Weber 5G15/Fender 6G15 reverb grounding

[Greg_L]

it is working and everything is fine?

Then stop fixing it.

Hey I know you told me to stop fixing it, but I have a question.

I'm running a 6V6 driver tube in this thing. I also, maybe foolishly, used a Hammond 1750A reverb transformer I had laying around. I didn't even consider this little transformer is for a 12AT7 reverb driver tube in a blackface as opposed to my current 6V6. Poor researching on my part.

22800 primary impedance
8 ohm secondary

I think this is maybe wrong for a 6V6? The secondary side matches my 8ohm 4AB3C1C tank, no problem. The spec calls for an approx 7k primary impedance for this circuit, so I'm way off!

So, what is actually happening here? Can anyone tell me?

[Enzo]

You are discovering that these are just guitar amps and FAR from precision circuits. So what if the impedance is not "optimal"? Do you hear anything that sounds wrong? The last thing we are doing is trying to squeeze every ounce of performance from the 6V6. It is loping along, reading a book. And since it is driving a spring, it means we likely won't really hear any distortion that might result.

We have all seen amps that will take both EL34 or 6L6 power tubes, with nothing more than a bias adjust. Same transformer serving both. Some small single ended amps boast pretty much any octal power tube with the common base arrangement will plug in and work.

[Greg_L]

You are discovering that these are just guitar amps and FAR from precision circuits. So what if the impedance is not "optimal"? Do you hear anything that sounds wrong? The last thing we are doing is trying to squeeze every ounce of performance from the 6V6. It is loping along, reading a book. And since it is driving a spring, it means we likely won't really hear any distortion that might result.

We have all seen amps that will take both EL34 or 6L6 power tubes, with nothing more than a bias adjust. Same transformer serving both. Some small single ended amps boast pretty much any octal power tube with the common base arrangement will plug in and work.

Understood, thanks.

Do I hear anything that sounds "wrong"? No, not really. I'm just wanting this project to be the best it can be, and learn more in the process. I have noticed the dwell control can get pretty wild at modest settings. Granted, I do have a long decay tank and I like the wild reverb. I don't have that much experience with these 6G15 clones though, so I want to make sure I'm doing anything I can to help it be what it's supposed to be.

[Mick Bailey]

FWIW, my own dwell control really spanks the tray from about 1/3 up. I could change the pot taper, change the pot and put and additional series resistor in but I sometimes like to turn it way up. The tray doesn't seem to mind one bit.

[Justin Thomas]

Not trying to be a smartass here, but this is part of the joy of DIY/kits/clones. I have an aversion to hacking/modding/exploding vintage gear, even the dreaded Silverface Fenders. This way, even if you DO blow something up, there's nothing that's dreadfully expensive to replace. I think the prices on vintage Fender Reverb Units are definitely higher than "the sum of the parts," so yeah, build yer own, twiddle with it- crrank it up, blow it up, enjoy the process, lather, rinse, & repeat!

Glad you're loving it, & I wish I could build anything THAT clean!

Justin

[Greg_L]

Not trying to be a smartass here, but this is part of the joy of DIY/kits/clones. I have an aversion to hacking/modding/exploding vintage gear, even the dreaded Silverface Fenders. This way, even if you DO blow something up, there's nothing that's dreadfully expensive to replace. I think the prices on vintage Fender Reverb Units are definitely higher than "the sum of the parts," so yeah, build yer own, twiddle with it- crrank it up, blow it up, enjoy the process, lather, rinse, & repeat!

Glad you're loving it, & I wish I could build anything THAT clean!

Justin

Haha thanks.

I'm not trying to be excessively nitpicky, but I do want to learn. Pure dumb luck can only go so far! If I used *technically* the wrong output transformer for this reverb unit, but it still works anyway, that's cool and all, but what is actually going on with the signal? Could it be better with the correct primary impedance? I'm going to scope the output today just to look. I'm more than happy to just play it, and I absolutely will, and have been. It just spent several hours last week cranking out surftastic drip without a single hiccup. But I'd still like to learn more about it.

[Enzo]

Could it be better with the correct primary impedance?

In any discussion like this, it begs the question: what is "better"? What does better mean here? In this example, perhaps we could get an extra few percent efficiency from that driver. But what does that gain us? We might get an extra (making a number up) 20% power driving the pan, but then we can already overdrive it as it sits. SO is that better? I don't know, honest question.

My explanation is by analogy. Imagine you have a 10 foot long plank sitting on cinder blocks. It is a bench seat. Now I can sit too far to one end and my left cheek hangs over the edge. I can sit too far the other way and my right cheek hangs over the edge. I can sit anywhere I want within the rest. I can sit near one end or the other, or I can sit in the middle. In all cases, I can sit there with my ass fully supported. No overhang. SO when we have small transformer circuits like this, I envision such a wide range of useful impedances that most any transformer we find will likely fit into that range.

[Greg_L]

In any discussion like this, it begs the question: what is "better"? What does better mean here? In this example, perhaps we could get an extra few percent efficiency from that driver. But what does that gain us? We might get an extra (making a number up) 20% power driving the pan, but then we can already overdrive it as it sits. SO is that better? I don't know, honest question.

I mean "better" as in healthier for the tube, OT, proper operation of the circuit, that kind of stuff. If it doesn't matter, it doesn't matter, and I'm thrilled with that. I'd just like to know why.

