Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jaymz77 on August 04, 2016, 11:14:18 pm
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Hello fellow amp builders. It's been a while since I posted but here goes...
I am building myself a 50w "Brown Eye".
The original amp that I reverse engineered and the schematic I have shows the use of a transformer with 3 secondaries plus the heater taps. The HT, obviously, a tube rectifier pair which is used for the relay supplies and the bias pair.
However the 50w PT that I have only has 2 secondaries plus heater taps. HT, and a 13v pair.
Should I be using that 13v pair for the bias supply or should I take the bias supply from one side of the HT before the rectifier?
I could then either use the heaters to supply the relays or use the 13v supply as well.
All help is much appreciated.
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... the 50w PT that I have only has 2 secondaries plus heater taps. HT, and a 13v pair. ...
The "13v pair" means 7.5v-0-7.5v (13vac total)? Or 13v-0-13v (26vac total)?
I guess it probably doesn't matter, since 26vac * 1.414 = ~36vdc before filtering, and you'll need -40 to -50v of bias for your output tubes.
... The original amp that I reverse engineered and the schematic I have shows the use of a transformer with 3 secondaries plus the heater taps. The HT, obviously, a tube rectifier pair which is used for the relay supplies and the bias pair. ...
You'll be doing without the tube rectifier in your version?
Also, it seems unlikely the 5vac winding powered the relay supply; more likely the 6.3vac heater winding did. The rectifier's 5vac winding will float up near full B+ voltage. As well, switching 5v relays usually requires starting with somewhat more than 5vac (often 6.3vac), rectifying and filtering (maybe regulating) down to 5vdc.
... should I take the bias supply from one side of the HT before the rectifier? ...
If you're convinced the power supply and output stage of the amp your trying to copy is substantially like a Marshall, you could copy the way many Marshall amps derive the output tube bias from the high voltage winding. Be prepared to be wrong about particulars and need to adjust the bias supply before installing output tubes once the build is complete.
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Thanks for the reply.
I should clarify. The PT is 350-0-350, 3.15-0-3.15 and 0-13 on the secondaries. The 13v tap has no centre tap. I'm not sure the exact specs of the pt in the 100w schematic other than it has an extra, non centre tapped secondary that feeds the bias circuit.
Therin lies the problem. Without knowing the voltage of the bias tap on the original pt I don't know how to supply my bias circuit.
To me, the obvious solution is to redesign the bias circuit to resemble any ht fed Marshall style bias circuit. I just thought I would throw it out there to see what those more knowledgeable than me had to say.
I can post the schematic if need be. I don't think it's a big secret. If I can get it off the net, others could too.
Cheers
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Just use the Marshall style bias circuit. It's simple, reliable, and easy to tweak for the voltage you need.
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Exactly what voltage do I need for 2 x el34's?
Cheers
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An adjustable range of 30 to 45 should do it. Look at Hoffman's Plexi 50 schematic.
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Exactly what voltage do I need for 2 x el34's?
EDIT: Sluckey beat me to it. :icon_biggrin:
That depends on your B+ and screen voltage.
You want your bias supply to be able to dial-in as much as -(Screen Volts * 0.1). So if your screens are at 450vdc, it would be good to get as much as -45v of bias (or more) to one end of the bias pot range, and somewhat less (-30v) at the other end.
A bias of ~10% of screen voltage should be able to drop output tube current to very low values, and assures you won't run the tubes too hot.
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Thanks guys. Much appreciated
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Ok, i need the help of the brains trust again...
I turned the amp on for the first time last night and got instant red plating of the output tubes.
My first guess is the bias circuit that i had to modify above.
I need to work out what the bias output to the tubes is on the original amp with a supply voltage of 100v into the bias circuit.
Then i need to work out how to achieve that same output with an input voltage of 350v (the B+ i had to use due to there being no dedicated bias tap)
I would really appreciate some assistance with this math if anyone is willing to help.
The second guess would be some issue with the OT. And to be honest, I'm thinking this may be a possibility as well.
I desoldered the OT primaries and measured between them and to the CT.
Red-CT=35ohms, White-CT=40ohms, Red-White=40ohms.
This is clearly an issue when the readings should be in the Megohm range, correct?
The OT issues is something i will have to take up with the manufacturer, but if anyone can chime in with some bias circuit assistance i would be much appreciative.
Cheers all.
