Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jordan86 on August 18, 2022, 09:16:59 pm
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I’ve posted on this amp conversion before. Tucked it away though and finally finishing it up tomorrow. Adding two 12ax7 tubes for a V1 preamp stage (parallel triode) and basic Vox LTPI on V2. Perplexed on how to wire my 12v heaters though. Since original glass was all 6v, Hammond grounded one half of the 6v winding. Never seen that before.
*See tab #5 on the PT. It goes to Pin 2 on a 6V6 socket but it’s tied to pin 1 on the 6v6 socket too and grounded. And ll the 6v6 sockets are connected at Pins 1&2.
So not sure how to wire up pins 4,5 and 9 on my 12ax7s??? Can I just run the single 6V tap from the Pin 7 on the 6V6 to pins 4,5, and 9? Or do I need both sides of the 6v winding from the PT?
Here’s the scheme
https://el34world.com/Forum/index.php?action=dlattach;topic=28902.0;attach=98486;image
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break the filament connection to ground, add a pair of 100r artificial center tap resistors (one from each filament tap to ground), and run your heater string normally (with one lead to pin 9 and the other to pins 4 and 5). will be a lot less hum that way
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Lots of old amps did the 6VaC heaters that way. The non-grounded end of the heater winding goes to one side of the 6V tube filament pin (e.g. on your 12AX7, to socket pins 4+5 -tied together). The other side of the 6V filament pin (i.e. Pin 9) goes to ground (the chassis acts as the connection for the return current for the 6V winding.
Re-wiring the sockets with a AC pair from the heater winding (and setting up an artificial ground reference with 2 x 100R resistors) would probably get things a bit quieter (now that you are adding another pre-amp tube to the mix).
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> Never seen that before.
That's how about all 6V-heat radios were wired. Chassis return. Works in radio and high-level audio. (This Hammond took signal at the several-Volts level.) Saves a wire.
Yes, you need the chassis return.
Wire 12AX7 socket for 6V heat, like we do in almost all g-amps. Now you have two leads. One goes to chassis easiest way. Other to that live 6V point.
But: if you are going to add a preamp, you will probably end up with 6V CT like every other >$29.95 guitar amp ever made.
> goes to Pin 2 on a 6V6 socket but it’s tied to pin 1 on the 6v6 socket too and grounded.
So what is pin 1? See datasheet. It is the metal shell. No, you don't have any metal 6V6, but Pete does. If there is an internal melt-down, the metal shell could be electrically "hot". So you tie it to chassis to short-out any wild voltage. Pin 2 and pin 1 do different things, and could be wired differently. But here it is simple and expedient to just chassis-tie both.
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Thanks all. Yeah I am only familiar with the “Fender way” with an artificial CT or built in CT in the winding itself.
For now, I think I will do the expedient option of grounding Pin 9 on the 12ax7. Will just get it working and then evaluate the noise floor and maybe save that artificial CT for a rainy day.
If I do eventually want to break the chassis ground side of the 6V winding and add the artificial CT, I should disconnect Pin 2 from Pin 1/Chassis on all the 6V6 sockets, yes?
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disconnect pin 1 from pin 2. as mentioned, pin 1 would be grounded in the event that you run metal envelope 6v6
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> any metal 6V6
Found a picture: