Guys,
I have a 1978 Marshall 2204 that I have been fixing up and when I got it had a number of minor problems all of whihc I fixed but I have found the following problem with V4 - power tube which ahs me foxed.
When the amp is switched on and idle, all the tubes come up as normal and everything looks stable. All voltages look correct etc.
After switching stand by on and with both the master and pre-amp volumes on zero I am getting the following on both V4 & V5
pin 3 = 374
pin 4 = 374
pin 5 = -32
(please note this amp does not have screen resistors ... yet... and I have recently fitted 5.6k grid resistors and 1 ohm bias resistirs as it did not have any)
So i start running a 1Khz test signal through the amp. If I turn the volume up to around 2/3 with the pre-amp on 2-4 etc the tube in V4 starts to red plate etc. So I have to reduce the volume asap or switch to standby to stop either the tube failing or the HT fuse (500mA) blowing.
I have tested all components on the board and I have tested the OT (just in case) for shorts etc and apart from a dodgy feedback resistor, plate load resistor (which I have replaced) and installing some cathode 1ohm resistors on the power tubes everything is normal.
I have tried to bring the volume up slowly and and measured the pins on V4 & V5 etc and I have noticed that the current is not staying stable on the tubes when a signal is applied (presumably normal) but V4 is drawing a lot more current and I have also noticed that I am reading AC on V5 on pin 3 but not on V4 pin 3? NB:this only happens when a sufficient signal starts to go through the amp.
There must be something fundamental going wrong somewhere but I (currently) lack the experience to find the source of the problem.
Please note, when I pull V1 tubes and let the amp idle with master up to 10 everything is ok, it is only when a high enough signal goes through that things start to deteriorate?
Any advise / pointers greatly appreciated...
Regards
Sean-
________
Ifa dicussion
Problem.... or not? - red plating issue - advice appreciated
Moderators: pompeiisneaks, Colossal
Problem.... or not? - red plating issue - advice appreciated
Last edited by badfan on Fri Feb 25, 2011 5:38 am, edited 1 time in total.
Re: Problem.... or not? - red plating issue - advice appreciated
First off, I'd try known good power tubes.
Second, put screen grid resistors on there, for goodness sakes! A pair of 1K5 5 watters will make the power tubes much happier!
The reason signal makes it happen, is that the power tubes idle at very low current, and signal makes them conduct. In a sense, it's a bias issue, but it may be the tubes, not the bias settings.
Second, put screen grid resistors on there, for goodness sakes! A pair of 1K5 5 watters will make the power tubes much happier!
The reason signal makes it happen, is that the power tubes idle at very low current, and signal makes them conduct. In a sense, it's a bias issue, but it may be the tubes, not the bias settings.
Rich Gordon
www.myspace.com/bigboyamplifiers
"The takers get the honey, the givers get the blues." --Robin Trower
www.myspace.com/bigboyamplifiers
"The takers get the honey, the givers get the blues." --Robin Trower
Re: Problem.... or not? - red plating issue - advice appreciated
Do you have access to a 'scope? Could be oscillation. You said you had trouble in the feedback loop - are you sure everything is OK there now?
W
W
Re: Problem.... or not? - red plating issue - advice appreciated
Thansk Guys
I will try some screen resistors and get back. The feedback loop resistor was measuring down to 4.4k? where it should have been 100k (according to the schematic)
Ill let you know how I get on and I am getting the scope out to see if I can find anything else...
Cheers
Sean
________
Motorcycle tires
I will try some screen resistors and get back. The feedback loop resistor was measuring down to 4.4k? where it should have been 100k (according to the schematic)
Ill let you know how I get on and I am getting the scope out to see if I can find anything else...
Cheers
Sean
________
Motorcycle tires
Last edited by badfan on Fri Feb 25, 2011 5:38 am, edited 1 time in total.
Re: Problem.... or not? - red plating issue - advice appreciated
you can't measure the feedback resistor when its in the circut.
As wayne posted ,might lean more toward an oscilation. Swap the tubes,and see if the red plating stays with the socket,or other tube shows the same symton in that socket. You may have to use some shielded cable, or check your lead dress.
As wayne posted ,might lean more toward an oscilation. Swap the tubes,and see if the red plating stays with the socket,or other tube shows the same symton in that socket. You may have to use some shielded cable, or check your lead dress.
Re: Problem.... or not? - red plating issue - advice appreciated
Don't change anything, mods should only be done to functioning amps. Find out the problem, resolve that, then measure the characteristics of the amp so you have a baseline set of measurements to refer to when modding.
