First amp build (Ceriatone Plexi All Access 51)

[pietro.castelli]

As you’re only supply the GZ34 anodes with 290VAC, then no.

Does it mean my power transformer is weak?
When I tested voltages on the rectifier anodes without any tubes installed I had 335.9v

Whenever undertaking a voltage survey off the LBL, it’s a good idea to include the mains and heater VAC measurements, at the wall outlet and between terminals 2 and 7 of the first output valve, eg V5 here.

Mains at the wall outlet are already noted in the chart: 227v (mains are wired to the transformer 230v primary).
I'll take measurements for the heaters and V5 p2-p7 tonight.

Thanks for the help as usual!

Cheers

Pietro

 

[Pete Farrington]

Mains at the wall outlet are already noted in the chart: 227v (mains are wired to the transformer 230v primary)

Oops, sorry

Does it mean my power transformer is weak?
When I tested voltages on the rectifier anodes without any tubes installed I had 335.9v

320VAC or more is more usual at the rectifier anodes, with the amp idling.
What info have TAD got about the PT?

 

[pietro.castelli]

320VAC or more is more usual at the rectifier anodes, with the amp idling.
What info have TAD got about the PT?

The transformers are from Ceriatone, such as the entire kit. Unfortunately, they don't provide any specs.
The only information I can gather from their voltage charts (for tube-rectified Plexi amps) is that the GZ34 anodes should normally be fed with 338-345 VAC

I can try emailing them and see if I can get additional insights

 

[Gene Ballzz]

@pietro.castelli & @Pete Farrington ,
I don't recall there being any discussion about voltages when switched to solid state rectification mode? Did I just miss it? Should we expect those voltages to be noticeably higher?
Simply Curious?
Gene

 

[Pete Farrington]

@pietro.castelli & @Pete Farrington ,
I don't recall there being any discussion about voltages when switched to solid state rectification mode? Did I just miss it? Should we expect those voltages to be noticeably higher?
Simply Curious?
Gene

With solid state HT rectification, I expect the rectified VDC to be 10-20V higher.
What's a bit strange here though is the VAC from the PT's HT secondary being somewhat lower than is often typical.

 

[pietro.castelli]

I've done some more investigation:

I think the problem lies in the fact that this amp utilizes a transformer meant for a SS rectified Plexi with 325-0-325 HT secondary.
The other Ceriatone models with GZ34 rectifiers (e.g. JTM45 and JTM50) rely on power transformers with 350-0-350 HT secondaries.

So I guess the slightly lower plate voltages on the power tubes are a compromise I will have to live with If I want to use the tube rectifier.

I've also noticed that if I let the amp warm up for a couple of minutes the EL34 plates tend to stabilize on 415V, which is probably good enough.
Heater voltages on Pin 2 and Pin 7 of both power tubes are at 2.99 VAC.

If I switch to the solid state rectifier instead, plate voltages bump up to 450V.

Cheers

Pietro

 

[pietro.castelli]

Schematics below for reference:

GZ34/SS Plexi 51 All Access (My amp)
SS Plexi 67 Lead 50W
GZ34 JTM50
GZ34 JTM45

 

[Pete Farrington]

Mains voltage fluctuates constantly, it’s best to monitor it, as all voltages in the amp will track it.

Going from your previous measurement of 227, your PT’s secondary voltages are almost 5% below the intended nominal.
It’s as if there’s too many turns on it’s primary, eg a 240V spec transformer.

It’s better to measure and use the HT voltage (ie at the rectifier output), rather than anode voltage. Probing an output valve anode can easily induce oscillation.
eg see https://www.tdpri.com/threads/blues...ltage-problem-1300-vdc-on-el84-plate.1107031/

 

[pietro.castelli]

Mains voltage fluctuates constantly, it’s best to monitor it, as all voltages in the amp will track it.

Going from your previous measurement of 227, your PT’s secondary voltages are almost 5% below the intended nominal.
It’s as if there’s too many turns on it’s primary, eg a 240V spec transformer.

It’s better to measure and use the HT voltage (ie at the rectifier output), rather than anode voltage. Probing an output valve anode can easily induce oscillation.
eg see https://www.tdpri.com/threads/blues...ltage-problem-1300-vdc-on-el84-plate.1107031/

You are right, this morning I was in a bit of a rush and I just took just a few measurements quickly.
I'll post an updated full voltage chart later on today.

