4x 6550 bass amp design

[stevesamps]

I have an amp running a Fender twin power transformer
that has some extra current at least 450ma
but I wish I had more B+...

I built this bass amp and its a dirty affair, likely typical
results for a "guitar amp guy"

all of a sudden I am reading tube manuals and
double checking transconductance
and primary Z ...

what is the best primary Z at about 400 410V for 4 6550?


I was thinking of using the SUNN model T
as a guide for the poweramp anyway

but I would settle for a louder cleaner fender twin
like beast

I like the sound of the preamp I built it is a
"clean amp by my standards

I used this output transformer I got on Ebay
from Risson its absolutely massive
NOS Fender from the CBS days

funny I also CBS'd the
PI a bit like we need to "silverface the amp hah theres a new one!

This Transformer
however I noticed is 5k, is that not way too high for 4 tubes
the twin OPT is 2k,

I know maybe I can crank up the bias till the tubes are near melting
and get more out of it clean or.. I also noticed as I add speaker cabs
and lower the plate to plate, it gets cleaner, but not that much cleaner

anyway sorry to ramble

I guess I want it to get loud first then dirty later.. I may just need more
B+ at this Primary Z or a lower primary Z.. am I thinking about this
the right way round as the brits say..?

It took me a while to shave the gain kinda everywhere in the PI and the preamp.. but, then it never makes a whole lot of power without the grind

Dont get me wrong its cool if you are in a Grand Funk cover band.


I am also starting to understand why those 8x10 cabs rule for tube bass
it seems the more baskets the better

Thanks for reading all this..

Steve

[Bob M.]

Steve,

I would check out a Sunn bass amp, like the 2000S power amp (4x 6550). Their power supply with dual chokes are also great and pretty heavy duty. You could marry this design to a Fender preamp or a Bandaxall preamp or even a Sunn preamp (I'd do all three). OTOH, you could do much worse than just copying the 2000S design. These are very nice, albeit heavy to carry around, bass amps.

Bob M.

[stevesamps]

Thanks for that , that is a better reference than the Model T

I have repaired a few of those, and they are not very inspiring..

I did some up with a cool mod for one that adds clean headroom

270k to ground right at the channel mixer the two 470k's
this dumps some signal and cleans the amp up

weird how they have
two stages in series and then the tonestack
I guess they were going for some "gain?

that amp also grinds it out and the distortion is
well.. awful its a better clean machine

[Steve Conner]

I bought some transformers from Risson too! Do you have the Dual Showman one, or the 300PS one?

[stevesamps]

not sure which one

I think he said it was 160 to 200w

I am starting to think it was designed for 6 tubes

[Steve Conner]

Well, the 300PS one was rated 300 watts, and if you dropped it on your foot you'd need a whole new foot. It weighed about 15lbs.

You probably have the somewhat smaller one.

[Wilder Amplification]

Of course if you really wanted to build a bass amp that utilized 6550s to their fullest potential, instead of running the screens on the same B+ rail as the plates you could use a power transformer that has a separate screen winding to reference the screens at 1/2 the plate voltage, which will allow you to get the plate voltage up there higher without taxing the screens where you could milk 100 watts out of each pair of 6550s. You'd also wanna consider a proper plate-plate load in that design though. Just a thought.

[Steve Conner]

Wilder has a point. You can probably reconfigure that PT to give you twice the voltage. If it was meant for a rectifier tube, use a bridge, and if it was meant for a bridge use a doubler. In both cases you can still get output at the original 400v for the screen supply and preamp.

If the primary really is 5k, then the extra voltage will help a lot, you'll get more power and headroom. You won't want to bias the tubes too hot, though.

[andrew_k]

And if you can get closer to 680V plate /340V screens then your 5K OT will be a good match for a pair of 6550 putting out 100W. My 2x6550 amp uses 5.5k primary OT with those voltages and easily achieves 100W of "clean". A full-wave voltage doubler will allow you to tap the screen/pi/pre B+ from the middle of the two caps in the doubler circuit.

[imaradiostar]

two thoughts:

1) buy an antek inc toroid to use as a PT- you could use an AN3T275 with a bridge rectifier/center tap arrangement to get 375 for the screens and almost 800 for the plates. A smaller 2T230 version would do about 310 screens and 620 plates, give or take.

