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#1 2010-10-02 03:04:29

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

JJ 7027 Bias / Disipation

I'm now at the point where I'm working on biasing my V4b; I have a set of JJ 7027s that seem pretty well matched.  The first set I got, one of the tubes was about 6-8ma off the others...with the stock 75k bias resistor, that set were pulling current in the mid 30ma range, but a second set sent to me from the vendor, and these have far lower markings.  Sure enough, with the stock 75k bias resistor, these were pulling about 18ma current, though they're all pretty close closer as far as I can see.  But I obviously need to adjust it.

So first off - I don't have a trim pot on hand to put in the bias supply.  Anyone have one in particular you'd recommend?

In the meantime I started calculating and paralleling large value resistors to see about where I'd want to be with these tubes, for now.  Paralleling a 1Meg got me to ~70k and 21ma. paralleling a 330k with the 75k got me to about 33ma.  That got me thinking - about how much current do I want?

A deeper look, and I notice the JJs 7027 datasheet says they can take higher plate voltage than the 6L6GCs, but the 7027s are listed at 30 watt tubes just the same.  Hence some confusion has ensued...

So that being the case, how many ma do I want?  I've seen Steve suggest that he keeps closer to 60% with 7027s - which is probably hotter than the factory from what I can see, but still colder than the typical 70%.

Given that JJ is saying they're 30 watt tubes like a 6l6GC is, rather than 35 watt,  should I stick to that 60% idle bias suggestion for 30W or 35W, or 70% of 30W?  At 540V ~34ma of current is about 18W for a 30 watt tube, which is ~60%. But for a 35W tube 33ma/18W is only 50% - unnecessarily cold.  39ma would put me at about 21 watts - 70% of 30W, or 60% of 35W.  Is this what I want? 

I'm not so much scared to have to get a new set of tubes in a year or two.  I'm not going to be using it everyday (or else I'd be more conservative and get a backup set of tubes!).  But I initially have had some highly random redplating despite hours of sitting and staring at various tubes without any sign redplating.  I've now replaced all the caps in the bias network and am going to replace C11/C13 this weekend too. But I'm still paranoid about redplating again, especially once it's finally off the bench - while jamming away from home or just when leaving the room, or worse, have it do major damage in the process of blowing a tube or tubes.

I'd definitely tweak the bias by ear if I could play it in my apartment at volume and had a trim pot to compare the differences, but I can't play it loud while it's on the bench and I have no trim pot to do on the spot adjustment anyway so it's hard to compare as it is. I mostly have to make a technical decision and stick to it for now.   sad

Right now I'm inclined to split the difference and shoot for 36ma - !19 watts which is 65% of 30 watts and 55% of 35 watts, but I wanted suggestions on how you guys with the JJ 7027 run them from a technical standpoint.  Thankfully the difference in clean output power is not of much consequence to me - I imagine it will be plenty loud either way - at least 75 watts clean idle?  Safety, tone, and tube life are the major factors.

Last edited by Liquids (2010-10-02 09:56:16)


Matthew

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#2 2010-10-02 22:19:22

Liquids
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Posts: 491

Re: JJ 7027 Bias / Disipation

Any takers?  big_smile


Matthew

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#3 2010-10-03 22:32:00

hangman
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Re: JJ 7027 Bias / Disipation

I would use the 30 watt plate dissipation rating for the JJ. 

in regards to how hot to run them,  i think 60-65% is a great range to run them.  I think 70% in the V4 is a little hot for modern tubes... (unless you don't mind replacing them a lot.)

At 60% plate dissipation the amp puts out 100watts clean. i'm not sh*tting you.

The only reason i recommend not running the tubes at 70% is because the tubes are already being pushed pretty hard.  the maximum plate voltage is being exceeded,  the maximum screen voltage exceeded,  i just feel that running the tubes hot in addition to those things is just asking for problems.
that being said,  we've modified V2s to run KT-88s and we bias those puppies at 70% without even thinking twice.  and the amp sounds killer.  68watts clean!
anyway...  don't overanalyze it,  go with your gut.  if you like the way it sounds at 70%  go for it.  if the tubes fail in 3 months,  then you'll know the cost. if they work great for 4 years, then we'll know that its not that big of a deal.

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#4 2010-10-03 22:59:40

Liquids
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Posts: 491

Re: JJ 7027 Bias / Disipation

Thank you Steve.  I definitely understand your logic - with most amps I'd stick to at least %70  too, but I get why 60% makes sense with this amp.  I just got tripped up wondering, whats 60% or 70% of what (watt)? big_smile

In the meantime I bread boarded up a way to parallel a pot/trimpot and resistor and try and tweak bias by ear watching my meter via the 10ohm plate resistors, see if I could hear anything. This was at conversation volume, breaking the rules in my apartment though.

