Hi I have an old 60's italian made 50w bass amp on the bench with some issues. Basically there's hardly any sound out of channel 1 and nothing at all out. I have replaced all the electolytics, new valves and some of the audio caps in the preamp section. I tested the OT primaries and they're giving me 98 and 102 ohms but after further checking I've noticed that there are 5 wires for the OT primaries instead of 3. If this OT were blown could I replace it with a standard marshall 50w OT or would it have to have 5? It runs on 2 el34s and 5 preamp tubes.
Thanks
J.
Thanks
J.
5 wires on primary usually indicates the OT has taps for UL-mode. The two extra wires are either connected to the tube's screen grid, or left floating. Floating as in fastened somewhere safe but not connected electrically to the circuit.
Sure you can use Marshall OT for replacement. Id make sure the OT is not only for regular guitar but also bass since it should be rated 50watts to 40Hz and not just 80Hz.
I asume u have an oscope and have checked u have signal all thru the chain...? If not start at the input and scope till the output. So many places something can be broken.
Sure you can use Marshall OT for replacement. Id make sure the OT is not only for regular guitar but also bass since it should be rated 50watts to 40Hz and not just 80Hz.
I asume u have an oscope and have checked u have signal all thru the chain...? If not start at the input and scope till the output. So many places something can be broken.
> What makes you think the OT is blown?
People's minds often go to the big expensive part.
A friend thought he needed a new water pressure tank (well, pump). Very low flow from kitchen faucet.
Well, the pressure tank just holds pressure (for short bursts), doesn't MAKE pressure.
Also the flow in the bathroom was fine.
I took the bubbler-screen out of the kitchen faucet. Yup, crud. Blew it out, flow was much better. I left it to him to either clean it better or swap-in a much nicer pre-owned faucet he had in the yard. (Which may also be clogged, may be why it followed him home...) At some point I guess I better put a filter in line.
People's minds often go to the big expensive part.
A friend thought he needed a new water pressure tank (well, pump). Very low flow from kitchen faucet.
Well, the pressure tank just holds pressure (for short bursts), doesn't MAKE pressure.
Also the flow in the bathroom was fine.
I took the bubbler-screen out of the kitchen faucet. Yup, crud. Blew it out, flow was much better. I left it to him to either clean it better or swap-in a much nicer pre-owned faucet he had in the yard. (Which may also be clogged, may be why it followed him home...) At some point I guess I better put a filter in line.
Jimmy74,
You should have More voltage on the Screens than the voltage on the Plates. That is for traditional UL output transformers.
Unless . . .
1. There is a resistor from the UL tap to the screen Or,
2. There is a separate winding for the UL taps, and its center tap is connected to a lower voltage; or a separate UL winding has Much higher DCR.
3. Is it possible someone swapped the plate and screen connections?
A typical example of a traditional UL transformer is: 40% UL
DCR: Center tap to UL tap #1 40 Ohms; Center tap to Plate tap #1 100 Ohms. Center tap to UL tap #2 40 Ohms; Center tap to Plate tap #2 100 Ohms.
Note: Digital Multimeters often have trouble reading DCR of a transformer. So be sure to put a short across the secondary winding, example a wire from 8 Ohm tap to the Common tap.
You should have More voltage on the Screens than the voltage on the Plates. That is for traditional UL output transformers.
Unless . . .
1. There is a resistor from the UL tap to the screen Or,
2. There is a separate winding for the UL taps, and its center tap is connected to a lower voltage; or a separate UL winding has Much higher DCR.
3. Is it possible someone swapped the plate and screen connections?
A typical example of a traditional UL transformer is: 40% UL
DCR: Center tap to UL tap #1 40 Ohms; Center tap to Plate tap #1 100 Ohms. Center tap to UL tap #2 40 Ohms; Center tap to Plate tap #2 100 Ohms.
Note: Digital Multimeters often have trouble reading DCR of a transformer. So be sure to put a short across the secondary winding, example a wire from 8 Ohm tap to the Common tap.
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Yes its an Elka bass amp been looking online for a schematic but can't find anything. Yes there is a resistor between the ul taps and the screen
Yes the tubes are all new... I am getting unstable dc readings on most of preamp pin 2 and 7 lugs....jumping around constantly when I put the multimeter on the mv selection and very low readings when I use the V setting. These unstable readings are signed by an asterisk.
V5 is a 12au7 the other 4 are 12ax7's. Tried installing a new output jack and before soldering the wires on again, I tested the common and 8ohm tap secondary and I'm getting 1.3r
Yes the tubes are all new... I am getting unstable dc readings on most of preamp pin 2 and 7 lugs....jumping around constantly when I put the multimeter on the mv selection and very low readings when I use the V setting. These unstable readings are signed by an asterisk.
V5 is a 12au7 the other 4 are 12ax7's. Tried installing a new output jack and before soldering the wires on again, I tested the common and 8ohm tap secondary and I'm getting 1.3r
3.3VAC heaters? Shouldnt this be 6.3 for EL34s and either 6.3 or 12.6 for preamp tubes?
Or is this 2x3.3VAC measured on each half of a GND (or bias voltage)?
The still unanswered question is about this: Screen 508V Plate 512V
Why is the screen voltage 4V less than the plate voltage?
Swapping plate and screen connections can do that. And that will give a lot less power out.
Either there are resistors from the UL taps to the screens, Or, There are separate Screen UL windings and Plate windings. (that is a very expensive way to build an output transformer). How many guitar amp manufacturers will pay the extra $$$?
Most UL output transformer windings are essentially electrically connected this way: Plate winding all the way to the UL tap, and then continuing on to the Center tap. Another winding from the Center tap, to the UL tap, and then continuing on to the plate winding. That way, there is more B+ voltage drop from the Center tap to the plate tap, than there is from the Center tap to the UL tap. From the Center tap to the UL tap, has Both the screen current plus the plate current. From the UL tap to the plate tap, there is only the plate current. Ohms law. Does that make sense?
Why is the screen voltage 4V less than the plate voltage?
Swapping plate and screen connections can do that. And that will give a lot less power out.
Either there are resistors from the UL taps to the screens, Or, There are separate Screen UL windings and Plate windings. (that is a very expensive way to build an output transformer). How many guitar amp manufacturers will pay the extra $$$?
Most UL output transformer windings are essentially electrically connected this way: Plate winding all the way to the UL tap, and then continuing on to the Center tap. Another winding from the Center tap, to the UL tap, and then continuing on to the plate winding. That way, there is more B+ voltage drop from the Center tap to the plate tap, than there is from the Center tap to the UL tap. From the Center tap to the UL tap, has Both the screen current plus the plate current. From the UL tap to the plate tap, there is only the plate current. Ohms law. Does that make sense?
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