Are you testing with power tubes in? I also think you're better off testing voltages with both channels connected (at least at V4).
Did you previously verify voltages at all the power supply nodes? Basically just working your way back from the OT center tap to each dropping resistor/filter cap.
It's disconcerting that measured voltages seem to keep changing each time. Perhaps this is Schrodinger's Amp. Are you always connected to the same ground point when measuring?
Firestorm wrote:Are you testing with power tubes in? I also think you're better off testing voltages with both channels connected (at least at V4).
Did you previously verify voltages at all the power supply nodes? Basically just working your way back from the OT center tap to each dropping resistor/filter cap.
It's disconcerting that measured voltages seem to keep changing each time. Perhaps this is Schrodinger's Amp. Are you always connected to the same ground point when measuring?
Yes, I'm testing with tubes in and using the same ground. The changes seem to be due to connecting R14. I'm regrouping to have another run at it. I'll try your strategy. I'm so tempted to pull the board to see what's under it and clean things up.
I would not be quick to pull that board. I had no choice because the i/s tranny blew and I had to R&R it twice. I got lucky and found an OEM part of eBay for cheap. The generic Hammond replacement was poo, but I diverge. Pulling the board is complicated, and I don't think it will move you ahead. IMO, there is only a remote possibility of a short under the board. Save this for much later.
For grins, will you put R14 back in the circuit and lift R30 (68K) and see what things are like with only ch 1 on line? In addition to voltages, I'm curious to know if there is signal and your judgment on how loud it is.
Well, the first thing I would probably have done is draw up a layout of where everything was connected and started selectively pulling things out. But before doing anything like that, having a set of node voltages (with all tubes in and both channels joined) would help.
The other useful set of measurements would be to see how much voltage is dropping across each plate load (just the basic preamp for now). You would connect your meter's negative lead to the plate pin and the positive lead to the other side of the plate resistor: These would be (from the EA schematic): R5 and R13 on the normal channel and R19 and R29 on the reverb channel. If you want, you can verify that the supply voltage on each resistor (the side of the resistor away from the tube) matches the voltage at each appropriate power supply node.
Measuring the voltage across these resistors tells us how much voltage is being dropped by each plate resistor and therefore how much current is flowing in each triode. This should help narrow down potential problems.
Again, we're interested in the basic preamps here. Later we can look at the tremolo oscillator and the reverb circuit. We are also ignoring the driver tube for now.
Relax, Phil... I'm not going to pull the board unless ya'll recommend it. I was venting. For the first time, I feel stuck.
For grins, will you put R14 back in the circuit and lift R30 (68K) and see what things are like with only ch 1 on line? In addition to voltages, I'm curious to know if there is signal and your judgment on how loud it is.
Ok, I did this, and the good news is that CH1 is MUCH louder and is now on par with CH2. Both channels are equally loud now. I also disposed of the notch filter as you described.
One question. How do you get a good mechanical connection with these solder cups? I don't feel that just poking the wires in is getting me a good connection.
Working with the solder cops is not difficult. There is an X sliced into the bottom. The objective is to push the component lead through the one of the slits, which go up the sides towards the top of the cup. You'll feel it. If you have been adding solder, you may want to use a solder sucker to remove some solder when it is liquid, as it is also a good idea to add a bit of new solder when necessary. A smaller chisel point on your iron will make working in those cups easier.
We are beating a dead horse. The good news is that the problem appears on both channels, more or less to the same degree. V4 voltage, plate is 2x schematic and cathode is 50% of schematic.
At this point, I am inclined to attach a jumper (wire with gator clips) from C4 (left side) to the input to the volume pot (left side of R eight). and then from C11 to R25, one a time. Do this so that C4/C11 are in circuit and not bypassed. This will bypass the tone stack.
Before you do this, let's see if Firestorm or anyone else out there concurs.
Here's my thinking. The bias for the triode is controlled either by the cathode resistor or what's riding on the grid. It appears that Rk is OK, so I want to change what the grid is seeing. Maybe that will have an impact on plate and cathode voltage. I still can't fathom the problem being nearly identical on both channels.
I continue to wonder if T3 is partially shorted. I think that is the same tranny as my Minuteman, which has a turns ratio of about 1:2.5. I think we may need to test it (again?).
And someone ought to fix the emoticons so R and the numeral eight can be used as a literal.
It might be simplest to eliminate the grids' influence by simply grounding the grid pin on each triode sequentially. This will eliminate any positive voltage that may be is leaking in and let us read the tubes' bias condition with just plate and cathode active.
Firestorm wrote:It might be simplest to eliminate the grids' influence by simply grounding the grid pin on each triode sequentially. This will eliminate any positive voltage that may be is leaking in and let us read the tubes' bias condition with just plate and cathode active.
So I'd jumper the grid pins to ground (two grids per triode, pins 2 and 7), each pin in turn, and take readings on all the pins?
Ground the grid, measure plate and cathode, then move on to the next. One triode at a time. It's as if each tiode had its own volume control turned down to zero. Eliminates any "signal" getting in and let's you see just the relationship of cathode and plate. If it differs appreciably from normal, it tells you something is coming in on the grid that shouldn't. Just V1 and V4 for now.
Firestorm wrote:Ground the grid, measure plate and cathode, then move on to the next. One triode at a time. It's as if each tiode had its own volume control turned down to zero. Eliminates any "signal" getting in and let's you see just the relationship of cathode and plate. If it differs appreciably from normal, it tells you something is coming in on the grid that shouldn't. Just V1 and V4 for now.
V1-4 1v
V1-6 -188v
V1-7 174v
V1-9 1.2v
V4-1 jumps between 2mv and 7mv
V4-3 277v
V4-6 1.5v
V4-8 172v
Last edited by Travst on Sun Feb 06, 2011 6:31 pm, edited 2 times in total.
Are you always connected to the same ground point when measuring?
