I think it depends on perspective. You should bare in mind that I'm enjoying the process of all this trial and error! And you could argue that I've already fulfilled the objective by constructing a working amp.
If I want the heaters from mains power, all I need is another, separate small transformer which isn't expensive at all. Or, I can carry on using a separate DC supply for the heaters. No problem.
I don't have much spare time at the moment but when I get some down time I'll carry on 🙂.
Thanks again for everyone's input. What a fantastic forum this is!
J.
If I want the heaters from mains power, all I need is another, separate small transformer which isn't expensive at all. Or, I can carry on using a separate DC supply for the heaters. No problem.
I don't have much spare time at the moment but when I get some down time I'll carry on 🙂.
Thanks again for everyone's input. What a fantastic forum this is!
J.
By 'highway to nowhere' i was referring to expecting a 6VA transformer to supply 8W of rectified and smoothed DC, excluding dropping resistors because the voltage was not correct.
Little update. I finally got it working from just a 20v DC supply.
I'm actually waiting for a band new MT to arrive from primary windings. The one I've ordered is rated far in excess of what I need for this amp but I thought it might be useful in other future projects also.
I also picked up 2 very reasonably priced NOS 6BM8's to use. I had been keen to use the PCL82/6's as I already have a load of them but it made sense given the price of the 6BM8's. I can use the 6.3v winding on the new MT for the heaters.
In the mean time, being impatient, I decided to try something else with the amp. I'm off work with covid and can't go back until I test negative so....
I spotted a little "DC-AC Inverter Power Supply" on Amazon. At £5.84 delivered next day, it was worth getting just to experiment with.
It outputs 220v AC at 40w (allegedly) from a 12v DC input.
I also took a punt on some better looking buck converter modules which use an LM2596, (also cheap) to play with.
I decided to try using 1 of these to drop the 20v laptop power brick voltage down to 12v to feed the inverter board with a bridge rectifier after the AC output to provide HT. I used another one set to 16v for the valve heaters (still using the PCL82's for now).
And... It works! I ran it for around 2 hours last night and it sounded ok. Nothing has overheated (yet!). Had it on a fair bit today again and seems very stable. It's absolutely silent in terms of hum.
However....Under load the voltage drops quite a bit, I'm getting 160v HT with the valves in. I think I'm hearing distortion when the volume is turned up, way before I'd expect it to distort i.e, while the volume isn't all that loud.
I'm guessing this is down the the low HT voltage?
I still have the option of ditching the laptop charger supply all together once the new MT arrives. But, I do like the idea of running this little amp off the 20v DC supply.
I'm toying with the idea of another inverter to give 1 per channel to increase the HT voltage available. Interested to hear your thoughts on that.
I've been having a go at building a plinth/chassis as well. The plinth still needs some sanding and varnish and some more holes cutting out.
The aluminum chassis will be painted to match my Lenco Goldring turntable which uses a similar wooden plinth setup.
👍
I'm actually waiting for a band new MT to arrive from primary windings. The one I've ordered is rated far in excess of what I need for this amp but I thought it might be useful in other future projects also.
I also picked up 2 very reasonably priced NOS 6BM8's to use. I had been keen to use the PCL82/6's as I already have a load of them but it made sense given the price of the 6BM8's. I can use the 6.3v winding on the new MT for the heaters.
In the mean time, being impatient, I decided to try something else with the amp. I'm off work with covid and can't go back until I test negative so....
I spotted a little "DC-AC Inverter Power Supply" on Amazon. At £5.84 delivered next day, it was worth getting just to experiment with.
It outputs 220v AC at 40w (allegedly) from a 12v DC input.
I also took a punt on some better looking buck converter modules which use an LM2596, (also cheap) to play with.
I decided to try using 1 of these to drop the 20v laptop power brick voltage down to 12v to feed the inverter board with a bridge rectifier after the AC output to provide HT. I used another one set to 16v for the valve heaters (still using the PCL82's for now).
And... It works! I ran it for around 2 hours last night and it sounded ok. Nothing has overheated (yet!). Had it on a fair bit today again and seems very stable. It's absolutely silent in terms of hum.
However....Under load the voltage drops quite a bit, I'm getting 160v HT with the valves in. I think I'm hearing distortion when the volume is turned up, way before I'd expect it to distort i.e, while the volume isn't all that loud.
I'm guessing this is down the the low HT voltage?
I still have the option of ditching the laptop charger supply all together once the new MT arrives. But, I do like the idea of running this little amp off the 20v DC supply.
I'm toying with the idea of another inverter to give 1 per channel to increase the HT voltage available. Interested to hear your thoughts on that.
I've been having a go at building a plinth/chassis as well. The plinth still needs some sanding and varnish and some more holes cutting out.
The aluminum chassis will be painted to match my Lenco Goldring turntable which uses a similar wooden plinth setup.
👍
Assembling it tomorrow as it is, with the DC supply and inverter board. It's plenty loud enough before it starts to distort. It sounds great as it is and there's no unwanted noise at idle at all.
