Dealing with High B+

[chinrest]

I'd like to get opinions on how to deal with high B+

The amp is a JTM50 with a plate voltage of 525vdc (loaded). It freaks me out a little that the first two filter nodes (F&T 50/50 500v) are operating over max voltage. The caps are on the cold side of the standby. I could have sworn that the last time I read the plate voltage on this amp (ten years ago?) it was 501vdc.

Should I:

Zener?
Mount another cap can (totem pole) inside the chassis (I'm not punching another hole)?
Dropping resistor?
Other tricks?
Wear safety gogles, say a prayer, and let it rip?


I'd appreciate your feedback.

[bob p]

Voltage doesn't climb for no good reason. I have a vintage Super Twin that's supposed to be at 500 but it's at 530 now for the same reason -- climbing wall supply voltages.

I wouldn't let the caps continue to operate beyond their rated voltage. That's just begging for a bad outcome to occur. So I'd do something about it.

Other options that you haven't listed would include:

- external bucking transformer
- voltage divider on the B+

Personally, I'd try to avoid raising the impedance of the supply rail, so I'd bring the B+ down a bit with a bucking transformer and use a totem pole.

[Gtr_tech]

CE offers 525v "can" caps. They are 4 section so you'd end up paralleling them to get 60/40u sections. If it were me I'd use series caps with balancing resistors like 2 100u at 350v with 220k parallel resistors a'la what Fender did. That way you have a 700v range so powering up with no output tubes shouldn't exceed the voltage rating.

I'm one of the few who actually prefers a high plate voltage. It makes the amp punchier with a faster transient response and lessens mush out on low notes.

A customer of mine brought in an old plexi for rehab some time ago. IIRC it had a plate supply of 560v. This thing had so much balls it would just about knock the wind out of you. Scary loud too....

[booj]

You could put a resistor between the power supply diode bridge and the B+ supply. Sort of get an unintended rectifier tube emulator. 100 ohms would probably be more than you'd need.

[pdf64]

Suggest that you take comparitive readings of line, heater and B+.
An issue with very high VB+ like this is that EL34 may be pushed over their limiting value of plate dissipation.
Pete

[kleuck]

Use a 25 volts Zener (5 watts or more)

[pdf64]

'Use a 25 volts Zener (5 watts or more)'

My feeling is that a 5 watt rating may not be sufficient; at 400V 3k5 load, this http://www.mif.pg.gda.pl/homepages/f...129/e/EL34.pdf suggests 267mA plate+screen current at max clean, so guess that may increase to 377 when overdriven; then it's being run at a higher voltage, so current draw likely to be even higher still, maybe close to the 500mA fuse rating.
So a 15 or 20 watt rating may be beneficial for good service life.
Pete

[defaced]

Last item on this page, B+ reducer. MOSFET amplified zener.
http://www.geofex.com/Article_Folder...osfetfolly.htm

[Steve Conner]

I'd vote for the "bucking transformer" aka RG's Vintage Voltage adapter. Your line voltage may well have gone up that much in 10 years. Vintage Voltage Adapter

Or if you like the sound of the increased plate voltage, do as suggested earlier, change the filter capacitors to a series arrangement of 350V caps.

[chinrest]

Thanks everyone for the great input. I've not read a whole lot about bucking transformers on this forum, so that's a nice bit of knowledge to add to the toolbox. Something about it makes me contemplate getting old -- like high blood pressure meds, hip replacements, pacemakers, a foley bag. "The poor old girl needs a little help gettin up the stairs now." But I digress. Do people mount these inside a cabinet, or do they stand alone between amp and outlet (or wheel it around like an oxygen tank or IV)? A device like this seems most useful for vintage amps or ones you wouldn't want to mod. It's a bit more pricey, but replacing the PT on a homie brew makes more sense to me.

I've always enjoyed the headroom and the whoop-ass that this amp delivers. I'm leaning toward beefing up the filter cap voltage capacity, but I'm wondering what the most non-invasive and economical way to do it is. The amp has three 50/50 500v cap cans (shared ground) and I have at least one extra lying around. Hmmm....

[bob p]

Do people mount these inside a cabinet, or do they stand alone between amp and outlet ...?

Yes and Yes. I have a bucking transformer that I've built into a junction box that can be used with many amps, and I've built them into an amp cabinet as well. I think it's handy to have one built into it's own box that can be used on a benchtop or in a home environment with many amps. but when it comes to dragging gear out to a gig, I think it's easier to build it into the amp head or combo cabinet, so that the amp can just be plugged into any available outlet and it will take care of the voltage conversion on it's own without having to drag around an extra box that might get forgotten. If you size the transformer for the current that's needed by the amp and nothing more, then they're small, they don't take up much room, and they'll fit into the amp.

[chinrest]

Suggest that you take comparitive readings of line, heater and B+.

Voltage doesn't climb for no good reason...
Personally, I'd try to avoid raising the impedance of the supply rail, so I'd bring the B+ down a bit with a bucking transformer and use a totem pole.

It's starting to sink in... A combination of bucking and totem poles. Yeehaw! Thanks guys.

[kleuck]

'Use a 25 volts Zener (5 watts or more)' My feeling is that a 5 watt rating may not be sufficient; at 400V 3k5 load, this http://www.mif.pg.gda.pl/homepages/f...129/e/EL34.pdf suggests 267mA plate+screen current at max clean, so guess that may increase to 377 when overdriven; then it's being run at a higher voltage, so current draw likely to be even higher still, maybe close to the 500mA fuse rating. So a 15 or 20 watt rating may be beneficial for good service life. Pete

Yep, don't know what i calculated, they can be used in serie too, say 4*6,2 volts/5Watts.

[R.G.]

Yes and Yes. I have a bucking transformer that I've built into a junction box that can be used with many amps, and I've built them into an amp cabinet as well. I think it's handy to have one built into it's own box that can be used on a benchtop or in a home environment with many amps. but when it comes to dragging gear out to a gig, I think it's easier to build it into the amp head or combo cabinet, so that the amp can just be plugged into any available outlet and it will take care of the voltage conversion on it's own without having to drag around an extra box that might get forgotten. If you size the transformer for the current that's needed by the amp and nothing more, then they're small, they don't take up much room, and they'll fit into the amp.

+1.

Here's a build-in I made in the vein of the "Vintage Voltage Adapter" Note the sizes of the ACin and AC out sockets. That's a 4x4x3 Bud box as I remember.

[g1]

Looks good R.G.! Any plans for a production stand alone unit? I think it would be a hit, especially with antique radio guys.
High B+ usually gets immediate attention due to the immediate damage it can do, but overvoltage in general seems to get overlooked.
Heater voltages up around 6.8V or more do increase noise and decrease lifespan of the tubes. And more volts in to the PT means more watts dissipated in the PT so it runs hotter. For the real vintage unobtainium PT's, this can be critical.
And not just for the vintage stuff, I've seen modern amps that are running class A way too hot, with too high heater voltages and filter caps right at their voltage limit. Put them on the variac and dial up 115V, everything goes back to normal. The PT's in these cases are usually offshore and labelled 115V. Maybe they are told to build to that spec., I don't know, but they seem to believe we still run on 115V. (I've mentioned this before and I think someone on the forum twisted it around to "they're sending us amps with 100V transformers for Japan!!" )
Anyway, I try to always check heater voltage as a way of "taking the amps temperature", and I think the bucking transformer is usually a better solution than dropping the B+, except when the B+ is the only thing high.