bias voltage range for a 4x6L6 amp question

[shooter]

....nothing.....was that right

depends on what you mean by right
your leads were placed correctly, your outcome was not.

power down, pull preamp tubes, OHM from pin 3 to ground and from 8 to ground.  If you have an open fix, if you have say 100k instead of 1.5k, fix

[sluckey]

I would suspect a missing ground wire if all cathodes read zero volts.

[Shack]

So I wake up this morning and im thinking, why would none of my preamp tube cathodes not be conducting.....and I feel so foolish, because it hit me like a ton of bricks. When I redid my input jack, I never put a ground wire back on the ground buss on the board....so its not connected to ground.

I will do that after work and see what I get 

[PRR]

"nothing" is a bad meter.

Did it read ZERO?

[Shack]

I would suspect a missing ground wire if all cathodes read zero volts.

How can I put that new build problems link in my sig, so I never forget it again ......it really bothers me that I overthink something as if I know what im doing , when if I checked all the grounds I would never have had to post anything at all....

In my noviceness, I thought that the voltages from the power rail  would read like a schematic if I did those first, and when they didnt read right, I freaked , without first making sure I didnt forget something.

So.....IT IS ALIVE, and sounds actually pretty good through the mismatch of 6L6, biased cold I think, and old chinese 12ax7, through a Fane speaker in a Crate amp, with the negative feedback disconnected.



I will fix all that, but one question, since this is my first build of this circuit.


The tone stack as the amp sits, doesnt seem to alter the frequency response at all, to my ears.

I made the bright channel plexiesh.....and it is too bright.....should I just change the bright cap ( 500 pf) or change cathode resistor and cap to the normal Fender values that Doug has on the Bassman circuit?
I do favor the other channel immensly, and I am used to playing on Fender amps.....just wanted a Marshallish amp lol

PRR, When I said nothing, I meant that none of the cathodes appeared to be conducting


I need to stop thinking that I know more than I really do, and follow new build protocol

[shooter]

I do favor the other channel immensely

does "that" channel use the same TS?  Are both your channels blend-able?
I built a plexi / blues Jr, 2 channel, but mixable, either by themselves sorta suck, blended, she rocks!

glad you're up n running, the high-five forehead moments will get less the more you work at it.

[sluckey]

Time to post a schematic.

[Shack]

yes ....blendable. And I guess ill make a schematic, Steve. I do like the sound of this amp. The schematic will be fun to make tho

[sluckey]

Gotta say, it sounds backwards to make the schematic last. Sure glad my house plans were required up front. 

[Shack]

Yea, but for me, I use the schematics I look at as guides, this is a blend of the Peavey, Dougs Bassman , and the Plexi circuit, mixed with the twin reverb power section, and your layouts for the plexi 6v6 input and bright channel.


Maybe that sheds light on why I sometimes have issues when its built, but I will always stay on it til its figured out.

With your patient guidance of course

[Shack]

I will work on my backwards ways....promise

[Shack]

Oh, forgot to add, redid the plate wires and hooked the negative feedback wire back up, but wife is sick of hearing me play, so guess im done for the evening lmao

[sluckey]

Yea, but for me, I use the schematics I look at as guides, this is a blend of the Peavey, Dougs Bassman , and the Plexi circuit, mixed with the twin reverb power section, and your layouts for the plexi 6v6 input and bright channel.

I know that you must have several documents that you are working with and your amp is a mixture of these documents. But the type issues you are having would be much easier for us to lend a hand with if we could see the as built blueprint. The preamp can have so many variations that a schematic is really needed to understand what you are asking.

For example,

I made the bright channel plexiesh.....and it is too bright.....should I just change the bright cap ( 500 pf) or change cathode resistor and cap to the normal Fender values that Doug has on the Bassman circuit?
I do favor the other channel immensly, and I am used to playing on Fender amps.....just wanted a Marshallish amp lol

What is plexiesh? One picture here (schematic) would make it crystal clear what you are asking and would make it easy to offer suggestions. There are quite a few folks here that have done extensive testing with tone shaping. They can offer some very good suggestions once you put together a schematic.

You did a good job on the power amp schematic. Preamp is not much more involved, especially since you are working from several proven schematics. We just need to see all your improvements.

[sluckey]

Oh, forgot to add, redid the plate wires and hooked the negative feedback wire back up, but wife is sick of hearing me play, so guess im done for the evening lmao

Take advantage of your quiet time. Grab a pen and some paper, or a mouse. It will tell you what to do.   

[Shack]

LOL....I will post schematic of the preamp....im really having fun making them too 

[PRR]

> How can I put that new build problems link in my sig, so I never forget it again

Click your name, right of your post.

(Since I can't edit your profile, I'll show mine. See Attach.)

There is Info and Modify, you wanna Modify, "Forum Profile".

You have been here before, to set your Picture. Keep going down. There is a text-box for Signature.

