New 100W Build Complete- Low Output Power Problem

[tobykea]

Hi there, I've just completed my second Dumble amplifier, this time a 4 tube 100w version following the ODS101 schematic.

The amp sounds fantastic at the moment and much better than my previous build as that had a few compromises in it as far as layout and component types went.

The only problem is that when I did my power output measurements I only seem to get around 57 Watts before the onset of clipping.

I'm using Mojo transformer Iron for both the Power and output Transformers, Mojo 763EX and Mojo 775 respectively.

Here are some of my observations I made last night during some testing:

Power tubes biased with 20mA on each. I measure 40mA accross the 1ohm resistors fittted to the cathodes of each push-pull pair.

I have also biased them up to 40ma each tube without much change in power output.

V on Plates at no signal in= 456
V on plates at onset of clipping= 426V
V on screens at no signal= 455
V on screens at onset of clipping=417V

V at speaker output 4 ohm dummy load= 15.2V rms which equates to around 57watts rms. I was expecting a whole lot more than that!

Heater voltage is rock solid at no signal through to full output.

I have tried the various other taps available on the output transformer thinking that the tap I was using was not the correct one however this gave me no more power out.

My grid input signals to the power tubes look good and are not prematurely clipping.

Doe'snt make a whole lot of sense to me right now but when I get home I'll start poking around again and come up with some more measurements.

The layout I followed is pretty much following what Normster has done in the past although I have taken a few liberties with the placement of my rectifier diodes and Bias supply due to real estate shortages within the chassis.

I'll take some pics shortly along with a clip or two also.

Any ideas on why I'm only getting this much "clean power" or are my figures consistent with what you guys have measured on your own 100w builds???

For what it's worth, the preamp voltages are all within 10V of where most people on this forum believe they should be.

Any comments would be great.

Thanks,

Toby

[heisthl]

Are you sure you're using the 4 OHM tap of the transformer? Mojo uses heyboer as a supplier and they can have non intuitive tap colors. If in doubt measure the resistance from ground least = 2 OHM etc.

[Bob-I]

I'd measure the voltage swing at the PI input and output. I don't know what it SHOULD be but post it, someone here should know, or I can measure one of my amps and see if you're in the ballpark.

Using the wrong tap would only drop your power slightly, not 40% like you're seeing. I typicall measure about a 10-15% drop when I'm on the wrong tap.

[tobykea]

Double post

[tobykea]

Double post

[tobykea]

Double post

[tobykea]

Double, Triple, Quadruple post...Sorry, damm browser playing silly buggers with me.

[tobykea]

Well, I've finally managed to take some pics of the new amp as well as some pics of the amp I built last year.

Thanks for the tips on my low output power, it turned out that the transformer tap I was using was the incorrect one despite me following the wiring diagram from MOJO.

Power output is still not 100w though. I'm getting around 80w now. I'm starting to think that these mojo transformers are far from the best out there and that the impedance match is not the greatest. Just guessing right now but will confirm later with a multi-tap transformer I have which should be able to provide a pretty good match to my dummy load.

I'm also thinking the power transformer has something to do with it. At full power output My B+ Voltage sags to around 417V. I don't think it is possible according to the 6L6GC spec sheets to get 100W out of 4 tubes with a B+ and subsequently screen Voltage that low. At no signal my B+ voltage sits around 454V.

As a test I used the 220V tap rather than the 230V tap on the primary side which gave me more B+ voltage. I fed the heaters from a separate 6.3V transformer and measured the power and it was up by about 7W.

Why am I being so pedantic??? Don't know really because I know I'm not really going to hear the difference between 80 and 100w. Just a sanity check really to rule out something I may have stuffed up in the build.

Just curious to know what you guys have measured on your own builds as far as power output and B+ voltage at full power output.

Anyway, I've gigged with it once now and it sounds bloody amazing! I love these amps!

Here are some pics of the latest build followed by some pics of my previous 50W version. You will see that I compromised alot with the build of the 50W unit. I used an old amp chassis I had lying around and basically used what I had on hand.

I posted a clip in a previous post from last year that you should still be able to find on the forum. It is a clip of the 50W unit during a gig. They sound similiar although the 100w unit seems to have a lot more clarity. Probably down to the Dales and the cleaner layout/grounding.

I'll post a clip of the 100w once I get my soundcard back online.

The first shot is of the 50W version I built last year. No vishay Dales, pretty much all metal film resistors.

Cheers,

Toby

[Luthierwnc]

At 417 on the plates, 80 watts is about right. Somebody correct me if I'm wrong but the generic replacement trannies on the market are based on blackface specs which I think is 85 watts. Some of the silverfaces got up to 100 before they radically change the circuits. You'd want to be north of 450VDC, under load, to get in the 100 watt range.

Cheers, Skip

[dobbhill]

Lot of voltage drop under load???? Also, way under-biased. Should be running 30-40 mA per tube at that B+ voltage. Just my opinion. Your mileage may vary.
D

[Luthierwnc]

Thanks Dobbhill. I meant to mention that too. 34 ma draw would be where I'd start. Then season to taste. sh

[tobykea]

Double post

[tobykea]

Thanks for the info guys. You've reassured me that it's not my build with some wierd problem.

I did forget to mention that each tube is currently biased for around 35-40mA each and the voltage figures I posted were for that amount of biasing.

I'm currently rewinding a power transformer that I have lying around that will easily deliver the required power once complete. I'll bung that in once I'm done winding and see how much power I can extract from the thing then.

One thing that bothers me a bit is that when going from clean to PAB, I seem to loose alot of Bass, or maybe it's that I have more mids in the mix when in PAB engaged not sure. Just seems to be more Bassy on clean and would like to keep a similiar tone in PAB but with the boost.

Anyone every face this problem??

Cheers,

Toby

[heisthl]

Thanks for the info guys. You've reassured me that it's not my build with some wierd problem.

I did forget to mention that each tube is currently biased for around 35-40mA each and the voltage figures I posted were for that amount of biasing.

I'm currently rewinding a power transformer that I have lying around that will easily deliver the required power once complete. I'll bung that in once I'm done winding and see how much power I can extract from the thing then.

One thing that bothers me a bit is that when going from clean to PAB, I seem to loose alot of Bass, or maybe it's that I have more mids in the mix when in PAB engaged not sure. Just seems to be more Bassy on clean and would like to keep a similiar tone in PAB but with the boost.

Anyone every face this problem??

Cheers,

Toby

Everyone - breaking the treble pot from the bass still funnels bass and mids to ground it just makes the amp louder because the preamp is not loaded down, unfortunaltely the only freqs passing through are ones that can get through the treble cap. A good alternate method is to hardwire the wiper and P3 of the bass pot, hardwire P1 of the treble pot to P3 of the bass pot, hardwire the 10k off the bass pot and P1 of the mid pot and use a relay to lift the bass and mid pots away from ground. You can temper it with a resistor (I like 470K) if the boost is too big.

[FUCHSAUDIO]

I had a similar problem when using a variac ! I use an isolation transformer and variac on my bench for 120 240 testing and safety. When conducting power output measurements, you must make sure the line voltage is not sagging.

I got a double whammy on a new amp model (my clean machine) when Heyboer (for reasons unknown) decided to change the output tranny color codes on the speaker leads AND I had line voltage sag. I went from 85 watts to the expected 130 watts on a 4 X 6550 amp, by making those two changes !