Dropping B+ to simulate the equivalent of a 5Y3, sans sag?

[dehughes]

Hey all,

So, I have my amp all together, but running a 5AR4 just kicks the voltages up too high across the board. A 5Y3 is perfect, but I don't like the "squish".

Is it possible for me to a priori determine what value of resistor I could put in line after the resistor (say, somewhere between the CT and the standby switch) that would allow me to run a 5AR4 but keep the voltages where they need to be?

So far, with a 5AR4, I'm getting 419v of rectified DC. With a 5Y3, I'm getting 363.

I suppose the main question is, would putting the resistor in line actually cause there to be some of the squish I'm trying to avoid?

[Structo]

Another thing you can do is use a high power rated Zener diode on the center tap to ground.
Use a stud mount that has the cathode to the casing.
Just pick one for the voltage you want to drop.

I did that on my Hammond organ amp.

[dehughes]

Another thing you can do is use a high power rated Zener diode on the center tap to ground.
Use a stud mount that has the cathode to the casing.
Just pick one for the voltage you want to drop.

I did that on my Hammond organ amp.

Yeah, I'd thought about that, but was wondering if a resistor would do just as well...

[M Fowler]

Try a 150-220k 5 to 10w off the standby switch. What amp are you working on?

[Firestorm]

Class A or Class AB? In Class AB the current demands fluctuate so much a resistor would introduce a huge amount of sag. It would be less of a problem in Class A, but the resistor would have to pretty large (in terms of wattage). To figure the value in ohms, you need to know how much current the whole amp pulls downstream of the resistor.
The Zener trick is definitely easier.
You can certainly lower preamp voltages with resistors; and if you are using pentodes or tetrodes you can lower the screen voltage with a resistor (screens draw limited current and don't fluctuate as much). The plates will still be at the higher voltage, but won't behave as if they are if the screens are dropped.
Or maybe just power scale the amp.

[dehughes]

Try a 150-220k 5 to 10w off the standby switch. What amp are you working on?

Oh, that Brown Deluxe/Constellation build I've been yacking about for the past few months. Started out simple enough, then things got complicated.

[dehughes]

Class A or Class AB? In Class AB the current demands fluctuate so much a resistor would introduce a huge amount of sag. It would be less of a problem in Class A, but the resistor would have to pretty large (in terms of wattage). To figure the value in ohms, you need to know how much current the whole amp pulls downstream of the resistor.
The Zener trick is definitely easier.
You can certainly lower preamp voltages with resistors; and if you are using pentodes or tetrodes you can lower the screen voltage with a resistor (screens draw limited current and don't fluctuate as much). The plates will still be at the higher voltage, but won't behave as if they are if the screens are dropped.
Or maybe just power scale the amp.

Cathode biased...about 80% of max...pair of 6V6s.

I don't have room for a power scaling device....though that'd be nice.

Tubes are one 6SN7, two 12AX7s, and a 6V6 pair.

Maybe the Zener trick is the way to go, huh. Hmm... Does it matter where I put the Zener? That is, should it be close to or far away from certain ground points, etc.?

Would this be the proper device?

http://www.mouser.com/catalog/specsheets/DO5%20SPEC.pdf

[M Fowler]

Pick a clear spot as the outside of the zener is larger than the part inside the chassis. So don't drill through something Just kidding.

Just need to match your voltage drop with right zener AK and if it is say 25v it doesn't drop exactly 25 volts.

Mark

Edit be sure to buy one that is cathode to base the AK designation.
Edit again I see that in this case the link you gavie show they give the following instructions on that issue: Standard polarity is anode-to-case
• Reverse polarity with cathode-to-case by designation R suffix in part number, e.g. 1N3305RB, etc.

[Firestorm]

And make sure you get the reverse polarity type.

Ooops. I see Fowler beat me to this point.

[dehughes]

Pick a clear spot as the outside of the zener is larger than the part inside the chassis. So don't drill through something Just kidding.

Just need to match your voltage drop with right zener AK and if it is say 25v it doesn't drop exactly 25 volts.

Mark

Right on.

So, if I'm looking to drop, say, 50v off of what I'm getting on the OT's CT, then would I pick a 50v Zener? Or, is the Zener's voltage drop going to change the PT's secondary, and therefore require more calculation?

