Hiwatt DR501 low output

[booj]

DR501 that puts out about 20 watts. I've checked voltages, swapped output transformer, output tubes, preamp tubes, replaced a suspiciously browned output tube socket. I checked voltages under load compared to no load. No big difference. The amp works normally except for low output and a ratty distortion sound when pushed. Any suggestions are welcome. I'm out of ideas.

[Helmholtz]

Schematic?

[vintagekiki]

Links to Hiwatt schematics

https://hiwatt.org/

https://hiwatt.org/tech2.html

https://hiwatt.org/Schematics/

https://irationaudio.com/tag/hiwatt/

https://www.schematicsunlimited.com/h/hiwatt

https://elektrotanya.com/?q=showresult&what=hiwatt

http://4tubes.com/2-SCHEMATICS/Music-amps/Hiwatt/

https://easyeda.com/musicgarage/working-hiwatt-lead-30

https://el34world.com/charts/Schematics/Files/Hiwatt/Hiwatt_Schematics.htm


Question: Hiwatt DR501 or DR504?

[Chuck H]

I checked voltages under load compared to no load. No big difference.

Does this mean with tubes in and without tubes in? Or are you saying you checked voltages with the amp conducting signal and not conducting signal? Or are you saying you checked voltages with and without a speaker connected (bad idea)?

And what were the voltages? How did they correspond to what was expected?

[Jon Snell]

What is the grid DC voltage on the bias supply?
Work backwards from the phase splitter and follow the signal with a signal injector.
These are very basic amplifiers and apart from bias and anode load resistors, rarely give trouble.
By the way, it was called the 504, not 501, as the HT on the EL34s is 504 volts DC, when run at the correct mains voltage. I have seen many over the years.

[Helmholtz]

First thing I'd do is check screen voltages directly at socket pins.

[Chuck H]

Well the reason I'm interested in the nature of "under load compared to no load" (which I think probably means with and without output tubes installed?) and how that voltage corresponds to what is expected is because it seems likely something other than the tubes may be loading the power supply significantly.

[booj]

Chuck H. Under load meant driving a signal to the point of clipping. No load meant no signal. I was talking about the load to the power supply. I was expecting a say, 10 volt drop but didn't get that, which makes sense since the amp is only putting out less than half power.
Helmholtz Screen voltages looked normal
John Snell Bias supply was within two volts of the schematic. The output tubes run plenty hot. Upper 300's bulb temp. The signal at the output of the phase inverter and the grids to the power tubes look healthy, but I don't know what voltage they are supposed to be at full output. You are correct sir! Model 504.

[Helmholtz]

The output tubes run plenty hot. Upper 300's bulb temp.

Celcius?
What are idle currents, plate dissipations?

The signal at the output of the phase inverter and the grids to the power tubes look healthy, but I don't know what voltage they are supposed to be at full output.

As always, the PI should be able to deliver a clean signal of twice the grid bias, so around 80Vpp.

[mhuss]

I imagine you would have said something, but the B+ is around 500vdc?

[vintagekiki]

... swapped output tubes, preamp tubes ...

Question:
Are the swapped tubes new or "like new"
Is the after replacement output tubes bias correctly set (idle current 25mA per tube)

[Mick Bailey]

I would check the screen voltages under full signal conditions to see if the voltage is collapsing, which would limit the output. If the voltage is dropping, measure the voltage across each screen resistor in turn to see if you have a screen supply or resistor problem.

[The Dude]

It might be helpful if tube voltages were measured and put up here in the thead. "Normal" doesn't really tell us much.

[Jazz P Bass]

Low output!
Measure the Vac input voltage to grid of the output tube before clipping.
With that measurement & the plate voltage you will have the answer to the output voltage.
From there it goes to the OT.
One other measurement will supply the wattage.