Hey Pete - no worries on speed - grounding is probably one of the most important aspects in terms of reducing noise... and you can't mess around with the :wife:'s birthday... :D
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Hey Pete - no worries on speed - grounding is probably one of the most important aspects in terms of reducing noise... and you can't mess around with the :wife:'s birthday... :D
OK Guys, I need your help.
Last night I fired her up. First I checked that all the heaters worked ok. Then flicked the standby switch with no tubes in. I checked the voltages on all the tube bases etc. Got the following: B+ = 450V,
Bias adjustment swing -20V to -39V.
I then put in a full compliment of tubes and turned the bias pot to maximum negative swing. I used the old stock HRD tubes in case any damage occurred, and connected the speaker. I flicked the standby switch with a very shaky hand and dived for cover.
There was no big bang, no blown fuses and no apparent damage to anything. However there was a loud 50 cycle mains hum through the speaker which just got louder as the bias pot was turned.
Voltages were now as follows:
B+ 389 V, Bias -37.7 V. I suspect something wrong with the copper cap arrangement unless of course I've done something idiotic. I recall JAM suggesting that the export PT might not have the stamina to cope with the copper cap so I'm going to try Full Wave Bridge Rectifier tonight. I should have measured the ripple on the caps last night but it was late and I wanted to tackle this with a fresh mind and maybe some advice from you guys.
Here's a few pics, you may be able to see something I missed.
http://images.lilypix.com/displayima...at=10425&pos=0
http://images.lilypix.com/displayima...at=10425&pos=1
http://images.lilypix.com/displayima...at=10425&pos=2
Pete
That's it. You should be getting 60 cycle hum. You need to move it over here. :odie:
I'm betting you put the electrolytic in the bias supply in backwards. + should go to ground.
Just a thought.
Originally posted by Kapn:No, the cap is connected with positive to ground. Reversing electrolytics usually results in a big bang followed by a spiral of aluminium foil and electrolyte. If you expand the pic you can just see the + is connected to ground at the yellow wire. The yellow wire is grounded to the PT chassis at the tagstrip.Quote:
I'm betting you put the electrolytic in the bias supply in backwards. + should go to ground.
Pete
And a really bad smell.Quote:
Originally Posted by TONESEEKER
If the cap is overspec'd for voltage, that doesn't always happen.
If you're getting a decent B+ then your rectifier
wiring is probably ok.
How are your filter cap grounds? Is the hum affected by the controls?
Tommy.
Kapn:Yeah, point taken. That cap is rated at 22microfarads 450 Volts. I've just taken delivery of some more parts today so I'm gonna replace both the bias circuit caps tonight. However as I've said, they are not incorrectly installed.Quote:
If the cap is overspec'd for voltage, that doesn't always happen.
Pete
This is the type of job where a o-scope is really quite handy.
No, I don't have one either. :bonk
Yeah, my first thought last night: A scope, a scope my kingdom for a scope. I reckon there must be some significant measureable ripple voltage on the B+. I wish I had thougt to check that last night. Tommy, I have used a 3 star grounding scheme: B+ cap , Bias circuit, and heaters are all grounded at the PT. Phase inverter and OT secondary are grounded midway down the chassis. The rest is grounded nearer the inputs. This 50 cycle hum is really loud. It's got to be Power supply.
Pete
I know you said it's set, but you may want to recheck the ground reference on the heaters. Those and the bias supply (half wave) are the usual sources of 50/60Hz hum. Power supply should be 110/120Hz.
I'm reasonably confident that the bias circuit is grounded ok. The heaters are grounded via 2 x 100 ohm resistors (Simulated centre tap). I will of course recheck everything tonight.
Pete
The reason I asked is because I had similar problem on my 5E3 build that ended up being a bad ground at the first filter cap.
Tommy.
Good point Tommy, I'll check that tonight.
Pete
I'm so glad you got a shot of the export PT...I was talking to someone in Norway recently in regards to the PT wiring and needed to see one.
Your tube socket mounting looks great. What did you use for the holes? A chassis punch or step-bit for drilling?
Yo TJ,
The sockets were a long tedious process of drilling and filing. Next time I'll use a Greenlee chassis punch. The PT wiring looks a bit of a mess. I cut out the small isolated area on the original PCB that contains the voltage selector spades. I mounted that on a standoff with a 5 pole tagstrip, centre tag to ground, other 4 terminating the heater and bios taps. Grounded the heater supply there with 2 x 100 Ohm resistors humdinger. Looks like my hum hasn't dinged.
Pete
How do you have the CC rectifier wired? The PS was designed for a FWB.
Hi JAM,
Pin 1 to ground, 2 x BA159 diodes from pins 4 and 6 (cathode). Anode to pin 1. PIN 2 to standby switch.
Pete
I had a problem with 60 cycle hum in my Bassman clone, it turned out that I had a tin whisker on one of the heater pins that was sending AC to the pin next to it and thereby directly into the OT.
Good point Wingnut. I'll check for that tonight. Thanks.
Pete
I dont know if it makes a difference in a CC, but Teds layouts with the CC always have the B+ coming off of pin 8. Pin 2 would normally be the other side of the filament on a 5U4/5AR4 etc rectifier.
