Simple and tested circuit. OPT is 6k6. Output power with 450 V supply voltage is some 30...35 W.
GNFB is set with R5 and frequency response with C3.
If the transformer has low leakage inductance, then C3, C8 and R21 are not necessary at all.
GNFB is set with R5 and frequency response with C3.
If the transformer has low leakage inductance, then C3, C8 and R21 are not necessary at all.
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6L6 Amplifier
The best circuit for an amplifier that you could use a 6L6 in is on page 696 of the RCA Receiving Tube Manual RC30.. although the circuit uses a 7027A (these are hard to find) a 6L6 would do the job just as well. see below circuit....I will send you the parts list if you want. Note I built this amp about 15 years ago and it is really a great performer.
The best circuit for an amplifier that you could use a 6L6 in is on page 696 of the RCA Receiving Tube Manual RC30.. although the circuit uses a 7027A (these are hard to find) a 6L6 would do the job just as well. see below circuit....I will send you the parts list if you want. Note I built this amp about 15 years ago and it is really a great performer.
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That is a 'Williamson' type topology; with a lot of NFB. Interestingly the OPT is not very good quality; the local NFB is needed to maintain stability. You could run that circuit with an EDCOR or organ amp OPT. LOVE that late tube era all pentode design though. The regulated G2 supply provides great performance also.
Many circuits coming up. At risk of going slightly OT, perhaps to define "best"?
Lowest distortion? Most 'musical' sound? Simplest? Some circuits will 'sound' rather nice - because they generate benign 'distortion'. Not all distortion is strident! Many excellent sounding amplifiers achieve that by having a few % 2nd harmonic and somewhat lower 3rd h. distortion, all known to make music sound richer.
Locally we have defined two concepts: (1) high-fidelity, as in 'closest approach to the original' (dictionary definition), and (2) high-nicety, as in musical/warm as described above. Thus hi-fi and hi-ni amplifiers for short.
There are many forms of the two typical popular topologies; the one first used by Williamson and the other first used by Leak. Both are excellent, with few disadvantages each. The values of components more often determine the sound quality than the topology.
My own thoughts would run as follows:
(1) It is impossible to design a power amplifier without audible distortion, without some degree of NFB. 'Hi-Ni' yes - 'Hi-Fi' no.
(2)Input stage:
I would prefer somthing where NFB stability is not influenced by the feeding impedance (as in Miller effect influence). If a low output impedance pre-amp is not guaranteed, that puts triodes second to pentodes. Further, at low signal amplitude pentodes have significantly lower distortion than triodes, although this % contribution may be small.
(3)Phase inverter:
The concertina (cathodyne) has excellent characterisitics, but not high amplitude output ability. Its unity gain also stresses the input stage, which must deliver the full peak grid-grid voltage for the power tube, unless followed by a p.p. gain stage as in the Williamson. Here also the Schmitt (LTP) has advantages (as per the Leak topology).
(4) Driving a power stage low rp tubes have significant advantages over high rp types regarding broadband/low h.f. phase shift, when later coming to applying NFB stably. This places pentodes second as well as the 12AX7 types, in favour of 12AT7 types - I later found the 6DJ8/ECC88 to be somewhat more linear; 12AU7 not so linear.
(5)Power Stage
Here the distributed load or UL can hardly be improved upon; best of both triode and pentode worlds.
For myself these factors should be considered if a hi-fi rather than a hi-ni amplifier is preferred. But that is individual choice. I believe the days of 'hi-fi rules' are over, It would appear that 'hi-ni' is often preferred - which is everyone's right!
Apology for length of post.
Lowest distortion? Most 'musical' sound? Simplest? Some circuits will 'sound' rather nice - because they generate benign 'distortion'. Not all distortion is strident! Many excellent sounding amplifiers achieve that by having a few % 2nd harmonic and somewhat lower 3rd h. distortion, all known to make music sound richer.
