Rectifier Tube Swap

[zozoe]

hi all~ I get it, & you can't simply swap them around w/o some knowledgeable surrounding value changes, but I have an modded Bell & Howell amp the uses 2x6V6, a 6SC7, a 6J7, & a 5Z4 that you could cook a steak on, & the tranny seems to get real toasty as well, & I was thinking of the potato masher 5R4 WGA/WGB rectos, especially due to their over-built-ness,,,, I know that 5U4's are not so easily subbed due to some electronic config or another, BUT sound & gainwise, what changes could I expect using that particular 5R4 or any other trusty JAN tube or a 5AR4, or whatever,, For whatever it's worth, I'd like to nudge just a touch more gain out of the amp, and temper all that excess heat.. Thoughts & experiences!
thnx in advance!!

[TUBEDUDE]

If a small amp had a toasty tranny, I'd solve that first. Before any mods, be sure of correct operation. Are the filter caps OK? Does the transformer have a shorted turn? Correct functioning of the amp will make it easier to judge and fine tune your mods.

[Leo_Gnardo]

a 5Z4 that you could cook a steak on, & the tranny seems to get real toasty as well

I doubt there's improvement to be had by swapping out rectifiers in your amp. With the PT & rectifier getting (over?)heated, I expect you're getting all the POWER it can possibly generate. But power isn't gain. Where to find more gain? You'll have to post the schematic. Nobody can guess how, without it.

Also, pay attention to what TUBEDUDE's telling you. Not much point in modifying an amp that's sick. Let's make sure the power supply & all the rest are working right first. Who knows, you might find some fault(s), correct it/them, then find your amp is now delivering the goods the way you want.

[johnnyreece]

5v4 is a more direct replacement. From what I've seen, the 5z4 drops 20v, the 5v4 drops 25, and only 10 for the 5AR4. If you're pushing the tolerance on your components, the 5AR4 may put out too much voltage. It does have *slightly* less current draw on the filaments (1.9A vs. 2A), but I don't know if that would be enough to help with your temperature issues.

[Stevem]

Does pin 8 of both output tubes go to one resistor that then goes to ground?

If so then what is the voltage you find on pin 8 and pin 3 and pin 4 of either output tube?

[Craig B]

I love these builds cause I had so much fun with my first one-a Newcomb record player conversion. The beginning of an addiction.
The heat issue and more gain issues are complicated to answer. The amp running too hot could result from modern line voltage being higher than when the power transformer was designed. Or from too small value on the power tube cathode resistor. Or from cabinet/chassis interference with the cooling airflow. Increasing gain is even more complex because of the wide range of definitions of "gain". Do you want more clean volume? More distortion? More sustain?
More information could lead to better answers. I would love to see a schematic, pictures of the cabinet and chassis, and voltage readings on all tube pins. Is any of that doable?

[Stevem]

I found a schematic on line for what I think is your model Bell & Howell.

Here’s what it says.output tube wise pin 8 on both tubes should be at 19 volts.
Pin 3, 335

Pin 4, 325.

The resistor on pin 8 should check in at 225 ohms.
How does it test out?
To me even if it does test out at 225 that’s too low and will caused the outputs to idle high ( ie draw a lot of current) and make the power transformer run hot.

I would change this to a 250 ohm 5 watt resistor.

Also the 50 uf bypass cap across this resistor if original could be going leaky.
This will place resistance in parallel with that 225 ohm resistor and make it seem like a smaller resistance to the output tubes which like I posted make the power transformer run hot.

If the simple act of snipping out one end of this cap lowers temperature then you have found sourse of the problem!

If this unit is a model M1, then the correct rectifier tube is a 5Y3.

If this is not the case then this could also be the whole basis of your problem with heat.

Is yours a model M1?

[zozoe]

cool, yeah, what I'm not looking for is more clean headroom, but just a c***hair's more breakup when the amp is dimed,,, It has 1-vol, 1-tone & that's it!! This amp is fresh off the bench & THOROUGHLY brought up to modern, safe specs, but it wasn't until I got it home & cranked, did I realize where I wanted this thing to go,,, If I'm getting my fabulous tone on '10', I want to take it to 11 or 12.... FWIW, it's a BELL & HOWELL model #12634 amp that used to play back some old oldtimer's father's 8mm/16mm b&w's, but is now at the top of my happy list of my top 10 best homebrews, all done by my local tech, after only give them to him w/full schematics & an audible vision,,, He, the dirt farmer who's worked on all things A.C. powered since he was 13 in the late 50's, & me, the happy but never happy guitarist that's still (but believes he already owns!) looking for the holy grail of mono, tube powered shit boxes, with some just so happening to put to shame, or at least keep up, w/my Supros, Marshalls, Voxes,,,, & ALL but my Morgan AC20 DELUXE!!!
So again, I just want to get this fantastically modded creation a tad closer to 'that' edge w/o lighting up the midnight sky, and/or with my tranny, recto, & power tubes contributing to any furthur local global warming warnings~ And if on any level, a recto swap 'could' affect the gain in my favour, while bringing down the overall 'heat' of things, then I'd love to hear!!
thnx all~

[R.G.]

