I would go straight to a current source.
I honestly could detect no downsides to this and it certainly improved the hum.
What I diod was...6.3v CT into two diodes (fullwave) then using a 10,000uF cap to ground, then the LM317 on a nice big heatsink, but reference the 'set' pin to the out pin via a suitable R and not to ground (as you would in a voltage reg). Philip (pre65) has a piccy somewhere.
Take the output to one side of the cathode and the other to ground.
My very first breadboard
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Cressy Snr
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#47
Hi Nick
Just shows how out of touch with DHTs I have allowed myself to become.
I can't believe I did that. Doh!!
Steve.
Just shows how out of touch with DHTs I have allowed myself to become.
I can't believe I did that. Doh!!
Steve.
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Cressy Snr
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#49
Thanks Andrew/ Philip
Nick
I've just sorted the heaters out with a 0.56R resistor on each leg of both the PX25s
The bloody thing is now virtually silent. Thanks
What a freaking idiot I am sometimes
Now to regulated supplies. What sorts of benefits are there to be had by going this route?
Steve.
Nick
I've just sorted the heaters out with a 0.56R resistor on each leg of both the PX25s
The bloody thing is now virtually silent. Thanks
What a freaking idiot I am sometimes
Now to regulated supplies. What sorts of benefits are there to be had by going this route?
Steve.
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Cressy Snr
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#51
Hi Simon,simon wrote:Good stuff Steve, really looking forward to a listen. Just remembered - do you want a couple of 4 pin sockets? BTW, particularly like the Danger of death warning!
Yes... a couple of 4 pin sockets would come in very handy, especially if as planned I try 26s as drivers fed by a 76 input stage.
Now I've got rid of the hum I'm settling down very nicely with the amp.
You are welcome round at my place for a listen if you want to bring the sockets sometime.
Steve
#52
More quiet, but if its quiet now, maybe not a big deal.Now to regulated supplies. What sorts of benefits are there to be had by going this route?
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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simon
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#53
Hi Steve,
It'll be a few weeks before I can make it - not till I get back from hols. Just thought, are you after UX4s or B4/B5s? I have spare B4/B5s but I don't have any spare UX4s sadly.
It'll be a few weeks before I can make it - not till I get back from hols. Just thought, are you after UX4s or B4/B5s? I have spare B4/B5s but I don't have any spare UX4s sadly.
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Cressy Snr
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#54
Hi Simon,
It was a couple of UX4 bases I wanted. I have a couple of pairs of B5 bases for if I decide to use the ML4 rather than the 76 as an input valve. I'll get a couple of UX4 bases off Langrex, they are not that expensive.
The amp is working just fine at the moment as a two-stage amp driven by 6A6s. These really do seem to be quite decent drivers for the higher gain output valves such as the PX25.
Tossie of Valves World Japan fame used a 6A6 as a differential phase splitter to drive a pair of 2A3s in push-pull. This is where I got the idea to use it as a straightforward paralleled driver for the PX25s. It is a hi mu triode, gain of 35, with very straight, shallowly sloped curves, about half way between a triode and a pentode, if you know what I mean. It looked to be quite a linear valve.
The data sheet recommends its use, either as a power valve driver or as a zero bias, class B push-pull output stage for old radios, so I thought, "what the hell" and tried it out. It works well. The 6a6 is electrically equivalent to the 6N7 and the Type 53. The 6a6 idles at 7mA when paralleled so for a 300B it might be a bit low in current drive capability to allow one of these to work at its best. Then again I have no experience of building 300B amps so I might be wrong there.
The sound of the whole package is sweet, smooth and detailed with slam where needed and the bass goes down and down, allowing the two subs to produce quite well controlled, floor rattling subsonics, where these are present. Dynamic contrasts are better than the 6336A amp though that might be a function of the amp's higher power delivery into the Metronomes, which though efficient, are no match for a Scottmoose MLTL or a Nick or Steve open baffle. In fact, I think Steve would like this amp through his speakers, even with the modern production PX25s.
