DIY Audio Projects Forum

When assembling the 300b set is it advisable install fuses in the internal power/signal circuit to protect tubes and transformers and speakers. Information on where best to place them and how to approach sizing the fuse if that is the case would be appreciated. Cheers!

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John

This is a contentious question in the amp building world. You can see some earlier comments in this very thread, here.

The key to using a a fuse in this manner is finding the proper value. Fuse are rated by their (I^2)*t value. This is a constant for the fuse so the greater the current, the shorter the time to fail. You really want the fuse in the B+ supply to the power stage so that it doesn't see the AC current variation. Some people put them in the cathode path but I'm not a fan of this arrangement.

Maybe some others will chime in with opinions.

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog

Thanks Matt. Reading further I saw you also wrote above about using the 250 mA fuse and the I^2t. Would the fuse need also to be rated above the 400 or so volts dc B+? The fuses I see at the local electronics shop are rated for 250v AC. Additionally I see Mark's fuse is on the plate connector of the 300b. Is that correct or is it preferred to precede the opt primary winding and after the final smoothing capacitor to also protect the opt. Apologies the questions are probably droll. Regards.

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John

Personally I don't like to submit the fuse to the widely varying current inherent in that location. Since it is protection against bias overload, I would recommend that the fuse go in the B+ line before the output transformer load for class A amplifiers. I would put it before the last filter capacitor for class AB, B, & C amps.

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog

Thanks Matt. Will implement this. Will check on the appropriate rating for the fuse first with Littlefuse. I think there is also a SWE-check fuse company with an office in Melbourne I can check which seems to have a largish range. Not all accommodate DIYers though.

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John

Hi, I agree with Matt on the selection of the fuse etc.....but would like to address some of the more contentious issues. My experience with tube amps has been that when there is a failure it is catastrophic. There is nearly never a small failure that is sufficient to trip any fuse that is not already audible in some way which would lead the user / builder to check out the piece of equipment. Thus all my DIY and commercial designs only fuse the AC mains as it enters the piece of equipment. Looking at the types of failures likely I think this makes sense. First if a power output tube fails internally such that it presents what amounts to a short to the power supply it will consume as much power as the power supply can deliver. For an instant often quite a bit, but in fractions of a second it will be limited by the impedance of the power supply and the dc resistance of the output transformer. It will none the less be quite sufficient to blow a main fuse in the AC mains (if that was selected correctly). The tube since it is already bad needs replacement but in nearly all cases nothing else will. In an amplifier with fixed bias that fails the output tube or tubes will again drain the power supply in a similar manner. Again the fuse would blow. The logical question is what happens to the tubes? Tubes are really quite robust. For the length of time involved they will not be likely to sustain any harm. If you check the surge rating of most tubes it is quite huge compared to the average power they can handle. I you are using $500 tubes then maybe some precautions are warranted, but for typical tubes especially new production ones I don't believe it is warranted. If some other part of the equipment fails then it would most likely be something like a shorted filter capacitor, shorted rectifier, shorted transformer (either power or output). In each instance there would be a significant drain on the power supply and in turn on the AC mains feed. The fuse if properly sized would blow. A similar situation would occur in solid state equipment even though the voltages are lower.

In the situation where some component gets out of tolerance, say the output tube's bias degrades over time, but not to zero. An increase in demand on the power supply would certainly occur. Since it would likely be gradual over time it would not cause any fuse in any part of the circuit to blow until something got so far out of tolerance that it failed (for example a power tube over dissipation rating and subsequent failure). This goes back to the earlier scenario

I personally think that putting fuses in the B+, heaters or tube anodes or cathodes is more likely to cause problems than solve them. For example if you fused the output tube and it failed, then the B+ would increase significantly and may cause failures in other parts of the circuitry because of excess voltage. Plus as fuses age they may corrode in the holders and cause intermittent noise and even fail for no reason other than fatigue. Then you will probably think that another component has failed and spend a lot of time and energy trying to figure out what happened and when you can't find anything put a new fuse in forever wondering if you will have it quit again.

Just my two cents

Good listening
Bruce

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Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Thanks Matt and Bruce. Lots to ponder.

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John

Will the gxse15-8-3.2k opts work ok with this circuit or would there be some limitations? this is what i am using at the moment and for most recordings the sound is exceptional i would say for especially jazz. double bass even low notes and drums are quite fine. however on some of my favourite rock albums i notice a dryness and sameness in electric bass which is not quite right. i wonder if the opts are at their limit or maybe something i have missed or operating points a bit off. I do have a low level hum so i could have a bad earth somewhere. i have followed this circuit to a large degree - my b+ to the 300b is 405v and to the 6sn7 gtb only 240v (my dropping resistor calculation / guess wrong). also i re-used my edcorusa 380v power transformer and 8H choke from the last effort. Also waiting on a 250k potentiometer- 100k temporarily installed which is likely suboptimal. thanks for any suggestions.

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John

Scratch the above concerns. The amp is fine. I was just listening to Bill Bruford and Michiel Bortslap In Two Minds -just wonderful.

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John

I see that the Rk-Ck are connected to pin 4 of the 300b filament with the positive of the filament voltage. Not sure why but i have used pin 1. should i move it to pin 4 or is either side ok. Thanks for assisting.

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John