Here are pictures of my 5b6 build in progress. The goal for this build is to get it done fast and cheap and see for myself what the fuss is all about concerning this circuit. I did splurge for Hammond transformers and PEC pots from AES when I probably could have used salvaged organ or HiFi iron. I'm using a 273BX for the power transformer and a 1750M for the output. I have an unfinished plywood cab made and given to me by a friend who deemed it too rough looking for his own less amateurish amp builds. What little else there is in this amp is from whatever was laying around in the basement. All told I expect to have a little less than $200 in it. I'm using the Tino Zolota layout stretched out to fit my big chassis an am adding grid resistors to the 6sc7.
So far it's been a fun, easy build and has come together very quickly. I should have time to finish it sometime in the next couple of days. If it sound as good as everyone claims, I think this amp could be for steel guitarists what the 5e3 Tweed Deluxe is for regular guitar: An unusual, good sounding early Fender circuit that is easy for a beginner to build as a first or second amp project, and remains a useful part of an amp arsenal. I'm hoping for an amp that is good for recording and lower volume gigs. I'm no pro amptech/builder by any means so feel free to comment and critique!
I could be wrong, but I thought the PT and the OT orientation should be at 90 degrees.
(Smacks forehead with hand.....) You're right! Did I mention I'm no pro? Good thing I'm not too far along. Since I have my power transformer wired up and I'll have to desolder the output transformer anyway I'll take this opportunity to try what is known "the headphone trick". This involves wiring up one of the secondaries to the otherwise unhooked output transformer to a 1/4" jack, plugging in a pair of headphones and putting wall voltage into the power transformer. The OT is the turned to the angle which allows the least amount of hum rejection. I'll have to re-drill the holes but I have chrome hole plugs and this amp was never destined to win any beauty contests anyway. Thanks for giving it a once over. I'm sure I'll make a few more obvious mistakes before its all put together.
Nice build... someone's already pointed out the transformer orientation... but your filament wires (although nicely twisted) are laying amongst the rest of the wires. Best practice is to have them flying straight up from the sockets as far from the other wires as feasible.
Looks good. That's an interesting project. Why did you choose that particular output transformer? I would have thought you would want one with a higher primary impedance. Say 6600 ohms. Output tubes are cathode biased, right?...Jerry
Cathode biased 6l6s. I used a step bit to drill my holes.
Everyone seems to be using an output transformer around 4k for this build. It is what the Tino Z book calls for, what Mark Fowler used successfully, and also what many Fender Tweed Bassman and Bassman head output transformers seem to be wound at. Tough to argue with what works! Data sheets seem to suggest something around 4k as well. That said, I've seen OTs for 6l6s as high as 6.6k. 4.2k will give me a little more power.
Stephen, I'm doing kind of the opposite of the Fender heater style here. The heaters will run along the bottom of the chassis and everything else comes in from above. Check out Ken Fisher's Trainwreck amps for an example of this. The Komet 19 also has some of this kind of heater wiring going on:
Looks good so far Matt, I too run the heater wires close to the chassis. I first saw that used on a Fender "94 Twin, so I tried it, and it is very quiet. The "94 wires weren't twisted either, just a zig-zag between tubes. Her is how I ran the heaters on my "Twin" clone. It is very quiet!
I always do mine the way Stephen suggests, but the other ways are also valid. Dr Z does not twist his heater wires either.
I do not reference my heaters to ground on my amps. I tie the two 100ohm resistors from the pilot lamp to the cathode bias resistor and reference them to about 35-40VDC.
I built one version of my Steel amp with DC on the heaters - that sucker was quiet! You can have every knob turned up to 10 and not even notice that it was on in a room.
I usually do the dc heater elevation as well. I ground each leg through a 300 ohm pot with the center lug attached to the top of the cathode bypass cap of the power tubes. However, with this amp I am trying to see how much I can get away with, so I'm using the center tap provided on the power transformer. I'll change it if it is noisy.
Not done yet. I got the output transformer flipped 90 degrees the day before yesterday and also did a little more of the wiring. Hopefully I'll finish it up tonight after work.
Can you supply me with vendors you used for Trany's and Caps? I need a 100W output trany and I have a super reverb, a super bassman, an epiphone w/2 6l6 and a few other amps that need caps.
Lots of progress! Everything is wired up as per Zottola and the amp is working and is pretty quiet. However, my preamp voltages are very low for some reason. Here are my voltages with the Zottola values in parentheses.
I double checked all my plate load resistors with the schematic and all the values seem correct. Anybody see anything that I'm missing? The amp actually sounds ok as it but definitely breaks up way too much for steel. I also have some ghost notes around the 12th fret. Even with the way-off voltages the octal tubes have a nice sensitivity to variations in right hand technique. I'm going to triple check everything tomorrow and then start adjusting plate load resistors.
Do you have a scope? Or another way to check for parasitics? The plate voltages on the PI are troubling... and do you have a link to the schematic you're using? Here's what I could find at SkemHeav:
If the values check out, its possible the current demands are too high and causing a voltage drop. I assume you tried another PI tube? The 6sc7 is getting its B from place and its pretty close.
Thanks guys, I have a few different 6SL7s I can try. I also have a scope but I want to get those voltages right before moving on to the ghost notes. Stephen, that schematic is mostly the same as the Zottola one. He uses 270k resistors where the original Fender uses 250ks. I'll work on my photographic technique and try to get some better pictures up.
I would think that 18W is not enough for a good clean sound, but at a low volume it might be just the ticket.
double check the PI resistor values - that is the first place I would look if there were ghost notes. Either that or maybe the output tubes are not matched. Or the PI triodes are not matched.
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