Parallel output tubes

[Klimon]

Please help me understand

I'm planning to build a double dc darling but when redoing the designer's homework (checking bias and loadlines) the bill just won't fit.

Why do the 1626's have about 400V at their anodes? With regards to further bias-variables I'm aware of the different cathode resistor value for parallel tubes (namely half) but it still doesn't make sense. It would be a whole relief if someone could explain in a few lines how this amp is biased or what I'm not getting...

Here are links to the schematic (second one on the linked url) and the loadlines:

http://www.geocities.com/TimesSquare/1965/jeremy.html

http://www.mif.pg.gda.pl/homepages/f...049/1/1626.pdf


Many (many) thanks!!!!!

Cheers,

Simon

[arnoldc]

The 1626 is biased at around 27V (154V - 127V).

This is a direct-coupled design and is the reason for the high B+ but the plate voltage on the 1626 is somewhere in the region of 235V (394V - 154V - 5V) considering the OPT voltage drop.

[Klimon]

Bless you, Arnold

[arnoldc]

You're most welcome

[drj759]

Klimon,

In Direct Coupled designs, the voltages of the 1626 have to be shifted to provide the proper grid bias voltage since the grid is held at 127 volts by the plate voltage of the 8532 driver stage. There is no coupling capcitor to block the DC of the first stage as there is in the first schematic. The levels also need to be shifted to maintian safe operating conditions for the 1626's to keep from exceeding the max plate to grid and max grid to cathode voltage ratings.

The 3200 ohm, 10W resistor is there to elelvate the cathode to a level to provide the proper grid bias of -27 volts (127V - 154V = -27V). The higher 394V is needed to compensate for the 154V drop across the 3200 ohm, 10W cathode resistor for the 24 ma current through each 1626 to maintain the 240 V 1626 plate voltage. The operating point for the Double DC Darling 1626's is going to be 394V - 154V = 240V at 24 ma per tube which is very close to the same operating point as the cap coupled version in the first schematic.

Don't forget to tie the heater to the cathode as shown in the DC Darling schematic to keep from exceeding the heater insulation breakdown voltage since the cathode will be at 154 volts.

Good luck.

[Klimon]

That's been very helpfull drj, I noticed your post only now while rechecking the thread to calculate current draw (24 mA/tube). The tied cathode/heater totally slipped my mind...

Simon

[drj759]

Simon,

I’m glad the information was helpful.

Your original post was timely as I'm in the process of building up a double DC darling myself. I had just gone through the same calculations about a month prior.

I’ve built a breadboard version of the original darling and a cap coupled double darling. The next logical step is to try the DC double darling.

The driver 8532’s sounded decent but the ones I have are a bit microphonic.

My posts are still under moderation so there will be a delay before the moderators accept this post. Hopefully they will remove my posting restrictions soon (Hint ).

Dave

[Klimon]

Hi Dave,

How do the cap-coupled darling and ddarling compare? I find the doubled power (and aesthetics) of the 4 output tubes very attractive but also heard stories about loss of focus and parallel matched tubes quickly drifting apart.

I'm already in the phase of laying out the chassis of my double dc (will build it directly without breadboarding) but it can take another while before I've collected a decent potmeter etc. Let us know how the double RC works out and stacks up against it's siblings.

Welcome to the forum (if they'll let you in..)

Simon

[drj759]

Hello Simon,

(A bit lengthy post)

I’m a newbie to valves as of last April with the original Darling as my first effort. I’m still relatively low on the learning curve so please factor that into my comments. Also, my listening experience on valves has been limited to just my own projects. I hope someday listen to some other valve systems to compare with my efforts.

I can’t say that my efforts to date on the Darling have been a complete success for me. I blame my inefficient speakers. Resource constraints have limited my speaker choice to a pair of vintage 1970’s Large Advents that I recently re-foamed and re-capped. I probably need to use a more efficient speaker but I’m reluctant to go down the DIY speaker rabbit hole just yet.

My complaint with the Darling on the Advents is listener fatigue after about an hour of listening since I had to drive the 1626’s hard to obtain a reasonable volume level. The Double Darling with the additional output power helped to reduce the listener fatigue some but not completely.

I did notice some nice characteristics of the Darling so I want to continue experimenting with it. I thought the soundstage was very good on the Darling. I had never experienced the sensation of presence as I did with the Darling. Jazz and Pop sounded good, Classical did not fair as well as I would have liked. I was surprised how much bass I got out of the Darling with the Advents. It was a strong bass but very controlled. My Double Darling trial was done rather hastily so I’m reluctant to make any further comments on that configuration. So I want to give the DC Double Darling one more try now that I understand a bit more about valves.

Your comment about the loss of focus on the Double Darling has raised my curiosity. I’ll have to rebuild the original Darling configuration for comparison to the DC Double.

[SY]

The Advents just scream for a good 15-50 watt push pull amp. They were a classic match with Dyna ST-70 or MkIII back in the day.