Have a guess - what's wrong with my '73?

[Matthews Guitars]

They're out there, but not always easy to find. If all else fails I could mail you a set. I really do have plenty of them.

 

[neikeel]

I've checked my stash
I only have the Cinch that I need for JTM resto stuff
The new ones are out there:
https://www.ebay.co.uk/itm/20280064...rand=Unbranded&_trksid=p2351460.c100667.m2042

https://www.ebay.co.uk/itm/12485773...1dvZpyWCBDbp8T4BVprUAU1usv|tkp:Bk9SR-yu-u_TYQ

Bit if Matthews Guitars has an NOS pair that he can sell you I would ask him nicely as it would be ideal, particuarly if your tech is neat and tidy with putting it back together again with original hardware (may like to go with new Welwyn 2.5w 1k screens but still use Iskra or Piher 4k7 or 5k6 swamps whilst he is at it?)

 

[Quinny]

Hi folks

Bit more advice please.......

After a lot of back and forth I had the amp back up at the tech today. It came back to me last trip with some transient voltage suppressors in place - plugged in, sounded great low/clean, then turned up to my usual 5/6 sweet spot and things went wrong again. This time I ended up with the amp still on/'working' but with a horrible scratching/fizzing kinda noise alongside the original tone. HT fuse didn't blow though, so seemed like progress of sorts and suppressors suspected. They're rated at 1.2kV so would have been surprised they were the issue. OT was also previously tested at 3kV and fine. When opened up a blown resistor on one of the power tube sockets was spotted (pic below shows the blown resistor in place). The suppressors were fine, but have now been removed.

Resistor was swapped, Siglent signal generator was plugged in and, as with last time or two, all well. Plugged guitar in however - again OK on low/clean, but start to crank up and the replacement resistor on the same socket blew. At one point in the testing the HT fuse went again - I'm not familiar enough to detail the testing which was done exactly.... my job was 'ok, now play the guitar again' and every time I did something failed around 5 on the volume. As things stand the power/standby switches have been swapped (power one because it was needed, standby just to match the new power), power tube sockets have been replaced.... everything measures fine in various stages internally with a signal generator plugged in..... but I'm still in the position where use it as an actual guitar amp with the volume from 4+ and things go south. With the Siglent plugged in the measurements/peaks internally measure as expected - but plug a guitar in and it becomes too much. The amp initially had a 1A HT fuse in instead of the prescribed 0.5A, so a previous owner was stretching a tolerance for probably similar reasons - but I'm loathe to do the same and move the problem to a more serious one elsewhere.

The main kicker is this.... ultimately the tech says everything which can be checked has been and looks OK. He thinks maybe I'm just running the amp too hard and it maybe doesn't play well with the attenuator (Fryette PS100 reactive load, some measurements looked a little lower when connected to his own resistive load). Whilst I understand that in terms of the amp's current 'turn it up too loud and blow a resistor or fuse' status, it doesn't feel like a satisfactory conclusion. I mean, I should be able to run the thing flat out if I wanted, should I not? I use it around vol 6 on ch1 upper input..... is there any logic in that just being 'too much' for the amp? Feels to me like something still isn't right, some signal not being effected as it should somewhere internally then hitting power or OT section too high. I have an ISO cab which I can play it through without an attenuator - but as things currently stand I'm fearful of plugging it in at all....

At a bit of a loss what to do next currently, would appreciate your experience/advise as always. Thanks.

 

[Matthews Guitars]

For reference I also have a '73 Superlead and it'll take being dimed and hit in the front end with a Tube Screamer set to max level, all day long, without a hitch. So definitely it SHOULD be able to be run at 4, 5, or 11.

The first thing that would get the stink-eye from me would be the crappy original impedance selector. They were not great when they were new, and 50 years later I'd use anything else. I would be willing to guarantee that by now the spring tension on the contacts is close to zero so the power handling capacity is close to zero.

Solve THAT problem by taking the selector out of circuit. You should have four speaker jacks. Hard wire one jack to the 16 ohm tap. Hard wire two jacks to the 8 ohm tap. Hard wire one jack to the 4 ohm tap. LABEL THEM.

Now retest. If the problem is gone, then that's that. If not, you've still eliminated the weakest likely link.

 

[neikeel]

Ok
Those MO resistors are the biasing resistors (1ohm on the cathodes to ground) they can act as fuses.
You appear to be missing the screen resistors I mentioned earlier. The yellow wires should be moved to pin 4 and the 1k resistor installed between pin 4 and pin 6.
In these circumstances you make be getting oscillation on the outputs so a 4k7 or 5k6 swamp on pin 5 would be advisable
Just my 2c but I bet it works.

 

[Athana]

If you search Fryette PS100 Marshall you may find using that thing has resulted in the fuse blowing in other cases.
Use the amp at high volume with out that thing...as the amp is intended to be used, does fuse blow?

 

[Quinny]

Ok
Those MO resistors are the biasing resistors (1ohm on the cathodes to ground) they can act as fuses.
You appear to be missing the screen resistors I mentioned earlier. The yellow wires should be moved to pin 4 and the 1k resistor installed between pin 4 and pin 6.
In these circumstances you make be getting oscillation on the outputs so a 4k7 or 5k6 swamp on pin 5 would be advisable
Just my 2c but I bet it works.

Didn't see any oscillation on the scope in current configuration - would that suggest this is a non-issue, or still worth trying? (Thanks for the info)

 

[neikeel]

The issue occurs when you start to drive the amp hard. Screens may draw excess current (hence needing the 1k screen resistors) which is more likely to occur during overdrive. The screen resistors MUST go on. The 4k7/5k6 swamps are recommended if you are having these sort of problems. The cathode resistor blowing is a symptom of excessive current draw by that tube not a cause.
Matthew’s guitars may have a general point re selectors but I am 99.9% sure it is not the issue in this case.
I urge you to implement both changes and report back.

 

[Quinny]

Any particular reason why this becomes an issue over time? Presumably screen resistors/swamps aren't stock and might be required due to general component drift over the course of a few decades? Speaking with tech now and getting it scheduled.

 

[neikeel]

Screen resistors became stock from 1972 onwards. Earlier EL34 amps used xf2 mullards and were not required.
I don’t routinely use swamps but in this situation the more stability the better.

 

[Quinny]

Got the amp back yesterday and it is running SWEEEEEET!! Sounds proper amazing, potentially a wee it better than it was before. Ultimately work done was:

Power/standby switches swapped for new (power switch was intermittent, standby swapped too just to match cosmetically).
EL34 sockets replaced for new ceramics.
1k screen resistors and 5k6 stoppers added.
Impedance selector left in place but bypassed and fixed to 8 ohms.
A previous owner had installed an 'earth check' for some reason. Disconnected but left in place, an interesting little part of the amp's history.

All original parts retained, trying to maintain a balance of keeping close to how it was 50 years ago whilst still being my main daily player. So happy to have this sorted, it really is a magical amp. Gonna treat it to a flight case.

Thanks for the advice and assistance with this, gratefully received indeed.

 

[matttornado]

Awesome! Glad to hear. Rock on!