Tube Amp Archive

groovtubin wrote:had a west "mini ir" amp in for repair circa 1971, has dynaco trannies!! also sports what looks ta be a DEF trainwreck inspired circuit!!!

Do I read this right? You're claiming that an amp made in 1971 was "trainwreck inspired"?

I have 50uF at the plates and screens (a series pair of 100uF Atoms at each node) and four 30uF nodes in the preamp. I also have a choke, since the amp mirrors the Komet and not the Express.

If anything, the 60 is more touch-sensitive than the Express. Much higher voltages throughout.

I built a dedicated K60 clone. Initially it was a 99% clone with the exception of the 500K pot replacing the voltage divider in front of the 'clipper' tube, as discussed above. There are some other differences...filtering is a little different, an extra node on the B+ rail so the first two triodes d...

Did you miss the previous post?

The 10K cathode resistor on the third triode controls a large portion of the response and feel of the amp. At 10K, that triode only has a gain of about 7, but it's biased pretty close to cutoff and it acts as an asymmetrical clipper - Marshall did the same thing with the 220x MV amps, although their...

The plate voltage is a little high for a KT-66.

???

The Express is in the low 400V range, is it not? The 60 is around 475V; I've used the Chinese KT66 up to 550V and the Saratovs to 500V with no problems. Unless the Concorde has REALLY high voltage I can't see KT66s being a problem

I've used KT66s and 6550s (original Tung-Sol) in my clone. It doesn't make the amp stay cleaner any longer, though - the preamp and PI clip well before the power tubes. The maxed-out tone with 6550s is a little tighter.

drz400, the one I "borrowed" to do this build had that sound you memntioned. I've noticed this as well. Removing the 220K between the third triode and PI reduces it; also small (I used 47pF) snubbers across the second and/or third plate load resistors help too. I tried adding cathode followers, a l...

there is a 220K/330K divider (220K series and 330K to ground) on the input of the third triode (selectable as 'gradual' or 'fast'). No. The K60 has the 220K/330K divider; the Concorde has no 'upper' resistor, only the 330K grid reference. In both cases, the 'fast/gradual' is a split on the plate of...

KGW wrote:Now, if you read my original post, it is not about the Airbrake.

My mistake. I thought this statement:

It looks as if the Airbrake is shunt attenuator - fixed series resistor and switchable shunt resistors (using taps of a big WW).

pretty much "is about the Airbrake".

I would disagree with the gain point. Dumbles have a lot more gain available than Express/Komet amps. The Express has that aggressive edge because the clipping is wildly asymmetric, but there's a lot less compression available.

But definitely, the Express is a lot easier to build.

KGW wrote:No, that is not how they work. Read this http://www.goldpt.com/how.html

I read it. Doesn't have anything to do with the Airbrake. If the schematic in the other thread is correct, the Airbrake is a fixed shunt, variable series.

I see it as the opposite - fixed shunt, variable series.

The graph is the load the Airbox shows your amp - when you have 16 ohms plugged into it, it's fairly close to 16 - the first attenuated setting is a little low, but the rest are closer. Whether they're "close enough" I can't really say.

Looks like that would be murder on your amp if you have 4-ohm out only. Not too bad for 8 or 16. Here's a chart of the load your amp will see at attenuation settings 0 (no attenuation) through 4 (last step below bedroom). It's DCR only but I think the trend is interesting. Series 1 is 4 ohms, 2 is 8...