New amp start up trouble

[Bieworm]

Hey guys! Hope you're all well..

 I have built an ampeg B15 and ran into trouble with the start up. Without tubes both the HT wiring measures 360VAC and the heaters for the GZ34 were both 2.6VAC, so that's good.
When i put in the rectifier tube the fuse blew when the tube started conducting. I know there must be a short somewhere.
Is it normal that the 5V heater winding measures zero resistance to ground? I have a 500mA HT fuse from pin 8 to the 1st filter cap. And even when i remove the fuse it gives zero ohm to ground at pin 2 and 8. Is that normal?

[Bieworm]

Solved. The 5V has a CT which I stupidly connected to ground. Disconnected it and.. tadaaa!!! No more lightbulbing😉

[Greg]

Yes it is normal! That winding is probably 1/8th of an ohm. If your 5 volt winding has a center tap, you can connect this to your B+/500mA fuse instead of letting it loose with shrink wrap. The yellow wires still goes to pin 2 and 8 and the riding B+ goes to your first cap/500ma fuse via the winding center tap.

[sluckey]

You're not the first!   

[Bieworm]

Yes it is normal! That winding is probably 1/8th of an ohm. If your 5 volt winding has a center tap, you can connect this to your B+/500mA fuse instead of letting it loose with shrink wrap. The yellow wires still goes to pin 2 and 8 and the riding B+ goes to your first cap/500ma fuse via the winding center tap.

Djeez.. that makes sense! Now it's got an artificial CT, with the actual CT shrinked off. Thanks for the enlightment!👍

[PRR]

The rectifier does not need a hum-reducing CT.

[pdf64]

… Now it's got an artificial CT, with the actual CT shrinked off. …

An artificial CT on the rectifier heater winding - could you explain the why and how

[shooter]

The 5V has a CT which I stupidly connected to ground.

[pdf64]

The 5V has a CT which I stupidly connected to ground.

Still confused sorry, regarding the why and how 

[shooter]

just grabbed the 1st tube rec pic i looked for...so
also, i;ve only built 1 amp with tube rec....so


i wouldn't think 5% ripple on the 5v line would be much at the 330+v plates, before filtration downstream removes what little there might be 

[mresistor]

… Now it's got an artificial CT, with the actual CT shrinked off. …

An artificial CT on the rectifier heater winding - could you explain the why and how

I found this on the music electronics forum and it helps explain it.


"Use the center tap as the connection to tap the DC voltage from the rectifier instead of using pin 8 of the tube. I read in one of O'Connor's books that there is some hum cancelling benfit to this method."

"Don't worry about which version of TUT it's in - the concept is older than KOC <grin> and that CT is only included for the specific purpose stated. The filament voltage adds a tad more hum to the rectifier's output so the CT is to balance this out. While the difference may be marginal - caps are so much cheaper than when the idea was created - it's the best use of the tranny and there is a certain "aesthetical" symmetrical balance to the circuit"

"Yes Bob that is correct. Rob is right when he says it only makes a minor difference. You would probably need a scope to detect it if you have substantial filtering. But I say use the CT if it is provided. It makes a bigger difference when you use it as the cathode connection for a directly heated power triode. I have TUT 2 and 3 and Principles Of Power, can't remember where I saw it."

"I'm not are familiar with the DHT mania as perhaps I should be (got a few original 2A3s and some 45s though) but I wasn't aware that any of them used a 5 volt filamentary cathode (but it does make sense to use the CT in this way for those tubes that do). In any case, as best I know, the 5 VCT'd winding is specific to the rectifier as it has higher voltage insulation and is, or at least should be, coded yellow with yellow/red center tap - at one time the standard for a rectifier winding. Non recto heaters should be brown then green etc."

[Greg]

I so much agree with PRR, the rectifier doesn't need a balancing center-tap with all the ripple at the reservoir cap of a typical guitar amp anyway. If I had a transformer with a 5 volt CT winding (like some of those fine Edcors) I would just use it instead of chopping it off or letting it loose with insulation (which is also correct). That's what I was suggesting. It would be unwise to start worrying about ''not'' having a center tap on a 5 volt rectifier winding.

