Push-Pull Amp with OTs from ST-70 or Fisher 500
May 17th, 2013, 11:35 am
Okay, last fall I had to rearrange my bi-amp system, and ended up with two Hafler 200 (unmodified) power amps. I don't really like how they sound. I may modify one for a bass amp, but I have to improve my voices and higher frequencies.
Huh, just remembered I have a Laff-a-Lot 550 tube amp (based on Harman Kardon Citation) with 7027s. Guess I could try that!
I'm thinking of using output transformers from a Dynaco ST-70, a Fisher 500 reciever, or maybe an Eico I have laying around as the foundation of a push-pull tube power amp. This is intended for the upper half (over 250 Hz) of my bi-amp system, so the bass extension is not critical, but will be considered.
Does anyone have opinions to share?
Thanks in advance!
Huh, just remembered I have a Laff-a-Lot 550 tube amp (based on Harman Kardon Citation) with 7027s. Guess I could try that!
I'm thinking of using output transformers from a Dynaco ST-70, a Fisher 500 reciever, or maybe an Eico I have laying around as the foundation of a push-pull tube power amp. This is intended for the upper half (over 250 Hz) of my bi-amp system, so the bass extension is not critical, but will be considered.
Does anyone have opinions to share?
Thanks in advance!
Re: Push-Pull Amp with OTs from ST-70 or Fisher 500
May 17th, 2013, 12:01 pm
Guy,
This will be fun......
How many watts do you need, above 250Hz?
Any of the OT choices you mentioned will work fine, but their primary impedance could end up dictating which output tube you use. Do you have a preference at this point?
If you can live with lower power, the Oddwatt/RedLightDistrict approach might be worth pursuing. The current source limits the output stage to Class A only. I know David built a variant with EL84s, and I think Joel did an RLD as well.
Why don't we have an amp shootout at your place?
This will be fun......
How many watts do you need, above 250Hz?
Any of the OT choices you mentioned will work fine, but their primary impedance could end up dictating which output tube you use. Do you have a preference at this point?
If you can live with lower power, the Oddwatt/RedLightDistrict approach might be worth pursuing. The current source limits the output stage to Class A only. I know David built a variant with EL84s, and I think Joel did an RLD as well.
Why don't we have an amp shootout at your place?
Guy wrote:Okay, last fall I had to rearrange my bi-amp system, and ended up with two Hafler 200 (unmodified) power amps. I don't really like how they sound. I may modify one for a bass amp, but I have to improve my voices and higher frequencies.
Huh, just remembered I have a Laff-a-Lot 550 tube amp (based on Harman Kardon Citation) with 7027s. Guess I could try that!
I'm thinking of using output transformers from a Dynaco ST-70, a Fisher 500 reciever, or maybe an Eico I have laying around as the foundation of a push-pull tube power amp. This is intended for the upper half (over 250 Hz) of my bi-amp system, so the bass extension is not critical, but will be considered.
Does anyone have opinions to share?
Re: Push-Pull Amp with OTs from ST-70 or Fisher 500
May 17th, 2013, 12:39 pm
I know I'm boring about this stuff but I spent years trying different front ends for PP amps and found that the classics sounded best to me (maybe that's why they are classic hmmm..). Then it depends on how much gain you need. If you need a lot use some version of the Mullard front end either like that of the Eico HF-60 or the Citation V with suitable modifications for what ever outputs you decide to use. The amount and compensation of feedback will need to be adjusted according to the output tubes and transformer.
If you need less gain versions of the Williamson still remain the best sounding PP amps I've ever heard. Implemented well they are amazing. I suggest a separate power supply for the input but if you use a choke input supply you could probably get away with a single supply for everything.
Charlie
If you need less gain versions of the Williamson still remain the best sounding PP amps I've ever heard. Implemented well they are amazing. I suggest a separate power supply for the input but if you use a choke input supply you could probably get away with a single supply for everything.
Charlie
Re: Push-Pull Amp with OTs from ST-70 or Fisher 500
May 17th, 2013, 1:10 pm
The RLD is class AB, so it's good for about 15W vs the oddwatt, and I'm sure I can bring it to the next "meet"
After hearing Charlie's mono blocks at the last CAF, I say try the Williamson front end, complete with 12AU7 and Duelund Cast caps.
After hearing Charlie's mono blocks at the last CAF, I say try the Williamson front end, complete with 12AU7 and Duelund Cast caps.
FerdinandII wrote:Guy,
This will be fun......
How many watts do you need, above 250Hz?
Any of the OT choices you mentioned will work fine, but their primary impedance could end up dictating which output tube you use. Do you have a preference at this point?
If you can live with lower power, the Oddwatt/RedLightDistrict approach might be worth pursuing. The current source limits the output stage to Class A only. I know David built a variant with EL84s, and I think Joel did an RLD as well.
Why don't we have an amp shootout at your place?Guy wrote:Okay, last fall I had to rearrange my bi-amp system, and ended up with two Hafler 200 (unmodified) power amps. I don't really like how they sound. I may modify one for a bass amp, but I have to improve my voices and higher frequencies.
