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jimmy74 18th September 2019 12:38 AM

OT question
 
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.

SemperFi 18th September 2019 12:55 AM

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.

JMFahey 18th September 2019 02:10 AM

What makes you think the OT is blown?

You must troubleshoot the amplifier: measuring DC voltages first, then injecting audio at the input and following up to the speaker itself.

PRR 18th September 2019 02:35 AM

> 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.

kodabmx 18th September 2019 03:31 AM

Same with my former auto teacher. Thought he had a bad fuel pump in his Fiero. He was ready to drop the tank and replace it. I told him that he taught us to start easy and work up to it... It ended up being the relay.

jimmy74 18th September 2019 03:41 AM

Yes they are connected to the screens...I have tested for voltage and bias all seem pretty normal around 508v screens, 512v plates, -45v bias and 3.3vac heaters. I'm getting around 100r between the UL windings

jimmy74 18th September 2019 03:48 AM

Don't have a scope...So just measuring with a digital multimeter

https://i1361.photobucket.com/albums...psoqlwnre1.jpg

6A3sUMMER 18th September 2019 05:37 AM

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.

Tesla88 18th September 2019 07:35 AM

Model of the Amp ? Maybe we can find a schematic to ease you the job

jazbo8 18th September 2019 07:42 AM

It's an Elka Bass amp.


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