extra on heater circuit running 4 KT88 verse the 4 EL34)
I'm assuming a .4a difference in heater from EL34 to KT88 shouldn't be a problem, also the amp
runs your typical VTL circuit, Paralell 12AT7 voltage amp cap coupled to a schmidt type LTP an
using a 12AT7 on out to the usual cap coupled outputs with the fixed bias mentioned.
(I have no schematic for the amp, just traced it out and the usual VTL stuff)
It uses 270k grid resistors for the EL34 circuit, if I go KT88 I'll lower those, also G3, pin 1 is tied
to pin 8 so it accepts a 7AC type pinout. It has 1K screen grid resistors and uses a PCB so I will
have to lift one end of the screen grid resistor to either go triode or feed another voltage source.
(nothing here difficult to do)
There isn't really any extra room inside this thing to try and say add another filter stage and drop
some B+ voltage, so I've all but given up altering the B+ (unless someone has a good idea there)
(there is plenty to feed the rest of the amp so dropping voltage wouldn't hurt anywhere though)
---------
Anywho am I off in thinking this original arrangement/voltage is just to stressful on the tubes here?
Any thoughts and or suggestions are welcome, (or any more info needed ask, I tried to give all)
Thanks in advance, (hope the post wasn't to long winded)
Keg
1/ 600V on the screens of an EL34 is basically criminal. I went head to head with Luke Manley about this at a hifi show once, and he said "You have to use our specially selected tubes in our amps".
"And where in Europe can I get your special tubes?" I asked.
"From us in California" was the glib answer.
I say BS, design ratings are design ratings and I've also had to service many blown up VTL amps because of exactly this.
One solution I had was to change the amps to cathode bias, and triode operation. That made them bias stable and sounding better. I don't know if an EL34 is universally safely usable with 600V on the plates & screens in triode, but as you say, far safere than in pentode! And these did run for sevearl years at least, until I lost contact with their owner.
The best solution would be to convert the PS to choke input, dropping the B+ down to circa 400V, but there isn't enough space on a VTL to add a decent sized choke.
I feel for you, and can't believe how a company such a VTL can make such badly design stuff and still keep their elevated reputation.
Regards, Allen (Vacuum State)
"And where in Europe can I get your special tubes?" I asked.
"From us in California" was the glib answer.
I say BS, design ratings are design ratings and I've also had to service many blown up VTL amps because of exactly this.
One solution I had was to change the amps to cathode bias, and triode operation. That made them bias stable and sounding better. I don't know if an EL34 is universally safely usable with 600V on the plates & screens in triode, but as you say, far safere than in pentode! And these did run for sevearl years at least, until I lost contact with their owner.
The best solution would be to convert the PS to choke input, dropping the B+ down to circa 400V, but there isn't enough space on a VTL to add a decent sized choke.
I feel for you, and can't believe how a company such a VTL can make such badly design stuff and still keep their elevated reputation.
Regards, Allen (Vacuum State)
VTL mods
I have modified VTLs over the years. One of them was VTL 300W monoblocks that run on over 600v on eight 6550s per channel at 30ma idle bias current. The owner wanted to change them to el34s because they sound better in other amps that can use both types. He also wanted to run them in ultralinear connection.
I too am shocked that VTL routinely run over 200v in excess of the 6550 and el34 max screen voltage, but the concern here is how much power the screen can dissipate. It will operate at 600v but the average screen current needs to be limited at this high voltage for best reliability.
I came up with a solution that uses high voltage TO220 mosfets to lower the screen voltage to 320v, with the gates of the mosfets operating on 40% of the anode to B+ audio voltage, to simulate ultralinear operation.
The amp put out about 200w with this mod and sounded sweeter and more harmonically resolved. As far as I know the amp is still running fine ever since, over 6 years.
Also, check the input stage power supply capacitors, they go open circuit over time. When the amp is switched on, before the tubes warm up, the voltage goes way over the cap's voltage rating until the tubes warm up.
Hope this helps
I have modified VTLs over the years. One of them was VTL 300W monoblocks that run on over 600v on eight 6550s per channel at 30ma idle bias current. The owner wanted to change them to el34s because they sound better in other amps that can use both types. He also wanted to run them in ultralinear connection.
