dropping B+ advice

[diddymix]

Hi guys,

Was hoping somebody could help me with a technical query on how to lower the B+ on my amps output tubes EL84's.. I'm not an experienced tech at all so plse bear with me! Basically I measured the voltage on the B+ that feeds the plates and it measured 415V. I'd like to lower it some down to around 360V.. Only looking at the schematic, all the filter caps for the preamp stages and PI are downstream.. Am I right in saying if I drop the B+ line that feeds the plates (HT) then that will lower everything else downstream.?
I'm looking at the schematic and I thought of this.. if I was to add a resistor from the output transformer CT (HT), am I right in saying I cant do that because that resistance would also drop the AC voltage that is the music signal?? How would I go about doing this?

Many thanks for your time and look forward to any advice at all

[dorrisant]

https://ampgarage.com/forum/viewtopic.php?t=27035

[tubeswell]

There are various ways.

You could install a voltage regulator in the HT rail after the reservoir cap, with the output voltage set at the desired ‘B+’. RG Keen has some MOSFET designs. Merlin Blencowe also covers this topic in his Designing Power Supplies book. If you use a big-ass enough tube, you could use a tube-based voltage regulator (but this will also require heater juice)

If the output stage is cathode-biased, you can just use plain old reverse-biased Zeners on the HT winding’s centre tap (or ground side of a FW bridge if the HT has no centre tap). (This trick won’t work for fixed bias output stages.)

[Phil_S]

You could replace the PT. A PT isn't that expensive. In the short or long run, it is the simplest, least messy, and quickest solution.

[frankdrebin]

El84s have no problems at over 400V on plates,the only problem is if you feed the screen grids with over 350V.
What i usually do is a zener follower after the first capacitor that feeds the plate,or a proportional mosfet regulator,that way i'm dissipating just the preamp and screen power,and i can avoid a choke,ripple is very well filtered.

[diddymix]

Thaks guys! All useful stuff.. I dont have any of these components handy or the chops to make those circuits.. Is there any way I could just use a resistor to drop the voltage from the output transformer HT feed?? That then feeds the primaries and I just saw that that may be OK in that the resistance is not in the way of the outgoing AC signal from the plates... COuld this work? Thanks

[R.G.]

It might work, but it's full of side effects. From an electron's point of view, that resistor is in series with each output tube as it tries to pull power into the transformer and therefore out to the speakers. The power supply sags in proportion to the signal voltage and you've introduced a lot of issues into your sound. Electrically, the resistor IS in the signal path.

Worse yet, as the amp pulls more current instant by instant, the resistor drops more voltage. If the amp is biased class A, it might be OK as current in a Class A doesn't vary as much as in other types of amps. But Class AB, most of them, has a power/current use that goes up from somewhat to substantially as signal is cranked up. So at low power/current, the resistor doesn't drop much voltage, and at high power it drops a lot, in addition to having tone issues.

Could it work? Sure - but it's likely to come with side effects. If you think about it, if dropping B+ was as easy as putting in a resistor, the other circuits would never have been needed and would not exist.

[Tony Bones]

El84s have no problems at over 400V on plates,the only problem is if you feed the screen grids with over 350V.
What i usually do is a zener follower after the first capacitor that feeds the plate,or a proportional mosfet regulator,that way i'm dissipating just the preamp and screen power,and i can avoid a choke,ripple is very well filtered.

I like that solution, but I'd probably just put a 5W zener in series with whatever dropping resistor is already feeding the screen node. You could branch the screen supply off separately from the preamp and PI if you don't want to drop the voltage there too. What some people call brute force others call elegant. Here's a 56V, 5W zener for $0.47. Other voltages are available. https://www.digikey.com/product-detail/ ... -ND/918050