"Probably too hi cathode degeneration ?
The sum of cathode resistors is appoximately equal to the anode load !
And the same current flows thru both.
If you drive'em with a dual widings IST, you could reference (AC only ! ) the 'cold' side of each winding to the respective cathode rather than to the ground.
If you can do that, you could also move the OPT between the 6080 cathodes and put a common bias resistor at CT.
I wonder if this kills Miller caps ?
Yves."
Tried that. Bad results. The current through the 6080 went up considerably, dropping the plate voltage availble to 80V. Overall much distortion and heat. May have done something wrong. took it out. Added cathode bypass to the 6080's.
In its case and final test configuration tried. I reduced the plate to plate feedback resistor to 75kohms. At 25mA driver current i have flat response down to 15hz (gone by 10hz) and now it extends flat to 30khz and tails off slowly to about 50khz. Very satisfactory. Can't tell you its final output power as I have only tested it on crappy speaker which distort badly above a few watts. Sine wave goes through absolutely clean. Square wave has a bit of overshoot and goes triangular above about 15khz. Still I am happy with the overall performance. The whole output stage amplifies what the driver stage gives it without adding any significant amount of distortion, and thats without any global feedback. Got about 5x voltage gain at this stage.
Still got plenty of case work to do so will be a few weeks before its completely finished and on test in my main system.
Got some nice russian coke bottle shape 6H13C and ECL82's.
Will keep you posted.
The sum of cathode resistors is appoximately equal to the anode load !
And the same current flows thru both.
If you drive'em with a dual widings IST, you could reference (AC only ! ) the 'cold' side of each winding to the respective cathode rather than to the ground.
If you can do that, you could also move the OPT between the 6080 cathodes and put a common bias resistor at CT.
I wonder if this kills Miller caps ?
Yves."
Tried that. Bad results. The current through the 6080 went up considerably, dropping the plate voltage availble to 80V. Overall much distortion and heat. May have done something wrong. took it out. Added cathode bypass to the 6080's.
In its case and final test configuration tried. I reduced the plate to plate feedback resistor to 75kohms. At 25mA driver current i have flat response down to 15hz (gone by 10hz) and now it extends flat to 30khz and tails off slowly to about 50khz. Very satisfactory. Can't tell you its final output power as I have only tested it on crappy speaker which distort badly above a few watts. Sine wave goes through absolutely clean. Square wave has a bit of overshoot and goes triangular above about 15khz. Still I am happy with the overall performance. The whole output stage amplifies what the driver stage gives it without adding any significant amount of distortion, and thats without any global feedback. Got about 5x voltage gain at this stage.
Still got plenty of case work to do so will be a few weeks before its completely finished and on test in my main system.
Got some nice russian coke bottle shape 6H13C and ECL82's.
Will keep you posted.
Thanks for the tip.
That would require a blocking cap in the secondaries return leg - which I didn't have. What sort of value would you think. What benefits would you expect to achieve as a result. If its down as low as 0.47uf I suppose that it would behave better than the cathode bypass cap which is 1000uf + 10uf.
Do you think it would work with the cathode bypass, or would it be better alone?
I am quite willing to try again if I can expect an improvement.
Shoog
That would require a blocking cap in the secondaries return leg - which I didn't have. What sort of value would you think. What benefits would you expect to achieve as a result. If its down as low as 0.47uf I suppose that it would behave better than the cathode bypass cap which is 1000uf + 10uf.
Do you think it would work with the cathode bypass, or would it be better alone?
I am quite willing to try again if I can expect an improvement.
Shoog
Yes, a blocking cap, in any leg, hot or return.
A grid leak resistor to ground is necessary of course.
The cap and the grid leak resistor form an hi pass, as in conventional cap coupled amp, here perhaps 0.5µ and 200K ?
The "good" is that this replace cumbersome and always critical cathode bypass.
The "bad" is that you no longer can push the 6080 in AB2.
To be checked
Yves.
A grid leak resistor to ground is necessary of course.
The cap and the grid leak resistor form an hi pass, as in conventional cap coupled amp, here perhaps 0.5µ and 200K ?
