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Output transformer for 845 SE triode amp

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Cifrocco

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Joined 2009
#1
I'm new to tube amplifiers. I have a schematic from Led magazine for a single-ended 845 triode amplifier which I would like to build. The specs for the output transformer are not given. The circuit has the following parameters:

Plate voltage: 1,000V
Grid bias: -155V
Plate current: 65mA
Internal resistance: 1,900 ohms
Plate resistance: 9,000 ohms
Output power: 21W

What should the output transformer specs be? Is it simply matching the plate resistance to the speaker impedance? Would that be a 9,000-ohm primary with a tapped 4/8/16-ohm secondary?

Thanks.
 
#2
That's a good starting place. Sometimes it's better to present a load that is somewhat different from that. You need to read up on the subject, as there are tradeoffs. These involve power output, distortion, stuff like that. Of course, make sure your transformer can handle the unusually high voltages you are using.
 
#4
I am not familiar with the amp in question, but from your post 1900 ohms is the plate resistance of the 845 tube. 9000 ohms is the primary impedance of the output transformer. This will give a pretty good damping factor for the amp as the primary imp. is over 4 times the plate res. of the 845. 65ma is pretty low for current through the 845. I would raise it to 75ma, or even more if you have NOS American 845 tubes. GM70 is also a good output tube that would work well at these specs and is a cheap tube and much more rugged than the 845.
Daniel
 
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#5
Forgot to mention, it is a Class A design, which seems to go together with the 9k-ohm plate resistance, since I saw another schematic for an 845 push-pull design, operated as Class AB1, and having the following specs:

Plate voltage: 1,000V
Plate current: 70mA
Grid bias: -170V
Plate resistance: 4,600-ohms

also mentions (Rk) grid resistance (?) as 2 x 2,400 ohms.

I guess I need to do more research since I'm a little lost. 🙂

Thanks for the replies, and the safety tips, yes, I intend to be v e r y careful, trust me. 🙂
 
#7
Here's a link to a blog I once started and never have the time to tend to, it contains the schematic of my 845 SE amp design which I redrew myself entirely in Visio.

Math is the World: 845 Single-Ended Amplifier Schematic

Everything is correct except I didn't number the component designators yet, they all say R3 or C1, I think, but their values are correct.

Looks like a fairly straightforward design, it's really frustrating that the article (which is in French) doesn't specify the primary impedance of the output transformer.

Hammond has three output transformer models they say are suitable for the 845, and they have 5000, 6500, and 10K-ohm primaries. I'm guessing the 10k-ohm one should do, having a current rating of 90mA, 30W and 88 Henrys.

I'd also like to splurge for an interstage transformer between the ECL86 and the 845 but I haven't got a clue about the specs, or if the circuit can accept it as is by substituting it for the coupling capacitor.

This is all very fun and exciting though.
 
#8
A capacitor between the ECL86 and 845... LAME! If you want the 845 to sing, direct coupling is a must. Either by using split supplies with a cathode follower or with a transformer, or Tubelab's "Power Drive" circuit. I have never liked using power triodes with RC coupling to the grid; they seem to sound wimpy to me.
Daniel
 
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#9
As for the Interstage transformer a 5K to 5K unit would work. Look at Electra-Print, they have units that will work in that aplication. You want a 1 to 1 ratio transformer, not a step up transformer there, if you can help it.
Daniel
 
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