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Maybe you can find interesting these links
https://robrobinette.com/Tube_Bias_Calculator.htm (https://robrobinette.com/Tube_Bias_Calculator.htm)
http://www.tedweber.com/webervst/tubes1/calcbias.htm (http://www.tedweber.com/webervst/tubes1/calcbias.htm)
Ciao
Franco
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I need to work out what the bias output to the tubes is on the original amp with a supply voltage of 100v into the bias circuit.
Then i need to work out how to achieve that same output with an input voltage of 350v (the B+ i had to use due to there being no dedicated bias tap)
We already told you what range of bias voltage you need. But you cannot use B+ to develop the negative bias. The input for your bias circuit needs to be one of the Power Transformer's HT 350VAC leads. Build the bias circuit exactly like the attached pic. Remove the power tubes to check the bias circuit by measuring the negative voltage on pin 5 of the output tubes. Set the bias pot for maximum negative voltage on pin 5 of the output tubes. DON'T PUT THE POWER TUBES IN UNTIL YOU HAVE GOOD BIAS VOLTAGE ON PIN 5 OF BOTH OUTPUT TUBES.
There is no math involved at this point.
Red-CT=35ohms, White-CT=40ohms, Red-White=40ohms.
This is clearly an issue when the readings should be in the Megohm range, correct?
OT primary resistance readings are typically a couple hundred ohms. Definitely not in the Megohm range. Lower than 100Ω may be OK for a big OT. You should get a reading from CT to either plate lead that are approx. equal. The plate to plate reading should be approx. twice the reading of the ct to plate readings. You can check these readings in circuit. Recheck yours.
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Ok,
I did say b+ but I have used the HT.
There is no need for the "I told you so's", I did build it exactly as I was previously told (can't see any attached pics) and IT DID NOT WORK. I have used a typical Marshall 50w bias circuit taken from one side of the HT.
So I would say it's time for some math.
I need to know what comes out at the point between R57 and R58 when 100v is applied to R59.
Thanks.
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Which schematic uses those resistor numbers?
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Sorry,
I thought I had posted a diagram above.
It's the BE100, but I'm making a BE50.
cheers
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And I got the numbers wrong too
The bias out to the tubes is between 56 and 57.
Bias supply comes into 58.
Cheers
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I'm starting to lean towards the floating centre tap of the PT secondary's.
The BE100 uses this setup. But comparing some 50w vs 100w drawings, I don't see any 50w amps using this setup.
I believe this is to raise the B+. Perhaps it is too high for the 50w.
This is probably a rookie error, but this whole exercise has been conducted to learn as I go.
Cheers
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The 100W uses a bridge rectifier and the 50W uses the center tapped full wave rectifier. You cannot interchange those with the same transformer.
The bias supply for the 50W would use a 220K resistor coming off of the HT tap.
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Thanks for that.
So I need to change my rectifier AND remove the floating centre tap setup?
Or just fix the rectifier?
Thanks heaps
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If you have a 350-0-350 transformer, you should have the centre tap grounded and diodes on each HT leg going to your B+. A 220K resistor comes off of one HT leg to feed the bias supply.
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There is no need for the "I told you so's",
I was not trying to be a smart ass with my answer. But you asked the same question that you asked previously and had already received a valid answer.
Here's the pic I forgot to post...
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So I need to change my rectifier AND remove the floating centre tap setup?
Or just fix the rectifier?
No, you can't just change your rectifier set up or just lift the CT. It will drastically change the B+dcv.
What PT do you have?
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Thank you very much.
I did do what everyone suggested above and changed the bias circuit.
However I obviously need to change more than just the bias circuit. I had that feeling all along TBH. I thought I needed to do something with those filter caps, but didn't even consider the rectifier.
Willabe, it's a Heyboer HTS11249/ HWPVT2/ DET 4939 thing...
Thanks again
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Well I'm pleased to report to all that assisted that the issue seems to be resolved.
I revised the power supply to be full wave rectified, not bridge rectified, along with the necessary changes to the bias circuit and I have no more red plating and my plate voltage is a respectable 464v with 37.7a of bias current.
Now on to the next issue... All the buzzing and noise. It's weird because one channel is fine, one is noisy and one is worse. I'm not sure which is which channel tho right now. The clean channel is the noisy one, It's either the BE or the HBE channel that is worse, the other one is fine. First guess, bad earth somewhere. If I need help I'll star a new topic for that one.
Anyway, thank you everyone for their help. You guys rock.
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Correction: 37.7mA
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Hello all... yes i need help...again.
