The power transformer that I'm using might be putting out too high voltages for the amp, but that doesn't seem to affect the Vinny part... Could that be enough reason for the oscillations? I've tested with another (similar) reverb tank, and neither gets signal.
Hi Bodhi and everyone,
I'm new to amp building too, and wanted to have a crack at building a standalone Vinny based on the shared schematic of the Dizzy Bird.
Noting your comments on PT voltage, can I ask what brand/type you are using, or if anyone has details on the type Benson use?
There also seems to be an absence of node and plate voltages on the Dizzy Bird so it would be super helpful if anyone has measurements they'd like to share.
The power transformer that I'm using might be putting out too high voltages for the amp, but that doesn't seem to affect the Vinny part... Could that be enough reason for the oscillations? I've tested with another (similar) reverb tank, and neither gets signal.
Hi Bodhi and everyone,
I'm new to amp building too, and wanted to have a crack at building a standalone Vinny based on the shared schematic of the Dizzy Bird.
Noting your comments on PT voltage, can I ask what brand/type you are using, or if anyone has details on the type Benson use?
There also seems to be an absence of node and plate voltages on the Dizzy Bird so it would be super helpful if anyone has measurements they'd like to share.
cheers,
Roman
I'm using an Indel transformer with solid state rectifiers since it's something I can get my hands on fairly cheaply. I built a VVR to tweak the voltages to whatever they need to be, but probably burned the mosfet either because of missing filtering before it or bad heatsink connections (worked once...). This all meaning that I haven't experimented with voltages yet to see if and how it impacts things, but I would assume that if you can locate voltages for any other single-ended EL84 amp they should be in that range...
I believe that Aaron mentioned earlier in the thread that "voltages in the Dizzy Bird were similar" or something like that, but all in all it's something I still need to wrap my head around. Similarly for current usage, if you're building only the Vinny you could check other SE EL84 builds to see what they specify, so you can figure out what the power transformer needs to be.
BTW, managed to get reverb signal in the Tall Bird section; it was bad wiring of the reverb send jack since I misunderstood input isolation here (basically didn't have a sink with the send jack ground floating). Wrt. tubes, having peered at the Dizzy Bird gutshot, I believe the other section of the 12au7 is actually directly after the input jack, but I'm still in the process of adjusting my testbed layout to check.
CraigGa wrote: ↑Wed Oct 06, 2021 10:04 pm
First things first Mike,
Is US mode with just the cap or the cap and resistor?
I could never be sure of that but figured US was the brighter of the two options.
Craig
Good point. On the original amp, the US mode has the heavier bass of the two settings. The UK setting cuts the bass.
This review on YouTube really captures the effect and tone of the amp. He talks about it right around the 10 minute mark. https://youtu.be/UG2n1vBzZI8
Mike
In that case my labels are upside down
So I can say that UK is just the cap and US is the cap and resistor.
At present I've got it plugged into a V30 in my Cornford cab but I'll try it into the Eminence Eric Johnson in my Boogie Fillmore later.
I've not decided on which speaker to use yet, a friend is going to build me a cab for it and I'm planning on putting a reactive attenuator in there to make it more wife friendly.
bodhi wrote: ↑Sun Sep 26, 2021 7:37 am
I'm using an Indel transformer with solid state rectifiers since it's something I can get my hands on fairly cheaply. I built a VVR to tweak the voltages to whatever they need to be, but probably burned the mosfet either because of missing filtering before it or bad heatsink connections (worked once...). This all meaning that I haven't experimented with voltages yet to see if and how it impacts things, but I would assume that if you can locate voltages for any other single-ended EL84 amp they should be in that range...
I believe that Aaron mentioned earlier in the thread that "voltages in the Dizzy Bird were similar" or something like that, but all in all it's something I still need to wrap my head around. Similarly for current usage, if you're building only the Vinny you could check other SE EL84 builds to see what they specify, so you can figure out what the power transformer needs to be.
BTW, managed to get reverb signal in the Tall Bird section; it was bad wiring of the reverb send jack since I misunderstood input isolation here (basically didn't have a sink with the send jack ground floating). Wrt. tubes, having peered at the Dizzy Bird gutshot, I believe the other section of the 12au7 is actually directly after the input jack, but I'm still in the process of adjusting my testbed layout to check.