[Bieworm]

Sorry for calling it an artificial CT. It's just a tap,no more than that.
So... after biasing the cathode for the 64 mode and the fixed one for the 66 mode I cant get rid of a hum. It's definately grounding related. It is dead quiet with both volumes at zero and gradually increases with the volumes turned up.
I used the grounding system with all preamp going to the ground at the input,using a ground buss for the board and one for the 2 volume pots.. straight to the input ground. I used an isolated jack input and a dedicated ground lug near the input. PI and power amp grounds, speaker ground, filter cap ground and bias all go to a star ground at the power transformer side. Plus a dedicated ground for the IEC mains inlet.
So preamp ground and power amp ground are as far as possible away from each other.
This is the method I always use abd never gave me trouble.
Now I read that the Ampeg B15 has only one ground point, apart from the mains ground. Otherwise there will be a ground loop. Is this making sense? Is there something special about the B15 that requires the grounding like that? If so... is it a good idea to lift the star ground at the power TX from the chassis and run a wire from there to the input ground?

Sorry for the lengthy post, but thanks in advance!

[Bieworm]

Is there any sense in it that the single ground point at the input Ampeg applies is better for this amp than the separate star grounds for preamp and PS+PA Merlin advises?

[pdf64]

Is there any sense in it that the single ground point at the input Ampeg applies is better for this amp than the separate star grounds for preamp and PS+PA Merlin advises?

You may be misreading Merlin, he advises a single point connection between the amp’s 0V common arrangement and the chassis. He’s not suggesting to connect each star point to the chassis.

[Bieworm]

Ok I made some progress with the hum.
Turned out to be a short in the shielded wire from the wiper of the heater hum pot. I attached the shield on 1 side to the back of the pot and clipped the other side + shrinked it off. On the 50V DC elevation I kept reading 0 V. Now I cut the shielding from the back of the pot and read 44V, so that's good.
Strange thing now is i can eliminate the hum when i turn the humdinger pot completely CCW. In the middle it hums, in ouder CW it hums even louder. I can't feed the heaters with 1V on one side and 5.3V on the other??? What is this telling us?

Oh, and i first pulled V1.. still hum. Put it back and pulled V2.. hum. With V3 pulled (PI) it makes no sound anymore

[sluckey]

Why oh why would you use shielded cable on a humdinger pot?

Does the filament winding have a center tap? If so, tape it off and tuck aside. Don't use it.

A correctly wired humdinger pot simply has one outside lug connected to one of the filament leads and the other outside lug connected to the other filament lead. Then the center lug either connects to ground or some voltage source. The original B15 connected the center lug to the output tube cathode. Later fixed bias B15s connected the center lug to the bias supply.

If you can't figure this out then post a schematic showing how your humdinger is actually wired. As always, hi-rez pics are helpful.

[Bieworm]

Why oh why would you use shielded cable on a humdinger pot?

Does the filament winding have a center tap? If so, tape it off and tuck aside. Don't use it.

A correctly wired humdinger pot simply has one outside lug connected to one of the filament leads and the other outside lug connected to the other filament lead. Then the center lug either connects to ground or some voltage source. The original B15 connected the center lug to the output tube cathode. Later fixed bias B15s connected the center lug to the bias supply.

If you can't figure this out then post a schematic showing how your humdinger is actually wired. As always, hi-rez pics are helpful.

Filament CT is sealed off. The 2 Filament wires connect to the humdinger. And now there is no shielding attached anymore, thus achting like a normal wire... going from wiper to 50VDC

[Bieworm]

What could be the reason of the hum pot acting like a hum volume pot with minimal hum on 1 side and maximum hum on the other side?
The hum is also barely audible with the volumes on zero

It looks like 1 heater wire gas all the hum and the other has none?

[pdf64]

As the heater circuit is elevated, I don’t understand how can the humdinger pot be beneficial?
My understanding is that the mechanism by which elevation operates is to saturate the current paths between heater and cathode, thereby making any modulation of that by the heater current impossible.
Just connect the CT or balancing resistors to the elevation point.

