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G'day,

First amp build/first post.

I'm having a great time and enjoying the build but have a problem.

With all tubes in and a load connected within 5 sec the speaker started to hum and the 4K7 (R22) started smoking.

Here is what I know:

The colour code of the resistor shows it to be more 470 Ohms(Yellow, Violet, Brown?, Gold) but it measures out at 4.58K Ohms on my meter??



Mains Voltage 241V (Australia)
With no rectifier in place:
V5 pins 4= 358V & pin 6=358V.
Across pins 2 & 8= 5.6V

V4 Across pins 2 & 7= 4.47V
V3 =4.45V
V2 Across pins 4-5 & 9= 6.46V
V1 =6.46V

Rectifier installed with no other tubes.
B+ =470VDC

Before powering up I checked the grounding and continuity allover. All being good.
I'm in the process of double checking everything and I will swap out R22 for a new one when I can get one.

Any ideas would be greatly appreciated.

Thanks

Ross

Welcome to the forum!

R22 sure looks like a 470 ohm resistor in your photos. Yellow-violet-brown. Did it always look that way or did the last band turn from red to brown when it smoked? With it removed from the board does it still measure 4.58K as you mentioned?

Smoking resistors mean something is drawing too much current. Usually that's caused by wiring errors, shorts or parts in the wrong place. Sometimes it could be a bad tube, too. In your case it could be just a bad 4k7 resistor, though.

Your heater voltage should read the same at V1, V2, V3, V4 and the pilot light. It looks low on V3 and V4 from your readings. Check those connections.

Thanks Mitch,

Yes R22 has always looked like that.
Haven't removed it yet but I will tonight as well as checking I have wired everything correctly.

I will check the heater voltages/connections. Of note, the pilot lamp was not working (no continuity on the globe) but I didn't check it before powering up so thats an unknown. I have a replacement globe, still chasing a 4k7 resistor.

Cheers

Good idea to measure the resitance before you install , but do measure it now. Also check that that stage is not shorted to ground. mitchm is correct , it means there is excessive current being drawn so if the R is 4K7, then you need to find the cause of the excessive current draw.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I have found my problem, I just have to fix the damage. After installing a new 4K7 resistor and having it do the same thing I went over my work again. I took me a while but I found that on V3 I put the 2 orange wires on pin 5 not pin 4 (wish I had seen this on my very first check haha). After fixing this error I got the amp going but with very reduced volume. Plugging into the normal channel with the volume on 3 and turning the bright volume all the way up I get no volume.

So I have checked all the resistors on the board and they seen to still be the correct values. I think I have cooked most of the caps though. Testing their capacitance (in circuit) I get bad readings with the caps nearest the 4K7 resistor being the worst. Testing their resistance (in circuit) they seem to be "open circuit". A good cap's resistance should start low and move towards infinity??
Does it sound like I'm on the right track? Do damaged caps cause volume reduction? Could I have damaged the 6V6 in V3 wiring to the wrong pin?

I was hoping I would get it all right the first time but this is a good way to learn!!

From my first post my heater voltages were all the same I miss typed the readings.

Thanks

To test the caps properly, you need to pull one end off the board. If they are OK, reinstall them and then start by measuring all the voltages with tubes in. That tells a lot.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I tested the 3x 16uF caps and they gave me readings of 15.5uF.

I found a bad solder join from the 68K to the low bright input and have fixed that.

I then checked voltages:

AC Mains 246V
B+ No tubes installed 400V
B+ All tubes installed 377V (JJ 5Y3)

V1, 12AY7
Pin 1, 126V
Pin 3, 1.9V
Pin, 4-5-9, 6.1V
Pin 6, 123V
Pin 7, 1.9V
Pin 8, 1.89V

V2, 12AX7
Pin 1, 144V
Pin 3, 1.35V
Pin, 4-5-9, 6.2V
Pin 6, 187V
Pin 7, 19V
Pin 8, 52V

V3, 6V6
Pin 2-7, 6.2V
Pin 3, 375V
Pin 4, 330V
Pin 8, 21V

V4, 6V6
Pin 2-7, 6.2V
Pin 3, 373V
Pin 4, 327V
Pin 8, 21V

The rectifier voltages are
Pin 2-8, 5.4V
Pin 4, 354V
Pin 6, 354V

On the board from left to right the voltages are, layout voltages listed parenthetically.

21V (22V)
325V (348V)
235V (255V)
52V (50V)
19V (18V)
186V (204V)
141V (158V)
1.3V (1.4V)
125V (130V)
122V (130V)
1.9V (2.0V)


Input jacks
I get continuity between the chassis and all 3 tabs (Ground, Hot and Switch)?
Is this correct?

Soldering
My soldering doesn't look great in places,especially on the board. Looks like I've got things too hot and melted the flux in the solder??
I've check the continuity of all joins, all being around 0.1 ohm so i'm thinking they are ok.

Thanks for any help.

With a cable plugged in, you should not have continuity to ground from the tip of the cable. No cable, the inputs are grounded.
Your voltages look fine. what is the issue?

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

From those pictures are the standoffs for the board installed?

I was't getting a good volume and the bright channel wasn't working but its all sorted now. I had a poke around with a chopstick and found some dodgy connections. One on the coax and another on the .1uF cap running up to the bright channel.

It sounds amazing, I am so happy with the amp and the grin on my face runs from ear to ear.
Stephen please put me down as another happy customer!

A big thank you to Stephen and the Forum for getting me to across the line.

I've got a VRM to install later and a cabinet to sort then I'll post a finished photo.

Cheers

Ross

Excellent! Happiness noted!

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps