Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: gar13 on January 08, 2008, 01:58:07 pm
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Anyone remember this post???:
Souelle:
"Slash used #36 to record AFD, not #39... The mod on #36 have been
made by Frank Levi and it was based on the #39 1959T Super Tremolo
Mod made by Tim Caswell.
#36 is a Standard 1959 Superlead with an extra pre-amp gain stage.
An additional (fourth) preamp tube was mounted in a hole drilled next
to the original (three) pre-amp tubes.
#39 have a switch to turn the mod on and off, #36 don't have that
switch, the Mod is always "on".
According to a post on the LP Forum, the usual Frank Levi Mod is :
Input jack to first stage - 34k resistance (the two stock 68k's in
parallel), or a 33k will do.
First stage cathode - 3.3k bypassed with 330mfd.
Out from first stage, through .022 into a 1 meg pot. Bypass the pot with
a .001 cap. Out from pot through 470k bypassed with a 570p cap, on to
second stage.
Second stage cathode - 2.7k bypassed by .68mfd (you'll notice this is
the stock 1st stage values - you can save some time/trouble by just
switching the wires on the first two stages).
Out from 2nd stage, through .022, into 1 meg pot (use one of the now
extraneous input jack holes). Bypass pot with a .005. Out from pot
through a 470k bypassed with a 500p, on to third stage.
Third stage cathode - 3.3k bypassed by a 330mfd.
Change tone circuit cap from 500p to 250p.
Everything else - leave as stock."
I gave this a try, and man did the amp screech and howl at first. After fixing some solder joints and fixing a wiring error in my tone stack (that must have been why the TS never worked so well on the JCM), I've got it working, sort of.....
It buzzes something fierce, even without the guitar plugged in. Some of the buzzing stops with the first preamp tube is pulled. (see below for the other cause of buzz). I've used Hoffman style ground buss for the preamp, only with cliff type jacks. I have a separate ground lug tied just below the jacks to the chassis.
It's freakin loud. I can't even but both volumes on 1 without feeling like it's on max. I've triple checked the volume wiring and it seems to be correct, and if I turn one volume off and turn the other up, the buzzing does increase (to afraid to do this with both), so I think the volume controls are working correctly, well at least wired correctly.
Without the speaker plugged in, the PT is humming. What causes the PT to Hum? I read that magnetic flux coupling (or something like that) can cause this but my OT is mounted at least 4" away and laminations are at right angles.
The voltages are a bit low for a marshall (430v on the plates). I did salvage this PT from an old organ, which worked great in the stock JCM circuit, but started buzzing after the mods.
Anyone have advice for taming noise in high gain amps? I was thinking of addive a PPIMV, DC filament or DC elevated preamp heaters. Any other advice?
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> Without the speaker plugged in, the PT is humming. ... The voltages are a bit low
You are pulling WAY too much current.
Why?
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Thanx, I'll double check everything for shorts, but I fired it up on a current limiter and the amp was ok. I'll try another transformer too.
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Trannie must have been pooched. Replaced it and things got way better. Old one hummed even when the secondaries were disconnected completely. Volume pots started working too, although I have no idea why.
I installed the proper feedback and presence circuit too for a 1959 PA, which decreased the feedback resistor from 100k to 47k, and I'm sure that had a lot to do with the extra gain. I didn't realize it was different until I looked at Doug's plexi layout, man they're easier to read than marshal schematics.
The amp is quite a bit closer, most of the hum is gone, but I've got some major squealing going on if I turn the volume up past 4 or so. Oh well, enough for tonite....
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Hello gar13.
Im Andrés. Just saw this post a couple of days ago so I decide to register and ask you how did you finally go with this project.
I found somewhere else Levi mods and the cascade seems to be done very close to what you post at the top.
Some values changed, nothing more.
So how did you end up with this??? Its sound OK now?
I believe that if you use proper shield it, shouldnt get noisy and squeal.
Saludos!
Andrés
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I just heard about this thread, but if anyone is still interested, here's my $.02. I built this circuit, and I
included it as one of the available circuits in my #39 amp, but I don't believe this is what was in #36.
