Bass guitar amp...

[Stormrider]

I still have a gutted Bogen CHB35a here that i haven't done anything with, frankly because i haven't decided what to do with it yet...
Schematic for the Bogen :

http://tdsl.duncanamps.com/pdf/bogen_chb35a.pdf

Up 'till now i've only been thinking about regular guitar amps, but really, i'm a bass player. I thought it'd be cool to have a guitar amp at my house for friends to jam on when they come over... but, i stumbled on some Ampeg schematics and now i'm thinking about building an SB-12 type amp. Schematic is here :

http://www.drtube.com/schematics/ampeg/sb127868-jp.gif

I have the power and output transformers, and the pair of 7868 tubes that were in the Bogen. I'm not positive the tubes even work, but if they don't i can just rewire for a pair of 7027a's.

I have to use solidstate rectifier because of the lower voltage transformer the Bogen uses. So, i'll just copy that basically.

The thing i'm unsure of is the bias voltage. It isn't labeled on the SB-12 schematic. The bias voltage for the Bogen was -23vdc, should i just start there and adjust it while looking at the output on a scope, looking for crossover distortion?

Any changes i should make to the Ampeg circuit? Thanks for any suggestions, and thanks to those who have helped me so far.

 

[kevinkr]

What you propose sounds feasible, the one potential issue I see is whether or not the existing OPT can handle low frequencies well or not.

 

[Stormrider]

Dumb question but, where would i put the pot for the bias control? In place of the 56k in the bias supply?

Another question... What does the "balance" pot between the power tubes in the SB-12 schematic do?

 

[hotbottle]

The balance pot is used to balance the bias between the output tubes if they are not exactly matched.

 

[Stormrider]

So, any ideas on the bias voltage needed for the SB12?

 

[Brett]

A pair of 7868 in PP pentode will give about 35W, nowhere near enough unless simply jamming quietly in a house without a drummer. Even with an efficient cab, probably not enough to accompany a jazz/blues guitarist in anything but a small club.

Could make an interesting studio or home practice amp though.

Look to 7591 designs like Fishers to get design ideas.

I've contemplated making a hifi amp using Fisher OPTs, 7591s and a slight modification of SY's Red Light District amp. A friend has the original amp and is going to restore it, so I never did the mod.

 

[Stormrider]

Could make an interesting studio or home practice amp though.

Exactly what this will be used for.

 

[Brett]

Exactly what this will be used for.

Sounds like a plan.
The SB12 schem above looks good at a glance. One suggestion would be to add a switchable Fender tone stack as well as the Bax for more tonal options. Cost is a few pots and passives and a switch.

I'd also ditch the 12AX7's for some 6SL7's or possibly 12AT7's. With most basses, you won't need the gain. The Ampeg B15N has a nice preamp.

 

[Stormrider]

I have a bunch of 12AX7's, AU7's and AT7's so i can mix and match later.

The Bogen doesn't have the space on the the front panel for that many pots, but i will definitely play around with the tone stack to see what i want there. The tone stack in the SB12 looks to be the same as the B15N.

I'll be doing the work on the chassis tomorrow. Bogen used rivets for everything so i have to drill them all out. Once i have the chassis completely stripped i will paint it, probably black. Then, i await the boxes of parts to build!

 

[Stormrider]

Yet another dumb question...

Should i put a stand-by switch in this circuit? The Bogen didn't have one, and the Ampeg doesn't show one on the schematic. If yes, where do i put it?

The power supply will be almost identical to the Bogen power supply so do i put the switch after the series diode in the rectifier, so the bias voltage gets to the power tubes, or right at the beginning where the ac comes out of the transformer?

 

[Brett]

For a practice/studio amp, why bother? I'd never implement a standby that way anyway.

 

[rickl]

I built a Fender Bassman clone for my practice room. It is cathode biased so it is about 25 watts. It sounds great with my 2x12 cab. We've even compared it to a black face Bassman and while not as powerful, it sounds better.

I'd look at a Bassman (or some other Fender) for a driver and use your output tubes and OPT. Looking at the circuit, I'd update the 12ax7 sections. You will need to decide your tone stack (single tone or bass and treble).

rick

 

[Stormrider]

I built a Fender Bassman clone for my practice room. It is cathode biased so it is about 25 watts. It sounds great with my 2x12 cab. We've even compared it to a black face Bassman and while not as powerful, it sounds better.

I'd look at a Bassman (or some other Fender) for a driver and use your output tubes and OPT. Looking at the circuit, I'd update the 12ax7 sections. You will need to decide your tone stack (single tone or bass and treble).

rick

This is going to end up a experiment/tweaker amp so I'm just starting with the basic Ampeg circuit and see where I go from there.

Thanks for the help guys. I'm about half-way finished with the wiring and now i'm just waiting for one last box of parts that should be in tomorrow... Pics tomorrow probably.

 

[Stormrider]

I powered the amp up for the first time today, and it didn't explode!

But it didn't really work either.

The high voltage supply is working fine, but i have no heater voltage. Nothing is flowing in the heater supply and i'm absolutely positive its connected correctly. Could a lead have disconnected inside the transformer? I find it odd because when i was using the Bogen in its stock form the heater supply was fine.

 

[kevinkr]

I powered the amp up for the first time today, and it didn't explode!

But it didn't really work either.

The high voltage supply is working fine, but i have no heater voltage. Nothing is flowing in the heater supply and i'm absolutely positive its connected correctly. Could a lead have disconnected inside the transformer? I find it odd because when i was using the Bogen in its stock form the heater supply was fine.

It is possible, but not too likely unless you were very rough with the leads. Check for possible shorts first, wiring errors (like you mis-identified two separate filament windings for example - I knew someone who connected two windings incorrectly in such a way that there was no voltage across any tube filament in the amp, ie same polarity end of both windings to one channel's tubes and the other polarity to the other channels) and then look for cold solder joints.

Remove all of the tubes and check for winding continuity at the first socket they connect to, if you have spliced to the original transformer wires check these too.

 

[hearforever]

Hey there,
I was just wondering how this project worked out. I have the same Bogen model and was thinking of doing the exact same thing with it! I was going to change the power supply caps, leave the voltage doubler and bias and then rewire everything else like the SB-12
Any advice?