Pull the power tubes and measure the values at socket, if all is right, install 2 of the "good" tubes and change your dummy load impedance to twice the tap of the transformer: 8ohm load on 4 ohm tap for example.
Set up the amp and test performance to make sure all is working ok. It is probably the red-glowing tube has a shorted element. An oscillation would not manifest its symptoms in one tube of a push-pull topography, they would all be red, even if the defect was in one tube(not likely, most tube defects lower gain not increase gain and phase shift) the rest would also be driven as hard by the oscillation.
If the amp checkout with two tubes, try the other two after pulling the good tubes(mark them to know which are already verified as working)
It the one tube glows again, switch sockets side to side and repeat, if it is the same tube in the different socket, you most likely have found the problem; an interelement short or loose element support which is changing the conduction characteristics of the tube.
When you have 4 working tubes reset the impedance tap to its original setting:8 ohm load on 8 ohm tap or whatever Z you are using.
Only after you have a working stock amp and made your meausrements as base line operation should you start modding the amp. Modding and troubleshooting at the same time is a waste of time and you will be unsure if you made improvements or gone backwards.
One of the base line measurements that is usually ignored but will save a lot of time in checkouts is the idling AC mains current. In the future, just plugging in the amp and noticing the idle current from the Variac current meter will tell you right away whether there is a general current draw that signals a defect before you even pull the cover.
The first rule of modifications:"There is no free lunch" That goes for amps, cars or anything else where hobbiests are second guessing engineers.
Every change has a range of influences, some good and some not. Generally, mods lower reliability, but some do not. Some change the tone in some operating condition while they also introduce undesired artifacts.
By knowing what compromises you are willing to accept to get an enhancement you will be happier with the results.
Document every change in the circuit, put any schematics showing the changes and notes in writing and put in an envelop stapled inside the amp case. A modded amp with no details of the changes is a very poor investment for a buyer so they would be reluctant to pay anywhere as much as for a stock amp. That is of course if you ever want to sell it.
[Edit] I reread the original message and realized it was a 2 tube power amp section. Ignore all references to changing Z and testing two tubes as a time. Try swapping tube positions and see if the red late moves with the tubes swap or stays in the same position. It is still not likely to be an oscillation, more likely a short or loose element inside that one red tube.
Pull the power tubes and measure the values at socket, if all is right, install 2 of the "good" tubes and change your dummy load impedance to twice the tap of the transformer: 8ohm load on 4 ohm tap for example.
Set up the amp and test performance to make sure all is working ok. It is probably the red-glowing tube has a shorted element. An oscillation would not manifest its symptoms in one tube of a push-pull topography, they would all be red, even if the defect was in one tube(not likely, most tube defects lower gain not increase gain and phase shift) the rest would also be driven as hard by the oscillation.
If the amp checkout with two tubes, try the other two after pulling the good tubes(mark them to know which are already verified as working)
It the one tube glows again, switch sockets side to side and repeat, if it is the same tube in the different socket, you most likely have found the problem; an interelement short or loose element support which is changing the conduction characteristics of the tube.
When you have 4 working tubes reset the impedance tap to its original setting:8 ohm load on 8 ohm tap or whatever Z you are using.
Only after you have a working stock amp and made your meausrements as base line operation should you start modding the amp. Modding and troubleshooting at the same time is a waste of time and you will be unsure if you made improvements or gone backwards.
One of the base line measurements that is usually ignored but will save a lot of time in checkouts is the idling AC mains current. In the future, just plugging in the amp and noticing the idle current from the Variac current meter will tell you right away whether there is a general current draw that signals a defect before you even pull the cover.
The first rule of modifications:"There is no free lunch" That goes for amps, cars or anything else where hobbiests are second guessing engineers.
Every change has a range of influences, some good and some not. Generally, mods lower reliability, but some do not. Some change the tone in some operating condition while they also introduce undesired artifacts.
By knowing what compromises you are willing to accept to get an enhancement you will be happier with the results.
Document every change in the circuit, put any schematics showing the changes and notes in writing and put in an envelop stapled inside the amp case. A modded amp with no details of the changes is a very poor investment for a buyer so they would be reluctant to pay anywhere as much as for a stock amp. That is of course if you ever want to sell it.
[Edit] I reread the original message and realized it was a 2 tube power amp section. Ignore all references to changing Z and testing two tubes as a time. Try swapping tube positions and see if the red late moves with the tubes swap or stays in the same position. It is still not likely to be an oscillation, more likely a short or loose element inside that one red tube.