The stuff from MEF about a CT for the rectifier heater being beneficial is in regard of directly heated cathodes, it doesn’t apply to indirectly heated rectifier types such as the GZ34.
You just need to take the rectified output from the cathode pin8 not the heater pin2. By doing that, there will be no interaction between the heater voltage and the rectified HT.

[Bieworm]

As the heater circuit is elevated, I don’t understand how can the humdinger pot be beneficial?
Just connect the CT or balancing resistors to the elevation point.

The stuff from MEF about a CT for the rectifier heater being beneficial is in regard of directly heated cathodes, it doesn’t apply to indirectly heated rectifier types such as the GZ34.

Thanks. Apart from the benefit (or not) the humdinger should behave the same as the heater winding CT when the wiper is in the centre, no?
I just can't wrap my head around it that the hum is only at one side of the heater circuit. It should be louder at either side of the middle position. Only thing that would make sense to me is that one or more of those 6SL7 tubes is troublesome at only one triode. But i don't have spares to check that. It's not the 6L6 tubes.. i already swapped those for known good ones.

[Bieworm]

Would there be problems if I let it like that, meaning one side ofvthe filament wirining at 1.5V and the other at 4.8V? It appears to be quiet then...

[sluckey]

I would replace the hum pot. It may have been damaged with your shielded wire short.

[Bieworm]

I would replace the hum pot. It may have been damaged with your shielded wire short.

The heater wirings unused CT to the 50V tap and wire the heaters from a standoff soldering lug might be a good option too then? PDF said there is no benefit for a hum pot when elevating the filament to a 50V DC reference...

[sluckey]

I would just connect the new pot wiper to chassis ground.

[Bieworm]

I would just connect the new pot wiper to chassis ground.

Not a fan of elevating heaters? Why? Just curious...

[sluckey]

I'm not the one having issues with elevated heaters.   

[Greg]

You can check your pot with amp off by removing all tubes and pilot light, just unsolder the wiper at the divider. Then, it should be half the resistance between wiper and each leg at the center of the rotation. You can mechanically attach the wiper wire to chassis ground and test the amp to hear if it is better without the DC bias.

I remember one time that the hum was killed not in the middle of the pot (and it had DC bias). That's why you have a pot there; with resistors you can't do that. I can't explain why it did that, I expected it to work in the center. However, the hum was killed in the center of the pot when I removed it from the elevated source and grounded it instead. But at the time, I needed the elevated source to ease for some high cathode voltage in another part of the circuit.

 Fender in the fifties had one side of the filament at 0V and the whole 6.3VAC on the other. The imbalance doesn't harm the tubes, you still have 6.3v across the heaters.

[PRR]

....can't wrap my head around it that the hum is only at one side of the heater circuit. .....

Makes sense if indeed the hum-pot was damaged in the accident.

What actual plan are you working from? Many things have been called "B-15". The attached is from a 1966.

Hum-pot and elevation are two different things. Elevation reduces stray emission from cathode. Hum-pot is about how physically un-balanced your heater wires are-- it balances the electrostatic fields which would cancel if the heater wiring could be perfectly symmetrical.

Treat them separately. Get a new hum-pot! Ground the wiper. Can you get a near-null near-center? If not, look where your heater wiring is way lop-sided. Wire way close to audio parts. Solder bridge. Nick in the plastic past a lug.

Try that way, grounded, and try that connected to the *slight* elevation at 6L6 cathodes.

The hum-pot CAN end up off-center, and different as you work the user controls. The later VT-40 on (single-sided!) printed circuit board could not make the heater 'wires' symmetrical. With all knobs down, null near 40%; knobs all up, humm/buzz was intolerable at any setting; at some 'normal' setting, 55% might be best for hum, 60% for buzz.

[PRR]

Also power off, disconnect the heater CT/Gnd, and measure the Ohms from each side of heater to ground. Should be infinity both ways. If you get like K-ohms, start pulling tubes until the ohms go to infinity. (You may even find a stray ground you forgot to remove in your trials.)

