Quite fancy turning Rocky into a PSE amp. Maybe get a few more watts.
The question is ...
Can i do this without changing the output transformers which are currently 5K? If it is possible, what needs doing?
[/img]Going PSE.
#1 Going PSE.
Right chaps.
Quite fancy turning Rocky into a PSE amp. Maybe get a few more watts.
The question is ...
Can i do this without changing the output transformers which are currently 5K? If it is possible, what needs doing?
[/img]
Quite fancy turning Rocky into a PSE amp. Maybe get a few more watts.
The question is ...
Can i do this without changing the output transformers which are currently 5K? If it is possible, what needs doing?
[/img]-
Cressy Snr
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#2
Hi Al
All I can say is I tried going PSE with the exact same circuit as you have. trouble was that although it sounded a bit more meaty, there was an accompanying hardness that pervaded the sound. This was possibly due to the high standing current being drawn by the four valves in single ended mode saturating the Hammond OPTXs I was using.
You'd need to rebias the EL34s for less current by upping the values of the cathode resistors somewhat. You'd also need to use closely matched valves to avoid one of a pair hogging all the current. Separate cathode resistors would also be a good idea to prevent current hogging by one valve.
The output resistance of a pair of valves is half that of a single one, so to maintain power you'd have to halve the impedance reflected through the output transformer by deliberately mismatching the secondary, ie connecting up the secondary as if you were using 16 ohm speakers. This lowers the turns ratio and lowers the reflected impedance from 5K to 2K5, matching the halved output resistance of the pair of valves.
My PSE Rocky didn't last long. I had much better luck by converting the output to ultralinear single ended.
If you have 43% ultralinear taps on your output transformers you can get a very useful increase in power by removing the 330R resistor connecting the screen grid to the anode and connecting the ultralinear tap from the OPTX to the screen grid instead.
You have to be careful with doing this that the valve can stand the full HT sitting on its screen grid. However, the EL34 was designed with this mode of operation in mind so it will not be a problem.
Using ultralinear the EL34 will not be a triode but neither will it be a pentode. Ultralinear is designed to keep the benefits of the benign distortion characteristic of a triode whilst having some of the power capability of the pentode.
All I can say is I tried going PSE with the exact same circuit as you have. trouble was that although it sounded a bit more meaty, there was an accompanying hardness that pervaded the sound. This was possibly due to the high standing current being drawn by the four valves in single ended mode saturating the Hammond OPTXs I was using.
You'd need to rebias the EL34s for less current by upping the values of the cathode resistors somewhat. You'd also need to use closely matched valves to avoid one of a pair hogging all the current. Separate cathode resistors would also be a good idea to prevent current hogging by one valve.
The output resistance of a pair of valves is half that of a single one, so to maintain power you'd have to halve the impedance reflected through the output transformer by deliberately mismatching the secondary, ie connecting up the secondary as if you were using 16 ohm speakers. This lowers the turns ratio and lowers the reflected impedance from 5K to 2K5, matching the halved output resistance of the pair of valves.
My PSE Rocky didn't last long. I had much better luck by converting the output to ultralinear single ended.
If you have 43% ultralinear taps on your output transformers you can get a very useful increase in power by removing the 330R resistor connecting the screen grid to the anode and connecting the ultralinear tap from the OPTX to the screen grid instead.
You have to be careful with doing this that the valve can stand the full HT sitting on its screen grid. However, the EL34 was designed with this mode of operation in mind so it will not be a problem.
Using ultralinear the EL34 will not be a triode but neither will it be a pentode. Ultralinear is designed to keep the benefits of the benign distortion characteristic of a triode whilst having some of the power capability of the pentode.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
#3
Thanks Steve.
Good explanation.
Sadly i don't have ultralinear taps, so it looks as though this idea will be filed away for the time being.
Maybe i'll do something with the 6J5 front end instead.
Sometimes this amp just runs out of steam a little, especially through open baffles.
Cheers.
Good explanation.
Sadly i don't have ultralinear taps, so it looks as though this idea will be filed away for the time being.
Maybe i'll do something with the 6J5 front end instead.
Sometimes this amp just runs out of steam a little, especially through open baffles.
Cheers.
#4
Hi Al
I'm mystified by your 'runs out of steam' description. I say mystified because I can only ever run son of rocky at half max input...there is always loads more headroom to spare. I get much the same result with ariels and vofos.
Not sure its of any use but what size room are you in and what is the efficiency on the drivers???
Ed
I'm mystified by your 'runs out of steam' description. I say mystified because I can only ever run son of rocky at half max input...there is always loads more headroom to spare. I get much the same result with ariels and vofos.