Locally we have defined two concepts: (1) high-fidelity, as in 'closest approach to the original' (dictionary definition), and (2) high-nicety, as in musical/warm as described above. Thus hi-fi and hi-ni amplifiers for short.
There are many forms of the two typical popular topologies; the one first used by Williamson and the other first used by Leak. Both are excellent, with few disadvantages each. The values of components more often determine the sound quality than the topology.
My own thoughts would run as follows:
(1) It is impossible to design a power amplifier without audible distortion, without some degree of NFB. 'Hi-Ni' yes - 'Hi-Fi' no.
(2)Input stage:
I would prefer somthing where NFB stability is not influenced by the feeding impedance (as in Miller effect influence). If a low output impedance pre-amp is not guaranteed, that puts triodes second to pentodes. Further, at low signal amplitude pentodes have significantly lower distortion than triodes, although this % contribution may be small.
(3)Phase inverter:
The concertina (cathodyne) has excellent characterisitics, but not high amplitude output ability. Its unity gain also stresses the input stage, which must deliver the full peak grid-grid voltage for the power tube, unless followed by a p.p. gain stage as in the Williamson. Here also the Schmitt (LTP) has advantages (as per the Leak topology).
(4) Driving a power stage low rp tubes have significant advantages over high rp types regarding broadband/low h.f. phase shift, when later coming to applying NFB stably. This places pentodes second as well as the 12AX7 types, in favour of 12AT7 types - I later found the 6DJ8/ECC88 to be somewhat more linear; 12AU7 not so linear.
(5)Power Stage
Here the distributed load or UL can hardly be improved upon; best of both triode and pentode worlds.
For myself these factors should be considered if a hi-fi rather than a hi-ni amplifier is preferred. But that is individual choice. I believe the days of 'hi-fi rules' are over, It would appear that 'hi-ni' is often preferred - which is everyone's right!
Apology for length of post.
This circuit looks very familiar. Can you explain where you got your inspiration?
That is a simple cathodyne circuit. It is not direct coupled so now one is dealing with an inherently unstable situation. That is why traditionally the practice was to use direct coupling from the voltage amp stage to the phase splitter. A pentode front end is able to swing lower and it has higher gain so that is why they did it that way. Cathodynes also have the advantage of not requring matched tubes. The only other front end that share this trait of not requiring matched sections is one of my faves...thefloating paraphase. That 6si7 would easily drive the 6l6 ouyputs in a floating paraphase because both tubes are ground referenced and see the full b+
6L6 PP Amp
I read your lengthy post with interest. It strikes me that the only reason one would want to use an existing design is due to their lack of knowledge on the subject. Reading your post you do not appear to fall into that category.... why therefore don't you design from scratch your own circuit. The joy and interest in Hi-Fi is not only in the building but the design as well?
I read your lengthy post with interest. It strikes me that the only reason one would want to use an existing design is due to their lack of knowledge on the subject. Reading your post you do not appear to fall into that category.... why therefore don't you design from scratch your own circuit. The joy and interest in Hi-Fi is not only in the building but the design as well?
I have built a lot of 5881/6L6 pp-amplifiers with different different topologies.
Now I wanted to make simple and all octal-base version with good performance. The gain of 6SL7 is sufficient for typical line-level sources giving up to 1 Vrms signal and some 10...12 dB GNFB.
I made a pcb. for this amplifier and it can be directly used with 6V6, 5881, 6L6 and EL34. The output transformer and supply voltage must be of course
selected according to the used tube.
The direct connected cathodyne is usually a compromise in terms of high output level and low THD. If the direc connection between voltage amplifier and phase splitter is used, then one or both stages operate in not optimum condition. When the THD of 1st stage is optimized then the anode voltage of this stage is a bit too high for optimum condition of the phase splitter.
Therefore I use ac-coupled cathodyne only.
If the required voltage level is 20 Vrms or lower then direct connected cathodyne is fully OK.
Below the amplifier pcb. with 6P6S (6V6) tubes.
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Thanks for the compliment, CV6045.
