Adjusting 300B filament voltage

[Deke609]

@PB - I've got the Beepre up and running again. With today's 119.5 line voltage, PosIn on both boards are measuring at approx. 13.45V (previously 13.05V and 13.15V @ 118V AC in). [Edit: and PosOut values remain unchanged: 9.82V on one side, 9.85V on the other]


You previously suggested that I could try bumping up the 931R to 1K.  Is that still your suggestion in light of the new PosIn voltages?


Many thanks,


Derek

 

[Deke609]

Follow-up on my previous post. My Mouser package arrived this aft, so I had to try it. I added 68R 1/2W in series with the 931R Rset2 to get approx. 1K.  Result: 10.48V out of the filament regulator and 5.05V dropped across the filament.  Great success! But the schematic shows +10V bias applied to the cathode / 10V dropped across filament and heater resistors - with no approx. symbol. Is the 10V a hard/optimal target?  If so, I will try a 30R in series with the 931R.


many thanks, Derek

 

[Deke609]

Further follow-up. I added 28.7R 1/2W in series with the 931R Rset2 and now get approx 10.1V out of the filament reg board (maybe closer to 10.05V with the EML 45Bs 300Bs - I got 10.1V with the EH 300Bs).  Tweaking the cathode resistor values a bit (adding a 68R to one side and a 75R to the other), I now get approx. 4.95V dropped across the EML 45B 300B filaments. 


Unless someone tells me this is overtaxing the SR circuit or doing something else bad, I am calling it good -- in fact great!


[Edit - mixed up my amps in my head and mistakenly referred to EML 45Bs when I meant the EML 300Bs in my Beepre]


cheers,  Derek

 

[Paul Birkeland]

Provided the regulator isn't dropping out (in which case you'll hear hum), there isn't much harm in running a BeePre like this.

-PB

 

[Deke609]

Many thanks PB. No dropouts yet.

 

[Deke609]

@PB or @PJ: what is the current rating for the 6.3V secondaries of the PT-7? 


many thanks, Derek

 

[Paul Birkeland]

3.5A AC.

 

[Deke609]

Many thanks PB.

 

[Deke609]

@PB: when a filament voltage regulator drops out and starts humming - does the output tube continue to amplify the signal (music) or does the music cut out and get replaced with hum?


I have an intermittent hum issue that I'm trying to troubleshoot that I thought for sure was the reg dropping out, but now I'm not so sure - I'll post more about this if I fail to figure it out myself. For now, it would be really helpful to know the precise sonic symptoms of the fil reg dropping out so I can compare them with what I'm hearing.


many thanks,


Derek

 

[Paul Birkeland]

It's hum on top of the signal.  Intermittent hum may mean that you're too close to the dropout region and as your line voltage wanders a bit, you're passing in and out of dropout.

 

[Deke609]

Shoot. Yeah, that makes sense. It also may explain why I can't get it to happen now. Gonna keep working on it - I did a fair bit of work taking out that PT last wknd - it's possible that something wasn't resoldered properly -- or maybe I toasted one of the B+ UF4007 rectifier diodes by desoldering and then resoldering them - I have some new ones that I can put in. 


It would be a real bummer if I couldn't use the EML 300Bs - they sound great. I've reverted to stock values for Rset2 on the reg board and for the cathode resistors - which puts me shy of the 4% filament voltage spec - but even with that I was still getting hum early today - but for the past 3 hrs, nothing. 

 

[Paul Birkeland]

You could also split the difference between the original resistor value and what you have now.  The UF4007 diodes are in the HV circuit, and if you have a damaged one the raw high voltage rail voltage will drop way down.

 

[Deke609]

Thanks PB.

You could also split the difference between the original resistor value and what you have now.

I tried that earlier today - I brought fil voltage to 4.8V -- exactly 4% low -- and still had intermittent hum. 

The UF4007 diodes are in the HV circuit, and if you have a damaged one the raw high voltage rail voltage will drop way down.

Yeah, I get that. I was just speculating that a slightly toasted UF4007 might still function, but become noisy - pure uniformed speculation on my part.


cheers and thanks, Derek

 

[Deke609]

Just trying to think things through here.


If the problem is the 1.3A draw of the EML 300B filament @5V, then aren't I a bit stuck unless the Shunt Reg can be made to handle more current? If the filament resistance is fixed, and I'm not supposed to go lower than 4.8V dropped across the filament, then I am stuck with a minimum current draw. 


If the current requirement of the EML 300B presently exceeds the current limitation of the shunt reg, is there a way around this ? E.g., adjust the C4S to allow more current - e.g., 1.4A? and then tweak the Shunt reg board as needed?


cheers, Derek

 

[Paul Birkeland]

You're mixing up the high voltage part of the circuit and the low voltage part of the circuit.  The high voltage shunt regulator has no idea what's going on at the filament.

If the filament is drawing 1.3A, then yes, you could trim down the paralleled 10W resistors to look more like a 3.8 ohm resistor instead of a 4 ohm resistor, but the additional current draw will also lower the output of the filament regulator, hence the desire to bump it up just a little bit.

 

[Deke609]

Many thanks PB. I think I understand what you're saying. I had wrongly assumed (without carefully looking at the schematic) that the fil reg was fed by a C4S, but I see that the C4S is used for the HV anode load. Apologies for that, I should have checked the schematic first.


So I think it follows from what you've written that the fil reg board is, subject to heat dissipation limits, not current limited, other than by the 3.5A ratiing of the 6.3V secondaries - which is plenty more than I need.


I will retry adjusting the Rset2 resistor. And it just occured to me that something else may be contributing to the hum issue.  On one channel I installed a pcb solder cup pin on one leg of the Rset2 (because I was testing only one side at first, and expected to make many resistor changes, and so feared ripping out the trace).  The pin diameter is a bit small so it's possible that during my previous Rset2 trial I unseated the pin, leading to a noisy contact. I will check and fix today.


Is there a max PosOut voltage that I should stay under? I ask b/c my first attempt at bumping up Rset2 resulted in about 10.45V out (IIRC) and I don't think I heard any hum at that setting - but did hear hum when I dropped PosOut to 10.1V. [Edit: I realize that in changing PosOut I am changing the bias, and am not sure how tight the acceptable bias range is]


Many thanks again for all your help,  Derek

 

[Paul Birkeland]

Regulator drop out should occur as PosOut voltage is increased, as the dropout occurs when the difference in voltage between PosIn and PosOut is not large enough.  As your bring the PosOut voltage up, the heat dissipated in the regulator will decrease.

I wouldn't worry too much about the bias voltage being off by a few tenths of a volt.  That will move the plate voltage around a little bit, but it shouldn't be very significant. 

 

[Deke609]

Ah, many thanks. So the reg board "reads" and regulates the difference between out and in? 


I must have been wrong about the 10.45V PosOut not humming, b/c I just tried 10.3V out and got instant hum on both channels - and both were dead silent before. 


I am going to try to hit exactly 10V out and then play with the cathode resistor value.


cheers and thanks, Derek

 

[Paul Birkeland]

Ah, many thanks. So the reg board "reads" and regulates the difference between out and in? 

The 1085 just regulates its output, provided the voltage at its input is high enough to do so.   

 

[Deke609]

Alright - so I'm guessing that the 1085 drops a minimum voltage doing so, and hence the need for a min difference between V-in and V-out. Increase V-out too high, and the regulator starves for voltage. 


No need to correct me if I'm wrong - fixing all my misunderstandings via forum posts could take a lifetime  ;D


cheers and thanks again, Derek