5V4 recifier with 6L6WGB pair

[pjd3]

Hello,

While I would normally not consider doing this, under these circumstances I'd be curious what you think

While I'm getting near the end of an amp rebuild that will really be like a Pro reverb with a pair of 6L6WGB's, I'm wondering if I could get away with using a 5V4 rectifier considering the low to lower-medium volumes I play at in the small to medium size bars and restaurants I gig frequently at. (plus, drummer uses a digital set so, we work for lower stage volumes because we can).

The 5V4 tube has been in my Slucky 6V6 plexi and of course has been fine with that amp since its feeding a pair of 6V6's. The reason I'm even considering trying it is that the 5V4 rectifier tube would likely bring the B+ down to the voltage that I'd like to see, and, I will be playing at the same volume level with the "Pro-reverb" that I have been playing at with my 6V6 Plexi. I'm making the assumption that given the same volume level, the "pro-reverb" wouldn't necessarily be demanding considerably greater power from the power supply/power tubes. The 5V4 has an output current max of 175mA. I don't think I will be coming close to that using the 6L6WGB pair at a consistent low to Low-medium volume.

Am I being reasonable? Wreckless?

Thanks for your input.

Best,
Phil D.

[nworbetan]

It sounds like it's pretty reasonable so far, assuming you're also using a power transformer that isn't over the 5V4's 375v rms limit.

What power transformer do you have in mind?

Another related question is: Do you know how to ensure the PT secondary resistance and reservoir cap don't over-tax the 5V4? That shouldn't be an issue either, adding a little series resistance if needed and using an appropriate reservoir cap are easy.

[pjd3]

Thanks nworbetan.

Is the taxing of the 5V4 referring to the turn on in-rush current? I could strap a 50K - 100K resistor across the Standby switch and use that as a soft-start mechanism. I did that to a new amp build I am doing and it was nice to see the first filter cap charge nice and slowly - 35 volts after one second, 70 volts after 2 seconds, etc... (but, the new project is a solid state rectifier.

The power transformer is a blackface bassman PT with a HT secondary coil of 356-0-356 vac. so, not quite there but, getting close.

I have not considered the PT secondary resistance and how that will function with the 47uF first filter cap. I assume that should there not be enough DCR in the PT HT coil that we are looking at high currents due to a low RC time constant? Can you clue me into what the limiting safe range specs to look for are?

Thank you for stopping by. I did already build a B+ Mosfet reducer that when connected to the HT Centertap will drop a little more than 20 volts. The fact that the 5V4 would also do that was an attractive temptation but, I don't want to be too precarious!

Best,
phil D.

[Stevem]

My 1954 Gibson GA77 uses a 5V4 and a pair of 6l6GCs .
It outputs 24 watts with a peak of 30 or so and is loud enough for a gig with its 1-15”.
My 5V4 is a NOS rca.

[nworbetan]

https://frank.pocnet.net/sheets/093/5/5V4GA.pdf

I don't know if this datasheet is the same model of 5V4 that's relevant, but it says typical use is a 10uf reservoir and 100 ohms per plate, at 375v rms.

A reservoir of 47uf feels like it could lead to some short rectifier tube lives with a 356vac transformer, but I don't know what math to do to quantify how far outside of the safe range on the datasheet is possible.

The GA77 looks like a real close match to what you'll want to aim for, and it uses a 20uf reservoir.

[pjd3]

Maybe it would be smart to stick to my original plan to use the B+ Mosfet reducer circuit to cut the B+ down a bit and stick to a good ol' GZ34 rect tube. Using a 5V4 seems like it may perform but possibly on the edge regarding input voltage and Filter cap max. I think it was certainly worth considering since there were a couple of definite benefits to using the 5V4 but, it's looking like there may be a couple of risks along with those benefits.

Maybe its a prudent idea to let the 5V4 live in my 6V6 plexi where its nowhere near being stressed and is performing fine.

thanks everyone. I'll try the B+ reducer circuit first.

Best,

Phil Donovan

[Stevem]

For a good SPL reduction which is overall what it sounds like your going for why don't you try out just runnng the amp on one of the speakers.

[sluckey]

Why do you think you need to reduce the B+? Blackface Bassman PTs get along just fine with 6L6s.

