Ampeg B-15N Schematics

[txstrat]

Go to this site for the OT: Hammond Mfg. - REPLACEMENT & UPGRADES - Tube Guitar Amplifier - Output Transformers and look for the 1760W or 1750R. They both deliver 100w with a primary impedance of 2000 ohms@447v. (good for four 6L6GC).

To tell you the difference of these and which one would be better for bass, one of the pros in this forum might chime in, cause that's beyond my knowledge.

Go to this site for the PT: Hammond Mfg. - REPLACEMENT & UPGRADES - Tube Guitar Amplifier - Power Transformers and look for the 291FX. It's got a secondary winding of 320v@518mA (including the 60v tap for the bias). Rectified you'll end up at around 450vdc.

[loudthud]

Hammond has significantly upgraded their offerings! The old models that we use in the US now have 125V primaries with a 115V tap. There are new offerings in the "Guitar Amp" listings with increased current ratings. Additionally, there are new models in the 300 series like the 373JX: 700VCT @ 863mA, 373FX: 650VCT @ 460mA, and 372LX: 600VCT @ 690mA. For a quad 6L6GC amp with a 1650T look at the 373FX or a Showman/Twin replacement 290FX.

[dpacek]

Thanks!! .. of course with this radical change it looks like I'm
pretty close to mating the B-15 pre-amp circuit with the Bassman 100 power amp circuit
wondering if thats exactly what I should do instead of attempting to mod
the B15 power amp circuit

[loudthud]

The B15 phase inverter is one of the key elements to the B15 sound. I'd keep that. When Ampeg moved away from that circuit in the B18 and B25, I think they lost something.

[dpacek]

thanks for the tip !!.. now I just need to decide if I'm going to go with KOC TUT3 design or Joe Piazza 1968 design..
and should I attempt to add bias adjustment ?

[txstrat]

The B15 phase inverter is one of the key elements to the B15 sound. I'd keep that. When Ampeg moved away from that circuit in the B18 and B25, I think they lost something.

I absolutely agree loudthud!!!

should I attempt to add bias adjustment ?

Yes, that way you can sort of dial in the sound you like...
I dig the '68 schematic, think it's more versatile.

[loudthud]

Schematic Heaven is down today so I don't know exactly what the Joe Piazza 1968 design is. In general I would say that cathode bias (like KOCs design) is not practical with only four 6L6s. You need a fixed bias design like a Fender Showman or Twin Reverb. Choke is optional. You'ld need two rectifier tubes so SS rectifiers should be used.

[txstrat]

Here you go: b15n68_portaflex.pdf

I would still love to see a layout plan.

[dpacek]

yeah I like the look of the porta flex layout with the transformers on each end
but KOC says you should avoid putting the input jack too close to the out put
transformer .. so I was thinking perhaps putting the input jack dead center in the middle
since I'm only building one channel.

Also since I'm hacking this schematic and need to create a new one, can anyone suggest some decent
schematic drawing software that runs on Mac or Linux ?
Hopefully not too expensive ..

[dpacek]

txstrat - stupid question but is that 100 ohm pot labelled "HumBal." the bias adj pot in the joe piazza scematic ? is that really 100ohm ?

[txstrat]

Yes, it's a virtual center tap for the heaters. I prefer two 100ohms/2w resistors from each heater side to ground. Both do the same. The idea of the pot is to equal out the heater hum - you turn the pot up to the point where the hum is at minimum.

[dpacek]

txstrat - on that 1968 B15 schematic I see a 5VAC tap off the PT that goes to the plates on the inverter section and the preamp section( 6sL7)
am I missing something ? shouldn't that be much higher DC voltage ? If I really need 5VAC to power those sections then I need a different PT and then
the connections to the ouput stage are need a little thought.

[txstrat]

You'll need that 5v winding if you go for a tube rectification, it's the heater winding for the rectifier tube from which the B+ is fed (that's how a tube rectifier works, but don't ask me why).
With a diode recto you can just replace the whole unit.
If the PT has a center tap, I'd go for three diodes (1N4007) in a row on each secondary tap (see pic).
If the PT has no CT just use a bridge rectifier.
With a diode rectifier the B+ will be higher, so watch for the ratings of the filter caps.

[dpacek]

Ok I copied the power section of the Fender Showman 763 which uses 4 6L6s and has bias adjust circuit.
(I added in resistors and test points to the cathodes to ease adjustment later.)

and mated it to the original B15 inverter circuit, but i have a question on the values of the 2 capacitors
that connect them... the Ampeg spec is that they are .022uf 400v while fender is .1 uf rated at 600v.
I was going to use .1 seem reasonable ?

also - connection from the cathodes of the inverter section to one side of the OT output
has a 10K resistor in the Ampeg design and all the fender schematics have a
resistor simply labeled as 820... is that really 820 Ohms or is that 8.2K ?
BTW - KOC calls for 8.2K in TUT3

lastly - the 1968 Piazza Ampeg circuit calls for the center of the hum balance
pot (across the AC heater mains) to attach to the -50 volt bias line.
Do I really need this since i have added bias adjust mentioned above ?

[txstrat]

OK good so far.
1. For the same frequency response as the B-15 I'd go for the .022 but I'd use a 600v rating.
2. The Fender label of the negative feedback is indeed 820 ohms but same as before I'd go for the Ampeg value and if you don't like it (or the amp don't behave) you can change that to different values.
3. If your PT don't have a heater center tap you're in the need of a hum balance either as a pot or two 100 ohms/2w resistors from each heater wire to ground (virtual center tap). If neither cures the heater hum you might have to lift the heater ground to a higher voltage level (we'll discuss that if necessary).