Building a 200W+ bass amp

[MrTransistorm]

I designed and built a few guitar/bass amps using a Williamson style design (ultralinear class AB) in the power amp section. I am using a quad of 6550s driven by 12AU7A drivers. The transformers are Hammond 1650T on the output and 372JX in the PSU. This gives about 400V B+ (1N5408 rectifiers into 100uF capacitor input) for 80 watts before clipping.

This works great for guitar, but I am well aware that for bass guitar I need a lot more power. I am currently using two of these amps (one for each cabinet) to be on an even level with the guitar and drums. For a while I have been wanting to build a higher power version of my amp. Now a friend of mine in another band has asked me to build such a bass amp for him.

I am considering using a quad of KT88s to achieve at least 200W. Now it’s a matter of finding the parts I need. There are a few dilemmas I have to consider.

1. Transformers: Looking at Hammond I see that they have the 1650W, which is a 280W version of the 1650T. With the 1900ohm primary I would need about 640V B+. There isn’t really a compatible power transformer in the 200, 300, or 700 series. I started looking at other brands, but it has become quite obvious that options for high power amps are very limited. I might even bite the proverbial bullet and have Sowter make custom power and output transformers. I have even thought about going all out and making this amp with Ampeg SVT transformers, but 300W is little too much power IMO. Any recommendations?

2. Reliability and Tube Life: My current amp design is running the 6550s rather conservatively. Even though the guitar player drives the amp into massive overdrive, the tubes are holding up quite well. My concern with the high power version is that a quad of KT88s may not last too long putting out that much power. I also wonder whether I should stick with ultralinear or run the screens at lower voltage.

3. Rectifiers: Dealing with 1400V peak to peak from the B+ winding means that I will either have to find rectifiers with sufficient PIV or use pairs in series.

I have more questions, but I will start with this.

I did come across the 8zylinder design, but there isn't much mention of the actual components used.

[Wavebourn]

For bass guitar you don't need lot more power, for bass guitar you need a power transformer that don't saturate on 41 Hz on a full power.
If you used such transformers for guitar amps you may use the same for bass guitar, but if you used designed for guitar transformers you defenitely need a bigger one.

[aletheian]

How much current will you need? Looking at the Hammond 279x, it should give you the voltage you need... 425-0-425 @ 150mA. Remember that those are rated at 115v, so if you have the typical 125v coming out of the wall, then you are really looking at a 460-0-460 tranny, which when wull wave rectified will give you around 660v... I just don't know how much current you will be needing.

[danFrank]

Hi,
300 watts is not much louder in db's than a 200 watt amp, but of course will take more wall current.
I would build a home brew SVT if I were you; but it will cost.
Use the 1650W Hammond output and 6 6550's or KT88's.
For power transformers, go to www.plitron.com
They have a 220 or a 240 volt output, 750 watt power transformer that you can configure as a voltage doubler to get 620-660 volts for B+.
Then you would need to get a filament transformer and a bias transformer. Plitron would have these also among other places.
The Hammond outputs are good for bass amps as they carry low frequencies quite well. They tend to blow if driven hard though.
SVT amps are IMO some of the best sounding amps for bass. Standing in front of a 8 X 10 cabnet, you definately feel it in your gut!
The bad thing about SVT's though is that they are like muscle cars. They need a lot of maintainance to keep them running good.
Daniel

[danFrank]

In this configuration, (voltage doubler) you would want to use the half rail (300-330v) of the B+ to feed the screens as 600+ volts is WAY too high for the screens.
Daniel

[aletheian]

Originally posted by danFrank
In this configuration, (voltage doubler) you would want to use the half rail (300-330v) of the B+ to feed the screens as 600+ volts is WAY too high for the screens.
Daniel

oooh! I forgot about the ultralinear part... yeah, you're gonna want to drop that voltage a bit. Shoot for the 550v range, or even 450v in ultralinear PPP should give you 150W into a 1.9k load i would think.

[MrTransistorm]

Originally posted by aletheian
How much current will you need? Looking at the Hammond 279x, it should give you the voltage you need... 425-0-425 @ 150mA.

I need more voltage and a lot more current than that. I was looking for about 460-0-460 @ at least 500mA. The Hammond 717 has 510-0-510 @ 500mA, but the voltage is a little high.

Originally posted by danFrank
Hi,
300 watts is not much louder in db's than a 200 watt amp, but of course will take more wall current.
I would build a home brew SVT if I were you; but it will cost.

I agree. If I'm at 200W then 300 isn't much more. I was just trying to avoid the inevitable cost increase of using even larger transformers and adding another pair of KT88s.

SVT amps are IMO some of the best sounding amps for bass. Standing in front of a 8 X 10 cabnet, you definately feel it in your gut!
The bad thing about SVT's though is that they are like muscle cars. They need a lot of maintainance to keep them running good.
Daniel

I have played through an SVT a few times before. It's definitely a force to be reckoned with. I like to play with a little overdrive, and it's a little difficult to achieve that on an SVT. Its circuit (especially the preamp) is designed to be very clean. Even when you crank it up, the overdrive is almost entirely in the power tubes. The onset of clipping is sudden and quite hard. It's not bad, it just doesn't suit my tastes. I have found that the best overdrive comes from mild clipping in each stage instead of just hard clipping one.

After looking around for a while I'm leaning towards the Mercury Magnetics SVT transformers. Considering this is just a one-off project at the moment, I'll stick with off-the-shelf parts. Custom transformers would require an extra level of design that I'm not ready to commit to yet.

Another friend of mine has a Magnavox era SVT. I think I'll try an experiment. I'll try plugging my preamp into his power amp then into my cabinets. It should give me pretty good idea of what the finished amp will sound like. Being that the SVT power amp design is not too far removed from the modified Williamson of my amps, I think my preamp design is what is going to keep the new amp true to the sound of my current amps. The same tone, just louder.

Originally posted by danFrank
600+ volts is WAY too high for the screens.
Daniel

Yeah, I realized that after I posted. I certainly wouldn't make that mistake when it comes time to build.

It does amaze me how many vintage guitar amps far exceed tube ratings. No wonder they eat tubes for breakfast.

[gingertube]

See this thread:
KT88 and Hammond Transformer Schematics
In there I referred jbldriven to Kevin O'Connor at London Power in Canada and suggested one of Kevin's books he should buy. Kevin has a whole range of Guitar and HiFi designs based on Hammond Trannies.

In his "The Ultimate Tone " Volume 3 (TUT3) he presents designs based upon the ICONs of guitar amp tone - that is designs to "clone" some of the more famous amps from history and at the same time adddress any issues they had.
He has a design for an SVT Clone but with a simplified Ampeg Bass preamp using Hammond 1650W for the output and using a Hammond 717 for main HT, a 167S6 for output tube heaters and a 272DX for screen supply and driver and preamp HT.

This gives 735V HT on the output anodes and 370V on the output screens and for the driver and preamp.

3 pairs of 6550 in pentode mode into the 1650W will give you close to 300 Watt.

Might be worth buying the book.

If you're into guitar stuff there are also designs for:
Fender Bassman, Champ
Marshall Plexi, 800
Hiwatt Bulldog
VOX AC30
Ampeg Portaflex, SVT
Traynor Bassmaster, Custom Special, Guitar Mate

The Traynor Custom Special is a 160W unit.

An alternative power amp section which may have a tighter Bass Sound is found in Kevin's "Principals of Power" book (the one I recommended to jbldriven). This is a design using 3 pairs of KT88 in Ultralinear mode to drive the Hammond 1650W giving about the same power of 280W but with the tighter Ultralinear sound. Uses the same 717 HT Supply Power Tranny with separate filament tranny and another again for the preamp.

For total shreders there is also a design to use 3 pairs of fan cooled 6550 driven by a 6L6 driven interstage transformer to give 435W BUT thats a bit "over the top" in my (modest) opinion.


Cheers,
Ian