Some voltage readings of my original 1989 Express...

[geetarpicker]

I thought I'd take a few voltage readings of my 1989 Express that many here are familiar with via my CD and/or Youtube videos. This sort of data is nothing new here, but I thought folks might want to see what's up number wise with this particular original Ken Fischer built Express.

I took voltage readings at idle, and then pushed hard into full saturation. This shows some insight as to why these amps work so well for clean to mean from the guitar, and where a good bit of the natural compression comes from. "Idle" was with the amp volume off. "Full saturation" was with amp volume full up, and hitting continuous sustained chords. For a load instead of speaker cab I used a Suhr reactive load box which is 8 ohms. I really dig the Suhr RL into a Two Notes C.A.B. for recording but that's another story!

I used a "Brown Box"to get the slightly hot line voltage around here down to 120v, just like the label maker on the back of the amp says to run. The output tubes are RFT EL34s, and the preamp tubes are Sylvania 12AX7s. Meter used is a Fluke 112.

GK

Input voltage: 120v AC
Output tubes biased at 43ma per tube

B+ at idle (measured at the B+ side of the 25w resistor): 386v
B+ saturated: 370v

Screen voltage idle (measured at EL34 pin 4): 364v
Screen saturated: 264v

v3 B+ idle: 254v
v3 B+ saturated: 205v

v2 B+ idle: 200v
v2 B+ saturated: 166v

v1 B+ idle 175v
v1 B+ saturated: 139v

[andresound]

Can you post cathode voltages as well please?

[martin manning]

Thanks, Glen! So, at full-tilt, the screen current is ~106 mA, or 2.5 times data sheet level at similar B+ (and the 1k 25W resistor is dissipating over 11W). That is a result of the ~2x nominal OT impedance, I believe.

[TUBEDUDE]

If I understand this correctly, the plate drops 16V, and the screen drops 100V! At this ratio, it seems the gain would drop dramatically reducing the compression effect. Since this is not the case, what am I missing? Thanks in advance.

[martin manning]

Due to the high Za-a, the load line is way below the Vg1=0 knee at idle. At full power, the 100V Vg2 drop actually has it in a more normal position, with little or no reduction in the available plate voltage swing.

[Roe]

Thanks, Glen! So, at full-tilt, the screen current is ~106 mA, or 2.5 times data sheet level at similar B+ (and the 1k 25W resistor is dissipating over 11W). That is a result of the ~2x nominal OT impedance, I believe.

Yes, a bigger screen resistor might be a good idea on higher voltage amps. Mine is running 420-30V at the plates. And my K60 has 475v with much less screen resistance (although the PT of the K60 sags more than an express PT)

[jelle]

I thought I'd take a few voltage readings of my 1989 Express that many here are familiar with via my CD and/or Youtube videos.

Was nice speaking with you on the phone this week!

That plate voltage matches with the batch of Ken Fischer transformers that I got a little while ago. With 119.6 VAC, I get 389VDC on the plates on one of the amps I made with this. The tubes are EH EL34.

Thanks for checking those voltages,

Jelle

[Roe]

Thanks, Glen! So, at full-tilt, the screen current is ~106 mA, or 2.5 times data sheet level at similar B+ (and the 1k 25W resistor is dissipating over 11W). That is a result of the ~2x nominal OT impedance, I believe.

Due to the high Za-a, the load line is way below the Vg1=0 knee at idle. At full power, the 100V Vg2 drop actually has it in a more normal position, with little or no reduction in the available plate voltage swing.

Martin, how did you calculate this? You assumed 106v/1000ohms=106ma right? But there is a 100v drop across a 1k shared 25w resistor in series with a 1k individual 5w screen resistor. We don't know the resistance the individual tube sees, except that it is between 2k and 3k (depending on bias).

Basically, the current is much less than what you indicated. It is between 35 and 53mA, depending on bias. So the express is not far off from the date sheet level at all

[martin manning]

Glen has measured some average DC voltage on the B+ side of the 1k dropping resistor, and some average DC voltage at a screen pin. So let's say that there is 3k ohms from B+ to screen pin (the 1k dropper is shared), which gives 35 mA average current per tube. If we say 2k (the dropper is not shared), then it's 53 mA per tube (106 total), as compared to the ~23 mA per tube shown on the data sheet (which is probably an RMS value). The real answer is somewhere between the two, since even with a sine wave input there is partial sharing.

[dblgun]

I thought I'd take a few voltage readings of my 1989 Express that many here are familiar with via my CD and/or Youtube videos. This sort of data is nothing new here, but I thought folks might want to see what's up number wise with this particular original Ken Fischer built Express.

I took voltage readings at idle, and then pushed hard into full saturation. This shows some insight as to why these amps work so well for clean to mean from the guitar, and where a good bit of the natural compression comes from. "Idle" was with the amp volume off. "Full saturation" was with amp volume full up, and hitting continuous sustained chords. For a load instead of speaker cab I used a Suhr reactive load box which is 8 ohms. I really dig the Suhr RL into a Two Notes C.A.B. for recording but that's another story!

I used a "Brown Box"to get the slightly hot line voltage around here down to 120v, just like the label maker on the back of the amp says to run. The output tubes are RFT EL34s, and the preamp tubes are Sylvania 12AX7s. Meter used is a Fluke 112.

GK

Input voltage: 120v AC
Output tubes biased at 43ma per tube

B+ at idle (measured at the B+ side of the 25w resistor): 386v
B+ saturated: 370v

Screen voltage idle (measured at EL34 pin 4): 364v
Screen saturated: 264v

v3 B+ idle: 254v
v3 B+ saturated: 205v

v2 B+ idle: 200v
v2 B+ saturated: 166v

v1 B+ idle 175v
v1 B+ saturated: 139v

Glen,
I know it's been a while since you had posted these measurements but I'm curious about the pre-amp/pi figures. You list B+ voltages for v1, v2 and v3 but are these measurements taken at the plate resistors as fed from B+3,4 and 5 or after the plate resistor(plate voltage)? These seem too low for B+ figures in some spots but too high for plates in others. Either way it appears that most of the folks builds vary greatly from amp to amp. As already mentioned I would also be interested in cathode and grid measurements as well.

Thanks,
Brent