new build -- suggestions/advice?

[jimipage]

OK, fellas, I'm trying my hand at a homebuild Express of sorts. I'll be basing it on the A4 schematic, except that I'm going for 6v6's in cathode-bias, and I'll be using a choke instead of the 1k/25W resistor, as well as a GZ34 in lieu of SS rectification.
Also using:
Hammond PT: 272FX, 300-0-300 150mA, 5v@3A, 6.3@5A
Deluxe OT, 6k6@8ohm
Choke, 9H/120mA

Are there any issues I should be aware of before I make this plunge? Does anyone else here have experience with using these substitutes? I know I've asked a couple of these questions before, but I'd sure appreciate any responses and help.
thanks!

[Shermock]

Hello!

A couple questions I would have would be: Have you made an Express before? Is this project an alternate "take" on something you've already done? If this is your first plunge into the Express world may I suggest you build a pretty straight-up version (Like the Kelly '90/Francesca schem) and then change it from there. You may be surprised at how cool it is!

Also... How much of a tweaker are you? Your plan will provide for many options, like SS vs. tube rectifier, etc...though you may find, like I have, that the original intent behind certain component inclusions was the right one.

For instance, I installed a choke as it was the better choice between it and the oddball 1K 25W resistor I had. I have since purchased a "proper" resistor, and I like what it does better than the choke, so out went the choke.

I toyed with the idea of tube rectification, but the SS rectifier has so much to do with the sound of an Express that I didn't want to alter that part of the mojo. The corresponding stiff power supply has a lot to do with the feel of the amp, and running a tube rectifier will necessitate less initial filtering that will "soften" the amp's feel.

So what I'm getting at is this. How much of a "real" Express do you want your Express to be? Deviations as you're planning will constitute all the ingredients of a great amp, but it won't necessarily be an Express.

Actually, I had all the same intentions you are proposing. My other 2 amps are tube-rectified, "choked", low-gain amps that have an amazing feel and are just an absolute joy to play. I figured I would install the same kinds of things in my Express build. Once I started the work I realized I didn't know anything about what an Express really is! So I shelved those ideas in favor of staying true to a Francesca-style build. I couldn't believe how much I loved it! It's by far my favorite amp now.

This may not truly answer your question about pitfalls to look out for, but I thought I'd throw in my $.02. HTH!




Shermock

[Allynmey]

Shermock, I agree. If you want an express, build it like an express. If on the otherhand, you want something different, experiment. The Express as well as all of KF's amp are built on the edge of stability. If you follow the layouts and parts closely you have a "wreck' sounding amp otherwise, the amp will probably sound good still but not the same.

[tubedogsmith]

The 200 series Hammond PT's are gonna give you over 7v's on your filaments unless they've changed them in the past couple years. I've believe they're built for 117vAC.

[jimipage]

thanks for the replies, guys
There's a couple of reasons I chose to build the amp earlier described:
I've never implemented fixed bias before, so I have to admit that I feel a little gunshy about it. As far as tube rectification goes, that's just a personal preference. Plus, I think using SS with the PT I have would make my voltages too high. Chokes I have, the 25w resistor I don't. Just a matter of what's on hand for me.
BTW, all I've ever used is the Hammond 200 series PT's and I've never had a problem with filament voltages, except for whatever was my own doing.
You guys have given me lots to consider. If anyone else feels like contributing about this, I'm all "ears"

[Shermock]

Just to let you know, I'm running the same PT w/SS rectification and my voltages are right in the ballpark. Right around 390-400 on the plates.

Have fun with this build! There is soooo much to learn from doing an amp like this, it's amazing!

Shermock

[jimipage]

thanks again for the reply, Shermock. Glad to know someone else has used that same PT with success. I spent most of the afternoon studying the schem's and re-planning my layout. Looks like I'm gonna go ahead and take the plunge building the "real" thing. Tomorrow is chassis cuttin' time!

[joe6v6]

I've never implemented fixed bias before, so I have to admit that I feel a little gunshy about it

I felt the same way for a long time but now that ive built several fixed bias amps I see that its a natural evolution of amp building to go from the simple to the more complex and it has given me a better understanding of how amps work in genral and how power tube bias works and affects the amps charicter. . There is a good explination of what a choke will do in this circuit in this thread - https://tubeamparchive.com/viewtopic.php?t=1778 . . My first express used all parts that I had on hand and a very non traditional layout & it turned out very good , ,Depending on your wall voltage since the 272fx is wound for 115v if your wall voltage is 122v you will wind up with 318-0-318 but with less current 150ma vs 300ma on the tone slut trannie the voltage will drop more under load.

