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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

FireFly Single Ended Tetrode

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Interesting point Ian.

It seems to correlate to Vds, the higher the Vds, the higher the voltage necessary to hit the plateau of the capacitance. For a 250V transistor such as the IRF644 it flattens out around 15V, so add that to twice the gate drive requirement of say 14V and you have a minimum requirement of 43V to insure drive to the 6P1P does not hit the non-linear capacitance region of the FET.

So, be judicious in picking your FET. The lower the Vds, the lower the -Vss supply has to be.

This is really useful when taking the overall design into consideration.

Back on the Firefly, I wired up a 12.6VAC w/ ct transformer for the 6P1Ps to see how offloading the main transformer would effect it. It should drop the dissipation from around 32.2W to 20.9W, so it will be interesting to see how much that drops the main transformer temp.
 
Temp only dropped from 124.5F (51.3C) to 117.4F (47.4C) for a drop of near 4C. Not as much as I expected considering the near 30% drop in dissipation, but still might be worth it to add the filament transformer and up the bias a couple of mA to decrease distortion.
 
I've started on the chassis (2), and have most of the components laid out.

However I have one minor issue. Does it matter how close the input twin triode is to the main power transformer (as long as it is not right on top of it).

I have the main transformer, choke, power supply, output transformers etc laid out with them spaced as far apart as possible to minimize magnetic coupling.

The tubes will be between the OPTs and PS transformers.

The tube on the left is the input tube, with the two output tubes on the right.

Does the input tube need to come forward?

I'd prefer to leave it where it is and put the volume pot right in front of it.
 

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I've started on the chassis (2), and have most of the components laid out.

However I have one minor issue. Does it matter how close the input twin triode is to the main power transformer (as long as it is not right on top of it).

I have the main transformer, choke, power supply, output transformers etc laid out with them spaced as far apart as possible to minimize magnetic coupling.

The tubes will be between the OPTs and PS transformers.

The tube on the left is the input tube, with the two output tubes on the right.

Does the input tube need to come forward?

I'd prefer to leave it where it is and put the volume pot right in front of it.

Here is a pic of my ugly little console SE 6N1/6P1. I put a Sharpie-fine on top for scale. No major induced hum from the open transformer (I actually found the noise problem that developed is a dubious socket under the 6N1).
Please keep us posted on how the amp sounds with the Edcor OPTs. They seem like a bargain if they sound halfway decent.

Oh and while we are posting tube amp porn :) Checkout this oldie I picked up recently. PP 6A3 driven by 6N7's (missing from the pic is the 5V4 rect). Sadly the OPT (a Marconi "Perfect Transformer"!) has an open primary on one side. The vintage Ferranti choke is scary dangerous with exposed HT thumbscrews.:eek:

Gary
 

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Layout suggestion:

Move output transformer that is front right to in between power transformer and other output transformer. Rotate choke so that its laminations are at 90 degrees to power transformer. Place three tubes along front.

Alternative:

Have narrow side of chassis as front. At back have power transformer and choke. In front of them, the two output transformers, in very front, the tubes.

Cheers,

Chris
 
I went ahead with the layout as is and probably should have tried one different for comparison, but this makes construction easier.

I made a shield out of ornamental Al sheet as well.

They should be ready to paint and go.
 

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Progress is pretty good other than the expected problem with the B+ wiring from the transformer not reaching the power supply board. I'm going to add a second 3 terminal barrier strip and mount the diodes on it, then run a wire from it to the caps on the PS board. That will solve this problem.

The two LEDs on the front are the two cheap red LEDs used for biasing the input triode stages. I figure I might as well put them where one can see them.

I had to order another inductor and a few connectors, etc from AES, so I'm only working on one amp right now.

My amp is Blue, SWMBOs will be black. I'm building mine first to make the appropriate screw-ups on it.

If anyone thinks it is worth it I'll make mechanical drawings. If no one speaks up I'll skip it.
 

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Nice work.

Another idea for open frame output transformer mounting is to take the pressed steel mounting bracket off and cut a hole in the chassis so that you can rotate the transformer such that the electrical connections are below the chassis and the laminations are parallel to the top plate.

Cheers,

Chris
 
Nice work.

Another idea for open frame output transformer mounting is to take the pressed steel mounting bracket off and cut a hole in the chassis so that you can rotate the transformer such that the electrical connections are below the chassis and the laminations are parallel to the top plate.

Cheers,

Chris

I wish I had thought of that. I bet it would work out great with the rotation of the laminations wrt the power transformer as well.

So far everything is mounting as I planned. I have to make one small change in the PS board as the power transformer leads won't reach it. I'll wire from the transformer to the diodes on a pair of terminal strips and wire from them to the PS board. I may have to extend the inductor leads as well as they are mounted on opposite ends of the chassis.

I'd like to make lables for the front, but he process I used in the past doesn't work with the ink from my inkjet printer.

Print artwork, spray with clear laquer, let dry and soak in water till the paper comes off the back. Glue transparency in place with more clear laquer.

The ink does not get absorbed by the laquer, and washes out and the paper is about impossible to remove from the laquer transparency. I may try better color ink, as this is standard color ink. I guess I need to find some cheap paper that will wash off easily.
 
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