Hi. This is my first thread. I don't have any formal electronics training but I was raised around high voltage in the 70's and I would buy tube amps at garage sales and try to keep them going. I usually knew where to kick them...
I've been breadboarding amps for about 5 or 6 years now. My audio systems that I listen to are what I breadboard together. They are all bench designed and they, so far, are all direct connected since that's where I had my first success in amp making.
I've been reading with much interest the plate to plate feedback in recent threads since I've had no experience with feedback and it seemed like a relatively simple approach. It occurred to me to apply it to a Loftin White type design with a high gain pentode for V1 since I could never get those to behave in the slightest.
I wired this up tonight and it sounds rather nice to me. It puts out a nice sine wave on the scope at a strong volume. The response isn't linear on the scope but music sounds natural and nicely balanced through it. The speaker is a transmission line loaded 5" dayton full range pointing up with the center cap removed and a mushroom shaped reflector mounted in the pole piece. I set it in the corner to help the bass out.
Thanks for looking. -Fred
I've been breadboarding amps for about 5 or 6 years now. My audio systems that I listen to are what I breadboard together. They are all bench designed and they, so far, are all direct connected since that's where I had my first success in amp making.
I've been reading with much interest the plate to plate feedback in recent threads since I've had no experience with feedback and it seemed like a relatively simple approach. It occurred to me to apply it to a Loftin White type design with a high gain pentode for V1 since I could never get those to behave in the slightest.
I wired this up tonight and it sounds rather nice to me. It puts out a nice sine wave on the scope at a strong volume. The response isn't linear on the scope but music sounds natural and nicely balanced through it. The speaker is a transmission line loaded 5" dayton full range pointing up with the center cap removed and a mushroom shaped reflector mounted in the pole piece. I set it in the corner to help the bass out.
Thanks for looking. -Fred
Attachments
frequency sweep
I think I was incorrect about the lack of linearity. Unless I'm completely missing something. I did a sweep from 0 to 30 kHz with the oscilloscope on the speaker leads with the speaker as the load and there seems to be minimal change in the peak to peak voltage. There looks like a bit of a peak as the frequency climbs but not nearly as much as I'm used to. I posted a youtube video of it.
Simplified Loftin-White EF184-6550 with plate-plate feedback - frequency sweep - YouTube
Here it is playing some music.
Simplified Loftin-White EF184-6550 w/ plate-plate feedback - music sample - YouTube
Thank you. -Fred
I think I was incorrect about the lack of linearity. Unless I'm completely missing something. I did a sweep from 0 to 30 kHz with the oscilloscope on the speaker leads with the speaker as the load and there seems to be minimal change in the peak to peak voltage. There looks like a bit of a peak as the frequency climbs but not nearly as much as I'm used to. I posted a youtube video of it.
Simplified Loftin-White EF184-6550 with plate-plate feedback - frequency sweep - YouTube
Here it is playing some music.
Simplified Loftin-White EF184-6550 w/ plate-plate feedback - music sample - YouTube
Thank you. -Fred
I'd be a bit concerned about the rating of the 4.5k resistor between the 6550 cathode and ground. If I'm reading the voltages right it is dissipating around 10 watts. If you would like to increase the linearity you could increase the current through the ef184. I really like direct coupled pentode and UL amps with plate-plate feedback.
Hi. Ah yes. I miss labeled the schematic. There's a 25 watt rheostat under the cathode of the 6550. It gets pretty warm.
And yes, there was about 2 ma running through the EF184. I adjusted the resistance under the cathode of the EF184 and was able to bring it up to about 5.5 ma before the entire circuit went berzerk and started drawing tons of current. After a bit of tweaking and a 15k grid resistor on the EF184 it settled down. Running the EF184 hotter increased the gain, which is awesome. It doesn't seem to want to run any more than 5.5 ma through the EF184 without distorting at the moment.
Thank you guys so much for looking at it. I read your replies this morning at a friend's house. It really made my day! Thank you. -Fred
And yes, there was about 2 ma running through the EF184. I adjusted the resistance under the cathode of the EF184 and was able to bring it up to about 5.5 ma before the entire circuit went berzerk and started drawing tons of current. After a bit of tweaking and a 15k grid resistor on the EF184 it settled down. Running the EF184 hotter increased the gain, which is awesome. It doesn't seem to want to run any more than 5.5 ma through the EF184 without distorting at the moment.
Thank you guys so much for looking at it. I read your replies this morning at a friend's house. It really made my day! Thank you. -Fred
Made a few more changes: Removed the resistor between the cathode of the 6550 and the transformer which increased the voltage across the 6550; increased the B+ to 560 volts; decreased the EF184 cathode resistor to about 100 ohms which increased the current through it to about 8.5 ma.