[pjd3]

Hi Stevem,

Actually, this amp only ever used or has one speaker. I love the sound of 2 speakers but, in my older age I'm trying to keep things a bit more compact and lighter. Everyone in my bands yells at me for having to lug big 50 watt amps into the trucks. I laugh at them but, I guess its gradually becoming an issue. So this is where I'm headed - at having a collection of different type amps that don't break the back but serve the sound and stage and venues. My amp tech let me use his real 1965 Fender Pro reverb for a couple of weeks while he was working on my gigging amp. I didn't want to give it back for obvious reasons but, yeah, it sounded really nice. Bitch to carry. I wouldn't let anyone else carry it though. Liability!

Thank you, it is great to know there are more than one option to address a situation and not always easy to decide on which one to go for first. I'd ultimately like to try all of them at some point or another.

Best,

Phil D.

[pjd3]

Hi Sluckey,

My idea to do that was based on Mike at KCANos (who sold me the matched tube pair) said that I probably shouldn't be putting more than 470vdc on the plate of these (they are the Phillips JAN 6L6WGB). I realize that if I don't do anything the B+ with the Mojotone Bassman PT will be around 455-460 which while not exceeding 470vdc, is starting to get pretty close. I had to wonder if it would be wise to just knock the voltage down just a bit to give a tad of breathing room for the plate voltage. Maybe the tubes could give me a tad more time and keep them out of any inadvertent trouble. After some reading recently I see that the JAN Phillips 6L6WGB have a reputation for being quite a bit more robust than say, the Sylvania and other manufacturers of the same type of tube, and there are a few that claim they have been playing on the JAN Philips 6L6WGB with 500vdc on the plates and maybe more with no issues. These nice little tubes are really getting a good dollar now and I'm willing to go the extra yard to make them work for me in terms of longevity.

But also, I am still in the juvenile green novice stage of my tube amp build era so, its also a matter of discovering things and proving to myself that these are tactics that actually work (or don't). It's learning and gratifying to make something work that use to mystify me, especially if its successful and useful.

Thanks again everyone for your generous input here.

Best,

Phil D

[nworbetan]

You're one or two decisions away from making it work, and where the adjustments need to be made depends on which parts you're already set on. It sounds like you have a pair of 6L6WGB that you're designing for. Are there any other parts that you already have on hand that you're planning on incorporating?

Off the top of my head I'm not sure if there were any blackface bassman amps made with tube rectifiers, so maybe look for a PT with a secondary around 300 to 325 or so and also has a 5vac winding? The Hammond 290CX looks like it's in the right ballpark, for one example. https://www.hammfg.com/files/parts/pdf/290CX.pdf But you'd need to add 60 ohms or more in series with each rectifier anode, and maybe a bit more than that if you want to use a bigger reservoir cap.

[pdf64]

…
I'm wondering if I could get away with using a 5V4 rectifier considering the low to lower-medium volumes I play at in the small to medium size bars and restaurants I gig frequently at…

I suggest to use a GZ34 with a sag resistor, eg 100R, in series with the rectifier output. By that you’ll get all the benefits of an indirectly heated rectifier plus by reducing current, you’ll be extending its life.
Fit 1200V of silicon protection diodes in series with its anodes of course.

[pjd3]

Hi and thanks so much for your informative and generous responses.

I have always wondered why I've seen the standard dropping power node resistors and choke for screens and preamp tubes but, never anything in terms of a resistor between the Rect tube and the first filter cap. Due to the total absence of ever seeing that I must have assumed that there had to be some detrimental electrical aspect to doing so. Off the top of my head, I figured power tubes benefited from as low impedance voltage source as possible. But, I would be able to tell pretty quick if there were any sonic alterations from the additon of a series resistor. It actually sounds worth a try.

I'd be happy to try out a few different values of lower value resistors and see what I get for an initial B+. Unfortunately, I'm not in a position to be purchasing transformers on the fly as I'm still a guy with a handful of dependents and priorities that are not about amps. (for which Im grateful).

Well, let me ask the question I have not asked. -

Would you feel good about placing 455-460vdc on the plates of your 6L6WGB's ? Maybe I should stop fussing about it and just do it. After all, thats the way the amp was for the first 7 or 8 months that I used it, and civilization as we know it didn't cease. I thought RG's Mosfet reducer was a neat little idea so, was happy to build one and employ it. Its there and build for the taking.

I will have a little time to decide, hopefully this amp will be gig ready in a couple of months. Until then, Sluckeys 6V6 plexi is holding up just fine!

Thanks again,
Best,
Phil