Hum level controls on amps? Vintage Bell (6V6) tube amps.

TheChairGuy · « **on:** 23 Jul 2006, 02:48 am »

I bought(actually, traded for, with Tim 'Tubes 4 Ever') my second pair of vintage amps....Bell Integrated Mono's with a whopping 10watts a side of 6v6 power

It is for a hoped-for second system in the upstairs rec room.

I've had them re-wired (silver cryo wire), recapped, re-rca'ed, and throughly cleaned by Brad at Revelation Audio Labs (new captive line cord, too). He's had them a few months and does a bit in between jobs....anyway, they are almost done now.

Fact is, I haven't even seen them yet...they were shipped off to him direct from Ohio when Tim and I swapped goods with one another.

Sooo, my question is...what da' heck is a Hum Control (or 'Hum Balance' as you can see in bottom pic) as these Bell's have it.

Is it the same as a bias control? I understand they moderate hum levels, but is this a separate function from bias adjustment? These were taken from a console - and I cannot find service manuals for them about, tho I've seen a couple mono integrated 6V6 Bell manuals (with other model designation) out there.

I'm going to have them re-tubed (all of them are pretty cheap if you shop around), as well.

I coulda' put asked this question in The Lab, but I feel completely out of my element in that circle

So, some kind soul out there kindly explain to me (in laymen terms) what a 'hum balance' is and how it differs from a bias control (if it does)?

Here they are as of January 2006:

avahifi · « **Reply #1 on:** 23 Jul 2006, 11:36 am »

I don't know about the Bell amps, but Dynaco had a hum adjust on the old SCA-35 tube integrated amp that actually just injected raw power supply ripple into the output of the amp out of phase with the audio signal.

The theory was that this would cancel the hum.

Of course all this did was to mask the gross effects, did nothing for the basic design limitations and the audio non-linearities this caused.

With good design and layout, there should be no hum to cancel at all. All the wonder parts in the world won't help this at all of course.

Frank Van Alstine

JohnR · « **Reply #2 on:** 23 Jul 2006, 12:44 pm »

I'm a bit surprised to hear about hum control on a 6V6 amp. (Indirectly heated pentode and all....) John, when you know the exact model number you should get yourself a schematic for your new babies. Look on EBay for "Sams photofacts." I recall reprints can be bought new somewhere but it's been a while... look forward to hearing more about them.

[Edit] D'oh!! Model 310 D. I should learn to look at the pictures

Although.. the front panel says 201...

JohnR · « **Reply #3 on:** 23 Jul 2006, 12:50 pm »

BTW a bias control adjusts the amount of DC current flowing through the output tubes.

TheChairGuy · « **Reply #4 on:** 23 Jul 2006, 03:41 pm »

In the deepest recess' of my parched memory - I had thought I had remember seeing/hearing of a tube amp or two with both bias and hum level controls....so I was pretty sure they were distinctly different.

Other than poor or ineffective design - why would a manufacturer integrate such a control in an amp? As Frank mentions, it does seem counterintuitive to me to have it. I mean, just design the dang things to NOT hum to begin with, right? Or, is there some other purpose to having a hum control?

And, should I futher take it...these guys have fixed bias that need a tech to re-bias when they make it back to me?

TheChairGuy · « **Reply #5 on:** 24 Jul 2006, 03:12 pm »

Hello, helpful (tech or tube knowledgeable) souls out there......

BobRex · « **Reply #6 on:** 24 Jul 2006, 06:16 pm »

In a triode (or triode strapped) amplifier, the bias is the amount of standing current applied to the cathode. The signal then modulates this current. The hum adjustment tries to balance the AC voltage on each leg of the heater. Most SETs are have AC heaters and this pot is added to reduce the offset. Amps with DC heaters don't need to balance the filament voltage so they will not have a hum pot. The same should hold for tetrode/pentode amps. Mt old cj MV75 has the ability to bias each output tube, and that's to match the opposite sides of a push pull pair. My current SET amps have a permanent bias, via a cathode resistor, but they do have a hum pot. Completely different functions.

With the Bells (as well as any other PP amp) the hum pot is to balance the heater/filament voltage between the output pair. It probably reduces leakage voltage to the OpT.

