#26 pre amp

[dhtrob]

If I understand you correctly, you want to put both filaments in parallell? Of course that is possible, be it that you will have to modify your Coleman to deliver 2.1 amps..

Rob

[tcqanh]

Originally Posted by Rod Coleman

Rob, nice idea!

you can do this with the "Coleman" regulators, because they do not couple the + and - outputs.

But, any other heating method that has a capacitor at its output will directly mix L and R channels, (large crosstalk) so please take care.

Could I use only 1 board DC filament regulator for 2 tubes with 2 filaments are connected in serie ?

so, maybe nomore crosstalk ?

[dhtrob]

Thomas,
Thanks for your advise. What secondary load would you recommend given the details of the Tribute? I can certainly try again..
Rob

[tcqanh]

Originally Posted by dhtrob

If I understand you correctly, you want to put both filaments in parallell? Of course that is possible, be it that you will have to modify your Coleman to deliver 2.1 amps..

Rob

I have 2 separate board DC regulators for filament and only one winding AC of transformer for heater. So I want to strap 2 filaments in series following your way.

[Vinylsavor]

Hi Rob,

Originally Posted by dhtrob

What secondary load would you recommend

I would try no load first. Actually that means the input impedance of your power amp will be the load.

Frequency response will suffer a bit, but in my experience this sounds better. You can then work your way dwon from there and stick with what you like best

Best regards

Thomas

[dhtrob]

Hi Thomas,
Thanks.
To be honest, you got me into re-engineering the 26 preamplifier.
Starting of with no load is what I normally do, then implement load if response is bad. You triggered me by stating that gain was probably low. That was correct.
I tried several loads, but could not get the thing going with more gain using the Tribute IT. I than swapped the 26 for a 101D (same filament amperage) and got nice frequency response with no load..
So, diagram is useable, only a sufficient Line-out should be chosen for the 26. Then again, chances are small that someone has the same IT lying around..

..stupid me..

Rob

[tcqanh]

Hi dhtrob,

How do you evaluate AC-DC adapter for laptop/ iphone which is used for DHT filament ?

When I use Sealed acide battery 5-6 Ah for filament of 26 or 71A, so how long does it work ?

[andyjevans]

I think it was me that was using the 12v AC-DC adapter (1 amp). I had some lying around and they work a lot better than you'd think into Rod's boards! I was running the 26 at 95v anode with a 5 ohm cathode resistor, slightly starved filament. That's similar to dhtrob if my memory is correct. I didn't "evaluate" the 12v supply - just sounded OK to me, even though it's probably switch mode. It's sealed so didn't look inside.

Andy

[Rod Coleman]

Originally Posted by tcqanh

Hi dhtrob,

How do you evaluate AC-DC adapter for laptop/ iphone which is used for DHT filament ?

When I use Sealed acide battery 5-6 Ah for filament of 26 or 71A, so how long does it work ?

1. AC-DC adapter ("off-line" converter). For DHT 26, the filament is 1.5V 1.05A, so you need a converter with 1.5V output, or a linear regulator placed between the converter & the filament.

Problems: Off-line converters have leakage current between mains-line (L) and Safety-Earth (E). It varies hugely between models. Your 26 preamp must have a connexion between Safety-Earth and 0V-GND for safety reasons.
This means that mains leakage through the filament supply, through the cathode resistor and into the chassis will happen. Adding a linear regulator will not kill the leakage, it is common-mode current.

The value of the leakage can be higher than the current due to the music signal in some cases.

This leakage current has high noise content.

Off-line converter has cheap electrolytic in its output. There is a music-signal voltage across the filament, and the cheap cap will short it out. Result: signal travels through cheap cap!

Off-line converter has high differential noise (as well as common-mode noise). Noise is very wide bandwidth, and it will be very difficult to remove.

If you want to try, be sure to have a spectrum analyser available, to measure the increase in noise, and test the effectiveness of your filters and regulators.

2. Sealed Lead-Acid Battery. You will probably need to measure the noise of these too. The chemical activity which releases current is not necessarily quiet.

26 filament requires 1.5V long term value. In steady state, it should be within 1.425V to 1.530V (ie -5%, +2%) for longest life of the filament.
Meanwhile a single-cell lead acid battery provides 1.0 to 1.3V across the discharge cycle, or 2.0 to 2.6V for 2-cell. So SLA needs a low-noise regulator also.

When the battery discharges, and the filament voltage drops below about 1V, the filament will be underheated and may be damaged. you must have a low-voltage detector circuit which shuts of the B+ and the filament supply, to preserve the life of the tube.

In both types of power source, you need extra linear regulation and filtering to even have a chance of successful heating.

You will find it to be lower cost overall to buy a good transformer, and quality filament regulation.....


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[disco]

Whoops. post #2000
Congrats gang