I too am shocked that VTL routinely run over 200v in excess of the 6550 and el34 max screen voltage, but the concern here is how much power the screen can dissipate. It will operate at 600v but the average screen current needs to be limited at this high voltage for best reliability.
I came up with a solution that uses high voltage TO220 mosfets to lower the screen voltage to 320v, with the gates of the mosfets operating on 40% of the anode to B+ audio voltage, to simulate ultralinear operation.
The amp put out about 200w with this mod and sounded sweeter and more harmonically resolved. As far as I know the amp is still running fine ever since, over 6 years.
Also, check the input stage power supply capacitors, they go open circuit over time. When the amp is switched on, before the tubes warm up, the voltage goes way over the cap's voltage rating until the tubes warm up.
Hope this helps
Bigwill,
That would be an option, there really isn't much room in this thing and
being a PCB adding in other circuits really isn't that easy, but I might..
Thanks for the suggestion..
---------
Allen I am seriously leaning the cathode bias triode deal, glad I'm not
only one thinking this is pretty insane on these tubes, I'd like to try an
keep say a small (like 5-10 volts) adjustable bias still to balance tubes.
And I think going to KT88's is another step in the right direction as well.
Thank you for weighing in with your experience..
That would be an option, there really isn't much room in this thing and
being a PCB adding in other circuits really isn't that easy, but I might..
Thanks for the suggestion..
---------
Allen I am seriously leaning the cathode bias triode deal, glad I'm not
only one thinking this is pretty insane on these tubes, I'd like to try an
keep say a small (like 5-10 volts) adjustable bias still to balance tubes.
And I think going to KT88's is another step in the right direction as well.
Thank you for weighing in with your experience..
ColinF
Thank you as well, and I to was concerned for those caps, with being a
solid state rectified amp yah that voltage ramps up there for a while till
things start conducting.
Would you happen to have a schematic/layout showing the type solution
you mention, it does peak my interest, I have to say..
Thank you as well, and I to was concerned for those caps, with being a
solid state rectified amp yah that voltage ramps up there for a while till
things start conducting.
Would you happen to have a schematic/layout showing the type solution
you mention, it does peak my interest, I have to say..
hey-Hey!!!,
Allen has it well thought out. The ground-cathode voltage to idle a KT88 at 35-40 mA with B+ around 600V is going to be 60-65V I'd bet...which puts you down around 540 plate to cathode...a useful step.
You can burn even more B+ with the Blumlein Garter bias/balance circuit in the cathodes...even more useful I think. Balance, and effectively dropping B+.
He's also right about the criminality of the whole exercise...somebody needs their doo-dads put in a vise for that sort of practice, let alone for thinking it is a fine idear.
cheers,
Douglas
Allen has it well thought out. The ground-cathode voltage to idle a KT88 at 35-40 mA with B+ around 600V is going to be 60-65V I'd bet...which puts you down around 540 plate to cathode...a useful step.
You can burn even more B+ with the Blumlein Garter bias/balance circuit in the cathodes...even more useful I think. Balance, and effectively dropping B+.
He's also right about the criminality of the whole exercise...somebody needs their doo-dads put in a vise for that sort of practice, let alone for thinking it is a fine idear.
cheers,
Douglas
Hi
Sorry, I just saw this post, maybe a little late ?
What I would do with this amp is decrease the screens voltage to 450 or 500V max, preferably 450V
This way, the primary reflected impedance by the OT will stay good with the circuit. If you use a triode connection, it will be not very optimal ...
Secondly I'd put out the 10 ohms cathode resistor, and put the cathode at ground potential !! If you actually have the g3 at ground, which I suppose is the case, it's a very bad thing, because even if the positive voltage is very small on the cathodes, it's positive relative to g3, and it's really not good for the tube, especially at high voltages ... This mod will make biasing a little longer, but personnaly I bias with a scope and my ears, so it doesn't change anything
And third I'd put out the g3 from ground, which is the case I suppose ? and connect the g3 to the bias supply (before the bias resistors)
And finally rebuild the bias supply
This way is I think the safest one to use EL34 on high voltages, and you will be surprised how robust the amp will become, the tubes too !!