JOE

[jimipage]

good stuff, joe -- thanks for the info and reassurance

[jimipage]

OK, fellas, I just finished prepping the chassis. I know I've got a busy day for myself tomorrow mounting components, filament wiring, etc., but I already have questions regarding bias, so here goes:
1) Looking at version A1 I see that the bias pot is rated for 20k. All I have is a 10k bias pot. Are there any component value changes I can/should make to make biasing easier, or does it not matter that I have a 10k pot?
2) I've been using the Ceriatone Express layout as a rough guide(though my layout is somewhat different) and I've noticed that the pot wiring in the layout is different than the A1 schematic. Any reason for that?
3) Is the bias pot as shown in the A1 schem wired like a typical pot? Meaning, looking at the pot from behind with the tabs facing up, ground is the left tab, wiper is the middle? I ask because, again, the Ceriatone layout is confusing me.
As you can see, I'm still a little nervous about trying fixed-bias, but I'm still gonna go for it. I just really want to do it right the first time.
Thanks for the help!

[mdroberts1243]

OK, fellas, I just finished prepping the chassis. I know I've got a busy day for myself tomorrow mounting components, filament wiring, etc., but I already have questions regarding bias, so here goes:
1) Looking at version A1 I see that the bias pot is rated for 20k. All I have is a 10k bias pot. Are there any component value changes I can/should make to make biasing easier, or does it not matter that I have a 10k pot?
2) I've been using the Ceriatone Express layout as a rough guide(though my layout is somewhat different) and I've noticed that the pot wiring in the layout is different than the A1 schematic. Any reason for that?
3) Is the bias pot as shown in the A1 schem wired like a typical pot? Meaning, looking at the pot from behind with the tabs facing up, ground is the left tab, wiper is the middle? I ask because, again, the Ceriatone layout is confusing me.
As you can see, I'm still a little nervous about trying fixed-bias, but I'm still gonna go for it. I just really want to do it right the first time.
Thanks for the help!

I just finished building a Ceriatone Express... I think the size of the pot will be important as the range of the bias adjustment is already a bit narrow... I've seen other posts recommending a larger than 20k pot and adjustments to the other values in the bias circuit to compensate.

In looking at the Francesca pics I noted that the bias pot was set up as a variable resistor with the wiper connected to one end of the pot. The Ceriatone layout leaves one end of the pot disconnected... this worried me as I couldn't tell what would happen if the wiper contact became intermittent or entirely open... so I connected the open end of the pot to the wiper.

I think other differences between the layout and schematic are minor and in general the Ceriatone layout has been updated to be very close to the Francesca.

-mark.

[jimipage]

hi, fellas. I had to take a break from building my Express for a bit, but I'll be back in action tomorrow. I was looking over some of the wiring I've done and I have a couple of questions:
1) as you know, I'm using 6v6's for this build and I've decided to go with fixed bias. so,... is it OK/safe to connect pin#8(cathode) of each 6v6 to each other and then run it to ground? or should they go to ground separately?
2) my PT has a 6.3v center-tap: should I leave it going directly to ground, or disconnect it and elevate the heaters with a pair of 100ohm resistors?

thanks!

[paulster]

1) as you know, I'm using 6v6's for this build and I've decided to go with fixed bias. so,... is it OK/safe to connect pin#8(cathode) of each 6v6 to each other and then run it to ground? or should they go to ground separately?

Why not just ground each one using a tag to one of the respective tube socket bolts? But, yes, it should be fine so long as you're not running the connection miles to a ground point and risking some oscillation in the power stage.

2) my PT has a 6.3v center-tap: should I leave it going directly to ground, or disconnect it and elevate the heaters with a pair of 100ohm resistors?

The 100R resistors are a bit of a kludge for transformers that don't have a centre tap on the filament winding. You can connect the centre tap direct to ground or, if you want to elevate your heaters, to wherever you're going to get your elevated reference from (although this is usually the power tube cathodes in cathode-biased amps).