Excellent point!!
That gave me a good place to start. I suppose it's not just a little parasitic capacitance or leakage inductance that causes such an oscillation. So I tried disconnecting the negative feedback. The first thing I noticed was the lower output without the NFB. That didn’t seem right. It looked like I may have reversed the phase of the output transformer (?) (I had the blue and blue/yellow wires in the primary of the Hammond 1608 connected to the top tube in the schematics, and the black wire in the secondary connected to ground.)
Reconnecting the amp to the B&W 804 and the oscillation is gone. I tried reconnecting the NFB, but I couldn’t get the oscillation back again to confirm that it indeed was the problem. I suppose things got changed just enough with me moving things around that the amp is now stable even with the positive feedback (?)
I tried reversing the feedback connection with the green wires on the secondary connected to ground. I got a slightly lower output as expected. But the amp runs fine without the feedback. I’m leaving it out for now.
Another issue I had was the fuses. The HT switch helps, but still not enough. I now use a few 33ohms in parallel to limit the current on power up and use another switch to short out the resistors after a minute. I’m going to order a CL08 inrush current limiter. Hopefully that’ll keep the fuses from blowing.
All in all, things well pretty well for my first attempt at DIY. I’m now thinking about building the other channel. I’ve put a few pictures of the amp on the following website. I’d appreciate any comments on the layout, components, etc.
http://cheunglai.smugmug.com/gallery/1336901
That gave me a good place to start. I suppose it's not just a little parasitic capacitance or leakage inductance that causes such an oscillation. So I tried disconnecting the negative feedback. The first thing I noticed was the lower output without the NFB. That didn’t seem right. It looked like I may have reversed the phase of the output transformer (?) (I had the blue and blue/yellow wires in the primary of the Hammond 1608 connected to the top tube in the schematics, and the black wire in the secondary connected to ground.)
Reconnecting the amp to the B&W 804 and the oscillation is gone. I tried reconnecting the NFB, but I couldn’t get the oscillation back again to confirm that it indeed was the problem. I suppose things got changed just enough with me moving things around that the amp is now stable even with the positive feedback (?)
I tried reversing the feedback connection with the green wires on the secondary connected to ground. I got a slightly lower output as expected. But the amp runs fine without the feedback. I’m leaving it out for now.
Another issue I had was the fuses. The HT switch helps, but still not enough. I now use a few 33ohms in parallel to limit the current on power up and use another switch to short out the resistors after a minute. I’m going to order a CL08 inrush current limiter. Hopefully that’ll keep the fuses from blowing.
All in all, things well pretty well for my first attempt at DIY. I’m now thinking about building the other channel. I’ve put a few pictures of the amp on the following website. I’d appreciate any comments on the layout, components, etc.
http://cheunglai.smugmug.com/gallery/1336901
Toroid transformer
ctaudio - if that's your first DIY amp, colour me impressed! Obviously this isn't your first circuit on a perf board though.
Yeah, I was going to suggest a thermistor, but you've beat me to the punch...
Anyway, I was wondering about toroidial output transfomers, which I found on eBay (oh oh). I can get a really good price on some 10W 40% UL toroidials, but I hear they (toroid output trafos in general) have a problem when there's a DC current imbalance between the two tubes. Do we have any opinions on this? I don't have any idea how to tune current output of the tubes - is an imbalance caused by mismatched tubes, or mismatched resistors, or other parts? Can it be fixed with a pot somewhere? Careful parts selection? Could I simply put a DC blocking cap between the EL84 and the trafo, or is this a no-no? Should I just go away and read about circuits for a long time and figure it out for myself?
Jeremy
ctaudio - if that's your first DIY amp, colour me impressed! Obviously this isn't your first circuit on a perf board though.
Yeah, I was going to suggest a thermistor, but you've beat me to the punch...Anyway, I was wondering about toroidial output transfomers, which I found on eBay (oh oh). I can get a really good price on some 10W 40% UL toroidials, but I hear they (toroid output trafos in general) have a problem when there's a DC current imbalance between the two tubes. Do we have any opinions on this? I don't have any idea how to tune current output of the tubes - is an imbalance caused by mismatched tubes, or mismatched resistors, or other parts? Can it be fixed with a pot somewhere? Careful parts selection? Could I simply put a DC blocking cap between the EL84 and the trafo, or is this a no-no? Should I just go away and read about circuits for a long time and figure it out for myself?
Jeremy
Thank you, Jeremy. This is indeed my first tube amp (my first analog anything, for that matter). But you are right. I have done a good number of digital boards in the past.
I've been listening to the Baby Huey (in mono) in the past hour or so. I must say I am very impressed. I can't wait to complete the other channel. I may have to retire my ML...
