High power bass amp

[maciek]

Hi,

I'm designing my first amp for bass, I've had some encouraging successes with preamps, so I need to add a power amp section now. A couple of questions, sorry if they're lame:

1. I want around 250-300W RMS - would parallel push-pull be the best pick?

2. What would be the best choice for tubes. Pentodes, tetrodes? Would say 8xKT88 give me desired power output?

2. If so - how to connect tubes in parallel push-pull? In single ended deign it's a no-brainer. I guess it may be the same for push-pull, but I thought I'd better ask. Has anybody got an example?

3. Is there anything specific to watch apart from the obvious matching of voltages, currents, plate dissipation etc. (for example as compared to designing a single ended)?

Or perhaps someone can point me to a good schematics for a 300W all-tube bass amp? That would solve all the problems :-)

Any help is greatly appreciated. Take care.

Maciek

[Enzo]

amps are amps, look at most any amplifier for a sample circuit. A Fender Twin had four power tubes in push pull, an AMpeg SVT has six. There is nothing cosmic about multiple tubes at all. Look up standard designs. The difference between guitar and bass amps is more in the preamps. Other than most folks have no desire for a 300 watt guitar amp.

The Ampeg SVT was a 300 watt bass amp, they also made the V9, which was the exact same amp with a different preamp in it, for guitar.

As they say in other parts of the world, tubes are valves. They don;t make power, they just let it through from your powr supply.

In other words putting 100 watt tubes in a 50 watt amp won;t convert the amp to 100 watts.

[maciek]

Thanks.

I've got a preamp which sounds quite good with my bass, so I'm happy with this part.

Meanwhile I found this:

http://mhuss.com/Hiwatt/Schematics/DR_400wOutput.gif

Looks like something worth trying out.

[Steve Conner]

There have been lots of good threads on high-power bass amps before. Try the search function. Also, schematics of things like the Ampeg SVT, Marshall VBA400 and Fender 300/400PS are kicking around. And I've seen 300PS output and driver transformers for sale cheap on Ebay, in fact I have a set of them

Two important things I can think of:

Triodes are a no-no, all the power tubes you can get at this level will be beam tetrodes.

"100 watt" tubes need something like 700-750V HT to get 100W per pair. You can't feed that to the screen grids, so you need a separate lower voltage screen supply.

Parallel tubes are more vunerable to parasitic oscillations than single tubes. A stopper resistor on every electrode (anode, screen grid and control grid) of every power tube is a good place to start. Mount them right on the pins of the tube sockets to minimize "un-stoppered" wire length.

[maciek]

(...) Ampeg SVT, Marshall VBA400 and Fender 300/400PS (...)

Ampeg SVT has about half a million parts for some unknown reason. I prefer British designs such as HiWatt or Orange (couldn't find schematics) because they are much simpler and still have killer tone. I guess that shows how little you really need to get a good sound. No offence to Ampeg, but why half a million parts?

(...)"100 watt" tubes need something like 700-750V HT to get 100W per pair. You can't feed that to the screen grids, so you need a separate lower voltage screen supply.

Ok got that. If you look at this diagram:
http://mhuss.com/Hiwatt/Schematics/DR_400wOutput.gif
Is is H2 that we're talking about? (I mean has to be lower than HT1).

Thanks a lot for your help.

[Steve Conner]

Why not half a million parts? Simpler doesn't always mean better. Also, that schematic looks simple because it's incomplete: the PI stage is missing.

Yes, HT2 must be lower than HT1. It's fairly common to use a voltage doubler kind of power supply, so HT2 will be half the voltage of HT1: 350V and 700V.

There are a few different ways of doing this, again, look at schematics of existing amps. Two popular ways are the full-wave doubler as seen in the Fender 300PS, and a bridge rectifier used with a centre-tapped transformer. The SVT (IIRC) used a completely separate winding and rectifier for HT2.

[maciek]

Why not half a million parts? Simpler doesn't always mean better. Also, that schematic looks simple because it's incomplete: the PI stage is missing.

Yes I know - it assumes that balanced singal is fed. I planned to use a trasformer rather than a tube PI.

Re: complex vs. simple. My experience so far taught me to stay away from complex designs for DIY. At least for the expensive projects. Perhaps after I built a few amps I could try to have a go at SVT replica. Also - I don's think SVT sounds any better than Orange, but let's not go there

I think Fender is really the old Sunn 300T. I read somewhere that the design wasn't that great. It was prone to failures apparently although I couldn't confirm or deny that. Have you built a replica?

There are a few different ways of doing this, again, look at schematics of existing amps. Two popular ways are the full-wave doubler as seen in the Fender 300PS, and a bridge rectifier used with a centre-tapped transformer. The SVT (IIRC) used a completely separate winding and rectifier for HT2

That's no worries, I planned to use centre-tapped transformer although I need a few different voltages around the circuit, so I may need a custom-built transformer anyway. I found a shop that can do it for me.