DRRI bias sweep.

[Sea Chief]

Hi- why is my bias pot only allow me max 20 one way, and 2 or so t'other? Im reading mV on the DMM which I think is right (read 6.7 max on the mA setting.. cant be right). Does this low max signify something amiss?

Its a Hammond 291BEX PT which pumps out a whopping 480v b+ on the plates with a GZ34.. so Ive whacked in a 5R4 to get me 413v b+ on both 6v6 tubes (at my brown and blue wires from OT > 6v6 points).

Thx SC.

[Sea Chief]

Hi Leo thanks for the spiel-

well I dont think the 6v6's are old & knackered.. they should be 1/2 life if not more left. Ive never pushed them, ever really. Odd thing is I had a 5U4 in (423v b+), then amp was briefly checked by a pro, who inadvrtantly shorted 2 lugs on the bias pot (i diagnosed it by an educated guess, whilst he got rather flustered trying to find why the 6v6's went red-platey!). The tubes were left for a period of time red-plating (say a min or 2 whilst he poked trying to find what the cause was- then powered off asap when he couldnt find it).

After I got it back and redid R69 (next to a diode in the bias circuit) it worked fine. But before he fried R69 there was considerably more bias range -albeit with the 5U4 I had in at this stage- Im sure able to get up to 30+.

Actually your suggestion of taagging on another 22K or so onto R59 (22k here) I already did, from you guys' advice on here.. which did the trick fine. So thst adds to the odd max of only 20mV I can find on the pot.

Im measuring across a 1.6r I added onto the 6v6 on pin s'thing/ the correct pin anyway. And Im measuring mV (not mA) which i think is right?

Would the 5R4 instead of the 5U4GB make any difference to the bias range? cant think it should, eh?

thanks SC.

[Mick Bailey]

+1 on the JJ 6V6S. They can be a little microphonic, but will stand very high plate voltages and idle dissipation. Not a traditional 6V6, but I think they sound good and my customers think so too.

[Sea Chief]

Pin 5 of 6v6 reads -33v.. what do you think?

I think the chap was reasonable with regard to the time red-plating, he did seem to try and find the cause quick and power down soon.. then wait, and repeat only a couple of times before thankfully I said check the bias circuit and bingo.. a fried resistor was it. Phew that was a relief (& a bit concerning as he did go in rather ham-fistedly/ it was friday very end of day..).

I get the analogy of the bowling ball. I need things like this without -any- experience of a tube red plating before you see. Id no idea whatsoever thats something they cant take for long/ I might have thought they 'look a bit red, dont they?'

They are EH 6v6GT's which i believe do have a reputation for being pretty rugged too.

[Mick Bailey]

The EH 6V6 (older ones, at least) are pretty reliable. With any tube though don't ignore screen voltage. That can be more of a tube killer than a high plate voltage.

[Sea Chief]

The EH 6V6 (older ones, at least) are pretty reliable. With any tube though don't ignore screen voltage. That can be more of a tube killer than a high plate voltage.

Thing is I have no reference point re. screen voltage apart from 'dont let it go below 30v' which its not but is -33v 'too close' or 'very good' or 'normal' or a catasptrophe around the cornner.. Ive no context at all. TBH I think its the 1st time Ive ever measured screen V on any amp. What are yours for eg? thats a start at least.

[Sea Chief]

Its like Mick Saying the JJ's are a bit microphonic.. but without knowing what a microphonic tube is or sounds like or looks like, I could not say whether I had one or not.

Also whats all this about bias settings meant to make the amp sound different? ive read folks want there's ~2mA perfect... but I couldnt tell an iota of difference 8mA to 19mA, apart from sounding louder. 2mA? bizarre.

[Sea Chief]

Hi Leo- good spiel, I can understand some. Some not: where does the 0.0125A figure spring from? Thes next section of plate power, watts, and current I just dont comprehend (Ive read watts many times mentioned, and dissipation, but alas I just dont understand it however many times its explained or I read it). I wouldnt know how to take measurements & do calculations therefroe, other than measure bias and my plate V. As for how one would double the plate current, & why, are two Q's far out of my reach or understanding. Apologies.

The 2mA figure I wasnt suggesting as a bias useable figure, but just the margin by which folks seem to want to get their ideal bias figure. I just dont get how on earth they could be so specific when i cant hear any difference within almost a full sweep of my pot (approx 8mA to 19mA). Another mystery. I just dont think amps and me mix! shame as I can hear good amp tone a mile away like a whale 10ks away from a farting squid. Or something like that.

[Sea Chief]

Hi Leo- I can understand the analogy i think tho Im not a motorhead. But however many times I read this stuff it makes no sense. Its not the maths itslef thats not hard, its just I dont know why were measuring this, or that, or why we want 70% or what 70% is, or how watts suddenly spring into the mix.. why you want to do the simple equation; yes its simple, but without having the framework or knowledge of why were doing x & y its all floating stuff with no meaning to me.

Dont worry its just not for me- all I want to do is set it for an average/ why I dont know it just seems logical, as I thought I found an anomally/ an issue with my very sim pot range.. but perhaps it is normal & no cause for concern, I dont know. I'll just put it to max & assume its ok.

