BE50 build... Problems

[Jaymz77]

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.

[HotBluePlates]

... 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.

[Jaymz77]

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

[sluckey]

Just use the Marshall style bias circuit. It's simple, reliable, and easy to tweak for the voltage you need.

[Jaymz77]

Exactly what voltage do I need for 2 x el34's?
Cheers

[sluckey]

An adjustable range of 30 to 45 should do it. Look at Hoffman's Plexi 50 schematic.

[HotBluePlates]

Exactly what voltage do I need for 2 x el34's?

EDIT: Sluckey beat me to it. 

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.

[Jaymz77]

Thanks guys. Much appreciated

[Jaymz77]

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.

[kagliostro]

Maybe you can find interesting these links

https://robrobinette.com/Tube_Bias_Calculator.htm

http://www.tedweber.com/webervst/tubes1/calcbias.htm

Ciao

Franco

[sluckey]

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.

[Jaymz77]

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.

[2deaf]

Which schematic uses those resistor numbers?

[Jaymz77]

Sorry,
I thought I had posted a diagram above.
It's the BE100, but I'm making a BE50.
cheers

[Jaymz77]

And I got the numbers wrong too
The bias out to the tubes is between 56 and 57.
Bias supply comes into 58.
Cheers

[Jaymz77]

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

[2deaf]

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.

[Jaymz77]

Thanks for that.
So I need to change my rectifier AND remove the floating centre tap setup?
Or just fix the rectifier?

Thanks heaps

[2deaf]

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.

[sluckey]

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...

[Willabe]

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?

[Jaymz77]

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

[Jaymz77]

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.

[Jaymz77]

Correction: 37.7mA

[Jaymz77]

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. 


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.    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.

[Jaymz77]

No love for me?

[silverfox]

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.

[Jaymz77]

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...

[Jaymz77]

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

[Jaymz77]

**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.

[shooter]

As expected, the earth hum didn't change a bit.

Can you determine hum frequency?  that will help which rabbit hole to focus on?

[Jaymz77]

As I said above it's 50 Hz
Cheers

[Jaymz77]

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.

[smacbride]

It could be related to the wiring in your house or shop.  Try it in a different room that's on a different circuit.

[Jaymz77]

Please read my earlier posts

[sluckey]

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.

[Jaymz77]

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.

[Jaymz77]

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.

[sluckey]

Can you actually measure the frequency with a dmm or scope connected to the speaker jack?

[Jaymz77]

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.

[Jaymz77]

I suddenly get this feeling that when I do measure it I'll make a fool of myself and my 20 years experience...

[Jaymz77]

I can't get a reading from the speaker jack using a dmm.
Funnily enough I'm getting 100hz on the OT primaries

[sluckey]

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. 

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.

[Jaymz77]

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.

[shooter]

i end up making a fool of myself.

ahh, but that's how we gain wisdom

[Jaymz77]

Too true Shooter

[shooter]

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?

[sluckey]

OK, so do you think we may be chasing a 100Hz rabbit? 

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?

[Jaymz77]

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

[Jaymz77]

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