Thanks Bodhi, great advice
Be interesting to see how your reverb issues pan out
the output buffer 1m and 1.5k resistors are indeed tied to ground through a shared 100k resistor as noted earlier in the thread.
The 12au7 input seems to make the dry channel less dependant on the dwell pot setting, meaning it works better individually and seems to push more signal balancing the channels out (wet/dry pots work better). In turn the shared 100k resistor feels like it effects the dry channel more (in my setup where the reverb send is still problematic), so tweaking this from separated grounds to shared 10k (misremembered the earlier note) to shared 100k allows the dry channel to come through loud and clear. It even seems to go above unity where it starts to push the Vinny section, though this is more volume than distortion with the dwell pot values I can achieve so far without it starting to squeal.
My cathode resistor seems to measure 430R on the 12au7 reverb driver (I can tweak this easily), and I've been using a FB reverb tank, so on top of doing the math on my voltages those are the current leads I have to go on in my build.
Edit: Tried a 500R resistor, no meaningful change. Also tried a AB reverb tank I had on hand, but it had similar issues with squealing and distorting regardless of hearing almost no reverb signal, so probably need to have a look at the voltages finally...
mikeywoll wrote: ↑Mon May 03, 2021 4:30 am
I built the amp in the chassis and cabinet, and scalped the transformers and a few parts from a Blues Jr tweed model. This transformer lacks a center tap, but with a bridge rectifier (diodes scalped from the BJr circuit board) it puts out 335V at the power tube node. All of the voltages match the schematic well. The reverb tank is an 800 ohm tank and works (it's not a great tank but gets the job done). I added a VVR (I'm a fan), which causes predictable problems with the cathode follower driven reverb when turned down, but overall I like the VVR with this amp.
I also built the amp with vvr, no reverb circuit yet. Would it be possible to power scale the whole amp except for the 12dw7 tube, and get better reverb performance at heavy scaling?
jow21 wrote: ↑Wed Oct 13, 2021 5:44 pm
I also built the amp with vvr, no reverb circuit yet. Would it be possible to power scale the whole amp except for the 12dw7 tube, and get better reverb performance at heavy scaling?
The short answer to that is maybe it can be done, but it's not worth the hassle.
You have to split off the power supply at the input to the VVR, so you have a parallel power supply node that bypasses the VVR. Then you need to figure out the correct dropping resistor and cap combo to get voltage to that cathode follower that is close to the "default" plate voltage. You'll also have to split the plate resistor off for the reverb recovery stage and power it from the original power supply node that is subject to the VVR, since you'll want that to be scaled, maybe. The trouble is that a "normal" strength signal will heavily overdrive a scaled gain stage. So, enough signal to drive the reverb may be too much for the recovery stage. If you don't scale the recovery stage you will likely overdrive the second gain stage, or maybe only have usable reverb from 0 to 1 on the knob. Even if all that works out, I'm not sure how well the DC coupling of the first gain stage to the reverb cathode follower will work when the first stage is voltage scaled. You may end up in the same boat with too low a signal to power the reverb tank.
A pedal made way more sense to me. Especially since this amp returns the reverb fairly early in the pre-amp anyway. It gets that mushy overdriven reverb thing if you push the amp hard when you use the onboard verb. It can be cool, but there's not much of a difference between the onboard reverb and a good pedal in front of the amp. That first gain stage basically doesn't overdrive unless you hit it with a really hot signal. The rest of the signal path is shared from there through the speaker.
I mentioned earlier that my build was sounding a bit fuzzy on low notes from neck humbuckers when it was being driven.
I set about increasing the cathode bypass capacitors on the PI and output and that made a subtle change but nothing worth leaving them in. Then I changed the coupling cap between the preamp and PI to 0.047uF and that has made a big difference for the better. The UK/US switch still has a usable range too.
Just passing it on, in case anyone else hears the same.
