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Newbie question- Ampeg B15N
- Thread starter slavax
- Start date
Crap! I changed pages before saving. Now I have to start over!
The RCA tube manual states that with 400+ on the plates, a pair of 6l6GCs should supply around 50W into the load. Mine is feeding 500V+ into the amp and I measure a tod over 60W before clipping. Ampegs actual ratings are garbage.
JimmyM try testing your Plethora of B15s on a bummy load and a scope. I think you might be surprised!
And don't think I haven't noticed your much more informed answers you're posting since I sent you that book. Kudos Brother and very well done!
Now to finish off my Sleemans Honey Brown! My last in the house!
Bed time, so goodnight one and all and to all a goodnight!
I hate Iranian air flights! Cost a friend of a friend and her young son their lives. And there is nothing left to collect for berial! Maan that sucks bigtime!
Paul
The RCA tube manual states that with 400+ on the plates, a pair of 6l6GCs should supply around 50W into the load. Mine is feeding 500V+ into the amp and I measure a tod over 60W before clipping. Ampegs actual ratings are garbage.
JimmyM try testing your Plethora of B15s on a bummy load and a scope. I think you might be surprised!
And don't think I haven't noticed your much more informed answers you're posting since I sent you that book. Kudos Brother and very well done!
Now to finish off my Sleemans Honey Brown! My last in the house!
Bed time, so goodnight one and all and to all a goodnight!
I hate Iranian air flights! Cost a friend of a friend and her young son their lives. And there is nothing left to collect for berial! Maan that sucks bigtime!
Paul
Thanks Paul. I'm to the point where I understand the very basics, but it's not the strong part of my brain where that knowledge goes to, for sure. Soldering is going to be my big downfall. I suck at it, and for as much as I've tried, you'd think I could get it by now. Man, my dad could solder. He soldered something and it never broke. He doesn't do it much anymore but he could fix anything. I have none of his natural abilities.
BassmanPaul:
I figured the 2-6L6 output was higher than 25-30W from having run my SF Twin Reverb on 2 tubes and one speaker- which is supposed to be around 60W instead of 100W One thing I've noticed recently is that the B15 NF doesn't have a bias adjust pot (but the SB-15 7868 does.) Another thing I've noticed is that Joe Piazza's schematics show the B15 hum balance control connected to the 6L6 bias supply @ the junction of the 100uf cap and the 47K resistor while the original schematic (see post #43) shows it connected to ground- is this right? It seems that the hum balance pot should come from ground since it's acting as an artificial CT.
Jimmy-I believe there's a link to good soldering instructions on the Geofex home page.
I figured the 2-6L6 output was higher than 25-30W from having run my SF Twin Reverb on 2 tubes and one speaker- which is supposed to be around 60W instead of 100W One thing I've noticed recently is that the B15 NF doesn't have a bias adjust pot (but the SB-15 7868 does.) Another thing I've noticed is that Joe Piazza's schematics show the B15 hum balance control connected to the 6L6 bias supply @ the junction of the 100uf cap and the 47K resistor while the original schematic (see post #43) shows it connected to ground- is this right? It seems that the hum balance pot should come from ground since it's acting as an artificial CT.
Jimmy-I believe there's a link to good soldering instructions on the Geofex home page.
- Feb 13, 2008
- 1,247
- 1,916
- Disclosures
- Owner, Vintage Blue (repro cabinets)
It looks like Joe was using a later B15NF circuit as his reference. I have copies of 2 B15NF schematics here; 1 from 9-65 and 1 from 1-66. The '66 schematic seems to match what Joe drew, while the '65 schematic looks like it matches the one in David's post (#43).
Looks like another example of Ampeg making it a bit difficult to determine what is 'right' with their vintage amps!
Attachments
I would like to make the bias on the B15 NF circuit adjustable. Can I do that by substituting a 50K lin. pot and say a 25K resistor for the 47K resistor on the anode side of the bias diode or is there a better way of doing this? From looking at different schematics it seems like there's several varieties of bias adjust circuits. Also, do you think that a 1N4007 diode will suffice as a bias diode in this amp? Thanks
I lost this thread! 1N4007 will be fine for the bias circuit. The circuit shown is a half wave rectifier. I prefer to use full wave which is basically the components repeated from the second plate of the 5AR4. Negative end of both bias caps join at the bias cap and 47K. This give you 120Hz ripple which is easier to filter out.
Your idea is workable. See what the voltage is currently, you're going to need something around minus 45V on the grids so if you can get a range of minus 35 to 55 volts that should give you all you need. You do not need the tube in to work this out.
