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

?Mark · Post by **?Mark** » Thu Jan 22, 2015 11:43 pm

I built a JTM 45 last FEB.

This is not my 1st build

I followed start up procedures.
120ac
Pwr on
approx. 325 VDC with GZ34 at standby.
-40 at pin 5 V4 and V5, bias full CW

V1 and V2 voltages are within spec.

V3 I have concern for Plate and Anode voltages
V3
pin 1 271VDC
pin 3 & 8 155VDC
pin 6 266VDC
Cathodes are way out from Spec of 48VDC

(and yes there's a 470 resistor in R18)

With the kt66s installed and warmed up I took it out of standby.
and my current limiter starts to glow!!??

with the exception of V3 concerns, (stated earlier)
all is fine till I apply HT

Marstran PT is 1202-55
OT is 1324/103
OT primary
V5 Blk start
Brn to cap choke
V4 Red finish
Secondary
16 Blue
8 Green
4 Orn
common Yel

Parts all in spec
I've been over and over the schematic
and everything is in its place and the right value.

1st attempt to test and power up was back in March 14".
Followed procedures, got to the put in the pwr tubes and
switched on HT part and it made little noise and unable to
turn up volume. I went thru schematic and board couldn't
find anything out of place or of wrong value.I went thru start
up again blew the mains fuse.

(I now use a 200Watt lamp in series
for limiting and safety. Works great!)

I set it aside till this week.
I started again with start up, (this time using the 200 watt
series current limiter). All was fine till HT was applied with the kt66 in sockets.
The current limiter starts to glow!
Something is causing it to draw current with the Power tubes in and the HT on and I don't know what/why?
HHHHHHHHHHHHEEEEEEEEEEEEEEEEELLLLLLLLLLLLLLPPPPPPPPPPPPPPPPP!!!!!!?????

Please, I'm going nuts on this one.
Thank you in advance I appreciate
any help
I'll take responses at madnis45@cox.net

neikeel · Post by **neikeel** » Fri Jan 23, 2015 11:50 am

Please post some clear well lit pictures of your build.

I am sure one of us will be able to help

?Mark · Post by **?Mark** » Fri Jan 23, 2015 12:28 pm

Thank you Neil

Images being made after this reply... stay tuned.

?Mark · Post by **?Mark** » Fri Jan 23, 2015 1:49 pm

ok Neil here they are
I've got the bias points on back of amp
I've moved the bias adjust to top
Both fuses are on back
I don't want to have to open it up
to change any thing.
If you want different or better view of something let me know
To date: I think Cathode side of V3 (to bias adjust) is to high 170VDC!?
spec calls for 50VDC
Everything, all other voltage test points read in spec.
Both KT66 are good
V4 V5 sockets look good parts correctly mounted no funky joints
So problem seems to be in out put and only with pwr tube in place
and HT applied. Something in the out put??

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no... 16Z and FB aren't soldered to center lug, pic angle makes it look that way
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Bias points for both V4 and V5 moved to back of amp
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no 470ohm resistor isn't touching or connected to filament pin 7 pic just makes it look that way
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both fuses are on back now
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?Mark · Post by **?Mark** » Fri Jan 23, 2015 2:31 pm

Neil we have pics

danman · Post by **danman** » Fri Jan 23, 2015 5:17 pm

How much glow are you getting? Are you blowing fuses with the tubes in? When I start up a new build I am using a 60watt bulb in my limiter. Without tubes I only get a brief flash while the caps charge. With tubes it does glow a bit because of the current draw of the heater filament. Are you getting a very bright glow from yours?

?Mark · Post by **?Mark** » Fri Jan 23, 2015 5:54 pm

Yes... before using Current Limiter it blew main fuse.

Only happened after I installed the KT66s and switched out of standby applying the HT. Then the lamp glows, not bright but visible orange.

Using the 200Watt incandescent lamp, (the bigger the better for purpose of protection) gives no glow, pulls no visible current draw on lamp filament but lets me "see" a large current draw and alerts to problem.

danman · Post by **danman** » Fri Jan 23, 2015 6:14 pm

Have you tried these power tubes in another to check for proper operation? Have you tried another set of known good power tubes? Have you tried inserting only one power tube and taking it out of standby and then the other to try and isolate any issues to a particular socket? Sorry for all the questions but it will help us to understand all the variables.

?Mark · Post by **?Mark** » Fri Jan 23, 2015 6:36 pm

No need to applogize for questions
I need all the help
I can get right now
So, Thank you! I appreciate
your help danman.

?Mark · Post by **?Mark** » Fri Jan 23, 2015 6:59 pm

Ok so, either tube, by itself, in either socket gives me a very LOUD hum
from spkr used for load when the HT is applied. ( Not having had to do this before,
when trouble shooting, I don't know if this LOUD hum is normal?) I've never run
a PP amp with only one pwr tube installed.
No controls, volume etc. are on by the way.

Ok I tried both KT66 out in an SE (at 60% dissipation) Plate at 305VDC.
Any who, they are fine! It's not the KT66! One down hopefully not to many
more to go.

I put in the 6L6 WGC and switched out standby and not as LOUD of a hum
but loud none the less and the tubes got brighter for a second.(Didn't wait
to see if CL glowed just powered off immediately if not sooner.) I have to
admit that LOUD hums and brighter than normal tubes bothers/scares me
when trouble shooting, mostly because I've never experienced them before.
I'm afraid it spells disaster is seconds away. I enjoy things going boom as much
as anyone just not my expensive amps. Well it will be if It ever works properly.
Right now it's just a LOUD boat anchor. It's something in output stage?

Marstran OT model is 1324/103
Can anyone confirm this info.

OT primary
V5 Blk start
Brn to cap & choke
V4 Red finish
Secondary
16 Blue
8 Green
4 Orn
common Yel

danman · Post by **danman** » Fri Jan 23, 2015 8:39 pm

I am wondering if perhaps you got a bad power tube socket. I have seen them with mislabeled pin numbers which can throw you off if you are wiring them strictly by the numbers on the socket. If you actually counted the pins themselves as you were wiring them then there is probably less chance of an error. Another thought that I had may be a bad E-cap on possibly the screen supply since this issue only shows up with the tubes installed. I forgot to ask earlier whether you had taken a bias voltage reading on pin five of each power tube socket. You should be seeing somewhere around -50vdc even with the amp on standby.

?Mark · Post by **?Mark** » Fri Jan 23, 2015 8:48 pm

I'll take a look at socket
don't know about cap, will look.
and yes, with Bias full CW I get -40VDC
Spec call out aprox -45VDC, -40VDC is fine.

I'll pick up again 2marO.

danman · Post by **danman** » Sat Jan 24, 2015 4:42 pm

Could you try to get a different camera angle on the power tube sockets? Maybe one from the opposite side and one from above. I'm trying to see exactly how the 1ohm biasing resistor is attached and the plate and screen connections.

danman · Post by **danman** » Sat Jan 24, 2015 4:54 pm

One more question...are the KT66's metal base or plastic?

?Mark · Post by **?Mark** » Sat Jan 24, 2015 5:23 pm

1ohm R37 R38 are attached across pin 8 and pin 1, with pin 1 lead to gnd
470 across pin 4 and 6 with B+ on 6
5.6k grid stoppers on pin 5
PRI 1 blk on pin 3 V5, PRI 1 red on pin 3 V4
2 & 7 filament both
every thing is as it should be and good/clean joints
If i may ask, danman.
Are you looking at/for connectivity, part placement or
quality of connectivity?
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