Search found 914 matches

by sepulchre
Tue Apr 23, 2019 1:25 pm
Forum: Technical Discussion
Topic: First Stage Hiss
Replies: 15
Views: 1973

Re: First Stage Hiss

It's been awhile. Been busy. Anyway, Martin - no DC on that pot. I replaced it to no effect. RG - I'm pretty sure I did replace the coupling cap, but I'll make sure. This is so frustrating because the first one I built is dead silent with nothing plugged in to it, even with all the knobs dimed. Can'...
by sepulchre
Mon Feb 25, 2019 10:46 am
Forum: Technical Discussion
Topic: First Stage Hiss
Replies: 15
Views: 1973

Re: First Stage Hiss

R.G. - I routed the Csnub to ground and added a 0.1uF across the 1m input ground. There's still some hiss but Way less. It's not as quiet as #1 but I think I just lucked out somehow with that one. I'm going to see if I can borrow it back for some more in depth study; try to figure out what my secret...
by sepulchre
Sun Feb 24, 2019 6:24 am
Forum: Technical Discussion
Topic: First Stage Hiss
Replies: 15
Views: 1973

Re: First Stage Hiss

Here are a couple of pics of #1. One pic is earlier on in the build, the other is the final as noted in the pics.
Stages 1+2 a.jpg
Stages 1+2 c.jpg
by sepulchre
Sun Feb 24, 2019 5:55 am
Forum: Technical Discussion
Topic: First Stage Hiss
Replies: 15
Views: 1973

Re: First Stage Hiss

xtian - The 100k anode resistors are indeed metal film, Vishay Dale to be specific. I'm waiting for a 10k to try on the input grid. I did try another, I think it's metal oxide. If there's any difference it's only that the hiss is smoother but I can't guarantee that - maybe just hoping. Malcolm - Yes...
by sepulchre
Sat Feb 23, 2019 5:27 am
Forum: Technical Discussion
Topic: First Stage Hiss
Replies: 15
Views: 1973

Re: First Stage Hiss

I use 1w throughout, but no metal film. I give it a try.

Thanks!
by sepulchre
Fri Feb 22, 2019 7:19 am
Forum: Technical Discussion
Topic: First Stage Hiss
Replies: 15
Views: 1973

First Stage Hiss

I built another 50 watt amp, sort of a Marshall 2204 with a Bunch of mods. The first one I built is amazing - guess I lucked out. It sounds really nice but what's really special to me is that with no guitar plugged in you can dime everything and it makes NO noise. Ok, a tiny bit of air sound, like s...
by sepulchre
Mon Jan 28, 2019 4:07 am
Forum: Technical Discussion
Topic: Bias Blues
Replies: 32
Views: 2680

Re: Bias Blues

Thank you Soo much, Martin! I got it biased a little on the cool side and after a bit of tweaking on the 1st two stages it sound Great! And thanks to everyone. Once again with all the help from you guys what looked like a disaster turned into a success! Plus, I have some really interesting stuff to ...
by sepulchre
Sun Jan 27, 2019 4:57 pm
Forum: Technical Discussion
Topic: Bias Blues
Replies: 32
Views: 2680

Re: Bias Blues

The Classictone 40-18015 has no CT. So, although I see and understand using voltage control inline with the main HT, could a Zener be used between the "back" of the rectifier bridge and the ground?
by sepulchre
Sun Jan 27, 2019 2:21 am
Forum: Technical Discussion
Topic: Bias Blues
Replies: 32
Views: 2680

Re: Bias Blues

Sorry, didn't get to finish that last post. I get a fairly steady 120v from the wall sockets. The 470v main I quoted was with so EL34s in it because before I fixed the Bias resistor I saw the 5881s (Tung Sol, incidentally) get a Tiny bit of red plating going on and turned it off IMMEDIATELY, of cour...
by sepulchre
Sat Jan 26, 2019 4:57 am
Forum: Technical Discussion
Topic: Bias Blues
Replies: 32
Views: 2680

Re: Bias Blues

Ten Over wrote: Fri Jan 25, 2019 4:06 am
sepulchre wrote: Fri Jan 25, 2019 2:32 am But, the rectifier produces around 470 odd volts - too much for the 5881s I'm using.
Is there any load (e.g. tubes) when you measure that 470V? What is your wall voltage? What kind of 5881's do you have?
by sepulchre
Fri Jan 25, 2019 2:32 am
Forum: Technical Discussion
Topic: Bias Blues
Replies: 32
Views: 2680

Re: Bias Blues

Okay I FINALLY got logged in. The system rejected me since my first post. That's why I haven't replied. Thank all you guys for your answers. First, I replaced the mistaken 220K resistor with a 1.5K. MUCH better! So yes to 9000 +ones. However, this amp has some other issues that I am working out now ...
by sepulchre
Sun Jan 20, 2019 8:40 pm
Forum: Technical Discussion
Topic: Bias Blues
Replies: 32
Views: 2680

Bias Blues

This amp was meant to use 5881 power tubes. The PT is a Classictone 40-18015. The mains are 300v @ 200ma, 6.3 @ 3.75a and 39v @ 3.5ma for bias. I wired it up exactly as I have several others with minor tweaks for the power tubes, etc. The bias circuit looks like this. But there are 0 volts coming fr...
by sepulchre
Sun Dec 30, 2018 5:47 pm
Forum: Technical Discussion
Topic: Synthesizing a PT center tap
Replies: 1
Views: 362

Re: Synthesizing a PT center tap

Okay, I just saw that the 40-18015 has a 39v bias winding. (duh)

Problem solved.
by sepulchre
Sun Dec 30, 2018 5:23 pm
Forum: Technical Discussion
Topic: Synthesizing a PT center tap
Replies: 1
Views: 362

Synthesizing a PT center tap

I have a transformer with no center tap. It's a Classictone 40-18015 - 300v, 200ma. So I'm trying to set up bias voltage. Unfortunately, using a rectifier bridge yields too much voltage; my power tubes are 5881 and even if I used other tubes the OT is only 40w. A couple of diodes would work fine for...
by sepulchre
Thu Jun 07, 2018 6:13 pm
Forum: Technical Discussion
Topic: Is a PT secondary center tap a must?
Replies: 10
Views: 2946

Re: Is a PT secondary center tap a must?

Hey, keeps us coming back, eh? Frequent checks of this place could never be a bad thing.