A few months ago i undertook the build of a BE50. Not having a 50w schematic i have built it based on the 100w schematic i was able to find.
I posted a while ago that i was having issues with biasing and red plating output tubes. Anyway, long story short, the brains trust in this forum helped me figure out that i needed to change the the rectifier setup from a bridge to a full wave, as well as the necessary changes to the bias circuit. As advised by this forum, i followed a 1987 power section. Problem solved. However, i then had unwanted noise.
From what i can tell there is some sort of preamp noise as well as an earth hum.
I then compared the BE100 and 1987 schematics closer and removed a lot of other components that the BE100 has but the 1987 doesnt. This had no effect anyway.
My dad and i thought then that it was a PT problem because you could hear the hum just by standing next to it when it was on (not through speaker), so we tried running and earth wire to the screen on the PT, no change. I then got a new power transformer. While i was wiring it in i thought i may as well put back in all the extra components that i previously removed, turn it on... no change and an apparent waste of money. :BangHead:
I decided to tackle the earth hum first.
I thought it might be a building earth issue so i tried lifting the chassis earth (yes i know it is unsafe, however i am an electrician and i know how to work safely with electricity) and it has no effect.
I thought it might be an induced hum from the heaters so i rewired them with tighter twists as well as tried DC elevation, no change. Ive since figured out that the BE100 has dc elevation but into it already and some of the components i removed and then replaced were part of that circuit, at least thats what i think.
I have pulled the preamp tubes one by one and the earth hum is definitely in the output section or power supply section. I have also tried using different power tubes. The ones i have in it are old ones that i keep for testing purposes, no change. The preamp tubes are new.
I am at a bit of a loss right now, also its getting late and I'm tired so i am probably not thinking clearly or making much sense. i apologise for that.
I really need some ideas of what to look at next. Maybe there is more to this 100w to 50w conversion than i first thought. :dontknow: I thought it would be as simple as mating a 50w power section and power supply to the preamp section. Maybe not.
I feel like I'm forgetting some things i have done, but as i said, I'm tired. Actually, one thing with the preamp noise, when i plug a lead into the input the noise gets worse. I just touched the tip to see if there was any signal getting through, which there is. However if i touch the shield or any other metal part while the cable is plugged in the amp seems to shit itself momentarily and no signal gets through at all, only noise, but then goes back to normal (some signal plus noise) after 5 seconds or so. Strange.
I would really appreciate some input from those much smarter than me. I am at that point where i can't think and am possible running around in circles.
Ive attached the 100w schematic as well as the layout i created. Keep in mind that the power supply section is unchanged in the layout, it represents what is on the schematic.
Thanks in advance.
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No love for me?
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First thought that comes to mind- Is your relay ground isolated from the preamp ground?
edit- Looking more closely at the layout diagram vs the schematic, the schematic definitely shows a regulated and separate ground based relay control circuit. Is that how you wired the amp? Did the transformer have a 12 volt tap that you used for the relay power?
silverfox.
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Hi Silverfox, thanks for the reply.
Yes i am using a seperate tap on the PT for the relay supply. Its is actually 13v ac that gets rectified and regulated to 12v.
The relay side of things is wired as per the layout, which my old man (a retired engineer) helped me develop from the schematic. Doesn't mean there can't possibly be mistakes though...
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Something i just thought of,
the earth hum does not change. I can remove the preamp tubes and the hum is the same. Something i didn't mention in my original post is that i tried disconnecting the OT primaries and listening for magnetic coupling of the OT and PT. This i did through my dummy load, which i am using for testing, connected to the speaker out that has an auxiliary speaker out that i plug a set of headphones into. Essentially i performed the "headphone trick" that is well documented for positioning of the transformers on the chassis. No hum. So that hum must be coming from the power supply or the output section.
However the preamp noise is at its best on the clean channel, which is activated by one of the relays in its energised state.
The other 2 channels are about the same, noise wise, and one of them is activated (not sure which, BE or HBE) when both relays are in their normally closed position, i.e., the coils aren't charged.
Cheers
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**UPDATE**
Your comment about the relay circuit got me thinking so I just disconnected the relay supply to see what would happen.
As expected, the earth hum didn't change a bit. However the preamp noise, with no lead plugged in, was probably 90% better. But when i plugged a lead in it was about the same.
This is quite the head scratcher... It seems to me, as i said in the original post, that there at least 2 seperate issues, but they seem to be interacting with each other somehow.
