Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: alange5 on February 21, 2011, 06:34:07 pm
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Hello all. First post here.
I'm looking for some help with a recent 5f2a build. I have not yet powered it up, but I thought I'd check in here before I did. FYI I have previously built a 5f1 and an 18 watt clone and I'm familiar with how to safely drain the filter caps before tweaking.
I'm using a Weber PT and a Hammond 125ESE OT.
My questions:
1.) I'm a bit confused as to what to do with the OT primaries. The original Fender layout I'm using shows only one lead connected to pin 3 of the 6V6. The more modern Weber layout, however, shows the second lead connected to the positive side of the first 16uf filter cap. I connected the brown wire to the 16uf cap and the blue lead to pin 3 of the 6V6. Is this correct??
2.) My PT does not have a tap for the heaters. I attempted to do the artificial center tap method, but I want to make sure I did it right. My 6.3v green pair runs from the PT to the terminals of the lamp holder (one wire to each terminal). Also attached to the lamp terminals are two 100-ohm resistors going to ground. FYI I grounded these resistors at the same spot that I grounded the negative end of the filter caps. The green pair then extends from the pilot lamp terminals to the heaters. Did I set this up properly?
3.) Another area of concern is the grounding. I am not too familiar with proper star grounding methods, aside from the general concept of keeping the preamp and power section grounds separate. In my build, I ran the ground from the 25uf cap to the brass plate near the guitar inputs. I have the artificial center tap, filter taps, and power cord grounded at one lug of the PT, and I have the red/yellow PT wire and .05uf "death cap" grounded at another lug of the PT. Any suggestions to improve grounding?
4.) Last but not least, the speaker jack wiring. I have the yellow OT secondary and the lead from the 22k resistor both connected to one side of the jack, and the black OT wire connected to the ground side.
A lot to swallow, I know, but any help anyone could provide would be much appreciated.
PICS: http://www.flickr.com/photos/27321442@N05/sets/72157626105873764/ (http://www.flickr.com/photos/27321442@N05/sets/72157626105873764/)
Thanks!
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Original 5F2-A layout and schematic:
http://www.el34world.com/charts/Schematics/files/fender/PRINCETON_5F2A.pdf
1) Hammond wiring diagram
http://www.hammondmfg.com/125SE.htm
2) Heater wiring and virtual center tap sounds ok
3) Either follow the Fender layout (except heaters) or figure out how to do modern grounding.
http://www.el34world.com/charts/grounds.htm
Get rid of the "death cap" NOW.
Hot power cord => tip of fuse holder
side of fuse holder => switch
switch => PT hot
Power cord neutral => other black PT lead
(you can use one of the unused pins on the rectifier tube socket or a terminal strip as an anchor point - don't do a flying splice)
The safety ground (you are using a 3-prong cord, right?) should be grounded by itself, as close to the cord entry as possible. Make sure that wire is the longest one so it would be the last one disconnected if the cord ever got pulled out somehow.
Ground the two 16uf filter caps plus the first 8uf cap together (just like Fender). Ground the PT at the same point, or even better on the negative end of the first filter cap (reservoir cap to some).
Or just follow the Weber layout:
https://taweber.powweb.com/store/5f2a_layout.jpg
Hope this helps,
Chip
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Get rid of the "death cap" NOW.
Hot power cord => tip of fuse holder
side of fuse holder => switch
switch => PT hot
Power cord neutral => other black PT lead
(you can use one of the unused pins on the rectifier tube socket or a terminal strip as an anchor point - don't do a flying splice)
The safety ground (you are using a 3-prong cord, right?) should be grounded by itself, as close to the cord entry as possible. Make sure that wire is the longest one so it would be the last one disconnected if the cord ever got pulled out somehow.
Ground the two 16uf filter caps plus the first 8uf cap together (just like Fender). Ground the PT at the same point, or even better on the negative end of the first filter cap (reservoir cap to some).
Or just follow the Weber layout:
https://taweber.powweb.com/store/5f2a_layout.jpg
It's unbelievable the number of threads about being shocked, among the many amp forums. There must be a rash of these going on. People just don't understand the safety behind a good power feed (incoming line cord) ground. And, the confusion between that and signal ground. Well said, on your part! Thanks for posting.
Jack
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Thanks chip. Already yanked that cap from fuse holder to ground. As of right now my power cord ground is grounded at the same lug as the center tap for the heaters. I'll isolate it. Where should the artificial center tap be grounded in relation to the PT and the filter cap grounds? All at the same spot?
I fired it up to a loud squeal. I reversed the OT primary leads and now it sounds very nice. Hum is a little louder than I'd like. I'm planning on adding shielded wire connecting to the 68k resistors before the 12ax7 and re-doing the grounding. Hope that should fix the problem. Otherwise it sounds great! Very clean and responsive.
Could anyone point me in the direction of some standard voltage readings for the 5f2a?
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Could anyone point me in the direction of some standard voltage readings for the 5f2a?
Go to this page...
http://www.el34world.com/charts/valve/oldvalvedata.php
...Then search for 5F2A on that page. Several people have posted voltages there.
For the heater artificial CT, just ground it at the nearest PT bolt.
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I've never found a 5F2-A schematic from Fender with voltages on it. You can look at the 6G10 Harvard (http://www.el34world.com/charts/Schematics/files/fender/HARVARD_6G10.pdf) which is the exact same circuit but in a bigger cab with a 10" speaker. However, those are more like Blackface voltages IMHO. The 5F2 Princeton (http://www.el34world.com/charts/Schematics/files/fender/PRINCETON_5F2.pdf) has a choke filter before the power tube plate node, but 300-320 VDC on the 6V6 plate seemed to hit a sweet spot for me in my experiments with this circuit. Likewise, 150-170 VDC on the preamp tube plates sounds really nice (to my ears).
One thing to be aware of is that current-production 5Y3 rectifiers don't have the correct internal architecture. This is one case where you really need an NOS tube because the new 5y3s drop 25 volts less than an NOS would. Fortunately, NOS 5Y3GT rectifiers aren't terribly expensive.
I've attached a chart showing my initial voltages with a Sovtek (new) 5Y3, an NOS 5Y3, the 5F2 and the 6G10 specs from the schematics.
BTW putting a choke between the first and second 16uf filter caps makes a big difference in the noise level on this amp. I used a Hammond 156L (5 Henries, 75ma, 135 ohms resistance). Also a 6G10 Harvard sized cab and 10" alnico speaker.
Cheers,
Chip