Mesa preamp

[Stevem]

Hi all!
I have a Boogie who's preamp uses 1N5303 diodes in place of the cathode resistors on some gain stages, what other diodes can I use in place of these?

[pdf64]

Just to clarify, is this in regard of a repair or a mod/experiment?

[martin manning]

It doesn't look like there are any readily available equivalents. I'm guessing you could substitute a red LED.

[Stevem]

I had replaced the diodes on the first two gain stages with resistors because one was bad, and now the owner after having the amp back for 4 months says that the amp has too much gain and too much middle, why he does not use the 5 band EQ for the mid issue is beyond me!

[gingertube]

Any standard silicon power diode can be used.
0.7 V nominal forward voltage at typical operating current, this is the bias voltage.
The diode has low AC impedance and so it acts like a resistor with a bypass cap.

Replacing the diode with a resistor of appropriate value should result is LESS gain (unless you added a bypass cap as well).

A Red LED has more like 1.4V forward voltage so tube would bias colder. It also has lower AC impedance so better "bypassing" and higher gain.

Would be tempted to leave the resistors but ditch any bypass caps if you previously added them.
Cheers,
Ian

[Ten Over]

1N5303 and 1N5304 are current regulative diodes. Way different animals than LED's, zener's, or power diodes.

Try Semitec E-152 or E-202.

[Phil_S]

This?
https://www.ebay.com/itm/SOLID-STATE-IN ... 1209!US!-1

[gingertube]

1N5303 and 1N5304 are current regulative diodes. Way different animals than LED's, zener's, or power diodes.

Try Semitec E-152 or E-202.

Ah - my mistake, chased up the data
1.6mA is its current, 1.65V minimum across it to get 80% of that 1.6mA, at which point dynamic impedance is 92K.
With more volts across it the current settles to that 1.6mA +/- 10% and the dynamic impedance rises to 475K.

These diodes MUST have bypass caps else you would get no gain.

Given the data above you could easily use a "Ring of Two" transistor current source in their place. Here is one I prepared earlier!!

BH_SF-CCS.jpg

That 499 Ohms sets 1.3mA want 400 Ohms for 1.6mA

Cheers,
Ian

[roberto]

why he does not use the 5 band EQ for the mid issue is beyond me!

that's because if you push mids on the early stages, and build the distortion with this modified eq, removing them after the distortion will sound totally different rather than having less mids since the beginning.

[Ten Over]

I had replaced the diodes on the first two gain stages with resistors because one was bad, and now the owner after having the amp back for 4 months says that the amp has too much gain and too much middle, why he does not use the 5 band EQ for the mid issue is beyond me!

If you replace the CRD (current regulative diode) with a resistor that puts the triode at the same idle operating point, the performance of the triode is identical. The frequency response with a 25uF cathode bypass capacitor is dead flat from 80Hz to 8KHz in both cases. The owner may be delusional.

CRD Bias for Triode.png

[Stevem]

I forget which position it was in , but one of the 2 AX7s I replaced was flat out shot and I think this guy's amp was this way since he got it.
Once I got the dead channel working and new tubes in it now the amp sounds like it should which he is just not use to!