How to spec a replacement reverb transformer?
Bluesbreaker
with 2 chassis (1 for the power and 1 for the preamp). After giving
it a cap job and replacing the output transformer, the normal channel
is now sounding pretty darn good! Whoo-hoo!
The trem/rev channel makes a loud pop when the volume is above 3 and I
pluck the guitar string hard (particularly the G string). The channel
just sounds pretty bad in general though -- it uses a 6GW8 to drive
the reverb, and I can see the tube socket arc from pins 5-6 (I
think). Anyway, I've replaced the socket and the tube, but still the
same symptoms. This happens even when the reverb tank is not
connected.
Gerald Weber was kind enough to inform me that the arcing is caused
from an impedance problem -- the reverb transformer is bad.
Can I use a standard Fender-type reverb transformer, meant for a
12AT7?
I was reading the specs on the following transformer:
http://triodeelectronics.com/tf160.html
It states "Matches 15K plate load to 8 ohm reverb tank input."
The 12AT7 has Ra = 11K ohms. The 6GW8 has Ra = 48K ohms.
Does this mean I should use a different transformer to match the 6GW8
to the reverb tank? If so, any suggestions?
WB
> Does this mean I should use a different transformer to match the 6GW8
> to the reverb tank? If so, any suggestions?
The spec also mentions " but could be used with tubes such as 6BQ7,
12AU7, or 6SN7 " ...
In other words, for $15 I'd try it.
Bluesbreaker
Thanks -- I guess "try it" qualifies as a suggestion, but not exactly
what I was hoping for. The other tubes mentioned all have Ra values
pretty close to 15K -- pretty different from the 48K Ra offered up by
the 6GW8 tube. A 33K mismatch is rather large and probably not very
efficient.
Other suggestions anyone?
WB
Refresh my memory .. Ra means ... A 15kZ load in parallel with a 8ohms
is just about the same as a 48kZ load ... trust me.
Phil Allison
"Bluesbreaker"
>
> Can I use a standard Fender-type reverb transformer, meant for a
> 12AT7?
>
> I was reading the specs on the following transformer:
> http://triodeelectronics.com/tf160.html
>
> It states "Matches 15K plate load to 8 ohm reverb tank input."
>
> The 12AT7 has Ra = 11K ohms.
** But Fenders run the two sections in parallel - making Ra 5.5kohms.
> The 6GW8 has Ra = 48K ohms.
** But the rated load for best power match is 7000 ohms.
All totally irrelevant to driving a Hammond or Accutronics reverb unit where
the max input power is less than 50 mW.
.... Phil
RS
Eh? 8 ohms would load down both types of tubes, but the transformer
will have a much higher primary impedance than 8 ohms.
OP: The driver stage in a 6GW8 is a pentode, and the plate resistance
can be expected to be much higher than that of a triode like the
12at7. The 'ideal' matching impedance is also calculated differently,
so for instance you'll often see the same type of output transformer
used for both triode and pentode versions of an amp (or an amp that
can be switched to triode mode) despite the considerable ratio between
the effective rp's.
There are calculations in the old tube tech books at the Millet site.
Just noticed that there are two different pages that come up, one via
a misspelling. Check both in case they're different (no time to do
that at the moment). Try Radiotron 4E first.
http://www.tubebooks.org/technical_books_online.htm
http://www.tubebooks.org/tecnical_books_online.htm
Phil Allison
"Bluesbreaker"
> I'm resurrecting a Guild Thunderbird Superstar. Its a 1 x 15" combo
> with 2 chassis
** This looks like the schem for the pre-amp.
The cathode resistor for the pentode part of the 6GW8 is not legible.
What value is it ??
... Phil
Bluesbreaker
>
> What value is it ??
Phil,
It looks like thats the schematic for the Guild Superbird.
Here's a link to the Superstar.
http://www.prowessamplifiers.com/schematics/images/Thunderbird-Superstar%20FMIC.pdf
The 6GW8's R(k) = 170R/2W
Thanks,
Andy
Phil Allison
"Bluesbreaker"
> The 6GW8's R(k) = 170R/2W
** Ok - forget the " 022921 " Fender reverb tranny - it will never work
in that circuit cos the standing DC current is WAAAYYY too high.
Go for the " 022905 " Champ output tranny instead.
Available from New Sensor and others.
.... Phil
Bluesbreaker
I have a Hammond ESE125 on hand (http://www.hammondmfg.com/
125SE.htm). Would that be safe to try? It looks like the expected
primary impedance (2.5K to 10K) is a good bit lower than 6GW8's plate.
I'm still confused as to how to properly determine a good replacement
transformer. If matching the impedance isn't really necessary for
driving the reverb tank, do I simply need to make sure the transformer
can handle the power rating? If so, could I use a Hammond 125BSE
rated at 5W?
Thanks again,
Andy
jh
Hi Andy,
the difference is that the standard Fender circuits are driven by
triodes. The optimal loading for a triode is rather different from that
of a pentode.
The 6GW8 likes to see about 7-10kOhms Plate load. The 48k figure is only
the "internal resistance" of the tube itself.
A SE OT which would work for a champ is pretty much in the ballpark of
this circuit. This is also the OT the 6G15 reverb unit uses for a
6K6/6V6. I bet your hammond fits in there nicely
http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/ECL86.pdf
regards
Jochen
jh
> On Sun, 18 Apr 2010 12:37:48 -0700 (PDT), Bluesbreaker
> <bluesb...@gmail.com>wrote:
> the driver and 5 watters on the secondary you'd be well off to use a 5
> watt xfmer if you have it on hand.
??
Phil Allison
"Bluesbreaker"
> Phil, thanks for your help and advice.
>
> I have a Hammond ESE125 on hand (http://www.hammondmfg.com/
> 125SE.htm). Would that be safe to try? It looks like the expected
> primary impedance (2.5K to 10K) is a good bit lower than 6GW8's plate.
** Should be fine - use the 4 ohm output.
> I'm still confused as to how to properly determine a good replacement
> transformer. If matching the impedance isn't really necessary for
> driving the reverb tank, do I simply need to make sure the transformer
> can handle the power rating? If so, could I use a Hammond 125BSE
> rated at 5W?
** Yes - it would be OK too.
The main issue is the DC current being drawn by the 6GW8, which in your case
is about 36mA.
The Fender reverb tranny is NOT designed to take anything like it and would
overheat PLUS the iron core would be heavily magnetised resulting is very
high distortion.
.... Phil