I agree with the direction the advice is going.
First, if it's getting hot, fis that first. If the amp has not had a re-cap job in 5-10 years, just do that first. Replace every polarized electrolytic in the amp with nearest modern rating. Electro caps getting leaky can eat more power than the rest of the amp. A leaky cap across the cathode resistors can make the tubes eat more too.

I'm an agnostic regarding rectifier tubes changing sound. Of course they can - but they do it by the method of changing the tube in turn changing the equivalent resistance of the rectifiers. I have read many, many reports of people replacing a rectifier tube entirely with solid state diodes and a series resistor to fake the internal resistance of the tube they pulled out. I have yet to hear that someone can even tell the difference between a well concocted SS rectifier plus resistor matched to a tube. It ... might ... be possible to tell, but remember that the whole point of the filter caps is to destroy any AC component left over by rectifiers.

But I digress...

Fix the amp first, including a re-cap if needed. It just may sound a lot better and maybe not overheat.

The AC line overvoltage is a serious problem, as mentioned. Stuff designed for 110Vac, has a hard time eating today's nominal 125Vac line. The power trannies make more heat because they're flirting with the edge of saturation from the higher voltage. You might like to read the article "The Vintage Voltage Adapter" at geofex.com.

[johnnyreece]

I'm an agnostic regarding rectifier tubes changing sound. Of course they can - but they do it by the method of changing the tube in turn changing the equivalent resistance of the rectifiers. I have read many, many reports of people replacing a rectifier tube entirely with solid state diodes and a series resistor to fake the internal resistance of the tube they pulled out. I have yet to hear that someone can even tell the difference between a well concocted SS rectifier plus resistor matched to a tube. It ... might ... be possible to tell, but remember that the whole point of the filter caps is to destroy any AC component left over by rectifiers.

A side benefit of diode rectifiers is, you would no longer be using the 5V tap on the PT, which should save some stress (and therefore heat) on the PT.

[Leo_Gnardo]

The resistor on pin 8 should check in at 225 ohms.
How does it test out?
To me even if it does test out at 225 that’s too low and will caused the outputs to idle high ( ie draw a lot of current) and make the power transformer run hot.

I would change this to a 250 ohm 5 watt resistor.

Also the 50 uf bypass cap across this resistor if original could be going leaky.
This will place resistance in parallel with that 225 ohm resistor and make it seem like a smaller resistance to the output tubes which like I posted make the power transformer run hot.

If the simple act of snipping out one end of this cap lowers temperature then you have found sourse of the problem!

zoezoe, what stevem says here should be checked out, and maybe altered to taste. In similar amps I've gone to 270, even 300 ohm cathode resistors to tame excess bias current. ALSO, the bypass cap better not be leaky or shorted. Selecting a value for that cap by ear might also lead you to your amp nirvana. Heck leaving the bypass cap out might even be the solution - some kool old amps have none and sound terrific.

But, if your heart is set on swapping rectifier tubes, and you think that's the only path, I'll leave you to it.

Also, we still don't see a schematic. Perhaps TAG brain trust might offer another solution but we're left navigating in the dark without a map.

[pdf64]

A side benefit of diode rectifiers is, you would no longer be using the 5V tap on the PT, which should save some stress (and therefore heat) on the PT.

But the HT voltage will increase a fair bit, especially at full loading. Given the same OT, that will cause the output valves to draw more current, thereby stressing everything - PT, OT, output valves, speakers - more.
So yes, if the (fixed bias) amp’s idling or just pottering along, there could be a benefit. But if it’s working hard, any VA saved on the valve rectifier heater will probably be overtaken by additional HT loading.

[zozoe]

STEVEM,, The chassis has these cool placards for each tube, & the recto is a 5Z4,,,,

[Stevem]

Ok fine, then the gear your dealing with is not the model I think it is when I was snooping around for you,but none the less I bet the output stage is darn close to the schematic I was looking at!

[R.G.]

But the HT voltage will increase a fair bit, especially at full loading. Given the same OT, that will cause the output valves to draw more current, thereby stressing everything - PT, OT, output valves, speakers - more.
So yes, if the (fixed bias) amp’s idling or just pottering along, there could be a benefit. But if it’s working hard, any VA saved on the valve rectifier heater will probably be overtaken by additional HT loading.

Not exactly.
If you just toss in solid state rectifiers, no "tube faking" resistors, yes, the HT will be higher, because less heat is wasted in the rectifier tube or added resistor. However, if you add the series resistor, you can tune it to nearly exactly the HT you were getting before with a rectifier tube. Or, you could get more power out of the same amp and transformer by rebiasing the outputs taking int account the new, higher HT. Either way.
Spending less of the usable PT heating in the 5V winding just makes more of the heating-withstanding available to other windings. And having less power burned up in the transformer core doesn't increase the HT voltage from the HT winding. That's set by the turns ratio and winding resistances, not how much loading is done.
So you can get the same output HT under the same conditions by modelling a tube style rectifier with SS diodes and a series resistor. The disuse of the 5V rectifier winding will either make the trannie run cooler, or let you load the trannie more for the same heating.
Given that the tube rectifier modelling resistor is used, then it does not follow that the amp working hard or loafing will be any different with the SS rectifiers plus resistor than it would be with a rectifier tube.