This is how I do it:
http://www.tjadamowicz.com/amps/gall...-Simulated.jpg
Sorry JAM,
B+ is coming off pin 8. I got the circuit from TJ a while back.
Edited: Yeah that's the one TJ.
Pete
No problem.
It this case pins 2, 3, 5, and 7 should not be used.
Hi guys just got in - just gave Pete a ring - the scenario is thus - he's now got 423V B+ - max bias is -39V (not enough me thinks) the bias circuit is
http://images.lilypix.com/albums/use...l_JAM_bias.jpg
as per Joe's design for me (though Joe specified 25K where the 22K is - we can get 22K or 33K as prefered values here - I wound up using 2 51K's in parallel) - the HRDx hasn't got a CT anyway so disregard that - I think he needs more range at the lower end (ie -50V or so) but he didn't mention measuring what he's got on the cathodes (I know he's got at least one 1Ω resistor on both cathodes though I did recommend putting one on both) so I'm not entirely sure where he is with respect to acheiving 70% dissipation - it was a slightly hurried conversation but I think he could do with advice on what to do to adjust the range of his bias... :D
He's got a Deluxe PT, Rob. The Bias tap is 39VAC on this one and on the DeVille it's 27VAC. After rectification, the Deluxe should have -53VDC and DeVille should then have -74VDC
If he matched your component values, he'd be off the mark. Also, in my search for the right bias, I found that it affects B+ too, so expect that number to change.
After the diode, I have a 1.5K resistor and a 33K after the 25K-L trimpot and that gets me where it needs to be.
I'm using that same design in my build, but as others have posted, you need to tweak the values depending on how much the bias tap (or winding) puts out. In my clone, the resistor off the diode is 2.2k for example.
Is that the Bassman-in-Bandmaster build, JAM?
Yep. The one I call the 5F7-A.Quote:
Originally Posted by NTBluesGuitar
Originally posted by Wingnut1:
Hey Wingnut! You were right on the money. I checked every socket visually and couldn't see any problem so ruled it out. However after tearing my hair out for a couple of hours I realised there was a voltage missing from one of the phase inverter plates. I still couldn't see the tin whisker but it was there. A quick wipe with a hot soldering iron and my problem was solved. Thanks. :smile:Quote:
I had a problem with 60 cycle hum in my Bassman clone, it turned out that I had a tin whisker on one of the heater pins that was sending AC to the pin next to it and thereby directly into the OT.
However I do have other problems.
As you will see from Rob's post methinks the bias circuit will need tweaking.
There is something wrong in the preamp section. I get's nothing out of that at the moment, but I'm sure I'll manage to fix that tonight or the weekend. Had to abandon my troubleshooting last night as :wife:
was feeling neglected.
One interesting thing about the copper cap with the export PT. Rob had trouble lowering his B+ whereas I had trouble increasing mine. I only had 402 V with the 240 V selector. I had to go back to 230 V to get 423 V B+. Still a little bit short methinks but in the ballpark.
Joe, what if I bring that 18 K resistor down to about 12 K ???
Or should I use TJ's config ?
Pete
You know I think you should be ok with that B+. The original 5F6A's B+ was 432VDC. I like the sound of my Bassman clone with the B+ around there, FWIW.
Tommy.
Hi Tommy,
Yeah I suspected it wouldn't be a huge problem. I 've got this book, A complete Circuit Analysis of The Fender Bassman and I got a bit hung up on accuracy.
Pete
Where do you have your OT center tap in relation to the choke?
The HRD choke is small, like a BF choke, and I had to move my OT CT to the B+ point before the choke, as it was causing some interesting bias issues.
I bet you have your OT CT after the choke, and it's affecting your bias like it did mine.
Hi TJ,
My OT centre tap is connected to the B+ Before the choke, I think.
Edited: Just checked my schematic and layout. Yes it's before the choke.
Pete
If your bias still is too hot, that would help, but you might need to go even lower, 4.7k or so. Or raise the value of the 25K. Changing the pot to 50k will increase the range.Quote:
Originally Posted by TONESEEKER
Is there any sense in using the same values as the hotrod deluxe for the bias circuit. I am after all using the same PT, OT and PSU ? The only difference is the grid resistors on the 6L6's are 100 k instead of 220 k to avoid blocking distortion.
Pete
Why so low on the grids? I'm using stock 220k and have no problems. That low loses something IMO.Quote:
Originally Posted by TONESEEKER
I used the stock HRD bias circuit in mine. I just dropped the 100K down to 33K.
JAM, if I put a 50K pot in place of the 25K, would I be able to bias hotter? That is, would I have more hotter ranges and less lower ranges?
Those are the values Rob is using. He had some problems with blocking distortion. He also used 0.022 uF coupling caps. I think your right, it does lose a little bottom end but avoids the distortion.Quote:
Originall Posted by JAM:
Why so low on the grids? I'm using stock 220k and have no problems. That low loses something IMO.
Pete
I'm not 100% sure the problem Rob had was blocking distortion. Certainly had the symtoms, but after seeing the pic of his first build, it could also have been some parasitic oscillation.Quote:
Originally Posted by TONESEEKER
I guess there's no harm in trying the original spec 0.1uF and 220 k along with the HRD Bias circuit values. I don't really want to lose anything from the tone. If I get blocking distortion Rob will say "I told you so." :D :D
Pete