What is the downside of low efficiency?

[KL7MJ]

I just finished building a classic bands (tr)uSDX. After a fair bit of tweaking, the best I can do is:
80m - 2.7w, 79% efficiency
40m - 2.6w, 76%
20m - 3.3w, 64%
15m - 1.6w, 40%
10m - 2.7w, 46%

The power is lower than I was hoping for (especially 15m), but good enough - I've already made one contact on 20m CW to test the rig.
The question is, what harm is there in having low efficiency? Unless I risk burning out the PA transistor (or some other type of damage to the rig), I'm pretty much ok with what I'm seeing.

Mike, KL7MJ

[Chip]

It pulls more power from the battery which would make it run out quicker. Pa also can blown out easier.

[SP9TKW]

What is a voltage you are using? Apart of lower efficiency I can see low output power. Is your powersupply able to provide 1.5-2A?

I would say 75% efficiency and more is fine, below 60% there is some more tweaking to be done. You should have no problem with 4W on all bands at 12V when efficiency is 70% or more.

[KL7MJ]

Based on other threads on the forum, I increased R shunt to 22 (PA bias was already at 128). It increased every band by about 15% efficiency, with marginal if any increase in power out. So now I'm at:
80m - 2.7/96
40m - 2.7/92
20m - 3.4/74
15m - 1.6/51
10m - 2.8/60

I'm going to keep playing with 10 and 15 to get them closer to 70%.
Thanks.

Mike, KL7MJ

[KL7MJ]

Based on reading the BOMs, I'm also going to increase PA bias from 128 to 160 and see how that impacts efficiency.

[dl6sez]

Hello OM,

please when you read something here read it exactly!
Adjusting Rshunt is only adjusting the resistor value for calculation of how efficiency is calculated!
This has nothing, absolutely nothing to do with your real efficiency,
you have to measure output power and measure current outside of (tr)uSDX and calculate, with your rx current substracted, your real efficiency.
Than adjust Rshunt in the menue to match your real efficiency... it is only a adjusting of the calculation to get it equal to reality!

Tx bias 160 is a must for classic and high band.

73 de Chris

[SP9TKW]

PA Bias needs to be 160 if you have PDT86256 transistor as a finals and 128 if you have 3x BS170.

99.99% of classic bands boards (80/40/20/15/10) have PDT86256 transistors. PDT transistor is located at the reverse (bottom) site of the PCB board.

[KL7MJ]

I have the PDT86256, and I just adjusted the bias to 160. The numbers look a lot better. 10m is more or less ok, but 15m is still too low, so I think I need to go back and work on it some more.

As far as the R shunt value, I checked my low band trusdx and it was set to 18 (not by me, it must be the default). Is the default R shunt value the same for all boards, or should it be higher for the classic/high band boards?

Thanks.

Mike, KL7MJ

[dl6sez]

Nope it is FDT86256
a part PDT86256 does not exist!

[KL7MJ]

Sorry for the typo - my classic bands trusdx has a single power transistor, not 3 BS170's.