12B4A Linestage

[Bryan]

Parts Questions

Hi guys.

I just recieved the "big" box of supplies, and all looks good.

The ASC oil caps I bought are HUGE! I have a question about these caps. I assume they have no polarity, correct? Also, what is the max DC I can put on these caps. They are 370 VAC. Would it be 1.41X370 = 520VDC?

Also, for the LM317T regulator, how hefty of a heatsink is required, assuming I'll have about 3-4 volts across the device? Finally, is it acceptable to add into the circuit via a switch parralell resistance with R1 to bring the heaters down to 63% of their standard voltage. I'm trying to think about standbty options.

Thanks,

Bryan

 

[analog_sa]

You're right about your first two questions. How did you come up with the mysterious figure of 63%? It is not a good idea to keep the tubes half warm. Full on heater with no plate voltage is the acceptable stand-by condition.

 

[Bryan]

Hey,


I was under the impression that a reduction of heater voltage (approx. 60-70% full voltage) and no HV was the proper standby condition.

I think I read this somewhere, though I can't remember the name of the book off the top of my head....

Anybody else?

Bryan

 

[agent.5]

Morgan's "Valve Amplifier" 3rd edition, has a circuit in which the heater circuit has a relay and run at 4V during standby. A good book to get from Amazon (especially if you sign up that BankOne credit card and get a $20 or $30 discount coupon).

 

[Bryan]

Agent.

Just the book that was escaping my mind. So this is a good idea, or is it unconventual?

I imagine it would increase tube life while avoiding the harsh conditions of firing up the fillaments from a cold state.... 99% of the time the lightbuld blows when you flip the switch!

Any thoughts.

Bryan

 

[agent.5]

I am not sure if it is a good idea. I have never used it myself. I do have a delay relay for the B+, and separate transformers for the B+ and filaments. So, all the filaments are powered about 30 seconds before B+.

 

[Bryan]

I'll have a seperate transformer for the fillaments and the HV/rectifier heaters as well. I'll get a slow start with the damper diodes in the PS, so I'm not too worried about having B+ applied without fillaments, as I would just switch the fillaments on before the B+, or use a relay. I was more thinking about when the amp is resting. I thought it would be nice to keep the tubes warm, but not hot, when the preamp is resting, as to avoid unecessary warm-up at the beginning of a listening session... or is this all a waste of time/effort?

I believe my ARC SP9-MKIII works this way???

Thoughts,

Bryan

 

[fdegrove]

Hi,

I imagine it would increase tube life while avoiding the harsh conditions of firing up the fillaments from a cold state.... 99% of the time the lightbuld blows when you flip the switch!

Let's not get paranoid here...

Assuming you stick to the tube rectifier, you have slow start built in.
The 12B4A also has controlled heater warm up ( that's what the suffix "A" stands for.

So, switch the preamp on about twenty minutes prior to serious listening and stop worrying.

If anyone has prove of prolonged tubelife by opting for a different work around for this case, I'd like to hear about it.

Cheers,

 

[Bryan]

Transformer Clarification

Hey Guys,

Just to clarify any confusion I may have...

The HV transformer should be 300VAC at 100mA, no center tap, correct? This is one of the advantages to this topology right, no need for the CT?

Would this be the same transformer as a 600 VCT transformer.

If I am having these custom wound, how should I specify this to the manufacturer?

I'm thinking I need a 300 VAC, 100mA transformer which will be used in full wave bridge configuration, right?

Thanks,

Bryan

 

[fdegrove]

Hi,

Would this be the same transformer as a 600 VCT transformer.

No.

A 300VAC xformer is just that, CT or no CT.

0-300V or 150-CT-150V are the same.

The CT is used with a duodiode tube rectifier in a full-wave configuration.

Have a look here:

TRANSFORMER

Cheers,

 

[Bryan]

Frank,

I understand that the output voltages are the same, but I was told by the manufacturer that the core type may be different depending on duty cycle of the transformer???

Which type would make more sense for my application, a 150-CT-150 or a 0-300, or would it simply not matter in terms of voltage output.

Using two monodiode tubes, would this still be FWB?

 

[fdegrove]

Hi,

I understand that the output voltages are the same, but I was told by the manufacturer that the core type may be different depending on duty cycle of the transformer???

60 or 50 Hz cycles? Never heard that before though.

Which type would make more sense for my application, a 150-CT-150 or a 0-300, or would it simply not matter in terms of voltage output.

If you use a tube rectifier then I recommend the CT.
Also keep in mind that these tube rectifier cause some voltage drop and you'll also lose some more in your filter section.

Using two monodiode tubes, would this still be FWB?

FW, yes but no bridge.

Cheers,

 

[Bryan]

Frank,

You're saying drop the hybrid rectifier, and go straight with a tube rectified PS, using a center-tapped transformer?

 

[fdegrove]

Hi,

You're saying drop the hybrid rectifier, and go straight with a tube rectified PS, using a center-tapped transformer?

Ah....Sorry about that....I had completely forgotten we had discussed that possibillity here.

Go for the hybrid one, I'm all for it.

Cheers,

 

[Bryan]

Frank,

So then, with the hybrid, I use a 0-300VAC non-center tapped transformer, as I have indicated in my schematic, correct? Or, could I use a 150-CT-150, and not ground the CT?

Thanks,

Bryan

 

[fdegrove]

Hi,

Or, could I use a 150-CT-150, and not ground the CT?

Both would work fine and if you go for the CT one, you can still ground the CT.

Cheers,

 

[Bryan]

Thanks

 

[Bryan]

LM317T Problem

Hey Guys,

Quick question/problem....

I've built up a little CRCRCRC-LM317T-fillament supply to power the 12B4A's.

I tested the circuit with my Fluke measuring the voltage between the Vout and the ground of the circuit. Everything was looking good. It showed a slow ramp up to voltage, at which point it stabilized, and would hold voltage steady when I would adjust the mains voltage with the variac. I thouhgt everything was fine.

Then I hooked up the circuit to a tube, and nothing happened. To be more exact, the voltage, when the load of the tube was applied, read 0.1 volts. I dont know what to make of this?

Any suggestions?

I will work over the circuit tomorrow morning, but it seemes to be correct.

The R values for the voltage divider are 220 ohm, and a 2.5Kpot set to about 1800 ohm, to set the voltage to 12.6...If this helps. I've also bypassed the pot with a 100uF cap, and added 2 1n4002 diodes as per the manufacturers recomendations.

Thanks for the help,

Bryan

 

[EC8010]

Why use CRCRCRC smoothing for a filament supply? The moment you draw any current, the voltage going to your 317 will collapse.

 

[Bryan]

Hi,

I was under the impression that I needed to properly rectify and smooth the voltage before the 317, is this not true?

What is the appropriate way to do this if you can not use a CRCRC... filter, just one cap in front of the 317?

Thanks,

Bryan