Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: HommeMarrBuckley on September 27, 2016, 10:23:06 pm
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Hello everybody,
I have a second Hammond A0-43 organ chassis with transformers and was going to build Steve's AB763 Deluxe Lite.
It's already underway but was wondering about adding a Master Volume. I tried googling adding a LARMAR style PPIMV to an AB763 layout and couldn't find much.
Anyone have a recommendation how I can add this to this circuit? I am somewhat dense. I could be off but it looks like it can be somewhat difficult/dangerous.
Just looking for general consensus---if it's been done frequently and successful, willing to give it a go,
but if not--and risks sound/safety, I have no problem nixing the notion and going forward without it.
Thanks in advance.
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I suggest you build the amp without it but plan to add it as a mod once the amp is up and running.
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The two .047 caps on the far left of the board.
Recommendations to stay with those or use .1?
Or won't really make a difference?
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I tried googling adding a LARMAR style PPIMV to an AB763 layout and couldn't find much.
Here you can find a well documented collection of mod for the AB763 that includes also a PPIMV
https://robrobinette.com/AB763_Modifications.htm (https://robrobinette.com/AB763_Modifications.htm)
Franco
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Robinettes info is priceless... great website .. nice person
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Robinettes info is priceless...
Rob may have the answers you seek as well.
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Been sidetracked a bit.
I need help.
Afraid to start setting up board and components without being sure.
So, in looking at Steve's schematic.
http://sluckeyamps.com/misc/AB763_Deluxe_Lite.pdf (http://sluckeyamps.com/misc/AB763_Deluxe_Lite.pdf)
(I feel dumb asking this because I think every amp has one but anyway) Does this amp have pre-amp and tone stack section?
If so, where on schematic, does that change over?
(I ground in sections and just trying to determine)
Would everything on V1-A to the left be considered pre-amp and all that follows to right (up to PI) be tone stack?
Would I then need to add another 20uf cap for the pre-amp and attach (+) that to R4 100k resistor?
Just saying because with the Plexi I finished, I grounded it differently so I needed to make adjustments to the build.
Like that schematic, I know this one is accurate but again I might have to alter what I do.
Thank you
PS I am going to add the MV mod with the push pull pot from Rob's site (Thanks Franco)
I did email him and, indeed, he's a good guy with a lot of info there.
Oh Steve, the pot that was in the Hammond (that I was thinking of using for bias) measured around 9k
Should that value have changed when I turned the pot while measuring?-because it stayed about 9k the whole time.
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Should that value have changed when I turned the pot while measuring?-because it stayed about 9k the whole time.
Resistance won't change if both meter leads are connected to the outside lugs.
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Does this amp have pre-amp and tone stack section?
Preamp is V1A, tone stack recovery stage is V1B, tone stack is in between, red box. Tone stack has 3 pots 'stacked' on top of each other in the schematic.
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I will check again on that pot Steve.
Thanks Brad.
Alright, so now.
Do you think I should add a 20uf cap with preamp section?
And then should I use the "D" 20uf cap for both tone stack and recovery sections?
OR
is the one 20uf cap like on schematic sufficient for these three parts? (I guess in a way also, is tone stack and tone stack recovery considered part of pre-amp section?)
Lastly, R14 270K is part of the PI, correct?
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In thinking about this,
I believe I should use the one filter cap as shown "D"
The 2nd shielded cable (by Volume pot) was throwing me off for whatever reason.
Am still alittle unsure but will ground all of those pre-amp, tone stack and recovery at one point at hi jack ground with bus
will take some accommodation, a good many points meeting there
So, prior post can be ignored,
But still leaves R14 270K-- that is part of PI? or "C" cap on schematic.
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But still leaves R14 270K-- that is part of PI? or "C" cap on schematic.
What does it say above C6, R13, R14? They are not part of the PI.
As far as adding another B+ filter cap node, you don't need it.
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Do you think I should add a 20uf cap with preamp section?
And then should I use the "D" 20uf cap for both tone stack and recovery sections?
OR
is the one 20uf cap like on schematic sufficient for these three parts?
It's not 3 parts, it's 2 gain stages and a tone shaping circuit. The tone stack doesn't need a filter cap, it's just a passive filter. It has no B+dcv going to it. Tubes that have B+dcv going to them need a filter cap.
(I guess in a way also, is tone stack and tone stack recovery considered part of pre-amp section?)
Kind of, the PI needs to see a certain acv (ac signal) so it can supply the output tube stage what they need to reach full output.
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Am still alittle unsure but will ground all of those pre-amp, tone stack and recovery at one point at hi jack ground with bus, will take some accommodation, a good many points meeting there
I like to do my grounding like this, D node B+ filter cap ground goes to this ground also because it's feeding the D node filter cap that feeds V1A and V1B.
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R14 is part of the preamp. If you look at the board layout you'll see that R14 is grounded with all the other preamp grounds.
If you build my board then these grounding questions will mostly be taken care of.
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Good info Brad. I will commit to memory but leads me to more questions but won't dog down this post with them.
