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sampleaccurate
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Joined 2010
2010-12-17 3:00 pm
#8
- 2010-12-17 3:00 pm
- #8
Building Soon
alexberg said:
Is it just simulation or real prototyped thing?
You need at least 140V amplitude from the driver and be still shy of getting 8A peak current into 8 Ohm load.
The driver can swing a lot more than 140V! The power section will clip first.
Right now it's a simulation using PSPICE. However, the parts are on the way, and I anticipate constructing this thing in January. There is still a lot of work to do, especially failure analysis so when a tube shorts or some other catastrophic event occurs, the amp doesn't blow up my speakers or itself. There is also the issue of start up. The voltages used in the LTPs are large, in excess of what the tube can handle if it's non-conducting.
For these problems there are a number of solutions such as gas discharge tubes, MOVs, and inrush current limiters.
Here are some response plots from the first schematic. The second schematic has better specs and very low distortion. It turns out it was the 12AX7 gain stage that was creating 90% of the distortion. Once I replaced it with two SRPP 6SN7 stages the sensitivity went up and the distortion went down. The bandwidth also went up.
These simulations are generally very accurate, with the caveat that they are only as accurate as the tube models used in the circuit model. So far I've been very pleased with the models I've found online. I find computer modeling an indispensible tool for designing amps or anything else for that matter. I may need to adjust the resistors by 5% or even 10%, but once I get the bias voltages and currents close to where the simulation shows they should be (and for which the amp has been optimized for) the amp will work - maybe!
There are still some pitfalls and traps to fall into. The amp could oscillate if it isn't constructed properly. I attempted to get the amp to oscillate by introducing simulated parasitic positive (as well as negative) feedback. I was able to get it to oscillate, but the coupling required was far in excess of what I expect to see in the actual build. The circuit seems very stable.
The response:
An externally hosted image should be here but it was not working when we last tested it.
The simulation uses a model of a 6AS7G, not the Russian equivalent 6H13C that I will be using. I scored the tubes very cheap in sealed boxes from Ukraine and Romania.
The output voltage with a sine wave input into an 8 ohm load. This looks better in the latter circuit revision.
Also note there is a DC offset. The DC offset is proportional to the amplitude of the output signal, but the time constant is such that it should be inaudible (it takes several seconds to develop) and the amplitude is such that the power wasted in the speakers will be less than 10% of the total input signal to the speakers. At idle with no input signal the DC offset at the speaker is less than 1mV and is easily adjustable:
An externally hosted image should be here but it was not working when we last tested it.
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6L6
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Joined 2010
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2010-12-17 3:14 pm
#9
- 2010-12-17 3:14 pm
- #9
This amp is really interesting!! I am looking forward to seeing all your progress!
Why do you want to build it as a Stereo amp? The amp itself is going to be huge, the PSU is going to be huge, and it seems like making it monoblocks will let you actually pick the thing up without requiring a forklift... 
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el156
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Joined 2010
2010-12-17 3:43 pm
#10
- 2010-12-17 3:43 pm
- #10
That amp would be very nice as amp for my ALL house , but also as a central heating system
How many watts will it consume?
Silvino
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SemperFi
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Joined 2002
2010-12-17 4:01 pm
#11
- 2010-12-17 4:01 pm
- #11
I will be following this I could use an amp like this in my house which is much too cold these days.
I agree simulations can be surprisingly accurate and your amp may very well perform. However, the simulations do not take into account the actual layout of components and how that interfers with an otherwise ideal assumption. You should give all tubes some random difference in mu or Gm to simulate offsets, as well as assume you'll have some unspecified temp drift.
I see you get farely symmetrical output swing even though you drive the output without compensating for the difference in the upper and lower tubes in the totempole. Only massive amounts of NFB will do that. That is my only caveat to real world performance.
Are you not going to use cathode resistors to help balance current sharing as well as offset drift?
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sampleaccurate
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Joined 2010
2010-12-17 5:28 pm
#12
- 2010-12-17 5:28 pm
- #12
Layout and Current Balancing
SemperFi said:
I will be following this
I agree simulations can be surprisingly accurate and your amp may very well perform. However, the simulations do not take into account the actual layout of components and how that interfers with an otherwise ideal assumption. You should give all tubes some random difference in mu or Gm to simulate offsets, as well as assume you'll have some unspecified temp drift.
I see you get farely symmetrical output swing even though you drive the output without compensating for the difference in the upper and lower tubes in the totempole. Only massive amounts of NFB will do that. That is my only caveat to real world performance.Are you not going to use cathode resistors to help balance current sharing as well as offset drift?
You bring up some good points that I haven't neglected. Certainly the amp must be constructed to minimize the potential for problems, and the layout is important. I've introduced some parasitic capacitance (but not inductance) in some key spots, and the circuit behaves rather well under simulation. It's difficult to get it to oscillate or misbehave without adding a lot of coupling in areas where there will be almost none due to careful layout.
There will be cathode resistors (not shown yet). Initially I intend to make those 1 ohm simply to allow measurement of the bias currents. I'll start with a lean bias and check each tube to see how much variation there is. The value of resistors required to keep the tubes reasonably well balanced will need to be experimentally determined. I considered changing gm and mu in the models to simulate "real world" tubes as someone suggested, but I have no idea how much variation to expect so I concluded it would be a waste of time and is best done experimentally.
The circuit is designed to bias the tubes well under their max power rating, and it's not that important for them to be balanced. It's more important to make sure that none of them exceed their dissipation rating and burn up. That I will be certain to do.
