Also check the NFB from secondary of the OT to the 12AX7. If wrongly connected at the wrong end of the winding you might get PFB instead and overdrive the stage. Have you tried my initial suggestion with the capacitor in series with the input wire? For test purposes any from 10nf to 1 uf will do.
Connect an Ohmmeter one side to chassis and then test all groung points of the circuit, do you get a low resistance reading at each ground point?
Can you describe the distortion? Is it humming as well?
Connect an Ohmmeter one side to chassis and then test all groung points of the circuit, do you get a low resistance reading at each ground point?
Can you describe the distortion? Is it humming as well?
No humming detected. Output is exactly as if one were to run any amplifier at maximum output to full clipping.
I am going to access some caps and eliminate or confirm your initial theory that the signal is being injected with excessive DC.
I note that when I power up or power down the amplifier the signal output exhibits normal sonics just prior to the tubes coming up to optimum running temperature and as they de-energize.
I will check the grounding points with a DCR meter as suggested but this chassis has not been modified in any way from original,,but perhaps a cold solder joint has materialized.
Thank-you.
Hi Fran....I went with your ground wiring configuration, no go, and then with cogsncogs suggestion that I take signal ground to 12AX7 ground at the 2.2k resistor. (no go either)
No, the light blue lines (signal grounds) were connected as illustrated.
No short at RCA's either. Continuity tests A-OK.
OK chassis is original so we can assume the wiring is OK. No humming would indicate that there is no hum loop so tha mplifier circuit grounding must be OK. Which then brings us back to the potentiometer. You noticed sonic improvements when powering up/down. I would check the anode supply voltages as a next step. Also, on the mains input check C64 C65 and R60 as there is a possibility of inducing a proportion of mains ac voltage. To eliminate this remove temporarily all 3 components and test again. Is the result still the same then?
Also consider the 100uf cathode capacitors on the el84 as a fault point. Old electrolyte tend to leak and bupass dc wise the cathode resistor which then would increase the current through the el84
Stef.
Also consider the 100uf cathode capacitors on the el84 as a fault point. Old electrolyte tend to leak and bupass dc wise the cathode resistor which then would increase the current through the el84
Stef.
Last edited:
No real Hum in output...just loud and clipped to distortion...so that would seem to eliminate ground loops Yes? No?
Perhaps I need to hook up a portable type CD Walkman with variable output and see if this amplifier behaves the same way...just as a part of the initial process of elimination?
Hi rongon...no intent to modify this amp until I learn more...my goal was simply to add an RCA input and a volume pot to control output and then study the whys, hows and where's of this particular little SET EL84 Amp workings.
I hit the textbooks and reference books everyday but much seems to elude my immediate comprehension
so I thought by experimenting with a very basic example I might learn enough so that the technical jargon would begin to mean more to me.Although implementing a volume pot has been problematic for me thus far, I am learning a lot more by doing than by reading.
I included the schematic for the amp for those in the know to have a look at and see if there were any issues related to its topology perhaps.
This type of partial schematic is what tube neophytes such as myself run into all the time..ie. the failure to mark on the other half of the 12AUX7, as if a beginner should just know that they are simply the other side of the triode and handle the other channel's signal...or the non-mention of pins 4 and 5 being bridged.
A beginner looking at the underside of the chassis and then the provided schematic would rightfully believe as I did that the schematic is only giving me part of the story. The fact that the physical layout of wires, caps, grounds etc (three-dimensional) looks nothing like the (one dimensional) schematic is something I'm working through as part of the learning process.
If my experiment with a variable output CD player confirms a continuing problem with over driven signal output then I will commence the process of elimination as per your suggestions and those of others kind enough to help a beginner.
Last edited:
To Mods: This part of rongon's original message didn't seem to make it on to this thread. Don't know why. But I am copying and pasting them from my original DIYaudio e-mail notification as I feel these comments are important to this discussion and any eventual likelihood of narrowing my problem down.
Hope this is ok with you rongon
QUOTE: "I'd expect these voltage readings (very approximately)...
