[Joe]

Hi all,

I have a Ampeg B-25 that I modified to run a pair of 6550's. This mod
involved removing the 5AR4 tube and using a 6L6 as a series voltage
regulator for the screen grids of the 6550 tubes. I copied this design
from an old Hi-Fi amp schematic I saw once. The rest of the amp is
original including the driver stages. The 6L6 voltage regulator uses
the 5.3 volt line for the filiment and the 6550's remain on the 6.3
volt line. As I remember a 7027A is the stock tube originaly designed
for this amp and those have a .9amp draw on the heaters. I am concerned
about filiment current. The heater voltage is currently at 6.37 volts
with 115volts on the primary. I adjust the screen grids to about 340
volts and idle the current on each tube around 40mils. The plate rests
around 520volts. This setup works very well giving over 70 watts out
before it clips. The amp seems to be able to produce a nice clean sine
wave right up to the last watt. Also adding a very SVT sound to the
amp. I did this mod before 7027's were easly found and used it for a
year like this, but now since the tubes are easy to get again and the
price of this vintage amp is rising, I feel the urge to "stock" this
amp back to original. I could add a filiment transformer to ease any
concerns, but i do not want to drill holes. I love the current tone of
the 6550's and have had several amps in the past that have used 5881 or
6l6 tubes. I really do not like the tone of the 6L6 for a electric bass
because they seem to sound passive, and I understand that a 7027 will
behave and sound like a 6L6. But I am also a fan of keeping an amp
stock if all possable given the re-sale price of this old amp is near
$550.

What do others think?


Joe

[DanF]

The 7027 is a 6L6 with a little different base pin-out. Just wire the
tube socket for a 6L6 but omit using pins 1 and 6 on the tube socket
and then you can use either the 6550 or 7027 or 6L6.
You are right, the 6L6 is kind of wimpy sounding for a bass amp, the
6550 is much better, but maybe I am biased to the 6550 because I think
that the original SVT is the best sounding bass amp I have heard, or
should I say, felt.
The 6L6 as a pass regulator is a cool idea... for a hi-Fi amp but kind
of counter productive for an instrument amp. The reason tubed
instrument amps sound so good is that their power supplies are not the
most rigid in terms of good regulation. They tend to sag quite a bit
under load.
I'd put it back to stock, if I were you and just build a bass amp to my
liking (6550's).
Just my opinion...Daniel



Joe wrote:
Hi all,

I have a Ampeg B-25 that I modified to run a pair of 6550's. This mod
involved removing the 5AR4 tube and using a 6L6 as a series voltage
regulator for the screen grids of the 6550 tubes. I copied this design
from an old Hi-Fi amp schematic I saw once. The rest of the amp is
original including the driver stages. The 6L6 voltage regulator uses
the 5.3 volt line for the filiment and the 6550's remain on the 6.3
volt line. As I remember a 7027A is the stock tube originaly designed
for this amp and those have a .9amp draw on the heaters. I am concerned
about filiment current. The heater voltage is currently at 6.37 volts
with 115volts on the primary. I adjust the screen grids to about 340
volts and idle the current on each tube around 40mils. The plate rests
around 520volts. This setup works very well giving over 70 watts out
before it clips. The amp seems to be able to produce a nice clean sine
wave right up to the last watt. Also adding a very SVT sound to the
amp. I did this mod before 7027's were easly found and used it for a
year like this, but now since the tubes are easy to get again and the
price of this vintage amp is rising, I feel the urge to "stock" this
amp back to original. I could add a filiment transformer to ease any
concerns, but i do not want to drill holes. I love the current tone of
the 6550's and have had several amps in the past that have used 5881 or
6l6 tubes. I really do not like the tone of the 6L6 for a electric bass
because they seem to sound passive, and I understand that a 7027 will
behave and sound like a 6L6. But I am also a fan of keeping an amp
stock if all possable given the re-sale price of this old amp is near
$550.

What do others think?


Joe

[Sun Dragon]

The B25 with some power supply improvements and 6550's is a very nice
sounding amp, compared to the Portaflex series it has a ton more tone,
remember even with the mods it still is a fairly low power amp, but using a
high efficiency speaker, with a W bin this amp can hold it's own. They came
with a very shallow 2-15 cab with CTS speakers. Those CTS speakers are a big
part of the old Ampeg sound, many of the later higher power Portaflex amps
people used JBLs, nice sounding, but it lost some of it's mojo IMHO.

I agree the SVT or V4 etc have the nicest preamps. The most powerfull setup
I have is an SVT preamp into a Fender 400 PS power amplifier, well over 450
watts at 60 hertz. The iron in these amps are much larger than the SVT.

With the 6550 its all about drive, and the Fender used a 6L6 to drive the PI
transformer, supplying current needed to light em up. The filaments also run
a bit hotter than 6.3v. (6.7v)

The amp doesn't leave the house unless I have prearranged a day laborer to
transport the damn thing.



Vin





"Bret Ludwig" wrote in message
oups.com...

DanF wrote:
The 7027 is a 6L6 with a little different base pin-out. Just wire the
tube socket for a 6L6 but omit using pins 1 and 6 on the tube socket
and then you can use either the 6550 or 7027 or 6L6.
You are right, the 6L6 is kind of wimpy sounding for a bass amp, the
6550 is much better, but maybe I am biased to the 6550 because I think
that the original SVT is the best sounding bass amp I have heard, or
should I say, felt.
The 6L6 as a pass regulator is a cool idea... for a hi-Fi amp but kind
of counter productive for an instrument amp. The reason tubed
instrument amps sound so good is that their power supplies are not the
most rigid in terms of good regulation. They tend to sag quite a bit
under load.


A bass amp is not a guitar amp. If you want the amp to mostly be a
distortion generator as most rock guitarists want then what you say is
well and fine but most bass players want definition, balls, impact and
lower extension because the bass serves a fundamental-no pun
intended-function in the music. So therefore they want more of a hi-fi
as opposed to guitar power amp.

If the stock amp was worth $6500 I would restore it but as is you
might as well just leave it the way you want it for $650. All the
"boutique" amps they sell cost several times this much.

What we found the most successful bass setup for the most people was
the preamp section of an Ampeg SVT driving a solid state or tube power
amp designed for PA or hi fi use. Hafler, Phase Linear, modified SWTPC
and several of the popular Japanese amps available in the late 70s and
early 80s were the more successful solid state amps and most people
liked them just fine. However, for tubes, McIntosh and Altec amps were
the most successful. The 1570B worked beautifully except several of the
lowest notes were cut off by the 60 Hz pass filter inherent in this
amp. (THis was before the 5 string with its low B, recall.) Some used
them anyway but you could replace the stock OPT with something else,
and I don't remember what, that would fix this. There was also the 260B
but it weighed so much as to be unroadworthy.

With the cannibalized SVT preamp-you used to be able to get them as a
service assembly for about $200, and use it with any amp you could tap
for B+ and heater voltage or cobble up a little supply-the big tube Mc,
Marantz and HK Citations we got in for service we would " operational
test" by hooking up a bass cab and this pre. I was never much of a
bassist but we had a guy in the shop that was the local answer to
Stanley Clarke, sort of, and he would go to town. His most impressive
act was to play two songs at once, like Chet Atkins, only on four
rather than six strings. He knew most of the
Porter/Mercer/Arlen/Gershwin/Berlin repertoire cold as well as the
latest fusion **** and was very impressive. He died a few years back
of a massive heart attack, they think, while working in the attic of
his house and when they found him the stench was so bad they let the
fire department burn it to the ground for training.