I was planning on grounding it like you have diagrammed. You wouldn't include the Lo jack in that pre-amp ground?
Steve, I copied your board, I really should have pushed myself to just use the schematic and try it on my own but you already did the work so why not use it.
The Supro build I always mention will come and that will be my test.
How to test the range/wipe of that bias pot? One lead to pot lug and other lead to ground?
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You wouldn't include the Lo jack in that pre-amp ground?
Yes, Lo jack is grounded with Hi jack. (Where else would you ground it? Lo jack is right next to the Hi jack. Just forgot to draw it in.)
Steve, I copied your board, I really should have pushed myself to just use the schematic and try it on my own but you already did the work so why not use it.
Yes, use Sluckey's layout and grounding. :icon_biggrin:
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How to test the range/wipe of that bias pot?
Just like you would check any pot with 3 lugs. One meter lead on the center lug (wiper). The other meter lead on either outer lugs. Turn the pot shaft.
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Alright, cool. Will do.
Here's my progress so far, have some more layout work to do and eyelets, looking forward to weekend.
I must say Steve when I was mounting the PT, I was smiling as I was putting the brackets on. Memory lane.
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There have been several of these amps built. The layout works well. But through the years some questions keep popping up. Mostly about grounding. When I did this layout, it was for a forum member that had good building skills and knowledge of layouts. The main purpose was to provide a BOARD layout. So my docs were pretty vague on grounding. I've just revised the documents and I think the grounding scheme should now be obvious to anyone building this amp. There are plenty of other ways to do the grounding for this amp and I'm not saying this is the one and only way to do it. But if I were building this amp (and I never will) this is how I'd do it.
As always with my drawings, the layout is a full size board design intended to be used as a template to actually build the board and make all connections between the board and chassis mounted components. This is the major portion of an amp. But my layout does not show how to wire the PT, or OT, or some other power supply components. You will have to refer to the schematic to build a complete amp, unless you have this knowledge in your head.
Anyhow, I hope you will find the changes helpful and easy to understand. You may have to refresh/reload the page to see the changes.
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I tested the pot and it is consistent so I am going to use it.
In regards to the filter cap layout, since I am building this as the lite, I assume I should use the values listed on the second filter board?
So the F&T blue 32/32 uf cap im using would be overkill, as the plate/screen cap?
And just to be sure that's (2) 220k resistors on the 10ufs
and a 1k and 4.7k between the 22ufs?
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The first cap board is for 6V6 amps such as the Deluxe and Deluxe Reverb. The second board is for the bigger 6L6 amps. Since you are using AO-43 iron, I suggest using the first board values.
I would use that cap can if I had it. In fact, I'd use two of them if I had them. Even better, use Doug's JJ 40/20/20/20 can.
Ain't no 10µF caps on those cap boards. The 1K and 4.7K resistor values are for the 6L6 amps. The 6V6 amps use 10K and 10K.
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The first cap board is for 6V6 amps such as the Deluxe and Deluxe Reverb. The second board is for the bigger 6L6 amps. Since you are using AO-43 iron, I suggest using the first board values.
Thanks for clarifying that for me.
I would use that cap can if I had it.
Great, I had already it mounted so glad to hear that.
Ain't no 10µF caps on those cap boards.
Yeah my mistake, missed the extra 0
I have the hi/lo jacks set. That shouldn't pose an issue, should it? I am just going to build how you had prior with hi/lo jacks.
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I am just going to build how you had prior with hi/lo jacks.
That's fine. The reason I changed to a single input was it seems to fit the "Lite" theme better. If you have any trouble with the hi/lo jacks you can always look at Hoffman's Tube Amp Library of information to see how they should be wired.
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Is this what you mean?
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Yes, but I was actually thinking of the Switchcraft jacks just above those Cliff jacks.
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In determining my PT wires.
I set my meter to ohms (for all, I applied one clip to one wire and other clip to wire of matching color)
one clip on each black measured 3.6 ohms (being black have to be primary)
one on each brown .3 ohms (have somewhat matching striped center tap) thinking this is heater
one on each (I dunno color--but appear same color) 141.8 ohms assume this is secondary (there is a very dark green which I assume is CT for this)
last 2 of matching shade measure .2ohms assuming this is 5V without CT
Would my assumptions seem correct?
ADDENDUM
Hold that--I believe I had my centertaps ID'd wrong--I was wondering why I couldn't put my black lead on the CT to determine resistance
ADDENDUM2
Laugh
frustrating but anyway I was reading that those that have CT will have continuity with each other(so all 3 wires will).
So Primary are done and easy-black.
I believe I figured out my HV because all three have continuity and are high in ohms--one is striped so it's the centertap
Now my heaters should have CT and have found three that all have continuity and are dark in color---2 darkish red brown and one forest green
thing is I am getting .2 ohms
but when I got to remaining 2, only have continuity with each other and dont have CT like they shouldn't so they should be my 5V but they read slightly higher .3 ohms
I would think that the 5V would read .2 ohms and the heaters measure slightly above at .3 ohms but this isn't the case but has to be right based on continuity of the wires, does that sound right?
