2204 build - do these PT voltages look ok?

[GregSt]

Hi, I’m about midway through a 50w 2204 build, I haven’t installed the board yet. I’ve just put in the PT (APD Heyboer) - it’s only wired up to the HT fuse, all heater wiring is complete to tubes sockets in 18G wire.

I’ve used the 240v tap on the primary of PT (it also has a 220v tap), mains voltage of 236v.

With tubes out, I get 6.33v at heater terminals, however with all tubes installed (there’s nothing else connected to tube sockets yet) this drops to 6.10v. I’m also only getting 297VDC from the HT fuse to ground, shouldn’t I be expecting 420-450v? The PT secondary is measuring 646vac, I get 590VDC between the rectifier +ve and -ve (which isn’t used).

Does this sound right? Thanks.

 

[Lowerleftcoast]

Are these voltages with or without a light bulb limiter in the line?

 

[Ten Over]

Do you have any filter capacitors connected yet?

 

[Ten Over]

I get 590VDC between the rectifier +ve and -ve (which isn’t used).

I don't understand this. Can you explain this a little more?

 

[Phrygian77]

It sounds like you are measuring unfiltered rectified DC.

 

[GregSt]

This is without the light bulb limiter (i originally had 5.8v with the limiter in circuit)

Yes, this would be unfiltered rectified voltage.

The -ve side of the rectifier isn’t connected to anything according to the layouts I have.

 

[Lowerleftcoast]

I would like to see a schematic of what you are working with (and maybe pictures).

I am unsure of what you are doing with the bridge rectifier. (Only using 2 diodes?)

Without the filter caps, voltages won't seem right.

 

[Ten Over]

This is without the light bulb limiter (i originally had 5.8v with the limiter in circuit)

Yes, this would be unfiltered rectified voltage.

The -ve side of the rectifier isn’t connected to anything according to the layouts I have.

Ah. You're only using half of the bridge rectifier. That makes sense now.

 

[King Fan]

As folks are saying, this may be a case of TLI: too little information. Do you know how to post a JPG or PNG file of the schematic? Maybe a PT schematic, too, or at least which APD it is? Maybe an amp layout? And some build pics?

If you're not sure, try the little 'picture' icon above. You can either click the box to browse your computer for images, or drag 'n' drop an image file to the box...

 

[Ten Over]

This is my guess for the PT:


It doesn't have a bias tap or a bias winding, so the white wire in the layout is undoubtedly going to the bias circuit.

 

[GregSt]

This is the PT and layout. Pretty confident once I connect the filter caps and board, the B+ will come right, it’s only powered up to the standby switch at the moment.

 

[Lowerleftcoast]

Once you have everything else hooked up, the B+ will likely be where you want it... in the low 400s. Unlikely for it to be 450v.

Have you planned out the ground scheme yet? (I hope you are not soldering to the back of the pots.)

 

[Paul G.]

 

[GregSt]

Once you have everything else hooked up, the B+ will likely be where you want it... in the low 400s. Unlikely for it to be 450v.

Have you planned out the ground scheme yet? (I hope you are not soldering to the back of the pots.)

I’m doing the Larry grounding scheme for this amp (no soldering wires on back of pots!).

 

[GregSt]

View attachment 1036969

Thanks for that, once everything is all connected and my B+ ends up below 400v, say 380v, should I try using the 220v PT primary tap to try and bring the B+ into the 400-450vdv range?

 

[GregSt]

View attachment 1036969

Thanks for that, once everything is all connected and my B+ below 400v, say 380v, should I try using the 220v PT primary tap to try and bring the B+ into the 400-450vdv range?

 

[Paul G.]

Stop stressing about things that haven't happened yet. Connect everything up properly, then see where you're at.

 

[GregSt]

Stop stressing about things that haven't happened yet. Connect everything up properly, then see where you're at.

Not stressing, just checking if everything is all good as I go (it’s been a while since my last two builds).

Update: I’ve connected the mains dual 50uF and I’m now getting 444VDC @ 236VAC mains and 450VDC with the variac at 240V. I’d expect this to drop a bit once everything is connected and loaded. Next step is installing the board.

 

[Lowerleftcoast]

should I try using the 220v PT primary tap to try and bring the B+ into the 400-450vdv range?

Imo, one would want to keep a closer eye on the heater voltage.

I agree with @Paul G. ... wait and see.

 

[dan40]

The 2204's from the late 70's ran a plate voltage of 370-380vdc. It wasn't until the JCM 800 line was introduced that Marshall bumped the plate voltage back up above 400vdc by using a different PT. Some folks love the sound of the early, low plate voltage 2204's while others prefer the higher voltage circuit in the JCM 800 amps. Higher plate voltages will give you a bit more headroom in the power amp and a tighter, punchier feel to the amp.

Thanks for that, once everything is all connected and my B+ below 400v, say 380v, should I try using the 220v PT primary tap to try and bring the B+ into the 400-450vdv range?

As Lowerleftcoast mentioned, you really need to see where your heater voltages land at once the amp is up and running. Whichever PT primary gets you the closest to 6.3vac on the heaters is the one you need to use.