Dual tap OT quandry

[SoulFetish]

Alright. Assuming Randall has worked out how to proceed...

What is the purpose of bypassing the 1 ohm resistor on the power tube cathodes in the schematic with a diode.?. Is that to limit the affect AC standing on top of the 1 ohm resistance may have on the mV reading? Since the measurement here is usually taken when the amp is at idle there shouldn't be any to speak of, right? Is it even significant enough for this extra diode?

They do that to save the resistor from opening in case of a short. If the current through the resistor causes the voltage across it to rise, the diode will forward bias and clamp the voltage, and present a low impedance path in parallel. In theory, this should prevent the max power rating of the resistor from being exceeded

[SoulFetish]

That's a conundrum for me also. I've seen diodes used for clamping or constant gm circuit/purposes (with a larger value cathode resistor), but the 1 ohm parallel resistor seems to make the diode a moot point. Maybe it's not intended for normal circuit operation, but rather if the resistor starts to open?

Right, it’s reverse biased in normal operation

[The Dude]

The diode would always be forward biased. Unless......, for some very strange reason, your cathode voltage went negative. There just wouldn't be enough bias for it to do anything unless the resistor started to open.

Edit: Re-thinking: The resistor is 1W 1 Ohm- so 1V 1A for a hot minute. I guess the diode would start conducting before the resistor reached max ratings...... So, the diode probably eventually shorts but the amp still works (shorted diode and all) after you change the bad tube and blown fuse?

[SoulFetish]

The diode would always be forward biased. Unless......, for some very strange reason, your cathode voltage went negative. There just wouldn't be enough bias for it to do anything unless the resistor started to open.

Edit: Re-thinking: The resistor is 1W 1 Ohm- so 1V 1A for a hot minute. I guess the diode would start conducting before the resistor reached max ratings...... So, the diode probably eventually shorts but the amp still works (shorted diode and all) after you change the bad tube?

yes, you're right. My bad. it would just be biased off.

[Helmholtz]

Wow, that's interesting, I've not seen that reported before; do you think they left the factory like that?

Here is an example:



More here: http://marstran.com/Historic%20Data%20Base.htm

e.g. http://marstran.com/64%201015.htm

[loudthud]

The purpose of the diode is to limit the Voltage at the back panel test point (where the user can touch it) in the case that the resistor fails open for any reason.

[g1]

That makes sense. I had thought they might be worried about exceeding heater to cathode ratings if the resistor opened, but safety regs. and liability would be a much bigger deal.

[SoulFetish]

The purpose of the diode is to limit the Voltage at the back panel test point (where the user can touch it) in the case that the resistor fails open for any reason.

That's another great reason for those resistors, perhaps a better one. But in the Fender HR amps there are no back panel test points for the user to come into contact.

[pdf64]

Without the diode, in the case of the fuse value being too high and a power tube shorting (neither of which happen often, of course), the resistor might 'fuse' and save the OT / PT etc from damage. Similar to 1W screen grid resistors.

[g1]

This is a little anecdote I've shared before, but it is possible the diode got implemented for this reason and just got continued on because 'that's how we do it now'.

This first mass-produced amp with 'user' bias adjust was the Fender "The Twin" released in the late 80's. The rear panel board had the test point receptacles, the bias trim pots, and the 1R cathode resistors. They did NOT have the diodes originally. Bad tubes would fry the 1R's, and often burn the board, the receptacles, and sometimes even the pots along with them. Lots of warranty claims for Fender I'm sure (we had several). The fix was to install the diodes across the resistors. It was a service bulletin and the instruction was that they were to be installed on ALL units that were in for warranty repair. Somebody that has all the service bulletins would have it.
The next revision of that model had the diodes 'stuffed' as they say. As far as I know all Fender product since that has those sense resistors has the diodes in parallel.

Maybe at this time someone realized the safety hazard loudthud mentioned, but I thought I'd relate the story due to them being used in models that don't have user accessible adjust, which SoulFetish brought up.
I do agree with pdf64 that having the diodes negates the fusing function, which seems counter-intuitive.

[Chuck H]

Glad I asked

[dmeek]

Here is the Service Bulletin.

https://music-electronics-forum.com/...1&d=1575168856

[Randall]

Looking at the schematic in post #12, the grey negative feedback wire is connected to the 4 ohm tap, it doesn't appear to be employed when using just the main speaker. Why is this the case? I am wanting to impliment this switching scheme in a Blues Jr.

[The Dude]

The 4 ohm tap is no longer used for speaker output, but it can still be used for NFB.

[Randall]

"The 4 ohm tap is no longer used for speaker output, but it can still be used for NFB. "

Ah, I see. So is the reverse OK as well, as in leaving NFB connected to the 8 ohm tap in the Blues Jr? There will still be NFB in 4 ohm mode?