Instrumental and harmonic textures are better portrayed than the 6336A, which this time is most likely to be a function of the performance of the PX25 DHT itself. This amp seems to be a very good match for my Metronomes in terms of the way it produces music. I have little doubt that it could be better, especially with a proper DHT driver valve such as the 26 but then I'd need, either three stages, or a separate preamp.
Change the drivers for 26s, remove the volume control and build a separate transformer coupled preamp using choke-loaded 76s, is probably the way I'll take this forward. Doing things this way I'll be able to incorporate a phono stage within the preamp too. Looking longer term I'll probably end up with a similar set-up to that used by Will, albeit using different valves. Don't hold your breath but after talking to Will at Owston and hearing his setup playing Muddy Waters and John Lee Hooker, I think I now have a more clear-cut direction in which I'd like to to take things.
Steve
It was a couple of UX4 bases I wanted. I have a couple of pairs of B5 bases for if I decide to use the ML4 rather than the 76 as an input valve. I'll get a couple of UX4 bases off Langrex, they are not that expensive.
The amp is working just fine at the moment as a two-stage amp driven by 6A6s. These really do seem to be quite decent drivers for the higher gain output valves such as the PX25.
Tossie of Valves World Japan fame used a 6A6 as a differential phase splitter to drive a pair of 2A3s in push-pull. This is where I got the idea to use it as a straightforward paralleled driver for the PX25s. It is a hi mu triode, gain of 35, with very straight, shallowly sloped curves, about half way between a triode and a pentode, if you know what I mean. It looked to be quite a linear valve.
The data sheet recommends its use, either as a power valve driver or as a zero bias, class B push-pull output stage for old radios, so I thought, "what the hell" and tried it out. It works well. The 6a6 is electrically equivalent to the 6N7 and the Type 53. The 6a6 idles at 7mA when paralleled so for a 300B it might be a bit low in current drive capability to allow one of these to work at its best. Then again I have no experience of building 300B amps so I might be wrong there.
The sound of the whole package is sweet, smooth and detailed with slam where needed and the bass goes down and down, allowing the two subs to produce quite well controlled, floor rattling subsonics, where these are present. Dynamic contrasts are better than the 6336A amp though that might be a function of the amp's higher power delivery into the Metronomes, which though efficient, are no match for a Scottmoose MLTL or a Nick or Steve open baffle. In fact, I think Steve would like this amp through his speakers, even with the modern production PX25s.
Instrumental and harmonic textures are better portrayed than the 6336A, which this time is most likely to be a function of the performance of the PX25 DHT itself. This amp seems to be a very good match for my Metronomes in terms of the way it produces music. I have little doubt that it could be better, especially with a proper DHT driver valve such as the 26 but then I'd need, either three stages, or a separate preamp.
Change the drivers for 26s, remove the volume control and build a separate transformer coupled preamp using choke-loaded 76s, is probably the way I'll take this forward. Doing things this way I'll be able to incorporate a phono stage within the preamp too. Looking longer term I'll probably end up with a similar set-up to that used by Will, albeit using different valves. Don't hold your breath but after talking to Will at Owston and hearing his setup playing Muddy Waters and John Lee Hooker, I think I now have a more clear-cut direction in which I'd like to to take things.
Steve
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Cressy Snr
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#55
Here are the two revised circuit diagrams for the amp. These have the correct voltages on them. On test, I increased the voltage to the driver stage by removing the 1K8 resistor from the HT line.
This has maximised the swing available with 2V from a CD player.
The overall HT was dropped to 373V, giving 363V at the bottom of the output TX, which gave me the correct 30V at the cathodes of the PX25s and gave the planned for current of 45mA through the valves.
They are operating at about 66% of max dissipation for longer life, so I'm probably getting around 5W of power out of them.
BTW those PX25s look gorgeous in the dark.
Steve
This has maximised the swing available with 2V from a CD player.
The overall HT was dropped to 373V, giving 363V at the bottom of the output TX, which gave me the correct 30V at the cathodes of the PX25s and gave the planned for current of 45mA through the valves.
