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In Reply to: Re: A suspicious voltage! posted by J Epstein on January 18, 2001 at 21:17:28:
No I am pretty sure the pins are right, I mean the numbers are ON the
sockets! That is something I DID pay attention to during assembly.
I am pretty sure the pots are wired right, I can see no other way to
wire them (basically the "left" connector on the pot is connected to
the "inside" wire on the RCA jack, the center tab is connected to the
grid of the 5691 and the "right" side is connected to ground(along
with the "outer" RCA) So when you adjust the pot you are inn effect
causing the resistence on the grid to ground to change, right? BUT I
am still unsure if my choke is in backwards or not?????
Follow Ups:
Pot is correct, choke is a subtle and fairly insignificant distinction which way you wire it, don't worry about that at all. I can look in my Hammond catalog tonight and advise you on that but it is NOT your problem.Hmm.
Well, is there any difference in the sound now that you have snipped out the extra carthode resistors?
-j
I have not tried it yet, I will after work. I was thinking, I know the Hammond 125E OPTs are very limited, but I HAD planned to use this amp just for my tweeters (bi-amping). I know it is totally hypothetical, but I'll probably need to buy better OPTs won't I?
Ok so I plugged everything in and tried again! NO DICE! First of all
once I had the input tube in I checked the voltages on it once again,
it looked right, anywhere from 130-210 VDC on the plate of the 5691.
BUT as all this was going on stange sounds where coming from my
speaker, kind of a popping spike-like sound. for a BRIEF second i
herad very faint and distorted music, BUT here is the kicker, the 45
cathode biasing resistors got VER VERY VERY VERY VERY VERY hot! So I
have to think that everything else seems right, EXCEPT the hole
cathode biasing affair! I think somehow it is wrong. I am not sure
how all this is SUPPOSEd to work with a DHT, what I have is 3
transformers, 1 for the main power, and 2 very small 2.5V 1.25A
filament transformers for the 45s, OK on the schematic it as "55V" on
the CT of the 45 filament transformers. So what the hell is this all
about?????????? Is there any way to DETERMINE if the bias is right? I
should add that during all this with the power in and the noises the
45's glow ould fluctuate severelly, becoming dim and then normal
bright! So what is next??
Hmm. Wow, this must be frustrating. It sure is confusing.Somehow your bias (on the 45) is screwed up. What you describe sounds like intermittent solder joints, now making contact, now open. And Doc B says that 90 percent of his kit builders' problems can be traced to that one problem.
So: re heat EVERY solder joint, starting with anything connected to the 45 socket or 45 filament transformer.
I'd especially be concerned about the 470K resistor from 45 grid to ground - if thise were intermittent I do believe it would cause the problems you describe.
As a double check, make sure you measure 470K ohms from that socket to ground with no tubes in.
Also verify the connections of the cathode bias resistors (1.5K from filament center tap to ground as you have it wired now.)
Finally, be sure all your electrolytic caps have teh (-) erminal to ground - these would be the 100uF cathode bypass caps on driver and 45. And be sure you are using a 100V rated cap on the 45 cathode bypass.
I'm just throwing a lot of stuff up against the wall. I hope some of it sticks!
-j
Ok so I re-soldered EVERYTHING. I replaced a few wires and re-routed
a few. When I first turned it on there was a lot of popping and the
45(I only had one in, one channel) started going through a series of
lighting up and then going dim all this accompanied by a spiking
sound from the speaker. AND looking into the 45 it looked like it was
sparking on the inside! kinda like a bug zapper!!!!??? Well I said to
hell with it and let it go, finally after about 15 seconds things
settled down, the 45 stayed lit, no more sparking noise and the music
got louder!!! Ok so I am STILL not there, it is still very quiet and
very distorted. BUT this probably isn't the amp is it??? I think
maybe it is not being driven hard enough. Don't laugh, but my source
is a discman! I guess the amp needs what? 1.2 V input for full
output????? And MAYBE my 45 is just old. I got it used from
Tubeworld. They said it tested good, maybe not good enough! OK and
one last thing, I measured the voltage across the plate and the grid
while it was running (the 45) and it was around 160 VDC. Does that
sound right??? That is not a steady state sort of thing right? I mean
that voltage varies depending on if signal is flowing, right?
I guess it is obvious, I need a better source (and maybe a pre-amp??)
well it is on the way (a new "real" CD player) I ordered one of those
Rik Stoet modified Marantz, the CD 5000. It will surely be better
than what I have now! Ok and maybe i need to put in a NOS or almost
like new 45? Any other suggestions??? ( am really interested on an
opinion of the whole popping behavior of the 45??? Is this an
indication that the tube is on its deathbed???)
The behavior of your 45 is not a really GOOD sign, no. Sometimes I have had a tube that will spark once and then settle down (it's bits of material getting caught beween two elements and getting a short-circuit through them) but not as a regular behavior. Is just one tube doing this? Maybe try and get your supplier to swap a good tube for the problem child.Measure these voltages, using the DC scale of your meter:
from pin 1 to ground - should be somewhere around 55V DC
from pin 2 to ground - should be somewhere around 325V DC
from pin 3 to ground - should be somewhere around 0 VDC
from pin 4 to ground - should be somewhere around 55V DCSo from pin 1 OR 4 to pin 2 should be about 270V DC
On the AC scale:
from pin 1 to pin 4 - should be 2.5V ACLet us know what you find. It certainly could be a bad tube. Also your source might be having trouble providing enough signal - it's not driving a headphone and it may not sound good driving a line level input. But this is a seperate problem to the tube's misbehavior. Try taking the output from a cassette deck or whatever you might have handy and see what you get.