Best regards
Sorry, I just saw this post, maybe a little late ?
What I would do with this amp is decrease the screens voltage to 450 or 500V max, preferably 450V
This way, the primary reflected impedance by the OT will stay good with the circuit. If you use a triode connection, it will be not very optimal ...
Secondly I'd put out the 10 ohms cathode resistor, and put the cathode at ground potential !! If you actually have the g3 at ground, which I suppose is the case, it's a very bad thing, because even if the positive voltage is very small on the cathodes, it's positive relative to g3, and it's really not good for the tube, especially at high voltages ... This mod will make biasing a little longer, but personnaly I bias with a scope and my ears, so it doesn't change anything
And third I'd put out the g3 from ground, which is the case I suppose ? and connect the g3 to the bias supply (before the bias resistors)
And finally rebuild the bias supply
This way is I think the safest one to use EL34 on high voltages, and you will be surprised how robust the amp will become, the tubes too !!
Best regards
Thanks everyone..
I don't have an easy way in this amp with it being a PCB to do much with the
screen voltage, and G3 is tied in the PCB, also it's a customers/friends amp so
using a scope to bias it won't really work here either.
----------
So yah the cathode bias and KT88 is the order of the day here.
Called VTL and as most would expect no support to address any issues nor give
any info to the heater amperage or high voltage amperage ratings on the tranny.
I don't have an easy way in this amp with it being a PCB to do much with the
screen voltage, and G3 is tied in the PCB, also it's a customers/friends amp so
using a scope to bias it won't really work here either.
----------
So yah the cathode bias and KT88 is the order of the day here.
Called VTL and as most would expect no support to address any issues nor give
any info to the heater amperage or high voltage amperage ratings on the tranny.
It's workin..
I'll post all the changes once I'm finalized in case anyone may find the info useful.
But running KT88's and 6550's I have roughly 60v on the cathodes for 46ma per tube,
I "massaged" the bias circuit to get about -8.5v with a range of about 2v, you can see
the mixed bag of outputs and I can balance each side within it's pairing dead nuts on.
I've got to up the cathode bypass caps and baring any disasters I think I'm done here.
I'll post all the changes once I'm finalized in case anyone may find the info useful.
But running KT88's and 6550's I have roughly 60v on the cathodes for 46ma per tube,
I "massaged" the bias circuit to get about -8.5v with a range of about 2v, you can see
the mixed bag of outputs and I can balance each side within it's pairing dead nuts on.
I've got to up the cathode bypass caps and baring any disasters I think I'm done here.
Attachments
Bozole,
You wrote several rather amazing things in your post above, and I'm wondering if you are willing to expand on your ideas?
>>Sorry, I just saw this post, maybe a little late ?
What I would do with this amp is decrease the screens voltage to 450 or 500V max, preferably 450V This way, the primary reflected impedance by the OT will stay good with the circuit. If you use a triode connection, it will be not very optimal ...<<
Not my experience. I have converted many pentode amps to triode, and find the OPT works just fine.
>Secondly I'd put out the 10 ohms cathode resistor, and put the cathode at ground potential !! If you actually have the g3 at ground, which I suppose is the case, it's a very bad thing<<
Why is it "a very bad thing" to have the cathode a few hundred millivolts more positive than g3?
>> because even if the positive voltage is very small on the cathodes, it's positive relative to g3, and it's really not good for the tube, especially at high voltages ...<<
Again, why?
>This mod will make biasing a little longer<<
A little longer, would actually make it almost impossible for a regualr enthusiast with a multimeter but nothing else.
> but personnaly I bias with a scope and my ears, so it doesn't change anything<
Very interested in knowing how to bias an amp with a scope - sure doesn't tell you how close you are getting to the tube's maximums IMO.
>>And third I'd put out the g3 from ground, which is the case I suppose ? and connect the g3 to the bias supply (before the bias resistors) <<
So while it's dangerous to have few millivolts of negative on g3, it's OK to have 50-100V on it, with respect to the cathode...please explain why?
>>And finally rebuild the bias supply<<
Why?
