Is img turned off?

[skyboltone]

[IMG1062]http://i25.tinypic.com/nl6ic4.jpg[/img]

NO

[bcmatt]

Whoa. Whoa! WHOA!!!! Hold on here.

Where the heck did this schematic come from?!?! I guess it's not at all a wreck style preamp, but it does very appropriate for me and my plans to convert my vibrochamp clone into something gainier using both 12ax7s. It's kind of neat- 2 inputs; the high with 4 gain stages and the low input skipping the first gain stage. I'm not familiar with very many schematics, so should I recognize this one from somewhere?
Now I'm rethinking my project to try this. Is it sort of like a an SE Plexi?

[skyboltone]

This was an Allen Amplification kit that is no longer offered. Brandon, (funkelishiousgroove) has from time to time recommended it as an option for folks looking for something with a lot of versitility. He uses one for his main gig amp in spite of being one of the brighter lights in the D clone world. PM him directly for more info.

I've got all the iron and stuff ready for one for myself one of these days.

And by the way, two "normal" gain stages (cathodes bypassed) followed by two stages with local NFB caused by the unbypassed cathode resistors, ("clipper" stages) is not that far off a wreck. The weirdness comes with the tone stack so late in the game. When I get around to building mine it'll have a 6550 at near 500 volts. I may use UR-12s power controll circuit for other tubes. Should be a do it all club amp.

Dan

[Stanz]

What trannys are they using?

[bcmatt]

What trannys are they using?

Good question.

Would my Ceriatone BF Vibrochamp Trannies work for this?

[dartanion]

Allen has his own transformers wound to his specs by Heyboer. I used this set in my Bastard Champ and they are great. I think he may have gone up a bit with the OT though as the OT I have from him is rated at 11watts. You'd be pushing that one really hard with a KT88.

It looks wreck like to me save for moving the tonestack to right before the power tube. This is the "plexi" tonestack position, whereas a wreck puts the tonestack directly after the first gain stage. The drawback of that is the small input signal is amplified, then sent through a high loss tonestack, then recovered in the second gain stage. Moving it to right before the power tube helps to reduce insertion loss. A plexi furthers this reduction of loss with CF directly before the tonestack.

Transformer Part Numbers:

TP25 Power
TO11C Output

[skyboltone]

I'm intending to use a 4.4K/8ohm leslie OT. It's about 25watt size. For PT I'm going to use a 375-0-375 140ma Hammond Organ AO-29 PT. I can change rectifier tubes if the Plate voltage gets too high for smaller tubes. Carbon comps all over the place and maybe even a choke. Altec 418B combo maybe. Should be a blues machine.

[dartanion]

I'm intending to use a 4.4K/8ohm leslie OT. It's about 25watt size. For PT I'm going to use a 375-0-375 140ma Hammond Organ AO-29 PT. I can change rectifier tubes if the Plate voltage gets too high for smaller tubes. Carbon comps all over the place and maybe even a choke. Altec 418B combo maybe. Should be a blues machine.

KT88? 6550?

[skyboltone]

I'm intending to use a 4.4K/8ohm leslie OT. It's about 25watt size. For PT I'm going to use a 375-0-375 140ma Hammond Organ AO-29 PT. I can change rectifier tubes if the Plate voltage gets too high for smaller tubes. Carbon comps all over the place and maybe even a choke. Altec 418B combo maybe. Should be a blues machine.

KT88? 6550?

Yup. should be fine at 500V and 4.4K. I think. Oh, and at the 220K/22K voltage divider to raise the heaters above ground, put a little 22uf/100V cap there.

[bcmatt]

Ok, so this is me goofing off today, while I intended to be getting something else done (working on a father's day sermon).

Anyways, I completed my first ever attempt at making a layout from a schematic. (done quickly and messily in a simple paint program with lots of pasting to avoid some free-hand drawing).

I started from my Ceriatone VibroChamp layout and kept changing it.

I'm a complete noob, so bare with me. (sory about the real mess of resisters around the filter caps.
I'm wondering if the 320-0-320 PT would actually even work for this? Do you know what the original PT would have been?

As you can see, I got confused at the voltage divider and am wondering about that 22uF cap and exactly where you would put it between. Do you mean that you would replace those resisters?

BTW, can you avoid the standby switch if intending to always keep in the 5Y3?

[skyboltone]

Looking Good Mister:
Loose those 100ohm resistors between the heater pins and the cathode of the power tube. One frequently sees 100ohm balance resistors between heater leads and GROUND but never that way. Besides, you're taking care of things over at that voltage divider with the green question mark.