Because, here's the thing: I'm getting about 330v on the plates of the recto tubes, which is fine (a 6G3 schematic shows 333v on to a 5AR4), but with a 5AR4 I'm getting 419v on the plates, as opposed to the 365v shown on the 6G3 schematic. I'm not sure why that is....but it just is...and I'm not a math-tech type of amp tinkerer, but I know there's a reason for it. Hence my desire to use a 5AR4 (for the sake of clarity, quickness, and tightness in the power section) but to keep the voltages in the amp down where they need to be.

[M Fowler]

I was going to write something else but I re-read your thread and found my answers. Get the value of secondary 290-0-290 or whatever yours is and calculate that against the type of tube your using the GZ34 which would be 1.4 times the secondary then you can figure out the value of zener you need connected to the CT.

Mark

[dehughes]

I was going to write something else but I re-read your thread and found my answers. Get the value of secondary 290-0-290 or whatever yours is and calculate that against the type of tube your using the GZ34 which would be 1.4 times the secondary then you can figure out the value of zener you need connected to the CT.

Mark

Okay. I'm not quite getting it...especially when I see the 6G3 schematic where the secondary is 333v, the recto is a 5AR4, and then 365v is the plate voltage on the 6V6. The 1.4 thing doesn't work there.

My PT has 320-0-320 secondaries with a 120 primary, though I'm constantly measuring 330v on the recto plates, and with a 5AR4 I'm landing at 419v or so on the 6V6 plates.

[dehughes]

So, I dropped the value of a dropping resistor, and now the preamp has voltages closer to if the whole amp was run with a 5AR4, but the power section is still nice and warm (as opposed to on fire...).

One question: I'm considering installing a "bleeder resistor" so that I'm not constantly having to dodge DC current when I'm working on this thing. What value is recommended, and would this change (drop) the B+ at all?

[Firestorm]

I don't know that I'd install a bleeder resistor just to dodge DC. It's pretty easy to keep a jumper connected between a preamp plate and ground. In order for a bleeder to load down the power supply (and drop voltages) it has to be pretty high power. The Fender 135 series had a bleeder to control voltages downstream of the main filters: it was 30K at 20 watts! A bleeder will make your rectifier tube work harder, too, as it winds up working all the time to keep voltages up.
The voltages are probably a non-problem in your case. I am absolutely certain that no 6G3 ever had 365 volts on the plates. Fender schematic voltages are notoriously unreliable. Take a look at the Deluxe Reverb AB763 schematic. It shows 325VAC on the plates of a 5AR4 but has a B+ of 420VDC. The 6G3 schematic shows 333VAC on a 5AR4 but only develops 365VDC? Something is wrong with this picture.
The problem is that rectifier output voltages are dependent on the input voltage AND the amount of current drawn through the recto. The manuals usually assume the maximum current (250mA for a 5AR4) and show the output voltage for that condition. But at idle, the amp is drawing nowhere near 250mA. I think Fender engineers wrote some schematics "by the book," and actually measured voltages for others.
In your case, the plate voltages are not unreasonable for a 6V6 AND since your amp is cathode biased, the effective voltages are actually lower. The tube only "sees" the difference between the plate and the cathode voltages, so if you have, say, 22 volts on the cathode, the plate voltage is "really" only 397V.
If you still want to drop the B+, get a 5V4GA rectifier. It can deliver 175mA (50 more than a 5Y3) and will drop 25 vots at 175mA compared to a 5AR4. It is indirectly heated like the 5AR4 so the plates and cathode can be closer together -- this makes it less "squishy." Get a NOS RCA or Sylvania tube (the current issues ones are usually mislabeled 5Y3s). They are dirt cheap because no one ever used them in guitar amps (until Matchless started doing it).t

[dehughes]

Thanks dude. Makes sense. I have several NOS RCA 5V4 rectos and love them. I think that, so far, I dig the 5Y3 in the amp enough to leave things be. I halved the value of the B+ dropping resistor to the preamp and that kept voltages high enough to keep me happy. I'll crank the amp up tomorrow or Monday and see what I think of the power section.

As for the B+ rating on Fender schematics...that makes sense... Still though, with plate voltages on the 6V6 of about 419, I'm still seeing almost 17w dissipation per tube! That's a bit much... Even with a 5V4 I'm seeing about 15w per tube...far more than I'm comfortable running....though it does sound great.

As for the bleeder resistor, I was thinking of it primarily as a safety measure to drop off voltage when the amp is turned off. The fact that it drops B+ was just incidental. That said, where is the best place for one to install a bleeder resistor?