Starting up a probable dormant vintage amp

[Chuck H]

That sounds really nice. Not just amazingly "period correct", but really very nice. Smooth but still crunchy. Just as it should be

[Leo_Gnardo]

I would use the PT tap that gives a heater voltage close to 6.3V

There's some excellent advice ^^^ . The amp doesn't so much care about what the line voltage is. BUT it does care what the internal voltages are. Measuring the filament voltage is a safe and convenient way to check whether voltages are on, or close to the mark.

[Gtr0]

I would use the PT tap that gives a heater voltage close to 6.3V

Can I smack myself for not thinking of that??????????

That sounds really nice. Not just amazingly "period correct", but really very nice. Smooth but still crunchy. Just as it should be

Thank you, sir. I am quite pleased with it. Up to today, my favorite amp I own is a JTM45 RI - I upgraded caps with Sozo etc - this Laney dwarfs it in tone - wonder if it's the old parts or tubes... likely both. An original JTM45 must be insane sounding.

[Gtr0]

At the moment I am getting between 3.06VAC and 3.1VAC on each heater pin when plugged into the wall and power trans is set at 250V! Set at 230V, the pins are 3.3VAC at minimum. Thoughts? I assume 6 to 6.8V is safe area - sound about right?

[Leo_Gnardo]

At the moment I am getting between 3.06VAC and 3.1VAC on each heater pin when plugged into the wall and power trans is set at 250V! Set at 230V, the pins are 3.3VAC at minimum. Thoughts? I assume 6 to 6.8V is safe area - sound about right?

I'd park it right there without another thought. 6.3V is the spec, you're at 6.16V.

Fact is, lots of amps run a bit high. Especially the older ones here in USA where line voltage used to average 115-117V. Now we often see 125V or so. Much to the detriment of the lifetime of output tubes.

Can I smack myself for not thinking of that??????????

I can tell you, I worked on tube amps for decades before this notion struck me one day. It was a wake-up moment for sure. No need to smack yourself. Now you know something of considerable importance. Anybody who has an amp with multiple line voltage taps would benefit from this information.

[eschertron]

...sound clip...

Thanks for the clip! Really iconic crunch!

First, the grounding scheme of this amp seems to be floating and while it is connected to earth via the bulgin power jack, there seems to be no chassis connection. Should I worry about this?

I'm not sure if I understand this? First, there should be a 3-wire grounded cable to the amp. the ground wire must go from the line cord to the chassis and be bolted on. If that hasn't been done, please do it before anything else. This protects the user against HV on the chassis, assuming the plug and service are correctly configured.

A connection between the chassis and circuit ground can be done a number of ways, usually to minimize hum and interference. If you want to move the chassis connection from the input jack to the reservoir cap ground point, that's up to you. I'm sure you can wrangle up plenty of discussion about what is "best".

[Helmholtz]

Using heater voltage as a reference is what I always do. You almost never see a dedicated voltage tube amp over here.

Typically tube can be safely operated at 6.3V +/-10%. But some datasheets mention that max. lifetime/reliability is achieved within +/- 5%.
You may notice that heater voltage at the power tubes is slightly higher than at the preamp tubes caused by wiring losses.
Also consider that your mains voltage may easily vary by +/- 5% in the course of the day.

Of course also B+ will change by around 8% between the 250V and 230V settings.

[Chuck H]

You may notice that heater voltage at the power tubes is slightly higher than at the preamp tubes caused by wiring losses.

Yes. Often overlooked and a good consideration here At lower voltages and higher current the resistance of wire becomes significant. I remember being a little surprised the first time I saw this in an amp. Inspiring me to do research and learn something

@ Gtr0 FWIW the filament voltage thing has been discussed here before. Assuming staying within spec, IIRC a little low is better than a little high for tube life. But tonal differences were also noted specific to filament voltage and not changes in B+. Like Helmholtz noted, there would also be an increase in B+ going to the 230V primary. Since both the 250V and 230V primaries are keeping the filaments in spec you could listen and choose. Or just go with the 250V primary for the potential tube life benefit.

[Enzo]

And a little nitpick, what matters is that you have 6.3v across the heater. 3v to ground from either side might be about the same, but if you get into that habit, what happens when you find elevated heaters? For heaters, measure pin to pin, not each pin to ground.

[Chuck H]

And a little nitpick, what matters is that you have 6.3v across the heater. 3v to ground from either side might be about the same, but if you get into that habit, what happens when you find elevated heaters? For heaters, measure pin to pin, not each pin to ground.

And also, this ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[g1]

the grounding scheme of this amp seems to be floating and while it is connected to earth via the bulgin power jack, there seems to be no chassis connection. Should I worry about this? Perhaps for this reason, there was not any shielding in the head cab either. But it's as quiet as can be I think - I'm more thinking of safety. IF for some unforeseeable reason a B+ wire jigged loose and contacted the chassis - I assume then the chassis would become "live" - is that correct? All of the amps grounds stem from the pot ground bus bare wire.

The bus wire connects to the pot casings, the casings are all bolted up to the chassis. So that is how the grounds connect to chassis. The Bulgin AC connector, I'm not positive about. Does the 3rd pin connect directly to chassis? Or does the green wire go to the bus wire which is then connected to chassis by the pots?

https://music-electronics-forum.com/...3&d=1586982570

[eschertron]

Does the 3rd pin connect directly to chassis? Or does the green wire go to the bus wire which is then connected to chassis by the pots?

If the PE wire doesn't go straight to chassis via a reliable connection (such as a dedicated bolt) it really aughta. To meet modern code and make the shortest, highest-current-carrying path to ground from a live chassis. I don't want to bet my life on a bunch of fiddly bits between deadly voltages and protective earth. Don't bet yours, or customers, either.

[g1]

I don't disagree with that at all. Just pointing out how the chassis is not 'floating'. Modern standards are much more stringent. But this thing was designed with chassis 'earthed', wasn't it? Like a Fender that has the 3rd prong ground going to a transformer bolt?
Not really acceptable, but definitely not 'floating'.

[Chuck H]

I don't disagree with that at all. Just pointing out how the chassis is not 'floating'. Modern standards are much more stringent. But this thing was designed with chassis 'earthed', wasn't it? Like a Fender that has the 3rd prong ground going to a transformer bolt?
Not really acceptable, but definitely not 'floating'.

When you consider that "earthing" a chassis was a secondary consideration to the electronics operation just a year or two earlier I'd say these guys were ahead of the curve. That doesn't mean it can't be idealized though.

[The Dude]

......Here's a short mp3 clip...
[ATTACH]58153[/ATTACH]......

That's a great sounding amp. Thanks for the upload!