Asteroid hissing in first stage?

[bnwitt]

Ok, so I've been playing this amp for a while (much to the chagrin of the neighbors since the airbrake parts aren't here yet) and I'm trying to decide which position I prefer the switch in, 104k or 82k I like the grind of the fast position but not the extra tube hiss that comes with it. I love the quiet and clean of the slow position but I miss the dirty breakup of the fast position.

What do you guys who have the amp prefer? Are you getting the low level added hiss from the amp when in "fast"? If so, any ideas how to eliminate it or is it just the nature of increasing the gain of the stage?

[Distortion]

It really depends on which Komet you are talking about.
With the Komet 60 I prefer the fast mode.
If you are talking about the Concorde or K50 I prefer the slow mode.
Regarding the hiss I haven't had any issues like you are describing. But it might have to do with the tubes you are using.

[Allynmey]

It really depends on which Komet you are talking about.
With the Komet 60 I prefer the fast mode.
If you are talking about the Concorde or K50 I prefer the slow mode.
Regarding the hiss I haven't had any issues like you are describing. But it might have to do with the tubes you are using.

+1

[bnwitt]

Well, I have at my disposal a virtual buttload (over 300) of different branded 12AX7s that I have subbed until I'm blue in the face to try to eliminate the hiss of which I speak. And, I've subbed all of the 12AX7's not just V1 where the variable plate load resistor value resides.

If you have a komet 60 handy, I would appreciate it if you crank it to 2 oclock on the volume knob, all other knobs at 12 oclock with no signal and put your ear near the speaker cabinet with the toggle in either position.

I find that on slow it is dead quiet, but on fast there is high gain hiss or swoosh that is common with other high gain amps. If I dial the cut control to full, it is reduced.

Let me know what you find. I would love for this to be something I can fix. I used all metal film resistors with only three exceptions so it's not resistor noise. Once again, it is only present in fast mode.

[bnwitt]

I think I may have just found the problem. I have a .068uf cathode cap on V1 instead of the .68uf of design. If my math is correct, that puts the frequency boost of the Rk/Ck at a very high frequency which would be perceived as hiss. Unfortunately, I don't have any more .68uf caps in the bin so I'll have to order some to see if that fixes the issue. I'll let you know if it does. I may just pop in a .1uf to see what happens.

[Allynmey]

I think I may have just found the problem. I have a .068uf cathode cap on V1 instead of the .68uf of design. If my math is correct, that puts the frequency boost of the Rk/Ck at a very high frequency which would be perceived as hiss. Unfortunately, I don't have any more .68uf caps in the bin so I'll have to order some to see if that fixes the issue. I'll let you know if it does. I may just pop in a .1uf to see what happens.

That will make a big difference!

[arwokc]

Hey Allyn,
Did you ever build a Constellation? Per your suggestion I got in contact with Jeanne over at BNP Lasers. She did a really nice job for me. Thanks for the ref. I'm still waiting on an exchange on my PT from S2 Amps. I guess he is really busy. I can't get an email response on two attempts after four weeks. I'll give him a few more days to reply, then give him a phone call. I got in on his MM group buy but failed to mention I needed the standup type transformer, so Steven is working to exchange for the correct transformer.

Andy

[Allynmey]

Hey Allyn,
Did you ever build a Constellation? Per your suggestion I got in contact with Jeanne over at BNP Lasers. She did a really nice job for me. Thanks for the ref. I'm still waiting on an exchange on my PT from S2 Amps. I guess he is really busy. I can't get an email response on two attempts after four weeks. I'll give him a few more days to reply, then give him a phone call. I got in on his MM group buy but failed to mention I needed the standup type transformer, so Steven is working to exchange for the correct transformer.

Andy

Andy, I have a chassis and the parts ready but, they are about 5 down in the build order! After the current amp I'm building for a customer is finished I'll relax for a week or two by....building my own amps!

On the list......

KF-50 Build
Hiwatt Build
D' Lite Build
Vibroverb Build
'67 Plexi build
Constellation Build
New Express Build
Deluxe Reverb Build

All parts for all amps are sitting in neat piles
These will be all mine!

[castor]

Hi Allyn,

you always make wonderfull works... and what a good list of amp!
Some months ago I modded my express pre completely according to a mk amp.. but it's not the same, and after that I come back to the original express scheme with your caps.
What do you think of sounds of standard kF, the concorde and the constellation?
the KF and concorde seems to sound very similar (according to mp found in internet)... and the constellation? will it sound similar to a rocket?
In future i'd like to build another amp for me.. and I'll have to choose one!

castor

[arwokc]

I love my Vibroverb build. It was my first. It was pretty complex for a first timer, but it sounds great.

[wsaraceni]

i like fast on my k60

[bnwitt]

OK, here is an update. I replaced the incorrect .068uf with a .68uf and I still have hiss. I pulled out the signal tracer and the hiss starts in the first stage V1a on pin 1 the plate. I have swapped tubes until I'm blue in the face and there is no difference. I have checked every other component value in the amp and all is correct. Of course the hiss increases as you get to each subsequent stage with the tracer. I will post a voltage chart shortly.

[tubetek]

Bad plate load resistor?? (can still measure OK)
Bad input jack ground??
I've had both with identical symptom...Do I win? LOL
tubetek

[bnwitt]

Bad plate load resistor?? (can still measure OK)
Bad input jack ground??
I've had both with identical symptom...Do I win? LOL
tubetek

Tubetek,

thanks for the suggestions. I've checked the input jack/V1 grid setup and with a dummy plug inserted everything checks out fine. 1meg+32k from pin 2 to ground, 1 meg from input jack to ground. With plug removed, 32kohms to ground from pin 2 and from input jack 0 ohms to ground.

I did check the V1a plate resistor and it is reading 102k ohms. I'll change it for jollies anyway. With the input jack grounded (no input) there are very few places this could be coming from. Power supply/plate resistor is my first suspect.

[rhubarb9999]

Barry,

Let me now if there are any readings I can take from mine that would help you track the issue down. Mine is dead silent in Slow mode and just a touch of hiss in Fast (but none of the swirling you are talking about).

Ed