Ampeg B15 build report (schematic with mods added)

[birt]

I made an Ampeg B15n, single channel. The donor amp was a Vortexion from the british army that i completely gutted. Re-used: OT, PT, choke, chassis, sockets, power/standby switch, tone switch (replaced the caps on the switch with new ones, the switch is in the circuit just before the volume) and the knobs.

It was quite a challenge because i made the layout while building it. With all controls, power, input and output very close to each other and to the first preamp tube. the power/standby switch for example is there but the PT and cap cans are on the other side of the amp. the long wire runs go on the side of the chassis and they're shielded. there is also an aluminium shield between the preamp circuitry and the mains wiring.

i did the first test late at night yesterday. the voltages are all normal, considering the B+ is 340Vac instead of 440Vac in a normal B15. i might change the rectifier (5U4) to something beefier to add some volts. but first i want to try some different 6L6 that i have since the plate voltage is ok for them and not only for 6L6GC.

the NFB isn't connected yet because i wanted to hear it and maybe install a switch. the volume at 1 was already VERY loud and over 5 resulted in a high pitched squeal (even without input). low volume sounded fine.

Front:

Back:

In:

Guts:

[balcorn80]

Dude! That looks awesome! Way to maximize the space inside! Now I want a Vortexian! I don't even know what one is! The 'Funk Machine' picture/logo is rad too!

[pullshocks]

That is so badass.  I love it.

[birt]

i've been testing a bit more. everything works as it should. i was playing around with different rectifiers and found that the 5U4GB that was in there gives me roughly 340V on the plates and i get a distorted rock amp. AC/DC and more. i tried some more, further increasing the voltage on the plates and eded with an old Mullard GZ34 that gave me 380V. a lot cleaner but still distorted if i turned up the volume. will i gain even more with a diode rectifier? it would be cool to be able to switch between a low voltage and high voltage rectifier.
i noticed a strange thing with a 5R4. only one of the filaments worked but the rectifier seemed to work fine. it got me about the same voltage as the 5U4GB.

with all rectifiers i tried i checked the sound with different NFB resistors. from the 10K stock value to 220K was a very nice range and i think i might use a pot for this.

[birt]

here's the schematic as it is now:

[kagliostro]

Cool

Nice amp

Kagliostro

[birt]

as it was built, it sounded dirty and very good for guitar in my opinion because of the low B+. i added the variable NFB and tried some mods to make a second voicing for bass. this is basicly getting the voltages on the plates as they were in the old B15's. on the schematic you can see 3 mods for this: parallel plate resistors in the preamp, diode rectifier and moving the ground reference of the output tubes from ground to -32Vdc.

i have an open 12V connection on the PT that was originally fed by a vibrator linked to a battery. i will use this tap to get the voltage i need for my relay. i just need to figure out what kind of relay i need here. does it have to be a special one suited for the high voltages?

[birt]

sorry for bumping this but i am really happy with this amp right now. i can use different power tubes and switch between 2 voicings (gtr and bass). i spent a lot of time on this trying to figure it all out and making it all fit. the chassis is really full now and i have no noise issues. i'm quite proud of that. the 230V goes all the way through the chassis in a shielding sleeve, same goes for the HT to the standby switch. the mains wiring around the IEC and switch is also separated from the circuit by a piece of aluminium. the input, tonestack and outputs are all on the front panel, away from the rest of the circuit. lots of shielded wire there too.
here's the schematic, i hope it can maybe help others as the switching between voicings is not so standard.

[HotBluePlates]

Now that's pretty hot! You got 10 pounds of sh!t in a 5-pound bag. 

Seriously though, I bet if someone peeked in there 10 years from now, they'd think most of that stuff was original to the Vortexion.

Am I seeing right that the relay simply determines if the cap and choke are in the return path? Tell me about what that does to the supply voltage. It's not clear to me, except for perhaps providing some extra filtering.

[HotBluePlates]

Oh yeah... the lunchbox form-factor of your build makes me think again about converting one of the HP AC meters I have into a small guitar amp.

[kagliostro]

With the same intention  I got this





K

[birt]

Now that's pretty hot! You got 10 pounds of sh!t in a 5-pound bag. 

Seriously though, I bet if someone peeked in there 10 years from now, they'd think most of that stuff was original to the Vortexion.

Am I seeing right that the relay simply determines if the cap and choke are in the return path? Tell me about what that does to the supply voltage. It's not clear to me, except for perhaps providing some extra filtering.

the relay switches the ground reference of the PA from ground to about -30Vdc, the other one switches to diode rectifier. so there is a lot more voltage on the power tubes in the bass setting. (the third relay raises the voltage on the preamp anodes for more headroom) the switch for the power tube bias disables one of the relays when set to 6V6 or 6L6 so these don't get too much plate voltage.

i'm curious about those test equipment amps!

[HotBluePlates]

I haven't actually mutilated one to create an amp yet.

The HP 400D is an a.c.-only VTVM. It's claim to fame is that it can measure voltages as low as 1mV full-scale (implies 10-100µV sensitivity), from 10Hz to 4MHz. I think my Fluke 87III starts rolling off at 200-300kHz.

But really, no one wants a big meter that does a.c. only, and if they want info on the low-MHz range, they'll use a scope.

The upside is the thing has a regulated power supply, which typically delivers enough current for a few-watt amp. Nice size, but you need a guitar cable with banana-plug ends. Not sure what to do with the meter movement that wouldn't be cheesy.