Premier 12 MB’s output setting

[goonybird]

So had a little mishap.
Wanted to change back to my backup GE6550a from the EH KT90 in at the moment.
Let amps settle to set bias when a replacement good tube Red Plated.
Turned amp off quickly.
Last time a GE 6550 Red Plated it took out the Bussman fuse as its designed to do.
This time it didn’t. So further investigation shows AC input 3At fuse O/C. Waiting for replacements as nothing in the spares draws.

So while the bottom panel was off, I checked the speaker impedance setting.
It is set for 8 ohms (factory setting 4ohms? ) So this may have been changed sometime in its past.
I can’t locate the feedback resistor? To check the value conforms to 8 ohm setting
Does anyone have a pic of the location or description.

Many thanks

[AnotherJohnson]

Check to be sure the big fuse is the right size. If you get red plating and they don’t blow, it can be that a prior owner put in what was available rather than what was correct.

If there was no smoke, and the resistors don’t look burned, you may have gotten it turned off quickly enough to avoid damage.

[admin]

Sounds like you got lucky but I agree with AJ, check on the fuses and do a good visual check.

We do have the Premier 12 in our schematics database so you may be able to trace out the location of the feedback resistor with that.

[goonybird]

Thanks. Have schematic. Fuses inside all original.
The Busmann fuses were from china but look the same, so I have a suspicion.
In that it should have blown. Will open one up to have a looksee
Fuses due today. And will use the Variac to power it up gradually

[admin]

Thanks. Have schematic. Fuses inside all original.
The Busmann fuses were from china but look the same, so I have a suspicion.
In that it should have blown. Will open one up to have a looksee
Fuses due today. And will use the Variac to power it up gradually

Definitely get 100% legitimate fuses. Arguably the single most important component in the entire unit. I would replace these "Chinese fuses" immediately and not power up the unit until that is addressed.

[AnotherJohnson]

Thanks. Have schematic. Fuses inside all original.
The Busmann fuses were from china but look the same, so I have a suspicion.
In that it should have blown. Will open one up to have a looksee
Fuses due today. And will use the Variac to power it up gradually

They often “look the same.” Do they have good continuity?

If you are not at the level of owning a multimeter and possessing skills to use it, put the covers back and take it to a technician. The DC voltage levels in these units can liters kill you.

[goonybird]

I do have over 50years as a hobbyist with repairing and rebuilding tube amps and have test kit if needed. best mistake was accidently touching the HT line on a Radford STA60/100 which threw me across the room (lol) note that amp ran nearly 1KV ht.
anyway fuses turned up tonite shipped from RS Components. Quite difficult to find in the UK and £15.00 for 10x but shipped overnight. Might get some 4 Amp that are in there as well

So all seems ok. Bench testing at the moment.
Note: as a tip... I am using a Thermal Thermometer left over from covid to read Tube Temperatures. Finding temperature still increases very slightly over 20 minutes which correclates that these amps seem to settle soundwise at about 20-30 minutes. Saves touching the Tubes to check warming up ok, and burnt fingertips!
Tip 2: is use a TV plastic adjuster to adjust bias.

[goonybird]

Ok Just traced back Feedback resistor and its 2.55kOhm which is for 4 Ohm setting. so I either replace resistor for a 3.92k vishay or change the Output Tx wiring link from white/orange to white/yellow. I seem to remember 4ohm is factory and also recommended setting.
Speakers now Kef R7 impedance plot from a revew.
So better 4ohm or 8ohm....?? Note Red Trace

[admin]

I think all CJ amps are wired for 4 ohm as standard.

[Big Dog RJ]

Yes, 4 Ohms is the factory setting. Every Tube power amp has been set at this spec before it leaves CJ's HQ. However, upon custom requests, CJ can organise a different Ohm setting if the owner wanted that. It used to be welcomed by the old crew... not sure how JF goes about that type of request these days.

In any case, I've found the 4 Ohms setting to be superior in the Output stages, it also allows the amplifier to drive better, with less strain and greater control.

A few custom orders placed in the good old days by only a handful of our customers for either 8 or 16 Ohm settings, we're mostly to do with driving very easy loads, such as Horns and other very high efficiency speakers. Horn loaded types, hybrids, Tannoy's dual concentric designs and field-coil single driver designs... very easy to drive with just 2 watts!

Other than that, I'd just stick with CJ's factory settings for 4 Ohms. Makes more sense for a wider range of speaker options.
Cheers, RJ

[goonybird]

Thanks.
Decided to revert it back to 4ohm.
Problem I have found is it likely needs a plasma cutter to desolder the speaker terminal
None of my Soldering irons were man enough for the job.
So going to replace binding posts due to Solder on them and looking a bit messy, - have some nice Cardas CCGR Binding Posts coming
and then will use M8 8mm "ring" terminals bolted to connector.
Much easier and neater (and easily reversable)