Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Shack on March 01, 2017, 12:01:10 pm
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I am now at the stage where im putting amp together, and I am testing each phase before moving on, to try and avoid stupid mistakes.
I just built the bias circuit and unloaded , the pot sweeps from -48v to -58v at the end of the 220k resistors that feed the grids.
The original Peavey schematic shows -55v coming out of the pot, so was wondering if that range sounds right for this stage of the build. If I need to change the bias range resistor, would rather do it now , but can wait, ...at least I am assured that everything works up to this point....and only preamp and PI left to build.
Thanks
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That's probably pretty close but I would feel better with a range of 40 to 60. Now is a good time to tweak it. Doesn't matter that it's unloaded because the load is usually very light, if any.
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thank you Sluckey.....I am using the 5F6A bias circuit from here. It has a 1k bias range resistor, and a 47k resistor going from bias pot ( 50k ) to ground , if I understand correctly....the 47k is what keeps it from going any lower in case the pot failed or I turned pot down too far, and the 1k sets the highest point of the range?
The Peavey used a 470 ohm for bias range, a 10k pot and an 82k from pot to ground..... I would guess though that I should go lower with both resistors , like 470 ohm and maybe 33k or so, but that would be me guessing , and I dont trust me that much,lol.
Had to edit, wasnt from wiper to ground, and I forgot about the 15k that goes from middle pin of pot to the diode, and I hope thats not the one to change, it was hard to fit in there.
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Refer to Hoffman's schematic. Change R33 from 15K to 10K. Then experiment with the value of R35 (up and down). Can you now get a wider range? Maybe something close to 40 to 60 volts?
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changed R33 to 10k, damn I hoped you didnt say that, was tough, but now I have -52 to -60 , so now I have to work on R35......thanks again Steve
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now I got -40 to -57 volts with a 22k resistor for R35.....but, I think the bridge rectifier that I bought died.....not getting the DC volts now for some reason. filament voltage and bias voltage are working good. getting about 368v AC between the red wires of the rectifier, but the DC volts on the standby switch arent there . I wonder if the self contained rectifier isnt as strong as using diodes to make it.
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thanks for the help Steve, but I think my insulator under filter board slipped out and I fried either the rectifier, main 100uf caps or both.....but bias voltage seems to be in a good range :sad2:
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We need full disclosure. Can you show us your working schematic?
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Steve, this is a project that I can only ask for help in a given area, because I took the Peavey chassis and kept main filter board.....wired a bridge rectifier that I bought here, to terminal strips....and wired up the power tubes like a twin reverb. All of that is good, and I replaced all caps on board, all that was good. but somewhere when I was trying to get bias right before moving on, I believe that the insulator under filter board may have slipped and I dont get B+ on standby switch or main 100 uf caps, but the 22uf caps had voltage on them lol, cause I felt it.
I cant do anything until I replace the rectifier and caps for now. I was working on bias circuit and that worked well, as I said, as well as the filament circuit. I just wanted everything to be as expected before moving on, and I wasnt getting DC voltages where I expected them, but tranny isnt hot so I suspect caps or rectifier.
here is original schematic
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Im just a little discouraged right now, but I believe the power tranny is good, im gonna put a smaller fuse in, and replace a few components before moving on.
This has turned into a scratch build, just using the Peavey filter board, and wiring up the output sockets like a twin reverb. and using the bias circuit from Dougs bassman layout. I got a respectable voltage range doing as you said, but need to get back to the same b+ voltages I had before what I think is the insulator coming out from under filter board while doing this. Unless my meter just sucks.
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Add that schematic with the bassman schematic, and thats where im at
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So I put it aside overnight to calm my frustration, and decided to test the power supply and this is what I have
AC secondarys ..385 volts AC
DC out of bridge rectifier.....385 volts DC
A , B, C, and D nodes all read about 525 volts
this is a schematic I drew showing all that is built thus far ( resistors and diode in power supply are from original amp)
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I think that -42v to -58v bias range will be fine. And you know how to tweak it if you need a different range.
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yes, thank you very much
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everything is good now, but I will have to find a way to make a better filter board before I fire this up again....the Peavey design leaves too much chance for something to touch ground now that ive changed the way the can is mounted.
