Transformers for a D'Lite 22 build, 18W OT ok?

[Snicksound]

EDIT: After some research I'm gonna go ahead and build it with what I have, meaning the Marshall 18W OT (Hammond 1750PA) mated to the Fender Deluxe Reverb PT (Hammond 290BX). I see the 18W OTs have been recommended a few times for 6V6 only builds. I know the Brown Note had a stouter PT, but I'm not gonna be running EL34s or 6L6s in this thing. My only worry is that the PT would be pushing over 450V with diode rectification. JJs are rated for 500VDC on the plate though, so we'll see. Probably won't be using a choke so should be easy to have the screens under 450VDC.

I have this amp I don't like (it started life as a Supro 24 type thing), I'm gonna build something new out of it. It has sockets for 3 preamp tubes, 2 power amp tubes (currently novals but I'll punch them out to octals), it's in a nice fat combo enclosure that sounds really good.
Power transformer is a Hammond 290BX, aka a Deluxe Reverb PT.

I was going to make something Blackface with it, use it as my clean amp next to my 2204 JCM800 build. But now... I'm really liking the idea of a D'lite 22 build (still a great clean platform, with the option of kicking in the overdrive circuit).

My worry is the output transformer. Right now it's a Hammond 1750E, aka Princeton Reverb OT (15W and 8.5k primary), but I also have a 1750PA I can use (for a Marshall 18W, much beefier, 8.4k primary).

I'm thinking it's probably not a big deal, but should I just go ahead and get a Deluxe Reverb OT (Hammond 1750H, 20W at 6.6k primary). I live in Canada and Hammonds end up being half the price of anything else because of shipping.
I'm not expecting this to be an exact replica of the Brown Note D'lite, let alone a real Dumble. But before I go through all this work, I'd rather start with the right OT.

[didit]

Hello --

Somewhat similar project here. Building a clean stout Princeton, with some extras. Robust 291AEX power transformer with silicon rectifier. At least Initially the output transformer tested will be replacement for Bandmaster/Vibrolux 1760J "wired" for 8KΩ, just using secondary providing at nominal 8Ω. If it works as anticipated it'll permanent. The 1750J would work too, but working with what's on hand.

FWIW.

Best .. Ian

[rootz]

From a technical perspective I can't see anything wrong with using either 6k6 or 8k primaries. Both will work. I'm guessing the 6k6 will be a better match for 450V, but you'd need to draw some lordliness on a datasheet to check this.

For 450V take the JJ's. Not the most genuine 6v6 there is, but they seem to take those higher voltages really well. IIRC they are rated for 450V on the screens. You'd be on the limit. You could use a 1k resistor instead of a choke. If the tube wants to run a away and draw excessive screen current, the voltage would drop drastically. Again, you'd need to check how much resistance you'd need here.

Other solution: use a tube rectifier. A GZ34 would drop 15 V, almost every other rectifier would drop more B+ under load. I'm confident that would cure your voltage problem when using a Deluxe Reverb PT and you end up with a Dumble with a tube rectifier. A GZ34 can take 50uF IIRC of capacitance/filter right after it. Respect that value! It would be more than sufficient for a 20 Watt 2x6v6 amp anyway.

For more guidance: Amplified Nation has one (or more?) 20 Watt builds in the discussion section of this forum.

Good luck and keep us posted on your progress!

[martin manning]

With 450V Va I would definitely go for 8k primary to keep plate dissipation under control. You might have to miss-match the speaker to find something that will work. Ian's example of a Hammond 35W 1750J (Tremolux/Vibrolux) with 4Ω output mismatched to an 8Ω speaker is an excellent option.

[Guy77]

Hi SnickSound. One thing about these transformers is that its easy to get them to sag a lot, not sure if that is what you interested in but I thought I would just share my experience. I built a 6L6 tweed deluxe with the 290bx a few years ago. I had the tubes going at 52 ma each and my B+ was at 385V with a solid state rectifier.The low MA rating on these (138MA) must have contributed to the easy sagging! A gentleman in Idaho owns it now I believe.

If you ran your D'lite cathode biased at around 30ma(with JJ) each you could probably get your B+ close to 410v, just an approximation.

Have fun with your project!

Cheers
Guy

[didit]

Hello -

With a Blackface Bandmaster output set up with 6V6 at a nominal 8KΩ, roughly 440V fixed bias, and lower plate current one would anticipate less sag. Less saturation and wider swing. Same approximate hunk of iron, different results.

Best .. Ian

[rootz]

Very good point Martin. You’d keep the tubes safer with a higher impedance.

Also a good point about cathode bias for the output tubes. Why didn’t I think about that? It would lower your affective plate voltage by 40-45 V. Yes, it does compress when you really let the output tubes work, the bias shifts. If you’d ever hear the difference compared to fixed bias depends on how much you push the amp.

Also, a resistor instead of a choke would almost certainly compress/sag more. When you push the output tubes, screen current rises and thus the voltage drop over the resistor increases, shifting the bias down. Keeps the tubes safer under heavy conditions. Also the preamp voltages will drop if you just replace a choke with a resistor and push the output tubes.. not sure if you’d hear that effect though.

138mA seems painfully light for a push pull 6L6 amp Guy. Any idea how much output power you can make with that amp? I guess the B+ will sag a lot more under heavy loading, greatly limiting available output power. Which isn’t a problem perse.

[Guy77]

Hi rootz. That 2 x 6L6 amp I built several years ago was putting out about 20 watts.

Cheers

Guy

[Snicksound]

Not sure why everyone is talking about 6L6, as per the original post that is not in the plans. 6V6 only.

Cathode biased is of course an option, I actually tend to build everything this way, even when it doesn't make that much sense. Put a big fat bypass cap, bias to less than 85%, and it sounds close enough to fixed bias to not matter. Makes swapping tubes easier, and you lose some efficiency which can be a good thing sometimes (we play with IEMs, we fill the room with the PA, loud stage amps create more issues than they solve).

A tube rectifier would also work, heck the socket is already there for it. But then I can't use the 5VAC tap for relays (no room for an extra transformer).

Anyway, gonna go with the Marshall 18W OT I already have (not the skinny Princeton OT that's there right now). This is a salvage build, so the more parts I can reuse the better.

[rootz]

I was well aware that you are going to use 6v6 outputs. The talk about 6L6 was in reply to Guy and off topic.

I think cathode bias like you propose will do the trick nicely: approx 410 V anode to cathode for your output tubes is fine. I have an amp (fixed bias) with 2x6v6 and 415 on the plates. I use an 8k 18 Watter OT too. Works like a charm.

I’m also a big fan of parts bin specials (PBS). I’ve made many amps with just spare parts. Always fun!

And agreed in the stage loudness. My sound guy frequently asks to turn my amp down a notch. I gig with an 20 Watt combo. Only on big stages I can turn it up a bit more. Safety regulations limit how much decibels we can make in about any place where we play.