The "good" is that this replace cumbersome and always critical cathode bypass.
The "bad" is that you no longer can push the 6080 in AB2.
To be checked
Yves.
"The "bad" is that you no longer can push the 6080 in AB2."
Mine is working in purely class A so its not really an issue.
By going down this path you would be losing one of the supposed advantages of an interstage transformer, namely an ultra low resistance earth reference for the grids, and the elimination of overload recovery of the blocking cap should the grid go positive. Though i'am not entirely certain the grid can draw any DC current from the interstage anyway.
On balance I think the cathode bypass might have fewer evils and allow the interstage to perform at its best.
Shoog
Mine is working in purely class A so its not really an issue.
By going down this path you would be losing one of the supposed advantages of an interstage transformer, namely an ultra low resistance earth reference for the grids, and the elimination of overload recovery of the blocking cap should the grid go positive. Though i'am not entirely certain the grid can draw any DC current from the interstage anyway.
On balance I think the cathode bypass might have fewer evils and allow the interstage to perform at its best.
Shoog
Moved the amp into the main system for final testing. Initial impressions are very favourable. Bass is deep and well defined and the top end is clean and well balanced. Good wide sound stage. I listened to some acoustic music first and I was impressed at how natural and detailed it all sounded. Putting on some Godzilla and the bass impressed. However if driven hard it did sound a little harsh on the top end. Could be overdriving my speakers. I will have to scope it up to find out whats going on. Kate Bushes Arial sounded fantastic.
I would almost go as far as to say that it sounds better, in almost all departments, than my 807 Parafeed SEPP amp. I will do a switch back tonight to confirm this.
O-yes very little hum to speak of, less than the SEPP
Shoog
I would almost go as far as to say that it sounds better, in almost all departments, than my 807 Parafeed SEPP amp. I will do a switch back tonight to confirm this.
O-yes very little hum to speak of, less than the SEPP
Shoog
I just did a side by side comparison between the new amp and my 807 SEPP amp. I was staggered by the difference. The SEPP sounds good and clean but the bass just doesn't have any punch and definition to it comapared to the 6080 amp. I took the SEPP down to my bench to see what was happening on the scope. The frequency response of both amps was roughly comparable, with the SEPP going down to a respectable 15hz and up to 50khz without significant roll off. Under 70hz and the SEPP was showing significant crossover distortion where as the 6080 was showing now.
This doesn't explain the lack of punch though. The only thing that the SEPP might have over the 6080 amp is slightly cleaner highs. Though the SEPP went into distortion much earlier than the 6080 PP.
I can only attribute this qualitive difference to the intrinsic difference between PP and SE. On the strength of what I have found so far I will have to improve the 807 SEPP by a huge margin or decomission it and build a 807 PP instead. The 6080PP sounds fantastic.
Shoog
This doesn't explain the lack of punch though. The only thing that the SEPP might have over the 6080 amp is slightly cleaner highs. Though the SEPP went into distortion much earlier than the 6080 PP.
I can only attribute this qualitive difference to the intrinsic difference between PP and SE. On the strength of what I have found so far I will have to improve the 807 SEPP by a huge margin or decomission it and build a 807 PP instead. The 6080PP sounds fantastic.
Shoog
Hi Shoog
Congratulations with the good results! SY will be in heaven knowing there is another one converted from SE to PP!
Erik
PS. I received the small mosfets for your version of the FVP5 buffer , but at the moment my whole 'hobby' room is a mess because of some changes in placement of tables and so... DOnt'even know where i put the mosfets...
Congratulations with the good results! SY will be in heaven knowing there is another one converted from SE to PP!
Erik
PS. I received the small mosfets for your version of the FVP5 buffer , but at the moment my whole 'hobby' room is a mess because of some changes in placement of tables and so... DOnt'even know where i put the mosfets...
I'am glad that your going to give my preamp circuit a try. I hope it works out as well for you as it has for me. Any questions just ask. Do you have any other valve preamps which you can compare it to. I recently built a CCS loaded 5687 preamp. The FVP5 clone is significantly better, though the 5687 could probably be optimised more. I will be very interested in your results and inmpressions.