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As expected, the earth hum didn't change a bit.
Can you determine hum frequency? that will help which rabbit hole to focus on?
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As I said above it's 50 Hz
Cheers
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Sorry, hope that didn't come across rude.
I'm just extremely frustrated. I am about ready to throw this thing off a cliff.
Nothing I have done makes one iota of difference.
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It could be related to the wiring in your house or shop. Try it in a different room that's on a different circuit.
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Please read my earlier posts
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As I said above it's 50 Hz
Cheers
50Hz would have to be associated with the line. Maybe filaments, transformer radiating coupling, signal wires too close to power transformer wires, etc. This should not be associated with 100Hz power supply hum, unless you have a half wave supply such as bias supply. And there is plenty of 50Hz noise in the air, especially if you have fluorescent lighting.
How did you determine it is actually 50Hz? You never mentioned 50Hz in this thread until this post.
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You're right. I must have chopped that bit out when I wAs editing that original rambling post so it made sense. Sorry.
I know it 50 Hz hum because I am an electrician of 20 years and have heard it many times before. I also have the second opinion of my dad who is an engineer of 40 years. Not trying to be a smart ass.
Its unmistakable.
I do have fluorescent lights in my garage but I've tried turning them off numerous times with no effect.
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I can pull the preamp tubes and turn the amp and standby on and then listen as the hum comes in as the tubes warm up. I have tried different tubes too.
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Can you actually measure the frequency with a dmm or scope connected to the speaker jack?
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I don't have a scope. I going to have to buy one soon.
I'll try with a dmm when I get up. I'm currently laying in bed on Saturday morning our time.
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I suddenly get this feeling that when I do measure it I'll make a fool of myself and my 20 years experience...
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I can't get a reading from the speaker jack using a dmm.
Funnily enough I'm getting 100hz on the OT primaries
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That's certainly not the point of asking you to measure. I've worked around 60Hz for 50 years. But rarely did I actually listen to it. I don't trust my ears either. But I've put a lot of faith in results measured with a scope or dmm. In fact, I've certified airway systems based on what I see on a scope or dmm for 40 years. I'm oriented to what I can measure with calibrated test equipment. What I hear is very subjective and I believe that's true with most people. Even if someone has perfect pitch, that doesn't mean much to me, unless they have a current calibration sticker on their head. :icon_biggrin:
This is not a personal attack on you. I don't know what you hear. Or what anyone else hears. But I trust good quality test equipment. Heck, for this kind of stuff, I even trust cheap test equipment most of the time. Many DMMs today are capable of displaying frequency. I've used my Fluke 87 to measure frequency as low as 2Hz and as high as 100Kz. Hopefully your dmm can do this also. It doesn't have to be accurate to the gnat's ass. But if it measures frequency, surely it will let you know for sure if it is measuring 50Hz or 100Hz. Hopefully it will reinforce what you hear.
You have a problem, maybe a couple unrelated problems, that can be difficult to diagnose in an amp such as you have. And since it has never worked right, there are a lot of possibilities. As said earlier, knowing the frequency will determine which rabbit to chase.
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No no no, none of it was or is taken personally. Its the Aussie way to put sh#t on ourselves, thats what i was doing after my bold "20 years experience" comment.
50% of the time i go out on a limb and make statements like that and make bold statements i end up making a fool of myself. This may be one of those times.
And i dig your comment about a calibration sticker because i am a quality assurance manager who looks after my companies equipment calibration, among other things..
I have a Fluke 117 and thats is what i used to measure. I DO need to get a scope.
It was reading about 3hz then jumping to 11hz on the speaker jack. I got about 99.7 on the primary.
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i end up making a fool of myself.
ahh, but that's how we gain wisdom :icon_biggrin:
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Too true Shooter
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you could hear the hum just by standing next to it when it was on (not through speaker),
with ALL tubes pulled do you still hear the hum?
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OK, so do you think we may be chasing a 100Hz rabbit? :icon_biggrin:
If so, the usual suspects would be power supply filter caps, or output tubes. I'm basing this on your earlier statement that pulling the preamp tubes did not affect the hum. Can you verify that the current in your output tubes is equal for each side of the OT? IOW, are the tubes really balanced?
Also, what happens to the hum if you pull a tube from each side of the OT and just operate with two tubes?
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you could hear the hum just by standing next to it when it was on (not through speaker),with ALL tubes pulled do you still hear the hum? I haven't pulled the output tubes with the amp on as i thought that would damage the OT.
OK, so do you think we may be chasing a 100Hz rabbit? Its starting to look that way isn't it?
Can you verify that the current in your output tubes is equal for each side of the OT? IOW, are the tubes really balanced? I will check the pins of each output tube and report.
Also, what happens to the hum if you pull a tube from each side of the OT and just operate with two tubes? Remember that i building a 50w amp from 100w drawings.
I have a lot of great, high res gut shots of an original BE100 and i was having a look over them and comparing to the schematic.
I did notice some clear discrepancies between the schem and the pictures, particularly in relation to the grounding scheme.
The schem shows the input stage all going to ground 1, and (from memory) the next stages (and pots) going to grounds 3 and 4. But my pics show the input grounded to the bus rail across the back of the pots. And instead of half the pots going to ground 3 and the other half going to ground 4, they are all tied together with a common bus rail.
Not sure what, if any, effect that will have
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ok, tube pin voltages from 1-8,
Its worth noting that pin 6 is used as a connection point for pin 4 and the resistor.
V5
0, 30v (DC elevated i believe), 450v, 448v, -35v, 452v, 30v, 0
V6
0, 30v, 451v, 445v, -35v, 451v, 30v, 0
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i thought that would damage the OT.
the source for the OT is the '34s, so it's doing nothing with tubes out
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I can confirm that i can hear the PT humming with ALL tubes removed.
Its not super loud but i can hear it.
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Its not super loud but i can hear it.
is it the same "loudness" as with tubes in? probably wrong rabbit hole since your fuse isn't blowing, follow Steve, i'll watch :laugh:
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Yeah i would say so.
If there is any difference it is so borderline that its hard to say 100%
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It would be nice if you could provide a schematic of your actual build.
Since you eliminated the preamp tubes as the source of hum, we need to concentrate on power supply and power amp. The voltages you measured on the output tubes don't tell anything about the current flowing thru them. Put a 1Ω resistor between cathode and ground on each power tube. Then measure the voltage across that 1Ω resistor. It will be less than 1 volt so set your meter to read millivolts. What reading do you get on each tube?
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Unfortunately i don't have a schem of the actual build.
All i can say is that it is the same except for the power supply section. I have ended up with a 1987 power supply up until the choke, then after the choke it is per 100w schem.
I don't actually know if i have any 1ohm resistors. Ill check my stash and if i do, ill do what you said. I could always put my DMM in line i guess... I do have clamp probes for the dmm.
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OK,
both output tube cathodes level off at about 46mA, but will very slowly creep up to about 51mA, at which point i stopped measuring after a couple of mins.
If i max the master they will jump to about 55mA.
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It is absolutely KILLING me that NOTHING i do changes ANYTHING
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troubleshooting is kinda a meditative thing, the more frustrated you get, the more gremlins smell it and invade! hang in there, you have 40yrs to catch up :icon_biggrin:
I believe your readings so far are close, but wait for conformation. Could you get a freq reading after the choke, anywhere in the PS, post choke.
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EDIT: Also, this is probably a wild HARE, but can you pull R53 or R52 (per schematic you referenced), then put the 34's in n power up, hum still there?
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Pulled r53, hum still there. Preamp noise gone tho, but no signal either.
Ive never been a good fault finder, i get frustrated too easily. Even when constructing a high rise building, if i have problems my tendency is to rip it out and start again.
I just read a thing on the Aiken page that said the OT secondary common should not be grounded to the chassis, so i tried lifting that so it only went to the sleeve of the speaker jack and the hum became worse.
I don't believe i should be able to hear the PT at all. Even if i rip it out and sit it on my bench, wire up the primaries and leave the secondaries hanging loose i can still hear the PT humming.
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Sometimes PT just hum. Laminations may be loose and are vibrating. Try tightening the bolts that hold the PT together. If that won't stop or minimize the humming, then there's probably nothing you can do about it.
Is the PT hum as loud as the hum we are chasing?
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Even if i rip it out and sit it on my bench, wire up the primaries and leave the secondaries hanging loose i can still hear the PT humming.
so now, is it a problem or normal?
Preamp noise gone tho, but no signal either.
Removing the R, removed power to the PRE.
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Sluckey, sitting on the bench? No but it sounds the same. When i hear it through my dummy load with headphones, it sounds louder.
Shooter, it hasn't changed. I just said that to make the point that there is inherent hum in the PT. I actually haven't done that with this PT, i did it with the previous one. But when i wired the new one in the problems were the same, so I'm assuming this one would do the same thing.