Here is the underlying "code" for the Sig that Sluckey is using:

Code: [Select]

New build problems? [url=http://el34world.com/Forum/index.php?topic=17701.0]Click here[/url].

[Shack]

Thank you.....I hope that keeps me from asking dumb questions lol

[Shack]

Here is my schematic.....pretty basic, but not sure what this amp is.....100 watt 6L6 plexi.... or a tweed twin with solid state rectifier, but it will be cool to play with

[shooter]

looks a lot like one I built.  I've had about 5 guitar guys play it and want it, but none can even cover parts cost

I'm not sure if There are any un-documented tweaks, the "reviews" basically say it's a great rock amp, not a lot of clean chimey, (that's in my later build), but the crunch is well liked, anyway, fwiw.

I didn't use the send return

[Shack]

Awesome, I still dont understand the use of different plate, and cathode resistors, but I hope to get there soon 

[shooter]

I still dont understand the use of different plate, and cathode resistors

my abstracted version, Rk helps set the bias point, say 1V, your input signal can now swing ~1v before it starts getting squished.  Rp  helps determines your plate voltage and how much the output signal can swing before getting squished.  you can calc the tubes current also by taking say your 1v/Rk = tube current

[Shack]

thank  you, ill try to absorb it 

[sluckey]

After seeing that nice preamp schematic I agree, that does look plexiesh!  If you change that top coupling cap to .002µF, it will look even more plexiesh.

I have two amps based on the Plexi 1987 preamp. My November preamp drives a PP EL84 power amp. My Plexi-6V6 preamp drives a PP 6V6 power amp.

I kept both preamp circuits true to the Plexi circuit except I permanently jumpered the inputs just as you did. It's my opinion (and I have a significant loss of hearing in the human speech range) that the top channel sounds like an ice pick in my ear. Very bright! The bottom channel sounds like a normal amp that has a thick blanket covering the speaker cab. Very dull! Neither single channel sounds good to me. But when you use the two volume controls to blend the bright channel together with the dull channel, you have a great sounding amp. I use the two volume controls as tone controls to get the tone I want. Then I use a master volume control at the output of the tone stack, just prior to the PI input cap, to set the actual volume of the amp.

Play around with blending the two channels together and see if you don't come up with a sound you like.

[Shack]

Actually, your November amp and 6v6 plexi schematics are the ones I used to make this preamp.

[shooter]

But when you use the two volume controls

+1

your November amp and 6v6 plexi schematics are the ones I used to make this preamp

A discerning thief doesn't steal junk

[Shack]

LOL, Shooter.... Steve, will it sound more plexiesh too?  The .002 cap

It needs a master volume, and matched tubes and....proper speakers.

It had more crunch when the negative feedback was unhooked, wonder if theres a spot for ONE more pot to make it variable.

If I pull two tubes and run it half power, will it hurt anything and will it really be half power?

[sluckey]

It had more crunch when the negative feedback was unhooked, wonder if theres a spot for ONE more pot to make it variable.

If you copied my amps you already have variable NFB. It's called presence.

If I pull two tubes and run it half power, will it hurt anything and will it really be half power?

Won't hurt anything and it will only be capable of producing half the power of four tubes. That does not mean it will only be half as loud.

[Shack]

oh, ok.....but it may be more balanced to bias til I get a quad of matched tubes. Right now, I have like 60 ma through one pair and like 80 through the other.

One last question, how responsive is this tone stack supposed to be, because im not finding it very responsive, and I hope I didnt mix up the bass and mid pots, since I have that bus wire soldered to them all.....I dont think thats the case, so just asking

[sluckey]

One last question, how responsive is this tone stack supposed to be, because im not finding it very responsive, and I hope I didnt mix up the bass and mid pots

If you wired your tone controls exactly as I did and used the same component values I did, then your tone stack is doing exactly what it should do. But, didn't I see a Hoffman sticker on your board? If you used my layout with a Hoffman board it's very possible to make a mistake. Hoffman's layout is quite different from mine.

I always thought the Fender tonestack gave a wider range of tones than the Marshall tonestack. Not necessarily better, just different.

[Shack]

its possible , I think I got it right, but since I used both schematics and layouts, I may have something mixed up....just expected more sonic changes with this tonestack than I have seen

[shooter]

....just expected more sonic changes

one of my guitar player amp tester was quite obstinate about the TS, I told him to LEAVE the pots on 5, adjust the gain pots till you like the tone, THEN, adjust the TS.  after about 10 min of play, he walked over and tweaked the treble pot 1-2 numbers then played for 30 more min and told me I had a great amp!

[Shack]

Alrighty mates, to add closure to this very helpful thread.........

So, I did have the bass and mid pots backwards after all. I rewired them, added a master volume, and only using 2 power tubes instead of four.

This thing sounds awesome . Cnt wait to get a 2 x 12 cabinet built for it....only wish I could make a faceplate because most of the silk screening on this is rubbed off, and the new controls are in different spots than the old ones.

I am willing to bet that the Peavey never sounded like this