I need to learn to make turret boards maybe
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this is a schematic I drew showing all that is built thus far ( resistors and diode in power supply are from original amp)
That's not how Doug has it drawn. He has the bias coming off of the wiper and you have it coming off of the top of the pot. Same bias voltage either way, but Doug's goes to the coldest bias upon wiper failure open and yours goes to no bias and serious red-plating.
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Referring to your schematic... I suggest you put a jumper between the pot wiper and the top terminal. Then you will be safe if the wiper fails.
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Doug's Plexi 6V6 and Plexi 50 could also benefit from a jumper between the pot wiper and the top terminal. In those, wiper failure leaves the bias circuit unloaded and it will charge up until the over voltage destroys the filter capacitors.
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:huh: Ahhh! I never noticed that until now.
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I drew that part wrong, because I have a free terminal on the pot like Dougs layout......is it safe for that dinky pot to unsolder it and add a jumper? and it would go from wiper to the unused terminal?
I build this stuff to further increase my knowledge, but every time you guys chime in, I realize how much I dont know still.
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There is no "unused" terminal on your schematic???
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there is on Dougs, and I said I drew that part wrong somehow.....sorry for that
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So how do you really have your pot wired?
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Sorry it took me so long......like this
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OK. Now put a jumper between the pot wiper and the unused pot lug. You'll sleep better. :wink:
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OK, im working on it now thanks Steve :)
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should that jumper change my negative voltage range?
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You aint put that jumper on yet? My, my...
You tell us if it changes anything.
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I did and it seemed to change the range, thats why I asked Sensai
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Well, you had -42v to -58v bias range before. What is it now?
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I read -36 to -50 , then I realized id been using my dim bulb tester,lol.
Now I just read -40 to -56.7
So I continue......I am going to order an eyelet filter board from Doug and make that area safer than the filter board that came with the amp. then ill sleep better at night.
Thank you again for helping me with the bias circuit, I never really paid enough attention to that part of the amps.
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My amp is built....sans the filter board , and I suspect that I really need to do it.....can I make a new build thread with pics and such or just keep it here.
I am using old tubes to test and only 2 output tubes for now. Current limiter is cool until I turn on standby and then I get a buzz through speaker and it seems to not dim down, I suspect that filter board.
I am not sure why the voltages are the same on all B+ nodes (unloaded) with the dropping resistors, and they are high, as far as im concerned,
Is there a possibility that I changed the phase from the original amp to this and need to just reverse the output transformer primaries?
I will look over everything to make sure all is wired up correctly cause ya never know, but im past the bias question and I can stay here or make a new thread on the build.
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I am not sure why the voltages are the same on all B+ nodes (unloaded) with the dropping resistors, and they are high, as far as im concerned,
Unloaded means no current flow. No current flow, no voltage drop. All B+ nodes will read about the same. That will change when you plug in the tubes.
Is there a possibility that I changed the phase from the original amp to this and need to just reverse the output transformer primaries?
That's always a possibility when you build an amp with NFB from the speaker. But you're not far enough along to know if that's a problem yet.
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ok, then with tubes plugged in, albeit only 2 output tubes, I cant test anything because of the buzzing through the speakers. I will delve into it tomorrow
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ok, then with tubes plugged in, albeit only 2 output tubes, I cant test anything because of the buzzing through the speakers. I will delve into it tomorrow
Maybe the NFB phase is wrong. You can reverse the OT plate leads to confirm. This is something you eventually need to do. Or, you can just disconnect the NFB loop and deal with NFB phase on down the road.
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OK, ill disconnect it tomorrow first......and ill post pics too, cause it did come out well. I still think ill redo the filter board, cause I want it as bullet proof as possible if I use it live
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So I unhooked the feedback wire from the speaker jack, and the buzz is gone, so I will have to reverse the primaries.
While looking everything over I noticed that the filament wire going to pin 4 of a preamp tube wasnt soldered in and fixwd it
I put in tubes ( a mix of 4 different 6L6 and 12ax7) to see if I could get the bias in the right area and listen for sound. While trying to measure for plate dissipation I noticed a 6L6 starting to red plate so I shut her down and looked over the socket and sure enought , the grid resistor going from pin 1 to 5, wasnt soldered to the socket, so I fixed that.
