New Build, Low Output Power, Any Ideas???

[tobykea]

Hi there everyone, I've just finished building my first HRM 50Watt Dumble with the Dumbleator built in.

The amp works well and sounds OK but I'm only getting around 30watts of output power as measured into a dummy load using an oscilloscope.

Plate voltages are as follows:

V1 210V

V2 220V

V3 1st Overdrive tube 230V

V4 2nd Overdrive Tube 239V

V5 Cathode Follower(Dumbleator) 275V

V6 Recovery Tube (Dumbleator) 220V

V7 Phase Splitter 300V

V8 Phase Splitter 310V

Output Stage tubes 460V

Screens 457V

Output transformer is a New Sensor Corp 018343 50Watt 2,4,8 ohm replacement unit.

On the clean channel I can get the preamp stages driving clean until the output stage goes into positive grid conduction and the output stage at this point is putting out 30 watts. Not good.

I'm wondering if there is a mismatch in the output stage with the output transformer I'm using. Also the screen supply voltage is very close to the plate voltage. Does this have much of an effect on output power?

At full power the supply voltages drop about 20v and i'm assuming that is OK.

I have a few other OT's lying around here so I might connect one up and see if i can get some more power.

Anyone have any ideas or suggestions.

Many thanks,

Toby

[llemtt]

Assuming 440v under full load on 6L6 plates you should get something like 55W rms at clipping onset!!

Is the dummy load connected to the proper OT tap?

Output tubes, are they ok?

Is bias correct?

How much screens sag under full load?

Teo

[mlp-mx6]

I'll ask a dumb question - please do not be offended.

What are the settings on your Dumbleator? Those act just like another master volume on mine.

[dogears]

Someone has not been reading the posts here.... You need the 150k load resistor to ground from the V1 supply node. That will drop your preamp plates into line.

This has no bearing on your problem, just sayin'...........

Did you copy the 1997 Humble schematic with its myriad of errors or the accurate ones here??

[Funkalicousgroove]

Are your tubes biased way hot??

[tobykea]

Thanks for the replies, much appreciated.

Output tubes are biased at 35mA each. I have experimented with 32mA and up to 40mA each with little change.

I'm getting around 46watts now with a transformer change so I think that was the problem. Should I be getting more power than that???

The output tubes I have a fairly old so it might pay to buy some brand new ones.

Thanks for the tip on the 150k resistor Dogears. I have been using the schematic OD-101 I think it is called that "Tonegeek" has posted on here recently although I found his resistor network in the power supply a little off in my amp and changed it. It's the one with the Dumbleator built in.

As far as the settings of the Dumbleator go, I have the pots set to a level where the output stage breaks up before any of the preamp does if that makes sense. I was really trying to find where the clipping was coming from so I had things set low on the dumbleator.

So now I've got 38V p-p into 4 ohms which equates to around 46watts.

The clean channel sounds great but the OD channel needs a bit of tweaking. I've got a bit of the fried egg sizzle that I don't like but should be sorted tonight.

I'm sure I'll have more questions so thank you all for your replies.

Can't wait to gig with the amp on Wednesday night!!!

Cheers,

Toby
NZ