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> troubleshooting is kinda a meditative thing
Zen and the Art of Motorcycle Maintenance (https://en.wikipedia.org/wiki/Zen_and_the_Art_of_Motorcycle_Maintenance)
https://www.amazon.com/dp/B0026772N8 (https://www.amazon.com/dp/B0026772N8)
www.arvindguptatoys.com/arvindgupta/zen-motorcycle.pdf (http://www.arvindguptatoys.com/arvindgupta/zen-motorcycle.pdf)
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Ha ha ha,how did you know I'm a bike man too?
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I'm starting to think I might have to redo the whole layout
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there were 3 books, (well 4 counting the Bible), that had a major effect on my life's path, Zen and the art of motorcycle maintenance, Siddhartha, and hitchhikers guide to the galaxy.
I might have to redo the whole layout
don't look at the amp as a whole, look at it as the sum of it's parts. The focus has been on the PS, based on hum. I think, (needs conformation) your DC part is good, same with the DC part of the power amp section. there's still the AC part, the connections part, the values part, yadeda
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Well, it's still humming away.
What do I do?
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>
www.arvindguptatoys.com/arvindgupta/zen-motorcycle.pdf (http://www.arvindguptatoys.com/arvindgupta/zen-motorcycle.pdf)
Dangerous and provocative information of a type one only discovers from life if they choose to be aware of self and surroundings.
silverfox.
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Anyone got got anything other than zen for me?
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i followed a 1987 power section. Problem solved. However, i then had unwanted noise.
I wasn't watching to close for this part. Did/do you have any guitar sound?
how loud is the current hum?, could you say put the guitar on 5 and make the hum go away? have you built a listening amp?, maybe we can figure out what parts are good. with PA tubes in only, did you measure AC volts on your power rail? (normal should be a few VAC after the reciter, going down to mV AC by the final tap.)
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I can get some guitar signal through it, but it's not great. That is a different problem ken I think.
The hum does not change with volume. The preamp noise gets worse with a lead plugged in.
What do you mean by a "listening amp"?.
I have just sat down at my desk at work on Monday morning. I'll try measuring the ac on the power rail tonight.
I'm actually thinking about buying a 1987x. Obviously to use but also for reference material. Ive never actually owned a Marshall yet I'm trying to clone them...
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Ken? Bloody auto correct
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What do you mean by a "listening amp"?.
since you don't have a scope, a listening amp will let you walk through the signal path to find out where bad things start. do a search here and you'll find how to make one. that should nail down your preamp noise.
with all tubes in a complete DC and AC voltage readings from each PS tap will help isolate the HUM.
Ken? Bloody auto correct
??
hang in there
dave
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Cheers mate.
I'm going to buy myself a cheap scope to make this and future things easier to locate.
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Ok,
so you mean to get voltage readings from both sides of the HT? I have 252.5vac none side and 253.5vac on the other side.
I took a reading from after the rectifier and read 462vdc, but i also read 2.3vac in what i assume is ripple.
To have 2.3vac at that point, is that normal or are the diodes possibly dodgy?
Cheers
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To test the filtering i just temporarily connected another 100u cap across the existing one, the hum definitely got lower in volume.
Kind of suggests to the dodgy rectifier diodes argument doesn't it?
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read 462vdc, but i also read 2.3vac in what i assume is ripple.
2.3VAC seems good at the rectifiers.
do the same after the choke, after R53, after R43, after R34. Those are your PS rail taps, often denoted as A,B,C, etc. measure, (both DC and AC), with all tubes in, nothing plugged into the input.
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> 253.5vac
> after the rectifier and read 462vdc
Does not sound right.
You actually have one 506V winding. If the _caps_ are good, this should give 708VDC(!!) at light load. 462VDC is SOOO far down it suggests the caps are not peak-catching.
> 2.3vac in what i assume is ripple.
Anything near 5% ripple at first cap is very reasonable and not a problem. You have less. Suspiciously less.
However most meters will NOT correctly read ripple riding on top of large DC.
I don't know BE50. Is this really a Seven Hundred Volt amplifier?? Or have you picked a way-wrong PT?
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You know, I better double check those numbers tonight. I could have had a brain fart.
It is a 350 0 350 Tx, and I bought it from Heyboer. I spoke with Phil and told him what I was trying to do. This is what he suggested.