I was looking at that filter 100uf/100uf can cap that was posted on page 28. The Monarch Reverb has obviously has some changes over the years. Looks like originally a first stage single 100uf/100uf can cap on the older one Monarch then a second 100uf/100uf can cap but only 1 capacitor side used. So filtering and dropping as
100uf --(1k)--> 100uf --(10K)--> 100uf
The newer ones looks to have 2 can caps at 50uf/50uf from some internet sleuthing of images on reddit. I can't quite tell but it appears that the new model has a 47uf cap paralleled to the 50uf. That'd give the first filter approximately 100uf (97uf). So newer and better reverb filtering as
I was looking at that filter 100uf/100uf can cap that was posted on page 28. The Monarch Reverb has obviously has some changes over the years. Looks like originally a first stage single 100uf/100uf can cap on the older one Monarch then a second 100uf/100uf can cap but only 1 capacitor side used. So filtering and dropping as
100uf --(1k)--> 100uf --(10K)--> 100uf
The newer ones looks to have 2 can caps at 50uf/50uf from some internet sleuthing of images on reddit. I can't quite tell but it appears that the new model has a 47uf cap paralleled to the 50uf. That'd give the first filter approximately 100uf (97uf). So newer and better reverb filtering as
Am I seeing this right? Has anyone tried the higher filtering like this? Just curious.
It's possible, but a 100uf reservoir capacitor on the tube rectifier is really big. Way over spec for that rectifier tube. If anything, I would guess it would be 50uf reservoir with 100uf on the screens.
EDIT: To be clear, I don't believe that the 100uf can cap is stock. The amp in question was being repaired for buzzing, likely someone boosted the caps at some point to try to quiet it.
We have a member here who has a newer monarch (not reverb) that otherwise matches the most up to date circuit. His has a 2x32uf JJ can and 50uf on the screens.
We also have the post where Chris Benson himself told us what the power supply looks like He's pretty clear... 22uf -> 1k -> 47uf -> 4.7k -> 22uf -> 4.7k -> 22uf. He even describes his reasoning surrounding feel and sound. FWIW, I used 30uf as a reservoir and couldn't feel a difference between 22uf and 30uf there. 100uf will be very different though, as that cap will really tighten up the amp and change the bass response, if it doesn't kill the rectifier.
Early in the process many people tried higher value filter caps and reported a different feel. They didn't seem to like it. You can always try it and report back.
If anyone else is having issues with popping when operating the voice switch then I've found that a 10meg resistor across the 250pF capacitor does the trick.
Hi Everyone - I've been following this thread pretty much from the beginning and have just taken the plunge to build my first amp having sourced all the necessary bits a pieces. I've been doing heaps of research re amp building taking a natural step off of making guitar pedals. Im planning on using BobL's layout (thanks Bob!) however I thought id double check before beginning whether there are any confirmed changes to that layout re the signal bleed? or any other recommendations?
The other Question I had was re the red/yellow CT wire and where most of you are connecting it? BobL's layout has it connected to the board however I've noticed heaps of pics have it connected to a ground lug..?
Also, my Hammond 290BEX (240v) doesn't have the green/yellow 6.3 CT so I suspect I use 100R resistors connected to the lamp to ground for an artificial tap?
I've been thinking of a way to implement a chimera "tonestack" in my monarch build. I tried the benson preamp approach but didn't liked the results. I suspect the chimera is done in a different way.
So I did some modifications that I really liked. I added a 1m pot in series with the existing 470k resistor and all that in parallel with the 250pf cap. The result came out pretty good, now I have the stock American mode and can shave off the lows but still wasn't getting the midrange quality of the British mode at extreme settings, as I suspect the Chimera also doesn't, based on YT demos (perhaps I should try a 2M pot?).
After that I modded the 1M pot to a no-load. Now I have the American mode and British mode available as well as everything in between. Sounds awesome!
Just wanted to share.
He's pretty clear... 22uf -> 1k -> 47uf -> 4.7k -> 22uf -> 4.7k -> 22uf. He even describes his reasoning surrounding feel and sound. FWIW, I used 30uf as a reservoir and couldn't feel a difference between 22uf and 30uf there. 100uf will be very different though, as that cap will really tighten up the amp and change the bass response, if it doesn't kill the rectifier.