My explanation is by analogy. Imagine you have a 10 foot long plank sitting on cinder blocks. It is a bench seat. Now I can sit too far to one end and my left cheek hangs over the edge. I can sit too far the other way and my right cheek hangs over the edge. I can sit anywhere I want within the rest. I can sit near one end or the other, or I can sit in the middle. In all cases, I can sit there with my ass fully supported. No overhang. SO when we have small transformer circuits like this, I envision such a wide range of useful impedances that most any transformer we find will likely fit into that range.

Lol. So in my situation, is the tube the ass and the transformer the plank?

I guess my confusion/curiosity comes from simply not fully understanding why a tube expects to see a certain impedance in the primary winding, and why that matters. If a single ended 6V6 "wants" a 5-8k primary impedance, why is my 22,8000 ohm impedance little transformer and 6V6 seemingly working nicely together?

Where can I read about this stuff and try to understand it?

[Helmholtz]

I guess my confusion/curiosity comes from simply not fully understanding why a tube expects to see a certain impedance in the primary winding, and why that matters. If a single ended 6V6 "wants" a 5-8k primary impedance, why is my 22,8000 ohm impedance little transformer and 6V6 seemingly working nicely together?

Unfortunately the matter is complicated. You may start reading here: http://www.valvewizard.co.uk/se.html

The optimum plate load as specified in the tube datasheet is the load impedance that allows for max. clean output power while not exceding dissipation limits.
Higher primary impedance reduces available signal plate current and thus means lower output power.
As increased plate impedance reduces average plate current it is safe for the plate.
But high load impedance increases screen dissipation and this may be a problem for the tube. I recommend to use a screen resistor of at least 1k to reduce screen dissipation.

Things get even more complicated considering that the actual load (the reverb transducer) is mainly inductive, meaning that its reflected impedance at the primary of the reverb transformer is not constant but changes roughly proportional to frequency.

[Greg_L]

Unfortunately the matter is complicated. You may start reading here: http://www.valvewizard.co.uk/se.html

Wow, you aint lying. That is complicated.

[Enzo]

And remember that your 20k primary is only 20k when the load impedance is the specified amount. If the rating is 20k into 3.2 ohm or a 16 ohm speaker, and you have an 8 ohm reverb transducer, well that primary is no longer looking like 20k to the tube.

If you want to learn the actual physics/engineering of the tube circuits , then by all means do so. Starting with the Valve Wizard pages is a good place. Merlin's books are well written and cover the theory I think pretty well. Helms here will give you good information, he is into all that stuff: load lines, calculating cutoff frequencies and so on. I approach things from a pro-technician point of view. What is needed to effectively keep these things running well. I have never in all my years needed a load line to REPAIR an amp. To design one from scratch? That is different. It depends upon what you are asking yourself. Are you asking WHY there is a 0.68uf cap for cathode bypass, or do you ask how to determine if the 0.68uf bypass cap is the problem. Different questions.

[Greg_L]

And remember that your 20k primary is only 20k when the load impedance is the specified amount. If the rating is 20k into 3.2 ohm or a 16 ohm speaker, and you have an 8 ohm reverb transducer, well that primary is no longer looking like 20k to the tube.

If you want to learn the actual physics/engineering of the tube circuits , then by all means do so. Starting with the Valve Wizard pages is a good place. Merlin's books are well written and cover the theory I think pretty well. Helms here will give you good information, he is into all that stuff: load lines, calculating cutoff frequencies and so on. I approach things from a pro-technician point of view. What is needed to effectively keep these things running well. I have never in all my years needed a load line to REPAIR an amp. To design one from scratch? That is different. It depends upon what you are asking yourself. Are you asking WHY there is a 0.68uf cap for cathode bypass, or do you ask how to determine if the 0.68uf bypass cap is the problem. Different questions.

Thanks, I do appreciate and value your practical input.

I guess my main issue, for now, is just what's actually going on in this reverb unit with my mismatched, although working, reverb tranny?
What would/could be different with the technically correct tranny?
Is the tube taking the brunt of the mismatch, or is the tranny taking the brunt of the mismatch?

I'd sleep better if I knew why I could just live with this.

[Helmholtz]

I guess my main issue, for now, is just what's actually going on in this reverb unit with my mismatched, although working, reverb tranny?
What would/could be different with the technically correct tranny?
Is the tube taking the brunt of the mismatch, or is the tranny taking the brunt of the mismatch?

I tend to think that the 6V6 (screen) will "suffer" most, especially without a screen resistor.

This would be the better choice: https://www.hammfg.com/electronics/t...s/classic/1750
(Hammond type 1750AX)

[Greg_L]

I tend to think that the 6V6 (screen) will "suffer" most, especially without a screen resistor.

This would be the better choice: https://www.hammfg.com/electronics/t...s/classic/1750
(Hammond type 1750AX)

Thanks. I'm glad you recommended that one. In my own hamfisted "research" I determined that's probably the technically correct OT.

I'm currently using the 1750A because I had it laying around and didn't know what I didn't know.

And you're right, there is no screen resistor. Screen voltage comes straight off the B+1 node. I just followed the schematic. So you say I should put a 1k resistor on there? What wattage?

[Helmholtz]

So you say I should put a 1k resistor on there? What wattage?

1k/0.5W or 1.5k/1W. Screen resistor will somewhat reduce tube gain. With the 1750AX it is not required.