DIY Audio Projects Forum

Something else I like about this particular amp is that I feel absolutely no need to adjust bass or treble. Clearly that's not even an option, but I feel like the range of frequencies are very balanced. Overall it's just a very balanced sounding amp..

Maybe I'll call it Balance.

Nice work. The chassis work is just like mine, unpainted Hammond aluminum. With the speakers you picked up it looks like you have pulled together a really sweet system pretty quickly. Probably took me 5 years to get that far.

Enjoy

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Thanks.

I kinda lucked into the speakers. I was originally looking at getting a local guy to build me cabinets based on the Fostex design, and picking up some 206en's, but ojaw had those ones already built up here.. I saw them, and thought, man those would be perfect, and then noticed he lived only 200kms away from me. Very fortunate.

I noticed that when I power the amp on and measure B+ on the plates of the 6AS7, as things warm up it climbs in voltage past the idling voltage of 244V up to over 300V, and then settles down to 244 after a minute or so.

Is this normal? I assume it's happening because the 6AS7 triodes take a while to get up to drawing full current.
Is there any danger with this, being that the max Voltage on the plates of the 6AS7 is 250? I'm not concerned about straining the filter capacitors as they're all rated for 450V.

I've been reading up on soft starting, and many people seem to think it's unnecessary as cathode stripping isn't actually a big issue when applying low-ish B+ to cold cathodes, but I'm kind of dealing with a different issue.

For me, because the tube is not drawing full current until the heaters are warmed up, the voltage drop's through my B+ line are low at the start, and then as current draw through the power supply increases, my voltage drops increase over the chokes and resistor and then my B+ settles in at what I want after about a minute. I wouldn't be concerned about that either except for that maximum voltage rating on the plates of the 6AS7 of 250V.

If I used a CL90 thermistor on one leg of the primary side of my power transformer, causing a ramp up of voltage on the secondary windings, would that potentially give me enough of a delay for the power tubes to warm up enough for full current draw before full B+ is applied?




OR.. Just thinking about an effective use of a standby switch of sorts.. What if I keep my power switch as it is, just switching main power for the power transformer on the primary side, but then use a DPDT for my "standby" switch which switches both the 5V and High voltage secondaries on when I feel that the heater of the 6AS7 and 12ax7 has warmed up enough?.. The power switch would effectively only be switching the 6.3V on, and then the standby switch would be switching the 5V and high voltage on. I'd still get the slow ramp up of B+ when turning the standby switch on because the 5V heater in the rectifier would only start to gradually warm up after that standby switch had been thrown, so no worries of a surge of current like a traditional standby switch would do by just cutting the B+ coming out of the rectifier.

Thoughts?

My 300B amp uses the $1200 a pair TJ Full music carbon plate tubes which I wanted to protect as much as possible. I just use a standby switch. I have the heaters for all tubes on for about three minutes then turn on the HT. Yes there is probably a rush because the HT has been in for that time but at least it's a rush to hot tubes. As an aside to protect the tubes against over-current I have fuses in the cathode path.

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Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |

Interesting. What if your standby switch was a switch for both the rectifier filament voltage as well as HT?.. Then you'd still have the slow start up on the HT.. Is that a poor idea?

This is pretty normal and within limits just OK. This is where the standby switch works because when the HT is dropped on there is a waiting load for it. Try a Sovtek 5Y3 rec tube. These have a cathode and take a while before they pass full HT. Usually the power tubes warm-up quicker. The power tube then already apply some load as the 5Y3 comes up. Other rec tubes e.g. 5U4 come-up a lot faster.

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Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |

I want to use the 5V4G because I've got 6 mint old stock of them right now . I'm interested in this idea of switching HT as well as 5V (with a DPDT switch switching one leg of each winding) with the standby switch, and wonder if there are any issues with doing that, and if it would accomplish what I want.

Stop! It's ok.

Your amp is perfectly fine as is. First, the maximum plate voltage listed in data sheets is the "maximum operating plate to cathode voltage at specified maximum current". Vacuum tubes are generally designed to handle around twice this voltage at zero current. This is why the maximum plate to cathode voltage on the 12AX7 is rated at 300v (Va) but the maximum B+ supply voltage is 550v (Vao). The 6AS7/6080, coincidentally, is rated at Va=250V and Vao=550v. Here is the cut out of the 1960 Phillips data sheet.
Second, the 6AS7 is, first and foremost, a voltage regulator and boost scanning tube. As such is it built "hell for stout" as they say. The peak inverse plate voltage is 1700v. The ability of the tube to stand up to much more than 550V at startup is virtually assured.

The behavior you describe is typical of most amplifiers. It is normal and will do NO damage to your tubes. They are designed for this. I strongly recommend you turn on your amp in the background and start planning your next project.

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Matt
It's all about the Glass!
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