Pins 1 and 6 of the 12AX7's to be around 150V to 175V (DC).
Pins 2 and 7 of the 12AX7's should have no DC at all.
Pins 3 and 8 of the 12AX7's should have about 1V DC or so on them.
Pins 4, 5 and 9 of the 12AX7's are the heaters.
Pin 7 of the EL84 should be about 250V DC.
Pin 9 EL84 should be about 250V DC.
[The schematic looks weird there. What's that capacitor doing between the 250VDC feed and pin 7 (plate) of the EL84?? Is B+ blocked from the plate but fed to the screen grid (pin 9)?]
Pin 3 of the EL84 should be about 10V.
Pin 2 of the EL84 should have no DC.
You should get 6.3VAC between pins 4 and 5 of the EL84's.
Can't tell how the heaters are wired for the 12AX7's, so don't know if they're wired for 6.3V or 12.6V operation.
If for 6.3V -- Pins 4 and 5 should be connected together (shorted), and 6.3V AC should be read between pins 4+5 and pin 9.
If for 12.6V -- 12.6VAC should be read from pin 4 to pin 5, and pin 9 should not be connected to anything.
I hope that helps.
--
PS - I can't tell from what you've posted, but are you familiar with taking voltage readings inside a live chassis? Just checking. Wouldn't want anything bad to happen...
***************END QUOTE
Thanks rongon.
Hope this is ok with you rongon
QUOTE: "I'd expect these voltage readings (very approximately)...
Pins 1 and 6 of the 12AX7's to be around 150V to 175V (DC).
Pins 2 and 7 of the 12AX7's should have no DC at all.
Pins 3 and 8 of the 12AX7's should have about 1V DC or so on them.
Pins 4, 5 and 9 of the 12AX7's are the heaters.
Pin 7 of the EL84 should be about 250V DC.
Pin 9 EL84 should be about 250V DC.
[The schematic looks weird there. What's that capacitor doing between the 250VDC feed and pin 7 (plate) of the EL84?? Is B+ blocked from the plate but fed to the screen grid (pin 9)?]
Pin 3 of the EL84 should be about 10V.
Pin 2 of the EL84 should have no DC.
You should get 6.3VAC between pins 4 and 5 of the EL84's.
Can't tell how the heaters are wired for the 12AX7's, so don't know if they're wired for 6.3V or 12.6V operation.
If for 6.3V -- Pins 4 and 5 should be connected together (shorted), and 6.3V AC should be read between pins 4+5 and pin 9.
If for 12.6V -- 12.6VAC should be read from pin 4 to pin 5, and pin 9 should not be connected to anything.
I hope that helps.
--
PS - I can't tell from what you've posted, but are you familiar with taking voltage readings inside a live chassis? Just checking. Wouldn't want anything bad to happen...
***************END QUOTE
Thanks rongon
.
Last edited:
Read this first..
Putting your pot to use..
Potentiometers (Beginners' Guide to Pots)
I guess you already know that the two rows of 3 pins on the pot are just two individual pots ie two pots in one part..
Looking at the input on your post with photos of the drawing...
The RCA input the two connections from the grids of the tubes IE top and bottom of the RCA connector go in two centre connections of the pots..
the ground of the RCA is connected to one end of the pot and linked to the same pin on the other pot.
The input from your source goes on the only other two non connected pins and the ground from your source connects to where the RCA ground is connected to the POT.
Link [ Gnd .....Grid ....Left signal in<<<<Three connections on one side of the pot
Link [ Gnd .....Grid ....Right signal in<<<<Three connections on the other side of the pot
If you get a volume increase in the wrong direction reverse the Gnd and signal in connections on each pot leave the grid connection as is in the middle..
If you connect direct to the RCA connections it should work if you have a ground loop then if using Phono connectors make sure they are isolated from the chassis so that only the RCA gnd earths them. If you are feeding it from a CD etc try disconnecting one of the input Gnd connections (on the incomming signal leads). as lond as one is connected it should work. Then again you could try dissconnecting both if the CD etc has an earth it will feed through the mains Gnd just to try it. if you get no signal then you need the signal Gnd connected.
