So I was given some output transformers from my father in law. (I(m also getting some Unitran 9U13 when he cleans some of his workspace)
They are a bit rusty but the innards are probably still good.
They where used with some big PA amps using 4 output tubes of the kt88/6550 sort.
So after a bit of looking I saw A-451 stamped somewhere. the spec tag was removed a long time ago.
After a bit of searching it appears that they are the Dynaco A-451 output transformers.
http://knob.planet.ee/kirjandus/books/Dynaco_Transformers.pdf
120w from 20-20khz and 240w from 30-15kHz.
They have screen taps so UL is possible.
Primary load is 2k2.
So I have 2 of these. Question is now what to do with them.
I was thinking they would make a nice amp for Bass guitar.
They handle 240w at 30Hz. So I think at around 40Hz this could be a lot more. As long as the tubes used can of course.
Another option would be a hefty Hifi amp. I could just use 4 KT88's in UL and be done and aiming for around 200w. Altough getting there needs and Anode voltage of 560v and using Taps for the screen I think the screen would get a hard life.
I also like to use some other tubes then standard🙂 EL156 has been long on the list to try
So have any of you guys have experience using this Dynaco A451 output?
They are a bit rusty but the innards are probably still good.
They where used with some big PA amps using 4 output tubes of the kt88/6550 sort.
So after a bit of looking I saw A-451 stamped somewhere. the spec tag was removed a long time ago.
After a bit of searching it appears that they are the Dynaco A-451 output transformers.
http://knob.planet.ee/kirjandus/books/Dynaco_Transformers.pdf
120w from 20-20khz and 240w from 30-15kHz.
They have screen taps so UL is possible.
Primary load is 2k2.
So I have 2 of these. Question is now what to do with them.
I was thinking they would make a nice amp for Bass guitar.
They handle 240w at 30Hz. So I think at around 40Hz this could be a lot more. As long as the tubes used can of course.
Another option would be a hefty Hifi amp. I could just use 4 KT88's in UL and be done and aiming for around 200w. Altough getting there needs and Anode voltage of 560v and using Taps for the screen I think the screen would get a hard life.
I also like to use some other tubes then standard🙂 EL156 has been long on the list to try
So have any of you guys have experience using this Dynaco A451 output?
Using a soft bristle toothbrush, gently remove loose rust from the lamination stack. Follow the brushing with a rust converter and, finally, paint. Abrasives are an absolute no/no, as insulation between individual laminations must be maintained.
Consolidate crumbling cloth wire insulation with clear nail lacquer. Follow with polyolefin heat shrink tubing. The composite is somewhat stiff, but it will last.
For HIFI PPP KT88s configured for approx. 130 WPC seems about right. If you use UL mode safety, current limiting, resistors between the screen grids and the "iron" is definitely in order. Full pentode mode, with regulated g2 B+, is an excellent alternative to UL.
BTW, if this becomes a bass guitar amp project, have the moderators move the thread over to the Instruments and Amps "board".
Consolidate crumbling cloth wire insulation with clear nail lacquer. Follow with polyolefin heat shrink tubing. The composite is somewhat stiff, but it will last.
For HIFI PPP KT88s configured for approx. 130 WPC seems about right. If you use UL mode safety, current limiting, resistors between the screen grids and the "iron" is definitely in order. Full pentode mode, with regulated g2 B+, is an excellent alternative to UL.
BTW, if this becomes a bass guitar amp project, have the moderators move the thread over to the Instruments and Amps "board".
I have a pair of NOS Dynaco A451, if you need measurements i can assist.
I think these are potted, it's only the outer casing that has rusted. I could be wrong, have to check them a little bit more.
And they are really "huge"
I was even thinking of using a sexted and use a little bit more voltage and regulated screen grids (I think in these days regulated screens can be easy and should be done if UL mode is not used). gives a little bit more headroom I think and the tubes will work less hard = less distortion
the secondary is 4/8/16 and if I remember there is a trick to connect 4ohm to gnd and use 0 and 16 for Cathode feedback.
The leslie 147 does this and it works very nice. Altough I don't know if it's a good idea I have read some mixed experiences about this.
I have been thinking some more and the 807 tube is one of the tubes I want to build an amp from. It has a very nice look to it.
Six of those (or maybe even 8) could be used with this transformer I think.
I remember reading about them and the screen grid can't handle much abuse.
Anode can handle High voltage and the tube is capable of class AB2.
For driving them I was thinking of mosfet followers on each tube with individual Bias settings. 300v regulated on screens and 600v on the plate (or maybe more....)
Don't know what the dynaco transformer can handle for voltage tough....(anybody an idea about this?)
Another benefit is that 807's are cheap.....
Six of those (or maybe even 8) could be used with this transformer I think.
I remember reading about them and the screen grid can't handle much abuse.
Anode can handle High voltage and the tube is capable of class AB2.
For driving them I was thinking of mosfet followers on each tube with individual Bias settings. 300v regulated on screens and 600v on the plate (or maybe more....)
Don't know what the dynaco transformer can handle for voltage tough....(anybody an idea about this?)
Another benefit is that 807's are cheap.....
The A451 transformer handles current production 6550 / kt88 tubes well.
If you insist using out-of-production tubes that cannot use UL capabilities
of the A451, then i suggest you get another transformer. The price differences
could be huge as you sell the A451
Just in case anyone is interested there is a nice article in the Sept. 1961 issue of Audio magazine titled Another Power Amplifier. It's a 2 chassis monoblock using 4 6550s, but you could easily use 6 807s instead. It's a little complex but I like it much better then the Dynaco Mark VI.