Re: Problem.... or not? - red plating issue - advice appreciated
Guys, thanks for the help and a couple of things to highlight:
a) The problem stays with the socket rather than the tube i.e. V4. If i swap tubes the same thing happens in the same socket. I have done this a few times so I am confident it is not the tubes.
b) I have documented all changes etc that I have done, unfortunately when I received the amp it had been modded already without the changes being documented however I am effectively putting the ampback to stock values (including correcting the input wiring, the low input had been disconnected and some of the resistors/caps in teh first 2 stages had been changed?)
c) Could somebody please explain why I am reading AC on pin 3 of V5 (i.e. as well as the DC) when it is not on V4? - pin 3 = OT primary white wire (standard Marshall colours and the OT is an original Drake...)
Thanks Sean
________
How To Roll A Blunt
a) The problem stays with the socket rather than the tube i.e. V4. If i swap tubes the same thing happens in the same socket. I have done this a few times so I am confident it is not the tubes.
b) I have documented all changes etc that I have done, unfortunately when I received the amp it had been modded already without the changes being documented however I am effectively putting the ampback to stock values (including correcting the input wiring, the low input had been disconnected and some of the resistors/caps in teh first 2 stages had been changed?)
c) Could somebody please explain why I am reading AC on pin 3 of V5 (i.e. as well as the DC) when it is not on V4? - pin 3 = OT primary white wire (standard Marshall colours and the OT is an original Drake...)
Thanks Sean
________
How To Roll A Blunt
Last edited by badfan on Fri Feb 25, 2011 5:38 am, edited 1 time in total.
Re: Problem.... or not? - red plating issue - advice appreciated
Deleted: double post
Last edited by km6xz on Mon Sep 08, 2008 6:49 am, edited 1 time in total.
Re: Problem.... or not? - red plating issue - advice appreciated
Now there is more to go on. So you now know that the circuit is at fault, and now possibly the tube that gets plugged into that socket.
AC on pin 3, the plate? Is that with signal applied or idling? If with signal, , the voltages you gave in the first post would indicate a shorted output transformer primary winding. High current and red plates only occur when driving the tubes with signal, yet idling the voltage and current seem right. There are few other explanations that would account for all that you have detected.
Power down and discharge the supply, unplug the tubes and measure the resistance from the transformer center tap to either pin 3 to verify that.
AC on pin 3, the plate? Is that with signal applied or idling? If with signal, , the voltages you gave in the first post would indicate a shorted output transformer primary winding. High current and red plates only occur when driving the tubes with signal, yet idling the voltage and current seem right. There are few other explanations that would account for all that you have detected.
Power down and discharge the supply, unplug the tubes and measure the resistance from the transformer center tap to either pin 3 to verify that.
Re: Problem.... or not? - red plating issue - advice appreciated
thanks again for the suggestions I will try them out when I get back ltr this evening,
yes the plate? that got me thinking the only place AC could come in is via the OT secondary but it only happens when signal is applied - makes sense? I recall testing the OT primary and secondary for shorts but I thought they tested ok ill check and post the results.
if this is the case on V5 then why is V4 red plating? is the bias and operating conditions being thrown off because of the AC on V5?
no wonder im confused
cheers
sean
________
Free Drupal Themes
yes the plate? that got me thinking the only place AC could come in is via the OT secondary but it only happens when signal is applied - makes sense? I recall testing the OT primary and secondary for shorts but I thought they tested ok ill check and post the results.
if this is the case on V5 then why is V4 red plating? is the bias and operating conditions being thrown off because of the AC on V5?
no wonder im confused
cheers
sean
________
Free Drupal Themes
Last edited by badfan on Fri Feb 25, 2011 5:38 am, edited 1 time in total.
Re: Problem.... or not? - red plating issue - advice appreciated
I might be confused by the different sockets so to clarify, the plate lead on pin 3 of the socket where the excess current is being drawn through the tube is suspect. At idle, the DC resistance of the the primary is the only resistance limiting current, both the good side and the hot side wil be pulling current based on the bias level on the grids and the DC resistance of the winding. As a signal is generated, normally sees the high impedance of the inductive component plus resistive component of the winding. The AC signal, if the winding is shorted, will not see the normally high impedance and act as if it is shorted even if it is not a direct short, just lower impedance than normal. The current drawn through the tube will be limited only by the DC resistance and the stiffness of the power supply....very high current when the tube is driven with a signal.