Not sure its of any use but what size room are you in and what is the efficiency on the drivers???
Ed
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pre65
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#5
Hi Al-what are you using for the pre-amp and is the volume pot shunted ?
The only thing necessary for the triumph of evil is for good men to do nothing.
Edmund Burke
G-Popz THE easy listening connoisseur. (Philip)
Edmund Burke
G-Popz THE easy listening connoisseur. (Philip)
#6
Ed and Phil.
I'm using 96dB drivers in open baffles. Losing a bit of efficiency here compared wth MLTLs, but not a huge amount.
Room size is 22' x 11'6 x7'6.
CD players straight into the amp with a 50k shunted pot which didn't affect the gain noticeably, after doing the mod.
Volume levels are acceptable with the knob at just below half its travel. But theres not very much extra volume to be had after this.(Before or after shunt mod).
I've been playing around with tube output stages, and depending on the CD player used, i've either got loads of gain or not enough. But thats something i can sort out.
I always got the impression that Rocky should have a bit more in reserve than it has.
I'm using 96dB drivers in open baffles. Losing a bit of efficiency here compared wth MLTLs, but not a huge amount.
Room size is 22' x 11'6 x7'6.
CD players straight into the amp with a 50k shunted pot which didn't affect the gain noticeably, after doing the mod.
Volume levels are acceptable with the knob at just below half its travel. But theres not very much extra volume to be had after this.(Before or after shunt mod).
I've been playing around with tube output stages, and depending on the CD player used, i've either got loads of gain or not enough. But thats something i can sort out.
I always got the impression that Rocky should have a bit more in reserve than it has.
#7
Hi Al
Your speakers are much the same as mine so I don't think thats the area, but your room is a good deal bigger than mine(18x10-12 low ceiling), which might explain the difference. Fwiw my circuit, particularly the driver, is completely different but looking at your circuit I'd guess youve got more gain than me anyway.....
doesn't look like I can do anything but waffle, sorry.
Ed
You could check out the circuit on my website, it may offer some clue to squeezing a bit more out.
Your speakers are much the same as mine so I don't think thats the area, but your room is a good deal bigger than mine(18x10-12 low ceiling), which might explain the difference. Fwiw my circuit, particularly the driver, is completely different but looking at your circuit I'd guess youve got more gain than me anyway.....
doesn't look like I can do anything but waffle, sorry.
Ed
You could check out the circuit on my website, it may offer some clue to squeezing a bit more out.
-
Cressy Snr
- Amstrad Tower of Power
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- Joined: Wed May 30, 2007 12:25 am
- Location: South Yorks.
#8
What you need to do if you want more drive is to up the HT on the driver stage to around 500V. This will give a much greater voltage swing into the EL34s than you have at the moment.
EL34s are pretty powerful tubes, and knowing a bit more now, than I did when I first put the circuit together, I would suspect that your drivers are not swinging enough voltage to drive the EL34s to the max power they are capable of, within the limits of your available HT. The fact that nothing happens after you turn the volume beyond half way lends further evidence to this hypothesis.
The *SN7/J5 family of valves thrive on being pushed hard and give a dynamic, smooth sound with low distortion when being kicked along with a high HT.
Because the valves are only small signal types you might get away with taking a branch off the power supply and using a voltage doubler to give the necessary HT for the drivers.
You'd have to recalculate the anode load and cathode resistor on the first valve and the cathode resistors on the cathode follower to match the new HT but it will be well worth it for the extra swing you'd get.
I think Nick used a voltage doubler in his 300B amp somewhere so he might be able to help with a suitable circuit. MJ, in his book, also gives a circuit for a Cockroft Walton voltage doubler using solid state diodes .
PS you will need to be using the GT GTA or GTB versions of the 6J5 These have a Va (max) of around 400V.
Steve.
EL34s are pretty powerful tubes, and knowing a bit more now, than I did when I first put the circuit together, I would suspect that your drivers are not swinging enough voltage to drive the EL34s to the max power they are capable of, within the limits of your available HT. The fact that nothing happens after you turn the volume beyond half way lends further evidence to this hypothesis.
The *SN7/J5 family of valves thrive on being pushed hard and give a dynamic, smooth sound with low distortion when being kicked along with a high HT.
Because the valves are only small signal types you might get away with taking a branch off the power supply and using a voltage doubler to give the necessary HT for the drivers.
You'd have to recalculate the anode load and cathode resistor on the first valve and the cathode resistors on the cathode follower to match the new HT but it will be well worth it for the extra swing you'd get.
