Recently I was interviewed about an amp I built, one that is my main gig amp. Steve Melkisethian of Angela Instruments played it about a decade ago and it left quite an impression apparently. He commented on the low end and how it held together nicely. Isn't your typical "Neil Young dirt box!"
Truth is it's all in the big knobs folks:
So, obviously that's not true. But there isn't much to it. First, a bit of background. I have no qualms with the stock 5E3. I'm a big fan of Neil Young and his sound and I've always liked the 5E3. I just wanted to build something a tad bit more modern for my own use. I prefer a tighter sound, something that is more clear. Most of my gigs I'm not using a mic. I've had little tube amps literally get swallowed up into the mix. I can't actually use that. I need to hear myself to have a good time.
The first thing I did was what I usually do: 47uf capacitor on the first stage, followed by a 22uf then an 8uf. That 47uf makes a difference in sag and compression. The original is 16uf. I also like that first stage to clean up any unwanted 60 cycle hum right off the bat.
The second thing I did was use a larger cathode bypass capacitor across the 250 ohm 10 watt resistor. You can experiment with values there, I used 100uf @100V. The Vox AC30 uses a 250uf cap there. Deep bass.
Photos of power supply and cathode bias arrangement.
Now these things helped me achieve my goal. But that's not all. The next thing I will freely admit I did not come up with on my own. I mean, same with what I discussed above, these are all things I just learned from others.......
I had a really good client when I owned Leighton Audio in Portland Oregon. He was a bit of a perfectionist. My eyes would roll whenever he came by cause I knew I'd work a little harder. But every time I worked with the guy I learned something new, so I always appreciated not only his business but also his "can we make it better?" attitude.
He brought in a 5e3 clone that he bought somewhere. Nothing special. It had a problem, bad tube or something. So when I got to it I noticed this 3.3Meg resistor that "shouldn't be there." I said in my mind "well, that's wrong" and yanked it on outta there.
He picked up his amp, got home and said "problem is fixed, but this amp had such a great low end and now it's flabby. I knew exactly what was going on, it was that mysterious resistor. So he brought it back in, I replaced it and we were both happy. Him to have his amp the way he liked it and me cause I learned what I wanted to do to my own amps.
So, what exactly is this mod? I can't say I know why it works, I can just say what the result is.
Take a 3.3Meg resistor from the last stage of the power supply and connect it to the grid of the phase inverter, the pin attached to the 1 Meg resistor. That's it. Voltage goes up on that grid and the cathode, amp sounds more clear, has tighter bass and if you like that hooray. If you don't simply remove it.
Photo:
I was asked if I would put it on a switch..... Switches aren't for me personally. I have enough on my mind when I'm on a gig. Volume, tone, guitar. That's as much bandwidth I need on my equipment so I can focus on the music. But if you prefer a switch, go for it and tell me your results.
Other bits about the amp... I used Mogami wire throughout the signal path. I learned this from working with an ace studio technician named Tim Hatfield back in 1992. It's nice to work with and sounds warm and clear. It's finally widely available again. I could find their shielded stuff from many sources but had to go through some channels to get this copper gold back in the day! I've used it for mic pre's, pedals....
Check it here: https://www.performanceaudio.com/mogami-w2514-hi-fi-red-hook-up-wire-by-the-foot.html
I also used nice Black Cat capacitors in the preamp along with Sozo caps as well. Good old mica cap for the treble....
Transformers are Heyboer / Mojotone Deluxe Reverb power transformer (lower voltage, more current, runs cooler!) and just a Mercury ToneClone for the output. I didn't want to go with bigger iron. For me it's a balance between high quality components and average. Black Cat caps aren't the best in other words, but they're then best for this kind of thing in my opinion.
And hey, this mod works on Princeton amps too! I like a Princeton, I've never loved them. They sound harsh and unbalanced to my ears when you push them. Not useful to me. I prefer my Deluxe Reverb for a 'modern' Fender amp.
Recently I had 2 Princeton Reverb amps come across the bench in one day. One was a museum quality 1965 that I was probably the first to turn a screw on. Original tubes, everything. Not a stain or scratch. I left that one alone.
The second was a butchered 1973 model. Someone had done plenty of "wha????" stuff to it. I needed to restore the thing so I figured I'd see if I am crazy thinking this mod is as good as I think it is.
Once I got the bugs sorted I blasted it and got that awful sound I usually get with the Princeton Reverb. Lovely at low volume, just awful to my ears at high volume. For me the 5e3 with it's simple volume and tone arrangement, that works for me. Not to mention the softer warmer cathode bias. With the Princeton's grid bias, scooped mids and more complex tone stack, it just never suited me.
Spot the 3.3Meg resistor:
It's right on the .022 cap feeding the phase inverter. The Princeton and 5e3 Deluxe have a very similar phase inverter.
Disclaimer: what you see above is not my work! It's sloppy but functional. This job was for a client who needed to keep the cost low. I would typically gut something like this and do it over!!!!
