solderjunkie
Member
Hello! I've been lurking on this fine forum for some time and have finally come upon a reason to join. I've recently begun re-doing a previously converted Curtis Mathes console power amp which I picked up on ebay and has been amplifying my son's ipod for the past few years.
I have been unable to identify the model of this amp or locate a schematic, but the transformer date is 1960 and it seemed like a pretty straight forward push-pull power amp. It is tube rectified with a 5U4 and there is a single RCA 7025 driver/inverter, (which is a 12AX7 variant and turns out to be a very valuable tube indeed. Tube Depot sells them for $180 ea.!) and four SBQ5/el84 output tubes.
My goals were:
There was nothing wrong with how the amp operated, that is, until after I started monkeying with it. The issues are to follow, but first some pics!
Here is what it looked like before...
As you can see, lots of unnecessary open-ended wiring originally connected to the preamp/tuner socket. Also both the input jacks and speaker binding posts are located next to the power supply. Another odd thing (odd to me anyway) is that one side of the 5V winding for the rectifier heaters was connected to the first filter cap BEFORE the rectifier tube. I'm sure there is a sound reason for this, which probably has to do with quieting hum, but it's not something I am familiar with.
Here is what the amp looks like now...
For a short time after the aforementioned changes the amp seemed to operate normally and idled very quietly with almost no discernible hum. I had it connected to an 8 ohm speaker load as it has been for years (although I think the output wants a 4 ohm load) and the new volume control worked perfectly. Gradually (after 30 minutes or so) the output reduced to barely a whisper. No distortion, just very little output. Also both the chassis and the power transformer felt unusually hot.
I opened the amp back up to check voltages and nothing seemed out of the norm compared to it's previous incarnation's voltages. B+, plates, grids and cathodes were unchanged on both preamp and output stages.
Here's what seems very strange:
I accidentally checked one of the heater pins on an output tube with my voltmeter set on DCV and it registered voltage. I thought I was mistaken and set the meter to VAC and it did read voltage but unusual numbers. One winding was putting out 7.5 VAC and the other 2.8 VAC. I then switched the meter back to DCV and it did actually read 4 DCV on one side and 3.6 DCV on the other side. This is not something I have seen before. The un-rectified, un-filtered heater windings are coming straight out of the PT and somehow putting out voltage that is both AC and DC. Can they be registering both AC and DC volts simultaneously? Also, I wonder if this higher voltage may prematurely burn out the tube filaments. Now, checking the voltage there it reads 4 VAC and 2.8 VAC, so fluctuating quite a bit.
Aside from adding a volume control and deleting a bunch of open-ended hookup wire, the only alterations I made to the previous circuit were
Here are the readings currently:
PT
HT Standby 325 VAC
On 311 VAC
6.3V heater windings 4.0, 2.8 VAC
5V windings
without 5u4 10.6/6.9 VAC
with 5U4 334 B+ DCV
V1
pin 1 (plate 1) 304
pin 2 (grid 1) .9 DCV (at full volume)
pin 3 (cathode) 5.0
pin 6 (plate 2) 304
pin 7 (grid 2) .9 DCV
pin 8 (cathode) 4.90
V2
plate 318
grid 314
cathode 11.0
V3
plate 321
grid 314
cathode 11.0
V4
plate 321
grid 314
cathode 10.1
V5
plate 321
grid 314
cathode 10.1
Any comments, ideas or reprimands are much appreciated.
~ C
I have been unable to identify the model of this amp or locate a schematic, but the transformer date is 1960 and it seemed like a pretty straight forward push-pull power amp. It is tube rectified with a 5U4 and there is a single RCA 7025 driver/inverter, (which is a 12AX7 variant and turns out to be a very valuable tube indeed. Tube Depot sells them for $180 ea.!) and four SBQ5/el84 output tubes.
My goals were:
- clean up the wiring
- relocate the inputs and outputs that were for some reason located on the power supply side
- swap the positions of the rectifier tube and can filter capacitors
- replace the radial electrolytics soldered to the old can with a new can and preamp filter
- add a volume control and more substantial power/standby combo switch
- add an external fuse holder.
- add heater supplied pilot light
There was nothing wrong with how the amp operated, that is, until after I started monkeying with it. The issues are to follow, but first some pics!
Here is what it looked like before...
As you can see, lots of unnecessary open-ended wiring originally connected to the preamp/tuner socket. Also both the input jacks and speaker binding posts are located next to the power supply. Another odd thing (odd to me anyway) is that one side of the 5V winding for the rectifier heaters was connected to the first filter cap BEFORE the rectifier tube. I'm sure there is a sound reason for this, which probably has to do with quieting hum, but it's not something I am familiar with.
Here is what the amp looks like now...
For a short time after the aforementioned changes the amp seemed to operate normally and idled very quietly with almost no discernible hum. I had it connected to an 8 ohm speaker load as it has been for years (although I think the output wants a 4 ohm load) and the new volume control worked perfectly. Gradually (after 30 minutes or so) the output reduced to barely a whisper. No distortion, just very little output. Also both the chassis and the power transformer felt unusually hot.
I opened the amp back up to check voltages and nothing seemed out of the norm compared to it's previous incarnation's voltages. B+, plates, grids and cathodes were unchanged on both preamp and output stages.
Here's what seems very strange:
I accidentally checked one of the heater pins on an output tube with my voltmeter set on DCV and it registered voltage. I thought I was mistaken and set the meter to VAC and it did read voltage but unusual numbers. One winding was putting out 7.5 VAC and the other 2.8 VAC. I then switched the meter back to DCV and it did actually read 4 DCV on one side and 3.6 DCV on the other side. This is not something I have seen before. The un-rectified, un-filtered heater windings are coming straight out of the PT and somehow putting out voltage that is both AC and DC. Can they be registering both AC and DC volts simultaneously? Also, I wonder if this higher voltage may prematurely burn out the tube filaments. Now, checking the voltage there it reads 4 VAC and 2.8 VAC, so fluctuating quite a bit.
Aside from adding a volume control and deleting a bunch of open-ended hookup wire, the only alterations I made to the previous circuit were
- replaced the pair of main filter caps from 68uf to 50uf (the original can had a 60 and 30 uf cap in it). I'll admit there were no total current draw calculations involved there, I just used what I had that was in the neighborhood.
- changed the preamp filter cap from 100uf to 33uf
- replaced one cathode bypass cap of same value.
- Moved the 5V winding leg to the rectifier before the first main filter cap.
Here are the readings currently:
PT
HT Standby 325 VAC
On 311 VAC
6.3V heater windings 4.0, 2.8 VAC
5V windings
without 5u4 10.6/6.9 VAC
with 5U4 334 B+ DCV
V1
pin 1 (plate 1) 304
pin 2 (grid 1) .9 DCV (at full volume)
pin 3 (cathode) 5.0
pin 6 (plate 2) 304
pin 7 (grid 2) .9 DCV
pin 8 (cathode) 4.90
V2
plate 318
grid 314
cathode 11.0
V3
plate 321
grid 314
cathode 11.0
V4
plate 321
grid 314
cathode 10.1
V5
plate 321
grid 314
cathode 10.1
Any comments, ideas or reprimands are much appreciated.
~ C
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