Current draw with KT66s in place...to much!

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danman
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Re: Current draw with KT66s in place...to much!

Post by danman » Sat Jan 24, 2015 5:36 pm

I just remember reading of issues in the past with certain types of KT66 tubes and pin 1 being grounded. It seems that some brands were using a metal base and shorting out when the bear claw retainer would grip the base. I wanted to have a look at the 1ohm resistor and how it is attached. One side of the resistor should be tied to ground. The lead on the other end should be tied to pin 8 and then bent over and connected to pin 1.

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Re: Current draw with KT66s in place...to much!

Post by ?Mark » Sun Jan 25, 2015 10:56 am

plastic base "Valve Art"
no retainer clip
1 ohm across 8 &1
with lead from 1 to gnd.

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Re: Current draw with KT66s in place...to much!

Post by danman » Sun Jan 25, 2015 12:04 pm

I'm not sure if the biasing resistor would cause any issue in that configuration. El34's and KT66's are interchangeable but have a slightly different internal pinout. The biasing resistor should not be between 1 & 8 but instead between 8 and ground with the extra lead connected between 1 & 8, bridging them together. The resistor could also be placed between ground and pin 1 with the extra lead tying into pin 8. Either configuration will work but I have never tried placing the resistor between 1& 8 so I cannot say if that would cause any issues at the socket. Maybe neikeel or another member will spot something that I have missed.

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Re: Current draw with KT66s in place...to much!

Post by neikeel » Sun Jan 25, 2015 2:10 pm

In this amp 1 & 8 can be tied on the valve socket base and then grounded to the mounting nut.

I think Danman was concerned that the pins were orientated correctly (not out by a pin all the way round) I share this concern and a pic from inside with tubes out an the pin arrangement vs the keyway position.

This is an issue as all your other readings look ok.
Neil

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Re: Current draw with KT66s in place...to much!

Post by danman » Sun Jan 25, 2015 2:22 pm

Would placing the bias resistor itself between 1 & 8 and then to ground cause any problems Neikeel? I have never seen it orientated this way and was wondering if you could still get a proper mv reading for biasing. Normally one leg of the resistor would tie together 1 & 8 and then the other leg would go to ground. I have never tried it between 1 & 8 to see if it would function properly.

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Re: Current draw with KT66s in place...to much!

Post by danman » Sun Jan 25, 2015 2:40 pm

I forgot to ask earlier if you could post some voltages for pins 3 & 4 on both v4 and v5 with the tubes out. Also with tubes installed if possible.

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Re: Current draw with KT66s in place...to much!

Post by ?Mark » Sun Jan 25, 2015 10:44 pm

No, I get it, so here are pics
The black line is the 1 pin
with key way between
pin 8 and 1 as is shown
nothing is off. God I wish
is was that simple. Probably
is/will be in the end :oops:

V4 & V5 pin 4 392VDC
V4 & V5 pin 3 393VDC

not able to get voltage with tubes in
because of the initial problem
"Current draw with KT66s in place...to much!"
and then blows fuse.

Hey Neil,
What about the 170VDC on the cathode of the PI?
surely this can't be normal!? Here is another
thought...ALL tubes get correct filament voltage
and warm up nicely,( which is another obvious
indicator that V4&5 pins,
sockets and the tubes are lining up right)
Problem again is switching on HT.
So I'm thinking the issue is else where
with the items connected to the pins&
sockets. I still need verification that
the marstran OT Primary wiring
is correct(please see earlier post).

1st 2 pics straight out of the bible Step # 5 :wink:

[iImage1_25_15 001 (1024x576) by https://www.flickr.com/people/130114472@N03/, on Flickrmg][/img]
[iImage1_25_15 001 (450x800) by https://www.flickr.com/people/130114472@N03/, on Flickrmg][/img]
[imImage1_25_15 003 (640x360) by https://www.flickr.com/people/130114472@N03/, on Flickrg][/img]
[imImage1_25_15 002 (640x360) by https://www.flickr.com/people/130114472@N03/, on Flickrg][/img]
[imImage1_25_15 001 (640x360) by https://www.flickr.com/people/130114472@N03/, on Flickrg][/img]

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Re: Current draw with KT66s in place...to much!

Post by danman » Mon Jan 26, 2015 6:23 pm

I have to admit that I have never seen the bias resistor mounted that way but electrically it seems to be the same. The voltages that you listed on pins 3 and 4 seem to be very low, especially since the power tubes are removed. With tubes I would expect you to be seeing around 440vdc. Without tubes it should be even higher. There seems to be something pulling the voltages down even without the power tubes installed. I have had this problem pop up in new builds before so now I have gotten in the habit of powering up and testing the power supply in phases as I am building. I have had a bad PT right out of the box that I was able to catch this way. You may need to work your way through the amp, disconnecting at each node and then powering up to test. You should be able to isolate the area that is causing the issue this way. The higher than normal voltages at v3 cathode are a little odd also.

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Re: Current draw with KT66s in place...to much!

Post by neikeel » Wed Jan 28, 2015 5:01 am

I have been looking through.

Yes your OT primary colours should be correct (always worth checking it out with a meter - start - finish should be twice start-CT and visa versa).
The high cathode voltage on V3 and low plate/screens voltages are wierd. What happens to your plate voltages if you fire it up without V3 in place?
I cannot see all of the pics on a work PC (they are cropped slightly).
Double check that the cathode resistor is a 470R and I presume the 'tail' of your PI is correct with 10k going to the NFB resistor via the short bus jumper and then to a correctly wired presence pot which in turn has a solid tight ground connection (I usually use dedicated black wire to the PI filter cap for the presence pot and exclude it from the rest of the pot bus wire).