TheChairGuy · « **Reply #7 on:** 27 Jul 2006, 04:00 pm »

Thanks for the explanation of this phenomenom, Bob

'OTL' chimed in a PM with this resource, as well....a bit technical, but I was able to gleen a bit from it:

http://www.oldradioz.com/manuals/rdh4/CHAPTR31.PDF

TheChairGuy · « **Reply #8 on:** 14 Oct 2006, 08:57 pm »

Sooo, these little Bell amps have returned after long away, completely re-tubed with vintage Sylvania (matched quad) 6V6's and a couple of (new, made in Hungary) EZ80 rectifier tubes. About $70.00 with shipping for the 6 tubes.

They returned from Brad at Revelation Audio Labs with a new captive cord and some re-wiring done within.

Alas, the caps were not replaced and both hum a little (despite tinkering with the 'hum balance' control)...one side more notably than the other. One side's noises can be trimmed a bit by jiggling the 'hum balance'; the other one has a bit of transformer buzzing (both hooked up to my balanced power conditioner). Clearly, those 50 year old caps need dire replacement. I'm not sure I will replace them yet...I kinda' like something in my house older than me

They're but about 6 lbs a piece...complete with 3 inputs including phono (I hope, magnetic 47K input, not ceramic)

I have them playing their little 10 watt hearts out on my Linaeum speakers...which do require a little more juice; but are an easy drive as they have very minimal crossovers. They are fairly faithful to the source (which is, right now, a cheapie Aiwa portable player)...Steely Dan's 'Two Against nature' has that trademark annoying high frequency content, Putamayo's 'Euro Lounge' sounds about the same as I remember it, etc. I have heard other tube amps, I guess most of the the ubiquitous EL84 power tube variety, and I do find the 'tone' on these to be quite pleasant (6V6's are apparently revered by many guitarists for their tone...audiophiles are less 'hip' to them, apparently, as they are a relatively rare entity nowadays)

The soundstage is quite wide....I really do see why you tube amp guys like these things. As I'm getting older I'm placing a higher emphasis on sheer listenability rather than some vague attempt at 'realism'.....something I don't think I can achieve in less-than-optimal surroundings, regardless. These little Bell 6V6 amps, traded with Tim/Tubes4Ever (another great AC'er

) for a used Alpha-Core interconnect, are quite a nice listen (tho seriously underpowered and burdened with old caps at the moment)

Bill Baker · « **Reply #9 on:** 14 Oct 2006, 10:26 pm »

Hello Chairguy,
In most situations, hum pots are supposed to be adjusted by ear but this does not always lead to the best setting or balance between channels. If at all possible, have a local tech adjust them on the scope.
I have had countless SET amps with hum pots come through where the consumer thought the pots were adjusted perfectly only to find out that after being adjusted via the scope, the amp(s) were dead quiet, equally balanced between channels and sounded much better.

TheChairGuy · « **Reply #10 on:** 15 Oct 2006, 02:03 am »

Hey Bill - thanks

Definitely, I'll take these to a tech for cap replacement, proper biasing, etc.

Tho they are suprisingly hard to find in a metro area of over 5 million, there are a few here in SF Bay area.

John / TCG

Berndt · « **Reply #11 on:** 15 Oct 2006, 04:22 am »

So, who is doing your work local?
Gonna make the diy fair tommorrow?
shameless plug...

TheChairGuy · « **Reply #12 on:** 15 Oct 2006, 05:19 am »

Quote from: Berndt on 15 Oct 2006, 04:22 am

So, who is doing your work local?
Gonna make the diy fair tommorrow?
shameless plug...

I thought it cancelled, Bill?

http://www.audiocircle.com/index.php?topic=32420.0

TheChairGuy · « **Reply #13 on:** 15 Oct 2006, 05:05 pm »

These really are very nice little amps....they make CD sound more enjoyable than I'm used to. Opera is really fun to listen to...but they handled fairly dense orchestral, too, last night. They are un-focused, a bit 'distant' sounding in their presentation right now (no doubt due in part to 50 year old caps)....but their is interesting sonic substance to them. They need an hour to warm up properly....about the same as solid state amps I've gotten used to.

What's the deal with tube watts? These are rated at 10 watts, but they are the mightiest watts I've ever experienced. 12 o'clock on the volume control is all the sound pressure I'll ever need. I've oft heard that 1 Solid State watt = 2 Tube ones.....but it seems like a 1 to 4 or 5 factor here

It's just seat-of-the-pants impression, no db meter here, but my office is my listening room and I'm in it (listening to music) up to 60 hours weekly (when I'm not traveling). So, there is not much in the way of day-to-day audible surprises to me.

I'm comparing them to 150 watt+ Odyssey Monoblocks putting out 100 amps....and these lilliputians are pushing the speaks just fine in a 12 x 15 x 8-12" room. Do we all generally just tend to buy too many watts for our needs?

