Pot on Output Valve of SE Amps?

We all start somewhere
Post Reply
David
User
Posts: 123
Joined: Wed Jan 05, 2011 3:12 pm

#1 Pot on Output Valve of SE Amps?

Post by David »

On an SE amp using one DHT output valve, what is the function of the pot connected across the heater connections?

Thanks
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#2

Post by pre65 »

It's normally called a "humbucking" pot.

With the pot around the central position hum should be minimal, twiddle to see if you can do it by ear.

Generally only used for AC filament heating.
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
David
User
Posts: 123
Joined: Wed Jan 05, 2011 3:12 pm

#3

Post by David »

Hi Phillip,
I thought humbuckers were only used on push pull output stages to balance the hum between the two valves?

Also, these output valves are running on DC. See attached schematic.
David
Attachments
Bowei 2A3-45SET2011.png
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#4

Post by pre65 »

No, SE amps use humbucking pots to null hum. Looking at your circuit I'd say they were humbucking pots. They can also be used with a "basic" DC filament supply, but there are a lot better ways to DC than you are using. :wink:
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
David
User
Posts: 123
Joined: Wed Jan 05, 2011 3:12 pm

#5

Post by David »

Hi Phillip, what would be a better way to use DC here then? :D
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#6

Post by pre65 »

David wrote:Hi Phillip, what would be a better way to use DC here then? :D
In my opinion the best way for DC on DHT valves is voltage and current regulated, like the Tentlab filament modules.

One of our members (Andrew L) has spent a lot of time designing his own version, and was offering PCBs or full kits to other members. I have not heard them myself, but those who have report very positive.

Only negative in your case is you would need another transformer to power them.

This is the topic, but it is 40 pages long. :wink:

http://www.audio-talk.co.uk/phpBB2/viewtopic.php?t=4729
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
David
User
Posts: 123
Joined: Wed Jan 05, 2011 3:12 pm

#7

Post by David »

Thanks. But I assume decent DC with a Pi filter would be a decent improvement /compromise?
David
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#8

Post by pre65 »

David wrote:Thanks. But I assume decent DC with a Pi filter would be a decent improvement /compromise?
David
Don't know. (honest answer). :wink:
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
Alex Kitic
Old Hand
Posts: 369
Joined: Mon Oct 14, 2013 3:20 pm

#9

Post by Alex Kitic »

I usually call the hum-bucker pot "hummer" pot, as a derivation from the German word "brummer" (brumm = hum) that we use in Serbian as well, "brumer".

The hummer potentiometer is particularly useful in equalizing the 50Hz (60Hz in USA etc.) on each leg of the filament, in order to effectively null the 50Hz. What you are left with is (remnants of) the 2nd harmonic, i.e. 100Hz, which is the commonly known DH tube hum.

Instead of the hummer pot you can use a pair of resistors that are matched as well as possible, for instance 47 ohms and 47 ohms - where it is not all that important whether it is 47.2 or 46.5, but whether the pair of them is perfectly matched. This is what I normally do - cheaper than the pot and of course more durable, while the result is identical to the best setting on the pot.

When using DC instead of AC, you should ideally stick with the hummer pot or resistor arrangement. In this manner you avoid unbalancing your heater circuit across the parasitic capacitance of the winding, etc. Keep that in mind as it is important, and it gets more important with each step you distance yourself from the "sophisticated" solutions (most sophisticated, probably, current regulated-voltage controlled, than just current regulated (would not recommend that), than voltage regulated, than unregulated with chokes in parallel or common mode... and last but not least, the simplest, and most sensitive to the above - plain rectification with caps...

It is another issue where would you put the bypassing cap, or caps... and it is also rather important.

This is why I usually state that DH tube amps are slightly more complicated to build than IH tube amps - they tend to have more complicated cathode circuitry solutions.
Alex Kitic
Old Hand
Posts: 369
Joined: Mon Oct 14, 2013 3:20 pm

#10

Post by Alex Kitic »

David wrote:Thanks. But I assume decent DC with a Pi filter would be a decent improvement /compromise?
David
Not exactly. IMHO, the best sounding solution is AC, if you can afford the residual hum.

If you cannot afford the hum, than at least voltage regulated DC (DC with Pi filter tends to sound rather dull), and particularly if you are a beginner, sourcing some sophisticated "readily made" solution on PCB is probably the best option.

That said, I am currently very much taken with HF AC, or "half wave oscillator circuit" - basically a modified electronic transformer for halogen bulbs. Not yet a bullet-proof solution, but some of us are working on making it predictable and simple.
David
User
Posts: 123
Joined: Wed Jan 05, 2011 3:12 pm

#11

Post by David »

Thanks Alex. I'm servicing /upgrading the amp with the circuit I posted in this thread so not planning on moving the bypass caps unless there's a good reason?
David
Alex Kitic
Old Hand
Posts: 369
Joined: Mon Oct 14, 2013 3:20 pm

#12

Post by Alex Kitic »

David wrote:Thanks Alex. I'm servicing /upgrading the amp with the circuit I posted in this thread so not planning on moving the bypass caps unless there's a good reason?
David
If you are just servicing the amp (is there reason to do that?), I guess you should leave everything as is, bypass caps in particular.

If you are contemplating ways to modify/improve (i.e. upgrade) the amp, I suggest you take good look at the RH2A3/45 schematics as posted in the RH-TTA article on my blog. The ECF80 is a noval tube, thus you can use an ECC81 in the same socket (using the second triode as cathode follower, instead of a different tube in octal socket like on my schematics).

I mean, you can look for ideas you might take from my blog (like CCS applied to different tubes in the same socket...), or just adapt (modify) the existing amp to suit the different schematics. You might be surprised at the result :)
Post Reply