At the risk of saying something you already know, there is no such thing as a BE50, only a BE100, as far as I know. I am creating my own BE50.
I am using a Fluke 117 dmm and in my experience these seem to pick up ripple even when I don't want them to. No other dmm I've used does it.
So maybe the caps eh? I wonder if my original power supply setup caused this. As my earlier posts say, I originally had the ps set up as per 100w and got instant red plating. I was reading 500 and something volts on the plates, maybe 560 or 570. The caps are 500v rated.
Cheers
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It is a 350 0 350 Tx, and I bought it from Heyboer.
That PT puts out twice as much voltage as you need for a full wave bridge. It is meant to be used with a full wave conventional rectifier.
In your other thread you said you converted to a conventional rectifier. Did you just forget to update the layout?
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Yeah exactly, just hadn't updated the layout.
I actually updated it last night. When I get home tonight I'll post it.
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OK, thanks. We were about to chase down that rabbit hole again. There's a lot of relevant info in the other thread. Do you want me to combine the two threads so no one has to chase info between the two threads?
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Yeah , probably a good idea.
Sorry, I am a bit naive when it comes to general forum etiquette.
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want me to combine the two threads
awe man, now I got to find my reading glasses :laugh:
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Just on a side note guys... I came to thinking in the weekend and realised that here I am trying to build Marshall clones (my first was a successful 2550) and I've never actually owned a genuine Marshall!
So I fixed that last night and bought a 1987x. I justified it to myself and my mrs by saying that I needed some reference material for my hobby builds... worked a charm!
Can't wait to receive the beast.
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Can't wait to receive the beast.
bonus, with all you've learned here, you can fix it when it arrives broke :l2:
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Why would it arrive broke?
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I've merged the two threads about this amp.
I'd like to see the updated layout and also some high rez pics of this build. Especially the power supply end of the board. I'd like to see where the PT wires are connected now that you have modified the circuit to use that PT with a full wave conventional rectifier.
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Its the Aussie way
Why would it arrive broke?
American humor, now go get pics for Sluckey
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Yes sir.
I am at work. I'll post them when I get home
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Here is that updated layout. I don't think I've missed changes.
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Oh and i just checked those voltages again.
Just as i suspected, i had a brain fart last night. The voltages are 352.5 and 353.5 on the HT, not in the 200's like i stupidly wrote.
B+ at the rectifier is correct though.
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Bump
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Testies testies one two three
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pics pic pics
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Oh crap...
It is per revised drawing. I'll get some tonight. Not sure how much new information they will give but
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volts DC and AC from each PS tap
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By ps taps do you mean the power transformer secondaries?
Shouldn't there there be zero dc on the power supply taps?
As far as ac goes there is 352.5 and 353.5.
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OK guys here are the pics you requested.
Its a bit messy from fault finding, but not too bad.
On a side not, my new 1987x arrived today and my first impression is WOW!
1, because its DAMN loud and 2, because it sounds awesome, it just screams AC/DC.
I mentioned in an earlier post that i made a dummy load with an aux speaker out for testing purposes. Anyway, after an unsuccessful attempt to use a passive volume control in the effects loop (it sounded like crap) i used my dummy load as a power soak so i could really push the amp for power tube break up. It lasted about an hour of noodling and 15 mins of ROCKING before burning out. LOL
Its all good, i knew it would. I just wanted to test the amp properly. I have spare parts to fix it.
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Another
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Another
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I don't see any obvious boogers.
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Here’s my basic steps, IF the amp powers up without flaming out;
A) measure VDC at power tube(s) cathode and plate (if your meter can handle it)
B) calc max plate to make sure I’m not frying my PA tubes.
NEXT
1. measure The power rail DC from after the rectifier to V1, recording each taps VDC value. Your power rail looks to have 5 taps
2. measure VAC at the same (5) points, record values.
3. measure VDC at the plate, grid and cathode of each tube.
Once I have these and know they are “close enough for now”
I plug in and see what sounds come out
If it’s screaming cats, or WOW, because I have a scope, I inject a signal, measure each plate starting at V1, playing with knobs as I “pass them” to see the tubes response, gain and distortion.
Once I have all that info, I scratch my head, tweak parts n pieces till I get stuck, then come here and ask for help, or smile with a big-ol grin and pester the cats and wife with guitar noise!
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Thanks for that, I'll try all of that from scratch.
It a real head scratcher because the power supply circuit is fairly simple, not many components yet it is humming like all hell.
