It is currently Tue Mar 17, 2020 1:01 am

All times are UTC - 7 hours



Post new topic Reply to topic  [ 11 posts ] 
Author Message
Post subject: Blues Jr with low output
Posted: Sat Nov 24, 2012 11:13 pm
Offline
Hobbyist
Hobbyist

Joined: Sat Nov 24, 2012 10:50 pm
Posts: 5
Hello

I recently aquired a Used Blues Jr that had some issues and a missing EL84. It had a short on the trace side where R31 is soldered. R31 lead end was bent over and touching a trace beside it that went to D11. After repairing that we installed all new tubes. The output is very low and distorted. If you remove either of the EL84 tubes the amp comes alive with no distortion, just a very slight hum. It isn't at full power but it sounds okay. Put the tube back in, and when the tube warms up, the sounds fades away again. We have done some voltage checks and all are normal so far. Any help would be appreciated.


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Blues Jr with low output
Posted: Sun Nov 25, 2012 3:13 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14049
Location: Province de Québec, Canada
I see R31 and D11 on the Blues junior schematic diagram named ; " Service diagram combined Blues Junior" . Not in the schematic named only " Blues junior schematic " .
R31 is a 220k used to send bias negative voltage to one EL84. D11 is used for "spike" on one side output transformer primary.


IMO you have a bias problem on EL84 tube or / and , R31 , D11 , or bias circuit could be damaged.

1- Remove both EL84 and take some reading on pin 2 on the socket, you should read close to minus 10 volts DC ( - 10 vdc ).

2- You must find this reading both side of each of the 220K resistors , R31 , R32

Note; If no voltage or very low voltage on 1 and 2 , check bias circuit

3- If voltages are good , do same tests with one EL84 in the amp and if good with both EL84.

4- For a test you can disconnect D11 , your amp don't need it to work fine.

You could also begin test by disconnecting D11 first .


Top
Profile
Post subject: Re: Blues Jr with low output
Posted: Sun Nov 25, 2012 4:23 pm
Offline
Hobbyist
Hobbyist

Joined: Sat Nov 24, 2012 10:50 pm
Posts: 5
R31 was missing when I got the amp so I put one in.

Checked these voltages with Fluke 179:


pin 2 of EL84 = -11.35vdc with no EL84's in amp
r31 & r32 = -11.44 one end & -11.31 on other end, with no EL84's in amp
r31 & r32 = -10.90 one end & -11.03 on other end, with 1 EL84
r31 & r32 = -10.90 one end & -10.76 on other end, with 2 EL84
pin 2 of EL84 = -10.50 with 2 EL84's in amp

All voltages in power supply section of schematic are within .5vdc.

D11 made no difference in or out of circuit.

All tubes are Fender or Groove tubes.


Top
Profile
Post subject: Re: Blues Jr with low output
Posted: Sun Nov 25, 2012 4:56 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14049
Location: Province de Québec, Canada
Power stage ( EL84 and power supply ) look good .....and you always have low and distorted output .

We need to know if it is sure all tubes are good. No tube tester please.
Also the Bias of each EL84 , cathode milliamp . Use a bais probe or put 1 ohm resistor between each EL84 cathode and ground and take some reading in millivolts , 100 mv scale on your meter.

Ok this could be in Phase Inverter circuit , tube before the output tubes. Check R21, 22,23,24

Check for wrong resistor value or printed circuit broken, solder defective.


Top
Profile
Post subject: Re: Blues Jr with low output
Posted: Mon Nov 26, 2012 5:13 pm
Offline
Hobbyist
Hobbyist

Joined: Sat Nov 24, 2012 10:50 pm
Posts: 5
Checked all the resistors and all were okay.

Both EL84 tubes are New Groove Tubes.

Inverter tube 12AX7 is new Groove tube. Old one checked bad on one side.

All check good on my pristine B&K 606.

Before I get into the bias checks, I would like to add this info. When you turn the amp on with a guitar plugged in and waiting for it to warm up, you can strum the strings and the amp will start to produce sound and then it will fade away when it gets warmed up completely. It will do this when you remove and reinstall either power tube too. With 1 tube out it sounds great, and when you plug the tube back in, the volume will rise while the tube is warming and then fade when it gets warmed up.The sound is good with no distortion. I replaced r31 with the right value and the distortion is gone.


Top
Profile
Post subject: Re: Blues Jr with low output
Posted: Mon Nov 26, 2012 5:42 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14049
Location: Province de Québec, Canada
The only thing warm up in a tube amp is....tube and some resistor. and transformers

This tube tester are not reliable to test amp's tube. Unless tube is almost dead or short , tube tester will show good.
If your tubes are really new , they must be good.

Bias test could help . This is one of the first test you might do. This show how output tubes work.
and if output transformer is short.
Voltage reading previously are still the same when warm up ( before sound drop ) AND after warm up ?


Top
Profile
Post subject: Re: Blues Jr with low output
Posted: Mon Nov 26, 2012 7:22 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
I agree with Stratele. A tube tester is limited in what it can tell you about output tubes (EL84, in your case). Most do not tests output tube under amplifier conditions (plate voltage and idle bias).

If you can pull one of the EL84 and the volume doesn't fade, I'd would venture a guess you have an unstable amp. Running one tube puts the amp into a pseudo single-ended topology. This really drops the output wattage.

When you plug in the other EL84, the amp reverts to push-pull output. And the output power goes up. The extra output can induce an instability not "seen" with the single tube.

I'd check your feedback resistor and/or cap network and the circuit board for damaged or charred tracings. A good tech maybe in order.

Good luck!


Top
Profile
Post subject: Re: Blues Jr with low output
Posted: Fri Nov 30, 2012 10:31 pm
Offline
Hobbyist
Hobbyist

Joined: Sat Nov 24, 2012 10:50 pm
Posts: 5
While doing some voltage checks I unscrewed the board and laid it out to get to the back side. I turned it back up and slid the controls back into the holes and put in a couple of screws to hold it in position. While I was calculating the plate current my son came in and plugged in his guitar and the thing worked perfectly. We wiggled and thumped cables and board and could not make it fault out. After all screws were installed and covers attached it still plays great. Let's see how long it lasts.

Thanks for all the info, I really appreciate your help. It may show up again since it is probably a loose connection or solder joint.


Top
Profile
Post subject: Re: Blues Jr with low output
Posted: Sat Dec 01, 2012 12:12 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
If the low volume issue pops-up again... you may need to reflow some solder points on the input section. And perhaps check that input jack, which is soldered to the circuit board.

Might be a good idea to draw up the OEM RoHS solder and re-flow some good Kester Tin/Lead.


Top
Profile
Post subject: Re: Blues Jr with low output
Posted: Sat Dec 01, 2012 12:58 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
BMW2002Ti wrote:
...Might be a good idea to draw up the OEM RoHS solder and re-flow some good Kester Tin/Lead.


+1

If the amp was made after 2005, it likely has crap...errr, I mean RoHS compliant solder, which sucks.

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


Top
Profile
Post subject: Re: Blues Jr with low output
Posted: Sun Dec 02, 2012 12:06 am
Offline
Hobbyist
Hobbyist

Joined: Sat Nov 24, 2012 10:50 pm
Posts: 5
Yeah, I'll redo the solder joints. I've used Ersin Multicore solder for 40 years, it's the best as far as I'm concerned.


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 11 posts ] 

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: