Stub Mandrel

I've discovered the amp is running of +/-50V ac not 40V. I suspect it's an early 250W one rather than 150W. This could explain why there were no external extension cab sockets. I'm going to fit an extra pair of mosfets - the PCB is ready for them, I just need to drill the heatsink. This should give greater reliability and safety margin if running at 4 ohms. Also, I found a thread, in Dutch, about someone with the same problem, except their DC offset was only 127mV. Much discussion and debate until in the last post they replaced TR201/2 with a matched pair and the offset came down to 5mV, then replaced TR203/4 as well and it came down to 3mV. I have high hopes swapping these transistors will cure my problem... There should be a translated version of the thread here: Thread on Dutch forum

Jamming with the guitarist from Impaler (gebuine signed band who had just put out their LP, guess the genre)and afterwards him telling the band I was a 'def' bassist. Probably had more to do with my HM2 pedal and sheer volume than any talent... 🙂

Old (analogue) Trace amps are conservatively rated for RMS not 'music power' or any other such misleading calculation, and have big hefty transformers with reasonable regulation. They also use a design for the power amps that gives hi-fi levels of quality, so you get maximum headroom with minimal distortion, possibly less than 0.1% at rated output. <Just looked carefully at my amp and see I've been talking bollocks so I'm editing this> I'd assumed it was a 150W amp because (1) everyone knows the 1110 combo was 150W and (2) the transformer is marked 39V 0 39V - the later AH150 was rated as 40-0-40 AC input. But I was just musing at how big the transformer is when I noticed the amp isn't actually powered from the 39V terminals, but from a pair marked 50V. That explains why the DC rails are at 69 - 0 - 69V. This probably also explains why a 150 watt amp is so loud... So... this amp is a very early 1110 combo, it has a MPB2 PCB marked 'Issue 2', with two mosfets and a fan. The issue 4 AH150 board has FOUR mosfets, 40-0-40V supply and no fan. The Issue 4 AH250 board has four mosfets, 46-0-46V supply and a fan. At the least I am tempted to add another pair of mosfets... they can drop straight into the board.

I think the PRS use a system based on a sample of venues that ensures the vast majority of royalties go to 'popular' artists. BTW you should be covered by the venue's licence.

The Running Man. I humbly refer you to the Utah Saint's Something Good '08...

I'm envious, but.. isn't a box set of unreleased tracks an oxymoron 🙂

Yes, but lessons considerably expanded the usable range of my voice. I hear 'it doesn't sound good' more often than 'my voice doesn't go there', lessons can help with that (and warming up can too! - I start a practice session by playing scales and speeding up as my fingers get into it, how many singers do that?) Ouch, found a real time pitch tester app....

A good question... It's a 1110 Combo, only proven by it being lightly scratched into the panel at the back. It has a GP11 preamp on the front and an issue 2 AH150 power amp at the back (only two FETS but with a fan - later issue 4 AH150s had four fets and no fan, the AH250 had four fets and a fan). Probably one of the oldest ones, not sure if Issue 1 or Issue 3 ever made it into the wild?

People have been in it a few times over the last 30 years. A resistor has been replaced and the speakers have all be replaced. They used Eminence Alpha 10s - not the ideal choice for a tuned bass cab, when they could have got Beta 10As for the same or less. I suspect they didn't have a clue what they were doing and assumed 'different' would be 'better'. I do need to check they are phased to match my extension cab.

The problem is that the MU doesn't distinguish between 'amateur musicians' who don't expect to do more than cover their costs and people who make music for a living. £221 is fine for full union support for your main job, but a big extra cost fif you are lucky to make a few hundred quid surplus a year. I can understand their problem - lots of such musicians paying, say £50 a year might not cover their costs, how do you decide who is a 'professional' and who isn't, and also some professional musicians may actually consider amateurs to be unwanted competition - as in how can they get decent paid gigs when the local four piece will rack up for £150 a night? <Edit> Just revisited their website. It really doesn't have anything that explains WHO they are for. Obviously if you are in an orchestra or a session musician, but what if you play half a dozen gigs a year in a pub band? Or you are in a small unsigned band? Looking at the list of benefits its really doesn't seem to be targeted at anyone not actually 'employed' as a musician as against just making (some) money as a musician.

