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Old 24th March 2010, 10:49 AM   #331
Richard is offline Richard  Australia
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Hi NG,
If you have the output transformer go for it, otherwise just build the plate choke version. Time spent not listening to a DHT 26 preamp is time wasted, in this life.

I’ll build Rod’s filament when I get time, but the LM350 ccs is fine to start with. Use choke input if you can or small cap, resistor then big cap and ccs.

The only major concern is finding quite tubes, although all 26’s are created equal, some are more equal than others and some are more microphonic than others.

Cheers,

Rich

PS, I don't recall anyone raving about Silvertone 26 globes, must be better than Arcturus!

http://cgi.ebay.com.au/Qty-3-Globe-S...item5ad7e1eb6c
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Last edited by Richard; 24th March 2010 at 10:56 AM.
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Old 24th March 2010, 10:53 AM   #332
Richard is offline Richard  Australia
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Hi Rod,

Is the value of the 5uF cap critical?
And what wattage rating of the resistors would you recommend?

Rich
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Old 24th March 2010, 11:04 AM   #333
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C2 not critical. reduce it all the way down to 470n if that's what you have at hand. Experiment with increased R3 if you do that. The C2/R3 filter is there to reduce hum level at the filament even further. The sound improves even when hum is inaudible, though.

R2 is burning about 1,5W for the 26, but you don't want it getting hot at all. I like the 20W TO-220 resistors here (see Farnell), attached to to chassis or heatsink. The others are low power, so any nonmagnetic will do - I use 1206 chip resistors so that the circuit can be built small - this reduces the chances of EM pickup of any kind, which is important in any audio circuit.
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Old 24th March 2010, 11:27 AM   #334
Richard is offline Richard  Australia
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That's great advice Rod, thanks one again for all the information. If you can, a photo would be great.

Rich
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Old 24th March 2010, 12:55 PM   #335
Salas is offline Salas  Greece
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Quote:
Originally Posted by regal View Post
Never could find sound impressions of this say in comparison to the CCS solutions.
He did not relay any news to me either since his last posts. Maybe he did not make the comparisons he was about to, yet.
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Old 24th March 2010, 02:57 PM   #336
NG is offline NG  Hong Kong
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Hi Rod:

Thanks for your advice. Just a little bit more about my PPP 2A3 : they are indeed Sovtek, matched quads for each monoblocks. I have connected the four 2A3's heaters in parallel on each monoblock and feeding them with 2.5 V tap directly from my PTX. Assuming that each heater takes up 2.5 A, you mean to say that the 4 FET (2 FET for gyrator and 2 FET for CCS) will be enough to handle the 10 A current, is that right ? Any other modification in your scheme ?

Second question, since I am using cathode bias, should I connect the cathode bias resistor to the "filament supply 0 V" of your circuit ?

Hi Richard:

Thanks for your advice too. I share your idea that I should start listening ASAP instead of continuously looking for the best circuit.

By the way, I will be getting a Silvertone ST triode 26 soon. Unfortunately, it is only a single one, but the day I have hear it, I will let you know how it sounds in comparison with the globe RCA/Cunninghams I have collected.

Regards,
NG
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Old 24th March 2010, 10:43 PM   #337
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Trying to design the PPP DHT heat is harder than it looked!

The negative side of the filament should be connected to the cathode resistor. This dictates a P device providing current source. Looking at the available FETs that will operate at 2V or so Vds (factoring in Rds(ON) at 5A) the capacitance is an absurd 500pF or more at the working voltage, which is going to let all the HF noise straight through!

A PNP transistor will give much better performance, but you must choose carefully. The Toshiba 2SA1887 will deliver 5A at 1V Vce, and 40mA base current.

Attempting the gyrator at 10A would be tough. Probably best to find a drum-cored choke that can take the current and use that instead. The diagram tells the tale.
Attached Images
File Type: png PPPdhtheat2010324.png (12.6 KB, 615 views)

Last edited by Rod Coleman; 24th March 2010 at 10:47 PM. Reason: typo
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Old 25th March 2010, 12:01 AM   #338
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Quote:
Originally Posted by Rod Coleman View Post
The Toshiba 2SA1887 will deliver 5A at 1V Vce, and 40mA base current.
Boy, after struggling with this same issue (and eventually resorting to standard LT1086 CCS heating), I really have my doubts that any BJT on the planet can accomplish this.

I recall my power electronics pseudo-professor (he was actually a design engineer in the real world) saying he always designs his BJT circuits assuming a beta of 10. Period, no exceptions. After much frustration, I am starting to agree with this formula. Especially when low capacitance is an important performance parameter.

That would be one heckuva transistor to accomplish this at even poor capacitance performance.
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Old 25th March 2010, 01:38 AM   #339
regal is offline regal  United States
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Rod , do you know if your Gyrator+CCS has been tried with high sensitivity headphones (SPL lmV =100dB), I know the newer low current Tent labs module has been tried and had no hum. These #26 Preamp discoveries naturally lead to a headphone implementation.
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Old 25th March 2010, 05:19 AM   #340
NG is offline NG  Hong Kong
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Hi Rod:

Thanks aaaahh lot ! I hope very soon I can finally start listening to some music after 2 years of building my PPP amp. The 26 preamp later on should be the "crown" !

Forgive me to ask this silly question, but I don't understand why there is only 1 drum cored choke L1 ? What should be the spec of the choke ?

Once again, thanks for your help.

Regards,
NG
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