weber princeton reverb clone a bit short on power

[el1psey]

hi,

i built a weber PR clone kit, which sounds great but seems short on power when voltage is measured across an 8 ohm non-inductive heat-sinked resistor. i tested the resistor with two meters, and it is accurate at all operating temperatures. with JJ's and sino 6v6's, i see 9v, and 10v respectively at the distortion threshold on my scope. so this is 9vrms^2/8ohm = ~10 watts, and 10vrms^2/8ohm = ~12.5 watts, right? this is with about 9 watts (per tube) idle dissipation in each case; 27ma*340v. it seems like i should be seeing more power/voltage across the dummy load. i expected 15-20 watts, is this reasonable? is it a result of my kit's lower plate voltage than the ~425v in the original PR? this is my second build; yes i'm a newb. please advise.

[txstrat]

What sort of OT is in there?
The tubes might be able to deliver 15-20 watts, but the OT has to be able too.
A too small OT wouldn't let through all the power, the tubes deliver.
In other words, see the OT as a bottleneck. If it can't let all the power (which the tubes deliver) through to the speaker, the rest goes up in heat.
On the other hand I think you wouldn't notice the difference between 12.5 or 15 watts.

[el1psey]

the OT is a weber w041318. the spec sheet is here: https://taweber.powweb.com/store/041318sch.jpg
it has a 6600 ohm primary winding, and is rated for 25 watts. i think 6600 ohms is a bit less that the textbook impedance for a pair of 6V6 -- the GE 6V6 datasheet recommends 8k, i believe. is the primary is rated plate to plate? so if i measured from the center tap to one end, it would be half the rating? anyway, it's the same trafo weber uses in the deluxe reverb kit, and it seems pretty beefy compared to those i have seen in other small amps. bass response is solid, and when it's running at full power on the dummy load on my work bench, it doesn't even get warm.

i'm not worried about hearing a difference in volume, i just want to learn. also, i'm building this for a friend, so i kind of want to see nominal performance measurements before i give it to him so that i know it is working properly.

[Chuck H]

Your right to think thats not normal performance. What are you using for an input signal (device, voltage, Hz.)?

Check the values of the feedback circuit too.

Chuck

[Amp Kat]

Another really cool tool is the Accubias meter that is on Bruce at Missions Amps site. It gives you a readout of plate voltage, tube current and wattage dissipation so you can see exactly what is going on and how much power you are actually putting out. Could be several things causing low power.

[el1psey]

thanks.

@chuck: my "function generator" is a laptop generating a 1khz sine wave, with the lineout plugged into the amp's input. i haven't measured the output level, i kind of just set the volume on the laptop so that the amp has about the same distortion threshold as with a guitar plugged in. i tried different combinations of signal level vs. amp input volume setting, and the level of power output just before the distortion threshold is constant; about 10-12W depending on which set of 6V6 i use.

i will measure the signal voltage soon, and report it.

the NFB loop has a 5K pot in series as a presence control, so it's kind of the same situation with different combinations of pre-amp volume/presence settings resulting a constant clean power output maximum of about 10 watts.

i notice that the GE datasheet for 6V6 is biasing to 35ma@285vplate, which is 10 watts dissipation per tube at idle, and claims 14 watts power. i'm only biased to 9 watts idle, which i chose with the 70% "rule": 13 watts max * .70 = ~9W / 340vplate = 27ma

i measured the plate voltate, resistance of the dummy load resistor, and idle current accross a 1 ohm cathode resistor with two different dmm's, both of which are cheap, and of questionable quality. the plate voltage is 340v, current is about 27ma at idle +- about .5ma. the meters don't perfectly agree, and perhaps the tubes aren't perfectly matched. the load tests at ~about 8.3 ohms, it's heatsinked, and never gets too hot to touch. my cheap meters don't agree with each other, but they both show more than 8 ohms. if they are close than i have even less than 10W, as calculated for an 8.0 ohm resistor, and the resistance would only go up as it heats.

actually, i'm half tempted to go buy a low end fluke dmm just to eliminate a variable here... anyone here tried a fluke 114 or 115?

i'm not considering screen current in my bias current calculation because i don't know how. i'm guessing it's probably 2-3ma, and i could increase the cathode current by that amount to achieve a chosen plate dissipation. is this a good guess? should i measure the current across my screen resistors and add it in?

my OT primary is 6K6 ohms, which is lower than the 8K primary listed in the datasheet example; with a lower impedance shouldn't i have more power (and distortion) than the example, instead of less?

a few thoughts after sleeping on it:
--maybe i get more watts at some frequency that isn't 1khz. is that ok?
--my crappy multi-meters may be lying to me.
--the datasheet is exaggerating (aka advertising) when it claims 14W@285v plate
--maybe i am confused about the relationship between load impedance and output power.
--my bias point may be too conservative.
--my dummy load resistor may not be 8 ohms.
--my 340v plate voltage may be too low to achieve 15 watts.

do any of these explanations seem likely?

[JoeM]

181431....

--my 340v plate voltage may be too low to achieve 15 watts.

My 65 Princeton gets about 16W, but the plate voltage is around 405V.

[el1psey]

k, i think i'm maybe dealing with some measurement error. those 1 ohm cathode resistors test as 1.3 ohm or 1.8 ohm depending on which meter i believe, and the dummy load is 8.3. hmm... gold tolerance band much?

chuck: i tried to measure the voltage of the input signal, but it's below the detection threshold of my DMM for ac.

i ordered a fluke 115 today, maybe it will give me different answers.

i switched to the 680v PT taps and rebiased. if i believe 1.3, and 8.3 ohms for the cathode resistors and dummy load respectively i get about 11.5^2/8.3 = ~16 watts with an idle point of 20ma*440v = 8.8W. this seems better. i'm going with 70% of 13W max... maybe i could get away with more?

anyone with a PR want to tell me their power output measurement and bias point?