Tube Amp Design Question...

[Langenbauer 1]

Hello everyone. I have lurked here for years. First I just gotta say this place is the best! Thanks to everyone who posts here with all the great ideas! I love building stuff and don't have much time, so I do stuff when I can, I'm glad I can finally post something...

I have read through some tube handbooks but nothing has really addressed this question directly. If you have said (http://www.pmillett.com/push-pull_kt...driver_pcb.htm) amplifier with two output tubes, what would you need to change about said amplifier to add two more output tubes? How would it bias out? Beings it's fixed bias, would you change the bias setup to accommodate for the two extra output tubes. Obviously you would have to change the power and output transformers. What else? Can anyone tell me exactly? Am I asking a dumb question? I'm really extremely sorry if I am... Any help would be much appreciated, I have been pondering this question for a long time... Again, if I'm going about this the wrong way please excuse me... I'm trying to learn the best I can...

[SY]

Pete is traveling at the moment, but he'll be back next week and, if I tell you anything wrong, he'll correct me. In any case, you souldn't have to change anything in the driver circuit. Are you running the output stage triode, ultralinear, or pentode?

In any case, I'd use separate bias controls for each output tube, if possible, running separate coupling caps from the driver board to each output tube.

[jaymanaa]

I've been having the same thought. Say you have a SET output TX with a pri. Z of 3k. If you wanted to run multiple 2A3s to get more power, can you do it by re-biasing, or would you "have" to get a TX of lower pri. Z?

[SY]

One trick is to connect the speakers to a different secondary tap. For example, if you move the speaker connection from the 8 ohm tap to the 16 ohm tap, the effective primary impedance will halve.

[nigelwright7557]

Originally posted by SY
One trick is to connect the speakers to a different secondary tap. For example, if you move the speaker connection from the 8 ohm tap to the 16 ohm tap, the effective primary impedance will halve.

Surely that will double the primary impedance ?

The output impedance is reflected back by the turns ratio.

So a 20:1 output transformer at 8 ohms output will give 160ohms input and 16 ohms output will give 320 ohms input.

But i could be wrong !

[SY]

Yes, you could.

The transformer winding ratios determine the impedance transformation by the square of the turns ratio times the load impedance. For a given turns ratio (which is fixed by the transformer construction), halve the load impedance, you'll halve the primary impedance.

For example, if a turns ratio is chosen to give 5000 ohms with a 16 ohm load, if you have the same turns ratio (same taps) and connect an 8 ohm load, it will reflect a 2500 ohm load. Connect a 32 ohm load, it will reflect a 10,000 ohm load.

[Langenbauer 1]

Originally posted by SY
Pete is traveling at the moment, but he'll be back next week and, if I tell you anything wrong, he'll correct me. In any case, you souldn't have to change anything in the driver circuit. Are you running the output stage triode, ultralinear, or pentode?

In any case, I'd use separate bias controls for each output tube, if possible, running separate coupling caps from the driver board to each output tube.

Cool! You guys are fast on here I want to run the amp in triode and ultralinear just like the design. With the switches in place. All I really want to do is use 4 KT88's as the output stage instead of 2. So what your saying is that (using http://www.pmillett.com/images/UNIAMP.PDF) you would make a circuit for grid 3 and grid 4 as well as cathode 3 and 4. Basically, mirror the circuits that are in place for 1&2 (C4 and C10, R14, R22, R16, and R19) Parallel the output tubes to the preamp tubes respectively???

[SY]

You got it.

Now understand that doubling the input capacitance of the output stage will cut your bandwidth in half. I haven't calculated it to see if it will be a problem for you, but it will certainly be much less of a problem in ultralinear than triode.

If your heart is set on triode and you calculate an unacceptable -3dB point, that can be overcome by adding a cathode follower or a source follower between the driver and output. That complicates things considerably, but it also provides an opportunity to modify the amp in such a way that prevents blocking distortion after overload. With UL, you're nearly certain to be OK.

[Langenbauer 1]

Oh man! Thank you so much! I'm off now... I'll post what I come up with when I get done... Thanks again!!!

[nigelwright7557]

Originally posted by Langenbauer 1
Oh man! Thank you so much! I'm off now... I'll post what I come up with when I get done... Thanks again!!!

I have a Simms Watts 100 watt valve amp and the output stage uses 4 off EL34 and they are in two pairs paralleled up.
The screens use seperate resistors tho.