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Don't fret over 0.1Ω.
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> Don't fret over 0.1Ω.
+1.1
There's "ideal" values where all CTs are dead-center and all winding Rs scale exactly with load.
And then there are real transformers where a few tenths either way makes no difference. Normally the resistance is "very small" compared to load so "small" differences are really very-very small. OTOH practical matters suggest some "unbalance". Usually half the winding is wound *over* the other half. Since they have the same number of turns, but the outer half is longer wire, it will measure some percent more Ohms. Also you may calculate "Gauge #16.78 and #17.89" for 6V and 5V windings, but you can only buy whole-numbers (and mostly even-number) gauges, and fewer gauge-changes means faster work, so you throw #16 on both windings.
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> Don't fret over 0.1Ω.
+1.1
:laugh:
Yeah, didn't anticipate some of these issues today. I hadn't looked at these second set of transformers since I got them a long while ago and just assumed that they would look similar but the PT colors were very different. I marked them when I got them and for whatever reason, thought to recheck those values today only to cause myself more headache, however, I believe they were right first time around. oh well, better safe than sorry.
In looking at schematic, im trying to plan out the layout and I believe I have the standby wrong. I remember Steve saying the standby is between the rectifier and the plate cap but maybe it doesn't apply here. I have been looking at this to help me because I am using diodes.
http://www.lynx.bc.ca/~jc/63PwrSupply.gif (http://www.lynx.bc.ca/~jc/63PwrSupply.gif)
Does this look correct? I feel comfortable with the power cable, power and fuse. My 5V will goto the 47 lamp.
Lastly heaters on the 4,5,9 pins of the 6V6 and on 2,7 on the at7 and ax7
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Reading ohms on a transformer (with no accompanying datasheet) is a little bit of an art form until you have some experience with it. When you are reading .2 ohms on some winding...your first thought is probably "wow, that's a short ".
With experience, you will come to find out that typical 5 volt and 6.3 volt windings on a tranny have incredibly low resistance. HV winding, oh, maybe 35-150 ohms per side, the 120 winding can be 1-2-5 ohms. Unfortunately, ohms readings this low do not necessarily tell you that much other than "there's continuity", or, assuming the tranny is good, "these 3 wires are the HV because I get 40 ohms from this [striped wire] to one side and 47 ohms to the other side so probably the middle one is the middle one".
Another approach which may give you more confidence: With an extra 6 volts transformer, connect the 6 volts to the primary black-black. Sometimes you can use a junk wall wart for this *provided* you are sure the wall wart puts out AC (most don't) and, some small-small wall warts will not be able to handle the load even in a no-load situation. Use a junk wall wart you get at a yard sale for 50 cents, not something you need to power something you use. You could also tack-solder two wires to an amp chassis and steal his 6.3 volts temporarily. You could use a doorbell transformer. Some of those 12 volts, some are 18 volts. The idea is just a low voltage AC source that will power up your mystery transformer but NOT place 700-800 volts on the HV secondary wires flying around as you are trying to take measurements.
6 volts is about 1/20th of 120 volts. First thing, measure the actual volts out of the "source" transformer. Get the approximate ratio of those volts to your line voltage. an 18 volts transformer would be 18/120 = 15%. A 6 volt tranny would be 1/20th or 5%.
The idea is just to not have huge volts on loose wires flying around. A 400-0-400 winding (800 VCT) would still have 120 volts on it using an 18 volts powering transformer, but that's a far cry from 800; and better would be just your ordinary 6 volt tranny which would put 40 volts on your HV secondary.
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Lastly heaters on the 4,5,9 pins of the 6V6 and on 2,7 on the at7 and ax7
That's backwards.
Unless you want the #47 lamp for looks, LEDs are so much easier to use (and drill out for!)
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> your first thought is probably "wow, that's a short ".
Your "Ideal" power source IS a "short", with Voltage in it.
Practical numbers. A 6V 6A winding supports a 6V/6A= 1 Ohm load. If the winding could be ZERO Ohms, there would be NO voltage sag from zero to 6 Amps of load.
Said another way: if my house transformer and power line were ZERO Ohms, my lamps would not dim when the well-pump kicked in.
But getting ZERO Ohms takes Infinite money (or more). And a little sag does not hurt. House economics suggests 2% sag in transformer, 2% in the line, 2% in each inside circuit. Space is not a big issue. In a amp-size transformer, wire-space is more precious, we don't have pump dimming lights, and it is reasonable to have 10% or more sag.
That's counting both primary and secondary. Assume half in each winding, 5%; though with many secondaries it does get complicated.
A DeLuxe's 6V winding is 6V at 1.5A or 4 Ohms. 0.2 Ohms in secondary winding would give ~~5% sag just in the secondary. An honest 0.10 Ohms is "too good" but entirely satisfactory. OTOH sub-Ohm measurements are tough to get great precision, so I read the "0.1 Ohm" as a ballpark.
