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Build a VT100?

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petera77

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Joined 2008
#1
Hello fellow forum members... 🙂


Question: Since I moved into a larger home, I need some more power to listen to music at levels I'm accustomed to. 35 wpc isn't cutting it anymore.

I have some experience doing mods and refurbishing my Dynaco ST70 (VTA driver board, SDS lab power supply), but admit that theory and circuit design are not my strong points.

Having said that, I've looked (and listened) to some fine gear and was wondering about the possibility of building a cloned AR VT100. Seems like it has enough power and would be a good match for my system requirements. I have no idea where I would source OPT, maybe Lundahl?

Any input appreciated.
 
#2
Do you really want to get involved with parallel push pull? A project of acquisition and overhaul of a vintage Harman/Kardon Citation II will yield an amp that O/Ps 60 WPC continuously and 120+ WPC instantaneously. That's plenty of power. Also, the O/P trafos in the "Duece" are among the very finest ever made. Dyna O/P "iron" aint bad, but compared to the Freed stuff H/K used, it's guano.

Jim McShane, AKA Mr. Citation, has everything needed to get a "Duece" back into tip-top shape.
 
#3
Eli Duttman said:
Do you really want to get involved with parallel push pull? A project of acquisition and overhaul of a vintage Harman/Kardon Citation II will yield an amp that O/Ps 60 WPC continuously and 120+ WPC instantaneously. That's plenty of power. Also, the O/P trafos in the "Duece" are among the very finest ever made. Dyna O/P "iron" aint bad, but compared to the Freed stuff H/K used, it's guano.

Jim McShane, AKA Mr. Citation, has everything needed to get a "Duece" back into tip-top shape.
Click to expand...

One thing ELI perhaps didn't make clear is that a properly working/restored Citation II sounds a heck of a lot better than you'd suspect for the vintage, and IMO sounds way better than the VT100 which is needlessly complex.
 
#4
Correct me if I'm wrong - I thought to hear a significant increase in volume that I would need roughly 3 times the power of my current amplifier.

I suspect my modded Dyanco is pushing closer to 40 wpc, so a Citation with 60 seems like it won't be enough increase for me notice.
 
#5
Despite the fact that the steady state power output difference between the Citation II and the ST-70 is in theory less than 3dB, the Citation has a lot more headroom over all, as Eli points out it is quite capable of producing more than its rated power on music. I've had both heavily modded ST-70's and Citation II, and the Citation II is a lot more dynamic, and has far better extension on the bottom end and top end, and plays way louder with a given speaker..

Frankly my old modded Citation II managed to sound louder and more dynamic on real program material driving a pair of Magnepan MQ1.4 and later a pair of 1.6QR than a 250wpc highly regarded solid state stereo power amplifier I owned at the time.
 
#8
Another question.... 😀


I'm trying to determine the correct characteristics for the output transformers in this amp.

Because I'm not good at reading circuits, I'm struggling a little bit. OK, a lot. 🙄

Near as I can tell, the 6550 like around 5K ohms of plate resistance. Would a Lundahl LL1620 be a good fit, given the circuit design of the VT100?
 
#9
I don't remember the exact pp primary z of the VT-100 opt, but my 120W PPP amplifier used a 3.2K transformer at 500V.. Typical values for a 4 tube PPP could run from about 1.6K to 3K or so depending on class of operation, operating point, and configuration. (Pentode/UL/Triode)
 
#11
Don't forget that you need output taps at 4, 8, and 16 ohms in order to supply the cathode feedback to the output tubes. I'm not sure if that can be accomplished with Lundahl transformers. I'm not saying that they cannot be used this way, just something to check into.
 
#13
Re: OK, so that idea is out

petera77 said:
After reviewing the schematic for a VT100 MKIII, I can see the OPT and other proprietary parts are going to be hard to source. 🙁

So now I'm considering a modified Williamson design:

http://home.att.net/~chimeraone/tubeampmodswilliamson.html


Any thoughts?
Click to expand...


Unless you use absolutely top notch O/P trafos in a Williamson amp, the phase shifts will KILL you.

Phase shifts are MUCH less problematic in a Mullard style circuit.
 
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