One thing I did understand & see in practice the workings of, was adding the 22k to the already 22k in R59 spot on the bias circuit. So I piggybacked a 22k onto the existing 22k when I went from the gz34 (470+v) to the 5U4 (430v). So here i can see the way this resisitor adds range to the bias... but you see without know what on earth bias actually is! so there is room for me to do certain things fine without needing to know the grander picture (Id prefer to know, but Ive tried & tried & it just doesn't compute for me). Another eg, I diagnosed the problem in my own amp (something in the bias circuit wasn;t right/ a fried R it turned out to be) and actually helped the pro engineer who has huge knowledge like you, but who couldnt find the prob. But not from knowledge of the bias circuit - I have no knowledge at all- just I remebered s'one else's amp did the same recently > their bias circuit red plating the tubes.

Alas the spotty herbert still has much more knowledge than me, & I think he just will- he & it is nerdery; Im better at a crisp drive for 4 through the covers (cricket).

[Sea Chief]

Are you suggesting piggybacking another 22k onto the 22k onto the 22k (3)? thats surely out of a madman's dream. Or incracing the piggybacked 22k, maybe to 44k or something instead-?

Again you see Im not even sure here -why- I want more range, or a figure of maybe upto 30mA to dip into.. it just seems the right thing to do! Im just following my nose completely aimlessly, & it might well sniff out a truffle, but I was only expecting erm.. a hazelnut? My analogies are utter sh*te.

[Sea Chief]

Stacked piggies 3 high sounds like a party . . . no need to go there. But swap say a 15K for one of your already paralleled 22K.

You know more than you give yourself credit for. The rest will sink in over time & reading some text on the how & why of vacuum tubes. I didn't get it at first & still learning.

And you're doing better than I would with the heating system. Mine looks like the iron box where Fred Flintstone & Alley Oop burnt dinosaur bones, an old coal burner nearly a century in use. Maybe more. Now it runs on piped in gas, beats shoveling coal every couple hours. Belongs in a museum of "can you believe people used to live like this?" items.

I thought the higher the R the wider the sweep? surely if I put on a full 22k onto another 22k, less K addition would have meant less sweep going in the direction as it does of less/ twds the orig single 22k. Pls dont make my ear hair grow even more. i cant cope today with watts and dissipation and room stats.

I swea Im not doing well with the boiler stuff- Im more confused than I was when I started: 1st I had only to really ask whether my side of an argumnet was ok, my friends not. Yes. But now introduce a 3rd goddam thermostat and ffs its utterly ridiculous, Ive never known -anything in my life- that makes so little sense even after a whole page of 'explanation'.

I dont understand tube amps- I really dont. I dont even know what a tube is frankly. I understand sections within it but even those Im not fully on board. Eg a PT is ok got this winding and that wnding for this & that.. then throew a curve ball its got a godam NEGATIVE winding just to rub my nose in it so I can never understand it completely.

[Sea Chief]

But Leo is it worth driving the -33v bias current down, in order to drive the 20mV bias current up (you see right here Im into floating wtf area right away/ up, down eh?).. then driving bias Voltage down.. but I thiught I wanted to drive the bias Voltage up? I cant understand a single word of it sorry.

Im not yet able to understand how a tube works (I wont ever be able to get this). I was simply asking "what is a tube". IE What is a tube for, or what is its job?

My ear hair has grown 3mm just from this thread today. Im totally more confused than when I started!

thx tho- you're a stirling chap for trying/ no Q about it. SC

[Sea Chief]

By turning bias voltage (not current!) down, you increase the current thru the tube. (That's why it's called a control grid.) With increased current, you'll read a higher voltage on the sensing resistor. Tubes/valves, that's what they do.

It's a cause-and-result situation. By adjusting the bias voltage = cause, you increase/decrease the bias current = result, and you read that result on your sense resistor.

Would be a shame to quit at this point, sorta like not bolting the neck on your Strat. If you can do that, and you did, you can do this.

Ok bias voltage down = current up. Right understand my mistake.. but then you have to add the fact that its -negtive- voltage.. then, I can actually hear my ear hair growing. It's like someone's designed all this to be the biggest asshole alive.

Ok undertand you middle paraG (apart from that is, the neg V is not mentioned it seems).

Oh I can do it in 3mins flat. Not a case of not being able to do it, its just why do something if I dont know -why- Im doing it. If I can hear no difference between the bias at 8mA and at 19mA.. I have to ask myself why im considering reducing the Voltage (or increacing the negative voltage..? another can of worms) in order to increace the bias current.

Im reminded of Neil in the Young Ones "Oh no, out of the frying pan,.......... & into another frying pan".

SC

[Sea Chief]

Im really confused now- I dont know which way we are meant to consider it in talk here if its one way for techies in absolute terms and the opposite in general terms with which we use to discuss on here.

I cant really cope with this sorry.

What are we thinking of doing. Getting the -33V to a lower number figure (IE -25V) or a higher number figure (IE -36V)?

Whichever one we opt for, what is this (in terms we discuss on a forum, whether or not thats different to other ways of discussing I cant decide/ my head cant cope): a higher negative voltage or a lower negative voltage?

[Sea Chief]

Hang on this contradicts itself. You agree that a "lower number figure" to 33 which is 25, but you say a good figure is a "higher number figure" of 35 to 40.

And which does this 25 "lower number figure" equate to (in terms we discuss on here): a higher or lower negative voltage to the number figure of 33?

Im exhausted with this.