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Alright, I think I understand how to wire up the standby, rectifier and the caps.
I believe this to be correct---the jumpers and resistor around the 32/32 uf cap seems odd but believe it's right and unsure about bias tap but looks accurate in terms of schematic.
My new dilemma is I have a board identical to the layout, but I would like to separate the bias supply and grounds....
(R36 10k and Cap 17, 18 50uf)
From the PI cap "C" and ground.
(R15 100ohms)
so they can be grounded with their dedicated filter cap ground
I am trying to determine which jumper(s) to cut that won't mess up the flow of the circuit.
Any suggestions--many many thanks
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...I would like to separate the bias supply and grounds.... From the PI cap "C" and ground...
Why???
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...I would like to separate the bias supply and grounds.... From the PI cap "C" and ground...
Why???
I swear not trying to be difficult or question people. I just know with my Bassman build that I had my grounds lined up on the buss correctly and all all centertaps, bias, etc by PT like I saw on other builds and I got major hum. When I broke it down in sections, and followed accordingly, it became quiet. I am not saying I don't believe in other methods--I just believe in this one because it has worked for me and don't want to test others if this works. The Plexi I just built, had minor hum with normal vol and bright controls, until I cut one jumper--I wanted to put that on my post.
I cut the jumper between the .68uf cap/820ohm and the 22uf cap and then ran a wire from the 68uf cap/820ohm to meet the tone stack ground and it went away. It's quiet--but Im thinking of trying that .0047uf cap on the bright switch because it's not that noticeable with the 470pf. All hum went away but some very bare sounds--dunno like when stylus touches record--very faint---not noticeable enough that I care --sometimes when turn pot can hear--but then goes away when I stop and "set" -thinking maybe it's a solder joint--I dunno. Anyway, I really love that amp---especially with MV feature--but I wouldn't have it if you wouldn't have put it online--so im just trying to figure out how to adjust this--like I did before.
this is what I am thinking to do---I dunno if it will work--been trying to trace path of circuit.
Cut the jumper between the 47uf(50) cap and 100ohm resistor.
Ground 2 will be the bias ground and run a wire from 100ohm to "C" PI cap ground.
Does standby, rec and filter layout look correct?
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> separate the bias supply and grounds
The wire from PT through rectifier to first main cap is a total train wreck of HUGE current spikes. This must not be in the midst of other grounds or it will inject crap.
Grounding input jack to chassis and power cap to chassis is dubious. It can be done, without a buss. It is often better to buss and let either power-end or input end float off-chassis.
There are several well documented grounding scheems published. I don't know why this wheel needs re-re-invention. Or even mix/match between different schemes of differing philosophy.
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The wire from PT through rectifier to first main cap is a total train wreck of HUGE current spikes. This must not be in the midst of other grounds or it will inject crap.
I believe my plexi is like that and I don't hear that. Now, that you say that---I might relook to see if that ground has it's own dedicated point before it goes to the bus or if I have it connected to the bias, power tube grounds, etc before it goes to the end of the bus---that might the reason for my subtle, faint record player sounds.
There are several well documented grounding scheems published. I don't know why this wheel needs re-re-invention. Or even mix/match between different schemes of differing philosophy.
Not trying to re-re-invent anything--just trying to adapt (what I believe came from "Merlin" of whoever in terms of grounding sections on a bus, in order, to the jack) to this schematic/layout. It has worked for me so that's why I have gone with that scheme.
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...I would like to separate the bias supply and grounds.... From the PI cap "C" and ground...
Why???
I guess my question should have been, "Why...would YOU like to separate the bias supply and grounds.... From the PI cap "C" and ground?" You didn't answer that question yet.
I revised my drawing specifically for you, to clearly show how I would do the grounds in this amp. But you can change it any way you want. Merlin is a member here. Send him a PM and ask how he would do the grounds in this amp?
Does standby, rec and filter layout look correct?
That's not what I had in mind. I was thinking use one 32 cap for node A and the other for node B. OT would connect to cap A, choke between cap A and cap B, and screens to node B. This would be a clean, straightforward way to utilize that dual can.
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That's not what I had in mind. I was thinking use one 32 cap for node A and the other for node B. OT would connect to cap A, choke between cap A and cap B, and screens to node B. This would be a clean, straightforward way to utilize that dual can.
Dang. Ok--will think about that.
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Ok,
this correct for one can? seems odd that both standby lugs just go to one point.
I guess my question should have been, "Why...would YOU like to separate the bias supply and grounds.... From the PI cap "C" and ground?" You didn't answer that question yet.
I wanted to separate the bias ground because I am going to put it on end of the bus at same point with the power tube ground, plate/screen grounds, CTs, and then put the PI cap ground with the 100 ohm resistor ground on the same point on bus some distance after/before (however, you're looking at it) that point mentioned previously.
So, not separate bias and PI completely, just their grounds so I guess I will just cut that one jumper.
But I wouldn't have got anywhere if you hadn't answered all of my elementary questions, so I am grateful for your help. :icon_biggrin:
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seems odd that both standby lugs just go to one point.