>>This way is I think the safest one to use EL34 on high voltages, and you will be surprised how robust the amp will become, the tubes too !!<<
I'm not knocking your statement here, it's just that I've been working with tube amps for 40 years now and these are very different ideas to what I know works.
Regards, Allen (vacuum State)
You wrote several rather amazing things in your post above, and I'm wondering if you are willing to expand on your ideas?
>>Sorry, I just saw this post, maybe a little late ?
What I would do with this amp is decrease the screens voltage to 450 or 500V max, preferably 450V This way, the primary reflected impedance by the OT will stay good with the circuit. If you use a triode connection, it will be not very optimal ...<<
Not my experience. I have converted many pentode amps to triode, and find the OPT works just fine.
>Secondly I'd put out the 10 ohms cathode resistor, and put the cathode at ground potential !! If you actually have the g3 at ground, which I suppose is the case, it's a very bad thing<<
Why is it "a very bad thing" to have the cathode a few hundred millivolts more positive than g3?
>> because even if the positive voltage is very small on the cathodes, it's positive relative to g3, and it's really not good for the tube, especially at high voltages ...<<
Again, why?
>This mod will make biasing a little longer<<
A little longer, would actually make it almost impossible for a regualr enthusiast with a multimeter but nothing else.
> but personnaly I bias with a scope and my ears, so it doesn't change anything<
Very interested in knowing how to bias an amp with a scope - sure doesn't tell you how close you are getting to the tube's maximums IMO.
>>And third I'd put out the g3 from ground, which is the case I suppose ? and connect the g3 to the bias supply (before the bias resistors) <<
So while it's dangerous to have few millivolts of negative on g3, it's OK to have 50-100V on it, with respect to the cathode...please explain why?
>>And finally rebuild the bias supply<<
Why?
>>This way is I think the safest one to use EL34 on high voltages, and you will be surprised how robust the amp will become, the tubes too !!<<
I'm not knocking your statement here, it's just that I've been working with tube amps for 40 years now and these are very different ideas to what I know works.
Regards, Allen (vacuum State)
600 volt screens on EL34 whoa! That is just nuts.
Apart from the long term life of the valves. What about the drive requirement of the EL34 with such a high screen voltage?!? So in one stroke you remove one of the major advantages of pentode outputs stages and run you valves into the ground.
Pentode output amps get lots of bad press - some justified some not - one of the reasons is the screens are often run at plate voltage or some case higher then the plate voltage (that is the plate is lower due to the resistance of the output transformer.) The screens should be not just be lower then the plate but in some cases much lower - in short picking the right screen voltage you can get lower distortion and also low drive requirements.
600 volt screens is just awful lazy design work. God the average DIYer could do better.
Cheers,
Anthony
Apart from the long term life of the valves. What about the drive requirement of the EL34 with such a high screen voltage?!? So in one stroke you remove one of the major advantages of pentode outputs stages and run you valves into the ground.
Pentode output amps get lots of bad press - some justified some not - one of the reasons is the screens are often run at plate voltage or some case higher then the plate voltage (that is the plate is lower due to the resistance of the output transformer.) The screens should be not just be lower then the plate but in some cases much lower - in short picking the right screen voltage you can get lower distortion and also low drive requirements.
600 volt screens is just awful lazy design work. God the average DIYer could do better.
Cheers,
Anthony
>>600 volt screens is just awful lazy design work.<<
Oh, no, no. Luke Manley himself told me it was perfectly fine, and how could he be wrong? He's certainly sold a lot more amps than all of us on this board put together.
You just have to use special VTL EL34's only obtainable from the factory in California...
Like as if myself in Europe, or you in Oz have instant access to such special devices. Plug in a set of JJ's and you'd most likely have a fire on your hands!
Regards, Allen (Vacuum State)
Oh, no, no. Luke Manley himself told me it was perfectly fine, and how could he be wrong? He's certainly sold a lot more amps than all of us on this board put together.
You just have to use special VTL EL34's only obtainable from the factory in California...
Like as if myself in Europe, or you in Oz have instant access to such special devices. Plug in a set of JJ's and you'd most likely have a fire on your hands!
Regards, Allen (Vacuum State)
"and how could he be wrong?"
