best o/p tranny for bass

[ernie_jr]

quick question,
building an amp with 2 6550's with about 525 B+. Which tranny would work best, a 55 watt Marsahll replacement (3400 ohm) 50 fender replacement (4250 ohm) or hammond 1560N (60 watt 4300 ohm)
Thank,
Ernie

[HotBluePlates]

What do each of them weigh?

For bass, pick the heaviest. That means it has the largest core, which will reproduce full power at the lowest frequency of the lot.

FWIW, we know the Hammond is 8 lbs and rated for full power down to 30Hz (I'm assuming you meant 1650N, rather than 1560N).

[triode]

HPB is correct for weight, but really, most (all?) for guitar built output transformers are designed
with a low frequency bandwidth of 70hz. Bass guitar (4 string, no drop tuning) does 39hz. If you
took a 100W guitar OT and fed 39hz into it, you will be way down in power output. For example,
the old dynaco A740 claims 35W at 20-20,000 hz, but 70W from 30-15,000hz. Ouch...
that is half power for 10hz.

Personally I have built a number of custom bass amps. I build them with HIFI iron. I used a 240W at
20hz output tranny in my last build (Dynaco A451). It weighs 23lbs. If you can find a Dynaco A431
(14lbs) it would be wonderful for a 100W bass amp. I use them in my Artemis amps. They will do 60W
at 20hz, 120W at 30hz. You can actually purchase new reproductions from Triodeelectronics (no affiliation).
I get mine from Mike at Magnequest.   

[RicharD]

http://triodeelectronics.com/dytr.html

[escopunk]

I use Mercury Magnetics with my amp, 6550/KT88, very tight low end, not flabby, very well defined. the one I have is MAR50-VM50-OS, $210 list, a little pricey, but I swear by them, I use them in all my designs.

http://www.mercurymagnetics.com/pages/mainframe.htm

[ernie_jr]

Thanks,
hammond 1650n best price $111, Dynaco a430 at triode is $134. Looks like the dynaco is the one to get.. I have 3 sunn 200s and I know the
dynacos work well.
now, do I go pentode or UL?
Ernie

[HotBluePlates]

And the Dynaco A430 going price is really 'same-as' the typical Hammond price (non-wholesale). That's a good deal for 20-20k.

As for pentode or UL, why not just have a switch to select between them? A double-throw switch can switch the screen resistor from UL tap to screen node of the power supply.

[triode]

As for pentode or UL, why not just have a switch to select between them? A double-throw switch can switch the screen resistor from UL tap to screen node of the power supply.

"2 6550's with about 525 B+"

He can not do UL with 6550 with 525V B+...

Note in the notes here: http://www.triodeel.com/6550ap3.gif

GE claims maximum screen voltage of 450V in UL PP. This is why Dynaco used KT88 for
70W in UL PP, as the 6550 is not recommended at that screen voltage (in UL). Go KT88/KT120
for UL 100W. Or a quad of 6550 and a different OT.

[HotBluePlates]

Thanks for the correction!

I blew right past the screen rating. Sorry, no UL for you. You'll need lower B+ for that.

[ernie_jr]

as far as the UL goes, I have 3 sunn 200s running sovteks 6550s that have between 525 and 500 B+ and I have had
no problems with the amps. I know it pushes the tubes, but so far they have held up well
Ernie

[triode]

as far as the UL goes, I have 3 sunn 200s running sovteks 6550s that have between 525 and 500 B+ and I have had
no problems with the amps. I know it pushes the tubes, but so far they have held up well
Ernie

Perhaps, but according to this: http://www.el34world.com/charts/Schematics/files/sunn/sunn_200s.pdf
it should be 480V at the plate, which would put the screens at like 450V.

Also, the Sovteks can perhaps take more abuse than a vintage GE. I would not stick expensive vintage
GEs in there... it is always a trade off when you start pushing tubes hard. Life vs sound. Also, we usually
push tubes over the limit for more gain and distortion. I thought this was to be a bass amp... (unless you
are shooting for that). The Sunn... I am not sure why it is like that. I have worked on a number of 200S
units... all with roughly 480V on the plate.

[ernie_jr]

Thanks for the input Triode. I use solid state rectifiers as Sunn did in the 190B amp ( a 200s with solid state)
So I am sure this is where is get the extra B+
Ernie

[ernie_jr]

scored a hammond 1650R on ebay. 100 watts, 5000 ohm,  (UL taps if wanted) 12 lbs, all for $90 and this includes shipping. Amp will have an Ampeg B25 preamp, sunn 200s PI, Sunn 200s power supply and KT 88's in pentode mode. Choke is a stancor 3.5 H 175mA. P/T is an old thordarson 400-400.
Will keep board posted.
Thanks for the advice,
Ernie

[PRR]

> but according to this:__ it should be 480V at the plate, which would put the screens at like 450V.

No, it shows 480 at the OT CT. Plates will be a few volts less. Screens will be between CT and plate. Say 480V CT, 470V at plates, 476V at screens.

Yeah, over-rating. Tung-Sol and GE assumed users of BIG amps would act conservatively and responsibly, and expect good long life. Ever since, everybody has been pushing the envelope.