I see a lot of contreversy about this topic in these forums. It is very simple to design around.
Yes, every coupling cap introduces poles. I studied electrical engineering in the 70s, and we were taught how to minimise this effect in practice. It may seem obvious, but I have seen a few people struggling with this issue here.
Solid state or tubes, it is the same. You want one pole to be dominant, and the other poles should ideally be a decade lower than the dominant pole. In other words, calculate the pole for the input coupling cap for a rolloff frequency of 15 Hz, as an example. Then calculate the interstage coupling caps or feedback loop cap for a pole of (ideally) 1.5 Hz. If using an output cap, making its pole lower than the input cap pole will minimise its effect on circuit operation. If you put this concept into practice, you will find that it is kind of a juggling game.
The whole idea is to make one cap/pole dominant and the other poles will be negligible in operation. This makes the amplifier's performance more predictable. If you stack up the poles (2 or 3 poles with the same or close frequency), you will not be in control of the frequency response and you will have a lot of phase shift. You may be disappointed and maybe scratching your head as to why your amplifier doesn't work like you designed it to. Of course, phase shift can cause other problems in a global feedback amplifier too. Employing the technique I described will make your amplifier performance more predictable and put you, the engineer, in the driver's seat with regard to parameters.
Warren - the 6N2P's will work fine in my amp and are a drop in replacement - just make sure you wire the heater of each valve (pins 4 and 5) in series (2 X 6.3V = 12.6V) if you are using my 12v heater arrangement. Also, make sure you ground pin 9 (shield) - in another thread I described my problems with these valves if pin 9 is not grounded.
All the best with your build
Hi Grant, I hope you're still following this old thread of yours. I have had some success with my build of your design, but tonight, the filament transformer I had used melted down. I'm using Russian 6n2p's instead of 12AX7s and so had used a Jaycar MM2005 (60VA multi-tapped) to get 6V AC to the heaters. Well... it wasn't enough. Lesson half learned.
How has your success been using the halogen transformer? I had read a post where the poster had observed quite varied voltages across the filaments of series-wired heaters. Do you have suggestions? With my 3 6n2p's I'll have to work something out to get the 6.3V to their heaters...
Murray.
How has your success been using the halogen transformer? I had read a post where the poster had observed quite varied voltages across the filaments of series-wired heaters. Do you have suggestions? With my 3 6n2p's I'll have to work something out to get the 6.3V to their heaters...
Murray.
I've just had a look at Evatco's site (I've bought stuff from Carol a few times) and maybe I should just bite the bullet and get a hammond 302AX.... 300-0-300 200mA 5v 3A, 6.3v 6A. I could use a hybrid rectifier, diodes into a 5u4 and get about 300v out along with 6amps of 6.3 volts. It also has a 50V bias tap. What do you think?
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Hi Murray - I'm a little perplexed as to why you had a "melt down" using the Jaycar MM2005 as it is rated at 12V at 5A - more than enough for the total load of 12v at 2.85A. Did you wire the heaters with a 12V arrangement or did you attempt to use a 6V setup? - the transformer would not have the capacity for the 6.6A of current with this configuration. If you use a series/parallel arrangement with 12V the heater current is halved to 3.3A and the M2005 would be fine. I have used 12V halogen transformers successfully and have even used an "electronic" transformer successfully for heater supplies.
Wow, do you sleep? Thanks for such a quick reply!
You guessed it Grant, I used a 6V tap. I was thinking, "I really should wire these up in parallel', but went ahead anyway. So I'll re-wire the EL36's in parallel, and the two phase-splitters as well. ( I guess I'm off to jaycar this weekend...) Do you have a solution for the single preamp tube (needing 6V)? Maybe I could use the tap from 24-30V to get 6V just for that one valve?
By the way, the amp sounds brilliant! I'm using a modded playstation 1001 for a source, and a pair of Frugalhorn IIIs with Mark Audio chr70s in them. I have a little more bass in one channel than the other which is a niggling annoyance. Overall, though, the system is so sweet to listen to. And, I built it myself with a lot of help from my (Diyaudio) friends!
You guessed it Grant, I used a 6V tap. I was thinking, "I really should wire these up in parallel', but went ahead anyway. So I'll re-wire the EL36's in parallel, and the two phase-splitters as well. ( I guess I'm off to jaycar this weekend...) Do you have a solution for the single preamp tube (needing 6V)? Maybe I could use the tap from 24-30V to get 6V just for that one valve?
By the way, the amp sounds brilliant! I'm using a modded playstation 1001 for a source, and a pair of Frugalhorn IIIs with Mark Audio chr70s in them. I have a little more bass in one channel than the other which is a niggling annoyance. Overall, though, the system is so sweet to listen to. And, I built it myself with a lot of help from my (Diyaudio) friends!
Just to update readers of this thread that I have had enquiries about increasing the power output of my 6CM5/EL36 stereo amp descibed earlier in this thread.
I have been recently discovering that significantly more power can be produced by a pair of EL36’s and recently built a EL36 mono amp that produced 75W in tetrode mode with a HT of 420V and a 2K plate to plate OT.
So I would suggest that if you upgrade the power supply (need to use transformers with double the current rating) - I suggest a 160VA 30-0-30 toroid from Altronics here in Aust and change the output transformers to 40W line transformers like the Altronics M1130 available here in Australia which reflect a 2K plate to plate impedance, you should easily get 40W a side from this amp in ultralinear mode.
Cheers, Grant
I have been recently discovering that significantly more power can be produced by a pair of EL36’s and recently built a EL36 mono amp that produced 75W in tetrode mode with a HT of 420V and a 2K plate to plate OT.
So I would suggest that if you upgrade the power supply (need to use transformers with double the current rating) - I suggest a 160VA 30-0-30 toroid from Altronics here in Aust and change the output transformers to 40W line transformers like the Altronics M1130 available here in Australia which reflect a 2K plate to plate impedance, you should easily get 40W a side from this amp in ultralinear mode.
Cheers, Grant
75W is probably in the "Tubelab" zone however, at this power level there was only a very slight blush on the plates in a very dark room. There is opinion out there that these valves can easily cope with 25W+ plate dissipation.
The standing current was approx 35ma apiece.
For more information on EL36 valves at high power levels, see http://www.chambonino.com/modify/mod1.html
however, at this power level there was only a very slight blush on the plates in a very dark room. There is opinion out there that these valves can easily cope with 25W+ plate dissipation.The standing current was approx 35ma apiece.
For more information on EL36 valves at high power levels, see http://www.chambonino.com/modify/mod1.html
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The thread is alive! I'm listening right now to Amy Winehouse wax on this amp. I used a Hammond PT and Raytheon blackplate 12ax7 preamp tube for my final build. Russian FT3 coupling caps and old Aussie miniwatt 6cm5s. And my friends are amazed by the sound. Thanks again Grant.
Many thanks for this design Grant.
Many thanks for this design Grant.
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