M Fowler wrote:Is the bias in good range, I've had amps where the bias was too hot and take out a tube.
By the way you did a very good job building this amp hope you or someone can identify the problem so you can enjoy.
Appreciate it Mark. I had the bias set a bit low as a precaution. About 25-30mA.
I'll figure it out with the help I'm getting here. Don't care how long it takes.
I will be changing the preamp, however, to a stock Concorde. I want a little more dirt and compression than I was hearing.
Do you have someone who can look at it on an oscilloscope? Possibly a high frequency oscillation? Do you have the original tubes that worked? Can you put those back in?
bruce egnater wrote:Do you have someone who can look at it on an oscilloscope? Possibly a high frequency oscillation? Do you have the original tubes that worked? Can you put those back in?
No, unfortunately, I don't but picking one up and learning how to use one was on my short list of things to do. Are the handheld scopes good enough just for amp use?
The original tubes were the 2nd to go after that first pair blew. Didn't have the white flash, but the blue glow got brighter for an instant and the fuse blew. They might have been fine but I tossed them.
The thing that causes a H.T. fuse to blow is too much current being drawn across the PT. This may mean that either the PT is shorted, or that there is a short to ground somewhere after one of the PT secondaries (such as a HT short through a shorted output transformer or a shorted filter cap causing the B+ to short to ground, drawing excessive current.
You have to diagnose where the short is. First thing is to see if the PT handles voltage without the tube installed (If it doesn't, then the answer isn't the tubes).
Could his bias circuit be failing or dropping out intermittently? If it does it would become cathode biased with the 1 ohm resistors to ground. The plates wouldn't last long in that condition.
The "ticking" mentioned in the first post sounds like charge is building up somewhere and then arcing to a lower potential. I would look carefully for carbon traces between pins on the power tube sockets, and check the coupling caps by powering up with no power tubes installed and checking for stable bias voltage.
Can you tell in what way the power tubes have failed? For example do the heaters still function? Are there shorts between pins?
+1 on checking the tube sockets for carbon traces. I had a tube flashover in one of my builds and I tried cleaning the socket several times but I kept blowing new tubes in that particular socket. I finally ended up replacing the socket and the amp has run fine ever since.
If the ticking is regular, with the same time interval between ticks, it's typically a capacitive effect from HT to ground somewhere. The most common place is actually the HT switch, you can bypass that to eliminate it. Fuseholders, supply caps, tube sockets, even one of the preamp tubes...there's a ton of other potential places for this to occur.
I've also seen it with chokes and output transformers, along with the other issues you have. The trouble is it's difficult to test the OT with low voltages as the arcing will usually only occur at high voltages under load.
Not sure if your build uses one, but chokes can create some weird and wonderful issues that are hard to track down. Temporarily replacing them with a 10W 300 ohm resistor is a good check.
HT fuses rarely blow if there is a problem with the power transformer providing the voltages you getting on the secondaries are correct. Even a dead shot of the HT usually takes the Power fuse out first, but that might not always be the case.
In the build photos of this (beautiful) chassis wiring, there appears to be a component or wire which is tightly-wrapped with yellow insulated wire -- such that there is shielding effect around that wire/component.
Paultergeist wrote:Very off-topic, but I just have to ask:
In the build photos of this (beautiful) chassis wiring, there appears to be a component or wire which is tightly-wrapped with yellow insulated wire -- such that there is shielding effect around that wire/component.
Might I ask what is going on at that location?
It's shielding for the wire running to the fast switch
Paultergeist wrote:Very off-topic, but I just have to ask:
In the build photos of this (beautiful) chassis wiring, there appears to be a component or wire which is tightly-wrapped with yellow insulated wire -- such that there is shielding effect around that wire/component.
Might I ask what is going on at that location?
It's shielding for the wire running to the fast switch
Thanks Cbass.......and now for my follow-up question......
If your looking for a technical answer I'm sure someone will chime in, otherwise go to the Komet web site for a description. Even better check the Komet Concode clips on YouTube by Pete Thorn. He demonstrates the fast and gradual modes in 2 separate videos.
Fast Mode: more gain. Louder. Faster attack
Gradual mode: softer. Not as touch sensitive. Less gain
Last edited by lgehrig4 on Mon Apr 28, 2014 5:53 pm, edited 1 time in total.
Paultergeist wrote:Thank you for the information.....I'll do some viewing on YouTube as you suggest.
Just realized that it sounded like I was giving that "search for yourself" answer that annoys me. I was really just trying to get you to check out those clips and not settle for just the technical jargon. Komets are amazing amps and both modes sound great but different IMO.
I'm still trying to grasp some of the troubleshooting tips I've received here. Rather than expecting everyone to dumb it down to my knowledge level I'll take it slow and try to get info from other sources and report back when I figure out what the problem is.
Or I can just give up and send it to someone like fusionbear
lgehrig4 wrote:If your looking for a technical answer I'm sure someone will chime in, otherwise go to the Komet web site for a description. Even better check the Komet Concode clips on YouTube by Pete Thorn. He demonstrates the fast and gradual modes in 2 separate videos.
Fast Mode: more gain. Louder. Faster attack
Gradual mode: softer. Not as touch sensitive. Less gain
It's all good, my friend. You have given me plenty of information with which to search for a few more insights, such that I can see if this feature might be a consideration for one of my amp builds.