Regards
M. Gregg
Putting your pot to use..
Potentiometers (Beginners' Guide to Pots)
I guess you already know that the two rows of 3 pins on the pot are just two individual pots ie two pots in one part..
Looking at the input on your post with photos of the drawing...
The RCA input the two connections from the grids of the tubes IE top and bottom of the RCA connector go in two centre connections of the pots..
the ground of the RCA is connected to one end of the pot and linked to the same pin on the other pot.
The input from your source goes on the only other two non connected pins and the ground from your source connects to where the RCA ground is connected to the POT.
Link [ Gnd .....Grid ....Left signal in<<<<Three connections on one side of the pot
Link [ Gnd .....Grid ....Right signal in<<<<Three connections on the other side of the pot
If you get a volume increase in the wrong direction reverse the Gnd and signal in connections on each pot leave the grid connection as is in the middle..
If you connect direct to the RCA connections it should work if you have a ground loop then if using Phono connectors make sure they are isolated from the chassis so that only the RCA gnd earths them. If you are feeding it from a CD etc try disconnecting one of the input Gnd connections (on the incomming signal leads). as lond as one is connected it should work. Then again you could try dissconnecting both if the CD etc has an earth it will feed through the mains Gnd just to try it. if you get no signal then you need the signal Gnd connected.
Regards
M. Gregg
Last edited:
Regards the above make sure you are testing for dead with a meter.
Before you touch..
If it holds charge for some time try soldering a (500K 2 Watt 500 Volt rated resistor) across the Power supply caps, It will bring the voltage down over a period of time after power off..
Just a point NEVER EVER remove a safety mains earth to stop hum, there are ways around it. If you remove a mains earth you could get killed.
Just thinking about your safety
Regards
M. Gregg
Before you touch..
If it holds charge for some time try soldering a (500K 2 Watt 500 Volt rated resistor) across the Power supply caps, It will bring the voltage down over a period of time after power off..
Just a point NEVER EVER remove a safety mains earth to stop hum, there are ways around it. If you remove a mains earth you could get killed.
Just thinking about your safety
Regards
M. Gregg
Last edited:
To Mods: This part of rongon's original message didn't seem to make it on to this thread. Don't know why. But I am copying and pasting them from my original DIYaudio e-mail notification as I feel these comments are important to this discussion and any eventual likelihood of narrowing my problem down.
Hope this is ok with you rongon
QUOTE: "I'd expect these voltage readings (very approximately)...
Pins 1 and 6 of the 12AX7's to be around 150V to 175V (DC).
Pins 2 and 7 of the 12AX7's should have no DC at all.
Pins 3 and 8 of the 12AX7's should have about 1V DC or so on them.
Pins 4, 5 and 9 of the 12AX7's are the heaters.
Pin 7 of the EL84 should be about 250V DC.
Pin 9 EL84 should be about 250V DC.
[The schematic looks weird there. What's that capacitor doing between the 250VDC feed and pin 7 (plate) of the EL84?? Is B+ blocked from the plate but fed to the screen grid (pin 9)?]
Pin 3 of the EL84 should be about 10V.
Pin 2 of the EL84 should have no DC.
You should get 6.3VAC between pins 4 and 5 of the EL84's.
Can't tell how the heaters are wired for the 12AX7's, so don't know if they're wired for 6.3V or 12.6V operation.
If for 6.3V -- Pins 4 and 5 should be connected together (shorted), and 6.3V AC should be read between pins 4+5 and pin 9.
If for 12.6V -- 12.6VAC should be read from pin 4 to pin 5, and pin 9 should not be connected to anything.
I hope that helps.
--
PS - I can't tell from what you've posted, but are you familiar with taking voltage readings inside a live chassis? Just checking. Wouldn't want anything bad to happen...
***************END QUOTE
Thanks rongon
It's all good! OK, I'm glad that was helpful. I didn't want to come off as lecturing you... And I didn't think the email notification would have all that anyway.
So, since that cat's now out of the bag.... Did your voltages check out? I'm especially interested in the voltage at pin 7 of the EL84. Is that 250V like it says it should be?
I'm also interested in what that capacitor is doing between B+ and pin 7 of the EL84. Is that a mistake?
--
Thanks,,,yes, I've read that guide to pots and will utilize yours and others detailed connection road map once I've eliminated the most obvious possibility that the problem lies somewhere within the amp, caps, tubes etc....etc....
Regards the above make sure you are testing for dead with a meter.
Before you touch..
If it holds charge for some time try soldering a (500K 2 Watt 500 Volt rated resistor) across the Power supply caps, It will bring the voltage down over a period of time after power off..
Just a point NEVER EVER remove a safety mains earth to stop hum, there are ways around it. If you remove a mains earth you could get killed.
Just thinking about your safety
Regards
M. Gregg
I appreciate your caveats and tips....and have a healthy respect for voltages when I'm mucking around on the underside of any chassis.
GETTING CLOSER?
Ok. As part of the learning curve for me and in some attempt to map out an orderly means of troubleshooting and analysis of what I'm working with, in respect to this simple little EL84 SET Amp, I did the following:
1)I removed my poorly attempted 500K dual gang log pot and all associated wiring.
2)Checked my installed RCA inputs once again.
3)Hooked up a portable CD Walkman with adjustable output to the RCA's utilizing a Y-Jack.
4)Double checked the underside of chassis for anything physically loose, burnt, or questionable.
5) Checked all chassis grounds.
6)Checked and tested the EL84/12AX7/and 6CA4 tubes in another live and working EL84 amplifier.
7)Hooked up a pair of 8ohm speakers to speaker outputs which are rated at 8ohms.
8)Powered up amplifier
9) Adjusted CD output to 50% of full output
RESULT?
Source Signal: Yes...
Output: Yes, but distorted at a reasonable listening level, no hum, but not clean output by any means.
My next step was to replace the two .33uf (400 volt) coupling caps marked C50 (see schematic) and the C32 cap (see schematic) 100uf (25 volt)
Powered up unit again
Source Signal; Yes
Output: Yes...much better, no hum. nice clean output from low volume to high...until clipping.
PROBLEM HOWEVER...after a few minutes the plate (anode) of one of the EL84s began to glow far brighter than the other.
Shut down amplifier immediately, then checked polarity of my recently installed caps (all ok) and pondered my next course of action.
So I take it my next step is to check the voltages of the amp as suggested by rongon?[/B]
Ok. As part of the learning curve for me and in some attempt to map out an orderly means of troubleshooting and analysis of what I'm working with, in respect to this simple
little EL84 SET Amp, I did the following:1)I removed my poorly attempted 500K dual gang log pot and all associated wiring.
2)Checked my installed RCA inputs once again.
3)Hooked up a portable CD Walkman with adjustable output to the RCA's utilizing a Y-Jack.
4)Double checked the underside of chassis for anything physically loose, burnt, or questionable.
5) Checked all chassis grounds.
6)Checked and tested the EL84/12AX7/and 6CA4 tubes in another live and working EL84 amplifier.
7)Hooked up a pair of 8ohm speakers to speaker outputs which are rated at 8ohms.
8)Powered up amplifier
9) Adjusted CD output to 50% of full output
RESULT?
Source Signal: Yes...
Output: Yes, but distorted at a reasonable listening level, no hum, but not clean output by any means.
My next step was to replace the two .33uf (400 volt) coupling caps marked C50 (see schematic) and the C32 cap (see schematic) 100uf (25 volt)
Powered up unit again
Source Signal; Yes
Output: Yes...much better, no hum. nice clean output from low volume to high...until clipping.
PROBLEM HOWEVER
...after a few minutes the plate (anode) of one of the EL84s began to glow far brighter than the other. Shut down amplifier immediately, then checked polarity of my recently installed caps (all ok) and pondered my next course of action.
So I take it my next step is to check the voltages of the amp as suggested by rongon?[/B]
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.