#26 pre amp

[CFT]

My sets arrived today, that was quick Rod!

Will report once they are built into the preamp and amp.

Thanks so much!

Cheung

[CFT]

My DHT preamp is now complete and thanks so much to Rod's regulators, no hum~! I'm loving it!!!!

As a reference, the hum level of my old 26 RC-coupled preamp connected to my Linkwitz Pluto (gainclones at last stage after the active x-overs) was absolutely unbearable (though when hooked up to a tube amp with higher input impedance it was acceptable), but hey almost no hum with these regulators yeah!!

The current setting is: two 26 globes and a 381 half wave rectifier. It is a transformer in, transformer out, with a 200H choke loading to a parafeed output design....all these transformers were wounded by a friend with vintage German iron and English (Litz configured) wire...supposed be musical I was told..... So a total of 9 transformers, yike, so heavy...1 power, 2 filter chokes, 2 inputs, 2 ouputs and 2 loading chokes.... .

Oh, oh, I am already impressed after a couple hours of listening even though it was only born last week ---- very euphoric......despite no burn in of the cathode caps and transformers.....and do keep in mind there could be placebo effect on me as well, hehe....

Photo - front: TVC volume control wounded by the same friend for mating with the preamp
Photo - back: The DHT preamp - for 26/ 01A tube- switchable with the jumper in Rod Coleman's filament regulators inside

Will post the inner construction after opening it up

[CFT]

My older 26 preamp with capacitor coupling out

[Original Burnedfingers]

Quote:

hi Hauke,

Yes, I will have them to sell.

I must prepare drawings and instructions first. Also, Power supply design recommendations, and heatsink compatibility. Then - they will be available!


Please drop me a line with cost and other info.

[eboz]

Originally Posted by CFT

My DHT preamp is now complete and thanks so much to Rod's regulators, no hum~! I'm loving it!!!!

As a reference, the hum level of my old 26 RC-coupled preamp connected to my Linkwitz Pluto (gainclones at last stage after the active x-overs) was absolutely unbearable (though when hooked up to a tube amp with higher input impedance it was acceptable), but hey almost no hum with these regulators yeah!!

The current setting is: two 26 globes and a 381 half wave rectifier. It is a transformer in, transformer out, with a 200H choke loading to a parafeed output design....all these transformers were wounded by a friend with vintage German iron and English (Litz configured) wire...supposed be musical I was told..... So a total of 9 transformers, yike, so heavy...1 power, 2 filter chokes, 2 inputs, 2 ouputs and 2 loading chokes.... .

Oh, oh, I am already impressed after a couple hours of listening even though it was only born last week ---- very euphoric......despite no burn in of the cathode caps and transformers.....and do keep in mind there could be placebo effect on me as well, hehe....

Photo - front: TVC volume control wounded by the same friend for mating with the preamp
Photo - back: The DHT preamp - for 26/ 01A tube- switchable with the jumper in Rod Coleman's filament regulators inside

Will post the inner construction after opening it up

Very nice! I would seriously recommend that you try battery bias of the grid instead of bypassed resistor in the cathode. If you don't like the idea of battery, LED could still be an option. I have found both battery and LED bias to be much much better than cathode bias in every way.

-MW

[TheShaman]

Rod, please let us know when you are ready to sell those kits!

[kevinkr]

Originally Posted by eboz

Very nice! I would seriously recommend that you try battery bias of the grid instead of bypassed resistor in the cathode. If you don't like the idea of battery, LED could still be an option. I have found both battery and LED bias to be much much better than cathode bias in every way.

-MW

I'll second that for most applications as one of the loudest and most vocal proponents here of fixed bias whether by battery or psu..

[eboz]

Originally Posted by kevinkr

I'll second that for most applications as one of the loudest and most vocal proponents here of fixed bias whether by battery or psu..

I am at a bit of an advantage in that I use a transformer based volume control, and can float the grid without a coupling cap. Hard to beat! You could try the same by isolating the input with a transformer instead of a cap.