They are operating at about 66% of max dissipation for longer life, so I'm probably getting around 5W of power out of them.
BTW those PX25s look gorgeous in the dark.
Steve
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Cressy Snr
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#56
Just a question for those experienced souls who have used nothing but DHTs for years.
Is there a rule of thumb about how hard to push them. For instance, the harder you push them the better they sound?
I ask this because, with the 400-0-400 Sowter transformer I have and the hefty PSU, I could easily unleash 400V across the PX25 meshies, (max dissipation) but am wary of doing this if it is going to drastically shorten their life. I'm erring on the cautious side at the moment.
Steve
Is there a rule of thumb about how hard to push them. For instance, the harder you push them the better they sound?
I ask this because, with the 400-0-400 Sowter transformer I have and the hefty PSU, I could easily unleash 400V across the PX25 meshies, (max dissipation) but am wary of doing this if it is going to drastically shorten their life. I'm erring on the cautious side at the moment.
Steve
#57
Well, it varies, they can sound good run a lot lower than max, but if that means that to get the volumn you end up driving them a lot closer to the limit you loose any gain you may have made. As long as you don't exceed the limits then they should be just fine.
One other thing I have been thinking about suggesting, is that power supply has a lot of resistance in it, why not change to choke input, loose the resistors, if needed, add a second choke stage, and if the voltage is a little low, just a smaller cap to peak it up a tad.
It might not sound any better, but it would be a interesting experiment.
One other thing I have been thinking about suggesting, is that power supply has a lot of resistance in it, why not change to choke input, loose the resistors, if needed, add a second choke stage, and if the voltage is a little low, just a smaller cap to peak it up a tad.
It might not sound any better, but it would be a interesting experiment.
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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Cressy Snr
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#59
Cheers Guys
I have bitten the bullet and dropped the first resistor to 68R to raise the HT to what I had on my first diagram. I now have 405V at the top of the PX25 with 30V and 45mA of bias at the other end. I lowered the cathode resistors to 600R to get back to the correct bias. I'm now at around 80% of maximum dissipation.
I don't know whether it is the increased headroom or the lowering of the resistance of the PSU but there seems to have been a slight improvement in harmonic texture and transient attack. It is not night and day but nonetheless is there.
The driver stage now has 250V at the anodes and 4V 8mA bias - very close to RCA's recommendations. It could be taken to a maximum of 294V and 5V of bias for ultimate stomp if desired.
Given the relatively low capacitances being used in the PSU I agree that a CLCLC supply with a first tweak cap would probably be a good move. An additional advantage would be the ability to get the supply free of electrolytics . Admittedly there is only one electrolytic in the PSU at the moment but it'll be nice to get rid of it just the same.
Those oil filled caps in the shiny cans are beginning to look like a realistic proposition now. You know, I quite like this breadboarding lark. Don't know why I didn't do it sooner.
Steve
I have bitten the bullet and dropped the first resistor to 68R to raise the HT to what I had on my first diagram. I now have 405V at the top of the PX25 with 30V and 45mA of bias at the other end. I lowered the cathode resistors to 600R to get back to the correct bias. I'm now at around 80% of maximum dissipation.
I don't know whether it is the increased headroom or the lowering of the resistance of the PSU but there seems to have been a slight improvement in harmonic texture and transient attack. It is not night and day but nonetheless is there.
The driver stage now has 250V at the anodes and 4V 8mA bias - very close to RCA's recommendations. It could be taken to a maximum of 294V and 5V of bias for ultimate stomp if desired.
Given the relatively low capacitances being used in the PSU I agree that a CLCLC supply with a first tweak cap would probably be a good move. An additional advantage would be the ability to get the supply free of electrolytics . Admittedly there is only one electrolytic in the PSU at the moment but it'll be nice to get rid of it just the same.
Those oil filled caps in the shiny cans are beginning to look like a realistic proposition now. You know, I quite like this breadboarding lark. Don't know why I didn't do it sooner.
Steve
#60
That makes two of usI quite like this breadboarding lark. Don't know why I didn't do it sooner.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.