The only things I haven't tried (that I can think of) is:
Not a good enough earth through the PT mounts.
PT primaries are dual 120's hooked in series for 240. Perhaps I didn't place the joined wires carefully enough.
Otherwise I'm out with of ideas.
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Ok guys, I've had a bit of progress
Its a little bit embarrassing but its not entirely my fault either.
When i bought the transformers, Phil from Heyboer face me the pinouts in an email, i then copied and pasted that email into a word doc for keeping.
As you know, when you cut and paste things, the formatting doesnt always come out as you want. I thought i had corrected it at the time, so that it was clear and easy to read. Obviously not.
I had the OT centre tap and one of the secondaries back to front.
The way it sits on the page after i printed it and stuck it on my pin board in the garage, makes it a bit un clear. When i connected it up, all i looked for was which one was the centre tap (obviously the other 2 go to the output tubes).
So that was the source of my hum. It still has a very faint hum, but within what i would call normal but possibly fixable.
So onto the next issue.
Immediately after resolving that, the amp started squealing. I pulled all the tubes one at a time and in various combinations. It only seems to do it when the PI tube is in. Also, it varies with the presence control. Worse when presence is low/ off, and basically gone when presence is on 10.
Any suggestions with that one?
Thanks
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Phil from Heyboer *GAVE* me the pinouts in an email.
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Actually, don't worry, Squeal gone.
It was crossed OT primaries.
Now onto the preamp noise...
Last time i checked it was somewhere around V2 i think.
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Alright! :thumbsup:
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How about a few pictures of the amp? This way we can see what the heck is going on. Especially mistakes in layout, lead dress, and grounding. Not to mention a plethora of other possibilities as you continue to keep finding...
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Nice work, don't be afraid to just swap parts in the effected area, you're deep into the fun part now :laugh:
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Pics have already been posted.
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Pics have already been posted.
Sorry, didn't see them before maybe from my phone but in any event I see numerous ground points of many various parts of your circuit of all your sections using the chassis as a ground plane? You have ground loops in an amp design where this is a highly important aspect. Not surprised to hear there's some preamp hum/buzz as likely parasitics too? These will change as frequencies and signal voltage changes...you will see in time once you get through and spend some time playing it? Good luck.
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The issue with the ground points is a fair call.
The problem is the schematic does not resemble the pics I have taken as far as grounding goes. It will probably be a game of trial and error.
The pics show the backs of the pots grounded. I always thought that was bad...
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You did a fabulous job in planning and building everything except a "grounding plan" which is right up there with component placement and lead dress IMHO. The only other thing that caught my eye was the relays' positioning near other components especially the inputs one which is most highly sensitive low level high Z area of the entire amp.
BTW, you can do things to help the grounding situation as it's "not too late" here. Play the amp for a while and see how things go. If you notice any further issues or something nags at you then you can approach the remedies later, and also be much more fresh and determined with a good plan going forward. Attacking it now could lead to further frustrations and doing things hastily. Not sure what you've read regarding the various grounding schemes but it's worth checking and reading them all in the meantime. Especially Merlin's - it takes several readings minimum to digest everything, and when you think maybe you've got it no problem, there's subtleties there that still could catch you by surprise. I'd gone through several builds before finally getting the hang of it all.
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Thanks for the kind words.
The "general" layout is the same as a BE100. If anything ive compressed it slightly. I've also designed my own chassis and I positioned the board a little too close to the pots and input. I'm also starting to get concerned about the way the OT secondaries run between v3 and v4.
At the moment the amp is not running well enough to plug a guitar into. There is some sort of oscillation going on I think. One channel is worse than the other 2.
I was concentrating on the hum first, and now that is fixed I'll work through the amp from the other end.
Cheers
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There is some sort of oscillation going on I think
fwiw
I've had a couple builds with parasitic osc., the low freq ones tended to sound like I had tremolo (when I didn't), the higher freq ones tended to sound like hissing.
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I think mine is low frequency. There is no real hiss. In fact it gets worse when I plug a cable in.
It sounds like when you ground the tip of the lead. That sort of thing anyway.
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it gets worse when I plug a cable in
most amps get worse when you do that, kinda like an antenna for noise.
once you've checked/fixed any grounding schemes, without a scope, the listening amp is a good tool, inject a signal at the input, listen at each stage, hear where things go south, well in Australia, I guess north :dontknow: :laugh:
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Nah it's still south to us. I think it's around v2