[CLS]

Hi Rod,

I've been watching for a while, about all the good words for your good works. Congratulations.

I'm not familiar with SS stuff, here's a very basic question for you.
Sorry for late to the party, the following quote is several pages back:

Originally Posted by Rod Coleman

...

KM75 is in the picture, tested for 24 hours at 6V headroom in a 300B configuration, where the heatsink reached about 85 deg C in a 25 deg C environment.

And I've also read in another post about a very high tolerable temperature of a transistor's case (not die), over 100'C IIRC...

A metal surface temperature higher than 65'C is very hot to touch already. 85 should be untouchable. SS parts which are too hot to touch always worry me very much. And, for almost all tube amplifiers, SS parts are mounted inside the chassis. So, components of such high temperature in the pot can not be a good thing - dried out electrolyte caps, crisp insulations... etc.

In your experiences, is it OK for such high temperature in long term stablity or safety? For the filament supply modules, and for the whole amps.

Thank you very much,
CLS

[Rod Coleman]

Originally Posted by CLS

Hi Rod,

I've been watching for a while, about all the good words for your good works. Congratulations.

I'm not familiar with SS stuff, here's a very basic question for you.
Sorry for late to the party, the following quote is several pages back:

And I've also read in another post about a very high tolerable temperature of a transistor's case (not die), over 100'C IIRC...

High case temperatures are possible for power-transistors, but this must be part of an overall design. Especially, with the isolated TO-220 type, there is a larger difference between die temperature and case temp. So, depending on the power level the temperature may not exceed 100 deg C, or less.

A metal surface temperature higher than 65'C is very hot to touch already. 85 should be untouchable. SS parts which are too hot to touch always worry me very much.

For the regulators, the target heatsink should keep the temperature below 65 deg C. Of course, a lower temperature gives a wider safety margin, and this is to be welcomed in an amplifier design. Using 3mm or even 5mm aluminium for the amplifier chassis (backplate, or baseplate) will present an easy way to keep your semis below 50 deg C.

And, for almost all tube amplifiers, SS parts are mounted inside the chassis. So, components of such high temperature in the pot can not be a good thing - dried out electrolyte caps, crisp insulations... etc.

In your experiences, is it OK for such high temperature in long term stablity or safety? For the filament supply modules, and for the whole amps.

No, it is not OK for the internal temperature of an amplifier to reach, say 70 deg C. You must prevent this by using ventilation, and heatsinking.

If your amp is very crowded inside, the heatsink should be part of the rear panel, so that the heat escapes to the back.

In any case, a sealed amplifier chassis is asking for trouble - transformers and rectifiers will also get hot, and ventilation must be arranged for these.

Be careful with electrolytic capacitors. Please use parts that have a Ripple-Current rating that is higher than the rms ripple in your design. The PSUDii software will show the rms value of ripple current in all of your power supplies, including the raw dc supply for the DHT regulators. If you want a rule of thumb - the rms ripple current = DOUBLE the dc current.

Many electrolytics give a specification for the lifetime of the capacitor versus ripple current, versus temperature. If you choose (say) an 85 deg C rated capacitor, and operate it at (say) 50 deg C maximum, and (say) half the rated ripple current, you will find that the capacitor will last a reasonable time - probably 10000 hours for a reasonable quality part. You must calculate all these parameters (or measure them), and choose your capacitor carefully.

If you choose unbranded capacitors that give no specification for ripple current, you will find that short lifetime, and venting of corrosive vapour into your amp chassis, will be the outcome.

The same approach applies to power resistors: these are specified for a temperature rise for the amount of power burned in them. Forget about "5W" resistors, find out the temperature rise and see if you can tolerate it. Use a larger part if marginal.

If you take this approach for all the parts in your chassis, and include plenty of apertures for ventilation, and draw heat away with well-positioned heatsinks, you won't have reliability problems. When the amp is built and working, be sure to monitor the internal temperature for a while, if there is any doubt.