Hagstrom 39

[Leevi]

My latest project is based on Hagstrom 39 amp. The amp sounds great but there is a
relative low ground hum. I have investigated it little bit and noticed that the hum is related
to heater circuit. In the PT I'm using is one 6.3v secondary only which means I have to use same
heater circuit both for rectifier tube (EZ81) and other tubes. If I disconnect the grounded 100 Ohm resistors
from the heater circuit the hum disappears but occurs very loud if I open the volume.

In the Hagstrom schematic below there is an own secondary for rectifier tube (EZ80).

Any ideas how I to get rid of that problem. I don't want to install a separate filament transformer since
there is no place for that.

Schematic:
http://www.hagstrom.org.uk/Schematics/Schempics/Hag39%20schematic.jpg

Sound sample on YouTube:
Stardust & Sultans of Swing played through Hagstrom 39 amp

/Leevi

[Ed_Chambley]

The man with the golden thumb.  Great!

[sluckey]

I don't want to install a separate filament transformer since there is no place for that.

You could always use silicon diodes.

I have a couple modified Hammond reverb amps that put the EZ81 rectifier on the same string with the other tubes. I haven't noticed any hum issues.

[kagliostro]

Try with an Humdinger

https://www.tube-town.net/ttstore/product_info.php/info/p5687_Fender-Potentiometer-Hum-Controll-100R.html

you can turn the pot in the position that gives the better result in Humm rejection

K

[HotBluePlates]

... In the PT I'm using is one 6.3v secondary only which means I have to use same
heater circuit both for rectifier tube (EZ81) and other tubes. If I disconnect the grounded 100 Ohm resistors
from the heater circuit the hum disappears ...

In the Hagstrom schematic below there is an own secondary for rectifier tube (EZ80). ...

There is probably leakage from the heater to cathode of the EZ81 due to the high voltage difference between them when you ground-reference the heater winding. That's part of the reason Hagstrom used a second winding and let it float (there would be a smaller, indeterminate voltage from EZ81 heater to cathode in this case).

Sluckey's got the best solution for you, if you can't use a different PT.

[HotBluePlates]

I wonder if the schematic has an error: seems like there could be a 1MΩ to ground between C6 and C7.

As it is, C6 and C7 form a 0.0025uF cap, and with the 1MΩ is -3dB at ~64Hz. If there is a 1MΩ between C6 and C6, there are two filters in series -3dB at ~32Hz for -6dB total and a steeper roll-off to keep out trem thumping.

[Leevi]

Code: [Select]

I wonder if the schematic has an error: seems like there could be a 1MΩ to ground between C6 and C7.
You are probably right. I interpreted it that the idea of these two caps is just to find a "right" capacitance value.
The amp is from year 1961 and because of lack of right components they have ended up to this solution.

/Leevi

[HotBluePlates]

I thought the same at first, but also figured a production amp could get the needed parts. After all, the 700Ω cathode resistor for the ECL82's are an odd value...

Great sounding amp! Would love to know about the speaker in the original.

[sluckey]

Leevi, is that you playing? Very nice and smooth.

I'm gonna study that schematic. That may be the answer for what to do with my extra Hammond AO-44. Thanks.

[Leevi]

Code: [Select]

Leevi, is that you playing? Very nice and smooth.
No, unfortunately not, I'm not able to play that well.

I must say that I succeeded to reach similar sound to my amp. I'm using Edcor 15W OT with ultra linear taps.
The tremolo is one of the best tremolos I have ever built.

Code: [Select]

Would love to know about the speaker in the original.
I found this. The original speaker has been manufactured by "Speaker factory" in Stockholm Sweden.

Code: [Select]

Some specs: The main speaker is a Sinus B-1051 8 ohm made by "Högtalarefabriken AB Stockholm Sweden"
/Leevi

[Leevi]

Code: [Select]

You could always use silicon diodes.
I tried that but it didn't help

Code: [Select]

Try with an Humdinger
This didn't help either

/Leevi

[HotBluePlates]

Code: [Select]

You could always use silicon diodes.
I tried that but it didn't help

Replaced the EZ81 with diodes, and used a center-tap (or artificial center-tap) with the 6.3v winding (now only connected tot he preamp/output tubes)?

You might try that arrangement, and if you still have hum, use a battery or d.c. supply to see if it's related to the heaters at all.

