Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 1blueheron on December 07, 2021, 07:54:04 am
-
This is the second Silvertone 157.47510 I have salvaged from old reed organs. I would like to do something more ambitious with this one than I did with the first. I am thinking I would like to convert it to a combination of Sluckey's Smokey and the "Producer" by J. R. Templeton.
Would appreciate thoughts and suggestions on this idea. In the study of this, I have noticed several different methods of running power to the output tube. What are the benefits and drawbacks of the different methods shown on the following three schematics, such as biasing the grid off the filament string.
-
The silvertone is the cheapest design of the three. No choke. And it will have the most hum because the plate is fed from the very ripply first filter node.
Smoky uses a choke and feeds the plate AND screen from the second filter node. Much lower hum. This works well for small 6AQ5 amps. The Fender 125C1A choke (4H, 90mA) is sufficient for Smoky. Of course you could use a separate filter node with dropping resistor and screen resistor for a small improvement, but I was trying to keep Smoky simple and I was using a small crumb catcher plate for a chassis. :l2:
The Producer uses a big, high current (expensive) choke and feeds the plate from the second node for much lower hum just like Smoky. Big high power tube screens are usually supplied from a different filter node and also use a screen resistor for tube protection. This design will also work well for a small 6AQ5 amp using the PT and OT from the silvertone donor. Buy a new choke.
-
Here is my first shot at a schematic of the merge. I used the 5879 rather than the EF86 or the 5654 since: 1) I have a 9pin socket rather than 7pin and, 2) I have a 5879 on hand but no EF86.
Planning to replace the old 40 + 40 cap can with a new JJ 40-20-20-20
I suspect plate resistors will need tweaked.
-
You need at least a B+ C node, for the CF + 5879 or/and a D node, 5879, would be nice too.
-
OK, I made a few of the suggested changes.
Working on trying to calculate voltages for each node.
I placed the 5879 on C node, and 7025 on D node as according to data it looks like 5879 may be run at higher plate voltages. Would this thinking be correct? Or would it be better to have it as D node? I probably will run it on the cool side to help avoid microphonics correct?
-
I suggest powering the 6AQ5 plate from node B for the reason I stated above.
-
I suggest powering the 6AQ5 plate from node B for the reason I stated above.
Sluckey,
Are you referring to node B in the first draft?
Smokey schematic shows the Plate and Grid being connected to Node A in your schematic. I notice your screen voltage is noted as higher than plate voltage (I assume that is due to the loss going through the OT. All other schematics show separate feeds for Plate and Grid with Plate being higher.
I understand the choke lowers noise, therefore Node B would be quieter than Node A as I have in Rev 1. drawing. Should I move node A to after the choke and feed both Plate and Grid as you did on Smokey? Essentially moving everything down one node and dropping D node.
-
Your rev2 drawing looks good. You may want to add a screen resistor but there will be little benefit in this small amp.
-
For those who like pictures, this is the subject I am working on as it was when I started with a little dusting off.
-
> Draft Rev2.PNG
470k for power tube cathode resistor?
-
Good catch!
-
Updated