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2010-12-17 6:08 pm
#13
- 2010-12-17 6:08 pm
- #13
sampleaccurate said:
There will be cathode resistors (not shown yet). Initially I intend to make those 1 ohm simply to allow measurement of the bias currents. I'll start with a lean bias and check each tube to see how much variation there is. The value of resistors required to keep the tubes reasonably well balanced will need to be experimentally determined. I considered changing gm and mu in the models to simulate "real world" tubes as someone suggested, but I have no idea how much variation to expect so I concluded it would be a waste of time and is best done experimentally.
I recommend 5 ohm 5 watt devices. If you use 1 ohm 5 watt units, if a tube arcs that will be the end of it. The 5 ohm resistors also sound better. The only issue is that if a tube shorts, it may not blow out so you do have to think about speaker protection. IMO, rating the B+ fuse properly should do the job.
The 100 ohm grid resistors should be rated at least 1 watt. That way they can survive a tube shorting/arcing.
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sampleaccurate
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Joined 2010
2010-12-17 8:49 pm
#14
- 2010-12-17 8:49 pm
- #14
atmasphere said:
The 100 ohm grid resistors should be rated at least 1 watt. That way they can survive a tube shorting/arcing.
I didn't consider that yet, but you're right, using small resistors there would be asking for damage.
I think I'll throw some 5 ohm cathode resistors in the simulation to see how it affects the performance. I'll post the results when I get them.
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2010-12-17 11:08 pm
#15
- 2010-12-17 11:08 pm
- #15
If all the tubes are rated the same, I doubt it would help in simulation, but in practice you can hear it easily. These resistors would appear to raise the output impedance, but in practice they actually lower it, because the output impedance otherwise has a lot to do with which tubes are hogging the current.
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sampleaccurate
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Joined 2010
2010-12-19 4:02 am
#16
- 2010-12-19 4:02 am
- #16
atmasphere said:
If all the tubes are rated the same, I doubt it would help in simulation, but in practice you can hear it easily. These resistors would appear to raise the output impedance, but in practice they actually lower it, because the output impedance otherwise has a lot to do with which tubes are hogging the current.
That makes sense. If only some tubes are conducting fully and others are near cutoff the output impedance would be higher than if all were conducting.
I'm in the process of modeling a circlotron version of the amp. I ran across some interesting reading material and now I'm seriously considering the circlotron topology instead of a totem pole output. I still want to use 12 tubes per side. Believe it or not the parts count isn't much different. I was even intending to use lots of parallel 100VA isolation transformers for power, 6 to 8 on each side of the +/-150V supply. Instead I'll use 6 -8 transformers per channel, 3 or 4 in parallel for two 300 or 400VA matched supplies on each channel. The circlotron ties both ends of the load to a cathode, and the perfect symetry of the design insures the negative signal excursions don't exceed the positive ones as they do with a totem pole.
If it works in PSPICE this will be a 12 parallel tube OTL based on the Circlotron circuit.
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alexberg
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Joined 2009
2010-12-19 5:20 am
#17
- 2010-12-19 5:20 am
- #17
The farther is goes the more wishful it gets.
Unless you get trafos with exact turns ratio or
connect primary/secondary in series
while secondary/primary in parallel at the same time
you will find balancing power between transformers really frustrating.
Or...
Are you going to use separate power source per tube?
Power supply tubes are "very particular":
datasheet shows that the fixed bias current is all over the scale i.e. 40...112 mA.
Attached is the speadsheet with cathode resistors to use per number of tubes in parallel
along with power dissipation per anode.
5 ohm resistor in cathode would be of no use, at very least.
So, as mentioned before, do you have hundreds of tubes on avail to match... some?
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sampleaccurate
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Joined 2010
2010-12-19 6:32 am
#18
- 2010-12-19 6:32 am
- #18
alexberg said:
The farther is goes the more wishful it gets.
Unless you get trafos with exact turns ratio or
connect primary/secondary in series
while secondary/primary in parallel at the same time
you will find balancing power between transformers really frustrating.
Or...
Are you going to use separate power source per tube?
Power supply tubes are "very particular":
datasheet shows that the fixed bias current is all over the scale i.e. 40...112 mA.
Attached is the speadsheet with cathode resistors to use per number of tubes in parallel
along with power dissipation per anode.
5 ohm resistor in cathode would be of no use, at very least.
So, as mentioned before, do you have hundreds of tubes on avail to match... some?
The power trannys are close enough in turns ratio to use with no balancing problems, although I wouldn't recommend mixing transformers from different manufacturers or of different sizes.
I'm well aware the bias will be all over the place. A less than 3:1 ratio (112 to 40mA) is perfectly acceptable. 5 ohm resistors are for current limiting more than balancing.
What's the problem?
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alexberg
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Joined 2009
2010-12-19 7:03 am
#19
- 2010-12-19 7:03 am
- #19
sampleaccurate said:
The power trannys are close enough
SKIPPED
5 ohm resistors are for current limiting more than balancing.
What's the problem?
Alrighty then.
Please disregard what I've posted.
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sampleaccurate
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Joined 2010
2010-12-19 8:09 am
#20
- 2010-12-19 8:09 am
- #20
alexberg said:
Alrighty then.
Please disregard what I've posted.
I appreciate the warnings about the pitfalls of constructing and balancing an OTL. What I'm attempting has already been successfully accomplished by others with less experience than I have.
One thing I intend to do is individually bias each triode grid to balance the bias current. That's an easy thing to do. It takes a lot of pots and caps, unless the tubes are all biased lean and pull up resistors are used on the grids to raise the bias to what it should be. I've seen several methods for balancing tubes that don't come close to matching.
When using parallel triodes, diversity and probabalistics tell us that the more triodes we use the closer the balance will be. Some tubes will conduct as you point out up to 3 times as much current as others, but if none are allowed to exceed their power rating there should be a close balance when using many tubes, or the tubes could be graded and chosen so that the sum of the currents will match.
Nobody said this would be easy.
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