Rats. This has got me singing along to Paul Robeson, I was doing OK until I found a song where he goes down to C2. I can get to F#2 fine, push to F2 and just to E2 then I start dribbling...

It was also beneficial to discover that one of the speakers was wired out of phase. This morning I moved the amp into my workshop which is better lit. After swapping the transistors not only did I notice the speaker I've been watching now moved in instead of out - so did two of the others but the third one moved out!

Sort of relevant to the thread title... I use all sorts of earplugs in my workshop, as well as ear protectors, they just plug my ears and reduce the noise. I bought some mid-price musician type ones off eBay. They are clear silicone with a black insert that has a filter inside it. They don't cut the volume as much compared to normal ones but do give a more natural sound. They are sold as 20 dB but looking around 12dB seems to be typical and roughly halving thr loudness seems about right. Similar to the ones on the right here. Are these likely to be of any benefit or am I better off sticking with my 'workshop grade' ones.

I'd love to have a go at singing, but I can't play and sing at the same time and I don't want to give up bass.

Now that's what I call festival music 🙂

It's cruel to say it, but... think most singers would benefit from a dozen singing lessons which would extended and even out their range. It's amazing what singing lessons can do...

Yep, seems to be these transistors, if I swap them (1) the direction of the offset reverses and (2) the amp gets noisier. Swap them back and the noise goes and the offset reverses.

I've found some help on another forum. Looked like a mismatch in the differential pair. I pulled the transistors this morning and their gains are 80 and 64. Enough to give hifi buffs the heebeejeebies, not sure if its enough to give the symptoms I'm seeing but I'm going to see if I can find a better matched pair rated 100v.

Talking to myself... but... Working on the principle that the collector and base voltages of the differential pairs should all be pretty much equal, the collector of TR204 is at -5.1V, 206 is at about -21V. [edit] This is supposed to be a 'current mirror'. I'm sure the collectors of TR203 and TR204 shouldn't be this far out of balance one is at -21.5V the other is at +5.0V. In an ideal world the collector of TR204 should be a couple of volts negative and I would expect TR203 to be somewhere near this with the amp quiescent. BUT the voltages across R208 and R211 are almost identical suggesting the current mirror is working.. Next looks the bases of TR201 and TR202 in the differential amplifier bit both at 0.4V. Reasonable enough, this suggests a base current of about 7 uA. Suspicion currently on match of R204 and R205 and TR203. But can anyone offer some advice?

Old fashioned 1980's combo 150W. Symptoms - intermittent noise, occasional cutting out. Noticed Transformer running VERY hot. The noticed speakers are all slightly forward of their normal rest position when amp switched on. Otherwise, sounds normal with apparently good headroom but hard to test in living room... Conclusion: problem with the power amp bias. Possible dodgy transistor in bias circuit? I do have a schematic + test gear. First check, 8V DC offset across speakers. Played with pot VR410, may have been part of the problem dirty as it instantly improved, now between 4V and 4.3V but still making speakers jump forward and noisy (NB this shoudl be adjusted to minimise crossover distortion). Discovered C409 was partially desoldered and 10uF (47uF specified). Tried both a 10uF and a 47uF and noise greatly reduced but still bias issue. Thing is, VR410 looks to me to be for adjusting the quiescent current to eliminate crossover distortion rather than the bias... Very simple amp, but I'm struggling to see what might be the problem. Have been over resoldering to make sure no dry joints. This is issue 4 of the MPB2 board, my amp is issue 2 and only two mosfets fitted + fan (but places for the other 2)

What makes Power Windows unique among Rush albums?

If those are genuine Di Marzios worth buying for the hardware and throwing the exquisite luthierie away... https://www.ebay.co.uk/itm/New-Personalized-Electric-Guitar-Small-Mini-Version-Of-Patented-Products/283146791957

I suppose if you have an inexperienced drummer it lets you balance out the levels 😉

Number of strings isn't a big thing, I'd like one of these too: Bit hard to hide it behind the sofa...