Hi all,
I'm trying to build a PSE amp using two paralleled KT88 for each channel, driven by a 12AX7 with the help of a PowerDrive circuit (Thanks to TubeLab).
I have a lot of surplus transformers, but no one with a suitable voltage for the KT88s heaters (6.3V).
I have instead some 12V transformers, so my thought was: can I place the two KT88 heaters of each channel in series (in a string)? This should give me a 12V string, and I could use some resistor, in parallel with each heater, to equalize the two voltages across the two heaters themselves.
Is this feasible?
Thanks,
Giovanni
I'm trying to build a PSE amp using two paralleled KT88 for each channel, driven by a 12AX7 with the help of a PowerDrive circuit (Thanks to TubeLab).
I have a lot of surplus transformers, but no one with a suitable voltage for the KT88s heaters (6.3V).
I have instead some 12V transformers, so my thought was: can I place the two KT88 heaters of each channel in series (in a string)? This should give me a 12V string, and I could use some resistor, in parallel with each heater, to equalize the two voltages across the two heaters themselves.
Is this feasible?
Thanks,
Giovanni
Obviously you can! Better would be to rectify the heater supply and regulate it to 12.6V precisely. Or better yet, with a CCS as I've seen. But this may be overkill... I think that simply putting them in series without any resistors and tricks will do good.
Btw, Giovanni, are you a member of some italian forums? I don't remember to have seen your nick anywhere else.
Edit: could you provide a schematic of your amp? Are you driving the KT88 in A2 with the powerdrive?
Btw, Giovanni, are you a member of some italian forums? I don't remember to have seen your nick anywhere else.
Edit: could you provide a schematic of your amp? Are you driving the KT88 in A2 with the powerdrive?
Yes, I have a matched quartet from JJ Tesla, coming from the same lot of production (at least this is what they told me...).
Giaime:
I have no schematic of my amp yet, I'm still trying different configurations and building some parts before to put all togheter.
However my amp will be a PSE (two KT88 for each channel), with para-feed output topology feed by a SS CCS, a PowerDrive to drive the two KT88s (modified in manner it can equalize continuously the current into the two paralleled tubes) and a not-yet-decided input stage (my favorite input stage at the moment is a long tail with a CCS).
The KT88s cathodes will be connected to ground, while the PowerDrive will provide the needed negative bias voltage.
Some sort of local feedback will be added, from the output stage to the input one (no feedback from the OPT, due to phase shift).
The B+ will be about 600V, regulated by an high voltage mosfet.
The PowerDrive will be powered by two voltage (Bias+ and Bias-), of about 24V and -120V (both of them regulated with mosfets).
This should permit me to drive the KT88's grid from about +20V to -80V, going into A2 zone.
I have built the four CCS (two with a current of about 5mA -input stage-, the others two with a current of about 150mA -output stage-).
At now I'm building the power supply, next will be the two modified PowerDrive boards and, at the end, the amp itself.
Ciao,
Giovanni
Rich if you know of any on diyAudio.com please let us know their whereabouts and how you detected them.
Although I'm implying Gaime got this on his system...(I don't want a fight with a nice fella) but the attachment http://www.danasoft.com/sig/Croccodillo.jpg contained a bug that upset my computer clock. If I'm wrong Ill take it all back but the forum mail was the only one I had in the "in box". I had opened other diyaudio threads, perhaps the villan is elsewhere.
richj
richj
Rich,
I can see you are not blaming Giaime or any particular person, but are suffering some indignation from having your system compromised.
Presumably you are thinking about the "wmf flaw", which was not patched at the time of the posts. I don't see what anybody (apart from Microsoft) could do about this, if indeed there was malicious code in the file. Searching for "Danasoft" and "wmf" or "exploit" returns no pertinent results in Google, so I doubt there was.
External and embedded web links do have a potential for misuse, but the buck must stop with the receiving system. No system is invulnerable, but some are a lot better than others.
I can see you are not blaming Giaime or any particular person, but are suffering some indignation from having your system compromised.