Check this first. Shame the bus wires are under the board as having them up top makes it a 30second job to confirm.
If this is not helpful we will need to double check the screens wiring and the orientation of the 8k2 resistor and the feeds to the PI and screens are not swapped over.
Neil

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Re: Current draw with KT66s in place...to much!

Post by ?Mark » Wed Jan 28, 2015 2:42 pm

Hi Neil, danman

danman,

All winding resistance measurements on PT show
no problems. I left it powered up, by itself, for
1/2 hour with no issues. (still cold to the touch)

Neil,

OT resistance is fine and wired correctly, check.

I took voltage readings at all points from Standby to
V1 (I can send them if you'd like.) Again,
these were taken with all tubes in,
EXCEPT KT66.

PI MEASUREMENTS

I measure 274 VDC on pin 1 V3A
I measure 269VDC on pin 6 V3B

I measure 100VDC at pin 2 V3A
I measure 109.7VDC at pin 7. V3B

I measured 157.9VDC at pin 3 V3A & pin 8 V3B.

A 470 ohm is in parallel with both 1 megs from pins 2&7

At this junction, before 10k, I measure 157VDC.

These are in series with the 10k and ALL are in parallel with
a .1uf cap from pin 7 of PI.

These then tie, with the 27k FB resistor (from OT 16 ohm tap)
to pin 1 on the 5k L Presence pot.

There, at pin 1 on Presence pot, I read 6.5 VDC.

A .1uf cap is across pin 2 & 3 of said pot
with pin 3 tied to case/bus for gnd.

Neil I like your idea and will put the Presence pot to gnd
at the cap instead of with rest of pot bus.

AT PWR TUBE SOCKET PINS

The green grid at pin 5
on V4 is -40VDC
and orange grid at pin 5
on V5 is also -40VDC.

BIAS CIRCUIT

I measure -41.2 at 68k junction with the 220k

At 15K anode side of diode I have -50VDC

At cathode side of diode and 220k junction I have 128.4 VDC.
Didn't measure Pin 6 on GZ34.

Have to say a 400VDC B+ is what I see for spec on the
Schematic and in the spec on "JTM 45 Amp Kit Instructions Testing
Part 1". At Plate and Screen 393VDC isn't to low. (We shall see later
when board is back in.)

I've removed the board in order to change out
resistors. ( I never liked what I'd installed. ) I wasn't
happy with any wires where I can't see them either. So
all wiring will be moved top side triple checked for
proper placement and connectivity.

If either of you see anything in my posted voltages
listed above let me know Thx.
I'll keep you posted when board gets re installed
and I go thru start up again. Keep fingers X. :mrgreen:

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Re: Current draw with KT66s in place...to much!

Post by danman » Wed Jan 28, 2015 4:18 pm

I am not personally familiar with that PT (Marstran 1202-55) so I could be wrong on the expected voltage. Most of the JTM 45 builds that members have posted were showing a B+ somewhere in the neighborhood of 440-450vdc so I was assuming yours may be the same. If your PT is suppose to be delivering a B+ of 400v with tubes installed, It's odd that you are only reading a plate voltage of 393v without the power tubes. Normally you will find the B+ to be about 10% higher when measuring unloaded. That's why I was suggesting that something may be pulling the voltage down even without the power tubes in place. I keep thinking that something may be up with one of the filter caps. Hopefully it will fire up with no issues after you pull the board and make your changes. Please keep us posted!

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Re: Current draw with KT66s in place...to much!

Post by ?Mark » Wed Jan 28, 2015 5:16 pm

yep,
thx I will.

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Re: Current draw with KT66s in place...to much!

Post by ?Mark » Sat Jan 31, 2015 5:16 pm

Well Gentlemen,
Success!
:clap: :clap: :rocker:

Thank you all who responded.
I corrected the problem, (with the
board rebuild) don't know what it was
but sure know the symptom.

I now have a working amp.
I know the schematic by heart!! :lol:

I do have one issue.
I have one bad tube!!!!!

(Yes it follows with a socket
swap)

One measures 42 ma
the other 27 ma.

Can anyone recommend a place to
purchase a guaranteed matched pair.
That's what I paid for! Get to go tube
shopping now... sh_t!!!

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Re: Current draw with KT66s in place...to much!

Post by neikeel » Sun Feb 01, 2015 9:27 am

Good 8)
I would play it as is for a while.
Some people want perfect matches but they often drift. Some people will send you a match if you give them your plate voltage, screen voltage and current draw (others will want the tubes to match too).
I like Golden Lions as new production KT66s but others like VA so it is down to opinions, price and availablity.
-keep the old ones as they will come in useful for testing on later builds :wink:
Neil

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Re: Current draw with KT66s in place...to much!

Post by ?Mark » Sun Feb 01, 2015 11:21 am

Thanks Neil,

I'll find another "close to" for one of them.
For right now they're not a problem. I'm going
to enjoy the fruits of my labor for awhile. A
little swirl as they say, isn't such a bad thing.

I did move all underside leads to the top without
any repercussion.

Both you and danman were very helpful and made
me actually think about the parts I was employing in
this build. I've been very fortunate in my past builds.
I've never really had bad parts, so I've never looked to
see if any were grossly out of spec or just plain broken,
as it were. I learned a lot in that aspect.

If I'd taken the time to match the schematic with parts,
placement and connectivity I would have found where I
went wrong. But then again If I'd taken the time to do that
in the 1st place I probably wouldn't be sitting here talking
about it now. So ultimately I'm the inattentive you know
what who caused my head ache. :palm: Mea culpa

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