If it is, an elevated heater reference of maybe 40-50vdc might help (this would help minimize heater-to-cathode leakage in the tubes).

[Leevi]

I start to believe that it is a ground loop hum and this time maybe the most difficult one I have met.
I have not mentioned that my PT does not have the center tap and I make the "CT" with diodes.
I'm using two grounding points: a star and bus connected to the pots

I have tried the following things without success:

- semiconductor diode bridge
- extra filter stage with resistor and choke
- choke both for the whole amp and and screens+preamp
- changed star grounding to different place
- positive voltage on the heater reference point even with separately made circuit with some ten volts
- pot instead of the heater resistors
- I'm measuring the voltage level of the hum and have noticed that by changing some groundings
some effect is visible but I'm still far away from an acceptable level

To be done?:

- battery, proposed by HotBluePlates
- new PT with center tap and two different heater winding
- isolate the pt from chassis
- use isolated jacks => one star grounding
- ...

/Leevi

[HotBluePlates]

... my PT does not have the center tap and I make the "CT" with diodes. ...

Try the battery or power supply first; if hum stops you know it's the heater circuit.

Then try using resistors instead of diodes to balance the heater winding.

[Leevi]

Code: [Select]

Then try using resistors instead of diodes to balance the heater winding.
That I'm using already. I make the CT for the EZ81 with diodes since there is no CT for the 220V secondary I rectify for B+.

If you look at the schematic there is B+ connected to one of the EZ81's heater  wires. What is the idea of this?

/Leevi

[PRR]

> B+ connected to one of the EZ81's heater  wires. What is the idea of this?

IF you have a separate heater winding dedicated to the EZ81, tying heater to cathode minimizes the heater-cathode voltage and possible breakdown of heater insulation.

But the main point of EZ81 compared to direct-heat rectifiers is that you *don't* have to have a separate heater winding for the rectifier. So that design misses the point of paying more for EZ81. And I wonder if the actual amplifiers were built like yours: EZ heater on the main 6VAC feed, more-or-less groundy.

[Leevi]

Try the battery or power supply first; if hum stops you know it's the heater circuit.

It's quite clear now that the root cause is not the heater circuit. I removed EZ81 and tried with SS-diode bridge once again
and the hum level didn't change at all.

I start to think the next move.

/Leevi

[terminalgs]

If it is, an elevated heater reference of maybe 40-50vdc might help (this would help minimize heater-to-cathode leakage in the tubes).

+1 HBP's suggestion.  The schematic shows 22v on the cathode?  maybe your problem is the 6.3v (CT referenced to 0vdc) to 22vdc difference.   

you could move the 6.3 CT to one of the ECL82-pentode cathodes. (--I think you can elevate to 1 of the 2, when the cathodes resistors are split like that, but I've never worked on such an arrangement.,, speaking of which, I'd be tempted to tie the two cathodes together and use a single resistor and by-pass cap).   

-OR- you can use your HT power supply with a resistor and series pot to ground to dial in any DC elevation for the CT you want. something above 22v, maybe 30v. with 1M total resistance between the HT and ground, there will be no real current draw impact to think about.

Also, you mentioned potential ground loops.  unplug instrument cables from input jacks with amp on.,  does noise go away?  if so, maybe a little RF is riding on the shield of the cord and its making itself available on the chassis (and getting amplified).

[Leevi]

I found the root cause
I moved the wires with chopstick and noticed that the wire going from 1M resistor to the lower ECL82 triode grid
was too close to the rectifier and caused the hum.

Thanks a lot for your help and tips
/Leevi

[Leevi]

Some pictures and sound sample:

http://i218.photobucket.com/albums/cc183/kz14100/RikstoneC9front_zps741aacb2.jpg

http://i218.photobucket.com/albums/cc183/kz14100/RikstoneC9panel_zps6e79d197.jpg

http://s218.photobucket.com/user/kz14100/media/RikstoneC9back_zps4e57a92c.jpg.html

http://s218.photobucket.com/user/kz14100/media/RikstoneC9back2_zpsb3e8537f.jpg.html

Sound sample:
Speaker: http://www.eminence.com/speakers/speaker-detail/?model=Legend_1028K

http://www73.zippyshare.com/v/2987363/file.html

/Leevi