Tube Amp Archive

Hi All! I picked up this amp chassis for $20 today. No tubes or documentation included. The only markings are a stamped circle with "32E" inside. The over-all circuit appears to be push-pull using some kind of octal power tube like 6v6, 6l6, etc (pin 1 unused, pin 3 [plate] tied to one side of opt p...

Thank you for your reply! I was 99% sure I was right about it NOT being the plate, your post confirms it. With the grid resistor changes, the tubes are now quite happy at all normal volumes. I assume the plate would basically be the entire plate surface glowing red, and grid would look like its comi...

I swapped out my 430-ohm sand-block grids for 1000-ohm 20-watt 1% units. At idle, screen voltage is now 479, with 3.7 volts across the grid resistors. This comes out to just under 2 watts, and that value measures/calculates the same for both tubes. With a 1-KHz sine wave test signal applied to the a...

Thanks for the reply! This is separate from the filament, which doesn't change in brightness, and based on your reply its not the plate. I've read elsewhere in this forum (several posts) about the screen grid glowing when the tube is pushed if the screen grid resistor is too low. Total idle plate cu...

Hi all!! Still fairly new to tube amp design/building, and have a question with what is probably an obvious answer. How do I tell if its the plate or grid glowing at high volume? I assume the plate would basically be the entire plate surface glowing red, and grid would look like its coming from furt...

The Bandmaster uses a capacitor input filter, not a choke input filter. A choke input filter will have much less DCV output than the cap input filter when a load is applied. Much less than you're thinking. The only reason the DCV is so high now is because there is no load connected. With no load cu...

TUBEDUDE wrote: ↑Mon Feb 27, 2023 9:55 pm That 35uF before the choke is a filter capacitor also.

Yes, used as the filter/reservoir for the power tube plates. They are not as susceptible to the power supply noise in this case since they run in push-pull configuration which tends to cancel those kinds of common-mode signals.

I connected a 3.18K test load, and found about 287V after about 5 seconds powered on. Will test it with solid state rectifier and see what it drops to later.

The Bandmaster uses a capacitor input filter, not a choke input filter. A choke input filter will have much less DCV output than the cap input filter when a load is applied. Much less than you're thinking. The only reason the DCV is so high now is because there is no load connected. With no load cu...

You keep calling this a choke input filter. A choke input does not have any cap between the rectifier and the choke. And the 6L6 plates would be getting their voltage ***AFTER*** the choke. The 5E5 schematic you linked to doesn't even have a choke! I think there's some confusion that could probably...

A smaller cap, 100-200nF should be enough to keep the magnetic field from collapse. Larger values run the risk of it becoming a capacitive input filter (if I'm not mistaken). Are we talking about the same cap, IE between the 5U4 and the choke? Connected to ground from that point, obviously, lol! An...

So you have a choke fed supply? With the B+ you mentioned I find that unlikely. Are you sure there's no cap before the choke after the rectifier? 100% sure, as I built the thing from scratch. I do plan on putting a low-val cap in front of the choke, something in the range of 2 - 10 uf or so. I did ...

I'm adding a 10-ohm, 20-watt resistor to the output of the choke before the capacitor to drop charging current a bit. That should make the 5U4 less unhappy.

Let me get this straight, you have for the first filter node after the 5U4 a 235 uf cap? If that's the case then you will eat up rectification tubes really fast! Thank you for your reply! I forgot to mention that the 5U4 dumps through a large choke first, then the cap. The choke is from the same Ba...

Are you sure it is for 6L6"GC" and not 6L6 "G", "GB", or "nothing" ? What is the B+ Voltage ? Its a GB, not sure why I typed GC, lol! For a supply I'm using a 5U4 powered from a center-tapped 700V transformer (develops a bit more like 730-750v since the mains are a around 120 volts here), 235uf wor...