PT for a 50W Plexi Style Amp (Ceriatone AFD #35)

[mhartman]

I'm looking for a PT for my Ceriatone AFD #35 kit. It requires a PT with 650v (325-CT-325), 6.3v and 5v taps and it mounts with 3.75"x3" studs.

Right now, the best that I've found are:

http://www.classictone.net/40-18073.pdf, which has the correct voltages, but has bolts that are a bit small (I would have to do some mod work on the chassis and a bracket of some kind)

OR

http://www.classictone.net/40-18054.pdf, which has the correct bolts, but doesn't match the recommended B+ voltages -- instead of 650v, it has 690v or 625v. With this one, I could try the stock B+ choices to see how they sound or I could lower the 690v tap with zeners to 650v (or where ever sound good).

Either way is a compromise. I'm torn. Which would you use? Or, is there another PT that you know of that you would use?

Thanks in advance,
Mike

BTW, here's the layout: http://ceriatone.com/images/layoutPic/ ... 5_50W.jpg .

[statorvane]

I'd go with the one with the correct voltages and do the required work on the chassis. It's not like you are trying to restore a vintage Marshall.

Figure out where you want the transformer, and mark the new holes for the bolts. If they are close to the old holes, maybe make a slot from each the new hole to the old one.

[martin manning]

If it were me, I'd use the Marshall replacement that fits the chassis. This circuit is supposed to be based on a Plexi, so why not use a Plexi replacement-type PT? On the voltages 690 vs. 650 is only +6%, and 650 to 625 is only -4%, so it's really not a big deal either way.

[rdjones]

If it were me, I'd use the Marshall replacement that fits the chassis. This circuit is supposed to be based on a Plexi, so why not use a Plexi replacement-type PT? On the voltages 690 vs. 650 is only +6%, and 650 to 625 is only -4%, so it's really not a big deal either way.

625VAC is fine and you might encounter slightly high supply voltages by a couple percent anyway.
A 690/625 switch might be a cool mod and interact nicely with the pentode/triode choice.

Also please note the safer way to wire a fuse holder.
You should also consider grounding pin 1 on the output tube sockets to accommodate EL34s.

RedDog Steve

[mhartman]

Thanks guys. I really appreciate the input!

Mike

[mhartman]

You should also consider grounding pin 1 on the output tube sockets to accommodate EL34s.

Just in case anyone is reading this, I wanted to mention that Nik e-mailed me and cautioned that:

Some 6550s have pin 1 connected to the base (ground). If this is the case, you will not have proper bias reading if you ground it, ie the 1R will be rendered useless, for the bias reading.

Regards,
Mike

[rdjones]

You should also consider grounding pin 1 on the output tube sockets to accommodate EL34s.

Just in case anyone is reading this, I wanted to mention that Nik e-mailed me and cautioned that:

Some 6550s have pin 1 connected to the base (ground). If this is the case, you will not have proper bias reading if you ground it, ie the 1R will be rendered useless, for the bias reading.

Cathode is on pin 8, pin 8 goes through the sensing resistor to ground.
Pin 1 by itself should go directly to ground.
In the Ceria drawing pin 1 is left floating.

6550s are intentionally designed this way to follow (now ancient) convention of having octal socket's pin 1 as a ground.
Fender came along and screwed this up by using pin 1 as a tie point.

You can have the suppressor grids (pin 1) of an El34 at ground potential and the cathode (pin 8) above as in cathode bias.
This will slightly increase the effectiveness of the beam forming suppressors.
You can even put negative bias on the suppressors to further increase the effect.

rd

[martin manning]

You should also consider grounding pin 1 on the output tube sockets to accommodate EL34s.

Just in case anyone is reading this, I wanted to mention that Nik e-mailed me and cautioned that:

Some 6550s have pin 1 connected to the base (ground). If this is the case, you will not have proper bias reading if you ground it, ie the 1R will be rendered useless, for the bias reading.

Why is it a problem to have the base shell connected to pin 1, and pin 8 and pin 1 going through the current sense resistor to ground? That won't ground pin 8 (bypass the current sense resistor) unless the base shell is grounded by some other path.

You can have the suppressor grids (pin 1) of an El34 at ground potential and the cathode (pin above as in cathode bias.
This will slightly increase the effectiveness of the beam forming suppressors.
You can even put negative bias on the suppressors to further increase the effect.

Good stuff rd, thanks for posting!

[jonnyfunk]

Which PT, OT and choke did you end up choosing? I just ordered a kit 1 AFD #35 as well! I'm excited. I like that it's based on the original Tim Caswell modded 1959 Tremelo with a mod to the tremelo for a hi gain channel.

[Structo]

Don't some 6550 tubes have a metal base?

So it would matter how you wire 1 & 8.

[mhartman]

Which PT, OT and choke did you end up choosing? I just ordered a kit 1 AFD #35 as well! I'm excited. I like that it's based on the original Tim Caswell modded 1959 Tremelo with a mod to the tremelo for a hi gain channel.

I went with the 40-18054 for the PT. The voltage on the lower secondary looks to be very close. I already had a 50w Heyboer that I'm going to go with. I can't wait to find time to finish it up.

Mike

[mhartman]

Don't some 6550 tubes have a metal base?

So it would matter how you wire 1 & 8.

I believe that this is the case and is the reason that it is wired the way that it is.

[rdjones]

Don't some 6550 tubes have a metal base?

So it would matter how you wire 1 & 8.

I believe that this is the case and is the reason that it is wired the way that it is.

Yep, I have a pair of KT88s here that also have the metal base connected to pin 1.
It also appears to be connected internally to one of the support rods.
The markings are rubbed off but I suspect they are Mullards.

In either case (6550,EL34) the Marshall the circuit is based on has pin 1 grounded.

reddog