New JTM45 build blowing fuses.......

[stelligan]

What’s the VDC at rectifier pin8?

70VDC

[pdf64]

The rectifier would seem to be bad?

[stelligan]

The rectifier would seem to be bad?

My thoughts as well. I just took delivery on a solid state alternative and 2 tube rectifiers. New Sovtek 5AR4 is in there now. Will try the solid state thing again and check volts. Maybe put a new socket in? I'm baffled...

Yes - just tried the solid state rectifier with same result - slightly higher shitty voltages

SSR-2.jpg

[pdf64]

Are you sure that the HT fuse is good?
I assume it’s on the HT winding’s CT return to 0V.
ie the VDC between CT and chassis should be minimal, <1V.

[stelligan]

Are you sure that the HT fuse is good?

Ah Hah! reads open. Will go digging in the fuse drawer now...

[stelligan]

Are you sure that the HT fuse is good?

Ah Hah! reads open. Will go digging in the fuse drawer now...

Ok. I'm fixed!! thanks to pdf64!! Where do I send the beer money? I need to save a little for new glasses and a pair of more closely matched KT66. These are way far apart...

[pdf64]

Haha, with regard to fuses, ‘looks good’ does not always mean ‘good’
Put the beer money towards the KT66 - you’ll need it, they aren’t cheap!

[stelligan]

Put the beer money towards the KT66 - you’ll need it, they aren’t cheap!

You are not kidding. I have probably been cheaping out with these Chinese Sino/Valve Arts things. This pair read at 32 & 67mv when this thing woke up. May have to pony up for Genalec again or something NOS.

Thanks again for the help!

[pdf64]

Dunno why, but even back in the early 80s when I began to notice such things, KTs always were expensive, maybe twice the price of their equivalents.
NOS - you’re kidding, surely? They’re crazy money these days!
I seem to remember @Roe recommending the Gold Lion KT66. Which I think are Russian made?

My perception / recollection is that KT66 have a creamier, smoother overdriven character than EL34 or 6L6. While my preferred pair of 90s GT KT66 are still good, I need to verify that I really can hear the difference between them and other 6L6 types. And that the difference isn’t just due to the LTP not being able to overdrive them as hard, due to their requirement for a greater magnitude of bias voltage, and hence more signal voltage required to overdrive them.
My ambition is to outlive those GTs, and if I can’t hear a difference, then I’ve got plenty of nice 6L6 types that I could replace them with

[stelligan]

NOS - you’re kidding, surely? They’re crazy money these days!
I seem to remember @Roe recommending the Gold Lion KT66. Which I think are Russian made?

Yes - I am kidding. Have been shopping online and NOS is a no go. I have 5881 in here now and it sounds fine. I have Gold Lions in a RedPlate BluesMachine 66 Tremolo. And they have never had a hiccup. Around $85 US for a matched pair. KT66 do seem to have a nicer high frequency breakup sound but I have that covered in the other amp. May just settle in with 5881s. I have a ton of them and can abuse them. I usually run this amp into a reactive load with channels jumpered and volumes 3/4 up. May be part of my problem..

My goal with building and owning amps has been to cover as many different tones as possible. This amp may just stay as is. For the sound I am after with it - this butters my biscuit just fine.

Thanks again. I can live with a $1 repair.

[norburybrook]

I have Chinese KT66's in my JTM45 and they're great..Shuangen or something like that, can't remember exactly. Really ,really nice. I've AB them against EL34's ,6L6's and the Chinese KT66's are just cracking tubes

they weren't that expensive either , about £40 IIRC some years ago.



M

[stelligan]

I have Chinese KT66's in my JTM45 and they're great..Shuangen or something like that, can't remember exactly.

M

Yes Marcus, that is what was currently in there. "Sino", "Valve Art", all Chinese from the Shuguang plant. Sound very good. Just been having some bad luck with them. I play this amp loud, though I guess.

Just ordered Slo Blo fuses for the HT - I have a fast in there now.....

[pdf64]

Provided you use a GZ34 but not standby, fast blow are fine for the HT fuse.
If an F500mA is blowing, there’s an actual issue.
Time to dust off your light bulb limiter.

[stelligan]

Provided you use a GZ34 but not standby, fast blow are fine for the HT fuse.
If an F500mA is blowing, there’s an actual issue.
Time to dust off your light bulb limiter.

I am thinking the 4 year old rectifier I had in before was bad & took it out. I will take to the tube tester later this week.

Like I said - I always thought something in the power section was amiss. Hope it's just substandard rectifier tubes.

[norburybrook]

Provided you use a GZ34 but not standby, fast blow are fine for the HT fuse.
If an F500mA is blowing, there’s an actual issue.
Time to dust off your light bulb limiter.

yes, I keep my standby permanently on and just use the main power switch. I remember reading about the poor design of the standby on these amps years ago so have always done that. I can't remember the actual technical reason though. Perhaps @pdf64 can fill me/us in on the reason


M