I've also heard good things about toroidial output transformers. I'd be interested in hearing what you find out.
I've been listening to the Baby Huey (in mono) in the past hour or so. I must say I am very impressed. I can't wait to complete the other channel. I may have to retire my ML...
I've also heard good things about toroidial output transformers. I'd be interested in hearing what you find out.
This amp design uses a bias servo circuit to maintain dc balance for the toroidal output transformer.
http://www.normankoren.com/Audio/TENA.html
http://www.normankoren.com/Audio/TENA.html
The two bias circuit at the cathodes of the EL84 are providing a constant and equal current draw. The absence of DC in the OPT will help to produce the excellent effects on the bass response Gingertube is talking about!
Another option, simple to implement, is the use of an LM317 or LM337. John Broskie shows it here;
http://www.glass-ware.com/tubecircuits/Tube_Auto_Biasing.html
Another option, simple to implement, is the use of an LM317 or LM337. John Broskie shows it here;
http://www.glass-ware.com/tubecircuits/Tube_Auto_Biasing.html
I think EL84 is a very good tube/valves with many good qualities, cheap, excellent sounding and easier to drive hence design. There are many types available out there to experiment as well.
Those who are interested in high powered amplifier should look at making a total of 8 tubes(in 4 per side in PP mode) per channel giving a total of about 35watts per channel, close to class A mode.
This I think is a good project for those who are keen. I am thinking of making one.
Those who are interested in high powered amplifier should look at making a total of 8 tubes(in 4 per side in PP mode) per channel giving a total of about 35watts per channel, close to class A mode.
This I think is a good project for those who are keen. I am thinking of making one.
2- EL84 tubes in PP operating in Pure Class A(operate at max limit) is about 12W(+/-1W), 4 tubes will give about 24 Watts, so design conservatively it will operate at 20W in pure Class A.
If you want more power, bias it in class AB mode it will give more about 35W easily or one bias the tube to suit one's liking.
One can get cheap and good quality russian EL84 or 6P14P-EV(military version) at about AUD$2-3 per tube. I just bought 4 Russian tubes (matched quad pairs for US$12.50) from Ebay.
I have not decided yet so don't put our hopes too high. I have experience in building and designing valve amp. Alternatively I am thinking of building a Nelson Pass Class A single ended, giving out 30-40 watts per side. I have all the info and parts are easily obtainable and able to get the PCB easily.... the circuit is simple to follow and soldered. The difficult is the heat generated by the mosfets, the heat sink is huge.
If you want more power, bias it in class AB mode it will give more about 35W easily or one bias the tube to suit one's liking.
One can get cheap and good quality russian EL84 or 6P14P-EV(military version) at about AUD$2-3 per tube. I just bought 4 Russian tubes (matched quad pairs for US$12.50) from Ebay.
I have not decided yet so don't put our hopes too high. I have experience in building and designing valve amp. Alternatively I am thinking of building a Nelson Pass Class A single ended, giving out 30-40 watts per side. I have all the info and parts are easily obtainable and able to get the PCB easily.... the circuit is simple to follow and soldered. The difficult is the heat generated by the mosfets, the heat sink is huge.
I can't help but think you may be further ahead running one pair of 6L6GC conservatively. Heater power (for a pair) is less than a quad of 6BQ5. With 360V plate in AB1 you'll get about 18W (out of RC-27). I'm sure 7591A's will perform similarly.
I'd stick with the 12~14 watt rating for a pair or move up to a more suitable tube for higher power.
-Chris
I'd stick with the 12~14 watt rating for a pair or move up to a more suitable tube for higher power.
-Chris
I have not owned a single ended amp before it would be interesting to build one and compare it with a push pull valve with the same power output.
There will be some differences in sound quality between the 2 amp. and each has its own characteristics besides the influence of components and contruction of the amp. I still cannot pin point the diff. characteristics(*) as yet as I have not owned or listened to one for a lengthy period of time. That is the reason I like to build one. I don't think one amp better than the another it depends on individual taste and mood at the time of listening.
(*)Characteristics I mean for example, pentode vs triode in push pull topology. Those who have compared between these 2 modes will know what I mean.
There will be some differences in sound quality between the 2 amp. and each has its own characteristics besides the influence of components and contruction of the amp. I still cannot pin point the diff. characteristics(*) as yet as I have not owned or listened to one for a lengthy period of time. That is the reason I like to build one. I don't think one amp better than the another it depends on individual taste and mood at the time of listening.
(*)Characteristics I mean for example, pentode vs triode in push pull topology. Those who have compared between these 2 modes will know what I mean.