Bassbreaker 30r schematic plz

[JMMP]

looking for schematic for this amp. Also what's the zener doing in this circuit? looking for schematic for this amp. Also what's the zener doing in this circuit? It parallels the cathode resistor/cap.

[christarak]

It’s a 30v Zener, so it will basically be “off” until the cathode reaches 30v. That’s 125mA through one 240R cathode resistor. It depends on how many EL84’s are feeding that cathode resistor. One EL84 would vaporise at 125mA, but 4 x EL84’s would be about 30mA per tube at idle. Very hot, but the Zener could provide a limit to the current through the tubes. The Zener can only tolerate about 40mA before it finds its maker. I’d just clip the Zener out, as I did on a Blues Breaker 15 with a dead zener. It worked just fine without it.
I’d be interested to hear other comments or suggestions as to the Zener's purpose.

[Jon Snell]

It is a 30volt 5Watt zener and designed to fail short circuit if overloaded.
This ensures the faulty valve that caused the issue will blow the HT fuse quickly and avoid a possible fire hazard from burning resistors and decoupling capacitor damage.
No service manual yet, too new but it's only an amplifier based on the Bluesbreaker 15 so can't be difficult to repair after all.
Under new legislation, all electronic and electrical items of a reasonable value MUST be repairable by engineers, after the warranty period has expired.

[Helmholtz]

Very hot, but the Zener could provide a limit to the current through the tubes. The Zener can only tolerate about 40mA before it finds its maker.

No, the zener doesn't limit tube current.
Zener current can be up to 160mA and in case of a shorting tube it will likely fail short as Jon Snell reports.

[JMMP]

Thanks for all the help, it's much appreciated. The diode was in fact shorting, just got a small supply of the diodes, now to get the tube board out, jeez!

[Helmholtz]

So you had a shorting tube?

[JMMP]

Yes, shorted and took the fuse as well. New el84 set installed, tested, all proper.

[glebert]

Seems like a very strange but somehow clever safety mechanism. Tube shorting? Well, then short it completely. Hate to think what happens if someone puts a 20A fuse in there.

[christarak]

No, the zener doesn't limit tube current.
Zener current can be up to 160mA and in case of a shorting tube it will likely fail short as Jon Snell reports.

Thanks Helmholtz, I didn't take into account that its a 5w zener (Imax of 5/30). If the combined current through the four EL84's (assuming they shared one cathode resistor) caused the cathode voltage to reach 30v wouldn't that grid to cathode voltage then effectively be a "fixed bias of 30v"? Yes, it's a very cold bias for EL84s, but I'd be interested to understand if it would be possible to have a cathode biased output stage convert from cathode bias to "fixed bias" by addng a Zener of say 12V..
Another question, how do we know that the diode won't fail open? I guess if it does, it's no better or worse than not having the diode at all.
Thanks mate!

[Helmholtz]

Thanks Helmholtz, I didn't take into account that its a 5w zener (Imax of 5/30). If the combined current through the four EL84's (assuming they shared one cathode resistor) caused the cathode voltage to reach 30v wouldn't that grid to cathode voltage then effectively be a "fixed bias of 30v"? Yes, it's a very cold bias for EL84s, but I'd be interested to understand if it would be possible to have a cathode biased output stage convert from cathode bias to "fixed bias" by addng a Zener of say 12V..
Another question, how do we know that the diode won't fail open? I guess if it does, it's no better or worse than not having the diode at all.
Thanks mate!

It looks as if each EL84 has its own zener. I think they were added to pass safety regulation.
The semiconductor manufacturer will have to guarantee that the zeners don't fail open.

Regarding your biasing questions I suggest that you start a new thread in "Theory&Design" to not derail this one.

[DanKolcun]

I have a 30r that I bought used a couple of years ago and it fizzes (for lack of a better term) for a minute or so as it warms up (without fail - every time). It eventually stops, and the amp plays fine. One of my buddies suggested a tube was going bad (I'm not an amp tech at all, though I'm not afraid of getting my fingers dirty...safely anyway). Would anyone here know if the 30r is self-biasing? I.e., can I replace all tubes (the EL84s and the AX7s), turn it on, and not have a problem?

[Horse57]

Reviving this thread as I've just had one of these in with a blown mains fuse. Shorted EL84 took out the Zener but I was getting around 50 ohms on that EL84 pairs cathode to ground. Turns out the Zener had failed and was reading around 50 ohms... this meant that that pair of EL84's was biasing up at over 150% plate dissipation. Good job the owner brought it in rather that just whacking some new valves in!

I'm tempted to just remove the Zeners altogether

[Delta362]

I had one in a couple months ago with a blown fuse, bad power tube, and the zener D13 (1N5366B, 39V 5W) was shorted as well.

[TheBigPill]

First I’d like to extend a big thank you to all of you who’ve already weighed in here. I too have one of these amps on my bench right now, with the exact same problem. I’ll spare you any rants about the evils of the modern world, but it seems, well, an avoidable weak point in Fender’s design of this amp; I imagine their own repair department is tired of it too. But I’ll get to my point: The unit before me has one notable difference to the one in the (fragment of a) schematic provided by the OP. Its cathode resistors are 150 ohms, not 240. And also, just in case anyone is still wondering, its EL84 cathode resistor/cap/zener combi is arranged not singly per valve nor all 4 together, but in pairs, i.e. two EL84s per resistor/cap/zener. Which totally made sense to me in view of the fact that exactly one half of its power tubes was red-plating after replacing the shorted one. Thanx again everyone, smile now, then have a beautiful day.

[g1]

The unit before me has one notable difference to the one in the (fragment of a) schematic provided by the OP. Its cathode resistors are 150 ohms, not 240. And also, just in case anyone is still wondering, its EL84 cathode resistor/cap/zener combi is arranged not singly per valve nor all 4 together, but in pairs, i.e. two EL84s per resistor/cap/zener. Which totally made sense to me in view of the fact that exactly one half of its power tubes was red-plating after replacing the shorted one. Thanx again everyone, smile now, then have a beautiful day.

All this depends on which Bassbreaker you have. The picture in the first post is from a Bassbreaker 007. At the time, I think that was all the OP could find, although it did not match his unit. So his question was just about the zener circuit in general.
The 15 has an arrangement like you describe, but with only a pair of EL84's using 240R cathode resistors for each. The 18-30 has four EL84's with a common cathode resistor for all (two 150R's paralleled for 75R).
I'm assuming you are talking about a Bassbreaker 30R which I don't have the schematic for, but attaching the other 3 variants.