Huh, just remembered I have a Laff-a-Lot 550 tube amp (based on Harman Kardon Citation) with 7027s. Guess I could try that!
I'm thinking of using output transformers from a Dynaco ST-70, a Fisher 500 reciever, or maybe an Eico I have laying around as the foundation of a push-pull tube power amp. This is intended for the upper half (over 250 Hz) of my bi-amp system, so the bass extension is not critical, but will be considered.
Does anyone have opinions to share?
Re: Push-Pull Amp with OTs from ST-70 or Fisher 500
May 19th, 2013, 11:13 am
Guy, unless things have changed in your rig, AFAIK, you have two sets of speakers, the D'Appolito ones and the Tympanis. If that is to remain, then the 15-18 watts probably won't do it, surely not for the Tympanis. Since changing from about 35WPC to 80 WPC on my Tympanis, above 360Hz, the macrodynamics and microdynamics were dramatically improved. So, I'd suggest that anything under 60WPC or so is a non-starter. Think KT88, transmitter or sweep tubes.
The major aspect of the sound of my system that has bothered me is the screechy treble. It seems each time I improved some other aspect of the system, the screech has gotten worse. Mostly on horn peaks or other high power high frequency sounds. I figured that like Accustats and ET speakers, perhaps this is a planar/ribbon/'stat issue. It is a challenge for lots of folks and I certainly know of some guys who won't use those speakers because of that sound and the power requirements.
Now, reading more about the PowerDrive (source follower) driver stage and the various forms of distortion resulting from RC coupling at the output grid, When I built an 18 watt SE SV572/100 amp for a friend, I used a source follower to drive the grid of the 572, which needs lots of current in A2. Tried it in my system. Guess what? Even with just 18 clean watts, no screech! Incredible treble resolution. Great sound. BUT, definitely lacking power for dynamic music at louder levels. Having heard the 572 amp and Dave M.'s Powerdrive amp, I really think what I was hearing with other amps, and what I hear in some other systems, is distortion due to the RC coupling. Tubelab George really explained a lot when he listed the various forms of distortion emanating from this scheme. Not just blocking, but in feedback amps, the distortion resulting from NFB attempting to "correct" gain lost at clipping, making the output even worse. Is this another reason we in general prefer amps without much overall NFB? Maybe.
So having said that, and having read extensively on PP amps with decent power, I'd suggest the amp build, customized for you of course, suggested on diyaudio, one which Tubelab George obtained ~60WPC from a single pair of KT88s using a source follower driver in TRIODE Class A2! That is an amp worth building.
should suit your needs. All the gain needed from two DC coupled 6SN7 cascaded differential amps, that feeding the MOSFETs. Neat method for DC balance is incorporated. Try it with the ST70 or Fisher 500 OPTs. They will be fine for power over 250HZ. If it works exceptionally, redo it using the KT550 transformers.
The schematic, with 6L6GC's is in post 460.
http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp-46.html
Check out post 272 for results without NFB.
http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp-28.html#post1883120
The major aspect of the sound of my system that has bothered me is the screechy treble. It seems each time I improved some other aspect of the system, the screech has gotten worse. Mostly on horn peaks or other high power high frequency sounds. I figured that like Accustats and ET speakers, perhaps this is a planar/ribbon/'stat issue. It is a challenge for lots of folks and I certainly know of some guys who won't use those speakers because of that sound and the power requirements.
Now, reading more about the PowerDrive (source follower) driver stage and the various forms of distortion resulting from RC coupling at the output grid, When I built an 18 watt SE SV572/100 amp for a friend, I used a source follower to drive the grid of the 572, which needs lots of current in A2. Tried it in my system. Guess what? Even with just 18 clean watts, no screech! Incredible treble resolution. Great sound. BUT, definitely lacking power for dynamic music at louder levels. Having heard the 572 amp and Dave M.'s Powerdrive amp, I really think what I was hearing with other amps, and what I hear in some other systems, is distortion due to the RC coupling. Tubelab George really explained a lot when he listed the various forms of distortion emanating from this scheme. Not just blocking, but in feedback amps, the distortion resulting from NFB attempting to "correct" gain lost at clipping, making the output even worse. Is this another reason we in general prefer amps without much overall NFB? Maybe.
So having said that, and having read extensively on PP amps with decent power, I'd suggest the amp build, customized for you of course, suggested on diyaudio, one which Tubelab George obtained ~60WPC from a single pair of KT88s using a source follower driver in TRIODE Class A2! That is an amp worth building.
should suit your needs. All the gain needed from two DC coupled 6SN7 cascaded differential amps, that feeding the MOSFETs. Neat method for DC balance is incorporated. Try it with the ST70 or Fisher 500 OPTs. They will be fine for power over 250HZ. If it works exceptionally, redo it using the KT550 transformers.
The schematic, with 6L6GC's is in post 460.
http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp-46.html
Check out post 272 for results without NFB.
http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp-28.html#post1883120