However there is an error of omission in the power supply schematic. There should be a connection between the 6X4 cathode and one of the 5U4 plate windings. Also, in the parts list, the UTC type S46 is listed as 2000-0-2000 volts. This is wrong! It actually is 1000-750-0-750-1000 volts per the UTC catalog. Since this transformer is long out of production you'll have to substitute it with another 1000 volt (or close) transformer.
However there is an error of omission in the power supply schematic. There should be a connection between the 6X4 cathode and one of the 5U4 plate windings. Also, in the parts list, the UTC type S46 is listed as 2000-0-2000 volts. This is wrong! It actually is 1000-750-0-750-1000 volts per the UTC catalog. Since this transformer is long out of production you'll have to substitute it with another 1000 volt (or close) transformer.
Nice article.
The input/phase splitter is something I have never seen before.
Isn't 800v on 6550 tubes a little bit to much?
On a 807 This would be fine as long as dissipation is ok because there is the top cap for the plate but on an octal.....
That amplifier is also designed with the way PA amps are rated today. (burst power)
And I can imagine having the capability of some extra peak power makes for a good sounding amplifier.
I was thinking of using the screen taps o the output transformer to maybe feed the driver stages and have something like an e-linair circuit I believe it's called.
And I think I have a 1000-0-1000v plate transformer somewhere and a big choke of 7H that will do 500mA at least.
The input/phase splitter is something I have never seen before.
Isn't 800v on 6550 tubes a little bit to much?
On a 807 This would be fine as long as dissipation is ok because there is the top cap for the plate but on an octal.....
That amplifier is also designed with the way PA amps are rated today. (burst power)
And I can imagine having the capability of some extra peak power makes for a good sounding amplifier.
I was thinking of using the screen taps o the output transformer to maybe feed the driver stages and have something like an e-linair circuit I believe it's called.
And I think I have a 1000-0-1000v plate transformer somewhere and a big choke of 7H that will do 500mA at least.
It's called a Van Scoyoc cross-coupled phase inverter. It's an old circuit from the 40's and it's not used in modern made amplifiers mostly because it requires an extra tube for little or no benefit to them. And like all circuits it has it's pros and cons. I like it for the symmetry alone. And being an old tube horder that doesn't bother me at all. I've used it in a slightly modified form with an NPN transistor in the cathode of each input tube. But that was well before John Broskie's blog about them. 2500 ohms is a tough load for a cathode follower. Although using a negative voltage source for the cathodes will allow the use of higher resisters so the tube will operate in a more linear area. And then there's a cross-coupled cascode inverter I also like.
Now, 800 volts on a 6550? Probably not. Certainly not with today's dubious ersatz copies. You might get away with it using a NOS GE 6550A. But if I were to build that amp, which I was once going to do, I'd use a little less voltage and settle for slightly less power and better reliability. And I would not use black sockets. I like miconal (brown) or porcelain sockets.
Thanks for this phase inverter. I like the symmetry. And if the amp has a negative bias supply the larger resistance for the kathode follower is not a problem.
Another benefit is that it is DC coupled so this could be good when using GNFB
I think 807's will handle 800V just fine. (I've seen local ham's using them at 1kv just fine) as long as the screen is within ratings and stabilized. because of the top cap there will also be no flash overs on the socket.
Using such A voltage that is allowed to sag a little under load makes for an amplifier that will sound very powerfull.
When I began making amplifiers and diy I was always convinced that an amplifier should be able to make the claimed power 24/7 continous.
But then I've noted some sounded more powerfull then others yet the continous power was the same.
So now I know that giving some extra dynamic headroom does a lot but I've never seen something written about this in early tube amp literature. nice to see this in that article.
Another benefit is that it is DC coupled so this could be good when using GNFB
I think 807's will handle 800V just fine. (I've seen local ham's using them at 1kv just fine) as long as the screen is within ratings and stabilized. because of the top cap there will also be no flash overs on the socket.
Using such A voltage that is allowed to sag a little under load makes for an amplifier that will sound very powerfull.
When I began making amplifiers and diy I was always convinced that an amplifier should be able to make the claimed power 24/7 continous.
But then I've noted some sounded more powerfull then others yet the continous power was the same.
So now I know that giving some extra dynamic headroom does a lot but I've never seen something written about this in early tube amp literature. nice to see this in that article.
I would hesitate to use 800V or above with a A451 transformer.
It's not designed for those voltages, and it is no longer in production, thus
a transformer failure will render the amp bricked.
What's wrong with a quad 6550 @500Volt as suggested by dynaco schematics ?
You have a good point there.
The thing is that at the dynaco datasheet of the A-451 it states:
120W:20-20kHz
240W:30-15kHz
so it should be possible to get 240W? would be nice,
Power will still be limited at 20Hz but there is not that much material in music between 20-30Hz.
That's why I am thinking how to get to 240W
I also am not a big fan of the 6550.
KT88 could be the way. I believe they would do 200W in PPP UL with around 560v on the plates.
I do like the look of the 807. But indeed it can't be used for UL because of the low screen specification.
So probably it will become the KT88.
The thing is that at the dynaco datasheet of the A-451 it states:
120W:20-20kHz
240W:30-15kHz
so it should be possible to get 240W? would be nice,
Power will still be limited at 20Hz but there is not that much material in music between 20-30Hz.
That's why I am thinking how to get to 240W
I also am not a big fan of the 6550.
KT88 could be the way. I believe they would do 200W in PPP UL with around 560v on the plates.
I do like the look of the 807. But indeed it can't be used for UL because of the low screen specification.
So probably it will become the KT88.