Anyhow, at least at that volume, the differences were surprisingly minimal to me, outside of biasing it so cold as to loose volume.  It was certainly clearer/crisper and less dynamic feeling at lower bias point, and a bit more meaty into 38ma and beyond.  Overall biasing hotter didn't seem like a major or necessary benefit at lowish volume.  I feel that with these new tubes the amp is so warm and fat sounding, I don't think I need the hot bias for the sake of tone like I would want on other amps. Once I can tweak bias by ear at higher volumes, I may change my mind.   Curious as to your perspective on the tonal differences between 50/60/70%  (~28ma / ~34ma / ~38ma?).

So I soldered in some resistors that I had that got it at ~34ma, though not knowing if that was 50% or 60% at the time.  That will do until I can get a nice multiturn trimpot in there.  I've seen guys suggest a 60k resistor +20k trimpot, but this set of tubes I got was drawing only about 17ma with the stock 75k resistor, so to get 34ma,  I'm at just about 60k...so maybe a 47k or 56k resistor is a better starting point in series with the trimpot, FYI.

I'm still concerned about getting the right trimpot with a safe wattage and voltage ratings in the meantime.  Trimpots seem so flimsy, and what a bummer to burn a trimpot suddenly here - result in an open circuit (cutoff) or a short circuit (maximum current draw)...so I'd like to get a good one so I can sleep at night!  big_smile

Anyhow, so I can expect to get 100 watts at 34ma?!  I'm going to a practice space tomorrow and now I especially I can't wait to crank this puppy up!

Last edited by Liquids (2010-10-03 23:02:20)


Matthew

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#5 2010-10-04 00:15:21

hangman
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Re: JJ 7027 Bias / Disipation

The Bias current will have more of a difference at louder volumes... but I've never been one to freak out about the differences in sound when biasing at 60 or 70%.  there is absolutely no audible difference in volume.  its more of a textural thing.  and I think you described it pretty accurately.  amps that are biased on the cold side tend to sound a little more solid, and less reactive,  while the hot amps tend to breathe more and have a little more grind to them. 
I always ask customers how they'd like me to do it,  but generally speaking... I bias most amps at around 65%.  Some amps I will bias more towards 60%.
its all a matter of reliability.  because I do this for a living,  The last thing I want to do is sell a customer a $100 quad of tubes, have him pay me to bias it at 70% and then see him back in a few months complaining that the tubes blew up.
its my job to keep my customers amps working AND sounding good.  Sometimes you have to balance the two.

I've never installed a trimpot in a V4.   if I change the bias i do it with a resistor on the trace side of the circuit board so that anyone that works on it in the future can instantly see the change.

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#6 2010-10-05 02:09:52

Liquids
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Posts: 491

Re: JJ 7027 Bias / Disipation

Okay, I've got an odd issue.

After hours of sitting and watching it this weekend, replacing screen resistors, plate resistors, C13/C11, C23/C15, getting these JJ 7027s bias set at 34 ma by paralleling a 82k/270k resistors... I fired it up today at my practice space.  I heard a hum and thought - that's odd - and sure enough, it was redplating like crazy!?

Turned it off, cooled it down, and it did it few times again for short time to check it out. I swapped locations, didn't matter, some start before others but they all redplate no matter the position.  Sounded clean when I plugged in and kept an eye on it, though I shut it off rather quickly just the same.  I quickly measured current across the plate resistors at the practice space (I'm so glad I leave a spare DMM there!) and when I saw it was easily drawing 60ma+, I switched it off again as usual.  Wall voltage was 122v rather than the 118 I get at home, that's the only variable I felt I couldn't isolate.

So I'm pissed since it seemed so very stable, all weekend, just hanging around running it waiting for it to act up again, and it didn't.  I took it out to play it and suddenly it acted crazy.

I got it home, and sure enough, it's drawing like 150ma?!  So I plugged in the old microphonic tubes it came with...they're drawing about 38ma - though one is drawing 47ma - not well matched, but no redplating?

Lastly, I've snipped the paralleled 270k bias resistor so that it's just a 82k in there, p[ut the JJs back in.  The tubes then draw 30ma, so I was thinking I might be ok...but then after a bit of time they're redplating - at 30ma!?

What the heck?!  I don't even know where to start.  My new tubes are probably just shot now, but where I do go from here?  Do tubes that redplate too long or too often just always redplate no matter how cold they're biased, after a while?  Maybe the screens are shot?

IS there anything should I be looking at or measuring to proceed and find the problem?  The only thing I can think to do is order all new diodes...what else merits replacement?  Intermittent problems suck.