Berndt · « **Reply #14 on:** 15 Oct 2006, 06:02 pm »

How do they drive the MMGs?

TheChairGuy · « **Reply #15 on:** 15 Oct 2006, 06:37 pm »

Quote from: Berndt on 15 Oct 2006, 06:02 pm

How do they drive the MMGs?

I'm pretty sure they don't, or won't. They do have a 4 ohm tap, but I can't fathom them actually driving them at 10 watts effectively. I might just try it for shitz and giggles, tho

Lately, I've been listening more to my Linaeums. I keep coming back to the conclusion that you need a HUGE room to fully enjoy Maggies - even the smallest ones (MMG's, SMGA's, etc). My 12 x 15 x 8-12' room is overwhelmed by them - sonically and visually.

Dang, I'm listening to Ben Harper now....he's coming across with all his growl, disdain, wonder, emotion and guitar licks as I'm used to....with only 10 watts. I'm so intrigued by how all this can be

TheChairGuy · « **Reply #16 on:** 16 Oct 2006, 06:36 pm »

Hey, Bill/Frank/Scott F./DMason or anyone that can jump in on this with comment....hooking up my TT (cartridge output a decent 5.0 mv) to these amps and the output was unacceptably low. Would that indicate the gain too low on these...were catridges in days gone by of higher output? Or, could these be well in fact for ceramic phono? It's circa 1955 or so amps - not entirely sure as I don't have or can find a manual for them.

Additionally, when using a phono pre available to me, run into 'Radio' or 'Tape' inputs (the two line inputs available to me), NOTHING emits. Nada, zilch, zero. How could that be as the signal has been stepped up by 40db? CD's have no such issue...they play great (fantastic, actually, tired caps and all).

I'm not sure if 6V6 tubes are particularly punchy, or I am at a stage in my life were I am more receptive to tube amps (or both)...but, even with languid caps and all, these may be the best amps I've ever owned. I'm simply shocked...they don't sound like 'tube' that I'm used to at all. The bass is NOT slow...all the sonic cues are there with a gigantic soundstage now before me. I really am shocked.

Nice thing is they emit no more heat than my (6 triode tube) Dynaco PAS-4 preamp - the heat really is uncomfortable on hot, sunny California Spring/Summers with most tube amps.

I'd have guessed these were 50 watters...Peter Tosh is simply deafening at 9 or 10 o'clock.

mcgsxr · « **Reply #17 on:** 16 Oct 2006, 06:45 pm »

Tube amps certainly can be interesting!

I am presently using a 45-50 year old Magnavox single ended pentode console pull amp, with my DIY Visaton b200 speakers. Apparently this amp is roughly 3-5wpc.

I have been using a 30wpc JVC hybrid digital, 50wpc gainclone, and have tried a 100wpc Panny receiver.

Darned if the 3-5wpc tube amp isn't easily as loud!

I use my Bolder SB3 as my pre/source, and run it around 6-15 of 40 steps in volume!

Fun stuff, and cheap as nails by comparison...

TheChairGuy · « **Reply #18 on:** 16 Oct 2006, 07:18 pm »

You ain't kidding, CanuckMark!

As you know, I;ve been down some of the same roads as you the past couple years, and I, too, am finding tube amps to be rediculously powerful for their rated wattage

I was reading recently that Peter Walker of quad made his classic Forty mono amps at 15 watts, more not needed, as he thought that completely sufficient to handle large scale orchestral (the most demanding music out there then as now...outside of, perhaps, DVD). I assume that's 15 watts on his own electrostatic speakers, as well, I think we're driving more efficient loads than that in our listening rooms now.

TheChairGuy · « **Reply #19 on:** 17 Oct 2006, 02:58 pm »

Anybody offer an expert opinion here??

Quote from: TheChairGuy

Hey, Bill/Frank/Scott F./DMason or anyone that can jump in on this with comment....hooking up my TT (cartridge output a decent 5.0 mv) to these amps and the output was unacceptably low. Would that indicate the gain too low on these...were catridges in days gone by of higher output? Or, could these be well in fact for ceramic phono? It's circa 1955 or so amps - not entirely sure as I don't have or can find a manual for them.

Additionally, when using a phono pre available to me, run into 'Radio' or 'Tape' inputs (the two line inputs available to me), NOTHING emits. Nada, zilch, zero. How could that be as the signal has been stepped up by 40db? CD's have no such issue...they play great (fantastic, actually, tired caps and all).