Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on September 03, 2022, 09:29:16 am
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I'm upgrading the original U.S. PT to a European one and met an issue with the heaters.
Silvertone is using 12.6V C.T. filament and my PT does not support that.
I have not seen this kind of wiring before where the power tube cathode is connected through the preamp tube heaters to ground.
Are there any ways to apply 6.3 V heaters without changing the characteristics of the circuit.I'm already planning to buy a 12.6V C.T. separate filament transformer that solves the issue.
https://el34world.com/charts/Schematics/files/Silvertone/Silvertone_1433_manual.pdf (https://el34world.com/charts/Schematics/files/Silvertone/Silvertone_1433_manual.pdf)
/Leevi
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If you're gonna buy a separate 12.6V CT transformer there's nothing special to do.
But if you want to use 6.3VAC from your replacement PT you simply need to remove the wiring from the two 6L6s, 6CG7, and 6AU6. Now just rewire these four tubes in the more common configuration using the 6.3 winding. Don't change anything about the two 12AX7 tubes.
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Rewire the power tubes, 6CG7 and 6AU6 in parallel as you would with any 6.3V filament string. Leave the 12AX7 filaments wired as-is - They are the cathode bias resistors for the power amp stage. The bonus is you get DC filaments for the high gain preamp tubes - less hum/buzz.
See attached for clarification.
--Pete
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Thank you sluckey and DummyLoad for your replies.
One additional question regarding 12AX7s. How do you get them heated? I don't see voltage supply for the heaters.
/Leevi
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The 6L6s are cathode biased and the two 12AX7 heaters are connected in series and serve as the cathode resistor. So, the 6L6s cathode current flows through the 12AX7 filaments resulting in 19V dc voltage at the 6L6 cathodes. If you remove the 6L6s there will be no current flowing through the 12AX7s filaments and they will not be lit.
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Yes, I was just a bit sceptic that 6L6 cannot supply the needed current.
/Leevi
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Silvertone has some other amps that use this same circuit. Works fine even though the current is less than the typical 150mA listed in the 12AX7 data sheets.
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Fisher and Bogen did this also. One 6L6 condition goes to 232mA total playing loud.
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I made a quick and dirty test and it WORKS!!!
The warm up time is longer than normally which is maybe depending on 12AX7 heating.
Hum 100Hz is still audible but I think I will get it more silent after I have improved wiring and grounding.
Thank you for the help
/Leevi
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The warm up time is longer than normally which is maybe depending on 12AX7 heating.
That's just a characteristic of this circuit. The 6L6s must warm up and conduct in order for the 12AX7s filaments to heat up.
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Yes. 11 seconds for the 6L6es to flow free, then another 11 seconds for the 12AX7 to admit pickup signal.
(On the Fisher, other than the Phono input, you were ready in one 11 second warm-up wait.)
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Hum is gone but there is a quite loud idle hiss that does not increase with volume. Most of the resistors are carbon. One critical part is the shielded signal cable from the preamp unit (upper) to the power amp. In general there is lot of room for improvements but have to take into consideration that the amp is 60 years old.
/Leevi