Trace V4 4xKT88 lower o/p into 4ohm than into 8

[Alex R]

Happy new year to all.

I have in a nice old Trace Elliot V4 with 4xKT88 outputs. Into an 8 ohm output it starts to clip at about 37vAC, about what I'd expect. But into 4 ohm it only puts about 25vAC before clipping. This is the case when a 4 ohm load is connected, whether the 4 or the 8 ohm winding is selected. Seems odd to me. The output waveforms are quite clean though the clipping is slightly asymmetrical at first both into 4 or 8 ohms. The amp came in because output into 4 ohms seemed a bit low. Not an OT fail mode I've come across. Any comments?

Service manual attached

Trace Elliot V4[1].pdf

[Leo_Gnardo]

171W to 8 ohms, 156W to 4 ohms, not all that much of a difference, 10%. Perhaps a small error in laying the secondary windings? FWIW I often find amp output wattages at clip don't come close to what the manufacturer claims. Perhaps amp manuf's are measuring their power at 10 or 20 or 30 - or 50% - distortion levels?

Same power into 4 whether taken from 8 or 4 winding? Is there really a separate 4 ohm tap as shown on the schematic? I've seen a couple UK made amps (Sound City comes to mind) which have one tap serving two output impedances, iow when you change the switch, you're really changing nothing.

One check you could try, run a small signal thru the amp, load the output with some high impedance say 100 ohms, set controls so the output is only 1 VAC or so, measure the AC voltage from 8 & 4, see if there's a difference in voltage from each tap. If they read the same, the 4 ohm tap is bogus.

Also, what happens if you run an 8 ohm load on the 4 ohm tap? If you get 37 VAC . . . then the only difference between the taps, is the labels.

[pdf64]

I suggest to check the winding ratios, eg disconnecting and feeding mains into the primary, and comparing the primary and secondaries Vac. Even if there really are 2 taps, there may end up being a slight discrepancy in the reflected primary load between the 2 secondaries, when they're correctly loaded.
Another possibility is that your load resistor values may be a bit off, eg perhaps the 8 ohm is a bit high?

It can't be a shorted turn, as I think that would cripple the OT; it's secondary and primary inductances would collapse, power output would be a tiny fraction of rated level.

[g1]

Are all your screen and other voltages at the power tubes good?
They are calling it a 220W amp, is that a bogus claim or is something wrong with it? (you are only getting 170W max.)

[Chuck H]

Have you checked the bias? If the amp is hot the tubes will self regulate somewhat when asked to produce that kind of power. Basically putting power below the crossover notch instead of the positive swing.