Think about that for a while.
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:icon_biggrin:
How could I forget my own quote. Ventured up to post 29.
Ok--so rec to standby
other standby to A+, red OT, Choke and 220k resistor leg.
So much for starting this weekend--oh well, I feel better about this circuit though.
It's IPA time.
:d2:
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Alright, spent a good deal of the weekend on this.
Should put a love/hate list in ampbuilding but don't want to bore anyone.
Alright, so my current situation is this.
I have installed the MV (trainwreck 3) as Franco put a link (post #3 here) to Robinette's page.
I am using the 1M push/pull pot connected to the 220k resistors, however, I realize now that I probably should have twisted the wires-but that can be corrected.
Anyway, while working, I was thinking, how will I be able to bias this with this mod? (with MV on or off?)
Perhaps, this mod is not choice for this circuit, I dunno
I spent past couple hours looking for someone that as done this similar and with my limited knowledge Im at a loss.
I believe someone mentioned that if the pot fails, I risk doing damage to the output tubes.
I realize many have utilized different types of MVs but I just want to know if this will work? then biasing bit and if it's true that if the pot fails, the damage to the output tubes.
Thanks
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It is just my humble opinion but the amp that you are building will be a challenge if it is your first amp of this level. It might make the amp build less stressful for you and easier for those helping you to just concentrate on building it exactly like the plan first and then after you have it up and working good, document all your voltages and all, you can easily add a master volume, or experiment with other mods. If you start in the middle of the build changing capacitor sizes and adding things that are not on the stock build and then have trouble it seems like it may be harder to troubleshoot.
I never had a MV on any of the first 4-5 amps I built but then later I went back and added them on the amps I wanted them on.
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You must use decoupling caps like here
(http://i.imgur.com/xEdplwl.jpg)
Franco
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I just want to know if this will work? then biasing bit and if it's true that if the pot fails, the damage to the output tubes.
Yes, it will work. You will still bias the amp just as you would without that simple crossline MV. Nothing will happen to the tubes if the pot fails, unless the pot actually shorts to chassis.
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Alright, great to know.
I will follow previous post's advice and make sure it works first, then check out applying the mv.
Have a bit to go, but will post voltages later assuming startup goes well.
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Finished up wiring and hooked it up to the limiter.
It is pretty late but was pleased to see illumination. I need to get shorter spacers--ones I had were too long--the board needs to be elevated about 3/4 - 1" off chassis. The bottom edge of the board sits on top half of preamp tube pins (6-9)
Even with board elevated, might be tricky when I measure those voltages. Angling the meter leads in there.
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Alright,
with limiter hooked up and in operate position, I started checking dv voltages and all readings are low and off.
Plate cap had like 4.4 volts.
Im assuming my wiring is wrong or I have messed up the PT wires.
I don't think it's my PT wires, but who knows.
Here's how I have it wired
I used the 32/32 F&T cap as the plate cap
2ndaries go to rec board---from the board they goto a point and meet wire from standby lug and 1st (+) 32 plate cap.
Red OT and red choke meet and goto other standby lug.
2nd(+) of 32 plate cap meets 220k resistor
Should I have a jumper across both (+) lugs of plate cap?
I don't think that's it, but I dunno.
Or I could have my heaters and 2ndaries wrong---when I turn it on--I have pilot light but there is absolutely no light through lamp limiter bulb at any point during switching on which seemed kind of odd.
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Well, switched the secondaries with the heaters and put a jumper across the (+) lugs of plate cap.
Turned it on--then flicked standby---limiter bulb lit up (didn't previously)--then a bit of smoke--shut it off quickly.
so perhaps I had the secondaries and heaters correct in the first place.
I just don't know with standby and plate cap wiring.
It's wired how I wrote--just now with that jumper which shouldn't matter.
Considering scrapping this--learning the hard way---without experience should go with a proven build.
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Alright, haven't given up quite yet. I believe my plate filter cap wiring is off.
Just asking anyone for help here.
If you look at this schematic of the power supply and to far right (2 70uf and 2 220k resistors)--that's what I want to try and replicate with my cap can.
http://www.lynx.bc.ca/~jc/63PwrSupply.gif (http://www.lynx.bc.ca/~jc/63PwrSupply.gif)
Im using F&T 32/32 cap can as the plate cap and have one 220k resistor (but have extra if I need second)
Could someone write or scribble pic how I should wire my can to replicate that?
I know one (+) lug meets rectifier board and goes off to standby
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If you look at this schematic of the power supply and to far right (2 70uf and 2 220k resistors)--that's what I want to try and replicate with my cap can.
Can't be done using only that cap can.
We covered this earlier. Back up and read this whole thread again.
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Can't be done using only that cap can.
We covered this earlier. Back up and read this whole thread again.
You're referring to your last comment in post#38, correct?
I honestly was thinking about that but didn't realize way I intended was not possible.
Alright, I am going to try and put your comment for using the can as both plate and screen.
Thank you for pushing me in right direction.