2) find a transformer (any of a number of random transformers) to add voltage to your B+ to achieve a higher plate voltage- use it with an addition bridge rect or whatever so your screen and preamp supply would be the output from the twin PT and the plate supply would be the smaller PT's supply stacked on top of the output from the twin supply. I have a an old o-scope PT at home with 150, 150 and 165 windings. By stacking them with bridge rectifiers I end up with about 400 for screens and a little over 600 for the plates- should keep 6550's relatively happy, might need largish screen resistors but nothing too severe.

jamie

[jrfrond]

Forgive me for being plain ol' down, dirty and quick, but it seems to me that if you purchase replacement transformers for a 100W Marshall e.g. model 1959, you could pretty much just drop them into your design. Unless you are looking to run ultralinear to squeeze every watt you can out of the 6550's, this will float your boat just fine.

[Wilder Amplification]

And if you can get closer to 680V plate /340V screens then your 5K OT will be a good match for a pair of 6550 putting out 100W. My 2x6550 amp uses 5.5k primary OT with those voltages and easily achieves 100W of "clean". A full-wave voltage doubler will allow you to tap the screen/pi/pre B+ from the middle of the two caps in the doubler circuit.

I actually had a question about that. I thought about going that route myself as opposed to doing the screen rail off its own winding and I'm curious to know if you would run into "power supply dragging" going that route? Do you happen to have any experience with amp circuits running that power supply config? I would think not if the winding was sufficient enough to supply the current while maintaining its "stiffness" but not sure as I've never built one with that setup.

@Steve Connor, I noticed you recommended to use a doubler in your post above. Won't going that route only get you 1/2 the current? Or would that not matter as it would still be 100 watts, just with a different voltage/current ratio?

[andrew_k]

Do you happen to have any experience with amp circuits running that power supply config? I would think not if the winding was sufficient enough to supply the current while maintaining its "stiffness" but not sure as I've never built one with that setup.

Not the "split doubler" config, no. As such it was irresponsible of me to suggest its use. I have built multiple amps from scrap using voltage quadruplers and there is a substantial drop in HT compared to a more traditional single transformer/rectifier config. The amp I was referring to above has a power supply designed with much help from forum members here, and uses a very affordable Antek AT230 toroidal. The final PSU schem with as-built voltages can be seen here -
http://notinteractive.com/stuff/guit....0-asbuilt.gif

Eminar amplifiers used the voltage doubler topology for their large amps, but even for their 6550 or KT88 amps they would use separate windings for the screen and plate supplies. I have attached a diagram showing the two types of power supplies most common in their large valve amps.

[Wilder Amplification]

Not the "split doubler" config, no. As such it was irresponsible of me to suggest its use. I have built multiple amps from scrap using voltage quadruplers and there is a substantial drop in HT compared to a more traditional single transformer/rectifier config. The amp I was referring to above has a power supply designed with much help from forum members here, and uses a very affordable Antek AT230 toroidal. The final PSU schem with as-built voltages can be seen here -
http://notinteractive.com/stuff/guit....0-asbuilt.gif

Eminar amplifiers used the voltage doubler topology for their large amps, but even for their 6550 or KT88 amps they would use separate windings for the screen and plate supplies. I have attached a diagram showing the two types of power supplies most common in their large valve amps.

I really like how you used 2 x 230VAC windings then used the screen supply to elevate the plate supply. Kinda the same thing as a 460VAC CT winding with the FWB doubler in a way, but done on the DC side of the rec circuit rather than on the AC side so the screens aren't really taxing the plate winding in that fashion.

Not to thread jack but...a couple of questions...would it be better to use the choke right after the BR as opposed to between the PI and the screen node so that you still get choke regulation on the screen tap? Or will the kick back from the choke damage the BR if it doesn't have a filter cap or resistor upstream of it?

Also, what's the purpose of the 820R/330K divider directly after the BR?

And why two seperate heater windings for power and preamp tubes? Any advantages to this?

Also dig the multiplier circuit on the bias winding.

EDIT: Here is a scheme for a proposed dual rail supply for my proposed 2 x 6550/KT88 @ 100 watt build I just drew up tonight. Voltages shown are my calculated "loaded" voltages. Think it's stiff enough?

[soundmasterg]

I happen to have an output transformer that was in a Sovtek MIG 100U with a quad of EL34's that is 1650 ohms and would probably work well for what you're doing. If you're interested in it, then PM me. I'll let it go moderately cheap. I have a choke too from that amp that I could also let go cheap.

Greg