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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 20th Jan 2023, 2:11 pm   #1
johnhoef
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Default Dynatron LF612a

Hello, everyone,
I tried to add my thoughts to a thread, but was not allowed. Further, looks like the three threads that interest me a closed? So, posting a new thread here. I sure hope I can get some help. Working on a Dynatron LF612a power amplifier. Ideally, I would like to have a copy of the schematic. I do have a schematic for a LF512a, and it looks similar, but the exact one would be nice. My issue is that it is blowing fuses. I did replace the power supply capacitors. Using the variac, I can get it working on, say, 80% power, and it sounds good. Idle current through the KT66 tubes looks within specs.
So, does anyone here have a schematic to share? It would be greatly appreciated!
John
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Old 20th Jan 2023, 3:01 pm   #2
Cobaltblue
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Default Re: Dynatron LF612a

Hi John
Welcome to the forum.

Threads close automatically if you wish to add to a closed thread use the report post button (the little red warning triangle) and request it's reopened with a reason.

Mods will then reopen unless the threads were closed for a specific reason.

Cheers

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Old 23rd Jan 2023, 5:29 pm   #3
Richardgr
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Default Re: Dynatron LF612a

I suppose one simple issue might be the right amperage fuse, but a fast blow fitted instead of a slow blow? Are the power supply capacitors the same spec as the ones replaced? Increasing the capacitance could stress PSU components, and the fuse blowing could be symptomatic.

If the coupling capacitors are leaking the tubes could be conducting more than intended, causing the fuse to blow as intended.

Any pictures? Sounds like an interesting item!
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Old 25th Jan 2023, 12:40 am   #4
johnhoef
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Default Re: Dynatron LF612a

Richardgr,

Thanks for your thoughts. The amp uses 2A fast blow fuses, as specified on the rear panel. The thought did cross my mind to use slow blow fuses. I replaced the power supply capacitors with same, so there shouldn't be any additional stress. The amp works very nicely with a clean output, total of 14 watts. All caps are new, including the filters. I also replaced the plate to grid resistors on the output tubes. They had drifted. Installed precision, so they are really 100 ohms! Cathodes are tied together, and through a 300 ohm resistor to ground. Measuring the voltage across that resistor, I get 32 volts, which translates to 53mA of idle current through each output tube. Since I have not found the schematic for this unit, I am using one for the Dynatron LF512A, which is very similar, however, it would be nice to have the correct schematic. So, the amp is working fine...just draws a lot of current, stressing the fuses! I'll follow up with some photos soon.

Here are a couple photos of the amp:
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Old 25th Jan 2023, 2:00 pm   #5
PJL
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Default Re: Dynatron LF612a

The resistors you have replaced are from anode to screen grid so the KT66's are running in triode mode.
The KT66 cathodes seem to be connected together but I can't work out where they go.

You should draw up the schematic yourself by comparing with the 512. I suggest you don't replace the fuse until you have worked out how it is biased and can check the anode currents.
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Old 25th Jan 2023, 4:03 pm   #6
Richardgr
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Default Re: Dynatron LF612a

Lovely clean construction! The sort of build I aspire to.

Is it configured for 250V mains? That could reduce the stress on the fuses. Also a CL-90 inrush current limiter on the on-off switch is a convenient way to buffer the startup jolt.
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Old 26th Jan 2023, 2:13 am   #7
johnhoef
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Default Re: Dynatron LF612a

PJL, As mentioned above, the idle current for each tube is 53mA, arrived at by measuring the voltage across the cathode to ground resistor. One thing I did realize is that the step up transformer I am using outputs 245 volts! So, I set the Variac for 230 volts, which is what the amp expects. That creates some relief for the blowing fuses. I also realized the other reason for fuse blowing is turning the amp on shortly after it is turned off. By doing so, the filter caps have discharged, but the rectifier tube is still hot enough to conduct, causing a high current flow to charge up the filters. Starting up on dead cold is not a problem, since the rectifier slowly comes on line, while slowly charging the filters. So, as far as I am concerned, the amp is working properly. Using my AC ammeter, I measure .5 amps being drawn, so well below the 2 amp fuse rating. The owner of this amp is aware of this issue. In his case, it's not a problem since he runs all his vintage tube equipment through a Variac, at 85% to 90%. So, Richardgr, in this case, I don't think an in rush limiter is necessary....good suggestion, though. Yes, I agree, a nice clean layout, neatly wired, and reasonably easy to trace circuits without a schematic.
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Old 26th Jan 2023, 8:29 am   #8
Synchrodyne
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Default Re: Dynatron LF612a

Circuit information for the Dynatron T99/LF612 combination and following suffixed versions thereof (Ether Conqueror V chassis) seems to be unobtainium.

There is some chance though that the LF612 is similar in terms of basic circuitry to the LF613, for which information is available. (Please PM me for a scan.) The T139/LF613 combination was the Ether Marshal chassis.

The LF613 did have a third KT66, acting as a cathode follower for the HT supply to the T139 tuner-control unit and the optional FM1 FM tuner. Otherwise though it seems to have followed the LF612 pattern with a pair each of Z77 (EF91) and KT66, all triode-strapped.


Cheers,
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