[Bieworm]

Well this is what I know now. The pot is removed and i wired the heaters in 3 different ways.
* H1 -CT to ground - H2 : loud hum
** H1 -CT taped off and 2x 100R to ground - H2 : loud hum
*** H1 - CT to 50VDC tap -H2 : less hum, but still too much

I also measured the hum pot for shorts or imbalances . All seems fine.

I'm wiring the hum pot back in until the new ones arrive. I can get the hum to virtually zero with the hum pot in way uneven position.. at any settings on the vol and tone controls. So maybe I should stop chasing the hum for now and focus on tolexing the cab.

Fwiw: i used the B15 Heritage schematic,  with both bias options and the '64 + '66 channels.
I also added 'Trinity's 50VDC circuit in front of the 1st filter cap. Mainly for the 2x 47uf/450V caps in series.. for i don't trust the 1st cap being only 500V when the B+ from the GZ34 is near 460 to 470V. The 50VDC was a bonus.

Sounds awesome, really great!!

[sluckey]

Fwiw: i used the B15 Heritage schematic

Would you post that schematic?

[Bieworm]

Voila

[Greg]

Now I understand. It uses 6SL7's... You will always have some residual hum with 6SL7 tubes because there isn't a center tap in their filaments.

Sluckey did mention that at some point, Ampeg did connect a negative DC bias on the heaters and apparently, it got better (because that doesn't make that much sense otherwise).

I once did glued a small 6VDC switcher PSU (1 amp or so) to power one 6SL7 in a previous amp. Maybe there's a place on the side of your chassis to glue it away from the heat. You could connect such a small switcher to power V1 and V2. V3 is not critical and you can use the AC that you already have for it and the power tubes. I do not recommend this if you are satisfied with the hum level at this point but it cured 100% of the hum in my amp.

[Bieworm]

Now I understand. It uses 6SL7's... You will always have some residual hum with 6SL7 tubes because there isn't a center tap in their filaments.

Sluckey did mention that at some point, Ampeg did connect a negative DC bias on the heaters and apparently, it got better (because that doesn't make that much sense otherwise).

I once did glued a small 6VDC switcher PSU (1 amp or so) to power one 6SL7 in a previous amp. Maybe there's a place on the side of your chassis to glue it away from the heat. You could connect such a small switcher to power V1 and V2. V3 is not critical and you can use the AC that you already have for it and the power tubes. I do not recommend this if you are satisfied with the hum level at this point but it cured 100% of the hum in my amp.

Thx Greg. I reinstalled the hum pot and dialed it in to an acceptabele hum level. Its super sounding

Btw.. this one is switchable between fixed and cathode bias. So it wouldn't work with the elevation on the cathode. Sluckey also advises against elevation on the neg bias

[Greg]

I guess the hum is acceptable enough that you can't hear it through the grapevine in those isolated James Jamerson sessions

[Bieworm]

I guess the hum is acceptable enough that you can't hear it through the grapevine in those isolated James Jamerson sessions

Lol. 🤘😀

[PRR]

Voila

OK. That one shows no elevation. (AND a 3-pin plug!) I'm inclined to trust Ampeg on this. Elevation masks leaky heaters, but you can just rotate the buzzers into high-level duty at V3 and use good bottles at V1 V2.

The lop-sided pot bothers me. Shouldn't have to do that. Did you post your Ohms each heater line to ground? A good picture of your wiring?

[pdf64]

Well this is what I know now. The pot is removed and i wired the heaters in 3 different ways.
* H1 -CT to ground - H2 : loud hum
** H1 -CT taped off and 2x 100R to ground - H2 : loud hum
*** H1 - CT to 50VDC tap -H2 : less hum, but still too much
…

So with respect to 0V common, what’s the heater voltage symmetry in the above 3 scenarios?
How about if all valves are removed, other than the rectifier?

[pdf64]

Now I understand. It uses 6SL7's... You will always have some residual hum with 6SL7 tubes because there isn't a center tap in their filaments.

