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B15-inspired Amp Head Build


JapanAxe

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Well that was interesting! The mains transformer I bought and installed is not the one I intended to get - it has a secondary of 300-0-300V at 200mA, whereas I meant to buy one that gives 360-0-360V at 400mA. I discovered my error when I checked the B+ voltages, which are about 10% down on what I expected. I used this online calculator to confirm that 200mA should be more than adequate for this amp with a 300V supply voltage - if it gets a bit squashy and saggy, no problem, this is for home use after all. Otherwise I'll have to drop another £120 on a PT that weighs 1.6kg more than the present one!

Plate dissipation in each 6L6 is 20W, or 67% of max - that should be fine.

With all valves fitted, there is a buzz at the output through my bench speaker but no signal is getting through. I noticed a bad solder joint at the 4ohm speaker output to which the NFB resistor is connected but fixing that made no difference. The 6SL7 preamp valves are the only ones I couldn't test, but I bought them as tested and working, the filaments light up, and swapping them round did not change anything, so I think there's still a wiring error in there for me to find.

I've got a busy weekend of gigs and a rehearsal coming up and I need to do some practice, so I'll have to leave it for tonight. In the meantime, here's the mighty valve line-up:

xd3J4Nq.jpg

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35 minutes ago, Beer of the Bass said:

I'm sure you'll get it there - it's fairly normal to need some troubleshooting. And if you do find it saggier than you wanted at the lower B+ voltage, trying a silicone rectifier could be an option to increase it a little.

Yep, just a matter of working through it until I find the issue(s). I've done this enough times not to be worried about it. I could always add a silicon rectifier as an option but tbh I don't really think it will be an issue.

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Posted (edited)
18 hours ago, JapanAxe said:

Yep, just a matter of working through it until I find the issue(s). I've done this enough times not to be worried about it. I could always add a silicon rectifier as an option but tbh I don't really think it will be an issue.

Looking good there! It's a pretty simple circuit, shouldn't take too long to scope things out.

We are finally getting a short break from the oppressive heat here (112F one day last week and over 100 for several days in a row) and I'm hoping to finally finish my new one up in the next few days. But of course our wonky Italian espresso machine chose last week as a good time to die, and the last round of replacement parts showed up in my mailbox this morning. Priorities...😎

 

Edited by Passinwind
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It lives!

Having confirmed that there was continuity everywhere there should be, I turned my attention to finding it where it should not be. I quickly discovered that the wiper of the volume control was connected directly to earth - the culprit was a stray whisker of shielding. Although I had stripped back the shielding and clipped it away (but not completely it seems), the action of sliding some heat-shrink tubing over it had dragged it into contact with the grid stopper resistor feeding the phase inverter.

The amp doesn't sound particularly loud through my test speaker (a 10in guitar speaker lying face-down on my work bench - I can't really give it much beans) but that's just to load the output and confirm that the amp is working - experience tells me it will be a different story through my BF One 10, but that will have to wait for now. There is a little bit of buzz that disappears when the volume control is maxed, but I can deal with that at my leisure.

I think my transformer purchase goof may have been a happy accident. Running the amp at full wall voltage (249V today) the HT voltages are only 7-9% below the Heritage spec. If I had fitted a 360-0-360V PT they would have been way above spec. Plate dissipation is now 21.7W in each 6L6. This is 72% of maximum, and perfectly fine for a cathode biased amp. The 10W bias resistor gets pretty hot considering it's only burning off 3.7W! I have been lucky with my guess about the PT secondaries i.e. the NFB resistor is not creating scary positive feedback.

I still have to fit the feet to the base plate and make sure they don't foul any of the electronics. I'll do this and fit the base plate before I sit it on my One 10. Watch this space!

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Posted (edited)
4 hours ago, Hellzero said:

No more TLRT's so here is one for you fantastic work ! 💙

Thank you!

My weekend started just now so I put a piece of cardboard on top on my One 10 and placed the head on that. Plugged in, it is plenty loud for my little practice room with the volume set at about 2/3 of maximum. At full volume it breaks up when pushed, but at that point there are quite a lot of items in the room vibrating!

The tone controls are very responsive, with plenty of plummy bass and crisp treble on tap. The tonal differences between my 3 bass guitars really stand out, and it copes with a low B without breaking a sweat. There is a little bit of crackle from the treble control, which is surprising as I bought all the pots new. I will check that there is no DC getting through to it.

The slight buzz that I mentioned seems to be at 100Hz rather than 50Hz, which points to noise being picked up from the HT rather than the heaters. I will try moving some leads around with a chopstick to see if that helps.

Edited by JapanAxe
Typo
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The good news: I finished putting it all together and applied all the labels.

The bad news. When I powered it up and took it out of Standby, the mains and HT fuses both blew immediately! I need to get stuff ready for this weekend’s gigs so I won’t have time to look at the amp for a couple of days but I expect it will be blindingly obvious when I open it up…

tX7cJkH.jpg%20

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This afternoon I sat down to find what might have caused both fuses to blow when coming out of Standby.

My first hypothesis was a (near-)dead short from the HT to ground downstream of the HT fuse. I examined all the cabinet fixings but none would have contacted any conductors, and there were no signs of arcing there or elsewhere. I then checked both 6L6 valves on my Orange Valve Tester and they were fine. Next I worked through all the components ancillary to those valves, and again found no problem. I measured resistances on all the transformer windings, so nothing has burned out there, and there was no short to earth from the first HT node.

I went away and sat down with the schematic, which was when I realised my error. Based on previous builds, I had fitted a a fast blow (F) 500mA fuse in the HT line. However the schematic calls for a slow-blow or timed (T) fuse. I expect this is because the first filter cap is a fairly chunky 47uF, which the HT rail briefly sees as close to a dead short when coming out of Standby - briefly, but long enough to blow an F-rated fuse.

For 240V operation the mains fuse should be a T2A (slow blow again) but I didn't have any slow blow 2A so I had fitted an (unmarked) Medium fuse, which had duly blown in response to the inrush current.

I have now fitted a T500mA HT fuse, and ordered a supply of T2A fuses. Once they arrive I will try again, initially with the dim bulb current limiter. In the meantime, if you suffer from insomnia you can read all about glass fuses here.

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The T2A fuses arrived today and the amp is now working just as it should, without blowing fuses. It draws around 0.35A (88W) from the mains when it has settled down, which is not unusual given how delightfully inefficient valve amps are - my Princeton Reverb draws 0.24A (61W). The steel bird cage gets quite hot above the valves, but my PF-50T did the same.

I'm calling that a win.

I just need a name now - any suggestions?

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1 hour ago, Richard R said:

Looks fantastic, the photo makes it look slightly menacing with the hot valves lurking in their cage and the red light  like the Terminator's eye.  

As for names, JapanAmp?

The pilot light has a white cover, but it's not the best photo. Nice suggestion though!

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