Thanks!
As cathode loads I have a pair of homemade chokes;
~600 turns of 0,5 (or 0,45)mm wire on 45VA EI transformer cores with about 0,5mm airgap.
I´ve done no calculations on these numbers, but it seems to work quite good.
Currently I´m using the same voltage (105V) for both heater and plate voltage in the output stage, but the power transformer has an additional winding that I´ll use to get about 120V on the plates.
Currently I´m running the output tubes at about 50% of the intended quiescent current, when everything´s finished I´m going to raise that to 500-600mA per channel.
Burning a couple of hundred watts to get (less than) 2x1W at the outputs might be a bit politically incorrect these days...
I see this as an introduction to OTL amps, and I will definitely dig deeper into this later on.
As cathode loads I have a pair of homemade chokes;
~600 turns of 0,5 (or 0,45)mm wire on 45VA EI transformer cores with about 0,5mm airgap.
I´ve done no calculations on these numbers, but it seems to work quite good.
Currently I´m using the same voltage (105V) for both heater and plate voltage in the output stage, but the power transformer has an additional winding that I´ll use to get about 120V on the plates.
Currently I´m running the output tubes at about 50% of the intended quiescent current, when everything´s finished I´m going to raise that to 500-600mA per channel.
Burning a couple of hundred watts to get (less than) 2x1W at the outputs might be a bit politically incorrect these days...
I see this as an introduction to OTL amps, and I will definitely dig deeper into this later on.
I was joking, but the efficiency is still plain horrible even compared to more ordinary SET´s.
Only the output stage consumes >120W (plates) + >60W (heaters).
I listened to this amp at full power (120V @ 500mA) today, and it seems to drive my FE167E´s quite well.
Only the output stage consumes >120W (plates) + >60W (heaters).
I listened to this amp at full power (120V @ 500mA) today, and it seems to drive my FE167E´s quite well.
hello again, too bad Fuling you gave up the 6c19pi project, looks like I have to learn to design it myself🙂 Can you still explain the heater problem you had in detail (newbie)? I can see it is somewhat power hungry set up, pulling 1A through each of the 12 tubes. Maybe I'll try that PL504 amp, if I find the tubes somewhere.
Poynton: First impressions: very open and clear sound, but it gets a bit "shouty" at high volumes. I guess the distortion figures are quite high, there is only one gain stage so the feedback ratio is quite low.
I´ll do some minor PSU changes before I put everything into an oak frame, right now there are no decoupling caps anywhere and I don´t like to have only electrolytics in the PSU.
JPPP: Yes, 6C19 would probably have been a better choice than PL504 but I´m not very happy having high current AC wiring close to the tube sockets. P-tubes are easier to deal with in this aspect since they are made for series heating and since I had a power transformer that delivered exactly the voltages I needed for both heater and plate voltage I gave it a try.
PL509 or 519 would be a much better choice than 504 if you have to buy new tubes, I took what I already had at home.
I´ll do some minor PSU changes before I put everything into an oak frame, right now there are no decoupling caps anywhere and I don´t like to have only electrolytics in the PSU.
JPPP: Yes, 6C19 would probably have been a better choice than PL504 but I´m not very happy having high current AC wiring close to the tube sockets. P-tubes are easier to deal with in this aspect since they are made for series heating and since I had a power transformer that delivered exactly the voltages I needed for both heater and plate voltage I gave it a try.
PL509 or 519 would be a much better choice than 504 if you have to buy new tubes, I took what I already had at home.
The PL504 SE OTL project turned out great, so why not build something even bigger following the same concept?
After all, 2x1W isn´t much when it comes to driving normal speakers.
A bunch of PL519 pentodes would be great in an SE OTL amp, but these tubes are getting a bit expensive and hard to find so I´m going to give 6S19 another chance. I ordered 20 tubes and 20 noval sockets a while ago.
To avoid any potential hum problems caused by the heater wiring I`m going to use a 12V 10A SMPS for the heaters.
The main PSU will be plain brutal:
2x35V 500VA toroid
35A vridge rectifier
3x 680uF 160V Rifa `lytics - 25mH 3A C-core choke - 3x680uF - Mosfet Capacitance multiplier (for slow startup) - 4700uF 100V Rifa.
After all, 2x1W isn´t much when it comes to driving normal speakers.
A bunch of PL519 pentodes would be great in an SE OTL amp, but these tubes are getting a bit expensive and hard to find so I´m going to give 6S19 another chance. I ordered 20 tubes and 20 noval sockets a while ago.
To avoid any potential hum problems caused by the heater wiring I`m going to use a 12V 10A SMPS for the heaters.
The main PSU will be plain brutal:
2x35V 500VA toroid
35A vridge rectifier
3x 680uF 160V Rifa `lytics - 25mH 3A C-core choke - 3x680uF - Mosfet Capacitance multiplier (for slow startup) - 4700uF 100V Rifa.
If it's "shouty" when driving your FE167E's that's pretty common with no correction circuit on the driver. What's the Zout of the amp? I may be able to help if you want to add correction, I've tweaked with that driver quite a bit with amps of several different output impedance.
Jeb-D:
Much has happened since I got the first impressions of the PL504 OTL. The amp itself has been burned in and tweaked (minor PSU changes) and my FE167E´s has been replaced with CSS FR125S´s in closed boxes (with stereo subs).
I do miss the sweet mids and the dynamics of the Fostex units, but I prefer the CSS´s because of their better tonal balance.
When I connect the SE OTL to my new speakers and drive it from my 12B4A-LL1671 preamp there´s no such things as "shoutiness".
What I need* is more output current (=raw power). The PL504 amp drives the speakers suprisingly well, but twice the current capacity wouldn´t hurt.
* I definitely don´t "need" another bulky, low power tube amp but hey, I´m doing this for fun!
Much has happened since I got the first impressions of the PL504 OTL. The amp itself has been burned in and tweaked (minor PSU changes) and my FE167E´s has been replaced with CSS FR125S´s in closed boxes (with stereo subs).
I do miss the sweet mids and the dynamics of the Fostex units, but I prefer the CSS´s because of their better tonal balance.
When I connect the SE OTL to my new speakers and drive it from my 12B4A-LL1671 preamp there´s no such things as "shoutiness".
What I need* is more output current (=raw power). The PL504 amp drives the speakers suprisingly well, but twice the current capacity wouldn´t hurt.
* I definitely don´t "need" another bulky, low power tube amp but hey, I´m doing this for fun!
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