I'm all for making your own mark in amp design and not following fashion. So normally an amp design that upsets the status quo I would be all for... but running tubes like this makes little to no sense. This is an amp meant to be sold to people who are not DIYers...nuts.
A friend had for many years a pair of VTL Tiny Tridoes and must say they sounded really nice, not perfect but they had many great qualities.....but two things always struck me about the amps; 20 watts out of a 4 EL84 in triode mode means they must be running really heavy into class AB but also the amps ran really hot which made a joke out of the Class AB bit of things. (They ran so hot you really couldn't touch them.) The transformers were so small it made almost no sense that this was a 20 watt hifi amp that cost serious money.
I think this all brings up a problem with valves - which is also wonderful - they are tough beasts that can be mistreated and still sound good....well when compared to sand not treated with kid gloves.
Cheers,
Anthony
I'm all for making your own mark in amp design and not following fashion. So normally an amp design that upsets the status quo I would be all for... but running tubes like this makes little to no sense. This is an amp meant to be sold to people who are not DIYers...nuts.
A friend had for many years a pair of VTL Tiny Tridoes and must say they sounded really nice, not perfect but they had many great qualities.....but two things always struck me about the amps; 20 watts out of a 4 EL84 in triode mode means they must be running really heavy into class AB but also the amps ran really hot which made a joke out of the Class AB bit of things. (They ran so hot you really couldn't touch them.) The transformers were so small it made almost no sense that this was a 20 watt hifi amp that cost serious money.
I think this all brings up a problem with valves - which is also wonderful - they are tough beasts that can be mistreated and still sound good....well when compared to sand not treated with kid gloves.
Cheers,
Anthony
Hi Allen Wright
Sorry to disturb you with my post, I'm going to explain my views, in the same order as you asked.
We speak about EL34 tubes, just EL34's
When you use a pentode as a triode, the optimal load impedance, considering the datasheet, is not the same in triode mode with same voltages as in pentode mode. It's not a big difference, but it is some, that's why I said it was not optimal. But it will work, you're right
For the cathode a few hundred milivolts more positive than g3, I don't think it's a problem with all tubes in all voltage conditions. But I think it can be one with actually poor EL34's used at high voltages (the old one were more robust ...), because this tube is more fragile in that way. The advantage of a negative voltage on g3 is that that way the g3's effectiveness is increased, efficiency is improved (in term of plate current vs grid voltage), linearity too, and finally reliability too : if one day you have a bias failure, the negative g3 will limit the uncontrolled anode current to sufficient safe levels ... I learned that from Kevin O'Connors writes, and others Internet sites, and in my experience it's perfectly right ...
For the bias setting, yes I use the scope and my ears, it's my preference, sorry for that. And after that I always verify that I'm not out of safe settings for the tubes using the shunt method, and I'm never out of safe settings, so how more bad is the scope method that another ?
That way, I find some advantages : first of all, I see the amp in action, at big power, and it may be very instructive to find some problems on the amp when it's in use condition (oscillations, bad coupling cap causing distorsion when the drive signal increases etc ...)
What do you do ? you use the 70% common rule of tube plate power dissipation to bias your class AB amp ? It's not always the spot point, sorry !! sometimes higher, sometimes less, depending on "how much" you go in class B regarding how much you stay in class A (depending on the OT's reflected primary impedance : if you change it, don't you think you will must change the 70% rule for biasing the amp ?).
And finally rebuild the bias supply (if necessary) because when you tie the g3 to bias supply, the tube is "more efficient", in that sense that at same bias voltage, you'll have more plate current, so you must be able to put the bias voltage a little more negative ...
Sorry not to have the same opinions as your's, that's life
Best regards
Sorry to disturb you with my post, I'm going to explain my views, in the same order as you asked.
