GZ34S vs 5U4GB --- voltage drop question, plus a discussion of SS diodes as backup

[xtian]

Practical example #2.

Changed the 47R resistor for 240R/10w.

At idle: 418v plates, 18mA current, 11.7v across 240R = 0.6w.

At max clean output, 15 watts, B+ sags to 365. Drop across 240R rises to 27v (3 watts).


So, with 47R sag resistor, max clean output is 21 watts and DC sag is 35v.
With 240R sag resistor, max clean output is 15 watts and DC sag is 53v.

[LOUDthud]

So, not much happening at idle. With dummy load and 1KHz sine wave at peak clean output (21 watts), B+ sags to 415v, power tube current rises to 65mA ea, and drop across the 47R resistor to 6.3v (0.84 watts).

That sag resistor really isn't doing much for me. Worth trying a larger value?

Once again, average Voltage should not be used to compute average power dissipation. Stick a 1W resistor in there and watch it smoke and burst into flames.

Look at this simulation:

290EX_sim_1.GIF

The 290EX is Hammond's version of a 5F6A Bassman PT. Simulation is run with the rated secondary current. The peak secondary current is 1.13 Amps even though load current is only 275mA. The Average Voltage across the 50 Ohm resistor is 13.75V which would make you think dissipation is 3.78W. The RMS Voltage across the 50 Ohm resistor is 24.882V which indicates power is 12.387W. It's not unusual for guitar amps to exceed a power transformer's current rating for brief periods.

Note: To get accurate RMS numbers from PSUDII, you need to run the simulation for a complete number of half cycles. 50mS gives you 3 cycles at 60Hz or 2.5 cycles at 50Hz.

[R.G.]

Practical example time!

I added pairs of 1N4007s in series with HT winding and GZ34 recto tube. Then I added a 47R/25W resistor in series with B+ exiting the rectifier. Wall voltage is 123v. After warm up, I set the bias: 450v plates, 18mA per tube (about 60% max diss). I find 2.3vDC drops across the 47R resistor (0.11 watts).

So, not much happening at idle. With dummy load and 1KHz sine wave at peak clean output (21 watts), B+ sags to 415v, power tube current rises to 65mA ea, and drop across the 47R resistor to 6.3v (0.84 watts).

That sag resistor really isn't doing much for me. Worth trying a larger value?

I think so.

Interesting results. I understand that the GZ34 is about 50 ohms internally, so a 47 ohm is getting it to intermediate with the 5U4, which is reputed to be about 150 ohms internally. The drop in B+ voltage is significant - with a GZ34 and a 47 ohm, your B+ is dropping by 35V. If it was me, I would
> short the 47 ohm resistor and measure the B+ idle-to-21W output drop for just the GZ34; this lets you know how much the 47 ohm resistor added to any B+ sag
> add another 50 to 100 ohms of resistance and repeat the test.

This gives you some numbers to play with. I would also play my guitar with each instance to get a feel for what tone changes (if any) I perceived.

The voltage sag from rectifiers is funny. The physics of diodes charging capacitors from AC sine waves makes for high peak currents near the peak of the input AC waves. The "harder" the diode (i.e. lower internal resistance) the peakier this gets, and also the bigger the input capacitor values, the peakier it gets. For SS diodes and high filter caps, the peak of the rectifier currents can be three to ten times the DC average load current. This is the primary reason that tube rectifier data sheets specify a maximum first filter capacitor. Making the first filter cap too big makes for big peak currents in the tube rectifier and this can damage the tube.