Paul
Your idea is workable. See what the voltage is currently, you're going to need something around minus 45V on the grids so if you can get a range of minus 35 to 55 volts that should give you all you need. You do not need the tube in to work this out.
Paul
Since you are building the amp from scratch, you should consider a separate bias adjustment for each 6L6GC. It helps to have this if you are using tubes that aren't perfectly matched.
Also, I like to use multi-turn pots in the bias circuit, or a pot that allows you to lock the setting by turning a nut. With a less expensive 3/4 turn pot, the bias setting can change due to vibrations. Some people apply glue to the pot shaft after it is set to avoid this from happening.
David
Also, I like to use multi-turn pots in the bias circuit, or a pot that allows you to lock the setting by turning a nut. With a less expensive 3/4 turn pot, the bias setting can change due to vibrations. Some people apply glue to the pot shaft after it is set to avoid this from happening.
David
Yes a resistor and pot serving as a voltage divider for each tube.
Here are two examples of how to apply a separate bias to each tube:
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The notion of designing the circuit to apply maximum bias if the pot fails is a good one. It protects the tubes.
Here are two examples of how to apply a separate bias to each tube:
Link Removed
Link Removed
The notion of designing the circuit to apply maximum bias if the pot fails is a good one. It protects the tubes.
Just saw this thread. Did you ever build it? how did it turn out?
I build tube guitar amps but this amp is gotten my attention.
I just finished a build of what I call is a 100Watt B15 head.
it's based on Joe Piazza '68 B15N schematic for pre-amp and splitter with
Bassman 100 output stage and power supply with mods from KOC TUT3 Portaflex schematic. Kept the Ultra-hi and Ultra-low switches and added a bright switch and presence control. Rear panel has individual bias pots for each of the 4 6L6GCs as well as a hum balance control and a ground lift switch.
it's based on Joe Piazza '68 B15N schematic for pre-amp and splitter with
Bassman 100 output stage and power supply with mods from KOC TUT3 Portaflex schematic. Kept the Ultra-hi and Ultra-low switches and added a bright switch and presence control. Rear panel has individual bias pots for each of the 4 6L6GCs as well as a hum balance control and a ground lift switch.
Attachments
Thanks, it's my very first build and I'm quite pleased with it. Took a lot of planning and parts searching but it was a great father and son project.
Total parts cost was probably close to $500 but I think it was worth it.
The inside wiring ain't the prettiest although modular (3 separate turret boards) but it's running so clean now I'm afraid to touch it
Some more build process pics here:
Invalid Link Removed
Total parts cost was probably close to $500 but I think it was worth it.
The inside wiring ain't the prettiest although modular (3 separate turret boards) but it's running so clean now I'm afraid to touch it
Some more build process pics here:
Invalid Link Removed
Wow. Absolutely killer. Great father-son project indeed. You're doing a cab too, right? Can't wait to see it.
Who did you use for trannies? Hammond? (just curious).
And you can run at 4 & 8 ohms?
Great job.
Great job for a first build. The fact that it actually works without a lot of hum and hiss is a testament to a good job. One question though... Your final wiring does not really look like that picture does it? If it does I would neaten that up a bit. It could be a problem down the road. But if it works , maybe leave it alone.Thanks, it's my very first build and I'm quite pleased with it. Took a lot of planning and parts searching but it was a great father and son project.
Total parts cost was probably close to $500 but I think it was worth it.
The inside wiring ain't the prettiest although modular (3 separate turret boards) but it's running so clean now I'm afraid to touch it
Some more build process pics here:
Invalid Link Removed
Since the power section is based on a Fender bassman/showman design I went with Fender replacement transformers.
The ClassicTone Vintage Fender Style 100W Output Transformer 4/8/16 ohm 40-18013
and
The ClassicTone Vintage Fender Style 100W Power Transformer 120V 40-18004
the PT provides a 60V Bias tap, but no center tap for the heaters.
But still cheaper than Hammond.
If you zoom in on the back panel pic u can see i have 4/8/16 ohm selector switch.
I'll let you know how well that works when I try and gang some cabs together.
All I have is 8ohm units.
The ClassicTone Vintage Fender Style 100W Output Transformer 4/8/16 ohm 40-18013
and
The ClassicTone Vintage Fender Style 100W Power Transformer 120V 40-18004
the PT provides a 60V Bias tap, but no center tap for the heaters.
But still cheaper than Hammond.
If you zoom in on the back panel pic u can see i have 4/8/16 ohm selector switch.
I'll let you know how well that works when I try and gang some cabs together.
All I have is 8ohm units.