Other components possibly in the plate circuit would be clamp diodes and possibly HF bypass caps, if they are installed. These devices if shorted would offer a path for B+ to power supply return, so signal or no signal they would pull high current if the tube was installed or not. So we are left with the only component in the plate circuit that is dependent on AC signal for loading the signal alone, and not the power supply directly: the half of the output transformer that is driven by that socket.
When testing the DC resistance of the transformer, comparing the two windings on either side of the center tap, the value is going to be within 10% in a proper transformer. Any more than that and it indicated shorted turns or improperly manufactured. There is usually a difference in DC resistance because of how a transformer is wound on a core, but not much difference in impedance. The DC resistance would be less than 100 ohms. for both sides while the impedance for a signal will be 5500-7500 ohms. That represents a very large difference in current flow if shorted turns removed most of the inductive reactive component from the load.
Other components possibly in the plate circuit would be clamp diodes and possibly HF bypass caps, if they are installed. These devices if shorted would offer a path for B+ to power supply return, so signal or no signal they would pull high current if the tube was installed or not. So we are left with the only component in the plate circuit that is dependent on AC signal for loading the signal alone, and not the power supply directly: the half of the output transformer that is driven by that socket.
When testing the DC resistance of the transformer, comparing the two windings on either side of the center tap, the value is going to be within 10% in a proper transformer. Any more than that and it indicated shorted turns or improperly manufactured. There is usually a difference in DC resistance because of how a transformer is wound on a core, but not much difference in impedance. The DC resistance would be less than 100 ohms. for both sides while the impedance for a signal will be 5500-7500 ohms. That represents a very large difference in current flow if shorted turns removed most of the inductive reactive component from the load.
Re: Problem.... or not? - red plating issue - advice appreciated
Here are the DC resistance readings from the OT with both power tubes pulled and no speaker load on the secondary
Primary (brown = CT)
Red-brown = 43.4 ohms
White-Brown = 44.6 ohms
Red-White = 87.9 ohms
Secondary (orange = ground)
Yellow-Orange = 0.4 ohms
Green-Orange = 0.6 ohms
Grey-Orange = 0.8 ohms
I will post some more readings later once I have added some screen resistors
Cheers
Sean
________
MOTORCYCLE TIRES
Primary (brown = CT)
Red-brown = 43.4 ohms
White-Brown = 44.6 ohms
Red-White = 87.9 ohms
Secondary (orange = ground)
Yellow-Orange = 0.4 ohms
Green-Orange = 0.6 ohms
Grey-Orange = 0.8 ohms
I will post some more readings later once I have added some screen resistors
Cheers
Sean
________
MOTORCYCLE TIRES
Last edited by badfan on Fri Feb 25, 2011 5:38 am, edited 1 time in total.
Re: Problem.... or not? - red plating issue - advice appreciated
Guys
Guess what sussed it.... turns out there was a connection to the OT secondary ground connection from pin 8 on V4. Obviously when I installed a resistor from pin 1 to ground I did not notice the connection on pin 8 which I should have removed? must be getting old or going blind
Anyway I have learnt a lot (again) from this exercise and can now get back to tuning the amp.
Cheers for all the help and pointers....
Regards
Sean
________
Babi Mac Break Away
Guess what sussed it.... turns out there was a connection to the OT secondary ground connection from pin 8 on V4. Obviously when I installed a resistor from pin 1 to ground I did not notice the connection on pin 8 which I should have removed? must be getting old or going blind
Anyway I have learnt a lot (again) from this exercise and can now get back to tuning the amp.
Cheers for all the help and pointers....
Regards
Sean
________
Babi Mac Break Away
Last edited by badfan on Fri Feb 25, 2011 5:38 am, edited 1 time in total.
Re: Problem.... or not? - red plating issue - advice appreciated
Guys
Guess what sussed it.... turns out there was a connection to the OT secondary ground connection from pin 8 on V4. Obviously when I installed a resistor from pin 1 to ground I did not notice the connection on pin 8 which I should have removed? must be getting old or going blind
Anyway I have learnt a lot (again) from this exercise and can now get back to tuning the amp.
Cheers for all the help and pointers....
Regards
Sean
________
No2 vaporizer
Guess what sussed it.... turns out there was a connection to the OT secondary ground connection from pin 8 on V4. Obviously when I installed a resistor from pin 1 to ground I did not notice the connection on pin 8 which I should have removed? must be getting old or going blind
Anyway I have learnt a lot (again) from this exercise and can now get back to tuning the amp.
Cheers for all the help and pointers....
Regards
Sean
________
No2 vaporizer