-j
ok pin 2 to ground I get 316 VDC pin 3 to gnd I get about -.44 VDC
pins 1 and 4 to gnd are EACH about -.003 VDC. Between 1 or 4 and 2
are both 305 VDC. And 1 and 4 is about 2.3 VAC.
SO! There is NO 55 VDC to be found (that is the bias voltage right??)
and accordingly NO 270 VDC (which is the B+ minus the cathode bias
55, right???) So if I have learned anything so far! I guess it is
STILL not biased right? (AND WHY NOT? I am using only 1 1.5K ohm
resistor on the CT of the filament supply bypassed with a 100 uf 100V
electrolytic with the lead with the minus symbols on it going to
ground! MAYBE i have bad resistors??????
Is this measurement with the 45 in or out?If the tube is out, measure again with the tube in.
If you still get ~ 0VDC at pins 1 and 4, then something is wrong with the tube. If you do NOT get ~ 0VDC at pin 3 (the grid) then your 470K grid resistor is not soldered correctly. Everything else looks good.
("~" = "about" - by ~ 0VDC I mean the type of numbers you are getting, less than half a volt.)
-j
All those measurments were made with the tube in. AND yes the tube IS
bad, or at least NOW it is! Once I realized that(the tube was bad(now
the grid will not light up AT ALL)) I put in ANOTHER 45, ok so it
started up and worked fine. BUT about 3 or so minutes into operation
IT started to make poping sounds and sparking on the
inside!!!!!!!!!!!!!!!11 SO I turned it off and took it out and put it
BACK in its box (the first 45 was a used 1940's RCA ST-14 type, all I
have left are two much older Cunningham 345 balloon type that are FAR
to expensive to even dream about buring up!!) The only thing I can
think is the cathode resistors are not working properly, maybe
getting too hot, maybe they need heatsinks???? I don't know. But that
is about all I can figure out! The resistance on them measures fine
UNTIL I put them in the circuit and run them for a few minutes. SO I
ordered some ceramic type 10 watters, once they get here I will
solder those in and try again!
Hmm. Don't kill the good 45's, I agree with you. Maybe your others are still OK, though, tubes are harder to kill than you think.I think you are on the right track with the cathode R's - they may be overheating. mayeb they deveop a short afetr that happens - I don't know typical failure modes for resistors. Anyway you always want your resistors in a tube device to be rated for more (I say at least double) than you run them at. Cheap insurance against this kind of crap.
Leave it aside and solder in ONE new 1.5K/10W per channel when you get the parts delivery. Then we'll try again. You may have killed these 45's though. I'd advise getting the amp running reliably with a relatively (yeah, I know, that's still not cheap) inexpensive pair of 45's (or 2A3's) first before you put the Cunninghams in.
-j
Well after having come to my last "conclusion" about the cathode resistors I decided to do some reading about biasing in general. I have a RCA recieving tube manual from about 1959 and I downloaded some pages from an earlier version that had detailed info on the 45. WELL as I looked closer and then recalled the pinout diagram that I DID use when I put everything together, one that was on the internet, some kind of tube info page, anyway once I read "pin orientation shown from below" on the RCA tube manual page and then looked at my tubes and then back at the manual then back again then I DID smack myself in the forehead! Yup, you guessed it, I had the plate and cathode switched on the 45s! Well getting that right makes a BIG difference(DUH)!!!! They sound just fine. I did burn up the 1 45 and I think the capacitors on the cathode resistor (100V and I guess with things backwards the B+ was running through them, so now they are all swollen and I suspect bad, I replaced them) But NOW it works! I need to make some more efficient speakers. And I was thinking of a Class A SE with a 6550 in it.
Well after having come to my last "conclusion" about the cathode resistors I decided to do some reading about biasing in general. I have a RCA recieving tube manual from about 1959 and I downloaded some pages from an earlier version that had detailed info on the 45. WELL as I looked closer and then recalled the pinout diagram that I DID use when I put everything together, one that was on the internet, some kind of tube info page, anyway once I read "pin orientation shown from below" on the RCA tube manual page and then looked at my tubes and then back at the manual then back again then I DID smack myself in the forehead! Yup, you guessed it, I had the plate and cathode switched on the 45s! Well getting that right makes a BIG difference(DUH)!!!! They sound just fine. I did burn up the 1 45 and I think the capacitors on the cathode resistor (100V and I guess with things backwards the B+ was running through them, so now they are all swollen and I suspect bad, I replaced them) But NOW it works! I need to make some more efficient speakers. And I was thinking of a Class A SE with a 6550 in it.
You'll only ever make THAT mistake ONCE!(Of course there are plenty of others to make!)
I must say I feel a little less puzzled which is good (although it doesn't un-fry your 45.)
Happy listening - it sounds like it would be safe to throw the Cunninghams in now!
-j
I am not sure how the whole cathode bias thing is supposed to work.
BUT in general what is the most common cause of little or no bias
voltage?????????
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