I think Nick used a voltage doubler in his 300B amp somewhere so he might be able to help with a suitable circuit. MJ, in his book, also gives a circuit for a Cockroft Walton voltage doubler using solid state diodes .
PS you will need to be using the GT GTA or GTB versions of the 6J5 These have a Va (max) of around 400V.
Steve.
Last edited by Cressy Snr on Mon Nov 19, 2007 1:20 pm, edited 2 times in total.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
#9
Its simple enough, tell PSD to use one to give you the ideaI think Nick used a voltage doubler in his 300B amp somewhere so he might be able to help with a suitable circuit.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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andrew Ivimey
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#11
Hear here about driving 6SN7s hard and of course voltage doubling can do the trick, as Mr Ramsey was enthusing, early summer - so I did just that! works too.
I'm also using a voltage doubler on some small trafos to drive 6B4gsPP output = 300volts out of 150 source. The amp is in the shed and it is pouring down but if you want, I'll go and have a look and draw it out for you. It is a version of what MJ proposes, if I remember. I don't like voltage doubling but needs must ....
I'm also using a voltage doubler on some small trafos to drive 6B4gsPP output = 300volts out of 150 source. The amp is in the shed and it is pouring down but if you want, I'll go and have a look and draw it out for you. It is a version of what MJ proposes, if I remember. I don't like voltage doubling but needs must ....
#12
No Andrew, i don't want you getting soaked.
There are plenty of voltage multipliers around on the web.
Anyway i may not need one........... i still have the power supply from a Maplin amp. Not state of the art but a useful 498v if i remember correctly. Hopefully, it will do to test the theory.
Got a couple more projects to sort out first, and since my productivity rate is well below yours, this could take some time.
There are plenty of voltage multipliers around on the web.
Anyway i may not need one........... i still have the power supply from a Maplin amp. Not state of the art but a useful 498v if i remember correctly. Hopefully, it will do to test the theory.
Got a couple more projects to sort out first, and since my productivity rate is well below yours, this could take some time.
#13
I decided to make a start on beefing up this Rocky amp with a higher B+.
I thought i should practice some load lines using the existing configuration, which is.....
B+ =334v, Plate voltage = 234v, with 22k resistor dropping 100v.
Should i take into account the effect of the grid resistor (1meg) on the second 6j5? Apparently this is in parallel with the plate resistor.
Bearing in mind this is fairly new to me, is this the best way of arriving at new component values?
I thought i should practice some load lines using the existing configuration, which is.....
B+ =334v, Plate voltage = 234v, with 22k resistor dropping 100v.
Should i take into account the effect of the grid resistor (1meg) on the second 6j5? Apparently this is in parallel with the plate resistor.
Bearing in mind this is fairly new to me, is this the best way of arriving at new component values?
#14
I wouldn't worry, its far less than 10% of the anode resistor.Should i take into account the effect of the grid resistor (1meg) on the second 6j5
Can I suggest if you are unsure why its in parallel, that would be worth spending a bit of time, as I think it will help your understanding.
Hint: think of it from the perspective of the anode.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
#15
Well Nick, i have to confess that i still can't see why it should be in parallel.
Best to admit stupidity when it occurs.
There again i have been quite busy.
Before i built the Lampizator valve output thing. I did a Google for SRPP, in order to find out what its all about.
One thing that came to light was a SRPP stage designed for 6J5 tubes. I printed it off and filed it away.
Anyway, the inevitable has happened and Rocky now has a SRPP input/driver section, making use of the full 340v.
This has gone gone some way to solving the problems which prompted me to begin this thread.
I now have a bit more gain, more use of the volume control and most importantly it now sounds like the speakers are working instead of idling.
The overall sound is now faster and more incisive. Difficult for me to describe but sometimes it sounds alarmingly different.
Question is......... will i get to like it?
Of course i also had my ears syringed last week so maybe thats got something to do with it.
Best to admit stupidity when it occurs.
There again i have been quite busy.
Before i built the Lampizator valve output thing. I did a Google for SRPP, in order to find out what its all about.
One thing that came to light was a SRPP stage designed for 6J5 tubes. I printed it off and filed it away.
Anyway, the inevitable has happened and Rocky now has a SRPP input/driver section, making use of the full 340v.
This has gone gone some way to solving the problems which prompted me to begin this thread.
I now have a bit more gain, more use of the volume control and most importantly it now sounds like the speakers are working instead of idling.
The overall sound is now faster and more incisive. Difficult for me to describe but sometimes it sounds alarmingly different.
Question is......... will i get to like it?
Of course i also had my ears syringed last week so maybe thats got something to do with it.