I was thinking about what I could do to improve the SEPP amp. It occurred to me that probably the best thing would be to convert it to a Class A tetrode 807 PP amp. This would give me a 36.5W output for about twice the input power. The power transformer I have has both a 900V-0-900V winding and a 350V-0-350V winding. This would happily supply supply the 600V plate if choke loaded. The second winding could happily supply the 300V screen supply. Exactly the same driver stage could be used as the 6080 PP amp, so retaining the benefit of the interstage transformer. The valve complement would go up by two overall. Because I would have a bit of +B to burn, I could implement a garter bias arrangement. This uses twice the cathode resistance to balance out current draw between the PP valves.
Practically the only thing I would need to replace would be the 10K a-a output transformers.
The only thing is I don't really need 36W of output power, or the 8x more gain !
Shoog
I was thinking about what I could do to improve the SEPP amp. It occurred to me that probably the best thing would be to convert it to a Class A tetrode 807 PP amp. This would give me a 36.5W output for about twice the input power. The power transformer I have has both a 900V-0-900V winding and a 350V-0-350V winding. This would happily supply supply the 600V plate if choke loaded. The second winding could happily supply the 300V screen supply. Exactly the same driver stage could be used as the 6080 PP amp, so retaining the benefit of the interstage transformer. The valve complement would go up by two overall. Because I would have a bit of +B to burn, I could implement a garter bias arrangement. This uses twice the cathode resistance to balance out current draw between the PP valves.
Practically the only thing I would need to replace would be the 10K a-a output transformers.
The only thing is I don't really need 36W of output power, or the 8x more gain !
Shoog
Hi Shoog. I am building an active crossover http://www.glass-ware.com/tubecircuits/Tube_Crossover.html with bipolar supplies. I have physically separated the crossover circuit from the buffer. So it will be possible to combine a certain buffer with a certain crossover. So far I have concluded some crossover circuits and some buffer circuits (with PCF80, the triode as buffer, the pentode as CCS.) I will make this one play first, and when it works fine (I am trying to make a 'universal circuit', but I have still to test some issues) I plan to compare buffer circuits: yours and the buffer circuit from the Aikido are the first that come to mind, but others are also possible (all will be running from from bipolar power supply).
now let 's get back to work
Erik
Hi there all,
I discovered why the SEPP wasn't performing to its best. Some time ago I had configured it as UL. On most material this was fine, but when there was heavy bass the OT was saturating below 70hz. I have now reversed this mod and things are a lot better in the bass region.
I SEPP has all of the classic SET virtues (clean sweet mids and treble) but I still have a gut feeling that the overall balance of the 6080PP is better.
Shoog
I discovered why the SEPP wasn't performing to its best. Some time ago I had configured it as UL. On most material this was fine, but when there was heavy bass the OT was saturating below 70hz. I have now reversed this mod and things are a lot better in the bass region.
I SEPP has all of the classic SET virtues (clean sweet mids and treble) but I still have a gut feeling that the overall balance of the 6080PP is better.
Shoog
Re: Idea for a 2Tube 6080 PP amp.
I had a crazy idea to build a 2 tube PP amp. The idea would be to have a beefy single pentode driver stage (choke/CCS loaded) and then to do the phase splitting by taking the signal for one of the 6080 triodes from the cathode of the other.
I have made 6080/6AS7 single tube PP. I used two pentodes for the driver/phase splitter. See my Web page below.
http://ja1cty.servehttp.com/6080/6080pp.png
http://ja1cty.servehttp.com/6080/
Sorry written in Japanese.
I had a crazy idea to build a 2 tube PP amp. The idea would be to have a beefy single pentode driver stage (choke/CCS loaded) and then to do the phase splitting by taking the signal for one of the 6080 triodes from the cathode of the other.
I have made 6080/6AS7 single tube PP. I used two pentodes for the driver/phase splitter. See my Web page below.
http://ja1cty.servehttp.com/6080/6080pp.png
http://ja1cty.servehttp.com/6080/
Sorry written in Japanese.