If I have to buy new tubes, I'm highly temped to go with new sockets, tube holers, and kt88s.  If so, anything I need to do to make sure they clear the transformer?

Last edited by Liquids (2010-10-05 02:12:23)


Matthew

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#7 2010-10-05 09:11:55

hangman
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Re: JJ 7027 Bias / Disipation

wow,  that is rough. 

when you set the bias did you use the plate resistors?  or what method did you use?

I have always had great luck with the jj7027s.  In rare instances I have had issues with tube sockets,  cleaning and retensioning the sockets could help,  but you did say that the problem followed the tube...
once a tube redplates pretty bad, there is no relying on it. 
I've had some last years some last days.... the latter is more often the case.   
the KT88s come within a mm or two of the power transformer.  some guys actually move the PT,  I usually just leave it.  that transformer is gonna get hot no matter how close those tubes are.

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#8 2010-10-05 10:39:05

Liquids
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Re: JJ 7027 Bias / Disipation

Yeah, I measured the voltage drop across the new 10k 5W plate resistors to measure the bias.  .320v = 32ma type of thing.

It seemed to follow the tube at first, but they all redplated in short time at 60 or 100+ ma obviously!   It's just which ones redplate first seemed to follow the tube. Back at 30ma, with it still redplating, I didn't get to deep into which ones.

I was also thinking of having the tube sockets replaced - or doing it myself - and going for the spring and hat style retainers while I'm at it, but it doesn't seem logical that the worn out RCA tubes I have aren't redplating at all at 38ma that I can tell if it was some short circuit on the tube sockets.  I'm stumped.  Any chance it's one of the transformers or diodes?  Diodes all test fine but I'm not afraid to replace 30 year old parts for stability and elimination of the variables...


Matthew

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#9 2010-10-08 09:46:09

Liquids
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Posts: 491

Re: JJ 7027 Bias / Disipation

Thought this might be valuable info, and it may help further diagnosis if need be - long story short, it turns out one JJ 7027 tube was the culprit.  It was hard to track down because, as best as I can now understand 1) MOst of the time two tubes were redplating and two weren't 2) it wasn't following sockets in particular, two tubes in particular, or an A+B pair.

Turns out that ONE tube (tube X) in particular seemed to go bad.  But whatever tube was on the same side of the transformer as tube X would also draw 150ma. However, if I moved tube X, the problem would follow both tube X and again whatever other tube shared that side of the transformer (left or right).  Really frustrating, especially after running it and having it be so stable for hours over the course of three days only for it suddenly to have a problem at my practice space?

Of course at this point that means I had a tube go bad under very normal conditions, the other three are no longer matched, and probably pretty shot due to a faulty tube.  It and makes me leery of the JJ 7027s (could be a fluke but I'm not sure), not to mention this as just one of multiple frustrating issues with the vendor at this point. 

I guess my question is - is there anything else I should keep an eye on given this kind of current draw one one side of the transformer?   I'll only have the old dead, micro phonic (loose mica spacers) poorly matched tubes to work with for a while now unfortunately.  And still a little scared to put any other tubes in there.  But I've replaced everything in the bias network at this point outside of the 56k resistor and the diode (I'm going to go ahead and do at least that before I put any other tubes in this thing).  Sigh.


Matthew

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#10 2010-10-08 16:30:09

hangman
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Re: JJ 7027 Bias / Disipation

sometimes bad tubes happen.

The JJ has been for the most part very reliable for me.   
where did you get yours from?

Ce distribution offers burned in sets for a couple bucks more...
otherwise winged C  or NOS would be my tube recommendations

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#11 2010-10-08 17:39:13

Liquids
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Posts: 491

Re: JJ 7027 Bias / Disipation

I currently don't want to throw the vendor under the bus in public (at the molment), but I can PM you if you're curious.

I realize Bad Tubes do happen. But I've ordered the parts to go for the Sovtek 6550s, which will be more work but make me more confident in the reliability given the plate and screen voltages, or maybe tax a fringe transformer and make it clear if it needs replacement - which would be more expense, but I have a back up amp and am bent on getting this thing reliable and optimal.


Matthew

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#12 2010-10-08 23:06:50

hangman
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Re: JJ 7027 Bias / Disipation

you didn't want to go for the kt-88s?

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#13 2010-10-08 23:16:31

Liquids
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Posts: 491

Re: JJ 7027 Bias / Disipation

I did, but I don't like the size of them of messing with clearance issues of the already hot transformer having a hot glass bottle physically in contact with it if not 'just so.'  I know you see it as negligable but I'm at the early stages of this venture, you've seen your fair share of V4s come and go big_smile

And again, I was thinking more about an easy upgrade from the 7027s than the 'ultimate' in reliability, which given my previous situation shouldn't be hard to beat.  I also was ordering from another vendor, with no assumption that I'll be refunded the money for the tubes from the original vendor, so price was an issue since I might have just blow a new set of tubes and may have had to eat that cost.

