AB763 PT Question

[mikeboone]

I built a single channel (normal channel) 2x6L6 AB763. I used transformers that I just happen to have in my stash.

The PT is a Heyboer 320-0-320, 150mA, similar to what would be used in a Bandmaster.

The OT is a Hammond 1650P 60W.

I am using a single 15" JBL D130F.

My question is... If I replace the PT with a higher current device, i.e the 200mA originally spec'd, will I gain any volume or clean headroom?

The amp is pretty loud and breaks up nice like it is, but I play with a big band with a 4 piece horn section and any more volume I can get out of this guy would be nice.

I am just asking advice before I commit the change.

I attached a pic for those interested.

[MWJB]

320-0-320 will only give you around 400vdc-ish after the rectifier with a GZ34. Maybe 420-430v with SS? If you're looking for headroom a higher B+ voltage would be good. Bandmasters can run up to 500vdc, or more, depending on year. Perhaps think about a B+ secondary of 345-0-345 or 350-0-350?

150mA should be enough for what you are needing, a bigger PT (morecurrent capacity)would have better pick up, but I think a higher B+ is your first port of call.

Is that 1.5K/25uf cap shared between v1a & v1b? I can't see another cathode bypass cap & resistor for a second stage? AB763 would normally have 820ohms at a shared cathode.

[mikeboone]

Ab763 Pt

Thanks for the input...

I think I will go larger on both fronts, I&E, if needed. The test will be after I play the first gig with this guy and whether or not I can cut through the mix. Even if it works, my curiosity will drive me to it anyway!

Oh, by the way my B+ is 396. Good call!

As for the cathode bypass resistor, I wasn't really sure which way to go, so I picked the 1.5K. I am very new to amp building. I just peeled the normal channel out of a Super Reverb schematic and went for it. I got really lucky because this guy is really quiet and sounds OK for a freshman effort. I perused a lot of kit photos and forums to get layout ideas and build techniques. This is a very cool new hobby.

Can you give me the nickel tour on the reason for the 820 in leiu of the 1.5 or point me to some literature that explains that circuit in depth. My guess now is since I am paralleling the bypass for both cathodes, I need to cut that R value in half as well. Yes, no??

Thanks,

Mike

[stokes]

If I may,MWJB,when using a "shared cathode resistor" to get the same effect as using a single 1.5k on each cathode,which is a very common value,you are "paralleling the 2 cathodes,if you put 2 1.5k in parallel,you get 750 ohms.The 820 ohm is the closest commonly available value.By using the 1.5k as a shared cathode resistor,it is like using a 3k on each,effectively reducing the gain of that stage.Check out www.aikenamps.com for some more detailed info about circuits in tube amps.

[mikeboone]

SS Rectifier

I have a SS rectifier (no sag resistor) that I could throw in there that would give me another 40-50V.

That may give me an idea of what more V will do for me.

Thoughts?

[MWJB]

Unless you are currently using a 5R4/WR4 rectifier and loads of plate current, subbing in a SS rectifier is not going to give anything like another 50v. 420-430vdc tops with an SS rectifier would be more like it, depending on plate current.

[mikeboone]

Thanks...

I am bouncing back and forth with a 5Y3 and 5U4 at present, just depends on my mood i suppose.

I have my first gig with it tomorrow. It may be fine, could be paranoia on my part. If not, putting in a new PT is not all that much work.

Thanks guys for the help. Oh and thanks Stokes for the link to Aiken, it has answered a lot of questions for me.

Mike

[MWJB]

Both 5Y3 and 5U4 have quite a pronounced sag, I'd definitely try 5AR4/SS before subbing the PT. A lot of plate current will pull down the B+ too, so try keeping it relatively cool, say 30mA?

[mikeboone]

SS Rectifier

I installed the SSR and re-biased at 30mA each. The amp stiffened up some but is not significantly louder. I think it will be fine.

My B+ went from 396 to 436, my pre plates(V1a/V1b) were 168 and 154 are now at 177and 165. The PI plates (v2a/V2b) were 206 and 208 and are now 221 and 225.

Should V1a and V1b be so different? I'd think they would be closer.

It sounds pretty solid and if these values are not anything out of the ordinary, I am going to leave it alone for a while!

BTW, my backgound is 20 years in satellite and microwave communications. I have recently crossed over into the tube amp world. I am less on the bench and more in the feild these days and I needed something to keep my bench skills sharp, hence the new hobby. Thanks for all the help guys!


Mike

[MWJB]

Typical preamp voltages for a AB763 style amp are more in the region of 220-260vdc. 177v is relatively low for this kind of amp, another 50v will give the amp a fair bit more presence & fidelity, which will sound louder.

[MWJB]

Also, it's a little odd that your PI plate voltages are higher than the preamp plate voltages, AB763s use a 12AT7 PI tube which runs at less plate voltage than 12AX7 in all AB763 designs.

If you are using a 12AX7 PI tube than that would explain the situation?

[mikeboone]

AB763 Voltages

I have been shopping around for transformers and I am finding that what is said to be the original spec for the Super Reverb is a 330-0-330 200mA transformer which is only 10V (each rail) higher than what I have. It still wont get me to that 450 range on my B+, will it?

Do you have any suggestion on a PT to use? Preferably one that is surface mount. I dont have any more room in my chassis for a tranny with a larger bell.

Thanks...

[MWJB]

That's not true, not if you are considering the AB763. A Super Reverb PT (AB763) should be giving you 375-0-375VAC...plus or minus a bit. The original schematic actually shows 360-0-360.

330-0-330 is more in line for a Deluxe reverb, Blackface bassman or AB763 Concert.

345-0-345 or 350-0-350 would probably give you a satisfactory boost (470-ish on the plates). This is tweed bassman territory.

Try Mojotone.

What PT are you using right now?

[mikeboone]

Excellent info, thanks. I'll look today.

I am using a Heyboer (don't have P/N in front of me) 320-0-320 150mA with bias tap.

I just happened to have it in my pile of stuff along with a Hammond 1650P 60watt OT. Since I grafted my layout from bits and peices of info from the Fender layout, I wasn't sure how it would work. So I didn't want to go out and spend the $$ for a new PT when I could test my layout with what I had at hand. It seems to be OK. The amp is relatively quiet. And so the quest for the proper PT begun. The internet is full of subjective information and wading through it to find truth can be troublesome if you aren't exactly sure what the truth is.

BTW, I played a gig with the amp last night. I was right at the high end of the clean headroom on volume to keep up. I used a clean boost pedal to get more volume and an OCD pedal for soloing. It sounds really good, but I do need to get the volume up a bit as I was in a very small club.

I'll have a look at Mojo and get this project to its final stage.

Again, thanks for your time.

Mike

[barfoden]

Old thread but feel the need to respond...

A fixed bias push pull 6l6GC amp with a stable power supply and a +B of 436V with a solid OT through a 15" JBL D130F with a 100dB sensitivity should be very loud indeed..
I would look into the PI as someone said.. An 12at7 should show siilar or lower voltages than the 12ax7 preamp voltage.
But if you want a big pedal style clean sound a 80-100 watt platform with 4 6L6GC or two KT88 might be more optimal..