Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Lectroid on April 08, 2025, 08:46:20 am
-
Greetings. I built a Princeton Reverb No-trem, using Hoffman's board and a Hammond 290AX for the PT. I substituted a one-tube reverb using a 12DW7 tube. Aside from that it's pretty faithful to Hoffman's parts values. This allowed me to dispense with one whole 12AX7 tube and its current draw. Btw, I did not post a schematic on purpose. I'm hoping for a general answer over an amp-specific one.
So--not surprisingly, the amp runs at higher voltages than I wanted or need. If I want to lower it somewhat, I can see two ways to accomplish it. Either:
1. Keep the 5U4 and change the PS dropping resistors to higher values?
2. Swap out the 5U4 and replace it with a 5Y3 to get the larger voltage drop?
I did swap in a 5Y3 and re-biased the amp, and all voltages dropped nicely. But I admit I don't know how to pick which method would be better in a given case. Are there any reasons one might use one method or the other?
How do you all decide in this situation? Change the rectifier or change the resistors?
Thanks in advance.
-
I'll await more knowledgeable members, but I frequently need to drop voltages given my preference for solid state rectifiers, and have never had a problem with increasing the values of the appropriate resistors.
Obviously, there is a practical limit to this approach. You don't want to ask a 290DX to do the job of a 290AX!
I'm not sure what the possible sonic effects are by doing this. I can't hear any, but that's not a reliable indicator.
-
I would change the rectifier tube and check your B+ to see if that's where you like the tone. If not use a dropping resistor and check your B+ and Bias again. I mostly always need to use a dropping resistor in some of the Champs.
-
... I'm hoping for a general answer over an amp-specific one. ...
The Right Answer is almost always circuit-specific. There's no one-size-fits-all answer for much of anything in electronics.
... a Hammond 290AX for the PT. ...
... the amp runs at higher voltages than I wanted or need. If I want to lower it somewhat, I can see two ways to accomplish it. Either:
1. Keep the 5U4 and change the PS dropping resistors to higher values?
2. Swap out the 5U4 and replace it with a 5Y3 to get the larger voltage drop?
I did swap in a 5Y3 and re-biased the amp, and all voltages dropped nicely. But I admit I don't know how to pick which method would be better in a given case. Are there any reasons one might use one method or the other?
You can do either. It mainly depends on whether you want the voltage to the output tube plates to drop.
- Choosing the different rectifier tube depends on the circuit, and how much current it will draw from the rectifier.
- A 5Y3 can support up to 125mA (https://frank.pocnet.net/sheets/093/5/5Y3GT.pdf) (Page 2).
- The Hammond 290AX (https://www.hammfg.com/part/290AX) is rated to deliver 100mA from the high voltage winding.
- If the 290AX is the right PT for the job, then the 5Y3 rectifier is fine.
But you can also add resistance in the power supply circuit to knock down voltage.
- Dropping voltage with a "Rectifier Tube" or with "Resistors" will use Ohm's Law and circuit-current to create voltage-drop.
- Dropping voltage with a rectifier tube can be done, as long as you don't pop the tube with too much circuit-current.
- You could install resistors to behave like higher-tube-rectifier-internal-resistance to drop the extra voltage.
Whether the outcome is palatable depends on the current-change of the circuit, and how much sag you can tolerate.
- If the circuit were Class A, circuit-current wouldn't change much (mostly at the output tube screens).
- With little current-change, the voltage-drop don't change much, and the amp doesn't seem to "sag."
- With Class AB amps, average circuit-current to the output tube plates changes a lot from idle to full-power; inserting resistance leads to a lot of current-dependent voltage-drop and "sag."
- Whether the above is objectionable depends on how loud you play the Class AB amp, the resistance-value used, and your preference.
-
While a 5Y3 can support a pair of 6V6s and a few preamp tubes, it will sound a bit ‘fuzzier’ or 'spongier' than using a 5U4 when you dime the amp and dig in to the strings, and you might not lose as much B+ voltage as you think (because the 5Y3 draws less current from the PT’s rectifier winding and this affects how much drop there is on the other PT secondaries). Try it and see.
-
While a 5Y3 can support a pair of 6V6s and a few preamp tubes, it will sound a bit ‘fuzzier’ or 'spongier' ... you might not lose as much B+ voltage as you think (because the 5Y3 draws less current from the PT’s rectifier winding and this affects how much drop there is on the other PT secondaries). Try it and see.
Odd you should put it that way given another issue here. At moderate volumes and above the amp's sound distorts, gets smoothly fuzzier, like an old Tone Bender. At low volumes the distortion almost disappears. At 7 it gets pretty loud. Otherwise, bias current on the 6V6s is 19.5mA / 21mA while B+ = 401V, bias voltage is just a hair above below the nominal -34V. All panel controls work.
I will sub the 5U4 back in and report back.
-
Changing the rectifier back to the 5U4 did not affect the fuzziness but did push the B+ up over 430V, harder than I want to push these Tung-Sol tubes. :laugh: So I think I'm back to the 5Y3 for the lower B+ voltages. My distortion must be a separate problem. I'll close this now and start a new thread for that.
Thanks again.
-
My distortion must be a separate problem.
Some distortion is normal for a BF (non reverb) Princeton.
But just in case yours is ‘more than normal’, check the high-tension voltages at each node on the power supply rail. You want to see progressively lower voltages at each filter cap node (as you move away from the B+). If you get 2 or more nodes measuring the same, you might have a problem with the decoupling of the power rail on those nodes (meaning filter caps may not be working/ not connected to ground return)
-
tubeswell,
PS nodes are:
--------
A: 402
B: 379
C: 310
D: 241
Here's the link to the new thread about this amp's distortion, with more detail and what I've tried.
link: https://el34world.com/Forum/index.php?topic=32654.msg362398#msg362398
Thanks again, everyone.