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Quickie upgrades & modifications

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Posted on October 10, 2009 at 20:03:27
Koda
Audiophile

Posts: 11
Location: South of Seattle
Joined: August 19, 2003
 I have been kicking around some ideas for my new Quickie, that by the way, sounds pretty darn amazing!
My main preamp is a passive TVC (Transformer Volume Control) that I built from parts. In a comparison the Quickie has more "punch" but I really like the transparency of the TVC, however it does not have the gain I need in my system.

I understand that a choke can sometimes load a tube better than a resistor, and therefore improve the sound quality.
Would something like the Hammond 156C be a good candidate for this duty without dropping a lot of money? In place of the RK or RL resistor?

My other brain storm (sometimes cloudy, with patches of clearing...) would be to graft the two units into one!
Could the transformers and switch replace the volume pot on the Quickie? Any downsides or other changes to consider?

I am open to other suggestions, as I am sure others are, on how to improve the Quickie without selling a vital organ to finance.
Thanks a lot for making this great kit available, and in advance for any help!
I can hear the potential in it just waiting to come out...

 

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RE: Quickie upgrades & modifications, posted on October 10, 2009 at 21:22:58
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 It should work well. Try it and find out! - and post your results! (When Doc B gets the expanded forum going, it will be a great addition to the Quickie forum.)

I like the TVC merging as well.

 

RE: Quickie upgrades & modifications, posted on October 11, 2009 at 10:31:37
Dr. Toobz
Audiophile

Posts: 31
Joined: September 8, 2009
 Could you briefly explain again why a choke would be better than a simple plate resistor of the same resistance? Does an inductor "hold" electrons in its magnetic field in such as a way as to smooth out what the plate "sees" in terms of a load? Is it better in terms of being able to present a more ideal load across a wide range of frequencies, like how a C4S board makes a tube's "curves" look more like straight lines on a plot?

Also, would the 3.7k resistance of the chokes linked to here change anything, circuit wise, over the 2k plate resistance in the stock build? Is more resistance on the plate always better (keeping in mind that there's no power supply noise to "reject" here)?

 

RE: Quickie upgrades & modifications, posted on October 11, 2009 at 11:52:31
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 A choke behaves like a current source (except at low frequencies), so as a plate load it makes the load line more horizontal and reduces distortion.

In the Quickie as preamp, the stock plate load resistor is 4K, equal to the tube's plate resistance. So it reduces the effective source impedance to 2K, good for driving cables. At the same time, it reduces the voltage gain and the peak available output voltage. Replacing it with a choke will double the gain (another 6dB) and double the output impedance (can drive half the cable length).

In the quickie as headphone amp, the output transformer and headphone is an additional load, further reducing the gain. So more output power is available at lower distortion if the 4K resistor is replaced with a choke.

In simplified SE amplifier theory, a plate load equal to the plate resistance (4K) provides the most output power per volt of input signal, assuming the input voltage is the limiting factor. If the plate supply voltage is the limiting factor, then a load of two times the plate resistance (8K) gives the most power.

(If plate power dissipation is the limiting factor as with a power amp, then the best efficiency is obtained with a high plate voltage, low current, and high load resistance. For example the SEX amp runs a low impedance of 3 to 4 times the plate resistance.)

For the example discussed here, the choke's DC resistance is the same as the stock resistor (3.7K vs. 4K) so it won't affect the operating point. A lower resistance choke (as usually used in power amps) will allow the DC plate voltage to be higher with the same power supply, increasing the available output power (unless the input signal voltage is the limiting factor). The choke will also store energy, allowing the plate to rise above the power supply voltage on positive peaks; a resistor or a current source will not do this.

 

RE: Quickie upgrades & modifications, posted on October 11, 2009 at 19:34:52
Dr. Toobz
Audiophile

Posts: 31
Joined: September 8, 2009
 Thank you for the excellent (yet still comprehensible) explanation! I had thought about building some sort of constant current source for the Quickie, since the C4S boards really have made my SEX amp and Seduction sound better than ever (as well as louder). It sounds like the aforementioned chokes would be a simple and relatively cheap way to get those same benefits in the Quickie.

I do wonder if the extra 16oz. of weight would be good for the acrylic plate if these were mounted to "hang" on the underside of the board, esp. considering that I have Specos in there already - maybe mounting these sideways on the inside the wooden base and running extra wire to them would be a better idea.....

 

RE: Quickie upgrades & modifications, posted on October 12, 2009 at 05:56:51
artsybrute
Audiophile

Posts: 751
Joined: March 19, 2002
 I'm running an extra set of iron on standoffs on either side of the output RCA's, hanging down. They are near the edges of the acrylic plate, so are well supported by the base. Seems to work fine.

Those aren't for the plate chokes, but crossover chokes. I have some LC low pass filters after the Speco's, before the RCA's. I do plan to add the Hammond chokes later, though. I think the acrylic can handle it, if not left in the sun for too long.

BTW for anyone interested, I need 500 ohms feeding the low pass, so I am using the Speco 1K taps for now (since it has an 8K input that we are using as a 4K input). Once I get the chokes in, I'll try switching to the 500 ohm tap. Output impedance should double back up to 500 ohms, and gain should bring the volume back up, too, I'm thinking.




Triamp... Take a load off!

 

RE: Quickie upgrades & modifications, posted on October 12, 2009 at 16:52:13
Dr. Toobz
Audiophile

Posts: 31
Joined: September 8, 2009
 Where did you get the standoffs from? I didn't think of that, though that's actually the way the power transformer is mounted in my newer Seduction.

I ordered the Hammonds last night - it will be interesting to hear any changes they bring to the Quickie once they're installed (likely towards the end of the week, I'd think).

 

RE: Quickie upgrades & modifications, posted on October 12, 2009 at 19:09:02
Sturmj


 
Sounds like this is the little additional gain I've been looking for. Thanks!

 

RE: Quickie upgrades & modifications, posted on October 13, 2009 at 06:04:22
artsybrute
Audiophile

Posts: 751
Joined: March 19, 2002
 I had a bunch of standoffs in my parts drawer (actually parts cabinets by now), so I pulled out two pair. I think the ones I used were from the original Paramour iron upgrade kit. They are plastic.

But you can purchase standoffs from any electronics house. I usually order from mouser.com. You can filter your choices on material, thread size, length, hex vs. cylindrical, threaded both ends or one end stud, etc. Same with digikey.com.

I usually order aluminum hex, threaded both ends, and length just a little longer than what I think I need (that's just me) unless space is cramped. Here it would be #4-40 female thread both ends.

Yes, please post on the sound with the chokes! I'm sure I'm not the only one interested.





Triamp... Take a load off!

 

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