Pietro

 

[pietro.castelli]

Hopefully, I've captured all the necessary voltages for a proper analysis now:





The power transformer has three primary windings: 120v, 230v and 240v. I've checked again and my mains are correctly wired to the 230v primary.

Cheers

Pietro

 

[dslman]

Beautiful workmanship, and soldering! I would never believe this was your first build. Thank you for sharing all the great pics!

 

[pietro.castelli]

Beautiful workmanship, and soldering! I would never believe this was your first build. Thank you for sharing all the great pics!

Thanks man! Much appreciated!

 

[Pete Farrington]

I've captured all the necessary voltages for a proper analysis now:

Good work!
Compared to what’s expected, the secondary voltages do seem to be about 5% low.
That’s obvious on the heaters, but though the HT winding VAC looks on target, bear in mind that the 325V applies at a heavier current draw, typically specified at 150mA.
The idle voltage will be higher than that, and the no load voltage even more.
eg see the Hammond equivalent https://www.hammfg.com/files/parts/pdf/290MAEX.pdf
It won’t cause operational problems as such, but it’s a bit annoying for it not to be right.

 

[pietro.castelli]

Good work!
Compared to what’s expected, the secondary voltages do seem to be about 5% low.
That’s obvious on the heaters, but though the HT winding VAC looks on target, bear in mind that the 325V applies at a heavier current draw, typically specified at 150mA.
The idle voltage will be higher than that, and the no load voltage even more.
eg see the Hammond equivalent https://www.hammfg.com/files/parts/pdf/290MAEX.pdf
It won’t cause operational problems as such, but it’s a bit annoying for it not to be right.

Thanks, I'll see how the amp sounds!

 

[pietro.castelli]

Well,
I've been on holiday for some days, and being back I'm eager to start testing the amp.
To play it safely I’ve decided to study the amp behaviour with a scope first.
This is my setup:



The signal generator injects a 200hz 200mV sine wave in the amp input:



With my d-lab dummy load connected to ch1 of the scope I can take a look at the output produced by the amp.
Clean first:



And then some nice distortion:



All controls seem to respond well on both channels and in jumper mode.
Master volume also does respond well.

 

[Gene Ballzz]

@pietro.castelli ,
You do some nice work there! And nice attention to detail.
Just Noticin'
Gene

 

[pietro.castelli]

@pietro.castelli ,
You do some nice work there! And nice attention to detail.
Just Noticin'
Gene

Thanks Gene!
You are a gentleman as always!

 

[pietro.castelli]

I'm so frustrated. I cannot play the amp because I'm still waiting for my headshell and cabinet.
I custom ordered them in May, and they were meant to be ready at the end of June.

Every time I ask for updates, the builder promises the cabinet will be ready "next week" and then disappears for a month or so.
At this point, I'm starting to think I've been scammed but I won't disclose the name of the builder, as I'm still hoping for a positive resolution.

With my rant out of the way, let's concentrate on something positive:

My life is seriously busy these days, but today I managed to find some time to test the amp with my Kemper Kabinet. It is using a 200W 4 OHM, FRFR speaker but it is still better than nothing.

Obviously, with a full range speaker, the amp sounds a bit "sterile", still, the voice is very promising, and I've been able to confirm all controls are working correctly.

I'll leave you with a picture of the amp and my Les Paul for a bit of "flexing"

 

[Gene Ballzz]

@pietro.castelli ,

Hey my friend, you've been MIA long enough that I was beginning to worry about you!

Here's a quick suggestion. Go out and find a fairly priced, suitable, used speaker cabinet. Buy it, own it and use until you get what you want sorted out. Then simply sell that cabinet for price similar to what you paid! Even if you take a small loss, that's a pretty cheap price for being able to nicely use your newly built amp. Heck, when you "flip" the cabinet, you even net a small profit! The real trick here is to buy used!

On another note: Do those templates I sent seem like they will be of any use to you?

Just Suggestin'
Gene

 

[Pete Farrington]

Good work!

And then some nice distortion:

There’s far too many cycles on screen to get good resolution. I suggest to adjust the scope controls to just display a few cycles. That should allow better assessment of the finer details.