60 ohm choke - EL34 AB1 pair - considerations

[pjd3]

Hello, hope some of you could comment on this.

I'll be sparking up an amp I built up on my "tube amp design station" within the coming week. It is pretty much a 3 stage preamp (Shiva clean channel - I think) with long tail pair PI into a pair of EL34's for testing. I may try a pair of KT77's later on following getting the EL34's going, but for now...

The only choke I have at this moment is a 15H - 60 ohm choke pulled from one of my old 50 watt amps. With a pair of EL34's with about 450v on the plates, do you think it wise to add a resistor in series with the choke? I was concerned that the screen voltage will be too high or maybe too close to the EL34 plate voltage.
I probably should mention that at this time I will be sparking up this amp using an undersized OT pulled from a hi fi amp that fed a pair of 6V6's. I did the testing and math on the OT to determine primary and tap impedances (winding ratio - impedance ratio - etc..) so I know how to pair the OT with the properly reflected load. Plus, I know better than to go any further than a comfortable bedroom volume as not to get anywhere near the max current capacity of the OT. The amp is for very clean guitar tone only so, absolutely no need for pushing things at this stage (or ever!). I'm only really testing for cleanish tone.

Do you think the EL34's and screens will be OK if I'm playing at very low volume? Or should I toss in say, a 100 or 150 ohm resistor is series with the choke?
Also, for screen resistors I was going to use a couple of 5 watt - 1200 ohm resistors.

Thank you, and appreciate any feedback on this!
Best,

Phil D

[sluckey]

I'd use the choke with NO resistor. The Hammond Plexi replacement choke has 80Ω DCR.

[Stevem]

Leave yourself the option of adding a resistor should you find after doing test and the math that your loading the screens too high.

Unless your running very tightly matched outputs the screens between the output tubes will pull a different amount of current and you want to compensate for that.

[martin manning]

...I will be sparking up this amp using an undersized OT pulled from a hi fi amp that fed a pair of 6V6's. I did the testing and math on the OT to determine primary and tap impedances (winding ratio - impedance ratio - etc..) so I know how to pair the OT with the properly reflected load. ... Also, for screen resistors I was going to use a couple of 5 watt - 1200 ohm resistors.

So your primary impedance will be ~4k? The 1200 ohm screen resistors will compensate for the choke's low DCR.

[pjd3]

Thank you for all of the informative responses!

I"m left with the notion that I'm good to kick off but just check where everything is along the way.

I had wondered if the 1200 ohm screen resistors would give some coverage for the screens. Last night was the first time the role of the screen resistor started to make some sense, that it would come more into play as bigger plate voltage swings were present and the screens would start to draw more current causing a larger voltage drop across the screen resistor that would occur, effecting the capacity of the size of the voltage swings. I imagine this would change the shape of the wave some and perhaps that's why some were commenting about increasing distortion and compressing at those moments with larger value screen resistors. That, and the chosen value of screen resistance being somewhat of a balancing act between offering current limiting when needed and allowing greater un-restricted output power to develop. Interesting little resistor that screen resistor is.

You will likely see another thread by me asking the best way to establish which of the OT secondary wires is to be considered the "common". Reason I am not sure is that the DCR values weren't telling me enough to make a call. What I did for now was to simply find the 2 out of 3 secondary wires that had the largest DCR (1.8 ohms) and had to assume that this was the whole coil end to end. I wasn't able to figure out which one was actually the other tap, and not the common. It was late, I probably was running on a small brain.

Thanks again for your generous and helpful input!
Best,
Phil D

[Roe]

how much current does the choke handle? 70ma or more?

[Stevem]

Can you post a picture of your OT and it's wires?

[martin manning]

…the DCR values weren't telling me enough to make a call. What I did for now was to simply find the 2 out of 3 secondary wires that had the largest DCR (1.8 ohms) and had to assume that this was the whole coil end to end.

Put some AC voltage on the primary and measure voltages on the secondary. You should be able to verify that you have identified the ends of the coil (the pair with the highest voltage) and sort the taps out.

[pjd3]

Thanks Martin,

Thats what I neglected to do. I did apply voltage (12vac) to the secondary to calc out the winding/impedance ratio of the full secondary but stopped there. It was enough to know I could put an 8 ohm speaker on that particular pair of wired. This is an old OT where the colors of the wires are indistinct plus there are two colors somehow on each of them. Must be 60 years old. I was surprised it still even worked and held the (unloaded) 480vdc from the PS.

The choke is likely to handle the current as it is from an amp that used a pair of 6L6GC's. Its big, much bigger that what you would see in even 40 watt Fenders and Marshalls.

I need to figure out how to post pics, and it this can happen using personal computer files.

Thank you everyone, I do have a little more work to do on this topic!

Best,

Phil D