Then I measured the current going through each pair from the 1 ohm resistors, and they are way off from each other, like 90 ma in one pair and 65-70 in other pair......maybe from the mismatch of tubes? The tubes arent even 6L6GC, so I probably shouldnt even be testing with them, since plate voltage is still like 500 volts...so before shutting it down, I plugged in a guitar, and there isnt any sound, except the hiss from the amp itself, but im not too worried, since im pretty sure that with a guitar jack plugged in, I shouldnt be reading continuity between the tip and ground, so ill fix that next.
I am running out of solder lol.
Here are a few pics of it, I am almost there. :icon_biggrin:
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I think that I wont be able to try to get sound out of it until I figure out the power supply.....This amp puts 470 volts on the plates of the preamp tubes. 500 on the output tubes. Glad im using old spare tubes to test it with....If I kill them I will survive.....maybe I already did, ill test them in another amp.
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This amp puts 470 volts on the plates of the preamp tubes
did you verify the tubes were conducting by measuring Vdc across the cathode R's?
Do you have another tap besides the 385v? something around 280v with bridge will get you around 400 at the rail.
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I will check
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no other tap
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Got my 6L n 6V confused, but the original schematic shows like 390 at Pre tap, dropped through 200+ plate R should keep you close IF the preamp tubes are conducting, otherwise they WILL read high.
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So besides altering my power supply with bigger dropping resistors.....maybe thats why the Peavey schematic used larger plate resistors.....because of the huge B+ voltage of the power transformer
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So besides altering my power supply
if your pre tubes are conducting and you have 470 at plate, you probably ain't gonna get there from here. The peavey shows like 390vdc before the plate R's.
does your PT have a CT for the HV winding?
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nope, thats why a bridge
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> 368v AC between the red wires of the rectifier
This should give a bit over 500V DC at the first cap. You got this.
It will also give 470V-500V at the small tubes' plates WHEN the small tubes are not installed or their heaters are cold (not sucking).
So do you have small tubes that are glowing?? Or are they missing/dead?
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All three preamp tubes and all 4 output tubes filaments are working and glowing. 6.5 volts AC between all filament pins.
I have 500 volts on plates of 6L6, 497 or so on screens, 475 on phase inverter plates and 470 on preamp tube plates.
I am using the complete power supply from the Peavey...only the bias supply is different.
All I can think with my limited knowledge is that maybe the 6C10 could handle higher plate voltages ( havent looked this up tho ) , and maybe thats why the Peavey had larger plate resistors.
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Sounds like your preamp tubes are not conducting. Measure voltage on the cathodes of all your preamp tubes.
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ok, thats a possibility, cause they are all spare chinese tubes.....ill let yall know
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so I put meter on DC volts....black lead to ground and red lead to the pos side of all 12ax7 cathodes ....nothing.....was that right?think I got voltage on PI tube, but not on the preamp tubes
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....nothing.....was that right
depends on what you mean by right :icon_biggrin:
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
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I would suspect a missing ground wire if all cathodes read zero volts.
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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 :cussing:
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"nothing" is a bad meter.
Did it read ZERO?
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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
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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.
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Time to post a schematic.
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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
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Gotta say, it sounds backwards to make the schematic last. Sure glad my house plans were required up front. :icon_biggrin:
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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
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I will work on my backwards ways....promise
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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
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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.
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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. :icon_biggrin:
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LOL....I will post schematic of the preamp....im really having fun making them too :icon_biggrin:
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> 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:
New build problems? [url=http://el34world.com/Forum/index.php?topic=17701.0]Click here[/url].
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Thank you.....I hope that keeps me from asking dumb questions lol
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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
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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 :dontknow:
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
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Awesome, I still dont understand the use of different plate, and cathode resistors, but I hope to get there soon :icon_biggrin:
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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
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thank you, ill try to absorb it :worthy1:
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After seeing that nice preamp schematic I agree, that does look plexiesh! :icon_biggrin: 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.
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Actually, your November amp and 6v6 plexi schematics are the ones I used to make this preamp. :icon_biggrin:
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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 :icon_biggrin:
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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?
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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.
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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
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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.
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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
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....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! :think1:
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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 :icon_biggrin: