First amp build (Ceriatone Plexi All Access 51)

pietro.castelli

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I’ve finally wired the effects loop and this officially marks the build as finished:

79199433-71CB-4CF2-BE83-A4A80F7AA690.jpeg

12DC19E8-A4B5-4E7F-AC7C-FE502D5C677C.jpeg

It is now to start thinking about the startup, and that means getting equipped with some proper safety/testing equipment:

5EE8C224-346C-4400-ADD6-D650216DFAEC.jpeg

I’ve purchased a used but good quality italian-made variac, that I modified slightly.
I’ve also built a light bulb current limiter and a dummy load (using the delay dummy head layout).

I’m now validating the entire layout arganist the schematic and it will soon be time to power it up!

Hope it doesn’t blow up 😅
 

pietro.castelli

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Today I've finished the sanity check of the layout. I managed to find three errors which have all been corrected already:

- V1 cathode bypass cap with the wrong orientation (thanks so much @Marcomel79 for spotting it from the pictures!)
- One diode with wrong orientation in the bias circuit
- Wrong effects loop out wiring to output jacks

Confident enough I've decided to attempt the first power-up, under variac and current limiter but no tubes: nothing blew up yet! :hbang:

With 231v on the primary winding of the power transformer I measured the following voltages:

- Rectifier tube heater: 5.56V
- Pre-amp and power-amp tube heaters: 6.47v
- Rectifier input to ground: 335.9v

The pilot lamp glows correctly!

Obviously, with no tubes, all the voltages will be higher than normal, but so far the amp seems healthy enough.
I'll move forward with plugging the rectifier tube and measuring voltages at the filter caps!

Cheers!
 
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mAx___

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Happy to hear that!
I was rushing through the thread trying to see if you could fix those mistakes before turning on the amp :D
Complimenti!
 

pietro.castelli

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Happy to hear that!
I was rushing through the thread trying to see if you could fix those mistakes before turning on the amp :D
Complimenti!

Thanks!
There might be more errors for you to find though :) ... It is still a long way before it makes pleasing sounds :D
 

pietro.castelli

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Today I plugged in the rectifier tube and proceeded measuring more voltages.
This was the test setup:

3C01911A-BDF2-479E-A275-EEB614CF0B63.jpeg

The light bulb limiter didn't glow and once again nothing blew up: GOOD 😀
I was able to get an healthy output at pin 8 of the rectifier tube, so I proceeded measuring voltages at the filter caps:

-1st stage: 457.5v
-2nd stage: 457.6v
-3rd stage: 434.1v
4th and 5th stage: 431.3v

Everything seems within reason, so I think I can proceed plugging in the preamp tubes.

Cheers!
 

pietro.castelli

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I installed the preamp tube and proceeded to measure DC voltages at each pin:

1661017773269.png

They all seems to be within reason, except for the 25v on V3 pin 2 and pin 7 which I don't think is normal (should read 0 according to ceriatone voltage chart).

I will have to dig deeper into that.

Cheers
 
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Dblgun

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What does p7 of V3 read if you pull the tube? I'm assuming the readings at p2 are 0?
 

pietro.castelli

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What does p7 of V3 read if you pull the tube? I'm assuming the readings at p2 are 0?

With the tube installed, p2 reads the same as p7 (25v roughly). If I pull the tube they both stay at 0v
I've updated the voltage chart in the previous post.

Thanks for the help!
 

Dblgun

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It looks as though the amp has an effects loop with a switch. If you toggle the switch with tube installed does it have an effect on the p2/p7 readings?
 

pietro.castelli

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It looks as though the amp has an effects loop with a switch. If you toggle the switch with tube installed does it have an effect on the p2/p7 readings?

I realized the effects loop was on while I was taking the previous measurements (must have bumped into the switch).
I tried to turn it off and measured again but it had no noticeable impact on p2/p7 voltages.
 

Pete Farrington

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They all seems to be within reason, except for the 25.5v on V3 pin 2 and pin 7 which I don't think is normal (should read 0 according to ceriatone voltage chart).
V3 is the LTP phase splitter?
If so, yes, your V3 voltages are reasonable.
Dunno why Ceriatone suggest differently.

I expect the V2 DCCF pin7 and 8 voltages to be a lot closer to each other though. To have the grid with a forward bias of 50V to its cathode is wonky.

It would be helpful to link to the documentation you’re referring to.
 
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pietro.castelli

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V3 is the LTP phase splitter?
If so, yes, your voltages are reasonable.
Dunno why Ceriatone suggest differently. It would be helpful to link to the documentation you’re referring to.

Thanks for your help!

I'm referring to the charts below. Ceriatone doesn't offer a voltage chart for the Plexi 51 all acess, so I referenced voltages for both their JTM50 and Plexi 50 models:

1661014752284.png

1661014813307.png

This is the layout of my circuit: Plexi 51 All Access

Unfortunately, Ceriatone doesn't provide a schematic, so that's one of the reasons I'm struggling.

Generally, all the voltage charts I've found on the internet for plexi style circuits, never show voltage on p2/p7 of the v3 tube,
so that's where my concern comes from.
 

pietro.castelli

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I expect the V2 DCCF pin7 and 8 voltages to be a lot closer to each other though. To have the grid with a forward bias of 50V to its cathode is wonky.

I managed to find the problem with V2, the lead coming to p3 broke off the solder lug.
Now I repaired it and voltages on pin7 and 8 are much closer:

1661017757042.png

I still have those weird 25volts on V3 pin2 and pin 7 though.

Cheers

Pietro
 

Pete Farrington

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I still have those weird 25 volts on V3 pin2 and pin 7 though.
Not weird, totally normal.
But it’s not beneficial to try to measure those voltages, because the measurement error is enormous. The real voltage on the LTP grids will be a volt or 2 below the cathodes, ie the voltage at the node of the 1M grid leaks, the 470ohm cathode bias resistor, and the 10k upper tail resistor.
If the anode and cathode voltages are ok, then the grid voltages must be too.
Hence they’re left blank, ie don’t bother attempting to measure them.
 
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pietro.castelli

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Not weird, totally normal.
But it’s not beneficial to try to measure those voltages, because the measurement error is enormous. The real voltage on the LTP grids will be a volt or 2 below the cathodes, ie the voltage at the node of the 1M grid leaks, the 470ohm cathode bias resistor, and the 10k upper tail resistor.
If the anode and cathode voltages are ok, then the grid voltages must be too.
Hence they’re left blank, ie don’t bother attempting to measure them.

Good to know! Thanks for the info!
 
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pietro.castelli

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Today I installed the power tubes and checked for voltages rapidly.
I've then removed the light bulb current limiter, proceeded biasing the tubes at 35mV and finally re-measured all voltages:

1661091449477.png

Should I expect higher voltages on the EL34 plates?

Cheers
 
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Pete Farrington

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

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