I've seen all kinds of complicated mods done to Princeton Reverbs. I've seen the tremolo scrapped so someone can re-do the whole phase inverter to be like a Deluxe Reverb. Ugh! The tremolo is the thing I like the most in a Princeton!! Try this one simple mod first if you want more headroom.
That's it today. Hope you got something out of this article. Please feel free to leave questions and comments below!
And here's me yammering on about this stuff on Skips podcast. I think he's doing a great service to those who like to play with capacitors! Thanks Skip!
-J
It’s nice to hear you get some well-deserved recognition on the international podcast circuit! Your blog is still more my speed, though.
ReplyDeleteI can’t wait to try this 3.3M phase inverter trick. I know a few players I think might really dig it.
Thank you! Glad you found it helpful. Let me know what you think when you try it. Best, J
ReplyDeleteA good mod option that 3.3M in 5E3, really. Do you know what is the (idle) voltage on the grid before and adding the resistor? I think the only thing that happens is that the voltage on the grid is raised to 25% of the B+ voltage of that tube, giving more headroom and better bass response.
ReplyDeleteI think you explained it really well here. That is likely what is going on.
ReplyDeleteA friend wants me to build him one after playing mine..... It's been years!
Thanks! J
One addition: since the PI triode does not (naturally) have a cathode bypass cap, the grid bias through 1M resistor follows the changes of cathode voltage. Taking the grid bias from B+ gives (IMHO) the same effect that having an infinite big cathode bypass capacitor. That is the reason for the better bass response. For a
ReplyDeletecomplete new 5E3 build you may consider to
a) connect the 1M resistor directly to ground instead of the 1.5k and 56k resistor joint to make sure the new bias does not affect the voltage swing at the cathode
b) try a bigger (e.g. 3.3k cathode resistor for even more headroom).
I had one other thought. Years ago I built a couple of hi fi amps using EL84 tubes, cathode bias, driven by a 12AX7 phase inverter. The PI was based on the Fisher SA16. Other than component values it's not too different from your typical cathodyne split load type of phase inverter like you find in a 5E3. The Cathode resistor on the second half of the 12AX7 is split though with a 50k resistor on the cathode then a 50k trim pot in series going to ground.The anode resistor is 85k. When I built those amps I was doing what I could to keep a simple signal path and avoid any feedback loops if possible.
DeleteOne of my mentors actually hipped me to this design. You could check it on the scope, adjust the trim pot and just see that distortion vanish when you hit the sweet spot. Look up the Fisher SA-16 schematic.
That's the kind of thing I love when I'm building a hi fi amp. On a guitar amp, the best thing I find is they only actually work so well. In fact, I'm not even all that fussy about matching tubes. If they're off that may be the ticket to Hollywood tonally speaking! The greatest guitar amps are pretty poor hi fi amps.
That being said, I would some day like to try this in a 5E3 type of amp. I actually do occasionally use my amp for my acoustic guitar on a gig I did pre-covid. It would be interesting to build something with a high quality 20-20k output transformer with a well balanced PI circuit. That will be a worthy experiment. It may fly or may just be "meh." Don't know till I try!
That sounds like a worthy experiment. I'll give that a try when I get a chance. I do think the arrangement as is also creates a bit of a feedback loop as well which would serve the purpose of lowering the gain some so I imagine using a bigger value resistor on the cathode may compensate for it.
ReplyDeleteThat being said, I myself am not necessarily going for any higher headroom than I already have. Trying to not get too far away from the 5E3 vibe which is just such a great amp to begin with. But these are really good ideas to experiment with so thank you for sharing!
That third filter cap,looks to be a 16uf, not an 8…?
ReplyDeleteHa! It's a 10uf. I likely ran out of 8uf when I built this.
DeleteCan this be done in a 73 deluxe reverb?
ReplyDeleteNope!
DeleteCompletely different circuit.
That being said it's not a bad idea to make sure your amp is in good health, biased properly and be sure it's the old AB763 circuit. Yours is probably an 868 circuit which is really similar. Remover those two 1500pf caps on the output tubes and you got it.
Thanks! I was looking at the 5e3, princeton and deluxe reverb circuits and couldn't make the connection. My deluxe runs a 5u4 and the 1500pf are gone. Running a k120 speaker sound real nice.
DeleteNice!
DeleteYeah, no amp really needs this mod but the Deluxe Reverb is already "tight." That is my amp of choice these days. I play a 1969 model for most gigs. This 5E3 is wonderful too.
I did work on a 1964 Princeton Reverb yesterday that was just incredible though. Shockingly clear, big tone, very open with a pleasant un-harsh overdrive. That is rare for me! I wouldn't change a thing on such an amp!
can you use Mogami interconnet cable for the grid inputs? Use it from the jack to grid and from the vol pot to grid? I'm curoius about trying it out? Thanks
ReplyDeleteMogami wire is high quality but at the end of the day it is just wire so the answer is yes. I use if for all of my signal paths. Mostly unshielded, sometimes I use shielded stuff if it's a high gain area or like on the first input stage in a black panel amp.
Deletethanks!
Delete