JapanAxe

Well this is both annoying and encouraging. Today I rebuilt the bias board with a voltage doubler, giving me a range of -56V to -36V. I still get a positive voltage at the control grids when I apply HT, and this time I got some sizzling and smoking from around V4. I wasn't able to determine exactly where, and there are no crispy components, but it's a start. I then removed all 4 output valves and dropped the mains to 180V. With the amp in standby, I measured around -14V on the HT downstream of the standby switch, and it changes as I move the bias adjustment pots. There should be no way for DC to make its way from the HT to the bias circuit. There's either a short or a leaky capacitor. I'm off to play sone music now to soothe my brain but I'll come back to sort this another day.

I think there’s a general lack of ‘fun money’ in people’s pockets at the moment.

Depends. But always remember - you can never be too smart on a gig. There are few worse feelings than that of being inadequately attired.

Not a scam but you’re not the only one to experience frustrating delays: https://www.thefretboard.co.uk/discussion/287054/

I’m a big fan of the sh!t sandwich. Start with something positive, then deliver the bad news, finish with another positive. But only if you’re asked.

Well that's annoying. I rebuilt the bias board with the correct component values but the result is the same - the bias voltage disappears when HT is applied. With 450V of HT there should be about -37V on the control grids accordoing to the 6L6GC spec sheet. I'm pretty certain that the tapping I am using for the bias is a completely separate secondary as I checked continuity when I first got the transformers out of the Bugera. It has a centre tap (unused) but the HT secondary doesn't as it went to a bridge rectifier (which is what I'm using on this build). I'll check that again, and then consider the following options: Voltage doubler on the 41V AC secondary. It may be that it just can't produce a high enough negative bias without this. Creating negative bias by dropping the HT with a potential divider. Merlin Blencowe goes into all this. Sadly it's looking less. likely that I'll have the amp ready for the SW Bass Bash

Yes. Yes it is.

Probably at least 2 hours per day, but I am retired and either a member of, or dep with, a ridiculous number of bands. Not many days go by that I’m not practising something or other, and sometimes I spend pretty much the whole day at it. I recently worked out that I was working on learning some 65 songs: - 15 jazz or similar numbers on guitar for a one-off charity gig; - 32 rockabilly and country songs (again on guitar) for a first rehearsal / audition - now joined that band! - 18 West Coast rock numbers on bass for a theatre tribute dep.

I love depping, although I frequently end up biting off more than I can chew! A lot of my opportunities have come through musicians who I’ve already played with, but if my diary is looking thin I put an ad on Bandmix. A couple of my long-term dep relationships have started that way. I always try to be the person that solves problems rather than creates them, whether that’s singing a couple of numbers, digging out cables from my spares bag, or giving a lift to another band member. Finally, I always thank the band for the gig, even if I have silently sworn to be unavailable next time they call - I’m pleased to say that’s a rare occurrence!

I’ll bring a roll of wide masking tape - have at it!

Well that's interesting. I've discovered that, when HT is applied, my nice negative bias voltage all but disappears, leaving the output valves to go full-on Fukushima. Which bit of this circuit is not a tried-and-tested design, but my own concoction? Yep, the bias supply. I consulted the writings of good old Merlin Blencowe, and discovered that my choice of component values may have left the output valves with too high a grid leak resistance. Time to re-design the circuit!

Not many people realise that, thanks to an oddity in the way the calendar is worked out, the 80s actually began in 1979.

I've spent some time yesterday and today, carefully commissioning the amp. I found a couple of screen grid connections that had somehow escaped soldering, so fixed that. The amp is working, in the sense that it produces output but I need to tweak the bias circuit as at present it's way too hot (over 200% of rated dissipation), even on the lowest pot setting and 180V mains supply. The over-hot bias is likely responsible for the hum I'm hearing (regardless of control positions). There is no audible oscillation when I raise the master volume, so the phase of the NFB connection is correct. I also used my oscilloscope to check for oscillation outside the audible range, and again there's nothing. I've done enough for today. Working on amps into the evening really messes up my sleep! A bit more work over the weekend and I'll be there.

I don't know, I know nothing about them!

I’ve never considered anything other than fuses to do the job of protecting the transformers from excess current. Every valve amp circuit I’ve ever seen has used fuses. The front panel may not look quite so clean when I label all the controls!

Tonight I trimmed the plastic face plates and re-glued them at the ends, fitted the knobs, and labelled all the DC power nodes (A, B, C, and D) on the board. I checked the circuit against the scematic and found a couple of issues: An output valve connection that had somehow escaped soldering, so that it was just hanging loose - not a great idea! Remember this post where I had a couple of 2.2nF caps but 'realised' I needed 22nF and had to order some? No, 2.2nF was right, I just wrote it wrong on the layout! I'm having some last-minute thoughts about fuse values. Based on the transformers from the donor amp, I think it would be T1.6A for mains and F2A for the HT. ALL BUILD PICS NOW IN POST DATE 01/11/2025

Done

The wiring is finished! Last night I wired up the master volume, and I've just now finished doing all the heater filaments. I had put the tag strip with the heater balancing resistors next to the pilot light but that turned out to be stupidly inaccessible for all the connections I had to make. I moved the tag strip to a better location and got to it. I'm a little way from plugging in though. Next session I will compare the built amp to both the layout and the schematic, and buzz out all the connections with my test meter. Then I'll think about applying power... ALL BUILD PICS NOW IN POST DATE 01/11/2025

The band plays rock’n’roll and a bit of country. ’Coward of the County’ was a massive crossover hit in 1979.

‘Cultural appropriation is the adoption of an element or elements of culture or identity by members of another culture or identity in a manner perceived as inappropriate or unacknowledged.’ - Wikipedia (my emphasis) I think that’s succinctly put, and I’d commend it to anyone who has concerns about performing a particular song. I’m more uncomfortable when one of the bands I dep in plays ‘Coward of the County’ by Kenny Rogers. The central events in the narrative are a gang rape, and the visiting of violent retribution upon the perpetrators. The song does not sit well with me as light entertainment!

Got some more done last night - PI and Normal channel. ALL BUILD PICS NOW IN POST DATE 01/11/2025