First, at the junction of the 220k and the 22k put the + lead of a 22uf/100vdc electrolytic cap. Run the - of the cap to the main PS ground point. This will decouple the AC hum from the rest of the power supply. Next, run a wire from that junction to the wiper (center lug) of a 100 ohm Fender hum balance pot. (tubesandmore.com) Run the two heater leads from the transformer to the two outside legs and then on to the tubes and pilot light.

Next, I don't like mixing the pot ground with the main PS ground. Run all the negative leads from your filter caps to one chassis ground point, then suspend a piece of #14 copper wire from the lugs on the pots with short pieces of wire where at each grounded lug. Tie the input ground to that buss and tie it to the chassis at that point and nowhere else. The way you have drawn the layout your tubes hang down, is that what you intend?

I'm thinking with a 5Y3 you'll see somewhere just under 400volts. A 5AR4 will give you a few more. I think it will make a great amp. If I was doing it I would put a 100K trimmer pot in place of the 56K resistor at the entrance to the tone stack just for grins. Start out with it set around 56K and experiment. This is called the slope resistor and has a huge influence on where the "center" of the tone stack works. Also on gain.

Put in a standby switch. You really don't want the amp talking CB trash during your sermon.

Also, I don't know why Allen put the 68K grid stopper on V1A. Make it a 34k the way he's got the hi/lo inputs wired.

Dan

[Sonny ReVerb]

The PT doesn't show a center tap for the heaters, so you could use the two 100 ohm resistors to create a virtual CT. Then you would tie them to the voltage divider OR the cathode as depicted (a la the p1eXtreme at AX84). Either method gives you an elevated 'ground'. The cathode method saves you two resistors. Leo would be proud

[bcmatt]

Hey Dan,
Thanks heaps!
I've been putting into play all of you suggestions, but I'm afraid that I am making my layout into a bigger and bigger mess. Have I sort of been getting the idea? I'll understand if you say it is too much of a mess.

This is the one thing that I haven't done yet that I know of:

Next, I don't like mixing the pot ground with the main PS ground. Run all the negative leads from your filter caps to one chassis ground point, then suspend a piece of #14 copper wire from the lugs on the pots with short pieces of wire where at each grounded lug. Tie the input ground to that buss and tie it to the chassis at that point and nowhere else.

I think I know what you mean. This is the ground for the input jacks, pots, speaker jacks, negative ends of the filter caps, and the rest of the preamp board, all connected to a copper buss mounted on the front pots, like I've seen on the Express. Right now I have a copper buss on the front edge of the turret board. Do you think it is important that it move up to on the pots? Or is it more important where on the chassis it is grounded?

Oh ya, the tubes hanging down is intentional. It's mounted up in the top of a 1x12 combo cab that I designed and my retired father built for my Vibro champ kit that I plan to butcher for this. (butchering the amp, not the cab)
Anyways, here's the mess:

[Sonny ReVerb]

You need to connect the center lug of the hum balance pot to the junction of the 220k/20k voltage divider instead of ground. You probably don't need the 22uF/100V cap in there. I suppose it's extra filtering for the voltage divider?

[skyboltone]

You need to connect the center lug of the hum balance pot to the junction of the 220k/20k voltage divider instead of ground. You probably don't need the 22uF/100V cap in there. I suppose it's extra filtering for the voltage divider?

It helps decouple the hum on the filaments from the rest of the totem pole. That's incidentally why we have separate caps for separate stages of the amp. So they won't talk to each other. Like Sonny said though, hook the center lug of the balance pot to the plus side of your 22uf/100v cap not to ground.

As far as the grounding/earthing system I'm just trying to promote two separate buss'. One for all the filter caps and one for everything else. The one for the filter caps gets the neg side of all filter caps, a wire from the grounded side of the power tube cathode resistor/capacitor, and a wire from the output jack grounded side, even if it's grounded there at the chassis. This puts all the high current return paths at one point. None of this is gospel but rather a scheme that has worked for me for minimum hum and hiss. You would do well to read Randall Aiken's grounding dissertation: it is found here http://www.aikenamps.com/ under tech info advanced. He discusses a plethora of schemes that are generally held to be "good practice". All of us have our favorites. Take a look at KF's Francesca for a general guideline of what I'm talking about.....but bear in mind that it is not necessary to solder the pot ground buss to the back of the pots. It can be suspended from lead wires going to each pot ground. Just remember that it hits the chassis at the input jack end and nowhere else.

Dan