6L6 fixed bias/PT question

[jimipage]

I have a customer who wants an amp with an output along the lines of a Bassman/Super -- basically a pair of fixed bias 6L6's. The beefiest PT I have at this time is a Hammond 273BX (350-0-350, DC ma 175). Rectifier will be a 5AR4, or possibly a 5U4; Pre/PI will not be more than three 12AX7's -- Does this PT have enough juice to do the job safely and permanently?

thanks!

[Andy Le Blanc]

a fender 50w pt makes around 140 ma.....
RCA quotes 47w AB2 at 205 ma ........... your in the ballpark....

just did a repair to a super today 19w static dissapation per tube was around
35 to 40 ma per tube.....

[Phil_S]

I think that the answer is, "it depends".

At Va=450, max plate is 210mA for the pair and that's above the rating. I recognize that many consider Hammond ratings to be very conservative. It's one thing when you build for yourself and another to build for a customer. With a customer, I think you really are required to mind what it says on the label. That's because the rules for commercial transactions come into play. As a consumer, what you do is up to you. As a business, you have continuing liability if you sell a product that is known by you to be unsafe. I'm not a lawyer, but I would think one definition of unsafe is running the circuit above the manufacturer's rating of the transformer.

Back to "it depends". Check to load lines on the 6L6 spec sheet. See if you can build it so that the power tubes max out around 160mA. This leaves over a little bit for the preamp tubes.

If you end up with something in the 40W range, I think this would be fine. If you are around 50W, you are probably pushing your luck. Do your math on this, as you will be responsible when the customer burns his house down. It won't matter if the customer misused the equipment and it was his fault. The insurance company will come after you and it won't be pretty. Their pockets are deep and they won't give up.

[Structo]

Yes, my D'Lite 44 ( 2 X 6L6) has a 300ma PT.

[jaysg]

Back to "it depends". Check to load lines on the 6L6 spec sheet. See if you can build it so that the power tubes max out around 160mA. This leaves over a little bit for the preamp tubes.

If you end up with something in the 40W range, I think this would be fine. If you are around 50W, you are probably pushing your luck.

This covers some points that have confused me no end. Part of what you're saying is, the OT matters in terms of selecting a good load electrically and sonically. I would guess that the Hammond is only going to create a 30 to 35W amp depending on the accuracy of that 175mA spec. As the thing is turned up, the power supply will sag and limit power. Is this what happens with any amp that's dimmed, as long as the OT can handle it? I wouldn't see the PT having any problem, regardless of how many 6L6s are out there, unless the heater supply is overloaded, in turn overheating the PT.

[Phil_S]

Let's look at some basic power transformer concepts here for a minute.

1) VA rating. This is what the total of all secondary windings can supply. This includes the filament windings. It essentially tells you when the PRIMARY will be overloaded. This is because the primary limits total supply.

2) The subject PT has a stated rating on the high voltage secondary of 350-0-350 @ 175mA. That limit applies only to the amplifier circuit without consideration of the heaters. It is the limit for that particular secondary winding.

Working with #2 above, estimated rectified B+ will be 1.3*350 = 455vdc for the 5AR4 and 1.2*350 = 420vdc for the 5U4.

455v*175mA = 79W
420v*175mA = 73W
175mA is the limit for all components (not heaters) taken together. Backing down from here, you have, perhaps 160mA available to a pair of 6L6. So, 455v*160mA=72W or 420v*160mA=67W. You've got plenty of Watts. The problem (simplified -- latitude please) is that current (amps) does the work.

If you run a pair of 6L6 at plate= 450v and screen = 400v, the spec sheet (RCA Manual, available for download, just Google for it) says zer0-signal plate current will be 116mA and maximum-signal plate current will be 210mA. Your actual results are likely to vary from the spec sheet, but in design work, that's what we have to go by.

Where you get in "trouble" is, if you should dime the amp and actually pull 210mA @ 450v, that's almost 95W. You don't have that kind of current supply available and you don't have the watts available in this particular PT. But that won't happen. 95W from a pair of 6L6 is unheard of. We expect 30-50W, so read on.

Study the spec sheet a little more. It says at Va=450vdc and Vg2=400vdc, the maximum-signal power output is 55W. How can this be? It suggests, as you draw more current in a fixed bias amp (spec says grid bias is -37v), plate voltage will drop. We can infer that, at 55W and 210mA (55/.21), plate voltage will sag to around 260v.

So, you see, the amp is not a static thing. To be thorough, you must look at the load line charts and then test your amp under operating conditions to confirm what's actually going on in the way of operation.

If we assume the book is right, then 55W from 2*6L6, you probably do have enough current available at 175mA.

You should also consider the choice of rectifier. The 5AR4 is capable of 160mA and the 5U4 is capable of 225mA. This makes the latter a poor selection for this particular PT. So, you'll get higher plate voltage and more sag with the 5AR4. I suggest this is what you need to go with here. Once you establish the 160mA capacity of the rectifier, then you've basically got things under control. You'll end up with that 35-40 watter.

Remember, you can't rely on anything I said above. I'm an amateur, not an engineer. I've simplified. I may have made an error. I might not have been as clear as I'd like. I'm hoping someone better informed will look at this and offer any appropriate correction.

In the end, you've got to check all of this yourself and satisfy yourself that the amp will be safe.

[John_P_WI]

Actually the 272 JX works perfectly. I have built many a bassmans with the 272 JX, both with and with out rectifier tubes.

John

[jimipage]

Thanks for the help, fellas. I feel more confident about this build now. Thanks, Phil_S, for making some things a bit clearer for me.

[VacuumVoodoo]

Where you get in "trouble" is, if you should dime the amp and actually pull 210mA @ 450v, that's almost 95W. You don't have that kind of current supply available and you don't have the watts available in this particular PT. But that won't happen. 95W from a pair of 6L6 is unheard of. We expect 30-50W, so read on.

Study the spec sheet a little more. It says at Va=450vdc and Vg2=400vdc, the maximum-signal power output is 55W. How can this be? It suggests, as you draw more current in a fixed bias amp (spec says grid bias is -37v), plate voltage will drop. We can infer that, at 55W and 210mA (55/.21), plate voltage will sag to around 260v.

You are confusing power supplied with output power. A class AB PP amp has at best 50% efficiency if all components are ideal. Taking into account efficiency of OT & "real life" expect less than 50% efficiency. The remaining 50% of supplied power gets converted to heat.