It definitely distorts at a lower point than it did with the little MT I was using though. I'm going to download a soundcard scope and hook the amp up to my soundcard and see what the output looks like.
I might still use the new mains transformer I have on order from primary windings with this amp, or I might try a second inverter and see if I can increase the HT for each channel.
Either way, the chassis is big enough to accommodate.
It definitely distorts at a lower point than it did with the little MT I was using though. I'm going to download a soundcard scope and hook the amp up to my soundcard and see what the output looks like.
I might still use the new mains transformer I have on order from primary windings with this amp, or I might try a second inverter and see if I can increase the HT for each channel.
Either way, the chassis is big enough to accommodate.
Nice work there it's looking good. I'd advise using the mains tfmr and bog standard linear power supply mainly because it would be far easier to build & fix if not working. For a small amp like this you want a rock steady HT/B+ supply of about 300v to squeeze every last watt out of the design.
Sorry to hear you got covid but it does mean you have time to build the amp though if your not feeling too rough.
Sorry to hear you got covid but it does mean you have time to build the amp though if your not feeling too rough.
Cheers, DA. I'm pretty much recovered now but yeah, had a few days once I was well enough to get some stuff done. Might as well make the most of it!
Looking forward to seeing it all assembled on the proper chassis now.
Once the MT arrives I'll get that fitted and swap over to the 6BM8's. I'll have easy 300v with more than enough Amps available to play with.
I'll keep the little plastic chassis for experimenting separately.
J.
Looking forward to seeing it all assembled on the proper chassis now.
Once the MT arrives I'll get that fitted and swap over to the 6BM8's. I'll have easy 300v with more than enough Amps available to play with.
I'll keep the little plastic chassis for experimenting separately.
J.
One thing I could use some advice as in using it as is while I wait for my new MT to arrive...
I've still got an issue with the heaters flashing bright on initial power up. How can I incorporate a soft start? I'm powering them from one of the buck converter modules. They get 16v DC as soon as it's powered on. I need them to come up to this voltage a bit more slowly.
Cheers.
I've still got an issue with the heaters flashing bright on initial power up. How can I incorporate a soft start? I'm powering them from one of the buck converter modules. They get 16v DC as soon as it's powered on. I need them to come up to this voltage a bit more slowly.
Cheers.
With a mains transformer, an Inrush current limiter like a CL-90 on the primary side of the power transformer works wonders.
Sometimes it can just be the tube, where a bit of heater is not inserted into the cathode. Maybe put a meter on there and see how the voltage changes across the heater? In case the regulator has an issue.
Sometimes it can just be the tube, where a bit of heater is not inserted into the cathode. Maybe put a meter on there and see how the voltage changes across the heater? In case the regulator has an issue.
Well, I think I'll consider it finished... For now 😏...
Really happy with how it turned out. The colour match to my record deck chassis is good and the plinth worked out well.
It's currently wired with another PCL86 circuit. It sounds great to my ears. I'm still using the laptop charger with an inverter module and diode bridge for HT.
I have some 6Bm8's and an EZ81 and the new MT from primary windings has finally been dispatched. I may rebuild it again or use that lot for a separate project and leave this as it is.
Still need to download the soundcard scope and have a play with that to familiarise myself with it.
Thanks again for all your input. It's been invaluable.
Really happy with how it turned out. The colour match to my record deck chassis is good and the plinth worked out well.
It's currently wired with another PCL86 circuit. It sounds great to my ears. I'm still using the laptop charger with an inverter module and diode bridge for HT.
I have some 6Bm8's and an EZ81 and the new MT from primary windings has finally been dispatched. I may rebuild it again or use that lot for a separate project and leave this as it is.
Still need to download the soundcard scope and have a play with that to familiarise myself with it.
Thanks again for all your input. It's been invaluable.
This is the MT I've ordered. Should give plenty of options for further builds
https://primarywindings.com/product/amp-maker-pwam01-275v-190v-160ma-6-3v3a/
I also spotted these little 4W OT's. Wondered if they'd be usable?
https://primarywindings.com/product/amp-maker-4w-push-pull-output-transformer/
https://primarywindings.com/product/amp-maker-pwam01-275v-190v-160ma-6-3v3a/
I also spotted these little 4W OT's. Wondered if they'd be usable?
https://primarywindings.com/product/amp-maker-4w-push-pull-output-transformer/
Well done mate, you've done a cracking job, had fun & learned stuff along the way: experience is everything.
I've used several OPT's from Primary Winding with good results, he makes good value well built transformers.
I've used several OPT's from Primary Winding with good results, he makes good value well built transformers.
Sorry, forgot to comment on heater flash on power up. A lot of valves do this, some people think it's an issue, EG Mr Carlson did a video on it. I posed the question on a respected UK vintage forum, the consensus being that it's not an issue, see - https://www.vintage-radio.net/forum/showthread.php?t=122438 I wouldn't worry about it, of all the 1000's of valves I've tested I've never found one with an open circuit heater.