I think that resistive divider that connected all the cathodes between B+ and ground on the original Loftin White was utilized for a sliding bias set up and a hum canceler. I finally got brave enough to just unhook it. It doesn't seem to suffer without it. It's pretty quiet and I don't mind the bias not changing with small and large signals.
The draw of the 6550:
There's 346 volts across the 6550 and 51.4 ma through it so I figure it's dissipating about 18 watts.
The power delivered to the speaker:
The peak to peak voltage at the speaker with no clipping is 20 volts @ 5 kHz. I used Volts^2 = Power x R with volts and power both RMS. Came up with 15 watts so I'm guesstimated it at between 10 and 12 watts since there must be some sort of phasing between voltage and current. Also judging just by ear how loud it is.
I don't quite know how to approach figuring out the gain. Would it be VA in/out or the voltage gain before the output transformer? I've seen gain on power amp knobs but I've never seen a unit given with it and it seems like you set the gain and measure the power delivered at the output.
Ran another sweep from 0 to 30 kHz and I didn't see any difference in the peak to peak voltage at the speaker leads.
It's stable now and sounds nice and stronger than before. It's really nice to listen to. Certainly the most polite sounding thing I've ever put together and I think I'm finally satisfied with the volume output. It sounds almost store bought at this point but it's not done yet I don't think.
I hope I answered everything. If I didn't or if I'm just completely off on something or lots of things please let me know. Thank you. -Fred
p.s. I hope this isn't a double post... here goes... close my eyes and push the button.
I think that resistive divider that connected all the cathodes between B+ and ground on the original Loftin White was utilized for a sliding bias set up and a hum canceler. I finally got brave enough to just unhook it. It doesn't seem to suffer without it. It's pretty quiet and I don't mind the bias not changing with small and large signals.
The draw of the 6550:
There's 346 volts across the 6550 and 51.4 ma through it so I figure it's dissipating about 18 watts.
The power delivered to the speaker:
The peak to peak voltage at the speaker with no clipping is 20 volts @ 5 kHz. I used Volts^2 = Power x R with volts and power both RMS. Came up with 15 watts so I'm guesstimated it at between 10 and 12 watts since there must be some sort of phasing between voltage and current. Also judging just by ear how loud it is.
I don't quite know how to approach figuring out the gain. Would it be VA in/out or the voltage gain before the output transformer? I've seen gain on power amp knobs but I've never seen a unit given with it and it seems like you set the gain and measure the power delivered at the output.
Ran another sweep from 0 to 30 kHz and I didn't see any difference in the peak to peak voltage at the speaker leads.
It's stable now and sounds nice and stronger than before. It's really nice to listen to. Certainly the most polite sounding thing I've ever put together and I think I'm finally satisfied with the volume output. It sounds almost store bought at this point but it's not done yet I don't think.
I hope I answered everything. If I didn't or if I'm just completely off on something or lots of things please let me know. Thank you. -Fred
p.s. I hope this isn't a double post... here goes... close my eyes and push the button.
20v p-p is 10v peak, so 0.707 of that is RMS, or 7.07vrms.
P = I^2 / R, 49 / 8 = 6 watts.
Gain would be the voltage in for a given voltage out converted to dB.
Not necessary to really know. Close enough to know how many volts at the input drives it to full output.
Hope the little amp is fun and you are enjoying it!
_-_-
P = I^2 / R, 49 / 8 = 6 watts.
Gain would be the voltage in for a given voltage out converted to dB.
Not necessary to really know. Close enough to know how many volts at the input drives it to full output.
Hope the little amp is fun and you are enjoying it!
_-_-
Your speakers are probably sensitive enough to need less than 2watts to go to unlistenable levels. If it was a commercial amp you would be wanting to squeeze as much gain out of the thing as possible, but since it is loud enough for you leave it at that. the only way to get more gain out of the beast is to add another stage - which will probably degrade the result for no overall benefit.
I have a 807 pentode PP amp which is theoretically capable of 20watts output, but with the 6AU6 driver it probably couldn't get anywhere near that. However since my speakers are over 100db/w sensitive it rarely strays outside the first watt of power.
Be very happy with what you have got.
Shoog
I have a 807 pentode PP amp which is theoretically capable of 20watts output, but with the 6AU6 driver it probably couldn't get anywhere near that. However since my speakers are over 100db/w sensitive it rarely strays outside the first watt of power.
Be very happy with what you have got.
Shoog
Thank you so much for the support and encouragement. I was actually feeling a bit proud of it as I responded until it ate the diodes, the 6CA4, and blew the fuse on the variac about an hour later.