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That's not what I had in mind. I was thinking use one 32 cap for node A and the other for node B. OT would connect to cap A, choke between cap A and cap B, and screens to node B. This would be a clean, straightforward way to utilize that dual can.
I follow
Can't be done using only that cap can.
We covered this earlier. Back up and read this whole thread again.
but now am not sure if you meant that I can't use that can solely as the plate cap or that I can't use it all
If I can use it:
1---where does my rec board (from diodes) line connect? go straight to standby?
2---then would wire from node A go to other standby lug?
3---Or can't use the can and go with 5 20uf caps? 2=plate 1 screen 1 PI 1 preamp
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Well,
After spending the last few hours unhooking the cap can and then creating a 2 20uf plate cap board and hooking it up---only to get dejected to see the lamp limiter bulb keep a dim light just like before--I didn't know what to do other than to look around.
I used Moogami shielded cable and it's very frustrating to use. The hot lead is so fragile and thin and tends to break so easy if you turn the cable. Things are hella close but couldn't see anything touching--lead to lead or to chassis. Then I looked at the power tubes thinking that I needed to have the 1 ohm resistors jumped from pin 8 to pin 1. That was a no but then I saw something. I forgot something. I didn't put in the 1.5k resistors across pins 1 and 5. :BangHead:
I can't believe it---If this solves the lamp limiter short--I will be more than ecstatic--but something tells me there will be something else--these pt wires--ugg but the lamp light is not that bright. If it does work----l might have to question reinstalling the cap can. Because it's better quality than the electrolytics I put in and they are in there super tight and close to many other components.
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........but the lamp light is not that bright.
There might not be anything wired wrong/shorting.
Depending on the the bulbs wattage and depending on how much current the amp draws the build will stay lit to some degree.
The lower the bulbs wattage and the more current the amp draws the brighter the bulb will glow even though the amp is wired correctly.
Try this, take a known running amp that is as close to the same power/wattage as the amp your building right now and use the light bulb limiter on it.
That way you'll get a feel for how bright the bulb stays after the 1st in rush current surge that charges the B+ filter caps and the heater current surge. The heaters draw something like 5 times(?) the current when they are still cold. As they heat up they draw less current until they settle in at their spec sheet current draw.
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In this case, there's something.
I let it run for a bit, but the limiter is lit more than it should be.
I was measuring the pre amp cap and got 439 volts and then I started smelling and then seeing smoke.
I shut it off quickly and saw that I fried the 470 ohm 1 watt resistor. (chopstick on right is pointing to it--grayish)
It's part of bias supply---on layout it goes to bias tap--I have it (purple wire) going to top half start of rectifier board (chopstick on left is pointing to it)
Any thoughts?
Also, in terms of wiring
red ot <---and --->choke ---------->standby lug
|> -----|> ----
\
rectifier ------X--> leg of 220k resistor ---> +20 uf cap ---> +20uf cap ----->standby lug
/
|> ----- |> ----
choke <---- B+ cap ----> pins 6 and 4 of V3 & V4
Down and tired but not out
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If it started to smoke then it's drawing too much current.
You could try this; take the power tubes out and disconnect that purple wire.
Now with the power tubes out power up the amp with the lamp limiter and see if the bulb goes dim.
If it goes dim then reconnect the purple -bias wire and work on the -bias supply. Something is pulling too much current through that 470R, you have to find it.
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I don't have the power or pre tubes in.
Was just going to check DC voltage with limiter at caps and tube pins.
:dontknow:
Wonder if I should add 2 more diodes to rec board?
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You can't use a 470Ω for that resistor because your PT does not have a bias tap. Replace it with a 100K 3W. Both of those bias caps are probably toast now from the several hundred volts negative voltage that they have been exposed to. They have certainly been stressed.
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I have a 2 watt 100k.
Should I hold out and get 3 watt or higher?
For the bias caps--I have some sprague 50uf/50v
Would that be satisfactory or need 100V or more?
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What is the voltage rating of the bias caps in the amp right now?
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2 47uf/160 V
Don't see any bubbles but what do I know.
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Try'em. They may still be good.
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AHHHH!!!! #$%$*#' RIGHT!!!!!!!!!!
No more lamp limiter issue!!! And numbers seem good! :headbang:
After using the Limiter--these numbers are from plugging right into the wall.
Caps V1 V2 V3 V4
Reservoir 454 Pin 1 445 447 Pin 3 453 453
Screen 447 Pin 6 446 446 Pin 4 452 452
PI 446 Pin 5 -26.3 -26.3
Pre 443
I believe my limiter staying lit issue was my prior filter cap layout. I did not have them properly in parallel and I was using a 220 k resistor that I believe that was not in order, as well. I should know better because I know that orientation and I don't know why I added extra nodes and strayed.
I went with (2)22uf for the reservoir caps which I believe equals about 44uf/900V capacitance and didn't use a resistor. The Robinette site is very good for this amp. I was reading it this morning. I lied to myself earlier (in the post) and have the MV put in so I am curious to hear how it is when I add tubes. I will go forward with biasing and to not exceed 12 watts for each 6v6. Time to clean up and and enjoy this is a bit. Will be fully satisfied when it's fully working.