I like your idea, its how I started at the concept stage of my PP amp. However I have to tell you that there are a few good reasons why it won't work particularly well. The 6080 is a very low MU valve and the self splitting arrangement you are suggesting really needs a medium MU valve to work satifactorily. It might work but you should consider it a SE amp with current compensated PP output transformer (ie the second tube is simply there to keep the PP output transformer from saturating). It might work and and it might not - my hunch is not. If it worked it would behave much more like an SE amp but probably with a punchier bass. If you go for it then seperate LM317 CCS in the tail of each output triode would be essential, in this case the capacitor coupled differential output stage becomes really attractive.
The other problem you will face is that the 6080 is a very hard driven tube, all but the meatist driver stage will sacrifice the top end. You might just get away with driving a 6AU6 hard into it but most small signal pentodes will wimp out, but again only experience would tell. Maybe a EL84 as a driver would work, but it seems a bit of a waste.
I finally dropped the ECL82 from my 6080PP amp. It just wasn't a happy fit and tended to sound a bit harsh. I replaced it with a parallelled 5687 with CCS anode load. I run this at a wopping 40mA. This still only gives 20mA per grid of drive current per output grid. Doing anything with a 6080 has to be considered as a brute force exercise. Well worth it in the end though.
If you are interested I can send you a copy of the schematic for the final version of the amp, I consider it a very optimised version of what is possible with this tube. Drop us an email.
Shoog
The other problem you will face is that the 6080 is a very hard driven tube, all but the meatist driver stage will sacrifice the top end. You might just get away with driving a 6AU6 hard into it but most small signal pentodes will wimp out, but again only experience would tell. Maybe a EL84 as a driver would work, but it seems a bit of a waste.
I finally dropped the ECL82 from my 6080PP amp. It just wasn't a happy fit and tended to sound a bit harsh. I replaced it with a parallelled 5687 with CCS anode load. I run this at a wopping 40mA. This still only gives 20mA per grid of drive current per output grid. Doing anything with a 6080 has to be considered as a brute force exercise. Well worth it in the end though.
If you are interested I can send you a copy of the schematic for the final version of the amp, I consider it a very optimised version of what is possible with this tube. Drop us an email.
Shoog
Shoog said:
I built a parafeed 6080, using a 12at7 driver. At first, I had the 12at7 with 100K plate load, but the 12at7 can't drive the 6080 satisfactorily.
I don't have any other tube that is better than the 12at7, so I lowered the B+ for the 12at7 and used plate choke for the 12at7, sounds a bit better. In the parafeed circuit, the 6080 is very promising!
I am still in the hunt for a better driver for the 6080.
Would a triode strapped 6688 work?
Shoog said:
My parafeed 6080 is not that heavy. I used a 2.54cm stack of 76 EI core for the plate chokes and a 50VA 230V pri, 12V sec Talema toroidal for the OPT. The power transformer is also 76 EI core with 5cm stack height. In all, I have EIGHT transformers in the amp, (power, psu choke, four plate chokes and two OPTs), but none of them is bigger than a 76 EI core. I wonder how it would sound with an interstage?
I got the idea of using toroidals from Shoog's post of using toroidal for OPT.
Where you using main power EI's or proper SE transformers. The main transformers perform very poorly in this capacity.
I was thinking that if I did build this amp I could use dinky little 30VA transformers as the outputs. What voltage and current are you running your 6080's at. I have found they work very well at low volts high current. How is it sounding and have you got any other SE amp to compare it t.
Shoog
Shoog said:
I was using an EI mains transformer at first for parafeed OPT, then switched to a toroidal mains transformer, and big difference.
I also built a 12b4 parafeed using an EI mains transformer for OPT, bass and treble rolloff (but still sounds good). I might get some more toroidal mains transformer to use for OPT on this circuit.
I have built Mikael's SE KT88 amp and I think I like my 6080 parafeed better. The vocals on the 6080 parafeed have more clarity, highs are better, but the SE KT88 has a bit better bass (not a lot better, just a bit better).
I am thinking of building a parafeed EL34 early next year.
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