Presumably you are thinking about the "wmf flaw", which was not patched at the time of the posts. I don't see what anybody (apart from Microsoft) could do about this, if indeed there was malicious code in the file. Searching for "Danasoft" and "wmf" or "exploit" returns no pertinent results in Google, so I doubt there was.
External and embedded web links do have a potential for misuse, but the buck must stop with the receiving system. No system is invulnerable, but some are a lot better than others.
Oh well apologies for the glitch... unwanted distraction from 88'topic.......some things are virtually beyond our means. However we all have to be on guard.
Back to the series heater. I spotted an abnomally with the 6550 series......not all vendors have identical hot resistances. I've some drawing 1.3A and some 1.8A at 6.3V.
Do the gizmo with ohms law and you will find at 12.6V one stews and the other runs a chill beyond the lower 10% tol..
richj
Back to the series heater. I spotted an abnomally with the 6550 series......not all vendors have identical hot resistances. I've some drawing 1.3A and some 1.8A at 6.3V.
Do the gizmo with ohms law and you will find at 12.6V one stews and the other runs a chill beyond the lower 10% tol..
richj
Richwalters:
I apologize if my "signature" has caused problems to you... But as far as I know Danasoft and its products (my signature is one of them) is a secure software house.
And my firewall/antivirus/antispam server does not detect anything from it.
I never heard too.
I'm a newbie, I'm trying to build my first amp.
The output power is not so high, I think I will have not more than 10-15W... I do not need a lot of power, and I do prefer to run the tubes well below their limits in manner to have lower distorsion.
Ciao,
Giovanni
I apologize if my "signature" has caused problems to you... But as far as I know Danasoft and its products (my signature is one of them) is a secure software house.
And my firewall/antivirus/antispam server does not detect anything from it.
I never heard too.
I'm a newbie, I'm trying to build my first amp.
The output power is not so high, I think I will have not more than 10-15W... I do not need a lot of power, and I do prefer to run the tubes well below their limits in manner to have lower distorsion.
Ciao,
Giovanni
If you want long tube life then forget about class A2, please!
You'll get about 10W in triode mode, that's the best way to use such big tetrodes. Or go ultralinear, or go in tetrode mode BUT only with NFB and regulated screen supply.
IMHO the simplest (and best sounding) configuration is plain triode, g2 connected to the plate.
You'll get about 10W in triode mode, that's the best way to use such big tetrodes. Or go ultralinear, or go in tetrode mode BUT only with NFB and regulated screen supply.
IMHO the simplest (and best sounding) configuration is plain triode, g2 connected to the plate.
Well,
THIS is a preliminary schematic of my amplifier.
It is really simple.
As you can see there's a long tail input stage followed by a para-feed output stage.
The output tubes are connected as triodes.
There are also two SS CCS, and a PowerDrive interstage.
Each CCS needs a separate supply (9V, 10mA), but a 9V 3VA transformer costs pratically nothing, about 2 Euro.
THIS is the PowerDrive schematic, and THIS is the CCS schematic.
THIS is a gallery of my first prototype.
You can see one 150mA CCS installed onto the large heatsink.
The output tubes was driven directly by my laptop soundcard output (no preamp stage), so the sound was not really loud.
The first prototype did not use PowerDrive, next step will be to build two PowerDrive PCB (i have already drawn them) and try them in the prototype.
Ciao,
Giovanni
THIS is a preliminary schematic of my amplifier.
It is really simple.
As you can see there's a long tail input stage followed by a para-feed output stage.
The output tubes are connected as triodes.
There are also two SS CCS, and a PowerDrive interstage.
Each CCS needs a separate supply (9V, 10mA), but a 9V 3VA transformer costs pratically nothing, about 2 Euro.
THIS is the PowerDrive schematic, and THIS is the CCS schematic.
THIS is a gallery of my first prototype.
You can see one 150mA CCS installed onto the large heatsink.
The output tubes was driven directly by my laptop soundcard output (no preamp stage), so the sound was not really loud.
The first prototype did not use PowerDrive, next step will be to build two PowerDrive PCB (i have already drawn them) and try them in the prototype.