It is overly gainy, with too much gain in the low frequencies. Rolling off the low end by backing off the 2 treble-bypassed gain controls makes it semi-usable, but it doesn't sound like what's on AFD to me. I had to go to extreme lengths to keep this from oscillating, and even so the gain controls need to be backed off
alot. pdburnette has a layout on Photobucket that is very close to my original #39. It is much easier to get to work, and I think sounds much closer to what is on AFD.
Cheers,
Tim Caswell
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Hello Mr. Caswell,
Very honored to have you post here, thank you for your comments.
I experienced the same results you mention, and I believe I had to add some tweaks to remove the oscillation. Really couldn't get that AFD tone. from this amp. The amp has sat on a shelf since then. I planned on harvesting the trannies for a plexi build at some point. The information was from another member and I thought I'd give it I try, but had no way to confirm it.
I found the layout on photobucket with a google search, thank you for the tip.
Dave
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I just heard about this thread, but if anyone is still interested, here's my $.02. I built this circuit, and I
included it as one of the available circuits in my #39 amp, but I don't believe this is what was in #36.
It is overly gainy, with too much gain in the low frequencies. Rolling off the low end by backing off the 2 treble-bypassed gain controls makes it semi-usable, but it doesn't sound like what's on AFD to me. I had to go to extreme lengths to keep this from oscillating, and even so the gain controls need to be backed off
alot. pdburnette has a layout on Photobucket that is very close to my original #39. It is much easier to get to work, and I think sounds much closer to what is on AFD.
Cheers,
Tim Caswell
Thanks for your post---lots of buzz around about these amps lately---- the info about the overly gainy response is very helpful...................gldtp99
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oh sweet...we got Tim Caswell? nice....
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I drew up a schematic of the preamp (without the switching) from the link I found on google. If I have offended by posting this, please let me know and I will remove it.
Interest in the amp is building since Marshall and Slash announced at NAMM this year that Marshall will issue a clone this amp sometime this year. They have set up an entire site around its construction. www.afd100.com (http://www.afd100.com)
This might be my next build, although I only have the trannies to do a 50w build :sad:
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You may be interested in this thread...
http://www.el34world.com/Forum/index.php?topic=8470.msg76508#msg76508
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there is a better caswell type mod that has 4 gain stages but unbypassed 10k's on two of the gain stages. I really like the motorized 39 at the NAMM. they were demoing an isolation box.
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Thanks for the kind words, guys. I guess the cat's kinda outa the bag on this, so no use cryin'.
That schematic looks good. I don't remember if the treble cap was 250 or 500p, and some of the 1959Ts I've seen have a .68uf bypass on the 820 cathode resistor in the tone driver, and some don't - I can't remember if #39 had it or not. Those choices are available as parts of the presets on my new amp. As for the AFD100,
John Rymas had loaned Slash one of my modded amps for nearly a year. Might it have made a trip to England?
TC
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I have built something like this last year when I created a lower wattage replacement for an old JCM800 that I used to own. It uses 6V6's but had and extra gain stage like that schematic shows. I had to end up making a voltage divider or two and especially in that first position's output (actually I have two seperate gain controls) not have the 470k bypassed w/ the 500pf. Too many of those bypass caps keeps too much high end early in the high gian circuitry and creates oscillations and/or parasitic issues. Not to mention way to much hissing background noise when the amp isn't being played or played very loudly. Not a very musical or wanted thing. Sorry, I'm not at home to reference the schematic I made but the amp can be dimed w/any & all gains and it pumps great sizzling shrieks of terror, lol w/out any issues. I really think that there's too many bypassing caps on that schematic though that's leading to all of those issues, IMHO. Don't just add them because someone originally drew it up that way. I did the same but found out otherwise. I hope this helps?
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> the cat's kinda outa the bag on this, so no use cryin'.
There's no magic in that schematic. Just over-the-top gain.
Everybody's done that.
So why is your #39 so special?
I suspect (irrationally) that some of the "sound" is in the HANDS that built it.
Maybe kharma. Maybe skin-sweat, even blood. The hifi nuts make far more outrageous irrational claims about sonics.
(Or maybe fine details of layout....)
There are other "#39 inspired" amps. But you can say that only the ones you sell have your personal involvement.
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Hi PRR, thanks for the good word. I'm familiar with you from other forums, and I have great respect for your knowledge, and your generosity with it. I'm not making any claims for the original #39 except that I did build it, and it was used on a lot of records and tours in the mid 80s and into the 90s (I saw it last at SIR LA around '95). It was high gain at the time, but by today's standards, it is actually rather tame. My goal with it was to keep the basic tonality of the 2203, but add some extra gain and sustain, especially on the high strings. That's what players were asking me for, and it's what I wanted for myself. As for the #36 specs that are floating around, I don't believe they are correct. I've built that circuit, and it's
a bear to make work. Even when it is marginally tamed, it doesn't sound to me like what is on AFD. If you must build it, use lots of shielded cable (connect the shield of the input cable to the plate of the 1st stage, float the shield at the input jack, put the 68K resistor at the grid end of the cable), and put
499 ohm resistors in series with the 330 uf cathode bypass caps on the 1st and 3rd stages. Leave out one of the treble-bypass caps on one of the gain controls (like I did on the George Lynch heads), tap B+
off the tone driver tube thru a 10k 1 watt resistor with a 47 UF/450v filter cap to run the extra gain stage,
and it will kind of work if you back off both gain controls. It's kind of fun, but I don't think it's particularly Marshall (or AFD) sounding. And remember, a great amp tone is inspiring to the player, but a great player will sound good thru anything!
TC
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My goal with it was to keep the basic tonality of the 2203
What an honor to have a renowned builder on our forum.
PRR is very respected on this and many other forums.
I have for the last 3 to 4 years tried to get rid of the " ice pick" type tones from the higher strings when playing solo's with the 2203 circuit.
when I adjust the tone stack slope R it gets to much bass and not enough mids for normal rythem playing.
so,
what's the trick to getting rid of the "ice pick" pierceing highs from the 2203?
i have already removed 470pf cap from gain pot and treble peaking 470R / 470pf network in the 2nd stage. i have tried adjusting the tone stack slope R.
all to no avail.
so please excuse my butting in on this thread, but this is a issue I would really like to resolve and would really honor and respect the suggestions of someone as highly respected as Mr. timcas......
It's a pleasure to have so much talent and knowlege to help us less educated indaviduals get our projects up and going.
many Thanks to all of you here.
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I don't mean to but in if you're waiting for someone else (timcas) but w/ little modification using low parts count and retaining much of the essence of your tone stack, I would try raising the slope resistor to 47k, lowering high end 470/500pf cap down to 330pf at first, then 250pf down to as much as 200 (but this will also lower some mids and/or high mids slightly getting down this low) and then also changing your mid pot from 25k to 50k to help regain & boost the mids you're talking about you'd like to retain from the other two changes. You need to do all three at the same time and doing one will change the reaction of the tone stack's response. Again, slope resistor from 33k to 47k, 470pf cap to 330pf, and mid pot to 50k.
There's other things to try before modifying your tone stack also such as smoothing caps on your plate resistors or maybe better yet put sm value caps across the plate to cathode, etc...as these will take off high end also w/out affecting your "tone" noticeably. Try 100pf first up to 1000pf or more using the lowest value that makes you happy and works. Keep doubling the value each time going up and when you go too far then back up you'll have it perfect.
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to tame the highs in a 2203 i have used bypass caps on the anoderesistors of the first 2 gainstages and it worked really well. i think it was 150pF
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Good advice on the tone stack and on the plate-bypass caps. I forgot to mention that in my post about
taming the circuit that started this thread. I used a .0047 cap across the 100k plate resistor of the extra gain stage, but maybe smaller caps on the 1st 3 stages would be better, especially in a lower gain circuit where you can get it to clean up by backing off your guitar volume control. In the 3 gain-stage circuit described at the start of this thread, it's not gonna clean up unless you switch out a couple of stages.
TC
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Thank you for the advice guys :smiley:
I thought about trying the snubber/smoothy caps after reading another post on the forum. I will try this out tonight and post my results.
just to clear things up a bit. This amp is a bassman 100 converted to a 2203. The only thing bassman is the clean channel, chasis and transformers. i have changed everything to 2203 specs with the ability to run 6l6's or EL34's. there's a certain quality of tone that i like with the 6l6's, but love the Britts mid bark EL34 overdrive.
sorry to have been intrusive on this post.
I very excited to hear the new AFD!
might even try the #39 circuit, high gain land
Thanks again and we or I truely am happy to have you here Mr. Caswell
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I have been studying the design differences between 6l6 and el34 power stages when used in the 2203 circuit.
now, my concern is that when using 6l6's I get piercing high end or "ice pick" sounds with the higher strings on the fret board (b and e strings above the 12th fret) as mentioned above.
could this be from the fact that the G1 R max value on the 6L6's is 100K?
with the 220K Bias R's + the 5K6 grid R's well exceeds the max G1 R value of the 6l6's.
could this be the cause of the piercing high end responce?
the smoothy/snubber R's do tame high end, but also alter overall tone of the amplifier.
I would like to keep the 6l6 option on the amp, but i dont like the snubber effect on overall tone.
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It was high gain at the time, but by today's standards, it is actually rather tame. My goal with it was to keep the basic tonality of the 2203, but add some extra gain and sustain, especially on the high strings.
Glad to have you here, Tim. It's always great to have people with first hand knowledge, and thank you so much for sharing it.
I have built a very similar circuit into my 1959 SLP Reissue. The only difference from the one posted above is that the gain control is between stage 1 and stage 2 and not between stage 2 and stage 3. I did not use a bright cap over the gain control - it really does not need one :grin:.Nevertheless, the sound matches exactly the above description. An awesome rock rhythm sound. I am still tweaking it a bit for more mids to get the single notes to sing better. My idea is to increase the treble peakers from 470pf to 1000pf.
Cheers Stephan
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the smoothy/snubber R's do tame high end, but also alter overall tone of the amplifier.
I would like to keep the 6l6 option on the amp, but i dont like the snubber effect on overall tone.
This is why I don't like using them unless other options are explored first. Did you try the plate to cathode caps? These affect tone less as I said earlier. Lastly, did you try the tone stack mod? This will help too and not affect the tone directly also.
I am still tweaking it a bit for more mids to get the single notes to sing better. My idea is to increase the treble peakers from 470pf to 1000pf.
You might try increasing that mid pot value, it will help & you can keep everything else the same. At halfway setting it will be like your mide pot full up, at 1/4 setting it will be your old 1/2 way setting, but over halfway you'll be extending your mids. All else will be exactly as it is/was.
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> with the 220K Bias R's + the 5K6 grid R's well exceeds the max G1 R value of the 6l6's. could this be the cause of the piercing high end responce?
No.
It may cause a small percent of 6L6 to red-plate and melt-down.
However when biased well below maximum dissipation, "most" fix-bias 6L6 are fine with 220K+5K, as shown by millions of Fenders alive and well.
The Marshall amp is bright. The 6L6 is hard. Maybe the two together are just too much?
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The Marshall amp is bright. The 6L6 is hard. Maybe the two together are just too much?
The 2204's with 5881's don't get as piercing as my 2203/bassman does.
I do agree that the 6l6 is more evenly pronounced through out the frequency range and EL34's tend to be more mid range centered tone wise.
when I switch over to the clean channel the 6l6's have a clean bell like chime that the EL34's just don't seem to get.
but,
the EL34's have a very distinctive overdrive on the 2203 channel that the 6l6's just don't seem to get.
I load the amp with the best matched set for the particular genre being played. (blues and jazz stuff 6l6's, rock metal stuff el34's)
I just wish I could tame the high piercing notes I get when using the 6l6's without loosing the "sparkel" off the high end.
the clean channel is just an extra 12ax7 with the bassman normal channel circuit and does not pierce my ears.
Maybe some type of lead switch that when engaged drops the high end responce?
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I am still tweaking it a bit for more mids to get the single notes to sing better. My idea is to increase the treble peakers from 470pf to 1000pf.
You might try increasing that mid pot value, it will help & you can keep everything else the same. At halfway setting it will be like your mide pot full up, at 1/4 setting it will be your old 1/2 way setting, but over halfway you'll be extending your mids. All else will be exactly as it is/was.
Thanks for the tip. Actually I keep the mid pot below the half setting anyway because when you turn up the mids you get also more treble. I already countered this by adding a 1000pf cap between the mid pot wiper and ground (something I took from the Diezel VH-4). Still the tone is very bright at high mid control settings so I need to find another way to fatten up the tone.
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The Marshall amp is bright. The 6L6 is hard. Maybe the two together are just too much?
The 2204's with 5881's don't get as piercing as my 2203/bassman does.
I do agree that the 6l6 is more evenly pronounced through out the frequency range and EL34's tend to be more mid range centered tone wise.
when I switch over to the clean channel the 6l6's have a clean bell like chime that the EL34's just don't seem to get.
but,
the EL34's have a very distinctive overdrive on the 2203 channel that the 6l6's just don't seem to get.
I load the amp with the best matched set for the particular genre being played. (blues and jazz stuff 6l6's, rock metal stuff el34's)
I just wish I could tame the high piercing notes I get when using the 6l6's without loosing the "sparkel" off the high end.
the clean channel is just an extra 12ax7 with the bassman normal channel circuit and does not pierce my ears.
Maybe some type of lead switch that when engaged drops the high end responce?
You must be over driving those power tubes? May be just the 6L6's your using.
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Thanks for the tip. Actually I keep the mid pot below the half setting anyway because when you turn up the mids you get also more treble. I already countered this by adding a 1000pf cap between the mid pot wiper and ground (something I took from the Diezel VH-4). Still the tone is very bright at high mid control settings so I need to find another way to fatten up the tone.
This is the kind of "amp tweaking" fun that is highly enjoyable & rewarding for me & many others on this site too. I can think of so many ways to "tweak the tone" but it's a little hard to do over emails or forum responses as it takes a number of questions/answers to narrow down things that can be done so much more quickly in real time.
The thing that comes to mind if I were wanting your description would be a "tweed mod" to the tone stack or rather bypassing the stack altogether. This would give an immediate gain increase w/ mids galore and would work great for a "lead switch". It could be w/ a push/pull pot, standard toggle, or be foot switchable. All it takes is a little filter network consisting of a coupling cap follow by a resistor & cap in parallel or just a plain resistor after the CC as high frequencies hate resistance and you'd be in high gain heaven w/ increased mids w/out the highs that you desire plus the added benefit of more volume & signal boost punch over the normal settings. I use a filter network such as this because if you straight bypass the stack you'll get way too much bass which muddies the tone up too much at high vol levels and it does tame the gain just slightly which you need too from overdriving the next stage too much sometimes depending on the circuit. Use values that make you happy and you're there in your own "solo-singing" heaven.
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Thanks for the tip. Actually I keep the mid pot below the half setting anyway because when you turn up the mids you get also more treble. I already countered this by adding a 1000pf cap between the mid pot wiper and ground (something I took from the Diezel VH-4). Still the tone is very bright at high mid control settings so I need to find another way to fatten up the tone.
The thing that comes to mind if I were wanting your description would be a "tweed mod" to the tone stack or rather bypassing the stack altogether. This would give an immediate gain increase w/ mids galore and would work great for a "lead switch". It could be w/ a push/pull pot, standard toggle, or be foot switchable. All it takes is a little filter network consisting of a coupling cap follow by a resistor & cap in parallel or just a plain resistor after the CC as high frequencies hate resistance and you'd be in high gain heaven w/ increased mids w/out the highs that you desire plus the added benefit of more volume & signal boost punch over the normal settings. I use a filter network such as this because if you straight bypass the stack you'll get way too much bass which muddies the tone up too much at high vol levels and it does tame the gain just slightly which you need too from overdriving the next stage too much sometimes depending on the circuit. Use values that make you happy and you're there in your own "solo-singing" heaven.
That sounds very interesting indeed. I believe this is similar to the bump section in the HOSO, only that it is fixed and not adjustable.
Meanwhile I replaced the treble peakers between the 2nd and 3rd and between the 3rd and 4th stages with 1000pf and I'm there. More mids without losing any clarity. The midrange control is more effective, too.
I also replaced the presence cap with a 0.68uf. Much better control than the 0.1uf to my ears.
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That sounds very interesting indeed. I believe this is similar to the bump section in the HOSO, only that it is fixed and not adjustable.
Meanwhile I replaced the treble peakers between the 2nd and 3rd and between the 3rd and 4th stages with 1000pf and I'm there. More mids without losing any clarity. The midrange control is more effective, too.
I also replaced the presence cap with a 0.68uf. Much better control than the 0.1uf to my ears.
Glad to hear you got it the way you like now. I also like to use a higher than standard value for the presence pot...
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Hey guys I found the cure!
I put the bassman 100's deep switch in the 2203 circuit just like fender had it in the stock bassmans circuit!
It works great.
I have a Marshall JCM 800 2204 with EL34's on the bench right now and I have been doing many a/b compairisons between it and the bassman/2203 conversion. I must say that the Marshall 2204 does have a nice tone and sings well, but it can be a litttle piercing.
The bassman conversion can go places the 2204 can't :smiley:
I contribute that to the other mods on the 2203 conversion though.
As far as true marshall crunch and mid range bark the, 2203 conversion sounds just like the marshall to my ears, but with the mods I have done (tube efx loop, 1st 3 stages cathode bypassed, treble peak caps removed, 2-channels bassman clean and 2203 dirty, EL34 or 6L6 switchable bias)
It gets much fatter and lush overdrive with much better harmonics!
the the deep switch gives it more of a mid shifted tone than the 2204 which is just enough shift lower to kill the "ice pick" highs and still retain the pronounced "mid bark" that Marshall is known for.
good stuff guy's thanks for all your help!!!!!!
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Here is my take on the #39 circuit. As mentioned earlier I built it into my 1959 SLP RI on a new turret board. I changed the stock output transformer to a Marstran C-1998 Dagnall clone.
I included two mini toggles mounted into two of the input holes. One adds a 0.68uf cap in parallel to the 10k cathode resistor of the third gain stage. I had the same arrangement for the 10k cathode resistor of the second gain stage but I greatly prefer to add a 10k resistor instead. This provides for a slight gain and low end boost that does not add as much noise as a bypass cap.
What can I say - the amp just freakin' rocks! No further tweaking needed.
Cheers Stephan
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DBM - does R9 really need to be there? On the presence control is the 4.7k there to lower the 25k pot's resistance down instead of using a 100k NFB & 5k pot? Lastly, the .68uF cap there instead of the usual .1uF - what difference do you hear w/ this?
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DBM - does R9 really need to be there?
It's not necessary for the circuit to function. I put it in because I read that it was part of the #39 circuit.
On the presence control is the 4.7k there to lower the 25k pot's resistance down instead of using a 100k NFB & 5k pot?
Correct.
Lastly, the .68uF cap there instead of the usual .1uF - what difference do you hear w/ this?
The .1uf affects only very high frequencies. The .68uf affects also the midrange. Since I have a 47k feedback resistor I thought the .1uf made the sound very shrill. On the other hand, the amp gets very loud with the .68uf when the presence control is fully turned up.
Cheers Stephan
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i built this afd amp and it is by far the best amp i have ever heard. i made some minor tweaks to it to tame it. i have 5 stages of gain plus extra gain on tap with the 4 cathode switches and no noise. all in all my amp turned out pretty quiet. i just ordered the weber switching relay's so i can foot switch the amp. although i still have some minor tweaking to do it, im really happy with the way this amp turned out. if anybody wants to know how i built mine or what i changed from the layout, just ask me.
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You should just assume that we all want to know the details and post at will :wink:
And preferably with pictures. Top-shelf magazines aren't just about the articles, right?
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here are some pics from my afd build.
let me know if you see anything wrong.
this amp rocks and oh my god is it loud!
thanx to all who have helped me build this thing.
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Looks great. :smiley:
What are all the knobs on the back for? Ways to control the beast?
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thanx for the comment, it was my first build. but as far as the knobs on the back: one is a presence, one is a resonance, and the other 2 are gain pots for the extra stage's. its also got some cathode switches and a tone stack switch on the back as well.
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Do you find yourself making adjustments to those extra pots while playing or do you tend to leave them alone once you've got it dialed in?
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i tend to just max the gains leave them alone, when i first built it i was using 1meg pots and it just squealed like a pig( had it way out of wack) so i used 500k pots and that put it back in spec with no squeal. but im getting ready to work on it again so i think i might just try to use a fixed resistor setting instead. because if i turn them down it affects the whole gain of the amp, so really i have 3 master gains when all the stages are active and i dont need that, so i need to fix that.
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So when we're saying AFD amplifier what are we saying? Which songs on the album had this amp utilized according to the story?
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So when we're saying AFD amplifier what are we saying? Which songs on the album had this amp utilized according to the story?
All of them but the most characteristic would be "Welcome to the Jungle" and "Nighttrain".