Do you like the EH 6550 'coke bottle' tubes?  I read that you did, and I'm informed that it's the Sovtek 6550 with real nice Kt88 shaped glass FYI...which was a factor for me.  I did order the spring hats for the 6550 mod, but I also ordered new high quality bear clamps, so that I can easily go back into 6L6 bottle sizes without the rivet issues.


Matthew

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#14 2014-02-13 23:51:18

doug_olitsky
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From: Gold Coast, Long Island, NY
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Re: JJ 7027 Bias / Disipation

Hangman

I'm replacing my old 7027's for some JJ's (the old Phillips and magnavox are strong, I just don't want to waste them).

Should I leave the bias resistors alone or do you recommend another value. (the v4 amp is working great as a bass amp played w/ a Kubicki Factor and a SVT810)

I'm not looking for any sound changes just don't want to kill new tubes.

What recommended value and what location are the resistors on the diagram. If you think its fine w/ the stock setup that's OK too.

EDIT- On schematic I see the Bias resistor is 75k at position R49. According to my notes I never bought a replacement, so I guess my amp is still stock 75k ohms

Last edited by doug_olitsky (2014-02-14 01:45:16)

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#15 2014-02-13 23:59:23

doug_olitsky
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Re: JJ 7027 Bias / Disipation

Oh yeah-

what wattage

metal film, metal oxide???

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#16 2014-02-14 03:00:08

hangman
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Re: JJ 7027 Bias / Disipation

You can leave it stock on the bias.  I always change the screen resistors (470ohm 5w). to 1k 5watt just to be on the safe side.

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#17 2014-02-14 04:48:47

doug_olitsky
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Re: JJ 7027 Bias / Disipation

OK I will do the same

order is in to tubes and more w/ my 7027a's

what effect does changing the 470ohm to 1kohm have... just curious

R 41,42,46,47  CORRECT???

Last edited by doug_olitsky (2014-02-14 05:04:54)

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#18 2014-02-14 07:49:50

hangman
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Re: JJ 7027 Bias / Disipation

Ok, if you have a non-distortion model they are resistors 41,42,47,48

These resistors are there to put resistance between the screen and AC ground.   If there is no screen resistor parasitic oscillations at very high frequencies can cause your tubes to basically be working at full capacity even though you can't hear anything,  that's because the transformers and speakers can't reproduce these high frequencies and even if they could it's well outside the range of human hearing.   Power amp instability.
Bumping up the resistors just gives a little more cushion.   You do loose about 3/4 of a watt at full power... But you can't really hear that so it's worth it to me, just to be on the safe side.

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#19 2014-02-14 21:29:58

doug_olitsky
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From: Gold Coast, Long Island, NY
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Posts: 45
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Re: JJ 7027 Bias / Disipation

Ahhh, just like old McIntosh ss amps I've worked on. I remember changing out some pwr resistors that were coupled to the output transistors on a MC 2200.

I Don't know if 99.25 watts of power will be enough :-)

Many thanks for the knowledge. I'm watching my doorstep for my Antique Electronics care package.


In the meantime I'm playing w/ a Fender Princeton 112 Plus I found in the trash. looks almost new but the speaker and reverb tank are shot. put a new speak in and waiting for the tank.

Amazing what people throw away. In the past I have found a Squire amp, vox pedal and a JBL monitor in the trash. All in working order and looking almost unused once cleaned up w/ some windex.

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#20 2014-02-15 09:25:16

hangman
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Re: JJ 7027 Bias / Disipation

99.5 might be a little conservative,  these guys can put out 105-110 watts clean.  It is pretty impressive.  But all the more reason to not worry about the small loss.

People do throw away very useful things. Reflection of our culture.  Everything is disposable these days,  most people don't remember a time when things weren't.

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#21 2014-02-22 01:18:11

doug_olitsky
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From: Gold Coast, Long Island, NY
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Re: JJ 7027 Bias / Disipation

Alright

Package arrived from Antique Electronics Supply.

All done and new JJ's in. 10 minutes on the amp and all OK

Thanks to all.

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#22 2014-02-22 02:00:12

hangman
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Re: JJ 7027 Bias / Disipation

Cool!  glad its back up and running

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#23 2014-03-03 03:03:03

doug_olitsky
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From: Gold Coast, Long Island, NY
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Posts: 45
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Re: JJ 7027 Bias / Disipation

It's easy to jump in and do these things w/ the resource you've created. So thanks goes to you and all that help keep these beauties going!

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