How low is too low?

[evh0u812]

How low can you go on a bleed resistor without affecting tone?
Is a 100k bleeder going to give a lower running voltage than say a 220k?
Or will it not affect the B+ whilst powered up?

[pdf64]

I can't see how any practicable value of B+ bleeder resistor would affect tone.
They won't change tube operating conditions.
Consider that the B+ is effectively ground for ac, which where tone lives.
Pete

[evh0u812]

Going too low in value has to affect the b+.
So at what value would you expect to lower b+?

[ToneMerc]

How low can you go on a bleed resistor without affecting tone?
Is a 100k bleeder going to give a lower running voltage than say a 220k?
Or will it not affect the B+ whilst powered up?

That question is relative to application and given parameters.

Yes, making the assumption that this bleeder would be hung off B+ rail to ground, yes a 100K will give a lower resultant reading than a 220K resistor. All you are doing is just creating a voltage divider.

For example, if you went to your last B+ preamp node and connected a 820K, 470K, 220K or 100K bleeder resistor to ground, you would see a decrease in node voltage as the bleeder resistor value dropped.

Thus, if decreasing preamp tube plate voltage affects "tone", then varying the value of the bleeder could result in such.

TM

[martin manning]

What is the context of this question? What purpose is the bleeder serving, and why do you want to reduce the value?

[evh0u812]

Ok a bit of context.
Ive just finished Friedman style Mashallesque build using a Heyboer jtm45 HV pt.
Im running a 500v single cap can paralelled for 100uf for the plates. I dont have the luxury of room to fit the usuall 4x cap cans series paralelled and my b+ is 485v loaded but 498v without tubes. Now this is with a 100k bleed resistor on that cap.
I cant afford time at the moment to get in there and tweak the values and take measurements so this was a general question to see if anyone else had played around with different values. As soon as i get the chance ill put a 220k in there and see if the voltage rises while in use. I might just end up zennering the ct to bring the ht back to sane levels for the poor caps!

[ToneMerc]

Ok a bit of context.
Ive just finished Friedman style Mashallesque build using a Heyboer jtm45 HV pt.
Im running a 500v single cap can paralelled for 100uf for the plates. I dont have the luxury of room to fit the usuall 4x cap cans series paralelled and my b+ is 485v loaded but 498v without tubes. Now this is with a 100k bleed resistor on that cap.
I cant afford time at the moment to get in there and tweak the values and take measurements so this was a general question to see if anyone else had played around with different values. As soon as i get the chance ill put a 220k in there and see if the voltage rises while in use. I might just end up zennering the ct to bring the ht back to sane levels for the poor caps!

Well, the fundamental issue is your cap voltage rating and I would not waste my time swapping that 100K resistor out. I'm curious as to how the amp is biased tnough as you only have a 14 volt delta between loaded and unloaded.


TM

[martin manning]

Say you put a 220k in there. It will draw less than half of the current that the 100k does, which will cause the B+ to increase by a few volts. Technically you're ok at 485V, but I agree it's cutting it close. If you have room to bias a little hotter, that could get you a little more margin. A bit of series resistance after the rectifiers will drop the B+, but you'll need a high-wattage resistor.

[Reeltarded]

Yikes

[Jana]

Does this PT have the 350-295-0-295-350 taps for the HV winding? If so, it sounds like you are using the 350 volt taps without a tube rectifier. Switch to the 295 taps.

[eddie25]

I had this exact same problem with the exact same PT. Ended up just swapping it for the low voltage one and used the HV one to make an amp with a CDE 525v cap can. I did run the amp for over year at 500v, but just couldn't keep doing it.

I tried a weber B+ dropping Zener, but it didn't work (I have terrible luck with their stuff). I also tried like six or seven 5 watt 10v zeners in series and they just burnt up almost immediately at high volume.

[Structo]

Yeah you have to use a high (power) stud mount Zener.

The chassis acts as a heat sink for the Zener.

[evh0u812]

It only has one secondary winding.
A gz34 would help the problem but a better solution would probably be a higher voltage cap can.
All that aside, theres no denying that high voltage make a huge amp. The punch and volume is massive. Its quite imposing.

I might have to squeeze in a small daugher board with 3 x 100uf caps to double my voltage rating.

[pdf64]

Your tubes are drawing maybe 75mA and they only pull the B+ down 13V (and that's ignoring the effect of loading the heater secondary).
Therefore your supply resistance is 13/0.075 = 173 ohms.
A 173 ohm resistor feeding a 100k resistor isn't going to drop much.
A bleed resistor would need to be a couple of orders of magnitude smaller before it pulled the VB+ down any.
Such a bleed resistor would dissipate several 10s of watts.
Hence it isn't practicable.

Consider what the effect of line voltage variation will be, eg the B+ will track it.

F&T make 550V axial caps.
Pete