Sluckey did mention that at some point, Ampeg did connect a negative DC bias on the heaters and apparently, it got better (because that doesn't make that much sense otherwise).

I once did glued a small 6VDC switcher PSU (1 amp or so) to power one 6SL7 in a previous amp. Maybe there's a place on the side of your chassis to glue it away from the heat. You could connect such a small switcher to power V1 and V2. V3 is not critical and you can use the AC that you already have for it and the power tubes. I do not recommend this if you are satisfied with the hum level at this point but it cured 100% of the hum in my amp.

I don’t think the valve heaters not being centre tapped is the problem cause here. Consider that we generally use 12A_7 valves arranged with 6V parallel heaters, rather than with them 12V series, centre tapped.
It seems more likely that it may just be due to a less advanced design of heating filament, not spiral wound or otherwise mitigating the radiated field.

My understanding is that elevation of the heater circuit can’t help with that, rather it mitigates for heater - cathode insulation issues. As you’ve found, the only solution for poor heating filament design is DC heating.

Elevation, positive or negative, works by saturating current paths in the imperfect heater - cathode insulation.

[Bieworm]

Well this is what I know now. The pot is removed and i wired the heaters in 3 different ways.
* H1 -CT to ground - H2 : loud hum
** H1 -CT taped off and 2x 100R to ground - H2 : loud hum
*** H1 - CT to 50VDC tap -H2 : less hum, but still too much
…

So with respect to 0V common, what’s the heater voltage symmetry in the above 3 scenarios?
How about if all valves are removed, other than the rectifier?

I did check that. Without tubes 3.2V symmetrical. With tubes 3.18 or 3.19V
it's not the windings or anything.

What could be inducing the 50Hz hum is the fact I put the HT CT on the star ground instead of the - of the 1st filter cap. That could imply that there's some heavy current pulses on the star ground in the return path of the 1st B+ node,no?

[Bieworm]

Voila

OK. That one shows no elevation. (AND a 3-pin plug!) I'm inclined to trust Ampeg on this. Elevation masks leaky heaters, but you can just rotate the buzzers into high-level duty at V3 and use good bottles at V1 V2.

The lop-sided pot bothers me. Shouldn't have to do that. Did you post your Ohms each heater line to ground? A good picture of your wiring?

I trust Ampeg as well, but why did Steven (trinity) decide to implement an elevation circuit? I know he's a talented guy.. so he must have his reasons. I also trust the series coupled 47uf/450V caps are safer after the rectifier than a 47uf/500V cap with these voltages. Dringend my research on B15 amps I often notice people replacing the original PTx for it has blown... higher voltage filter caps (600V) are really expensive

[pdf64]

…
What could be inducing the 50Hz hum is the fact I put the HT CT on the star ground instead of the - of the 1st filter cap. That could imply that there's some heavy current pulses on the star ground in the return path of the 1st B+ node,no?

No, those current pulses are at twice the mains freq, 100Hz

[pdf64]

… I also trust the series coupled 47uf/450V caps are safer after the rectifier than a 47uf/500V cap with these voltages. …

Indeed, but with the downsides of more hum and sag, as the effective reservoir capacitance is halved to 22.5uF.

[Greg]

I also trust the series coupled 47uf/450V caps are safer after the rectifier than a 47uf/500V cap with these voltages. Dringend my research on B15 amps I often notice people replacing the original PTx for it has blown... higher voltage filter caps (600V) are really expensive
[/quote]     

That's because the design mention to use either 5ar4 or 5u4. With the 5u4, the voltage might climb up to 550vdc before going down with the load.

[PRR]

....why did Steven (trinity) decide to implement an elevation circuit? I know he's a talented guy.. so he must have his reasons. ...

He had a good amp and was working to improve it. Your amp is currently(?) 'troubled', so you are not starting from a good amp. Elevation won't cure flaws.

[Bieworm]

Hey guys.. just wanted to mention I finished the amp. We had it running at practice yesterday and it sounded awesome IMHO.