I pulled out that scary high ratio plate transformer and put in a capacitor input supply. Was able to take a choke out which is nice. Also put an EL34 between B plus and the screen of the 6550 with the grid of the EL34 connected after the screen resistor with the hope of providing some regulation for the screen. There's about a 40 volt drop across the EL34. The thing's up and running again. Very similar performance to before.
I think I'm a little proud of what looks like the frequency response. Would be interesting to see what kind of response is coming out of the speaker. I don't think the speaker can take any more power as a full range and I'm looking at the speaker and amp as a unit.
I hooked up my old HP oscillator for grins. Up to 40K it's pretty even but then the voltage climbs until about 70k then builds again at about 140k and then finally dies down to zero at about 400k. 20 Hz is audible. Not powerful at all but audible.
I'm so happy to have all the great replies and suggestions etc. Made me consider things I wouldn't have which is great. Thank you. -Fred
I pulled out that scary high ratio plate transformer and put in a capacitor input supply. Was able to take a choke out which is nice. Also put an EL34 between B plus and the screen of the 6550 with the grid of the EL34 connected after the screen resistor with the hope of providing some regulation for the screen. There's about a 40 volt drop across the EL34. The thing's up and running again. Very similar performance to before.
I think I'm a little proud of what looks like the frequency response. Would be interesting to see what kind of response is coming out of the speaker. I don't think the speaker can take any more power as a full range and I'm looking at the speaker and amp as a unit.
I hooked up my old HP oscillator for grins. Up to 40K it's pretty even but then the voltage climbs until about 70k then builds again at about 140k and then finally dies down to zero at about 400k. 20 Hz is audible. Not powerful at all but audible.
I'm so happy to have all the great replies and suggestions etc. Made me consider things I wouldn't have which is great. Thank you. -Fred
-Fred
I think it's done for now.
Hi. I returned it to much like how it started and addressed the problems I ran into and kept the things that made a positive difference. The circuit seems to have reached a sort of equilibrium.
I've got a suspicion the choke input supply helps the bass out. I discovered that supply in my earliest tinkering and I always return to it. Hard to know if things are psychologically influenced sometimes.
The hump is gone from the response from 40k and above. It puts out a pretty even 20 volts to the speaker from 20Hz to 40kHz and then dimishes after that. In the super high range there's a bit of activity but nothing climbs really big like before. Something to study further I'm sure.
The feedback loop from the transformer really takes the fizz out of the sine waves. They look about the same on the scope but you can hear the overtones and color diminish as the resistance in the speaker loop is increased. I adjusted it to taste.
The sound is nice. The high frequencies have a slinky sound to them and the bass is full of tone and deep. Doesn't sound like I made it... or I guess it sounds exactly like I made it.
I named it Little Big Amp since it weighs about 30 pounds and eats up about 70 watts and all for about 5 watts to the speaker (oh. but the sound...). Thanks to Bear I'm closer to knowing how much my amps are putting out. I've been a lurker here for years. I'm glad I finally had an idea I felt worth sharing. -Fred
Hi. I returned it to much like how it started and addressed the problems I ran into and kept the things that made a positive difference. The circuit seems to have reached a sort of equilibrium.
I've got a suspicion the choke input supply helps the bass out. I discovered that supply in my earliest tinkering and I always return to it. Hard to know if things are psychologically influenced sometimes.
The hump is gone from the response from 40k and above. It puts out a pretty even 20 volts to the speaker from 20Hz to 40kHz and then dimishes after that. In the super high range there's a bit of activity but nothing climbs really big like before. Something to study further I'm sure.
The feedback loop from the transformer really takes the fizz out of the sine waves. They look about the same on the scope but you can hear the overtones and color diminish as the resistance in the speaker loop is increased. I adjusted it to taste.
The sound is nice. The high frequencies have a slinky sound to them and the bass is full of tone and deep. Doesn't sound like I made it... or I guess it sounds exactly like I made it.
I named it Little Big Amp since it weighs about 30 pounds and eats up about 70 watts and all for about 5 watts to the speaker (oh. but the sound...). Thanks to Bear I'm closer to knowing how much my amps are putting out. I've been a lurker here for years. I'm glad I finally had an idea I felt worth sharing. -Fred
Attachments
Continuing project on my blog
Hi. I've moved my project to my blog. Just posting here so as not to leave a lost leader.
Little Big Amp:
http://www.diyaudio.com/forums/blogs/homeskillet/1044-little-big-amp.html
Thank you. -Fred
Hi. I've moved my project to my blog. Just posting here so as not to leave a lost leader.
Little Big Amp:
http://www.diyaudio.com/forums/blogs/homeskillet/1044-little-big-amp.html
Thank you. -Fred
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