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Ok turned on with tubes and then switched standby to operate and got Hellish loud shrilly/squeal. Quickly shut off.
Hopefully this is just OT issue.
Should I switch the OT on power tubes? or can I wire negative feedback resistor wire differently?
Steve, I remember you did something to your jack instead of reversing the OT wires--dunno if it can apply here, would be great because it's easier fix.
Bright green is NFB with green ot, then jumper--->Black OT
Will put NFB with black OT on tip, green OT on ring with jumper going to ground.
Believe this is correct.
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Ok, switched the NFB wire on the jack and turned on and no squealing so believe that is solved.
But at a standstill now, went forward with biasing.
I got 457 on pin 3 of V3.
Went to check mV on pin 8 and got 0 which seemed ok because bias pot was turned the whole way counterclock wise (to left).
Slowly started turning the bias pot and still nothing, 0mV, if full sweep is 100%, I turned it to about 30% and still got 0mV.
Heard a bit of tiny crackling. At start, power tubes were dimly lit, as I turned they got bit brighter, plugged guitar in but no sound.
I know I damaged that bias resistor prior---so nervous about continuing to turn the bias resistor if something else in that path got damaged.
Is there a way I can check bias supply to see if something else got fried?
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Ok--Update
I checked the bias pot and it continues to sweep to 9+K I checked the resistors around the bias on the board and they were correct. to further check the bias pot, I checked pin 5 on V3 and was -26, (bias pot completely to the left) I turned it a bit to the right and checked again and pin 5 was 25.2, so I know I have it set correctly for maximum negative DC.
So, I turned switchby on and checked the bias and it read 49mv :huh: so I sure didn't turn it right to increase!
I had guitar plugged in, so strummed it and sure enough--sound--but super, super loud, barely had guitar volume up and normal volume on amp was probably at like 2--but there was sound so good sign! Just didn't expect it because it was dead quiet.
Oddly enough, I didn't mess around too much for fear of damaging the output tubes--but they didn't appear to be super red or overheating--but shut it off.
So Im thinking I need to add a resistor somewhere in the bias supply or those 2 bias caps aren't fully functioning.
I know I am very close.
Going to take a break on this for a bit (but not for long :smiley: )---been at it almost non-stop for past two days and been living like caveman. Still will be in my head though because honestly believe I am so close.
Anyone's thoughts, suggestions, troubleshooting ideas?
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You need to be able to adjust the bias voltage on V3 pin 5 from about -25 up to about -40. If the max you can get is only -26 then you'll probably burn up those 6V6s.
That 100K resistor you put in the bias circuit is the range resistor. Lower the value to increase the voltage on V3 pin 5.
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I think I will try a 470Ω :l2:
Im thinking a 2W/10K to start.
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NO!!!!!!!!!!! Try an 82K.
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Ok, I have 82K but not in higher wattage.
Should I wait until I get a 3-5 watt?
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82K may not be what you end up with. May even need 68K, or 62K, or 56K, or 47K. So I'd even try a 1/2 watt. When you find one that gives you the range of voltage that I suggested, then consider getting a 1 watt, or 2 watt, or 3 watt to use permanently.
You do not need any tubes plugged in to determine the correct resistor. Heck, your 100K may already be what you need. You say you get -26v with the pot turned all the way to the left. What do you get with the pot turned all the way to the right?
Once you can adjust the volyage on V3 pin 5 between approx. -25 and -40, then put the tubes back in and set the bias pot for about 25mV at your bias test points. Then measure plate voltage and calculate idle power.
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You say you get -26v with the pot turned all the way to the left. What do you get with the pot turned all the way to the right?
Yes, I thought for some reason I had the Hammond pot wrong--but no, when I started to turn it right the negative voltage went down, -25 to -24 and so forth. I didn't think it was a good idea to continue turning it because I figured it would just continue to decrease.
I might check it again but everything that happened seemed that's the case. High mV bias reading, super loud. Only thing not sure---was the tubes, but then again I never saw tubes burn up either so perhaps they were getting hot. Just didn't wait around to see that possibility. I am using JJ 6v6S they each say 17/2.3 on box--guess that's something with matching--never read up on that.
Good to know I can do without the tubes in and I will experiment with different resistors.
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You say you get -26v with the pot turned all the way to the left. What do you get with the pot turned all the way to the right?
Please answer.
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It slowly went down as I turned it right----I don't understand, as I continue, you're saying it's going to reverse at some point and the negative DCV will increase?
But if you believe and say it won't do any damage, I will go ahead and turn it the whole way to the right.
The tubes are out.
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He want's to know the -bias dcv at 1 end of the -bias pots rotation AND the -bias dcv at the OTHER end of the bias pots rotation. In other words, he want's to know the full -bias dcv swing available from the bias pot set full up CW to full down CCW.
Please post BOTH -bias dcv's.
Remember to do this with the power tubes uninstalled/removed.