I have in fact done this; see my post #9 near the beginning of this thread. The circuit is suggested as reliable and reproducable. (I have since turned to 6DJ8/ECC88 for the phase inverter with minor change in the common cathode resistor; more linear than a 12AT7.)
If I may further comment (not as lengthy as before!) The advantage of a pentode as input also allows for a lower 'good' anode operating voltage than with a triode, for direct coupling, as mentioned by Artosalo in the previous post.
So have I, right here on this forum. Chrish and I developed and designed a 6L6GC based amp even though we are on opposite sides of the world. Our only means of communication was the following thread, so all the details, decisions, and experiments are here for all to follow. Chrish built a pair of completed mono blocks which have been running for 2 or 3 years.
My version was used as a breadboard test amp and has been used with everything from 307A's (a DHP) to KT88's, 6L6GC's, and of course BIG sweep tubes. The amp has a great sound, with good dynamics, and can be run completely balanced from input to output.
After 2 years of inactivity, the big Edcors are on order, and a 100 WPC sweep tube amp will be completed in a rather new chassis design.
http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp.html?highlight=6l6+ab2
Oops!
Big designs appearing here, all well capable of doing their thing. My own suggestion was not intended to fall outside the simple clean 30 - 40W kind of bracket.
I would mention one more small point in favour of pentode input stages. One must remember that distortion/non-linearity introduced at the NFB-signal mixing/summing point, is not cancelled by the feedback. (This refers to where e.g. the NFB is introduced to a cathode and the signal to a grid, i.a.w. mixed electronically through a non-linear 'interface'.) Although input stage distortion is normally low, it, subject to the above, can become significant in topologies where a triode input stage is followed by a cathodyne stage. I.e. where the input stage must supply the full power tube grid signal.
Again an optimum here is greatly dependant on the component values, etc. - I usually 'play chicken'!
Big designs appearing here, all well capable of doing their thing. My own suggestion was not intended to fall outside the simple clean 30 - 40W kind of bracket.
I would mention one more small point in favour of pentode input stages. One must remember that distortion/non-linearity introduced at the NFB-signal mixing/summing point, is not cancelled by the feedback. (This refers to where e.g. the NFB is introduced to a cathode and the signal to a grid, i.a.w. mixed electronically through a non-linear 'interface'.) Although input stage distortion is normally low, it, subject to the above, can become significant in topologies where a triode input stage is followed by a cathodyne stage. I.e. where the input stage must supply the full power tube grid signal.
Again an optimum here is greatly dependant on the component values, etc. - I usually 'play chicken'!
I first built my earlier posted design many years ago...It started out with the EF86 connected as a pentode and the KT88's running ultra-linear with negative feedback to the cathode of the EF86. I spent a lot of time playing with the circuit with my only guide being listening. I didn't have anything to measure what I was doing. The circuit ended up all triode with no feedback, which just sounded best to me. Can't wait to get this thing re-built and measure it now. It could be a total disaster! Interesting stuff!
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Personally I think the best instrument you can use to measure with is your own Ears....if it sounds right then its right.
Personally I think the best instrument you can use to measure with is your own Ears....if it sounds right then its right.
I pointed to our design for an example of the design process. As you stated about 8 years ago "simple" and "best" are not always compatible. The design we created was slanted toward "best" with no consideration for "simple."
I ran my version of the amp I designed with triode wired KT88's and zero feedback for most of the time. That configuration also sounds the best to me too. The big amp will run sweep tubes in pentode with only Schade feedback around the output tubes to make them act like triodes.
After thinking about this some more there is another way to get very good performance with a relatively "simple" build. I have been tinkering with a pair of Pete Millett's new driver board, using a wide variety of output tubes.
He built a PPP amp using 4 Loctal 807's per channel which are a 6L6GB variant. They are triode wired and run very conservatively for 40 WPC. The circuit looks rather complicated, but the build is rather easy because of the driver PC board.
http://www.diyaudio.com/forums/tube...h-pull-driver-pcb.html?highlight=driver+board
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