HELP With My JTM 45 Build

waggclan

Par 4 Holer
Platinum Supporting Member
Messages
1,313
I have built about 5 or 6 amps (all 50w) so I should know by now. However, I am experiencing some weird problems. Checked all of the component values and continuity of the wires. All just fine. I am using a Classic Tone multi voltage PT that seems to be working fine 440v B+. My problem is, when the power switch is on, the right contact on the standby switch reads 440v and so does the rectifier tube. BUT, the dual 32uF can reads zero. Then when I turn on the standby switch the B+ drops to 27v and so does the rectifier; still no voltage at the cap can. Fuses are hanging in there and I feel I'm close to sussing out the problem. Have a Heyboer OT but installed a Classic Tone choke. Maybe I should change out the choke. The installed CT is a 3Hy but the Heyboer choke I have is a 7Hy.

Any help is appreciated.
 

waggclan

Par 4 Holer
Platinum Supporting Member
Messages
1,313
One other thing... I'm getting no bias voltage at the KT66's.

Thanks.
 
Messages
826
Double check how things are connected. Something must be wired up wrong. I don't think the choke can be the problem, unless it is wrongly connected.
 
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Dan40

Silver Supporting Member
Messages
1,387
We will probably need some clear pics and the layout that you used for the amp would also help. The differences in the choke(3hy vs 7hy) would not cause the issue you are having.
 
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826
... when I turn on the standby switch the B+ drops to 27v and so does the rectifier; still no voltage at the cap can.....
This suggests that the output side of the standby switch is seeing a low impedance to ground which should not be there, but is not connecting through to the cap.
 

pdf64

Member
Messages
7,369
Such initial power up and basic de-bugging should be done via a light bulb limiter; it would be highly beneficial to set one up and note the bulb intensity for the following.
1st power up is with no tubes - check everything that it's feasible to, including the bias supply; adjust bias supply to obtain the max negative voltage at power tube control grids terminal #5.
2nd with just the rectifier - check the HT etc.
3rd with rectifier and power tubes - set a suitable bias etc.
 

waggclan

Par 4 Holer
Platinum Supporting Member
Messages
1,313
Well... Found that I did not bridge the heater wire on V1 and thought I was done. Stupid busy these days, tax season. Now I'm getting voltage to the standby switch when it is off but when I turn it on it drop down to 27v. I'll take some pictures and post later today.

I'm starting to suspect a bad dual 32uF can cap. I don't believe switching the wires on the standby switch will do anything.
 
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waggclan

Par 4 Holer
Platinum Supporting Member
Messages
1,313






































If you guys see anything, or need another picture or close up, just give me a hollar. Thanks.
 

waggclan

Par 4 Holer
Platinum Supporting Member
Messages
1,313
You guys see anything? Project is a bit messy due to the sushi game out of the problem. No wire ties, etc. looking at my heater red and black wire twists kind of embarrasses me. I just don't know how Greg Germino (my hero) does it. Thanks again.
 

pdf64

Member
Messages
7,369
Are you powering it via a light bulb limiter?
If not (& why not?!) get the power tubes out of there until suitable operating conditions are verified.
 

waggclan

Par 4 Holer
Platinum Supporting Member
Messages
1,313
Are you powering it via a light bulb limiter?
If not (& why not?!) get the power tubes out of there until suitable operating conditions are verified.
Absolutely. Would never fire up a new build without using a limiter.
 

pdf64

Member
Messages
7,369
Was the bulb bright or dull when the amp was powered up and the above voltage measurements were taken?
 

waggclan

Par 4 Holer
Platinum Supporting Member
Messages
1,313
It seems that after I throw the standby switch the problem begins. Perhaps the gentlemen at Heyboer gave me the wrong wiring information. Thr red wire is supposed to be the common wire, but I just dunno. It is twisted with one of the choke wires and soldered to the dual 32 can cap. Also wired to that lug, is the red wire that is soldered to the side of the standby switch that is engaged when the switch is turned on. I changed the dual 32 cap, and even tried switching the other two OT wires between V4 and V5 but no dice. This is a doozie for sure...
 
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826
Assuming you are doing these tests with the rectifier tube in, but all the other tubes out, a mix up of the OT primary wires wouldn't cause the problem you are seeing.
The output side of the standby switch is just connected to three things? Namely, +ve terminal of the main 32uF reservoir cap, the OT primary centre tap and one side of the choke. I think you have already changed that 32uF cap. Mixing up the OT primary wires wouldn't cause this problem (assuming the power tubes are out). That only leaves the choke. Perhaps the choke has a fault to ground (seems unlikely) or perhaps something is wrongly connected at the other end of the choke?

Is the bias supply producing the expected negative voltage, when standby is switched on and off?
 
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826
Note that some mix up between primaries and secondaries of the OT could cause a problem since one of the secondary wires is grounded.

You can check which of the OT primary wires is the centre tap, by measuring the DC resistances (tubes out, powered off, caps drained, of course). You should get somewhere around 100 ohms from the CT wire to each plate wire, and double that resistance value from plate to plate.
 
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