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You don't answer my questions. I don't answer your questions.
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Ok, so I believe with this full sweep measurement, this will help calculate the new range resistor.
full left -26.55
full right -14
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Do you have a proper range of bias voltage yet?
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I haven't tried to swap the bias resistor, yet. Will get to it, just had some late hours and came home with no energy. Hopefully, will have some time tonight, I dunno. I will start with 82k and report when finalized.
When youre done soldering, how much time you let cool/sit, before you power up. I usually give half hour or more if I can. Just curious.
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Immediately! Sometimes I don't even turn the power off.
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Alright, Here's what I got with the 1/2 watt resistors I had on hand.
V3 Pin 5
82k -31.3
68k -35.7
56k -43.4
Go with 56k? or Perhaps I should use a 62k? Im thinking 62k might get it closer to -40
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Looking for a range of voltage when you turn the pot.
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Alright,
So, I should have been looking at the range.
Am I taking that reading on pin 5 or the 1 ohms? Or doesn't matter?
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On pin 5, and looking for a range of -25 to -40
56k -23.09 - 43.4
68k -18.9 - -36
Didn't bother with the 82k because of initial -31.3 reading
I just wish I had a 62k to check.
Thinking maybe to order 2-3 watt 62k.
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56K looks pretty good to me. That should let you set the test points to a safe 25mA and go up or down as needed.
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Alright, sounds like a plan, awesome!
Will update when I get ahold of them. Can't wait to install and put tubes back in and give this thing a whirl. Good vibes coming.
I was also wrong--this has been done
http://el34world.com/Forum/index.php?topic=20460.0 (http://el34world.com/Forum/index.php?topic=20460.0) (great audio and musicianship)
http://el34world.com/Forum/index.php?topic=9891.0 (http://el34world.com/Forum/index.php?topic=9891.0)
Perhaps more, but what I have discovered in working on this.
Only thing I am not sure is--I had my reservoir cap wiring all messed up--which wasn't getting me anywhere---but wanted to know If I could I have used the cap can wired in parallel as the reservoir cap?
Thanks
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I was also wrong--this has been done
What are you talking about?
but wanted to know If I could I have used the cap can wired in parallel as the reservoir cap?
Absolutely. But you can't wire that can in series like you asked about.
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What are you talking about?
I was whining about trying to do something above my comprehension---it was all there but I couldn't figure it out and couldn't find something to reference. It wasn't until I saw that euro version and started to look at Fender layouts that I realized what I was doing wrong.
But you can't wire that can in series like you asked about.
Yes, that makes sense now. I was just thinking of putting it back in place but couldn't remember why I asked about in series but I just remembered it was related to that link that showed 70uf in series. The robinette site mentioned not to exceed 60uf capacitance for the reservoir so regardless, the can in parallel would not be wise and in series would not work and be too low anyway. Just have to use it in another build.
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The robinette site mentioned not to exceed 60uf capacitance for the reservoir
That only applies if you have a rectifier tube. You can use any value cap you want with silicon diodes like you have. Marshall is fond of using dual 50µF cap cans tied in parallel for their filter caps. Your dual 32µF can will be fine. Look at this...
http://el34world.com/charts/Schematics/files/marshall/Marshall_JCM800_2204_Lead_Series.pdf (http://el34world.com/charts/Schematics/files/marshall/Marshall_JCM800_2204_Lead_Series.pdf)
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Well, my resistors arrived today so I installed the 56K and put the tubes in.
Here are the results:
V3 Pin3
442V x 21mV = static dissipation wattage of 9.282
V3 Pin3
429V x 21mV = idling static wattage of 9.009
And it is working!.... :grin:
but this thing is really loud---I mean I could not put the volume past 2 or 3
I couldn't get to any grittiness or any kind of breakup because it was so LOUD.
Also, the Master Volume doesn't work--if this mod applies I would have to attribute it to my wiring.
I think the Master Volume pot wiring is ok but Im thinking about the pot wires and where they meet the 220K resistors on the board.
Im having them connecting (on layout) at top of 220Ks where bias wiper meets as well.
I believe I need to have them connect at other end of 220Ks where they hook up with .1 coupling? caps.
When I compare what I have to that mod, that's one thing I see that I believe to have done wrong.
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Awwwwww Yeah!!!!!
Well, that was it. Connected to the other end of the 220Ks and rechecked bias just to make sure.
432 x 22.5 = 9.72
425 x 22.5 = 9.56
Push Pull pot down--No MV
Push Pull pot out--MV works!
Pretty cool--so, of course, I cranked the regular volume to about 8 and just slightly put MV bit past 1 and it was totally rad.
Great amp for bedroom or to play out. Can't say enough about the master volumes in these past two amps--plexi and this.
Now, on to make a cab to house this.
Many thanks Steve and forum members! :m11
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Hehehe, alright! Got her up and running. :blob8: :happy1: :bravo1:
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Congratulations! Everybody needs an AB763. And now you are the 'go to guy' for AO-43 conversions. :worthy1: