I'm planning to build a Fender AA864 Bassman, and then sell my vintage one.
The AA864 is diode rectified, and wants a B+ of about 422vdc. The Bassman schematic
appears to be 305-0-305, and Commercially available Bassman power transformers are 330-0-330.
I have a Bogen H-30 that I can definitely use as a donor for OT and choke, not sure about the PT.
The Bogen sports a whopping 780v CT, 380-0-380.
Question -- Looking at the Bogen schematic, even with the voltage drop of the 5U4, how is the B+ so low (435v)..?
AA864 Schematic is HERE
Bogen H-30 Transformer specs are HERE
Bogen H-30 Schematic (with voltages) is HERE
The AA864 is diode rectified, and wants a B+ of about 422vdc. The Bassman schematic
appears to be 305-0-305, and Commercially available Bassman power transformers are 330-0-330.
I have a Bogen H-30 that I can definitely use as a donor for OT and choke, not sure about the PT.
The Bogen sports a whopping 780v CT, 380-0-380.
Question -- Looking at the Bogen schematic, even with the voltage drop of the 5U4, how is the B+ so low (435v)..?
AA864 Schematic is HERE
Bogen H-30 Transformer specs are HERE
Bogen H-30 Schematic (with voltages) is HERE
If you already have the H-30, why not simply power it up and see what B+ is in it? Then you can determine which parts of the specs don't seem to agree with the other parts. For example, what if it were a typo, and the transformer was 680vCT instead of 780v?
In any case, regardless of the specs, if the Bogen works with this transformer, I see no reason a Bassman circuit wouldn't work using it. Push-pull pair of 6L6s either way. You could even keep the rectifier tube if you liked.
In any case, regardless of the specs, if the Bogen works with this transformer, I see no reason a Bassman circuit wouldn't work using it. Push-pull pair of 6L6s either way. You could even keep the rectifier tube if you liked.
Great suggestions, thanks..! I'm a little worried that a whole lot of the capacitors in the Bogen are shot. Can definitely give it a shot, see what it'll do when brought up through a current limiter and variac though.
Also -- I'm no expert on power supplies, and I don't know enough about what having a 5U4 (or other voltage-dropping tube rectifier) will do the sound/feel of the Bassman circuit to say for sure if I'm willing to go that route.
It'll sound like a Bassman with some additional sag. the recto tube loses some voltage, but then your new transformer seems to be higher voltage to start with. Maybe a wash. In any event, if you don't like the tube rectifier, simply replace it with a couple diodes and you are where you were starting anyway.
I have to see where this PT is voltage-wise under load. If it is where the spec sheet says it is (780vdc), then SS rectifying will put the B+ way high.
That is why you plug it in and FIND OUT what voltages are going to be. Since there appears to be a disagreement between specs, we need to find out what is what.
With an old RCA 5U4GB and some cheapo 6L6s plugged in (as well as all the preamp tubes), I get 408vac on each side of the center tap, and rectified B+ of 480vdc at the first filter cap.
SO - voltage is high, per the transformer spec sheet. I still don't know where their voltage readings are coming from on the schematic. I'm not sure I care though -- Looking ahead, it's
about how to get the voltage that I need out of it.
I'm thinking about using 5 (or so) zener diodes between the PT's center tap and ground to lower the B+. Will these 5W 12v ones get it done?
1N5349BG: ON SEMICONDUCTOR: ICs & Semiconductors
Other ideas?
SO - voltage is high, per the transformer spec sheet. I still don't know where their voltage readings are coming from on the schematic. I'm not sure I care though -- Looking ahead, it's
about how to get the voltage that I need out of it.
I'm thinking about using 5 (or so) zener diodes between the PT's center tap and ground to lower the B+. Will these 5W 12v ones get it done?
1N5349BG: ON SEMICONDUCTOR: ICs & Semiconductors
Other ideas?
Yes, there certainly are MANY amps running 6L6s on 480v B+. For that Bassman circuit, it is 10-15% higher than the schematic, but doesn't materially affect the circuit.
If you built it with this power supply voltage, nothing would stop you from going back later and dropping the voltage, if you found the high voltage had negatives.
If you built it with this power supply voltage, nothing would stop you from going back later and dropping the voltage, if you found the high voltage had negatives.
Thank you Turk and Enzo -- I started typing a response just before Enzo posted..
I have been trying to figure out how to drop the B+ because I am/have been afraid of frying or significantly reducing the useful life of the 6L6 tubes with ~500 volts. All the old data sheets indicate a max plate voltage of 450. (New JJs are reputed to handle 500). If you all think that the ~480 or so is safe, then I'll give it a shot.
Also -- not sure how to determine the current portion of the calculation for
resistance value and wattage rating of needed dropping resistors along the power rail. I absolutely need to be able to do this to get the right plate voltages to the right places.
Varying articles say to add up the draw for tubes 'upstream' of the resistor. Common assumption seems to be 1.1 Ma for 12aX7 triodes -- for power tubes, though, current draw seems to be a moving target based on the plate voltage and also amp class). Looking at the GE 6L6 data sheet for two tubes in AB1, idle plate current is 88ma at 360v and 116ma at 450v, and max-signal plate current is 140ma at 360v and 210ma at 450v.
Which current draw number to use..? AND, if my plate voltage will be even higher than the 450, how to proceed..?
I have been trying to figure out how to drop the B+ because I am/have been afraid of frying or significantly reducing the useful life of the 6L6 tubes with ~500 volts. All the old data sheets indicate a max plate voltage of 450. (New JJs are reputed to handle 500). If you all think that the ~480 or so is safe, then I'll give it a shot.
Also -- not sure how to determine the current portion of the calculation for
resistance value and wattage rating of needed dropping resistors along the power rail. I absolutely need to be able to do this to get the right plate voltages to the right places.
Varying articles say to add up the draw for tubes 'upstream' of the resistor. Common assumption seems to be 1.1 Ma for 12aX7 triodes -- for power tubes, though, current draw seems to be a moving target based on the plate voltage and also amp class). Looking at the GE 6L6 data sheet for two tubes in AB1, idle plate current is 88ma at 360v and 116ma at 450v, and max-signal plate current is 140ma at 360v and 210ma at 450v.
Which current draw number to use..? AND, if my plate voltage will be even higher than the 450, how to proceed..?
Since Bogen H30 used the same output tube types, why not just follow its schematic for the output stage, especially since the voltages are already close to what Fender used.
..because the H30 voltages as indicated on the schematic are not what I measure in the H30 itself, and are much more in line with the 780v on the spec sheet - so, I doubt the numbers on the H30 schematic.
The spec sheet/measured numbers for the H30 seemed to me to be pretty high for a Bassman, so I thought I'd ask for help from the more learned in the community.
I measured using 120vac through a variac. Measured:
- 120vac coming in to the PT
- ~408 vdc at pins 4 and 6 of the 5U4 rectifier
- ~480 vdc at pins 2 and 8 of the 5U4
- ~480 vdc at the junction of rectifier pin 8 and the first filter cap
- 120vac coming in to the PT
- ~408 vdc at pins 4 and 6 of the 5U4 rectifier
- ~480 vdc at pins 2 and 8 of the 5U4
- ~480 vdc at the junction of rectifier pin 8 and the first filter cap
The large (16-watt) 20K resistor started sparking and basically caught on fire before I could take detailed measurements on the power tubes. 😡
Here's the plan. I'm going to go with Turk and Enzo and build my AA864 and take some measurements. If it's crazy high, I can always drop it some.
Here's the plan. I'm going to go with Turk and Enzo and build my AA864 and take some measurements. If it's crazy high, I can always drop it some.
It isn't so much the voltage that kills a power tube, it is the dissipation. The Bassman didn't NEED 480v for its 50 watts, but 480v is just about average for many 100 watt 6L6 amps, 480-500v really. Set the bias so the tubes are not melting and they work just fine at 480v.
And while we are not talking 6V6 right now, Fender for decades ran 6V6s at 100v over the book maximum, and things like a Deluxe are not known as tube eaters,
And while we are not talking 6V6 right now, Fender for decades ran 6V6s at 100v over the book maximum, and things like a Deluxe are not known as tube eaters,
- Status
- Not open for further replies.