Why using an interstage?
There are many ways in connecting a driver to a power-tube, all with the same goal: to provide a signal which is able to fully drive the power-tube. The signal has to be low impedant combined with enough current. Low impedant to deal with "mr Miller" (high roll-off due to inter electrode capacitance in the power-tube) and enough current to be able to fully drive the power-tube at lower frequencies. Both have their math's, but that's not the subject now...
Plenty of drivers.
There are many drivers, each with their own pro's and con's and addict followers. For instance (1) resistor load, (2) choke load, (3) tube load (triode or penthode) and (4) FET load, all coupled using a capacitor. Cranks are (5) drivers with one of the mentioned loads but DC-coupled and (6) transformer load which also couples the driver to the power-tube. The latter is the interstage which also can provide in a (minor) amplification. If we take a look at the pro's and con's of each driver, we can eliminate the following: (1) due to a high impedance output (a low impedance tube has a relative high idle current which will cause a great voltage drop across the resistor) and (4) the FET(?) since I don't want a FET in my tube-amp (principles?). (2) May operate well if a (or almost) power-tube is used as driver (low plate resistance = low output impedance). (5) Also a good solution if the "Komuro method" is used (different "floating" powersupplies for each amplifying stage) to keep the drivers' powersupply voltage as low as possible. Remains (3) and (6): (3) can be SRPP, Mu-follower and Mu-stage. These are (almost) ideal drivers but to me their sound is not... Believe me, I've tried many with all the same result: too much "stressy" almost "artificial" sound...
What driver will do?
My preference is an "iron load": choke or interstage. This load provides in a most natural and easy sound. A chokeload is coupled using a capacitor (as said) and thus a grid resistor is used on the power-tube. A low value resistor sounds better here (almost always), but using a low value resistor results in increasing the value of the coupling capacitor. In my experience a higher value (>0.33mF) capacitor will result in loss of soundquality... No ideal match, so to say. Than the interstage. This is a transformer, having primary- and secondary windings, which can easily deliver the current to the power-tube. The secondary is the grid resistor (inductive) with effectively the wire's dc-resistance as actual resistance but for the signal (ac) a relative high resistance. Almost all power-tubes sound "relieved" when using an interstage! But a con is also here: a good interstage is hard to make as well to be found... Because of the relative high current of the driver an airgap is needed to prevent saturation. In doing this a bigger core is needed to create the neccesary induction thus increasing the length of the windings which will increase the winding capacitance... This will result in a limited bandwith. The "quest" for a good interstage starts...
Tango and Tamura, both Japanese firms, have a great reputation. Each of them have their own sound: Tango sounds a bit "Hi-Fi" and Tamura a bit "dark romantical". I think Tamura has a better tonal balance and Tango have more "wow!". Both have their own fans but one thing is in common: their price. In Japan almost "normal", when sent for really expensive! Post & package and customs are the culprits here. (for instance: a pair Tamura SS351 will cost you about Euro 600 in Europe). Lundahl has relative cheap (plm Euro 150 for one pair LL1660) interstages which sound good (i.e. LL1660), but when listened to after hearing Tamura or Tango, you'll know why they're relative cheap... Having not heared American made interstages, I can not review them..
A Dutch firm with world-class 1:2 interstages?
That would be nice! AE-Europe has none (yet?)... Tribute had none either. I spoke to Pieter (from Tribute) and asked him why? He told me that making 1:2 interstages means concessions in bandwith.. That's way he makes only 1:1 interstages. Since my driver- and power-tube choice is definitive, I need an extra amplification on the interstage. Tribute decided to "give it a try" and sent me prototypes later on that week. These prototypes explained to me that making a 1:2 must be really difficult, so I won't mention them again. My findings about these however meant much for Tribute since I quickly received another pair 1:2's. Now we're talking "interstage"! They are made on a double 0.05mm silicium steel core, primary impedance about 5K, secondary about 20K and can handle 50mA. They measure (-1dB) 18Hz-38Khz, which is better than the SS351. But: measuring is no indication in sound quality...
I have had a Lundahl LL1660 (3K5 - 1:2.125 @ 20mA) which gave good results (especially looking at the price!). Than the Tamura SS351 (10K - 1:2 @ 30mA) came, entered my power-amp and let me hear the "Tamura legend": Beautiful! And that despite a limited bandwith (-1dB 20hz-18Khz) compared to the LL1660.. Than Tribute (5K - 1:2 @ 50mA): Even better than Tamura! More "drive" and energy. Ofcourse Tribute has the advantage of a closer match to the plate impedance of the driver and and can handle more current so that is the main reason in sounding better than the Tamura.. But this is not what I mean. When the "Tamura legend" struck me, it was one of "ease and naturalness" compared to the Lundahl. In these qualities Tribute matches Tamura... And that at an fraction of the price of Tamura: Euro 190 p/pair... This is where Tribute is better!
|Lundahl||DIY Paradiso Belgium|