Trace Elliot, tell me about it

[Chienmortbb]

11 hours ago, bremen said:

C17 will isolate the input stage from the +v rail and any nasties thereon, eg 100Hz ripple or any DC variation as big currents into speaker drag it down.

Yes C17 is a decoupling cap, forming a high pass filter with R16 removing residual noise. Contrary to popular belief, this is likely to be at 100Hz (Europe et al) or 120Hz (North America, Asia) as the typical transformer/rectifier power supply adds noise at twice the mains supply voltage.

 

46 minutes ago, DGBass said:

That is a good question. Wish I could answer it with 100% certainty. I can only say from experience working on these type of boards. Tant's are as far as I am aware more expensive, have a longer life span, and from experience seem to have tighter tolerance than comparable electrolytics so are more stable. Yes, on these boards, and also on most Ashdown amps in recent years, tants are often employed as input couplers. Most schematics for these type amps specify components are replaced with the same original part specification, which for me at least is good enough reason to replace like with like.🙂

It was a rule of thumb when I was working that you did not use a "tant" unless you could not help it. They used to be very prone to going short circuit I believe. https://sound-au.com/articles/capacitors.htm

[HenryL]

Just to say I finally tracked down the problem with the GP7. 

The problem turned out to be the switched speaker socket that connects the amp to the internal combo speakers evidently not making a good clean contact through when nothing was plugged into it (the normal combo case).

It worked fine when I finally plugged in a separate cab to the jack socket.

 

I'm quite surprised it produced the kind of distorted sound it did - it wasn't a typical 'erratic contact' kind of sound but a quite regular fuzz (with the odd dropout) at low levels.

This is what threw me off the scent from the start - otherwise I'd have got to checking the socket much sooner.

Since there are two speaker output sockets offering permutations of 'parallel with the internal speakers' and 'interrupt the internal speakers' as a precaution I've rewired them so both sets of socket contacts are feeding through to the internal speakers in parallel, providing redundancy. It means if you want to disable the internal speakers while the amp is still in the combo case then both output sockets have to be plugged into, but it's not a setup I can ever see us using. 

Thanks again for the helpful input.

[Downunderwonder]

If I had a dollar for every time an interrupt jack socket was corroded.....

[DGBass]

I've never come accross an early 4x10 combo that had switched speaker sockets, or any speaker sockets for that matter, especially on the PM2-S output boards. They were usually wired direct from the output board via male and female spade terminals to the green/orange wires from the cabinet. The back panel usually had blanking plugs, probably to discourage owners from using extension cabs. The 4x10 cabs were also 8 ohm so these type combos were probably not pushing out much more than 100 watts and were never intended to power an extension cab. But they were loud. The PM2-S versions run a lot hotter than the PM4-S versions, and there was normally a large cast frame Papst cooling fan that ran permanently on. I guess that's just how it is when dealing with amps that are 40+ years old. You can never be totally sure what is original and what is not. Even the most reliable kit will need servicing at some point, which might include modifications that could cause unexpected reliablity problems in the future. 🙂

 

 

 

 

 

 

 

 

 

 

 

 

[HenryL]

10 hours ago, DGBass said:

I've never come accross an early 4x10 combo that had switched speaker sockets, or any speaker sockets for that matter, especially on the PM2-S output boards. They were usually wired direct from the output board via male and female spade terminals to the green/orange wires from the cabinet. The back panel usually had blanking plugs, probably to discourage owners from using extension cabs. The 4x10 cabs were also 8 ohm so these type combos were probably not pushing out much more than 100 watts and were never intended to power an extension cab. But they were loud. The PM2-S versions run a lot hotter than the PM4-S versions, and there was normally a large cast frame Papst cooling fan that ran permanently on. I guess that's just how it is when dealing with amps that are 40+ years old. You can never be totally sure what is original and what is not. Even the most reliable kit will need servicing at some point, which might include modifications that could cause unexpected reliablity problems in the future. 🙂

 

 

I was there when my friend brought it home new from the shop (probably Tiger Music in Brighton) then later it passed to me - who has had it since - so I know it wasn't swapped for the amp module from a standalone head, at least not since ~1983

Presumably they wanted to minimise the number of variants manufactured, although the printed instructions on the back panel next to the sockets are tailored to the combo case at least.  

The wording always seemed rather eyeball-crossing though, albeit correct, since it doesn't mention that socket B disconnects the internal speakers which would have made it more immediately obvious...

IIRC it just says  "If socket A is not used then a 4ohm cabinet may be connected to socket B otherwise use 8ohm only."  ( edit: oh I see yours has the same wording)

 

Re heat/fans - yes this was/is an issue.  When my friend first got it, it kept cutting out after a while due to the thermal cutout.  He took it back to the shop and it was modified by their tech "Oh yes sir, we do a lot of these".  So they fitted the aforementioned horrendous papst fan which can definitely not be put in the 'quiet fan' category.     When the amp passed to me I immediately removed the amp from the combo case, removed the fan, and installed the amp in a small separate flight case with more natural ventilation, where it was perfectly happy.     Some years ago, wanting the flight case back for other purposes, and irritated by hole in the top of combo cab, I fitted it back in its proper place but with a quiet 12Vdc Nexus fan run at 6V from it's own small PSU piggy-backed on the mains transformer.

Edited Thursday at 11:46 by HenryL

[HenryL]

[DGBass]

Thats a proper old school unit, thanks for posting a pic.  I must admit never having had any issues with overheating or fans on these types of Trace Elliot amps. My last 4x10 combo was a ( MK4) GP11 with the PM2-S setup and Papst fan which ran silently. It was probably one of the most original old TE's I'd come accross when I bought it. Same with the (MK4) AH150 head I still gig occasionally, its a PM4-S with the same Papst fan that runs silently. I did previously use an older GP7 head and the fan on that was noisy but I think that was just wear and tear / worn bearings from decades of use.

[HenryL]

As far as I can recall the fan they fitted seemed intrusive to me, from day one, but, looking back there is a little haze...