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Am I the only one who thinks it's dangerous to attach the input jacks shield wire to the plate?
If something happens and the shield and core short you'll have 200V+ at the tip of your guitar cable!
Wouldn't it be a good idea to put a cap between the input jack and the 33K resistor to block the DC just in case?
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Am I the only one who thinks it's dangerous to attach the input jacks shield wire to the plate?
You're not alone. I don't like connecting the shield to any voltage potential other than chassis or signal ground. I especially don't like this circuit because of the possibility of putting plate voltage on the guitar pickups. I realize this is a very high gain circuit, but if it's important to have the small amount of hi freq negative feedback provided by connecting the shield to the plate, just put a 47pF cap between the plate and grid and connect the shield to the input jack ground lug so the shield can do what it's supposed to do.
There's another case where I've seen the shield connected to the negative bias supply when putting a MV pot in a fixed bias amp. I don't like that either.
Even an experienced tech could get a nasty surprise with these 'hot' shields.
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If you want to make it safer then put a small cap between the plate and grid anything from 4pf to 47pf should work.
INSTEAD of shield to plate, not in addition to
1 more point this mod and any mods are for techs only, who know how to do them.
Like I have said on the schematic only try this if you are a tech.
That's good advise, but my main concern is even if wired properly by the best tech in the world, you're putting all your faith in the fact that the hot and shield of the shielded wire will never short.(manufacture's defect, user abuse?)
By the way what is the rating of typical shielded wire? Full out you'll have your full B+ on the shield.
It probally will never fail but that'll hurt like hell if it does.
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possibility of putting plate voltage on the guitar pickups.
Right, then you burn out your pickups so your guitar doesn't work so you unplug it now you're holding 200V+. You touch the tip with your thumb to see if you hear a bzzzt bzzzt through the amp. Zap!
Tone is good, safety's better.
47pf cap: $.70
Not getting one hell of a zap: priceless.
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... put a 47pF cap between the plate and grid and connect the shield to the input jack ground lug so the shield can do what it's supposed to do.
If this cap shorts you will also have the B+ in the guitar pickup. I would rather put it between the plate and cathode or across the plate resistor.
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Does putting it across the plate and cathode or across plate resister do the same thing though thats what Im wondering. sorry I know another stupid rookie question but I dont know . :angel
Thanks Bill
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Does putting it across the plate and cathode or across plate resister do the same thing though thats what Im wondering. sorry I know another stupid rookie question but I dont know . :angel
Thanks Bill
Sort of. Across the plate and cathode, it's passing the ac signal to ground through the cathode resistor and by pass cap. Across the plate resistor, it's not allowing the ac signal to develop across the plate resistor in the chosen frequencies that cap effects. The end result is the same.
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here are some pics from my afd build.
let me know if you see anything wrong.
this amp rocks and oh my god is it loud!
thanx to all who have helped me build this thing.
Rockgod212,
Do you have the most recent schematic for the amp build? I am looking to build my own and cannot access the Metroamp forum.
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It's been 6 years since Rockgod212 has logged onto the forum.
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I built DarkBlueMurder's (DBM) version of a #39 and that amp is fantastic, built a second with same results and onto a 3rd for myself because I can't stop smiling whenever I play the amp I sold to my friend.
It's a Marshall JCM800 2203 the 100w version with extra gain stage and cathode bypass capacitors switching for boost. I also add Resonance/Depth pot to get some bass back in.
Mark
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Thanks for sharing Mark.
If nothing else, it made me go back and save that schematic and look it over.
Kinda reminds me of Bogner's Ecstacy red channel preamp topology...how do your preamp voltages compare to these?
http://el34world.com/charts/Schematics/files/bogner/bogner_ecstacy.pdf (http://el34world.com/charts/Schematics/files/bogner/bogner_ecstacy.pdf)
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Thanks for sharing Mark.
If nothing else, it made me go back and save that schematic and look it over.
Kinda reminds me of Bogner's Ecstacy red channel preamp topology...how do your preamp voltages compare to these?
http://el34world.com/charts/Schematics/files/bogner/bogner_ecstacy.pdf (http://el34world.com/charts/Schematics/files/bogner/bogner_ecstacy.pdf)
Sorry I checked to see if I had a voltage chart on my two builds and do not. When I complete number 3 I will save a voltage chart and post.