We speak about EL34 tubes, just EL34's
When you use a pentode as a triode, the optimal load impedance, considering the datasheet, is not the same in triode mode with same voltages as in pentode mode. It's not a big difference, but it is some, that's why I said it was not optimal. But it will work, you're right
For the cathode a few hundred milivolts more positive than g3, I don't think it's a problem with all tubes in all voltage conditions. But I think it can be one with actually poor EL34's used at high voltages (the old one were more robust ...), because this tube is more fragile in that way. The advantage of a negative voltage on g3 is that that way the g3's effectiveness is increased, efficiency is improved (in term of plate current vs grid voltage), linearity too, and finally reliability too : if one day you have a bias failure, the negative g3 will limit the uncontrolled anode current to sufficient safe levels ... I learned that from Kevin O'Connors writes, and others Internet sites, and in my experience it's perfectly right ...
For the bias setting, yes I use the scope and my ears, it's my preference, sorry for that. And after that I always verify that I'm not out of safe settings for the tubes using the shunt method, and I'm never out of safe settings, so how more bad is the scope method that another ?
That way, I find some advantages : first of all, I see the amp in action, at big power, and it may be very instructive to find some problems on the amp when it's in use condition (oscillations, bad coupling cap causing distorsion when the drive signal increases etc ...)
What do you do ? you use the 70% common rule of tube plate power dissipation to bias your class AB amp ? It's not always the spot point, sorry !! sometimes higher, sometimes less, depending on "how much" you go in class B regarding how much you stay in class A (depending on the OT's reflected primary impedance : if you change it, don't you think you will must change the 70% rule for biasing the amp ?).
And finally rebuild the bias supply (if necessary) because when you tie the g3 to bias supply, the tube is "more efficient", in that sense that at same bias voltage, you'll have more plate current, so you must be able to put the bias voltage a little more negative ...
Sorry not to have the same opinions as your's, that's life
Best regards
Bozole,
I think Allen was looking at the amp from the standpoint of customer and tech.
(while also wondering about your suggestions)
As techs we should think in terms of what is in the best interest of the owner here.
(quite often means less major changes the better, and keep things simple)
The amp was brought in with problems, obviously means the owner does not know
much about amps or how to fix them, making radical changes outside the "norm" to
where the owner won't understand them or be able to check there own amp wouldn't
be advisable, if they have problems in the future it makes much more difficult to fix an
to explain to someone why and what was done.
Some of your suggestions may very well have merit, but in this context not what most
would suggest as being "proper" for the case at hand.
If this was a Diy/Scratch built amp by the designer then anything is fair game, but one
that is being repaired for someone usually it is best to stick with that more "standard"
approach/methods and hopefully improve the amp at the same time.
I think Allen was looking at the amp from the standpoint of customer and tech.
(while also wondering about your suggestions)
As techs we should think in terms of what is in the best interest of the owner here.
(quite often means less major changes the better, and keep things simple)
The amp was brought in with problems, obviously means the owner does not know
much about amps or how to fix them, making radical changes outside the "norm" to
where the owner won't understand them or be able to check there own amp wouldn't
be advisable, if they have problems in the future it makes much more difficult to fix an
to explain to someone why and what was done.
Some of your suggestions may very well have merit, but in this context not what most
would suggest as being "proper" for the case at hand.
If this was a Diy/Scratch built amp by the designer then anything is fair game, but one
that is being repaired for someone usually it is best to stick with that more "standard"
approach/methods and hopefully improve the amp at the same time.
Hi kegger
I understand his point of view, no problem, really !! And your's too, believe me.
But what I suggested here in this case is what I personnaly would have suggested to the customer (even if it implies mods, but I don't think really more than triode operation, lower the scrrens voltage etc ...), just because I think it's the mod that would keep his amp sound the most similar to the way it sounded originally (if your customer likes this amp), and fix definitively the bad reliability and the tubes consommation ...
Putting the stage in triode mode will make a real difference in sound, putting KT88's too, but that's just my opinion.
Have a good day, best regards
I understand his point of view, no problem, really !! And your's too, believe me.
But what I suggested here in this case is what I personnaly would have suggested to the customer (even if it implies mods, but I don't think really more than triode operation, lower the scrrens voltage etc ...), just because I think it's the mod that would keep his amp sound the most similar to the way it sounded originally (if your customer likes this amp), and fix definitively the bad reliability and the tubes consommation ...
Putting the stage in triode mode will make a real difference in sound, putting KT88's too, but that's just my opinion.
Have a good day, best regards
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