Ciao,
Giovanni
Operating a tube in A2 does not cause any shortening of tube life. Pushing a tube beyond its published ratings does. It is possible to operate a tube beyond its safe ratings in A1 or A2. An amplifier (using normal receiving tubes) can be configured to allow A2 operation. It will spend 99% of its time in A1. The only difference is that the driver circuit is designed such that it can drive the grid of the output tube positive on transients without distortion. The reason for this is to remove one of the common (but not well understood) sources of distortion in a tube amp. PowerDrive was conceived to eliminate this distortion, the extra power output and improved sound were side benefits.
Go to http://www.aikenamps.com/
Select Tech Info, then choose Advanced, then scroll down to What is "Blocking Distortion" for a full explanation. This site is about guitar amps, but the tech info is excellent, well written, and applicibable to all tube amps.
Most receiving tubes were never designed with positive grid operation in mind. Some are linear in this region, and some are not. I have not tested a KT-88 for operation with positive grid bias. If a tube is linear with positive bias, then you can adjust the amplifier to take advantage of this and gain some extra output power. If a tube is not linear in the positive grid bias region, a driver designed for A2 operation will still remove the distortion that occurs when the driver clips, and the severe distortion that occurs when a transient peak upsets the charge on the coupling capacitor which then takes time to recover (blocking distortion).
I have used A2 successfully on many amplifiers (but I have not tested a KT-88). The tubes that benefit the most and can provide extra power ( all pentodes tested in triode mode):
300B, 845, 211, 2A3, EL-34, 6LW6, 6AV5, 807 and the 833A
The 45 didn't provide any extra power with A2 capabilities, although the amp did sound much better with PowerDrive.
Some tubes were designed to operate with positive bias on the tube these REQUIRE a driver capable of A2 operation. The obvious ones are the 811A, the 3-500Z, and the 838.
Go to http://www.aikenamps.com/
Select Tech Info, then choose Advanced, then scroll down to What is "Blocking Distortion" for a full explanation. This site is about guitar amps, but the tech info is excellent, well written, and applicibable to all tube amps.
Most receiving tubes were never designed with positive grid operation in mind. Some are linear in this region, and some are not. I have not tested a KT-88 for operation with positive grid bias. If a tube is linear with positive bias, then you can adjust the amplifier to take advantage of this and gain some extra output power. If a tube is not linear in the positive grid bias region, a driver designed for A2 operation will still remove the distortion that occurs when the driver clips, and the severe distortion that occurs when a transient peak upsets the charge on the coupling capacitor which then takes time to recover (blocking distortion).
I have used A2 successfully on many amplifiers (but I have not tested a KT-88). The tubes that benefit the most and can provide extra power ( all pentodes tested in triode mode):
300B, 845, 211, 2A3, EL-34, 6LW6, 6AV5, 807 and the 833A
The 45 didn't provide any extra power with A2 capabilities, although the amp did sound much better with PowerDrive.
Some tubes were designed to operate with positive bias on the tube these REQUIRE a driver capable of A2 operation. The obvious ones are the 811A, the 3-500Z, and the 838.
croccodillo said:
Ah! deserter...... he's using solid state in the driver stage...I've always suspected this stage in any amp determines alot of the quality aspect more than the output tubes. I wouldn't be suprised after auditioning he does an about turn away from mosfets. Anyone else using such SState driver applications in thier designs ?
omo richj
Probably that big heatsink is not big enough....
The heatsink is where the two output stage CCS are mounted.
Give a look at the schematic: the amp is a para-feed, the B+ voltage is about 550V (I'm not sure if raise it up to 600V).
The output tubes will sit at about 350V at idle: that means that the CCS feeding the KT88s will work, at idle, with a voltage of about 200V and a current of 150mA (for each channel).
Thus, each CCS will dissipate a power of about 250V*0.150A=30W.
Having a stereo amp the two CCS will dissipate about 60W of power.
As you can see, I really need a big heatsink!
I made some calculation, trying to simulate the heatsink (I have no data about it, it came from a surplus stock), and it should handle the power and raise up to 60°C at idle...
It could seems a big amount of power, but the four KT88s will dissipate about 100W of power (not including the heaters)!
Ciao,
Giovanni
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